Symposium Organizers
Zhenan Bao Stanford University
Alejandro L. Briseno University of Massachusetts
Vitaly Podzorov Rutgers University
Iain McCulloch Imperial College London
II3: Materials and Patterning
Session Chairs
Wednesday AM, April 07, 2010
Room 3001 (Moscone West)
9:00 AM - **II3.1
Photolithography for Organic Electronics.
John DeFranco 2 , Alex Zakhidov 1 , Jin Kyun Lee 1 , Hon Hang Fong 1 , Christopher Ober 1 , George Malliaras 1
2 , Orthogonal Inc., Ithaca, New York, United States, 1 Materials Science, Cornell University, Ithaca, New York, United States
Show AbstractA critical step for the realization of organic electronics is the availability of patterning techniques that are compatible with these materials. Although great strides have been achieved in our ability to pattern organics, the techniques used in the mature and entrenched industry of silicon processing have made little impact in this field. This is primarily due to incompatibilities between chemicals used in photolithography and the vast majority of organics. Overcoming these incompatibilities promises a breakthrough in the manufacturing of organic electronics since it would provide for massively parallel output along with process knowledge and equipment already available from a very successful industry. We report on a few generic approaches for the photolithographic patterning of organic materials using sacrificial layers as well as photoresists that can be processed with solvents that are orthogonal to organics. We demonstrate the applicability of these approaches to the additive and subtractive patterning of several organic semiconductors, including polymers and small molecules. The application of photolithography to pattern various organic devices with micron-scale features is demonstrated.
9:30 AM - **II3.2
Organic Thin Film Electronics on the Molecular Scale.
Marcus Halik 1
1 Materials Science Dept., University Erlangen-Nurnberg, Erlangen Germany
Show AbstractSelf-organizing molecules are promising components to serve as active layers in high performance, low cost, and flexible electronic devices. The process of self-assembly can be used in or even outperform printing technologies in future device fabrication due to the molecular induced driving force of voluntary film formation – serving surface – and the local selectivity and full control on film thickness. This contribution highlights the use of self-assembled molecular layers in organic transistor development. We start with simple n-alkyl molecules as monomolecular dielectric in organic transistors. This concept is suitable for a wide range of semiconductor materials (p- and n-type, nanoparticles, polymers) and enables CMOS like organic electronics with reduced power consumption even on flexible substrates. An extended concept will be introduced, by which a mixed self-assembled monolayer containing aliphatic and electron-accepting components is employed as an ultra-thin molecular gate dielectric to facilitate reversible, non-volatile electronic memory functionality in organic transistors.Finally, organic transistors will be demonstrated, in which the dielectric layer and the semiconductor layer are created by self-assembly of one molecule. With specially designed molecules the realization of p- and n-type self-assembled monolayer field effect transistors (SAMFETs) is possible.
10:00 AM - II3.3
Flexible Printed Integrated Circuits Based on Organic Solution Processed Materials.
Ana Claudia Arias 1 , Tse Nga Ng 1 , Jurgen Daniel 1 , Sean Garner 1 , Gregory Whiting 1
1 , Palo Alto Research Center, Palo Alto, California, United States
Show AbstractWe are developing flexible sensor tapes to detect the occurrence of events that cause traumatic brain injury (TBI). TBI is a medical condition that is cumulative and triggered by events such as blast pressure waves, noise and acceleration. Inkjet printing, laser machining and lamination are employed during fabrication, with inkjet printing being the main material deposition method. The sensor tape has integrated sensors, signal conditioning electronics, non-volatile memory and a thin film battery. The sensors are based on piezoelectric polymers such as PVDF due to low-power requirements, low drift and relatively simple fabrication. The availability of n and p-channel solution-processed semiconductors enables the fabrication of complementary circuits which have the advantages of lower power consumption and simpler design compared with unipolar circuits. We have demonstrated all printed p-type TFTs with mobility of 1.6 cm2/Vs and n-type TFTs with mobility of 0.8 cm2/Vs. We have characterized the charge trapping rates for n- and p-channel devices and assessed the inverter gain and noise margin. All printed inverters showed a typical gain of 8 with VDD at 10V and -3dB cutoff at 150 kHz for a load of 0.02pF. We have integrated printed amplifiers with printed polymer based-accelerometers and pressure sensors and showed operation between 0-1000g and 5-100 psi respectively.
10:15 AM - II3.4
Highly-Aligned Semiconducting Polymer Films.
R. Kline 1 , Lee Richter 2 , Daniel Fischer 3 , Dean DeLongchamp 1 , Michael Toney 4 , Iain McCulloch 5 , Martin Heeney 5
1 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Surface and Microanalysis Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 4 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States, 5 , Imperial College, London United Kingdom
Show AbstractOrganic electronics has the potential to replace inorganic materials as the active layer in low-cost and large-area electronics. These applications cover applications across displays, sensors, lighting, radiofrequency identification, and photovoltaics. In each of these devices, the performance and properties of the materials will determine the market penetration. For thin-film transistors, the charge carrier mobility is the primary performance metric. The charge carrier mobility is directly related to the molecular packing within the film. A better understanding of how the underlying molecular structure of the semiconducting polymers affects the local packing of the molecules would greatly aid the development of new materials. We have applied a method called flow coating to obtain preferential alignment of molecules of poly(2,5-bis(3-n-alkyl-2yl)thieno[3,2-b]thiophene) (pBTTT) across large-area substrates. We have found the method to result in biaxially oriented films with the backbone of the polymer molecules preferentially oriented in-plane. Interestingly, the high degree of orientation does not occur immediately after flow coating. These films initially have only about a 10% net orientation in the direction of the flow. The large-scale orientation occurs after recrystallization of the films from the melt. We have used a series of insitu measurements of microstructure and orientation to determine the source of the film reorientation on recrystallization. As flowed films studied by grazing-incidence x-ray diffraction (GIXD) show a small, net in-plane orientation. Insitu GIXD revealed that the films maintain this small in-plane orientation in the “isotropic” melt. Upon cooling through the crystallization temperature into the mesophase region, the in-plane orientation increases and the lamellar spacing returns. Further cooling results in a gradual increase in the orientation with the degree of orientation maximizing in the room temperature crystal phase. Heating the films well above the melting temperature or holding the films in the melt for an extended period reduces the residual in-plane orientation and results in an isotropically oriented film after cooling. We will show how these measurements allow this method for in-plane alignment to be generalized to other semiconducting polymers. We will also show how these macroscopically aligned films can be used for single crystal diffraction for solving details of the packing structure not possible to determine for un-aligned films.
10:30 AM - II3.5
Organic CMOS Logic Papers Customized Using At-Home Inkjet.
Tsuyoshi Sekitani 1 , Koichi Ishida 1 , Naoki Masunaga 1 , Ryo Takahashi 1 , Shigeki Shino 2 , Ute Zschieschang 3 , Hagen Klauk 3 , Makoto Takamiya 1 , Takayasu Sakurai 1 , Takao Someya 1
1 , University of Tokyo, Tokyo Japan, 2 , Mitsubishi Paper Mills Ltd.,, Kyoto Japan, 3 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractWe have demonstrated the manufacturing of user-customized logic paper (UCLP)—paper in which organic CMOS logic cells are embedded. UCLP provides on-demand customizability to the users by making use of commercially available inkjet printing at home. For the logic cells interconnection, we adopted a newly developed technique for sintering conductive Ag nanoparticle ink at room temperature, which can be patterned using at-home inkjet printer [1]. Organic CMOS inverters customized by this technology can operate within 2 V and signal-gain is achieved to more than 30. Furthermore, stage delay of customized organic CMOS ring oscillators is 0.12 s, indicating excellent feasibility of the UCLP.UCLP comprises an array of vias and organic CMOS based logic cells. Organic CMOS are prefabricated and embedded in papers, and field customizability is provided by the at-home printed interconnects. Organic CMOS were manufactured by vacuum evaporation through metal-shadow masks. A gate dielectric was obtained as a combination of a thin layer of aluminum oxide (thickness: 3.6 nm) and a molecular self-assembled monolayer (SAM) (thickness: 2.1 nm) of n-octadecylphosphonic acid [2]. P-type pentacene or n-type F16CuPc organic semiconductor was used for channels. At 2 V, the mobilities of p-type pentacene and n-type F16CuPc transistors were 0.3 cm2/Vs and 0.02 cm2/Vs, respectively. For the logic cells interconnection using inkjet printing, we adopted a newly developed conductive Ag nanoparticle ink, which can be sintered at room temperature and exhibits the sheet resistance less than 0.4 Ω/square. The design rule of the printed interconnects is 50 μm. Organic CMOS cells are interconnected by printed Ag nanoparticle ink at room temperature to form organic CMOS inverters and ring oscillators, which exhibit excellent electrical characteristics, demonstrating excellent feasibility of this technology.UCLP can be utilized in a wide range of printable electronics products, including flexible displays, electronic papers, and solar cells; UCLP can also be used in education.This study was partially supported by JST/CREST, the Grant-in-Aid for Scientific Research (KAKENHI; WAKATE S), NEDO, the Special Coordination Funds for Promoting and Technology. [1] Ishida, et al., 2010 IEEE The International Solid-State Circuits Conference. [2] H. Klauk, U. Zschieschang, J. Pflaum, M. Halik, Nature 445, 745-748 (2007).
10:45 AM - II3.6
Hybridization of Conjugated Polymer and Functionalized Single-walled Carbon Nanotubes and Its Effect on Charge Transport in Printed Hybrid Transistors.
Do Hwan Kim 1 , Jiyoul Lee 1 , Bang-Lin Lee 1 , Byungwook Yoo 1 , Hyunsik Moon 1 , Jeong-Il Park 1 , Bon Won Koo 1 , Sangyoon Lee 1 , Seiichiro Murase 2 , Jun Tsukamoto 2 , Wi Hyoung Lee 3 , Kilwon Cho 3
1 Display Laboratory, Samsung Advanced Institute of Technology, Berkeley, California, United States, 2 Electronic & Imaging Materials Research Laboratory, Toray Industries, Inc., Shiga Japan, 3 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractOrganic thin-film transistors (OTFTs) have recently attracted considerable attention owing to their potential applications in large-area, low-cost, flexible, and printed electronics. To achieve OTFTs with high-performance, most of the recent research in this has mainly focused on new molecular design, interfacial control of dielectric–semiconductor and semiconductor-metal, and device optimization. However, the charge-carrier mobilities of conjugated polymer films are generally limited by hopping transport (lower conductivity) between polymer chains in disordered regions compared on inorganic semiconductors. Therefore, the use of conjugated polymer blends with inorganic functional nano-materials as a channel part has brought a new solution to enhance the electrical performances comparable to inorganic systems.In particular, the hybridization of conjugated polymers and single-walled carbon nanotubes (SWNTs) usually enhances the combined attributes of each component such as conducting properties and crystalline nanostructures. In other words, SWNTs reduce the effective channel length and the contact resistance as well as enhancing the charge generation at the interface of conjugated polymers/SWNTs and improving the nanomorphology and crystallinity in films (local epitaxy). Generally, SWNTs are dispersed by using surface modification or surfactants, which are likely to disturb the charge transfer between conjugated polymers and SWNTs. In contrast to these methods, in this study, we used poly (3-hexylthiophene), P3HT as an interfacial conducting channel between conjugated polymer and SWNTs because P3HT is a popular conducting polymer with high-crystallinity and solubility. Despite the improved field-effect mobilities exhibited by OTFTs with SWNT-based hybrid system, little is known about how the intermolecular packing and conformation between conjugated polymers and SWNTs in the hybrid thin films affect the charge transport in printed transistors. Here, with a view to enhancing the field-effect mobilities and elucidating the hole generation effect in printed hybrid transistors, we focused on the effect of the density of SWNTs on threshold voltage shift as well as charge carrier mobilities using ink-jet printing. We found that at the optimized SWNTs concentration, hybrid TFTs have high field-effect mobility of 0.3cm2/Vs. The enhancement of the field-effect mobility over that of the pristine film is due to enhanced intermolecular packing and the high conductivity of the P3HT-functionalized SWNTs which act as conducting bridges in the printed hybrid film. Furthermore, in this study, we have demonstrated that the charge density in the hybrid channel can be tuned by the surface dipole of dielectric layers controlled by varying the type of functional groups in self-assembled monolayers (SAMs).
11:00 AM - II3: Materials
BREAK
II4/GG6/HH8: Joint Session: Advancing Organic Photovoltaics I
Session Chairs
Zhenan Bao
Venkat Bommisetty
Garry Rumbles
Wednesday PM, April 07, 2010
Room 3001 (Moscone West)
11:30 AM - **II4.1/GG6.1/HH8.1
``Plastic” Solar Cells: Self-assembly of Bulk Heterojunction Nano-materials by Spontaneous Phase Separation.
Alan Heeger 1
1 Center for Polymers & Organic Solids, UC Santa Barbara, Santa Barbara, California, United States
Show AbstractThe achievement of 6% power conversion efficiency and the demonstration of quantum efficiencies approaching 100% have been demonstrated: Each photon absorbed leads to a (positive and negative) pair of mobile charge carriers, and all the photo-generated charge carriers are collected at the electrodes. Higher efficiencies will come from improved harvesting of the photons from the solar spectrum using new semiconducting polymers designed and synthesized for use in “plastic” solar cells.TEM studies of thin films, of cross-sectional images “sliced” from this films have revealed the details of the morphology in BHJ solar cells.Progress during recent months will be summarized.
12:00 PM - **II4.2/GG6.2/HH8.2
Bulk Heterojunction PV from the Acceptor Perspective.
Jan Hummelen 1
1 Stratingh Institute for Chemistry, University of Groningen, Groningen Netherlands
Show AbstractThere is ample room for power conversion efficiency improvement of plastic solar cells. Progress on improved donor polymer ingredients recently led to substantial efficiency enhancement. At the other side, much can be gained at the acceptor side as well. We will give an overview of recent developments on tailored and electronically optimized fullerene-based acceptor materials for bulk heterojunction application. We will address recent investigations on the dynamics of hole transfer in various polymer:methanofullerene blends. For the first time, time constants for hole transfer in these blends have been determined using ultra-fast optical spectroscopy. Furthermore, we report on our recent reconnaissance studies concerning advanced photon management (i.e. photon up-conversion) inside bulk heterojunction composites.
12:30 PM - **II4.3/GG6.3/HH8.3
The Effect of Three-dimensional Morphology on the Efficiency of Hybrid Polymer Solar Cells.
Rene Janssen 1 , Stefan Oosterhout 1 , Martijn Wienk 1 , Jan Gilot 1 , Jan Anton Koster 1 , Ralf Thiedmann 2 , Volker Schmidt 2 , Svetlana Van Bavel 3 , Joachim Loos 3
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands, 2 Institute for Stochastics, Ulm University, Ulm Germany, 3 Laboratory of Materials and Interface Chemistry, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractHybrid conjugated polymer – metal oxide photovoltaic devices have been prepared with AM1.5 energy conversion efficiencies of about 2% via an in-situ approach in which a metal oxide network is formed in the semiconducting polymer layer via reactive spin coating and moderate thermal annealing. High resolution three-dimensional experimental data on the morphology and phase separation of these bulk heterojunction solar cells will be presented that provide unprecedented insights into the actual heterojunctions present in the films. The phase separation will be related to the device performance via exciton diffusion and exciton quenching, and the presence of suitable continuous percolation pathways for photogenerated charge carriers to reach the respective electrodes in both phases. The data provide a unique new insight in the operation of bulk heterojunction devices and provides directions to further improvements.
II6: Poster Session: Charge Transport and Devices
Session Chairs
Thursday AM, April 08, 2010
Salon Level (Marriott)
9:00 PM - II6.10
Temperature and Density Dependent Channel Potentials in High-mobility Organic Field Effect Transistors.
Martijn Kemerink 1 2 , Toby Hallam 1 , MiJung Lee 1 , Ni Zhao 1 , Mario Caironi 1 , Henning Sirringhaus 1
1 , Cavendish Laboratory, Cambridge United Kingdom, 2 Applied Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractOver the years, a large number of fundamentally different models has been proposed to describe the charge transport in organic semiconducting materials. For amorphous, low-mobility materials such as poly(phenylene-vinylene)s there seems to be consensus on a description in terms of variable range hopping (VRH) of polaronic charges between localized states in a density of states (DOS) that is broadened by energetic disorder. The shape of the dominant low-energy tail of the DOS is usually assumed to be either exponential or Gaussian. For (poly)crystalline high-mobility materials like poly(thiophene)s, oligo(acene)s and oligo(thiophene)s the situation is less clear and consensus on the preferred model(s) has not been reached.In this work, we use high resolution, variable temperature scanning Kelvin probe microscopy to measure the electrostatic potential distribution V(x) across the channel of an organic field effect transistor while simultaneously measuring the electrical device characteristics. By working in ultra high vacuum spurious effects of absorbed water could be avoided. We show that the density dependence of the mobility gives rise to a V(x) that fundamentally differs from that of a constant mobility. Moreover, the temperature dependence of V(x) can be used to discriminate between various mobility models.The results allow us to evaluate different transport models. We find that the simultaneously taken mobility and potential data can consistently be described by a model based on variable range hopping in an exponential density of states, as proposed by Vissenberg and Matters. [1] Description of the data in terms of a mobility edge model [2] requires the use of a more complicated density of states. In either case, the shape of the channel potential V(x), and its dependence on temperature form an independent and direct proof for the density dependence of the mobility.
9:00 PM - II6.11
Organic Semiconductor Patterning by the Use of Negative Photoresist and UV Lithography.
Erik Roeling 1 , Erik Jan Geluk 2 , Wijnand Germs 1 , Rene Janssen 1 , Martijn Kemerink 1
1 Applied Physics, Eindhoven University of Technology, Eindhoven, NB, Netherlands, 2 COBRA Research Institute, Eindhoven University of Technology, Eindhoven, NB, Netherlands
Show AbstractA key issue in the fabrication of organic electronic circuits or devices is the patterning of the organic semiconductor. Patterning is important to prevent parasitic currents flowing within or between devices undermining the performance. Common UV lithography is usually not suitable since the solvents required for standard photoresists also tend to dissolve the semiconductor. Alternative methods for patterning organic materials have been proposed, but often suffer from lack of resolution or scalability, or require non-standard photoresists and developers. Here, we present results of a simple and scalable patterning method where conventional photoresist is used as a patterning mask. The method has a spatial resolution in the micrometer range.Bottom contact field effect transistors are fabricated on highly p-doped silicon wafers covered with a 200 nm thick thermal silicon-oxide (SiO2) layer serving as the gate dielectric. Ti/Au (5/25 nm) source and drain contacts are fabricated through a standard lift-off process. A two micrometer thick negative photoresist layer is then spun on top of the field effect structures and will serve as patterning layer. With UV lithography the photoresist covering the transistor channel area and the contact pads is removed. The characteristic feature of negative tone photoresists is that after processing the resist sidewalls show an undercut. When a small molecule material is subsequently evaporated, there will not be a connecting path between the material in the channel area and material deposited elsewhere. The same holds for the contact pads. The organic semiconducting layer is a 50 nm thick pentacene layer, which is thermally evaporated. Prior to this the SiO2 surface is passivated with a monolayer of hexamethyldisilazane. A reference wafer is fabricated without patterning layer.Electrical characterization is performed in a high vacuum probe station. An on/off ratio of about 105 is obtained for unpatterned transistors. The linear mobility was in the order of 3×10-2 cm2/Vs. For the patterned transistor with the same channel length on/off ratios exceeding 107 and a linear mobility around 10-1 cm2/Vs are obtained. Atomic force microscopy images show the characteristic large pentacene crystals and dendrites in the channel area for both transistors indicating that the improved device performance is due to the patterning of the pentacene layer. By scanning electron microscopy and electrical characterization methods it is furthermore found that the patterning mask can withstand temperatures up to 140 °C. At elevated temperatures photoresist layers have the tendency to flow, destroying the undercut of the sidewall profiles. Summarizing, we present a scalable, high-resolution structuring method for organic semiconductors that is based on standard negative UV photoresist. An increase in on/off ratio of over two orders of magnitude is realized without loss in other performance indicators.
9:00 PM - II6.12
New Terminal Functionalizable Conjugated Polymer Blocks With Evolving Energy Levels and Gaps.
Sam-Shajing Sun 1 , Cheng Zhang 1 , Thuong Nguyen 1 , Jianyuan Sun 1
1 Center for Materials Research, Norfolk State University, Norfolk, Virginia, United States
Show AbstractAvailability of a variety low energy gap (Eg), stable, processible, and terminal functionalizable acceptor (n-type) or donor (p-type) type conjugated polymers is essential for developing various types of donor-acceptor (D/A or p/n) junction based polymer optoelectronic (OE) devices, such as any semiconductor devices made of all-polymer p/n heterojunctions. In this work, a series of new main chain conjugated and terminal functionalizable sulfone-containing thienylenevinylene-based polymer blocks with evolving energy levels and gaps have been designed, synthesized, and studied. The HOMO/LUMO energy gaps of these polymers were in a range of 1.5-2.0 eV. The sulfone group greatly lowers both HOMO/LUMO levels, making these polymers potential electron acceptors (n-type) for general polymeric electronic/optoelectronic applications. The terminal functional groups (aldehyde or phosphonate) make these polymer blocks potentially ideal candidates for the development of D-A block copolymer supramolecular nanostructures for variety optoelectronic applications.
9:00 PM - II6.13
Synthesis of New Soluble Polyaniline Derivatives for OFETs Applications.
Amelie Champagne 1 , Mario Leclerc 1
1 Chimie, Université Laval, Québec, Quebec, Canada
Show AbstractSynthesis of new soluble polyaniline derivatives for OFETs applications.In the past decade, semi-conductors have reached much interest in applications like light-emitting diodes (LEDs), organic field-effect transistor (OFETs), radio frequency identification tag (RIFT), e-paper and so forth. Organic semi-conductors have many advantages over their inorganic counterparts such as low cost production using ink-jet printing or spin coating techniques. Many small molecules and polymers have been synthesized for those applications. Among all conducting polymers, PANI stands out due to outstanding properties, including good environmental stability and reversible acid/base doping/dedoping chemistry. Under doping, PANI exhibited reversible transitions (insulator-conductor) that can be useful in OFETs. The conducting state of PANI, the emeraldine salt state, can reach conductivity superior of some metals like germanium. Unfortunately, polyaniline is infusible and insoluble in common organic solvents. New soluble small molecules or polymers with PANI-like structures need to be synthesized in order to be used as active layer in OFETs. We prepared several PANI-like structures by Ullmann and Buschwald-Hartwig cross-coupling reaction with carbazole and indolocarbazole. Those materials have been used due to their planar core and the possibility to add alkyl chains on the nitrogen atoms. These materials should exhibit good electrical conductivities, good environmental stability and reversible acid/base doping/dedoping chemistry. UV-Vis analysis in function of pH proves transitions insulator-conductor. Moreover, X-ray diffraction on thin film and power has been made to study the organisation. Those new soluble PANI derivatives should have good mobility with high ON/OFF ratio.
9:00 PM - II6.14
Factors That Determine Intercalation of Molecules Between Side Chains of Conjugated Polymers.
Roman Gysel 1 , Nichole Cates 1 , Eunkyung Cho 2 , Dongwook Kim 2 , Jean-Luc Bredas 2 , Chad Miller 3 , Michael Toney 3 , Martin Heeney 4 , Iain McCulloch 4 , Michael McGehee 1
1 Materials Science & Engineering, Stanford University, Stanford, California, United States, 2 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States, 3 , Stanford Linear Accelerator Lightsource, Menlo Park, California, United States, 4 Department of Chemistry, Imperial College, London United Kingdom
Show AbstractWe have recently shown that several aspects of the behavior of a polymer in bulk heterojunction (BHJ) solar cells are largely determined by whether or not the acceptor molecules, which are usually fullerene derivatives, can intercalate between the side chains of the polymer. Intercalation enables rapid exciton dissociation and alters the electronic structure of the blend by modifying molecular packing. When intercalation occurs, it is necessary to put more fullerenes in the film because the fullerenes that go in between the polymer side chains are partially separated from each other by the side chains and do not carry electrons as well as the pure fullerene regions of the film. Films without intercalation typically perform best in solar cells with an approximately 1:1 volume ratio of polymer to fullerene while films with intercalation tend to optimize at 1:3.Using synchrotron x-ray scattering, we have identified many examples of molecules that intercalate and others that do not despite being small enough to fit between the side chains. We use quantum mechanical calculations and differential scanning calorimetry (DSC) measurements to explain why only some molecules intercalate. We hypothesize that the interaction of fullerenes with the polymer backbone stabilizes the intercalated structure and makes them perform particularly well in BHJ solar cells.For a comprehensive understanding and better control of BHJ formation, we have used DSC and synchrotron x-ray techniques with in-situ heating to generate full phase diagrams. We find that bimolecular crystals are stable up to 200°C. Above the melting temperature of the polymer, the intercalated fullerenes separate from the polymer and form crystals. If the films are cooled, the PCBM is not able to diffuse back into the polymer. It is therefore possible to make BHJs without intercalation by choosing certain kinetic pathways.This insight on molecular ordering is the groundwork for a step further in the understanding of organic photovoltaic devices and for new design rules of low-band gap polymer as well as new acceptor materials including the control on the pertinent molecular ordering.
9:00 PM - II6.15
Scanning Surface Potential Microscopy Study on Low-voltage Metallophthalocyanine Based Organic Field-effect Transistors.
Jun Du 1 , Jin An 1 , Zhefeng Li 2 , Qian Miao 2 , Jianbin Xu 1
1 Department of Electronic Engineering and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong China, 2 Department of Chemistry and Materials Science and Technology Research Center, The Chinese University of Hong Kong, Hong Kong China
Show AbstractLow-voltage operating organic field-effect transistors (OFETs)
1 based on vanadyl-phthalocyanine (VOPc) and copper hexadecafluoro-phthalocyanine (F
16CuPc) are demonstrated. The heavily doped n-type Si wafer is modified by octadecyltrimethoxysilane (OTMS)
2 self-assembled monolayer (SAM) which serves as the gate dielectric. Using the thin SAM dielectric
3, the devices can be operated with gate bias less than 2 V. The VOPc based p-type OFETs exhibited high performance with a hole mobility of 0.59 cm
2 V
-1s
-1, a threshold voltage of -1.09 V and an on/off ratio greater than 2.3 × 10
5. For n-type OFETs fabricated with F
16CuPc, the mobility, threshold voltage and on/off ratio are 0.18 cm
2 V
-1s
-1, 0.47 V and 1.4× 10
2, respectively. Meanwhile, we present a scanning surface potential microscopy (SSPM)
4,5 study on the correlation between morphology and electrical properties for the low-voltage VOPc OFETs. Low operating voltage is favorable for the surface potential measurements because the unwanted effect of the electrostatic interaction between tip and electrode can be minimized. We find that for the ultra-thin VOPc layer, the surface potential signal increases with the number of the VOPc layers, and can be tuned by the gate voltage, which directly manifests the charge accumulation at the initial several layers in the proximity of the interface between the organic semiconductor and gate dielectric.
Acknowledgment
The work is in part supported by Research Grants Council of HKSAR, particularly, via Grant Nos. CUHK4172/06E and CUHK02/CRF/08.
*Corresponding author: [email protected]References
1. Li, L.
et al. Battery Drivable Organic Single-Crystalline Transistors Based on Surface Grafting Ultrathin Polymer Dielectric.
Adv. Funct. Mater 19, 2987-2991 (2009).
2. Ito, Y.
et al. Crystalline Ultrasmooth Self-Assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors.
J. Am. Chem. Soc. 131, 9396-9404 (2009).
3. Klauk, H., Zschieschang, U., Pflaum, J. & Halik, M. Ultralow-power organic complementary circuits.
Nature 445, 745-748 (2007).
4. Burgi, L., Sirringhaus, H. & Friend, R. H. Noncontact potentiometry of polymer field-effect transistors.
Appl. Phys. Lett. 80, 2913-2915 (2002).
5. Mathijssen, S.
et al. Monolayer coverage and channel length set the mobility in self-assembled monolayer field-effect transistors.
Nature Nanotech, 1-10 (2009).
9:00 PM - II6.16
Inkjet Printing of Fluorinated Photoresists for Orthogonal Processing.
Carol Newby 1 , Jin-Kyun Lee 1 , Alexander Zhakidov 1 , Priscilla Taylor 1 , George Malliaras 1 , Christopher Ober 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractOrganic electronics holds promise for future low-cost, large-area, flexible electronic applications but commercialization of the technology has not yet been achieved because of difficulties with patterning organic electronic materials. The development of “Orthogonal Processing”, which uses non-damaging, highly-fluorinated solvents and photoresist materials[1,2], has provided solutions to the major part of such difficulties. However, problems remain with the deposition of these fluorinated materials, particularly when roll-to-roll processing is to be employed with flexible substrates. In this work we have explored the use of inkjet printing to deposit fluorinated photoresists for Orthogonal Processing of organic electronic materials.In this presentation, we demonstrate that deposition by inkjet printing is compatible with fluorinated resists and hence a viable alternative. Various patterns have been printed. A single dot of resist 100 µm in diameter can be deposited by simultaneous ejection from several nozzles. The dot formed is subject to the ‘coffee ring’ effect; the thickness at the edge is typically 600 nm while it is only 300 nm in the center. The resist can be made thicker if desired, by printing a second layer on top of the first which partially fills in the valley left by the first. This results in a dot of similar diameter but 1500 nm thick at the edge and 1100 nm thick in the center. Devices fabricated using inkjet printing alone may achieve reproducible features with dimensions in the tens of micrometer range. However, combining inkjet printing resist deposition with photolithographic patterning is expected to allow higher resolution in device fabrication. [1]A. A. Zakhidov, J.-K. Lee, H. H. Fong, J. A. DeFranco, M. Chatzichristidi, P. G. Taylor, C. K. Ober and G. G. Malliaras, Adv. Mater., 2008, 20, 3481.[2]J.-K. Lee, M. Chatzichristidi, A. A. Zakhidov, P. G. Taylor, J. A. DeFranco, H. S. Hwang, H. H. Fong, A. B. Holmes, G. G. Malliaras and C. K. Ober, J. Am. Chem. Soc., 2008, 130, 11564.
9:00 PM - II6.17
Transport Properties of Molecular Wires by Using First-principles Calculations and Nonequilibrium Green’s Function Formalism.
Hiroshi Mizuseki 1 , Sang Uck Lee 1 , Rodion Belosludov 1 , Yoshiyuki Kawazoe 1
1 , Institute for Materials Research, Tohoku Univ., Sendai, Miyagi, Japan
Show AbstractMolecular devices are potential candidates for the next step towards nanoelectronic technology. Polythiophene has a typical pi-conjugated system, then many polythiophenes and its derivatives are synthesized and several have been well characterized. Calculation systems based on neutral, doubly charged, and highly charged oligomers whose all aromatic rings are linked to have linear chains were studied as model for the polaronic defects in doped polythiophenes. The energetics of the aromatic and quinoid structures is investigated using the both ends of neutral oligomers substituted by dimethyl and dimethylen. To estimate the electronic structures, the difference between corresponding bond lengths along the C-C path of neutral, dicationic, and dianionic oligomers, were investigated. Calculations were performed on systems containing 16 monomers, by using B3LYP/6-31G(d) level of theory. We compare the electronic behavior of oligofuran, oligopyrrole, and PEDOT. In this presentation, we will present the transport properties of molecule wires using the nonequilibrium Green’s function formalism for quantum transport and the density functional theory (DFT) of electronic structures using local orbital basis sets. We investigate the coupling effects between the sulfur atom and the metal surface by adjusting their distance in a very small range, and calculate the I-V behaviors of self-assembled nanowires on silicon surface [2], Nitrogen-Doped Capped Carbon Nanotubes [3], porphyrin [4], metallocene, fused-ring thiophene wires, and so on.[1] http://www-lab.imr.edu/~mizuseki/nanowire.html[2] R. V. Belosludov, et al., Phys. Rev. B, 75, 113411 (2007).[3] S.-U. Lee, et al., Small 5 (2009) 1769.[4] S.-U. Lee, et al., Small 4 (2008) 962.
9:00 PM - II6.18
Infrared Signatures of High Carrier Densities Induced in Organic Semiconductors by Fluorinated Organosilane Molecules.
Omar Khatib 1 , Bumsu Lee 2 , Jonathan Yuen 3 , Alan Heeger 3 , Vitaly Podzorov 2 , Dimitri Basov 1
1 Physics, University of California, San Diego, La Jolla, California, United States, 2 Physics and Astronomy, Rutgers University, Piscataway, New Jersey, United States, 3 Center for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractIn recent years, fluorinated trichlorosilane (FTS) molecules have emerged as a unique and efficient method to increase the electrical conductivity of organic semiconductors by several orders of magnitude. Here we present a thorough spectroscopic investigation of several organic semiconductors that have been doped to a highly conducting state as a result of exposure to FTS vapors. Infrared spectroscopy offers access to details of charge injection and the electronic structure that are not always available from transport measurements, which can be dominated by defects and contact effects. In organic molecular crystals of rubrene, FTS molecules are confined to the surface where they form a self-assembled monolayer. In conjugated polythiophenes, however, the FTS molecules penetrate into the bulk, leading to a rich spectrum of electronic excitations in the mid-IR range. Additionally, treated structures can be doped to high enough carrier densities to approach the metal-insulator transition (~1014 holes/cm2), which results in the appearance of a Drude-like absorption in the far-IR
9:00 PM - II6.19
Fabrication of Organic Thin Film Transistors With Hydrophilic Organic Semiconductor and Self-assemble Monolayer by Solution-process in Air.
Yu-Ping Wang 1 , Tri-Rung Yew 1
1 Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan
Show AbstractSelf-assembled monolayer (SAM) plays a crucial role in enhancing the performance of organic thin film transistors (OTFTs). However, the effect of SAM on organic semiconductor layer has been limited to hydrophobic SAM studies due to the hydrophobic nature of organic semiconductor materials. In this study, the fabrication of OTFTs with hydrophilic anthracene derivative semiconductor layer and hydrophilic self-assembled monolayer (SAM) is demonstrated. During device fabrication, hydrophilic SAM layer self-assembled on silicon dioxide not only promoted the wettability of anthracene derivative solution but also enhanced the film crystallinity. The morphology and crystallinity of the patterned semiconductor films were characterized by high resolution scanning electron microscopy and X-ray diffraction spectroscopy, respectively. All the preparations of SAM and semiconductor layers were conducted by solution process in air. The electrical properties of OTFTs were also measured in air.
9:00 PM - II6.2
Phosphorescent Polymer-based Light-emitting Diodes by Using Diphenylbisindenoanthorazolines as Electron Transport Materials.
Taeshik Earmme 1 , Eilaf Ahmed 2 , Samson Jenekhe 1 2
1 Chemical Engineering, University of Washington, Seattle, Washington, United States, 2 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractOrganic light-emitting diodes (OLEDs) have been extensively investigated over the past two decades. In the past several years, intensive efforts in utilizing phosphorescence by metal-complex phosphorescent emitters in PhOLEDs have led to nearly 100% internal quantum efficiency. However, balanced charge carrier injection from both cathode and anode and balanced charge transport to the emissive layer still remains key challenges in developing more efficient organic light-emitting devices. Since hole mobility is typically orders of magnitude larger than electron mobility in most current OLED systems, the introduction of an electron transport layer can dramatically reduce the hole current and thereby result in more efficient OLEDs. In this work, maximum brightness and efficiency of spin coated polymer-based phosphorescent organic light-emitting diodes (PhOLEDs) were drastically increased when 4,9-diphenylbisindenoanthorazoline is used as an electron transport material. Devices with green triplet emitter fac-tris(2-phenylpyridine)iridium (Ir(ppy)3) dispersed in poly(N-vinylcarbazole) as the emissive layer and 4,9-diphenylbisindenoanthorazoline as the electron transport layer (ETL) showed almost 3-fold increase in brightness compared to device with widely used tris(8-hydroxyquinolinato)aluminium (Alq3) as an ETL even at lower turn-on and drive voltages. The high electron affinity and electron mobility of 4,9-diphenylbisindenoanthorazoline explain its effectiveness as an electron transport material in PhOLEDs. These results demonstrate the use of an electron transport material having a high electron affinity as a potential strategy for improving the charge injection and achieving substantial advance in performance of PhOLEDs.
9:00 PM - II6.20
Hall Effect of Solution-crystallized and Vapor-deposited 2,7-Dioctylbenzothieno[3,2-b]Benzothiophene Field-effect Transistors.
Masakazu Yamagishi 1 , Yuichi Takatsuki 1 , Junshi Soeda 1 , Yugo Okada 1 , Yuri Hirose 1 , Takafumi Uemura 1 , Shoji Shinamura 2 , Kazuo Takimiya 2 , Jun Takeya 1
1 Grad. School of Science, Osaka University, Toyonaka Japan, 2 Grad. School of Engineering, Hiroshima University, Higashi-Hiroshima Japan
Show AbstractOrganic field-effect transistors (OFETs) have been intensively investigated because of their capability of applications to flexible, light-weight, large-area and low-cost next-generation electric devices. In terms of the maximum field-effect mobility, development of organic single-crystal transistors disclosed that even the value of more than 20 cm2/Vs is reachable for practical OFETs and that the high-mobility transport is based on a diffusive transport. However, they have difficulties in producing commercial components because crystals are grown independently of substrates. Vapour-deposited polycrystalline thin films usually show one-order less performance, though they have moderate accessibility to large-scale application. On the other hand, solution processes are the most suitable to large-scale and low-cost fabrication but have suffered from low mobility. However, very recently, we develop a method to fabricate crystalline thin films from solution showing high carrier mobility exceeding a few cm2/Vs. In order to understand carrier transport mechanism of practically useful solution-processed crystalline films and vapour-deposited polycrystalline films, we measured Hall effect, which can directly examine that the microscopic in-grain transport mechanism separated from effects of grain boundaries. In the present experiments, we focus on recently synthesized semiconducting material, 2,7-dioctylbenzothieno[3,2-b]benzothiophene (C8-BTBT), which shows carrier mobilities as high as ~3 cm2/Vs in both vapour-deposited polycrystalline films [1] and in solution-processed crystal films [2]. To construct OFETs for Hall effect measurement, Hall bars were shaped in the films fabricated on SiO2 gate dielectrics. Results on both films show that inverse of Hall coefficient 1/RH (= IDΔB/ΔVH = ne) well corresponds to the charge amount calculated from the simple capacitance model. The observation means that diffusive band transport is realized equally in the solution-crystallized films and inside grains of the vapor-deposited polycrystalline films. Therefore, relatively high ordering is realized even in thermally deposited films inside the grains which lead to high field-effect mobility when grain boundaries do not affect seriously to restrict the carrier transport.[1] T. Izawa et al., Adv. Mater. 20, 3388 (2008).[2] T. Uemura et al., Appl. Phys. Exp., in press.
9:00 PM - II6.21
Raman Spectroscopy of TIPS-Pentacene FET Under Device Operation Condition: Effect of Stress Biasing.
Eunju Lim 1 , Riccardo Di Pietro 1 , Masahiro Kawasaki 2 , Masahiko Ando 2 , Henning Sirringhaus 1
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Hitachi Cambridge Laboratory, Hitachi, Ltd., Cambridge United Kingdom
Show AbstractFor practical applications of organic field effect transistors (OFETs), the presence of defects in the active layer is a serious problem. Mobile carriers are scattered by the defects and in some cases are trapped there. Hence the stability of OFET depends critically on defects [1]. To understand their role in the active layer of OFETs, Raman spectroscopy enables us to explore the relationship between defects and carrier motion in the active layer, thus providing information on the chemical states of the molecules, e.g., changed chemical bonding caused by defects and trapped charges. We have shown previously that Raman spectroscopy can be used to examine defects and traps in pentacene thin film transistors which were subjected to bias stress [2]. In this present study, we employ Raman spectroscopy technique to TIPS-pentacene FETs in a top-gate/bottom-contact structure with a 10 nm thick TIPS-pentacene coated with a 500nm cytop as a gate insulator and investigated the effect of stress biasing. We observed changes in the Raman spectrum after prolonged application of a negative gate bias stress and these will be interested in terms of structural deformations of the TIPS-pentacene performance unit cell induced by charge trapping.[1] Salleo, A., Endicott, F., and Street, R. Appl. Phys. Lett. Appl. Phys. Lett. 86, 263505 (2005).[2] Winfield, J. M., Duffy, C. M., Minakata, T., Ando, M., Kim, J. S., Friend, R. H., Sirringhaus, H., submitted.
9:00 PM - II6.22
Interplay Between Energetic and Kinetic Factors on the Air-Stability of n-Channel Organic Transistors.
Joon Hak Oh 1 , Ya-Sen Sun 1 , Ruediger Schmidt 2 , Michael Toney 3 , Dennis Nordlund 3 , Martin Koenemann 4 , Frank Wuerthner 2 , Zhenan Bao 1
1 Department of Chemical Engineering, Stanford University, Stanford, California, United States, 2 Institut für Organische Chemie and Röntgen Research Center for Complex Material Systems, Universität Würzburg, Würzburg Germany, 3 Stanford Synchrotron Radiation Lightsource, Stanford Linear Accelerator Center, Menlo Park, California, United States, 4 , BASF SE, Ludwigshafen Germany
Show AbstractThe effects of the interplay between energetic and kinetic factors on the ambient stability of n-channel organic thin-film transistors (OTFTs) were studied using two perylene diimide (PDI) compounds with distinctly different lowest unoccupied molecular orbital (LUMO) levels. Judging from the empirical energy level windows, one compound (N,N'-bis(2,2,3,3,4,4,5,5,5-nonafluoropentyl)-3,4:9,10-tetracarboxylic acid diimide (PDI-F): -3.84 eV) is at the onset region for air-stability, whereas the other (N,N'-bis(cyclohexyl)-1,7-dicyano-perylene-3,4:9,10-tetracarboxylic acid diimide (PDI-CN2): -4.33 eV) is in the air-stable region. Charge transport behaviors under inert atmosphere and in air were investigated as a function of active layer thickness. Charge transport in air was greatly affected by the active layer thickness for both compounds, an effect that has been overlooked so far. The ambient stability of the air-unstable PDI-F TFTs increased significantly for thicknesses over ~10 monolayers (ML). Surprisingly, the previously considered “air-stable” PDI-CN2 TFTs were not stable in air if the active layer thickness was less than ~4 ML. The molecular packing and orientation of the PDI thin films were investigated using grazing incidence X-ray diffraction (GIXD) and near-edge X-ray absorption fine structure (NEXAFS). We found that the minimum thickness required for air-stability is closely related to the LUMO level, film morphology, and film growth mode.
9:00 PM - II6.23
Chemically Crosslinked Ultrathin Ferroelectric P(VDF-TrFE) Films for Low Voltage Operation in Ferroelectric Field Effect Transistors(FeFETs).
Yu Jin Shin 1 , Youn Jung Park 1 , Hee Joon Jung 1 , Cheolmin Park 1
1 Materials Science and Engineering, Yonsei University , Seoul Korea (the Republic of)
Show AbstractPoly vinylidene fluoride-co-trifluoroethylene(PVDF-TrFE) has been well known as one of the representative ferroelectric polymer materials for organic non-volatile memory applications. Among the various architectures of ferroelectric memory devices, ferroelectric field effect transistors (FeFETs) with organic semiconductors have been of great attention because of nondestructive readout and large scale integration. Our group recently reported low operating voltage FeFETs by using PVDF/PMMA blends, which exhibits good data retention capability over 15h at an operation voltage of ±15V. Since, PVDF-TrFE has high coercive voltage of approximately 50MV/m, it is essential to reduce the thickness of PVDF-TrFE layer for low voltage operation. However, for very thin films less than 100nm, electric shortage frequently occurs in capacitors and at the same time, the high gate leakage is one of the main problems resulting in high off current state in FeFETs. In order to resolve the problem, many works have been done by introducing various types of interlayer such as insulating and conducting organic materials; poly(styrene-r-methylmethacrylate) (P(S-r-MMA)) and poly vinyl phenol (PVP) for insulting interlayer and polypyrrole and poly(3,4-ethylene dioxythiophene):poly(styrenesulfonicacid) (PEDOT:PSS) for conducting one.Here we suggest new fabrication method to obtain ultrathin films of PVDF-TrFE below 50nm which can be applied to the low voltage operating memory element, by using chemically crosslinkable ferroelectric thin films. We introduced thermal crosslinking(CLK) agent, 2,4,4-trimethyl-1,6-hexanediamine(THDA), which form molecular bonding between two carbon backbones of PVDF-TrFE. By applying the THDA, we can get uniform, very low surface roughness films based on spin-coating and obtained electrically, solvent resistive films from subsequent thermal annealing conducted at 170 degree for 30minutes. For elucidating the effect of CLK agent, the crosslinked ferroelectric thin films less than 50nm have fully saturated hysteresis loops with the coercive voltage and remanent polarization of 3.0 uC/cm2, 2.7V, respectively in MFM capacitor. Then we succeeded to fabricate the low voltage operating FeFETs with single-crystal TlPS_PEN organic semiconductor which show excellent interface between the crosslinked ferroelectric gate dielectric and the organic semiconductor single crystal. From these transistors, the ON/OFF bistability of the FeFET was about 1000 with low gate leakage current and low gate voltage sweep of ±10V. Furthermore, we applied conducting organic interlayer of PEDOT-PSS in order to fabricate bilayered structure of FeFETs to improve the memory performance.
9:00 PM - II6.24
Reduced Band Gap Dithienopyrrole-based Polymers for TFT Applications.
Konstantin Pokhodnya 1 , J. Wang 1 , J. Sandstrom 1 , S. Evenson 2 , S. Rasmussen 2
1 Center for Nanoscale Science and Engineering, North Dakota State University, Fargo, North Dakota, United States, 2 Chemistry, Biochemistry, and Molecular Biology, North Dakota State University, Fargo, North Dakota, United States
Show AbstractPolymer TFT transistors have attracted tremendous interest due to their potential applications in flexible electronics, i.e. OLED displays, RFID tags, etc. Despite the great progress that has been achieved in improving device performance, problems related to the low mobility and poor device stability hinder their wide application. The low mobility can be attributed to two factors, the amorphous nature of the material and interface quality. Therefore, the design and synthesis of new polymeric materials with potential high mobility and air-stability is of great importance. The family of air-stable poly(dithieno[3,2-b:2,'3'-d]pyrrole)-based conjugated polymers (polyDTP) with band gaps of ~1.6 eV have great potential in TFT applications. We have fabricated simple TFT device by spin coating of polyDTP toluene solution on OTS treated silicon dioxide surface, followed by thermal evaporating of two gold source-drain electrodes with a channel length of 20 micrometers and channel width of 1 mm. Promising field effect mobility, high on-off ratio, and near zero threshold voltage have achieved.
9:00 PM - II6.25
How Does the Orientational Disorder Affect Charge-Carrier Transport in Ordered Molecular Materials?
Akira Ohno 1 2 , Daisuke Uchiyama 1 , Jun-ichi Hanna 1 2
1 , Tokyo Insutitute of Technology, Yokohama, Kanagawa, Japan, 2 , JST-CREST, Kawaguchi, Saitama, Japan
Show AbstractAmorphous organic solids generate the energetic disorder of localized states responsible for charge carrier transport. This energetic disorder in such materials originates from the Coulomb interaction between randomly distributed dipoles and carriers localized in molecules [1,2]. This effect is reduced for molecularly ordered materials such as molecular crystals and liquid crystals because of molecular alignment and orientation, but it cannot be neglected because carrier distribution in the density of states easily shifts its depth, depending on temperature[3]. For example, a small energetic disorder, σ of ∼50meV is comparable to the activation energy, ∼100meV around room temperatures, which increases with decreasing temperature. In fact, this effect can be seen in organic FETs fabricated with polycrystalline thin films of organic semiconductors on the gate insulator having random alignment of dipole moment due to substituents such as -OH , or –OOH, even though this is not the case for the charge transport in the single-crystalline material of organic semiconductors. In this study, we have modeled the effect of dipoles on the charge carrier transport in such material systems described above including the orientationally ordered materials: we fixed angle θ1 between molecular long axis and direction of dipole moment, while the molecule freely rotates and the direction of dipole has random variation around the long axis; and the molecular long axis have orientational-order described with order parameter, S. In this model, we found that the energetic disorder does not always decrease with increasing the order parameter, but it increases with an increase of the order parameter at a certain range of the angle, indicating that importance of molecular design in terms of orientation of dipole moment. We discuss the availability of our model with experimental results on the energetic disorder in the charge carrier transport properties of liquid crystalline mesophases. And we show that our model directly gives us an insight into the effect of molecular orientation on charge carrier transport properties in the ordered molecular systems and how we can design a molecules exhibiting excellent charge carrier transport properties in the ordered states.
9:00 PM - II6.27
Controlling the Turn-On-Voltage in Low-voltage Al2O3 OFETs With an Amorphous Semiconductor.
Niklas Bjoerklund 1 , Fredrik Pettersson 1 , Ronald Oesterbacka 1
1 Center of Excellence for Functional Materials and Department of Physics , Åbo Akademi University, Åbo Finland
Show AbstractOrganic field-effect transistors (OFETs) have been studied intensively over the last decade and can now compete with amorphous silicon devices in terms of performance. The operation voltages need to be reduced significantly in order to increase energy efficiency and reduce costs. A lot of effort is also being put towards using roll-to-roll (R2R) fabrication methods, and this introduces several restrictions that need to be taken into account, such as flexibility and stability.Reducing the threshold voltage and the inverse subthreshold slope is the key to low-voltage operation. These transistor characteristics depend mainly on the dielectric. Consequently, the focus needs to be on increasing the capacitive coupling of the device. In a traditional OFET this can be achieved by increasing the permittivity of the dielectric. However, high-k dielectrics often have a high polarity which may have a negative impact on the dielectric-semiconductor interface. Reducing the insulator thickness is another approach, but thin films might increase the leakage current due to pin-holes.We have studied roll-to-roll compatible OFETs with an anodized Al2O3 gate dielectric and an air-stable organic semiconductor (Lisicon). To improve the performance we investigated the addition of different insulating layers on the Al2O3, such as an ultrathin polymer film or a self-assembled monolayer (SAM). Our transistors operate at – 4 V and show field-effect mobilities of 3*10-3 Vs/cm2 and on/off ratios on the order of 102. We have observed shifts in the turn-on-voltage depending on the different insulating layers used. In our work we strive to clarify the reasons behind the shifts and use this knowledge to control the turn-on-voltage in the devices.
9:00 PM - II6.28
Thickness Dependent Structural Order in P3HT Films – A Key Parameter for High OFET Mobility.
Benedikt Gburek 1 , Richa Sharma 1 , Torsten Balster 1 , Veit Wagner 1
1 School of Engineering and Science, Jacobs University Bremen, Bremen Germany
Show AbstractApplications of organic electronics require cheap and fast production methods on e.g. flexible and transparent substrates. Following these goals, organic field-effect transistors (OFETs) on flexible and transparent plastic foils made of polyethylene-terephtalate (PET) with organic gate insulator and semiconductor are analyzed. The organic semiconductor, regio-regular poly-(3-hexylthiophene (rr-P3HT), and the gate insulator, e.g. poly-methylmethacrylate (PMMA), are deposited wet-chemically by spin-coating under atmospheric conditions.Plastic devices in top-gate architecture were used to analyze the dependence of the device characteristics on the crucial parameter of semiconductor layer thickness. To ensure reliability of the results more than 10 transistors were measured and analyzed statistically for each examined layer thickness. The charge carrier mobility was found to be rather low for extremely thin layers of several nanometers only. However, with increasing layer thickness, mobility increases by two orders of magnitude until a “saturation thickness” of 50 nm, above which it remains constant. The strong variation in mobility is attributed to a thickness dependent change of structural ordering in the semiconductor film.Further details of the ordering were extracted from transfer curves recorded in the linear regime (VDS = -1 V) and at VDS = -20 V by analyzing according to the Vissenberg-Matters (VM) model. VM assumes charge transport in disordered systems with an exponential density of states. The model results in a gate-voltage dependent charge carrier mobility μ = μ0 ((VGS – Vth)/1 V)γ, where the exponent γ is directly related to the width of the density of states and thus to the disorder of the system. The analysis reveals that the disorder parameter γ decreases from 3.1 to 1.0 over the examined thickness range from 2 to 220 nm, which proves the higher disorder in thinner films and explains their low mobility by higher energetic disorder. Phase contrast AFM images confirm these findings, showing increasing domain size with increasing film thickness. Furthermore, increasing the film structural ordering by spin-coating from higher boiling-point solvents shows larger domain sizes and exhibits as expected an even improved disorder parameter γ, ranging from 1.7 to 0.8 in the same thickness interval. A further positive effect of the higher boiling point solvent is a significant reduction of the saturation thickness to 30 nm.The analysis of energetic disorder proves to be highly advantageous in comparison to the commonly used method employing linear fits at a limited gate-voltage range, as our fits reproduce the whole transfer curve, give better comparability and offer more physical insight for material analysis. Our study demonstrates the crucial role of active layer thickness tuning for improved film structure in order to achieve the optimum material performance.
9:00 PM - II6.29
A Novel Second-order Order–disorder Transition in the π-Conjugated Backbone of Highly Crystalline High-mobility Poly(bithiophene-alt-thienothiophene) Lamellae: Implications for Charge-carrier Mobility.
Li-Hong Zhao 1 , Jie-Cong Tang 2 , Jing-Mei Zhuo 2 1 , Lay-Lay Chua 2 1 , Peter K.H. Ho 1
1 Department of Physics, National University of Singapore, Singapore Singapore, 2 Department of Chemistry, National University of Singapore, Singapore Singapore
Show AbstractPoly[2,5-bis(3-alkylthiophene-2-yl)thieno[3,2-b]thiophene-2,5-diyl] (PBTTT) is a family of polymers that exhibits an unusual degree of lamellae order with high accumulation charge field-effect mobility (μFET) that provides a remarkable opportunity to study the effects of well-defined transitions in the polymer on carrier mobility. From extensive variable temperature structural studies using powder X-ray diffraction (XRD), Fourier-transform infrared spectroscopy (FTIR), Raman spectroscopy and UV-vis spectroscopy, we infer the existence of a ring-twist temperature denoted here Tr, analogous to the well-known glass transition temperature Tg in amorphous, in which a second-order transition in the π-conjugated backbone from a nearly planar state to an increasingly disordered ring-twisted state occurs. We identified this onset from the slope change in the temperature dependences of numerous properties, including backbone phonon modes, π–π* transition energy, and XRD crystallinity. This occurs at ca. 340 K for the tetradecyl(C14)-substituted PBTTT. Variable temperature μFET measurements show that carrier mobility shows three distinctive transport regimes: (i) well below Tr, μFET is dispersive (i.e., the differential carrier mobility varies strongly with carrier sheet density σ); (ii) above Tr, μFET becomes non-dispersive, but remains thermally activated despite increasing ring-twist disorder; (iii) far above Tr, μFET decreases with T when severe disordering sets in and both backbone rigidity and lamellae order are eventually lost. Increasing Tr may be key to achieving even higher mobility. From these measurements, it is also clear that side-chain disordering occurs at an earlier onset temperature of ca. 240 K, as confirmed by the behavior of CH2 rocking and bending modes. These results provide new insights into the structure of the polymer needed for high carrier mobility.
9:00 PM - II6.3
Organic-Inorganic Hybrids: A Route to Ferromagnetic Semiconductors.
Anne Arkenbout 1 , Takafumi Uemura 2 , Jun Takeya 2 , Auke Meetsma 1 , Thomas Palstra 1
1 Solid State Chemistry, University of Groningen, Groningen Netherlands, 2 Department of Chemistry, Graduate School of Science , Osaka University, Osaka Japan
Show AbstractWe study inorganic-organic hybrids [1,2] that are ferromagnetic and semiconducting. This combination is not common in conventional materials but can be created by combining conjugated organic materials linked with magnetic inorganic components in a hybrid network. We have synthesized various inorganic-organic hybrid network structures with the general composition MCl3+x(C6H5CH2CH2NH3)1+x, and analyzed the correlations between the crystal structure and magnetic and electric properties. Due to the formation of tightly linked low-dimensional inorganic arrays, strong magnetic interactions are present in these materials. In the hybrids with 2-dimensional perovskite like inorganic sheets, long range ferro- and canted antiferro- magnetic order is present with ordering temperatures as high as 100K.[3]The studied hybrids are semiconductors, with band gaps between 1 to 2.5 eV. We have fabricated field effect transistors, but they did not introduce sufficient carriers to enhance the conductivity significantly. However, interface doping with an electron donor TTF, can increase the conductivity by more then five orders of magnitude.[4]The inorganic-organic hybrids have many phase transitions which are not yet characterized and identified. Recently, we measured anomalies in the dielectric constant and pyroelectric currents at 330 K. X-ray diffraction studies showed that this behavior is related to a rearrangement of the hydrogen bonds, which results in a polar ordering. This suggests that the hybrids might constitute a new class of multiferroics.References[1] D. B. Mitzi, Prog. Inorg. Chem. 1999, 48, 1.; D. B. Mitzi, J. Mater. Chem., 2004, 14, 2355. [2] C. R. Kagan, D. B. Mitzi and C. D. Dimitrakopoulos, Science, 1999, 286, 945.[3] A. H. Arkenbout, A. Meetsma and T. T. M. Palstra, J. Mater. Chem., submitted.[4] A. H. Arkenbout, T. Uemura, J. Takeya and T. T. M. Palstra, Appl. Phys. Lett, 2009, 95, 173104.
9:00 PM - II6.30
Solvent Vapor Annealing Process for High Performance Solution Processed n-type Organic Field Effect Transistor.
Dongyoon Khim 1 , Kang-Jun Baeg 1 , Yong-Young Noh 2 , Dong-Yu Kim 1
1 Department of Material Science and Engeenering, Heeger Center for Advanced Materials (HCAM), Department of Nanobio Materials and Elctronics (DNE), Program for intergrated Molecular System (PIMS) , GIST, Gwangju Korea (the Republic of), 2 Department of Chemical Engineering, Hanbat national university, Deajeon Korea (the Republic of)
Show AbstractOrganic field effect transistors (OFETs) using conjugated molecules have been actively researched for application to low-cost electronics such as driver of flexible display and radio frequency identification (RFID) tag. Solution processing enables to realize cheap electronic devices, while those showed a bit lower performance than evaporated devices in a high vacuum chamber. Typical solution processable conjugated polymers and small molecules need a long time and high temperature thermal annealing process to improve crystallinity but this process is not suitable for a high speed roll-to-roll process which must be incorporated with high throughput solution process for reduction of manufacturing cost. Here we propose solvent vapor annealing process to improve performance of n-type OFETs. After solvent vapor annealing, the field effect mobilities of bottom gated n-channel OFETs transistor dramatically increased as high as 0.48 cm2/V s. To investigate origin of increased device performance, devices were characterized by X-Ray Diffraction (XRD) and Atomic Force Microscope (AFM).
9:00 PM - II6.31
Photo-crosslinkable Polymer-blend Gate-dielectrics for High-performance P, N-type Organic Field-effect Transistors.
Se Hyun Kim 1 , Kipyo Hong 1 , Hoichang Yang 2 , Chan Eon Park 1
1 Polymer Research Institute, Pohang university of science and technology, Pohang Korea (the Republic of), 2 Division of Nano-systems Engineering, Department of Advanced Fiber Engineering, Inha University, Incheon Korea (the Republic of)
Show AbstractIn present study, we reported the fabrication of photo-crosslinkable polymer blend gate-dielectrics consisting of two insulating polymers with distinct properties, and the optimization of the qualities of the blend gate-dielectric to maximize OFET performances. First, photo-crosslinking capability of the polymer blend was investigated because this property enabled the gate-dielectric to obtain solvent-resistance, electrical strength, and photo-patternability.In addition, we characterized variation of surface properties of the blend gate-dielectric according to the change of the blending ratio of the separate polymers. Based on the investigation on dielectric surface properties, we inspected the effects of structural and morphological features of the semiconductor layer grown on the blend gate-dielectrics (e.g., molecular ordering, crystallinity, size and boundary of the grain) on the OFET performances.Finally, we considered the device instability, such as hysteresis behaviour, of the OFETs employing blend gate-dielectrics, and confirmed the usefulness of our polymer blend as the gate-dielectric in the N-type OFET.
9:00 PM - II6.32
Effect of Traps on the Dynamic Response of Organic Field Effect Transistors.
Brian Cobb 1 , Laura Ferlauto 1 , Christopher Lombardo 1 , Ananth Dodabalapur 1
1 , University of Texas at Austin, Austin, Texas, United States
Show AbstractThe effect of trap states on quasistatic operation of organic transistors has been studied in detail. However, the effect that these traps have on the dynamic response of the devices has not been well characterized. The transient response of a device can lend insight into the mechanics of charge transport, including channel formation. It also serves to explain the operation of devices designed to operate in a non-quasistatic regime, such as high frequency voltage rectifiers for use in RFID applications. Traps also influence turn-off behavior in OFETs and this can strongly influence the turn-off characteristics of switching FETs used in active matrix displays.Two experimental methods have been employed to explore the large-signal transient response of these devices. The first measures channel formation in the time domain. The second explores the response of the device in the frequency domain. Both experimental methods allow for an extraction of carrier velocity and mobility for carriers within the channel. These values are determined under varying bias, and at temperatures down to 77K. In the time domain, these methods allow us to extract activation energies for multiple carrier concentrations within the channel. Trap states have more effect on the frequency response of the device. At higher frequencies, the large signal response of these devices becomes less efficient. The slope of this roll-off, however, is determined partly by trap density and lifetime. This data allows insight into the dynamics of charge transport through the channel. Capacitance-voltage measurements provide information on interface traps through measurements made at various frequencies. With certain dielectrics, this can be a significant source of trap states in the device. We will describe how these correlate with turn-off behavior, in particular on a Zirconium Oxide high-k gate insulator. These results will then be compared with the response when a low-k organic layer is used to modify the interface on top of the Zirconium Oxide, reducing the density of these interface traps.We are developing methods to fill traps within the channel without significantly altering the mobile charge carrier concentration. By testing the device under an intense light source, we produce bound exciton pairs. Due to the unipolar nature of transport within many of these devices (for example, hole transport in pentacene), these pairs do not contribute to the mobile charges within the channel. They do, however, occupy trap states, making these states unavailable to mobile charges. The aforementioned dynamic measurements are performed under these conditions.
9:00 PM - II6.33
In-situ Optical Spectroscopy of Two-dimensional Polarons in Liquid Crystalline Polymer Semiconductor Transistors.
Jiyoul Lee 1 , Do Hwan Kim 1 , Bang-Lin Lee 1 , Jeong-Il Park 1 , Byungwook Yoo 1 , Bon Won Koo 1 , Sangyoon Lee 1 , Kimoon Lee 2 , Seongil Im 2
1 Display Lab., Samsung Advanced Institute of Technology, Yongin-si Korea (the Republic of), 2 Institute of Physics and Applied Physics, Yonsei University, Seoul Korea (the Republic of)
Show AbstractSelf-assembled regioregular thiophene based polymer semiconductors with supramolecular two-dimensional structures are of interest owing to their relatively high mobility coupled with solution processibility, allowing printable low-cost large area electronic circuits. It is well known that these semiconducting polymer modified by the increased interchain stacking can bring the carrier delocalization leading to the high mobility. Therefore, self-ordering of the polymer chains in the bulk polythiophene film has been extensively investigated for high-performance organic thin film transistor (OTFT) applications. However, the research on the intermolecular packing and their electronic structures in the polythiophene semiconductor/ dielectric interface which significantly influence the device properties has been rarely carried out. This is mainly attributed to lack of experimental techniques with strong interface sensitivity to distinguish clearly molecular and electronic structures at the interface and in the bulk. Here, we used in-situ optical spectroscopy to investigate the molecular and electronic structures at the polymer semiconductor/dielectric interface. Monochromatic light was illuminated on active area of our polymer thin-film transistors while the transistor was operating. We used a novel donor-acceptor type liquid-crystalline semiconducting copolymer, poly(didodecylquaterthiophene-alt-didodecylbithiazole), PQTBTz-C12, which contains both electron-donating quaterthiophene and electron-accepting 5,5’-bithiazole units as a channel materials. The accumulated charges at a certain energy level were excited by the correspondingly energetic photons, leading to a relative threshold voltage shift. The relative threshold voltage shift was recorded at varying light wavelengths. The in-situ optical spectra of PQTBTz-C12 TFTs showed the π-π* absorption peaks between 2.2 eV ~ 2.5 eV, which were well matched with those in the UV-vis absorption spectra of the corresponding polymer films. More importantly, the spectra also showed an associated charge-induced absorption at energies below the π-π* absorption edge. The charge-induced absorption spectrum peaks appeared at around 1.4 eV, 1.7 eV, and 1.9 eV. These charge-induced absorption peaks might be related to the delocalized polaron states in PQTBTz-C12/dielectric interface, which is hardly observed in other analytic technique. With these results, we will discuss how the molecular and electronic structures are present at the interface and compare with those exist in the bulk.
9:00 PM - II6.34
Structural Optimization of Self-assembled Monolayers for Organic Transistors.
Kenjiro Fukuda 1 , Kazunori Kuribara 1 , Tomoyuki Yokota 1 , Takanori Hamamoto 1 , Tsuyoshi Sekitani 1 , Ute Zschieschang 2 , Hagen Klauk 2 , Takao Someya 1
1 Department of Electric and Electronic Engineering and Department of Applied Physics, The University of Tokyo, Tokyo Japan, 2 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractWe have fabricated pentacene thin-film transistors (TFTs) using self-assembled monolayers (SAMs) based on alkyl-phosphonic acids with five different alkyl chain lengths as the gate dielectric, and investigated the relationship between the SAM chain length and the electrical performance and stability of the transistors [1,2]. A SAM chain length of 14 carbon atoms provides a maximum TFT mobility of 0.7 cm2/Vs, along with an on/off current ratio greater than 105. High-performance organic transistors with low operating voltage were achieved within the constraints of a low-temperature, flexible, cost-effective, large-area, and simple fabrication approach. Organic transistors with SAM gate dielectrics were manufactured by vacuum evaporation and solution processes. A 25 nm thick Al layer was deposited as the gate electrodes through a shadow mask in a vacuum evaporator on a glass substrate. A thin aluminum oxide film with a large density of hydroxyl groups for molecular adsorption was formed by oxygen-plasma treatment (300 W, 30 min), and a SAM of phosphonic acid was prepared from a 2-propanol solution at room temperature. Five phosphonic acids with different alkyl chain lengths were studied: n-hexyl- (C6-SAM), n-decyl- (C10-SAM), n-tetradecyl- (C14-SAM), n-hexadecyl- (C16-SAM), and n-octadecyl- (C18-SAM) phosphonic acid. Purified pentacene was deposited to form 30 nm thick channel layers on the AlOx/SAM gate dielectric. Finally, a 50 nm thick Au layer was evaporated through a shadow mask to form the source/drain contacts.The transistor characteristics strongly depended on the alkyl chain length. Increasing the alkyl chain length from 6 to 14 carbon atoms causes the mobility to increase from 0.4 to 0.7 cm2/Vs, while further increasing the chain length to 18 carbon atoms causes the mobility to drop to 0.4 cm2/Vs again. The gate current in the TFTs with C6-SAM is approximately about 1 nA, which is same as that in TFTs without any SAM. As the alkyl chain length is increased to 16 carbon atoms, the gate current decreases monotonically to below 100 pA. This confirms the important role of a well-ordered SAM in providing a gate dielectric with low leakage currents. The on/off ratio improves from 104 for TFTs with C6-SAM to above 105 for TFTs with C14- and C16-SAM. The minimum gate current is 60 pA for the TFTs with C16-SAM, and the maximum on/off ratio is 4 × 105 for the TFTs with C14-SAM. We have evaluated the thin-film morphology of the pentacene films by atomic force microscopy and the surface roughness of the various SAMs by X-ray reflectivity. C14-SAM is substantially smoother than the C16-SAM and the C18-SAM and the pentacene on the C14-SAM has a grain size of approximately 3 μm, which is notably larger than on the C16- and C18-SAMs. These results may explain the larger grain size and the larger mobility of the pentacene films on the C14-SAM. [1] H. Klauk et al., Nature, 445, 745 (2007). [2] K. Fukuda et al., Appl. Phys. Lett. accepted.
9:00 PM - II6.35
Numerical Modelling of Dual-gate Organic Field-effect Transistors.
Jakob Jan Brondijk 1 , Mark-Jan Spijkman 1 , Paul W. M. Blom 1 , Dago M. de Leeuw 1 2
1 Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, 2 , Philips Research, Eindhoven Netherlands
Show AbstractOrganic field-effect transistors (OFETs) have a great potential for a broad range of low-cost, low-end applications. Dual-gate OFETs are very well suitable for sensor applications. To gain better understanding of the device physics of OFETs, it is essential to understand the charge transport in conjugated materials.Ideally, an OFET is an interface device and the charge transport can be described 1-dimensionally. In practice however, semiconductor bulk effects cannot be neglected and a 2 dimensional approach is needed. An analytical solution or approximation can be found in simple cases, but in more complicated situations, e.g. dual gate OFETs or doping in polymers, a solution has to be found numerically.To describe the charge transport in OFETs, we developed a numerical simulation package that has the flexibility to include user-defined parameter models, for example for mobility and recombination. This allows us to incorporate the specific physical processes that are characteristic for organic semiconductors, such as a density dependent mobility.By incorporation of the charge-dependent mobility the temperature dependence of p-type OFETs could be exactly reproduced. As a next important step OFETs with a p-type doping were investigated. For the first time we were able to reproduce the experimentally observed shift in pinch-off voltage, for varying acceptor density in the semiconductor.Next, we used the software to simulate a more complicated structure, a dual-gate OFET. In this device, an extra gate is added on top of the normal typical device structure to influence the semiconductor behaviour. Experimental data show that the voltage bias on the extra gate systematically shifts the threshold voltage of the OFET. This effect makes OFETs very well suitable for sensing applications. The shift in threshold voltage is perfectly reproduced in our simulations and can additionally be explained using simple electrostatics.An important feature of our simulations is that both electrons and holes are explicitly considered. Since organic semiconductors are primarily p-type, electrons are often neglected in simulations. Taking electrons into account opens up the possibility of studying ambipolar organic semiconductors and screening effects.
9:00 PM - II6.36
Influence of Ambient Atmosphere on the Performance of Organic Field-effect Transistors.
Andreas Klug 1 , Gerhild Wurzinger 1 , Arno Meingast 1 , Kerstin Schmoltner 1 , Michael Forster 2 , Ullrich Scherf 2 , Emil List 1 3
1 , NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz Austria, 2 Macromolecular Chemistry, University of Wuppertal, Wuppertal Germany, 3 Institute of Solid State Physics, Graz University of Technology, Graz Austria
Show AbstractMany conjugated polymers are solution-processable and can be deposited and patterned at room temperature. Moreover, the field-effect mobility of some organic semiconductors has already reached values similar to those of amorphous hydrogenated silicon (a-Si:H) on the order of 1 cm2/Vs. Both abilities make such materials favorable as active layer materials in low-cost, large-area electronics on flexible substrates. However, for many applications of organic-based devices also ambient stability of the constituting materials is crucial, in particular e.g. for organic sensors to ensure or enhance their sensitivity and selectivity with respect to a target analyte. For reliable organic-based circuits organic field-effect transistors (OFETs) should exhibit constant device parameters under ambient operation, ideally without the need for expensive encapsulation techniques. However, several available active layer materials like the well-established transistor polymer poly(3-hexylthiophene) (P3HT) show degradation when exposed to ambient air due to interaction with oxygen/moisture. On the other hand, OFETs with e.g. polytriphenylamine-(PTPA)-based polymer active materials were shown to be less sensitive to air, exhibiting rather stable device parameters.Here we report on a detailed investigation of possible reasons for ambient device (in-)stabilities of bottom-gate bottom-contact OFETs based on several different organic semiconductors. A gas and humidity measurement plant including a probe chamber with appropriate sensor equipment was developed and applied for the analysis of the ambient and shelf-life stability of the devices. The corresponding OFET current-voltage characteristics were recorded and analyzed under various conditions, including ambient air, oxygen as well as dry and wet nitrogen at different levels of relative humidity. The results were benchmarked against the behavior of P3HT and the small-molecule organic semiconductor pentacene, also comparing device parameters such as field-effect mobility, switch-on voltage and on-/off-current ratio.
9:00 PM - II6.37
Structure Solution of Thin Organic Films on Single Crystalline Metallic Substrates Through Growth Analogy on Fibre-textured Graphite: Perfluoropentacene on Ag(111), Au(111) and Cu(111).
Ingo Salzmann 1 , Martin Oehzelt 2 , Steffen Duhm 1 4 , Bernhard Wedl 3 , Dmitrii Nabok 5 , Norbert Koch 1
1 Institute of Physics, Supramolecular Systems, Humboldt University of Berlin, Berlin Germany, 2 Institut für Experimentalphysik, Johannes Kepler Universität Linz, Linz Austria, 4 Graduate School of Advanced Integration Science, Chiba University, Chiba Japan, 3 Institut für Festkörperphysik, Technische Universität Graz, Graz Austria, 5 Atomistic Modelling and Design of Materials, Department Material Physics, Montanuniversity Leoben Austria
Show AbstractIn a recent study the growth of perfluoropentacene (PFP) on Ag(111) was investigated by a combination of x-ray standing waves and specular x-ray diffraction (XRD). There it was found that PFP adopts flat adsorption geometry within the monolayer and shows a structural transition to a herringbone arrangement upon subsequent growth in the multilayer region. However, XRD revealed that PFP grows in a yet unknown polymorph on Ag(111), which cannot be solved directly by x-ray diffraction reciprocal space mapping (RSM) due to the single crystalline nature of the substrate. The straightforward alternative of using flame-annealed or polycrystalline gold films as substrates exhibiting a fibre-texture failed due to the high substrate roughness, which is exceedingly disadvantageous for RSM investigations.In the present study we demonstrate a generally valuable approach to solve unknown thin-film polymorphs grown on single crystalline metallic substrates through a combination of RSM, x-ray diffraction pole figure technique (XRD-PF) and an appropriate variable substrate choice. From XRD on PFP films on Ag(111), Au(111) and Cu(111) substrates a lying molecular orientation with respect to the substrate plane is evident. As analogously found in numerous cases for conjugated organic molecules on metallic substrates an identical growth mode of PFP on Highly Ordered Pyrolytic Graphite (HOPG) substrates was found via XRD. Taking advantage of both the fibre texture of HOPG and the extraordinarily low surface roughness we determined a structure solution of PFP/HOPG by RSM. Via XRD-PF performed on PFP films on Ag(111), Au(111) and Cu(111) using synchrotron radiation we demonstrate that the same (triclinic) PFP polymorph is present on these substrates. Finally, through a correlation with the substrate orientation via XRD-PF the epitaxial relationships of the PFP bulk with the Ag(111), Au(111) and Cu(111) substrates could be derived. This new approach may be of high value for analogous problems of structure/energetic correlations in thin organic films, where most often single crystalline substrates are being used.
9:00 PM - II6.38
Spectroscopic and Morphological Investigation of Buried Functional Interfaces in Transparent Organic Field-effect Transistors.
Stefano Toffanin 1 , Raffaella Capelli 1 , Gianluca Generali 1 , Antonio Facchetti 2 3 , Michele Muccini 1
1 Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, Bologna Italy, 2 , Polyera Corporation, Skokie, Illinois, United States, 3 Department of Chemistry and the Materials Research Center, Northwestern University, Evanston, Illinois, United States
Show AbstractThe development of polymeric dielectric materials is fundamental to the progress of organic electronics. Emerging display and labelling technologies based on organic field-effect transistors (OFETs) require fabrication on flexible substrates over large areas and via economical, high throughput processes. Realizing transparent flexible dielectric thin-films on ultra-thin gate electrodes in OFETs and organic light-emitting transistors (OLETs) can open up the possibility of fabricating active matrix, colourful and flexible electroluminescent displays. Correlating spectroscopic and morphological information at the nanoscale in field-effect devices is fundamental in providing a unique knowledge about photophysical and transport properties of organic semiconductors. Among all the techniques employed, far-field laser scanning confocal microscopy (LSCM) is the less invasive performing three-dimensional imaging with fast data acquisition [1].Herein we propose to study the relevant interfaces in fully-transparent top-contact and bottom-gate field-effect devices by using a LSCM configuration in which the active layer is excited and the photoluminescence signal collected through the transparent dielectric/electrode/substrate system. Since the mismatch of the refractive indexes among the different materials in the light optical path can be controlled, it is possible to gain information directly on the interface between the gate dielectric and the active material, main responsible for the charge transport. Moreover the region of the device channel underneath the electrodes can be directly investigated determining whether the metal deposition process can induce modifications on the organic semiconductor thin-film nano-organization. The semiconductor morphological features can be thus correlated, with an in-plane spatial 300 nm resolution, to continuous and picosecond time-resolved spectroscopic data.Finally, the buried interface morphology studied by LCSM is compared with the morphology of the outermost surface of the electrodes and active layer obtained by atomic force microscope. This helps us to describe how the morphologies of the metal and organic layers are modified during the growth process in organic field-effect transistors.[1]Loi M. A.; Da Como E.; Zamboni R.; Muccini M. Synth. Met. 2003, 139, 687.
9:00 PM - II6.39
Organic Electronic Devices Based on Solution Processable Small Molecules.
Scott Watkins 1 , Mark Bown 1 , Ming Chen 1 , Gavin Collis 1 , Chris Dunn 1 , Giovanni Fanchini 1 , Matthias Haeussler 1 , Katalin Hegedus 1 , Jacek Jasieniak 1 , Peter Kemppinen 1 , Andrew Scully 1 , Birendra Singh 1 , Gerry Wilson 1 , Kevin Winzenberg 1
1 Future Manufacturing Flagship, CSIRO, Melbourne, Victoria, Australia
Show AbstractThere is growing interest in the use of blends of solution processable small molecules in the active layer of organic solar cells and organic field effect transistors. Small molecules offer the advantage of easier purification and analysis when compared with polymers. However, the stability of small molecule-based blends and the morphology issues associated with these are distinctly different to those presented by polymers. In this paper, results from the use of solution-processable polycyclic aromatic compounds, such as dibenzocrysenes, in bulk heterojunction solar cells and organic field-effect transistors will be presented. The synthesis of these materials from commercially available Vat dyes and the implications of this for synthetic scale-up will be discussed. We will discuss the relationships between crystallinity, exciton quenching, charge mobility and device performance and highlight some of the issues that differentiate small molecule blends from polymer-based blends. The interactions of these small molecules with a variety of other semi-conducting materials, such as other small molecules and conjugated polymers, will also be discussed as will the analysis of these materials by photo electron spectroscopy in air (PESA).
9:00 PM - II6.40
The Simple Process of Fabricating High Performance Organic TFTs Using a Gate Insulator Mixture.
Hyun Doo Hwang 1 , Chang Ho Kim 2 , Chang-Jae Yu 1 2 , Jae-Hoon Kim 1 2
1 Electronics and Computer Engineering, Hanyang University, Seoul Korea (the Republic of), 2 Research Institute of Information Display, Hanyang University, Seoul Korea (the Republic of)
Show AbstractOrganic thin-film transistors (OTFTs) have been much interested due to their advantages such as simple process, flexibility, and printable characteristics. It is well known that the performance and stability of such OTFTs can be improved by treating the surface of the gate insulator with a hydrophobic primer such as octadecyltrichlorosilane (OTS), hexamethyldisilazane (HMDS), and a thin polymeric buffer layer prior to the deposition of organic semiconductors. The surface of the gate insulator treated by hydrophobic primer could alter the interfacial properties between the organic semiconductor and the gate insulator. The treatment of surface affects the initial growth mechanism of an organic semiconductor. Therefore, the control of the surface of a gate insulator should be considered by one of the key issues in order to make OTFTs of high performance.In this work, we have fabricated the high performance OTFTs have the highly hydrophobic insulator and simple process without any additional treatment. We mixed the H1 solution in the cross-linked (4-vinylphenol) (cPVP) gate insulator. The cPVP gate insulator layer was widely used because of the high performance in field-effect mobility. The H1 solution introduced in 2009 MRS Spring Meeting forms the highly hydrophobic surface. The surface was changed to the highly hydrophobic surface through the H1 solution in cPVP without any changing of the insulating characteristic. The results of X-ray diffraction show that the mixture can improve the ordering of the pentacene molecules. It is observed that the electrical characteristics of the OTFTs using the mixed insulator were higher than reference device. In particular, the field-effect mobility was significantly improved and the mobility calculated was about 2.33 cm2/Vs. We conclude that the cPVP insulator mixed H1solution contributes to the growth of the pentacene molecule and provides a most simple process to fabricate the high performance OTFTs.
9:00 PM - II6.41
Mesoscopic Self-organization of X-shaped π-Conjugated Molecules Based on Anthracene Derivatives and Their Application to Thin Film Transistors.
Suk Young Bae 1 , Ki Hwa Jung 1 , Kyung Hwan Kim 1 , Min Ju Cho 1 , Dong Hoon Choi 1 , Dae Sung Chung 2 , Dong Hoon Lee 2 , Chan Eon Park 2
1 Chemistry, Korea University, Seoul Korea (the Republic of), 2 Chemicaln Engineering, Pohang University, Seoul Korea (the Republic of)
Show AbstractOrganic semiconducting materials based on extended linear-conjugated systems have been very intriguing and significant development has been achieved in these materials over the one decade. In the exploration of the application as organic semiconductors in organic thin film transistor (OTFT) is an important component for developing future flexible displays. In organic semiconductors, the intrinsic carrier mobility depends critically on the degree of molecular orientation and on the extent of the intermolecular interaction. In this presentation, new crystalline X-shaped conjugated molecules will be demonstrated. Introduction of thiophene-based peripheral moiety into an aromatic core improve the solubility in organic solvent for facilitating the solution device fabrication. They showed intriguing mesoscopic self-organization of molecules inducing the anisotropic alignment. We investigated the photophysical and electrical properties of the new X-shaped molecules. Finally, we fabricated thin film transistor to investigate the carrier mobility and its photosensitivity under light illumination.
9:00 PM - II6.42
Polymer Thin-film Transistors With Improved Mobility and Air Stability.
Kengo Nakayama 1 , Mayumi Uno 1 , Jun Takeya 1
1 Dept. of Chemistry, Osaka University, Toyonaka Japan
Show AbstractPolymer semiconductors are attractive candidates for active-channel materials in organic field-effect transistors (OFETs) because of their compatibility with easy low-cost fabrication processes such as spin-coating, drop-casting, screen-printing and inkjet-printing. Recently liquid-crystalline polymer semiconductors have been reported to show field-effect mobility exceeding 0.5 cm2/Vs [1]. However, such high mobility is obtained only in an inert atmosphere and with relatively large operation voltage. Therefore, it is desired to develop polymer TFTs with high mobility and low-voltage operation in ambient atmosphere. We fabricated polymer TFTs consisting of liquid-crystalline poly(2,5-bis(2-thienyl)-3,6-dihexadecyltheino[3,2-b]thiophene) (pDA2T) and 100-nm-thick SiO2 gate insulator. The surface of SiO2 insulator is treated with ultra-hydrophobic F-terminated organosilane SAMs (F-SAMs) instead of commonly used CH3-terminated octadecylchlorosilane (OTS). In comparison to CH3-SAMs-treated devices, the transfer characteristics of F-SAMs-treated samples shifted in parallel to the direction of negative gate voltage, indicating presence of additional carriers at the semiconductor-insulator interfaces. The maximum mobility measured in air went up to 0.6 cm2/Vs, which is twice as large as that of CH3-SAMs treated devices. We note that the mobility is the highest among reported values for p-type polymer TFTs in ambient condition. As we tested the air-stability for a few days, their performances of high mobility and the on/off ratio of ~105 are maintained without serious degradation. Also, the devices operated in relatively low gate voltages in the range of ±30 V. The improvement of the device performances is partially attributed to the trap-filling effect. Electron-affine F-terminated SAMs electrostatically accumulates additional carriers due to their molecular dipoles, which result in enhancement in the ratio of the trap-free charges and in the field-effect mobility. We also speculate another effect of F-SAMs to hinder absorption of water molecules, which stabilizes the FET performances in air.Improvement of effective mobility is also obtained when using an ionic liquid (IL) for gating with extremely large capacitance exceeding ~1 μF/cm2, which enables to accumulate very high-density carriers. We have fabricated ionic-liquid-gated polymer transistors with pDA2T. Maximum field-effect mobility is 1.5 cm2/Vs in ambient condition and the operation voltage is only in the range of ±1.1 V, as the trap levels near the mobility edge is very efficiently filled due to the extremely large capacitance. The above results demonstrate that high-density carrier doping is useful to improve the performance of polymer TFTs.[1] McCulloch et al., Adv. Mater. 21, 1091 (2009).
9:00 PM - II6.44
π-Conjugated Semiconducting Molecules Based on Pyrene Derivatives: Electronic and Photophysical Properties.
Kyung Hwan Kim 1 , Tae Wan Lee 1 , Min Ju Cho 1 , Suk Young Bae 1 , Youn Sun Kim 1 , Mai Ha Hoang 1 , Dong Hoon Choi 1
1 Chemistry, Korea University, Seoul Korea (the Republic of)
Show AbstractOrganic semiconductor materials based on extended linear-conjugated systems have been very intriguing and attractive in the field of electronic and optoelectronic devices. Accordingly, a number of researchers have attempted to synthesize π-conjugated small molecules, dendrimers, oligomers, and polymers because of their strong potential applications to electronics and optoelectronics such as in organic light-emitting diodes, organic field effect transistors, and photovoltaic cells. In organic semiconductors, the intrinsic carrier mobility depends critically on the degree of molecular orientation and on the extent of the intermolecular interaction. In this study, new π-conjugated molecules based on pyrene derivatives have been synthesized through the Sonogashira coupling reaction. Introduction of thiophene-based peripheral moiety into an pyrene core improve the solubility in organic solvent for facilitating the solution device fabrication. They display a p-type semiconducting behaviors and their electrical properties are investigated in detail. By means of absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry, new materials were thoroughly analyzed for further device applications.
9:00 PM - II6.45
Modification of Gold Source and Drain Electrodes by Self-assembled Monolayer in Staggered n- and p-channel Organic Thin Film Transistors.
Damien Boudinet 1 , Mohamed Benwadih 1 , Yabing Qi 2 , Stephane Altazin 1 , Jean-Marie Verilhac 1 , Michael Kroger 2 , Christophe Serbutoviez 1 , Romain Gwoziecki 1 , Romain Coppard 1 , Gilles Le Blevennec 1 , Antoine Kahn 2 , Gilles Horowitz 3
1 LITEN/LCI, CEA, Grenoble France, 2 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 , ITODYS, Paris France
Show AbstractWe report on the effect of the deposition of self-assembled monolayers (SAMs) on the source and drain electrodes on the contact resistance and mobility in organic thin film transistors(OTFTs). Ultraviolet photoelectron spectroscopy (UPS) shows a variation of the work function of the electrodes depending on the SAM. OTFTs are fabricated on plastic substrates with solution processible Polyera ActivInkTM N1400, TIPS-pentacene and TFB, giving access to both polarities of transistors and to both crystalline and amorphous materials. The transfer line method (TLM) is used to separately estimate the contact resistance and mobility. A clear correlation is found between the work function of the modified electrodes and the corresponding contact resistance. In the case of poly-crystalline semiconductor, the mobility corrected for contact resistance is linked to the surface energy of the electrodes, as measured by the contact angle with water. For n-channel (p-channel) transistors, mobility is found constant for contact angles smaller (larger) than 90° and linearly increasing when the surface becomes more hydrophobic(hydrophilic). Optical microscope and AFM images is used to link this variation of mobility to the morphology of the semiconductor. In the case of amorphous TFB, the most affected parameter is the threshold voltage. We find a linear dependence between the threshold voltage and the work function of the electrodes.
9:00 PM - II6.46
Titanyl Phthalocyanine (TiOPc) Organic Thin Film Transistors With Highly π-π Interaction.
Huang-Ming Chen 1 , Yung-Hsing Chen 1 , Bo-Ruei Lin 1
1 Department of Photonics & Display Institute, National Chiao Tung University, Hsinchu Taiwan
Show AbstractThe objective of this research is to obtain uniform vacuum-deposition triclinic phase II crystal of titanyl phthalocyanine (α-TiOPc) films from various TiOPc crystal forms. The crystal structure and morphology of vacuum-deposited TiOPc films can be manipulated by deposition rate and substrate temperature. Crystal structure was determined by XRD and film morphology was analyzed by AFM and SEM. Highly ordered α-TiOPc film with an edge-on molecular orientation was deposited on octadecyltrichlorosilane (OTS) treated Si/SiO2 surface. All TiOPc crystal forms, such as amorphous, α and γ phases, provided the triclinic phase II crystal of TiOPc. The full width at half maximum (FWHM) of the peak at 7.5 degree in XRD spectra was 0.23, 0.27 and 0.29 for γ, α and amorphous powder when substrate temperature remained at 180 oC, respectively. The FWHM of the 7.5 degree peak can be achieved 0.22 deposited from all crystal forms at elevated temperature higher than 200 oC. The α-TiOPc deposition film exhibited an excellent p-type semiconducting behavior in air with dense packing structure due to the close π–π molecular packing. The devices’ field-effect mobility range from 0.02 to 0.10 cm2/Vs depending on various process parameters. The on/off current ratio (Ion/Ioff) is over 10^5. The TiOPc OTFTs will be applied as multi-parameter gas sensor in the near future.
9:00 PM - II6.47
Self-assembled Hetero-structure for Organic Non-volatile Memory Transistors.
Tomoyuki Yokota 1 , Tsuyoshi Sekitani 1 , Takashi Nakagawa 1 , Ken Takeuchi 1 , Ute Zschieschang 2 , Hagen Klauk 2 , Takao Someya 1
1 , University of Tokyo, Tokyo Japan, 2 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractWe have fabricated organic floating-gate transistors for non-volatile memories. The gate dielectrics (control and floating gate dielectrics) are formed using single-molecule-thick self-assembled monolayer. For programming, -6 V is applied between the control gate and the source contact, and threshold voltage is significantly shifted from 0.5 V to -2.1 V. To erase, +3 V is applied to discharge the floating gate and recover the initial threshold voltage (0.5 V). A threshold voltage shift of more than 2 V is obtained with a 200ms programming time, and the retention time is about 5 × 104 s in air. A programming cycle was more than 104 times. These transistors are fabricated by vacuum evaporation and solution processes. First, 30-nm-thick Al layer is thermally evaporated through a shadow mask as Control gate. Second, we form control gate dielectric layers. Then 30-nm-thick Al layer is evaporated on the control gate dielectric layers and formed floating gate dielectric layers. The control and floating gate dielectric layers consist of thin layers of aluminum oxide and molecular SAMs (n-octadecylphosphonic acid)[1]. A thin aluminum oxide layer with a large density of hydroxyl groups for molecular adsorption is formed by oxygen-plasma treatment (300W 30 min) and a SAM layers are prepared from a 2-propanol solution at room temperature. Purified pentacene is deposited in vacuum through a shadow mask to form a 50-nm-thick as an organic semiconductor on the floating gate dielectric layer. A 50-nm-thick Au layer is evaporated through a shadow mask to form the source and drain electrodes.This study was partially supported by JST/CREST and NEDO.[1] H. Klauk, U. Zschieschang, J. Pflaum and M. Halik, Nature 445, 745 (2007).
9:00 PM - II6.48
Energy Level Alignment at Sexithiophene-Fullerene Heterojunctions from Photoelectron Spectroscopy: Differentiating Between Intrinsic and Extrinsic Energy Shifts.
Raphael Schlesinger 1 , Hendrik Glowatzki 1 , Johannes Frisch 1 , Ralf-Peter Bluhm 1 , Patrick Amsalem 1 , Antje Vollmer 2 , Juergen Rabe 1 , Norbert Koch 1
1 Physik, Humboldt-Universität zu Berlin, Berlin Germany, 2 , HZB-Bessy II, Berlin Germany
Show AbstractUltraviolet photoelectron spectroscopy (UPS) was used to study the electronic structure of heterojunctions formed by thermal evaporation of sexithiophene (6T) and C60 on Au, Ag, and poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDT:PSS). 6T and C60 heterojunctions are prototypical for efficient charge separation structures in organic photovoltaic cells (OPVCs). Our results suggest that a monolayer of 6T on PEDT:PSS is lying flat, while molecules in multilayers stand vertically upright. The highest occupied molecular orbital (HOMO) of the 6T monolayer is less than 0.2 eV below the Fermi-level, while for multilayer 6T films the HOMO is pinned at 0.4 eV below the Fermi-level. This difference in binding energy of the frontier 6T levels is attributed to a charge-exchange reaction between 6T and PEDT:PSS, which is confined to the 6T monolayer only. The energy level alignment at the C60/6T interface is found to follow the vacuum level alignment model. We will further highlight that effects often assigned to band bending in organic molecule films may well be induced by the UPS measurement itself. 6T films on PEDT:PSS exhibited a non-reversible shift of the energy levels/work function to higher binding energies by up to ca. 0.5 eV as a function of illumination with ultraviolet light (UV) that is commonly used to excite photoelectrons in UPS experiments. The magnitude of the energy shift was a monotonous function of UV illumination time and flux, and not related to sample charging; the overall lineshape of the 6T UPS spectrum was, however, barely affected. C60 deposited on top of UV-modified 6T layers exhibited similar shifts, which could also be misinterpreted as band bending at such heterojunctions. Being aware of this possible extrinsic energy level shift effect in UPS is important to be able to clearly identify the reasons for energy shifts unambiguously.
9:00 PM - II6.49
Ferritin/Carbon Nanotube Composite for Anode of Biofuel Cell.
Ho Jin Shin 1 , Kwang Min Shin 1 , Ji Won Lee 1 , Seon Jeong Kim 1
1 Center for Bio-Artificial Muscle and Department of Biomedical Engineering, Hanyang University, Seoul Korea (the Republic of)
Show AbstractBiofuel cells (BFCs) are expected type of fuel cells that use renewable and sustainable biomaterials such as glucose or ethanol for energy source [1], and use biocatalysts such as enzymes or microorganisms that converts chemical energies to electrical energies [2]. Biocatalysts based BFCs are good candidates for implantable devices and sensors because they operate near physiological conditions and they do not need membrane separators because of their own substrate selectivity. Recently, biocatalysts based BFCs have studied for operating micro-scale implantable devices, and with nanotechnologies we can approach new design of BFCs. We fabricate a single wall carbon nanotube (SWCNT)/ferritin/glucose oxidase composite electrode for a biofuel cell anode, using a SWCNT/ferritin composite as an electron transfer mediator from the enzyme to the electrode. We characterize its properties for potential that can be used as an anode of BFCs.Reference[1] S. Topcagic, S.D. Minteer, Electrochim. Acta 51 (2006) 2168.[2] S.C. Barton, J. Gallaway, P. Atanassov, Chem. Rev. 104 (2005) 4867.
9:00 PM - II6.50
Dependence of Pentacene Growth on the Surface Conditions of Poly(3,4-ethylenedioxythiophene) (PEDOT) Electrode, and its Effects on Contact Resistance of Organic Thin Film Transistors (OTFTs).
Hyunho Kim 1 , Mohammad Amdad Ali 1 , Kyunghoon Jeong 1 , Hoesup Soh 1 , Jaegab Lee 1
1 Advanced Materials Engineering, Kookmin University, Seoul Korea (the Republic of)
Show AbstractWe had fabricated bottom contact (BC) pentacene organic thin film transistors (OTFTs) using the selectively grown poly(3,4-ethylenedioxythiophene) (PEDOT) source/drain electrodes on a pre-patterned (3-aminopropyl)trimethoxysilane (APS) monolayer. In BC pentacene OTFT, one critical issue is the contact resistance of between PEDOT source/drain electrodes and pentacene active layer. The contact resistance is a function of surface property (hydrophilic or hydrophobic), surface roughness, and crystallinity of pentacene on PEDOT films. The crystallinity of pentacene was significantly influenced by surface roughness and hydrophobicity of the PEDOT surface. The two different oxidants such as FeCl3, Fe(PTS)3 were used to polymerize conducting PEDOT films and investigate effects of the surface roughness of PEDOT film on crystalline structures of pentacene films grown on it; the FeCl3 oxidant produced rough surface PEDOT films with a root mean square (RMS) value of ~20nm for 60 nm thick film; the Fe(PTS)3 oxidant produced smooth PEDOT ones with RMS value of ~ 1 nm for 100 nm thick film. The smooth surface PEDOT films enhanced the crystallinity of pentacene films grown on PEDOT, as confirmed by XRR. In addition, the enhanced hydrophobicity of PEDOT film surface treated by CF4 plasma significantly improved the crystallinity of pentacene films. Finally, we fabricated the three different types of BC pentacene OTFTs based on the different surface conditions, and evaluated the electrical performances and compared.
9:00 PM - II6.51
Improved Electrical Stability and Characteristics Using Polymer Binder in the 6, 13-bis(triisopropylsilylethynyl) Pentacene Thin Film Transistors.
Jin Young Oh 1 , Young Bum Yoo 1 , Su Sang Chae 1 , Woo Soon Jang 1 , Won Jin Choi 1 , Ji Ho Park 1
1 , yonsei univ., Seoul Korea (the Republic of)
Show AbstractThe electrical stability and characteristics were improved by using two component blends of 6, 13-bis(triisopropylsilylethynyl)pentacene with Poly(alpha-methylstyrene) as the active layer in organic thin film transistors. The blend TIPS-pentacene film gives rise to semiconductor structure modification by the vertical phase separation. It is lead to the improved active layer uniformity and altered interface between semiconductor and dielectric. These unique characteristics induce the increased charge current without leakage current and decreased charge trap sites such as –OH group of oxide dielectric surface and defects of semiconductor. The enhanced active layer quality and reduced hysteresis is observed in current-voltage measurements of the blend TIPS-Pentacene TFTs. Consequently this blended semiconductor has high potential in applications as a simple and effective control of the semiconductor and interface between semiconductor and dielectric characteristics for high performance OTFTs.
9:00 PM - II6.52
Correlating Photocurrent Spectra and Electrical Transport Parameters in Organic Field Effect Transistors.
Beatrice Fraboni 1 , Riccardo DiPietro 1 , Anna Cavallini 1 , Piero Cosseddu 2 , Annalisa Bonfiglio 2 , J-Oliver Vogel 3 , Jurgen Rabe 3 , Norbert Koch 3
1 Physics, University of Bologna, Bologna Italy, 2 , University of Cagliari, Cagliari Italy, 3 , Humboldt-Universität zu Berlin, Berlin Germany
Show AbstractPhotocurrent (PC) spectroscopy is proposed as a reliable tool in the investigation of the transport properties of organic thin film transistors (OFETs). We report on PC analyses of the electronic density of states distribution (DOS) around the band-edge of pentacene films of different thickness (from 50nm to 300nm). We show how the modification of the DOS distribution observed in PC spectra can be clearly correlated to the effective pentacene layer thickness in OFETs. We also used photocurrent spectroscopy to study the electronic density of states distribution in thin films of sexithiophene and an alkylated analog, both at the band-edge and inside the band gap. The presented photocurrent spectroscopy analyses provide direct experimental evidence for a modification of the distribution of the electronic density of states in an OFET channel induced by the modulation of the effective carrier density by the gate voltage. We observe a correlation between a gate voltage induced increase of the carrier density in the channel and a broadening of the distribution of the electronic states around the band edge. We also detected the formation of deep electrically active states within the band gap of both pentacene and sexithiophene OFETs, induced by the exposure to air and light. These results are in good agreement with the evolution of the macroscopic transport parameters of the devices, i.e., carrier mobility and threshold voltage, as a function of the exposure time to light and atmosphere
9:00 PM - II6.53
Injection-Limited Current in High Mobility n-type Polymer.
Robert Steyrleuthner 1 , Marcel Schubert 1 , Zhihua Chen 2 , Antonio Facchetti 2 , Dieter Neher 1
1 Institut für Physik und Astronomie, Universität Potsdam, Potsdam Germany, 2 , Polyera Corporation, Skokie, Illinois, United States
Show AbstractThe understanding of processes determining the injection and motion of electrons resp. holes is of fundamental interest for the further advancement of electronic and optoelectronic devices. Therefore, the study of charge transport in conjugated polymers, especially in a device related thin layer geometry, has received increasing attention during the last years. Hole transport in p-type polymers has been extensively investigated by utilizing hole-only devices and field-effect transistors and various materials with large hole mobility are now available. On the other hand, electron transport in organic semiconductors is often considered to be trap-limited, with a substantial reduction in mobility. Recently, Facchetti and coworkers reported a novel n-type polymer (Polyera ActivInk PAI-N2200) with an exceptionally high electron field-effect-mobility of up to 0.85 cm2/Vs [1]. In these devices, electron transport is, however, exclusively parallel to the layer within a narrow sheet at the interface to the gate insulator. The purpose of our work was to study the electron transport in this particular polymer in the direction perpendicular to the substrate plane and at carrier densities relevant to optoelectronic applications. We have recently published a reliable method for the fabrication of electron-only devices [2]. The results from studies on two typical electron-transporting polymers were consistent with the model for space charge limited currents in the presence of an exponential trap distribution.Time of flight experiments on ca. 2 micrometer thick layers showed that electron transport in Polyera ActivInk PAI-N2200 is indeed fast, with a high electron mobility of 0.01 cm2/Vs perpendicular to the layer plane. To our very surprise, the current in our electron-only devices was contact-limited for a wide range of layer thicknesses. In fact, a transition to the space charge limited current regime was only achieved when increasing the layer thickness to several tens of micrometers. For thinner layers, injection limited currents were observed even when using low work function metals such as Ba or Ca. Such metals had been shown to form ohmic contacts with various electron-transporting polymers. Our results hint at the presence of severe barriers for electron-injection from these metals, and that these barriers become present in combination with a high electron mobility material. [1] H. Yan, Z. H. Chen, Y. Zheng, C. Newman, J. R. Quinn, F. Dotz, M. Kastler, and A. Facchetti, Nature 457, 679-U1 (2009).[2] R. Steyrleuthner, S. Bange, and D. Neher, Journal of Applied Physics 105, 8 (2009).
9:00 PM - II6.54
Quantitative X-ray Diffraction of Thin Films of Semicrystalline Polymers to Investigate Microstructure/Transport Relationships.
Leslie Jimison 1 , Jonathan Rivnay 1 , Ludwig Goris 1 , Michael Toney 2 , Alberto Salleo 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States
Show AbstractIt is well known that the microstructure of organic semiconducting thin films has a large influence on electronic properties. In the case of semicrystalline polymer semiconductors, electrical performance can depend on crystalline quality of the grain, grain size, and the characteristics of the grain boundaries. While studies using atomic force microscopy are very useful in visualizing the grain structure at the top of the surface, AFM data lack information on crystallite quality and orientation in the bulk of the film and at the buried interface. In this work, we use X-ray diffraction to characterize P3HT thin films with various molecular weights and regioregularities that have been deposited from solvents with differing boiling points. To supplement traditional specular and grazing incidence diffraction data, we introduce a novel method for the compilation of complete pole figures using images collected with a two-dimensional area detector and a high resolution point detector. To the best of our knowledge, this is the first time complete pole figures have been compiled for these beam sensitive semiconducting organic thin films (<100 nm). The pole figures are unique because they give information about P3HT crystallite orientation distribution from -90° to 90° with respect to the surface normal. By integrating the area under the poles, we can quantitatively compare the distribution of crystallites. Interestingly, we find that in most films, more then 50% of the film’s crystallinity is attributed to crystallites that are extremely well-aligned with the surface normal (within 1°). In addition, by comparing the integrated intensities of pole figures from different films, we can, for the first time, quantitatively determine the relative degree of crystallinity between films. We use complete pole figures to explain differences in electronic performance collected from temperature dependent mobility measurements analyzed with the Mobility Edge model, which separates the effect of traps from that of transport within the crystallites. Trap distribution within the bandgap is also measured using Photothermal Deflection Spectroscopy (PDS), a very sensitive technique for measuring absorption in the bandgap. We find that in our sample set, molecular weight has a larger effect on the energetic width of the shallow trap distribution than the degree of regioregularity, which is supported by the crystalline texture probed by the complete pole figures. Solvent choice has the largest effect on crystalline mobility, which is also supported by X-ray diffraction. We believe that the results presented here give valuable insight into the fundamental relationship between microstructure and charge transport in semicrystalline, semiconducting polymers.
9:00 PM - II6.55
Controlling Organic Semiconductor Crystallization and Morphology Using Solution Shearing.
Gaurav Giri 1 , Eric Verploegen 1 , Hector Becerril 2 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 Chemistry, Brigham Young University - Idaho, Rexburg, Idaho, United States
Show AbstractSolution deposition of organic semiconductors (OSC) is a leading contender for producing large-area, inexpensive, and flexible organic electronics. A recently developed solution shearing process has demonstrated better organic field effect transistor (OFET) performance compared to simple drop casting or spin casting methods.1 In this method, an organic semiconductor (OSC) solution is sandwiched between two substrates, a non wetting ‘shearing substrate’ and a wetting ‘device substrate’; the latter is heated to a controlled temperature. As the OSC solution is sheared by translating the upper shearing substrate, semiconductor crystallization occurs on the bottom device substrate. However, this method produced films with variable thickness and OFET performance. Using the small molecule trimethyl-[2,2’;5’,2”;5”,2”] quarter-thiophen-5-yl-silane (4T-TMS), the sheared films yielded OFETs ranging from non-performing devices to devices with charge carrier mobilities as high as 0.5 cm2V-1s-1.2 We have recently fabricated a new solution shearing instrument that allows for the control of the substrate gap with an accuracy of tens of microns and control of the tilt angle between the substrates. The temperature of the solution during shearing can be varied between -25°C and 220°C. We investigated the thin film formation of the 4T-TMS molecule. Our work explores the changes in thin film morphology due to controlled changes in experimental conditions. Variations in experimental parameters such as casting temperature, shearing speed, substrate gap width, and substrate tilt angle can be used to manipulate the thin film morphology, which in turn affects the charge carrier transport properties. By controlling the many variables present in the solution shearing process and understanding how each parameter affects the morphology and properties, it is now possible to create large, uniform, and highly oriented crystalline thin films (~7 cm2). These films show better charge carrier transport properties than films created through other solution deposition methods. Using controlled solution shearing, the morphology and properties of thin films consisting of molecules with similar functionalities to that of 4T-TMS, such as bis(trimethylsilane)-quarterthiophene (4T-2TMS) and trimethylsilane-phenylene-dithiophene-phenylene (P2TP-TMS) is explored as well. Finally, the relationship between the morphological anisotropy and anisotropy in the charge carrier transport properties is investigated. 1.H. A. Becerril, M. E. Roberts, Z. Liu, J. Locklin, and Z. Bao, Adv. Mater., vol. 20, no. 13,pp. 2588–2594, Jul. 2008.2.Z. Liu, H. A. Becerril, M. E. Roberts, Y. Nishi, and Z. Bao, IEEE, vol. 56, no. 2, pp. 176-185, Feb. 2009.
9:00 PM - II6.56
Ultrasound Induced Enhanced Supramolecular π Stacking in Regioregular Poly(3-hexylthiophene).
Avishek Aiyar 1 , Rakesh Nambiar 2 , David Collard 2 , Elsa Reichmanis 1
1 School of Chemical and Biomolecular Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractThe charge transport properties in solution processed poly(alkylthiophenes) are critically dependant on their ability to self-assemble into well-ordered thin films that enables enhanced π- π stacking between the conjugated backbones. We report an increased supramolecular packing in chloroform (CHCl3) solutions of poly (3-hexylthiophene) (P3HT), which persists in the solid state, through the application of low intensity ultrasound. The ultraviolet-visible (UV-vis) spectra of the ultrasonicated solutions are characterized by the development of low energy peaks at ~570 nm and ~620 nm, in addition to the π- π* absorption band at ~450 nm. The correspondence of these low energy peaks with similar features that appear in both, the solution spectra of methanol aggregated P3HT/CHCl3 solutions, as well as P3HT thin films (absorption maximum at ~559 nm and a more pronounced shoulder at ~605 nm), suggest that ultrasonication leads to improved π-π interchain interactions in the solution state, that can survive the casting process and persist in the solid state. In addition, the similarities of the photoluminescence spectra, showing a prominent red shifted peak representing the π-π interactions, between methanol aggregated and ultrasonicated P3HT solutions, also indicates molecular ordering. Based on an observation of the photophysical properties of P3HT in solution as well as solid state, we suggest that the application of ultrasound leads to an improved supramolecular packing of the polymer chains through π stacking induced molecular aggregation. This facile approach could have a critical impact on the charge carrier mobilities in P3HT field effect transistors, eliminating the need for dielectric surface modifications or additional processing steps such as thermal annealing.
9:00 PM - II6.57
Retention Time and Depolarization in Organic Non-volatile Memories Based on Ferroelectric-Semiconductor Phase Separated Blends.
Kamal Asadi 1 , Paul Blom 1 3 , Dago de Leeuw 1 2
1 Physics of Organic Semiconductors, University of Groningen, Groningen Netherlands, 3 , Holst Center, Eindhoven Netherlands, 2 , Philips Research Labs, Eindhoven Netherlands
Show AbstractResistive switches have been fabricated using a phase separated blend of the ferroelectric random copolymer poly(vinylidene fluoride-co-trifluoroethylene) with the organic semiconductor regio-irregular poly(3-hexylethiophene), contacted with Ag top and bottom electrodes. The ferroelectric polarization modulates the injection barrier yielding an injection limited OFF-state and a space charge limited ON-state. To study the effect of depolarization an additional polyphenylenevinylene-type semiconductor layer with HOMO energy comparable to that of rir-P3HT has been inserted in the diode stack. When the ad-layer is the injecting contact, the current modulation ratio goes to unity. The origin is a decrease of the effective band bending at the contact with increasing ad-layer thickness. When the electrode at the blend interface (the counter electrode) is the injecting contact the resistance of the diode can be switched, but the ON-state is only stable when the contacts are connected. Upon field-removal the ferroelectric depolarizes and the current drops to that of an unpoled pristine diode. We demonstrate the importance of the depolarization by capacitance-voltage and retention time measurements.
9:00 PM - II6.58
Synthesis and Characterization of New Rod-coil Poly(indolo[3.2-b]carbazole) Derivatives.
Philippe Berrouard 1 , Jean-Remi Pouliot 1 , Mariane Ouatarra 1 , Josee Brisson 1 , Mario Leclerc 1
1 Chemistry, Université Laval, Quebec, Quebec, Canada
Show AbstractThe interest in conjugated molecules is still growing; indeed some electronic devices can be made with organic molecules such as solar cells, light-emitting diodes (OLEDs) and field-effect transistors (OFETs). In OFETs the organization and morphology of the semi-conducting layer have a crucial impact on the electronic properties. Usually, the best performances are obtained with small molecules because of their capacity to organize efficiently. On the other hand, polymers usually have a lower degree of crystallinity but have better mechanic properties and are much easier to process which lead to lower cost for the fabrication of devices. To merge the advantages of the polymers and small molecules, we propose new rod-coil copolymeric structure. This type of structure has a tendency to self-assemble into supramolecular structures which can lead to interesting electronic proprieties. A wide variety of such copolymers were synthesized from indolo[3.2-b]carbazole derivatives, as the rigid segment, with alkyl chains, as the flexible segment. We thus modified many structural parameters of the copolymers to understand their influence on the organization and the crystallinity. The complete characterization of these materials was made by GPC, FTIR, XRD, DSC, SEM, and AFM techniques. From these results, we modified the molecular design of our polymers to optimise the organisation and enhance the π-stacking. The properties of some materials confirmed the synthesis of liquid crystalline polymers with good morphology. Other experiments are in progress to optimize the organisation and the crystallinity of our polymer in thin films to obtain good semiconducting material for OFETs. Recently, we were able to significantly improve the organization in thin film with Langmuir-Blodgett depositions and by annealing on silicium and germanium substrate. We will soon fabricate devices with our most promising materials to know their potential as semiconductors in OFETs.
9:00 PM - II6.59
Surface Molecular Vibrations as a Tool for Analyzing the Surface Interactions in the Cu-phthalocyanine Organic Nanocrystals.
Krzysztof Nauka 1 , Yan Zhao 1 , Hou T Ng 1 , Eric Hanson 1
1 Laboratories, Hewlett-Packard, Palo Alto, California, United States
Show AbstractComparison of the IR spectra of the Cu-phthalocyanine (CuPc) nanocrystals obtained using surface sensitive attenuated total reflectance (ATR) and bulk sensitive transmittance sample configurations revealed small but measurable changes of some vibrational frequencies of the molecules at the surface of nanocrystals with the outermost part of the surface CuPc molecules being the most affected. These changes are caused by electrostatic interactions between the polar components of the molecules on the surface of nanocrystals and external polar molecular species vicinal to the nanocrystals. The external polar species can be either chemically bonded to the CuPc nanocrystal's surface or they can reside in its vicinity without forming a chemical bond with the nanocrystal. Molecular modeling (DMOL3 - Materials Studio and Gaussian03 calculations) of the impact of selected external polar species vicinal to a CuPc molecule on the CuPc molecular vibrations confirmed experimentally observed changes in the vibrational frequencies of the selected CuPc molecular bonding configurations and provided detailed information on the forces involved in these interactions. The population of external polar species vicinal to the CuPc surface can be modified by washing the nanocrystals or by introducing polar molecular additives miscible with the CuPc nanocrystals. Reduction in the number of external polar additives was accomplished by either centrifuging the aqueous dispersion of the nanocrystals or by organic solvent-based Soxhlet extraction, while their number was increased by soaking (followed by drying) the nanocrystals in high and low pH aqueous solutions containing SO3- and OH- ions. These quantitative and qualitative modifications of the population of external polar species surrounding CuPc nanocrystals were reflected in the corresponding changes of the selected vibrational frequencies of the CuPc surface molecules providing an effective tool for not only recognizing the molecular species vicinal to a nanocrystal but also quantifying their concentration. Some of these modifications can also be observed with a naked eye in the form of noticeable color changes of the CuPc nancrystalline powder. This is due to the extremely high visible extinction coefficient of the CuPc nanocrystals causing that the impinging light is mostly absorbed/reflected within the surface region of the nanocrystals. Changes of the electronic structure within this region, caused by the interactions with the vicinal polar species, shift the vis absorption/reflection spectra changing the observed color of the nanocrystalline powder. Similar results were obtained for other molecular nanocrystals, including yellow chromophore molecules. Preliminary data indicate that the described analytical method of analyzing the molecular polar species vicinal to a molecular nanocrystal could find variety of applications ranging from molecular device fabrication to pharmaceutical materials.
9:00 PM - II6.6
Influence of Thiol Treatment on Bottom-contact Field Effect Transistors Based on n-Type Organic Semiconductors.
Jangdae Youn 1 , Geetha Dholakia 2 , Antonio Facchetti 1 , Tobin Marks 1
1 , Northwestern University, Evanston, Illinois, United States, 2 , NASA Ames Research Center, Moffett Field, California, United States
Show AbstractThe performance of bottom contact thin-film transistor (TFT) structures lags behind that of top contact structures owing to severe contact resistance.The major sources of the contact resistance for bottom contact TFTs are believed to involve a combination of non-optimal semiconductor morphology on the contact surface and the limited available charge injection area versus top contact structures. As a part of an effort to understand the sources of high charge injection barriers for n-channel TFTs, we have investigated the influence of thiol contact treatment on the molecular-level structures of such interfaces using hexamethyldisilazane (HMDS)-treated SiO2 gate dielectrics. We focus here on contact surface treatment methods for TFTs based on two archetypical n-type semiconductors, α,ω-diperfluorohexylquarterthiophene (DFH-4T) and N,N’ bis(n-octyl)-dicyanoperylene-3,4:9,10-bis(dicarb-oximide) (PDI-8CN2). We find that TFT performance is greatly enhanced--- to the level of top contact device performance in terms of mobility, on/off ratio, and contact resistance. To analyze the molecular-level film structural changes arising from the contact surface treatment, surface morphologies are characterized by AFM and STM. The high resolution STM images show that the growth orientation of the semiconductor molecules at the gold-semiconductor interface is maintained with the molecular long axes oriented along the SiO2 surface normal. As a result, the film microstructure is well- organized for charge transport in the interfacial region.
9:00 PM - II6.60
Seeding Crystallization of Conjugated Polymers in Solvent Swollen Thin Films.
Edward Crossland 1 , Kim Tremmel 3 , Sabine Ludwigs 1 , Guenter Reiter 2 , Ullrich Steiner 1 4
1 Freiburg Institute for Advanced Studies, University of Freiburg, Freiburg Germany, 3 Freiburg Material Science Center, University of Freiburg, Freiburg Germany, 2 Department of Physics, University of Freiburg, Freiburg Germany, 4 Department of Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractMuch is now known regarding the crystallization habits of high performance semicrystalline conjugated polymer semiconductors such as the polyalkylthiophenes (PAT)s. Optoelectronic properties of these materials depend strongly on both the configuration of a single polymer chain and how the chains are oriented relative to its neighbours. As such, the rather strong effects of even small changes in growth conditions on crystalline morphology, and the resultant effect on charge transport are of utmost importance in understanding how to optimise these materials for future applications. Simply slowing down the kinetics of chain assembly by slowing the evaporation of carrier solvent during film formation has in the past proved a remarkably successful method of increasing charge mobility via increased ordering of the chains. We instead start from a solvent swollen semicrystalline film in which we can track, in-situ, both the dissolution and recrystallization of PAT in-situ using UVvis spectroscopy and Polarised Optical Microscopy. Polymer concentration in the film is precisely controlled via solvent vapour pressure and substrate temperature. This approach is akin to the classical self seeding(1) of polymer solutions, in which all but the thermodynamically most stable PAT crystal nuclei are dissolved as the polymer concentration is decreased. On subsequent increase of polymer concentration, we create conditions in which these seeded nuclei grow in a controlled manner, while further nucleation is prohibitively slow. In contrast to typical nanofibrillar PAT crystallites, we observe the growth of long (ca. 100μm) crystalline fibres, some several hundred nm wide, in which the polymer chains lie parallel to the fibre long axis. Characteristic π-stacking of the chains is directed radially outwards, resembling a shish-kebab morphology. Nucleation can also be successfully seeded under similar conditions on the surface of pre-placed single crystal polyethylene lamellae. Charge transport mobility along the seeded fibres is expected to depend more directly on charge mobility along the PAT molecular axis.A swollen polymer film also provides an extremely well controlled environment to probe the effects of both microscale material flow and gradients of polymer concentration on the crystalline morphology. Material flow is induced for example during the growth of surface instabilities in the mobile film induced by external electric fields. Concentration gradients are produced by slow advance of the solvent flow over the substrate in the manner of a zone melting experiment. We describe the alignment of both seeded and non-seeded crystal structures in both vertical columns and in sub-micrometer lines and describe the consequences for in-plane charge transport.(1) J. Xu, Y. Ma, W. Hu, M. Rehahn, G. Reiter, Cloning Polymer single crystals through self seeding Nature Mater. 2009, 8, 348.
9:00 PM - II6.61
Molecular Junctions by Soft Lithography and Investigation into Electrical Characteristics.
Jeremy Niskala 1 , Wei You 1
1 , University of North Carolina at Chapel Hill, Chapel Hill, North Carolina, United States
Show AbstractAs electronic circuits are reduced to the molecular scale, the need to understand electrical transport in single molecules or a small collection of molecules becomes very important. To probe electrical properties of molecules, metal-molecule-metal junctions have been formed by a multitude of methods; yet, quite often the formation of these junctions is not trivial and results are often not in agreement. Here, we report the electrical characterization of metal-organic-metal junctions formed by nanotransfer printing (nTP) thin film metal electrodes onto self-assembled monolayers formed on metal electrodes. Au, Ni, and Co were transferred to Au/SAM, Ni/SAM, and Co/SAM electrodes and the current-voltage characteristics were studied by conductive probe atomic force microscopy (two probe measurement). Four point probe measurements were also performed under vaccuo and at low temperature. The molecules studied were alkane dithiols and alkane diisocyanides of varying length as well as biphenyldithiol and terphenyldithiol conjugated systems. The results will be discussed.
9:00 PM - II6.62
Comparison of Chain Morphology in Spin-cast, Drop-cast and Inkjet-printed Regioregular Polythiophene Thin Films by Variable-angle Spectroscopic Ellipsometry (VASE) and Near-edge X-ray Absorption Fine Structure Spectroscopy (NEXAFS).
LokeYuen Wong 1 , Rui Qi Png 1 , Lay-Lay Chua 1 2 , Xing Yu Gao 1 3 , Ke Lin 4 , Soo-Jin Chua 4 , Andrew T.S. Wee 1 3 , Peter K.H. Ho 1
1 Physics, National University of Singapore, Singapore Singapore, 2 Chemistry, National University of Singapore, Singapore Singapore, 3 , Singapore Synchrotron Light Source, Singapore Singapore, 4 , Institute of Materials Research & Engineering, ASTAR, Singapore Singapore
Show AbstractDespite its importance in determining the optoelectronic and transport properties of π-conjugated polymer organic semiconductors (OSCs), the chain morphology present in thin films, including conjugation length, segment orientation and polymorphic states, and how this varies with film deposition condition is little understood. We show here for a model semicrystalline polymer, regioregular poly(3-hexylthiophene) (rr-P3HT), that this can be obtained from the optical dielectric spectrum in combination with the C1s→π* NEXAFS spectrum, which we found to vary strongly between the air and substrate interfaces, and remarkable also between spin-cast (sc), drop-cast (dc) and inkjet-printed (ijp) films. We show that (i) polymer chains near the top surface of sc films tend to be more ordered than the bottom despite the faster drying, but this difference becomes less marked in ijp and dc films; and (ii) more surprisingly ijp films are better ordered, more isotropic and have a higher fraction of a close-packed polymorphic state than dc films. This unexpected morphology which we attribute to spatially inhomogeneous drying of the printed film on the substrate causes consistently lower field-effect transistor (FET) mobilities in ijp films. However, solvent-vapor annealing can recover these values to the dc levels, presumably through relaxation of this frozen-in stress and morphology.
9:00 PM - II6.63
Ambipolar Polymer Thin Film Transistors and Their Complementary Circuits.
Felix Kim 1 , Xugang Guo 3 , Mark Watson 3 , Samson Jenekhe 1 2
1 Chemical Engineering, University of Washington, Seattle, Washington, United States, 3 Chemistry, University of Kentucky, Lexington, Kentucky, United States, 2 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractOrganic thin film transistors (OTFTs) with ambipolar semiconductors can be operated in either n- or p-channel modes depending on the polarity of applied potentials. The multifunctionality of the ambipolar OTFTs in complementary circuits is a key property for reducing the number of patterning processes which is directly related to the fabrication cost. We report high-mobility ambipolar OTFTs fabricated from a solution-processable polymer semiconductor and gold electrodes, and their integration in complementary inverters. Standard bottom-contact, bottom-gate OTFTs were fabricated by spin-coating of polymer solution onto the hydrophobic substrates. Charge carrier mobilities are on the order of 10^-2 – 10^-3 cm2/Vs. The on/off ratios were on the order of 10^3 – 10^5 at large source-drain bias. A complementary inverter which consists of two identical ambipolar OTFTs showed excellent switching with the output voltage gains of 16 – 30, recording one of the highest gains among such complementary inverters based on the single-component ambipolar OTFTs. Our present results demonstrate that the utilization of ambipolar OTFTs is one of the promising approaches for applications in low-cost consumer electronics.
9:00 PM - II6.64
Ultraflat Nanoelectrodes: A Novel Platform for Fundamental Research on the Electronic Properties of Organic Monolayers.
Allard Katan 1 , Bas Hendriksen 1 3 , Bruce Harteneck 2 , Miquel Salmeron 1
1 Materials Science Division, Lawrence Berkeley National Laboratory, Berkeley, California, United States, 3 Institute for Molecules and Materials, Radboud University , Nijmegen Netherlands, 2 Molecular Foundry, Nanofabrication facility, Lawrence Berkeley National Laboratory, Berkeley, California, United States
Show AbstractOrganic electronics is a relatively new and exciting field of research, where the main goals are to understand the electronic properties of organic materials, and use these properties to fabricate functional devices. One important challenge in organic electronics is to make good contact from metallic electrodes to organic molecules. The charge transport in organic transistors has been shown to mainly take place in the first few molecular layers of the material. It is therefore assumed that ultrathin devices, only a few layers thick, will have certain advantages over more classical thin film devices, especially for sensor purposes. However, contacts to the fragile molecular layers are notoriously unreliable. The most success in making contacts to molecular monolayers has been achieved using top-contact electrodes deposited on top of the molecular film. One disadvantage of such a geometry is that the molecule-electrode contact is always buried, and cannot be accessed with scanning probe or other microscopy techniques for detailed studies. The shadow evaporation often used for making the electrodes, sets lower limits on the device size, so true nanodevices are rare in organic transistors.We have developed a novel platform to allow the study of electronic properties of ultrathin organic films down to single-molecule detail. This platform, dubbed Ultraflat Nano-Electrodes, is similar to the well-known bottom-contact geometry used in organic transistors, but has the metal source and drain electrodes embedded in the gate oxide. The surface of the metal electrodes has sub-nm roughness, and is also coplanar with the oxide to sub-nm levels. The source-drain spacing can be as small as a few tens of nm. Underneath the oxide is a back-gate electrode. This device presents a nearly atomically flat surface onto which molecular layers can be deposited, through vacuum deposition or with a Langmuir-Blodgett technique, without breaking the continuity of the film. By depositing the layer, an organic transistor is formed with a nano-sized channel, where all of the active material is exposed. This has the great advantage of allowing access of an AFM tip to the surface of the film, so that structural details of the entire active region can be imaged or manipulated with molecular resolution.We will show the fabrication method of our device, the so-called replica method, and show that we can reliably make devices with sub-nm roughness. Furthermore, we will present the results of AFM structural and electronic investigations, combined with transport measurements on monolayers of several organic semiconductors, which shed new light on the role of structural defects and grain structure in the charge transport through molecular films.
9:00 PM - II6.66
Spray Deposited Organic Thin Film Transistors: Structure and Electronic Properties.
Calvin Chan 1 , Lee Richter 2 , Brad Dinardo 1 , Cherno Jaye 3 , Brad Conrad 1 , David Germack 4 , Dean Delongchamp 4 , David Gundlach 1
1 Semiconductor Electronics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 2 Surfaces and Microanalysis Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Ceramics Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 4 Polymers Division, National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show AbstractOrganic thin film transistors (OTFTs) are attractive for large-area and low-cost applications such as display backplanes, environmental sensors, and radio-frequency identification circuits. Spray deposition of organic thin films offers a high throughput processing scheme in which large areas can be coated quickly. Although spray painted organic solar cells have been previously demonstrated, there is currently very little work in the literature discussing spray deposited OTFTs. In this work, fabrication of poly-3-hexylthiophene (P3HT) transistors by airbrush spray deposition is investigated, and their structural and electronic properties are compared to spin coated OTFTs. Despite the disparate processing conditions, both spray deposited and spin cast devices demonstrated remarkably similar transistor behavior with similar drive currents. The measured maximum mobilities were at least 0.01 cm2V-1s-1 for both films, which are comparable to values obtained for high molecular weight P3HT. Structural characterization was investigated using ultraviolet-visible (UV-vis) absorption spectroscopy, near-edge x-ray absorption fine structure spectroscopy, grazing incidence x-ray diffraction, and atomic force microscopy (AFM). AFM images demonstrated dynamic features including individual P3HT droplets and indications of complex film drying and formation processes. The other structural characterization methods showed that although bulk spray deposited P3HT films are more disordered than spin cast P3HT films, the polymer orientation at the device-relevant organic-dielectric interface of both films are at least comparable. This work demonstrates the potential of using spray deposition as a fast throughput method of making organic thin film transistors.
9:00 PM - II6.67
Controlling Organic Semiconductor Growth for High Performance Organic Transistors.
Ajay Virkar 1 , Stefan Mannsfeld 2 , Yutaka Ito 1 , Michael Toney 2 , Zhenan Bao 1
1 Chemical Engineering, Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States
Show AbstractIn organic transistors the interface between the semiconductor and dielectric is extremely important since it is where the vast majority of current flows. To optimize organic electronic devices it is critical to understand how to engineer the ideal dielectric surface. We show record charge carrier mobilities for more than 20 organic semiconductors on a crystalline octadecylsilane (OTS) monolayer surface. Mobilities as high as 5.0 and 3.0 cm2/Vs were achieved for C60 and pentacene transistors, respectively. [1,2] The semiconductor growth mode is the preferred 2D layer-by-layer growth on a crystalline dielectric modification self-assembled monolayer (SAM) whereas on an amorphous SAM the undesirable 3D growth is observed. 2D growth is preferred to 3D growth since less detrimental grain boundaries, which diminish current, are formed. SAM order and organic semiconductor growth mode and nucleation were critically investigated using atomic force microscopy (AFM), grazing incidence X-ray diffraction (GIXD) and Monte Carlo simulations. The interface-semiconductor interaction energy necessary to drive 2D growth was also calculated and important considerations about organic semiconductor nucleation density, stability, and thin film growth on OTS monolayer surfaces are discussed. [1-4]References[1]The Role of OTS Density on Pentacene and C60 Nucleation, Thin Film Growth, and Transistor Performance, Ajay Virkar, Stefan Mannsfeld, Joon Hak Oh, Michael F. Toney, Yih Horng Tan, Gang-yu Liu, J. Campbell Scott, Robert Miller, Zhenan Bao. Adv. Funct. Mater. 2009, 19, 1–9. [2] Crystalline Ultra Smooth Self-Assembled Monolayers of Alkylsilanes for Organic Field-Effect Transistors, Yutaka Ito, Ajay Virkar, Stefan Mannsfeld, Joon Hak Oh, Michael Toney, and Zhenan Bao. J. Am. Chem. Soc., 2009, 131 (26), pp 9396–9404[3] Organic Semiconductor Growth and Morphology Considerations for Organic Thin Film Transistors. Ajay A. Virkar, Stefan Mannsfeld, Zhenan Bao and Natalie Stingelin. Submitted to Advanced Materials[4] Energetics and Stability of Pentacene Thin Films on Amorphous and Crystalline Octadecylsilane Modified Surfaces. Ajay A. Virkar, Stefan Mannsfeld, and Zhenan Bao. Submitted to Journal of Materials Chemsitry – special issue on Interfaces in Organic and Molecular Electronics
9:00 PM - II6.68
Effect of Film Morphology on Charge Transport in C60-based OFETs.
Mujeeb Ullah 1 , Philipp Stadler 2 , Alexander Kharchenko 3 , Clemens Simbrunner 1 , Almantas Pivrikas 2 , Neyazi Serdar Sariciftci 2 , Helmut Sitter 1
1 Institute for Semiconductors and Solid State Physics, Johannes Kepler University Linz, Austria, Linz, Upper Austria, Austria, 2 LIOS, Johannes Kepler University Linz, Linz Austria, 3 , PANalytical B. V, 7600 AA Almelo Netherlands
Show AbstractThe critical factor that limits the efficiency of organic electronic devices is low charge carrier mobility which is attributed to various disorders in the films, electronic interactions, chemical defects and interface effects. In this work we study the effects of active film morphology on the charge transport in the Organic Field Effect Transistor (OFETs) devices. We fabricated the OFETs using different substrate temperature to grow different morphologies of C60 films by Hot Wall Epitaxy. Atomic Force Microscopy images and XRD showed increasing grain size with increasing substrate temperature. The influence on the field effect mobility was observed for different OFETs fabricated from C60 films with increasing grain size. The temperature dependence of the charge carrier mobility in these devices showed different Arrhenius activation energies. The temperature dependence of the mobility in C60 based OFETs follows the empirical relation named as Meyer-Neldel Rule. A shift in the characteristic Meyer-Neldel energy, which can be interpreted as a measure for the energetical disorder of the defects, was observed with changing the C60 morphology.References:1- Mujeeb Ullah, T.B. Singh, H. Sitter, N.S. SariciftciApplied Physics A, Materials Science & Processing (2009), in press2- H. Bässler, Physica Status Solidi B 175, 15 (1993).3- W. Meyer and H. Neldel, Z. Tech. Phys. 18, 588 (1937).4- I.I.Fishchuk, A.Kadashchuk, H.Bässler, and S.Nešpurek, Phys. Rev. B 67, 224303 (2003).
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Laser Printing of Polythiophene Layers for Organic Field-effect Transistors.
Marina Makriyianni 2 , Panagiotis Dimitrakis 1 , I. Zergioti 2 , S. Chantzandroulis 1 , P. Normand 1
2 Physics Department, NTUA, Zografos Greece, 1 Inst. of Microelectronics, NCSR Demokritos, Aghia Paraskevi Greece
Show AbstractThis work presents our recent results on solid phase Laser Induced Forward Transfer process (LIFT) polymer material for the fabrication of a fully operative organic thin-film transistor (OFET). The use of LIFT is a promising alternative for maskless manufacture of organic electronic components on flexible supports when usual techniques, such as inkjet or roll-to-roll printing, cannot be used. Furthermore, LIFT allows localized deposition of the polymer without altering its properties. The LIFT experiments were carried out using a pulsed Nd:YAG laser (266 nm wavelength, 4 ns pulse duration) and a high power imaging micromachining system. The LIFT technique was used to deposit the semiconducting layer, the regioregular poly-3-hexylthiophene (P3HT). The polymer solution was prepared at different concentrations P3HT/toluene and spin-coated on a quartz substrate (donor substrate). Typical bottom gate OFET architecture was followed. More specifically, a dry silicon dioxide layer 80-100nm thick was grown on an implanted p-type Si substrate (receiver substrate) acting as the gate oxide and the gate electrode respectively. Au/Ti metal pads were sputtered on the SiO2 layer through a shadow mask, forming the source and drain electrodes. The polymer material (P3HT) was transferred from a donor substrate onto the receiver substrate placed in between the source and drain electrodes and covering partially the metal pads. The distance between them defines the gate length of the transistor which was about 100μm. Upon irradiation, the donor and the receiver substrates are located in close proximity. The resulting transistors were characterized by parametric I-V measurements in order to determine its operating characteristics and performance. It was found that the mobility of the P3HT layer is similar to that of spin-coated P3HT. Furthermore, the source to drain current measurements under zero bias revealed that bulk conductivity of P3HT is due to a space-charge-limited-current mechanism.
9:00 PM - II6.7
Graphoepitaxy of α-Sexithiophene: The Fabrication of Highly Oriented Elongated Domains.
Yoshio Kanamori 1 , Susumu Ikeda 2 , Yasuo Wada 3 , Koichiro Saiki 1 4
1 Graduate School of Science, The University of Tokyo, Hongo, Bunkyo-ku Japan, 2 Graduate School of Science, Tohoku University, Sendai, Miyagi Japan, 3 Graduate School of Interdisciplinary New Science, Toyo University, Kawagoe, Saitama Japan, 4 Graduate School of Frontier Science, The University of Tokyo, Kashiwa, Chiba Japan
Show AbstractGraphoepitaxy is a promising technique for oriented growth on substrates with an amorphous dielectric top layer. Recently, we have proved that graphoepitaxy of α-sexithiophene (6T; C24H14S6) occurs on the SiO2 surface with artificial periodic micro-grooves fabricated by electron beam lithography. The aim of the present study is to increase quality and anisotropy of organic films by reducing the periodicity of grooves. The pitch and depth of the periodic grooves were 200 nm and 10 nm, respectively. 6T thin films were grown by molecular beam deposition using a Knudsen cell under ultrahigh vacuum conditions (≒10-7 Pa).The morphology of 6T films grown on periodic grooves was quite different from that on a flat substrate. 6T films consisted of isolated domains on grooved substrate, while 6T films were covering the substrate with randomly oriented regions on flat substrates. 6T domains on periodic grooves were classified into three types: (a) Elongated domains, (b) typically facetted domains, and (c) groove filling domains. The elongated domains extended along the groove direction upto 10 μm, with each edges facetted. The top of elongated domains was almost flat; less than one monolayer step per 1 μm. The typically facetted domains are surrounded only with low index facets. These domains were thicker than the elongated domains, implying stronger aggregation. The groove filling domain filled only one groove. The height of the domain was almost equal to the depth of the groove (10 nm). The typically facetted domains seem located on the groove filling domain.The crystallographic orientations of domains were estimated by their shapes. All of elongated domains aligned with the b-axis direction ([010] direction) parallel to the groove, while the b-directions of typically facetted domains were normal to the grooves. There were strong correlation between crystallographic orientations and domain shapes. It is likely that the substrate anisotropy affected not only nucleation at which the crystal orientation are determined, but also the growth kinetics of 6T film. The growth feature was affected also by surface modification. OTS-SAM treatment widened the elongated domains as compared with those on bare substrates. HMDS-SAM treatment increased nucleation density and suppressed formation of facetted domains.The optimum condition for the growth of elongated domains, which is the most characteristic of graphoepitaxy, was examined by varying deposition rate, growth temperature, groove width, and groove interval. Low deposition rate and high growth temperature lead to produce large elongated domains with the length of over 20 μm. We now plan to fabricate FET structures with these elongated domains to investigate electric properties, such as conduction anisotropy with respect to the b-direction.
9:00 PM - II6.70
Ultra Low Roughness Ferroelectric PVDF/PMMA Blend Films for Low Voltage Non-volatile Polymer Memory.
Seok Ju Kang 1 , Youn Jung Park 1 , Insung Bae 1 , Ho-Cheol Kim 2 , Siegfried Bauer 3 , Edwin Thomas 4 , Cheolmin Park 1
1 , Yonsei Univ., Seoul Korea (the Republic of), 2 , IBM Almaden Research Center, San Jose, California, United States, 3 , Johannes Kepler Univ., Linz Austria, 4 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractFerroelectric polymer memories are of interest not only due to their easy and flexible device fabrication based on solution processes such as spin coating but also due to their compatibility with common organic electronic fabrication technologies. Representative ferroelectric polymers such as poly(vinylidene fluoride) (PVDF) and its copolymers with trifluoroethylene (TrFE) possess a bistable dipole polarizations that can be repeatedly switched upon application of an external electric field. The basic ferroelectric polarization storage component in memories is a metal/ferroelectric polymer/metal (MFM) capacitor with a ferroelectric polymer thin film sandwiched between arrays of metal electrodes. Recently ferroelectric thin films have been employed as a gate insulator to form Ferroelectric Field-Effect Transistor (FeFET) device architectures due to its various advantages like non-destructive readout capability, small cell size and low operating voltage. The polarization state of the ferroelectric gate is set by the polarity of the writing gate voltage that controls the electrical conductance of the channel between the source and the drain electrode. In both device structures (MFM and FeFET), the rather large coercive field of the ferroelectric polymers (approximately 50 MV/m) requires the fabrication of films as thin as possible for low voltage operation.We investigated a facile route to fabricating thin ferroelectric poly(vinylidene fluoride) (PVDF)/poly(methylmethacrylate) (PMMA) blend films with very low surface roughness based on spin coating and subsequent melt-quenching. Amorphous PMMA in a blend film effectively retards the rapid crystallization of PVDF upon quenching, giving rise to a thin and flat ferroelectric film with nanometer scale beta type PVDF crystals. The still, flat interfaces of the blend film with metal electrode and/or the organic semi-conducting channel layer enable us to fabricate a highly reliable ferroelectric capacitor and transistor memory unit operating at the voltage as low as 15V. For instance, with TIPS-pentacene single crystal as an active semi-conducting layer, a flexible ferroelectric field effect transistor shows a clockwise I-V hysteresis with a drain current bistablility of a 103 and data retention time of more than 15 hours at ±15V gate voltage. Furthermore, the robust interfacial homogeneity of the ferroelectric film is highly beneficial for transfer printing in which arrays of metal/ferroelectric/metal micro-capacitors are developed over a large area with well defined edge sharpness.
9:00 PM - II6.72
Synthesis and Properties of Thiadazole-fused Indolo[2,3–a]Carbazole Based Materials.
Ganapathy Balaji 1 , Parameswaran Manoj 1 , Wong Shim 1 , Suresh Vailyaveettil 1
1 Chemistry, National University of Singapore, Singapore Singapore
Show AbstractAmong the five indolocarbazole(1) isomers, indolo[3,2–b]carbazole has been explored extensively for electronic applications. Extensive studies on indolo[2,3–a]carbazole based compounds are illustrated in biological and sensing applications. However, the material aspects of this isomer of indolocarbazole are poorly explored. In this regard, a new series of indolo[2,3–a]carbazole-based fused-ring compounds containing thiadiazole units were synthesized by using Cadogan cyclization as a key step(2). Synthesized compounds are stable at ambient conditions and soluble in common organic solvents. Crystal lattice of indolocarbazole-based compound showed sandwiched herringbone arrangement. The change in photophysical and electrochemical properties upon incorporation of acceptor moiety (benzothiadazole) in fused ring system was studied. The benzothiadazole incorporated indolocarbazole are found to have better properties compared to indolo[3,2–b]carbazoles. Synthesized compounds shows a close packing in solid state and low HOMO energy level. The unsubstituted compound was electro polymerized over ITO substrate to yield a polymer. Preliminary optical and electrochemical studies indicated that the synthesized molecules are potential candidates for electronic applications (OFET). References:1.Janosik, T.; Wahlstrom, N.; Bergman, J.; Tetrahedron 2008, 64, 9159.2.Balaji, G.; Shim, W. L.; Parameswaran, M.; Valiyaveettil, S. Org. Lett. 2009, 11, 4450.
9:00 PM - II6.73
Fabrication and Characterization of p-n Junction Based on ZnO and CuPc.
Ram Gupta 1 , Kartik Ghosh 1 , Pawan Kahol 1
1 , Missouri State University, Springfield, Missouri, United States
Show AbstractPulsed laser deposition technique was used for fabrication of inorganic-organic hetero-junction. Zinc oxide (ZnO) and copper-phthalocyanine (CuPc) were used as n and p- type semiconductors respectively. ZnO and CuPc were deposited using pulsed laser deposition and thermal evaporator techniques. ZnO was highly oriented along (002) directions, while CuPc was amorphous in nature. Current –voltage (I-V) characteristics were studied in details. I-V characteristic of the ZnO-CuPc junction showed rectifying behavior. Various junction parameters such as barrier height and ideality factor were calculated using I-V data and observed to be 0.64 eV and 4.0. Different techniques such as Cheung and Norde’s method were used to compare the junction parameters obtained by I-V characteristics.
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Chemically-fixed p-n Heterojunctions for Polymer Electronics via Covalent B-F Bond Formation.
Corey Hoven 1 , Huiping Wang 2 3 , Logan Garner 2 , Daniel Winkelhaus 4 , Mark Elbing 2 , Guillermo Bazan 1 2
1 Department of Materials, University of California, Santa Barbara, Santa Barbara, California, United States, 2 Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, California, United States, 3 State Key Laboratory of Supramolecular Structure and Materials, Jilin University, Changchun China, 4 Inorganic & Structural Chemistry, University Bielefeld, Bielefeld Germany
Show AbstractWidely used solid state devices fabricated with inorganic semiconductors such as light emitting diodes and solar cells derive much of their desirable function from the p-n junction. Such junctions lead to diode characteristics and are attained when p-doped and n-doped materials come into contact with each other. Achieving bilayer p-n junctions with polymeric semiconducting materials has been hindered by difficulties in the deposition of thin films with independent p-doped and n-doped layers. Metastable p-n junctions can be obtained in light emitting electrochemical cells, where a conjugated polymer is mixed with an ion conducting polymer and a salt. Under an applied bias the ions redistribute, which combined with injected charges, creates a p- and n-doped regions via in-situ electrochemical doping. That ion motion is required for the operation of these devices leads to long temporal responses and a dynamic electronic structure. We show on how to achieve permanently-fixed polymer based p-n heterojunctions by using a cationic conjugated polyelectrolyte (CPE) and an underlayer composed of a neutral conjugated polymer bearing anion-trapping functional groups. This approach required the design and preparation of a new conjugated polymer bearing chemical functionalities specific for anchoring anions. Application of an external bias leads to charge injection and anion migration from a CPE with fluoride counteranions into the neutral layer with fluoride trapping boryl-groups, where irreversible B-F bond formation takes place. Novel aspects of this method is that anion migration leads to the formation of a new anionic species, which is prevented from returning to its original layer due to its incorporation into a large macromolecular structure. After the initial charging, electrical doping and borate formation, one obtains devices with no delay in the turn on of light-emitting electrochemical behavior and excellent current rectification.
9:00 PM - II6.75
Carrier Transport in Small-molecule Nonvolatile Memory Characteristics.
Sungho Seo 1 , Woo-Sik Nam 1 , Sang-Yi Lee 1 , Kwang-Hee Park 1 , Jea-Gun Park 1
1 Dept. of Electrical and Computer Engineering, Hanyang University, Seoul Korea (the Republic of)
Show AbstractWe investigated the effect of electron-transport (Alq3) and hole-transport (α-NPD) small-molecule bi-layer on nonvolatile memory characteristics for small-molecule memory-cell embedded with Ni nano-crystals. The device structure is a bottom Al electrode / bottom conductive small-molecule bi-layer (α-NPD and Alq3) layer, Ni nano-crystals surrounded by NiO, upper conductive small-molecule bi-layer (Alq3 and α-NPD) layer, and top Al electrode. Alq3 layers were deposited on both sides of Ni nano-crystal layer, and α-NPD layers were inserted between conductive small-molecule layer (Alq3) and upper or bottom electrode.Nonvolatile memory characteristics for small-molecule bi-layer memory-cells were estimated as a function of the ratio of electron-transport (Alq3) layer thickness to hole-transport (α-NPD) layer thickness. The on-state current (Ion) and memory margin (Ion/Ioff ratio) decreased with increasing the hole-transport layer thickness. In addition, as the hole-transport layer thickness increased, the retention time was getting shorter and the standard deviation of the endurance cycles of erase and program was getting larger. These results indicate that the α-NPD layer in small-molecule bi-layer memory-cells behaves as an electron-transport layer rather a hole-transport layer. Finally, we present that the dominant transport carrier in those memory-cells is an electron rather than a hole.Acknowledgement* This research was supported by "The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
9:00 PM - II6.76
Influence of Side Chain Position on Photo-physics and Electroluminescence of Conjugated Polymer.
Dinesh Kabra 1 , Liping Lu 1 , Guoli Tu 2 , Sebastian Albert-Seifried 1 , Remeco Havenith 3 , Wilhelm T Huck 2 , Richard Friend 1
1 Optoelectronics Group, Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Melville Laboratory for Polymer Synthesis, Chemistry Department, University of Cambridge, Cambridge United Kingdom, 3 Theoretical Chemistry, Zernike Institute for Advanced Materials, Rijksuniversiteit Groningen, Groningen Netherlands
Show AbstractConjugated polymer have been of great interest of academic and industrial laboratories due to its low cost large area processing techniques and with the advantages of synthesis of tailor-made organic substances, which allows fine tuning of absorption and emission spectral range and charge transport properties. Side chains in chemical structure of polymers play an important role in processing technique apart from its effect on spectral properties and charge transport. [1] In this study, we demonstrate the role of side chain position on photophysics and optoelectronic properties of red emissive conjugated polymer poly((9,9-dioctylfluorene)-2,7-diyl-alt-[4,7-bis(3-hexylthien-5-yl)-2,1,3-benzothiadiazole]-2,2-diyl) (F8TBT). Pristine polymer films with side chain positions inside (from thiophene unit towards benzothidazole unit, F8TBT-in) and outward (from thiophene unit towards polyfluorene units, F8TBT-out) were studied using time correlated single photon counting technique and photoluminescence efficiency (PLE) measurements. Higher photoluminescence efficiency and exciton lifetime revealed F8TBT-in favourable use for electro-luminescence devices. F8TBT-in has different chain orientation as compare to F8TBT-out, which changes dipole strength between F8 vs. TBT unit and we observed corresponding change as blue shift in absorption (50 nm) and emission (20 nm) spectra. These measurements reveal poor charge transfer between these units which result in higher exciton lifetime and PLE. A computational simulation has indicated that there is a geometrical difference between the F8TBT-in and F8TBT-out. We found a distortion between T-B-T units, which results in a less efficient conjugation between π-π units and a wider band gap that leads a blue shift of the absorption in F8TBT-in. This new polymer was further investigated for electroluminescence in hybrid inverted diode structure. [2] We obtained lower operating voltages with almost an order higher current efficiency then F8TBT-out as preliminary results, which might be due to reduced injection barrier. We also suggest a further enhancement in their performance by using higher molecular weight polymer and more of F8 with respect to TBT unit [3], which is 1:1 in this study. Reference:[1] R. Joseph Kline et al Macromolecules, 2007, 40, pp 7960–7965[2] D. Kabra et al Adv. Mater. 2008, 20, pp 3447 - 3452[3] Qiong Hou et al, Macromolecules 2004, 37, pp 6299-6305
9:00 PM - II6.77
Novel Bibenzo[b]thiophenes With Reduced Bandgaps as Determined by Density Functional Theory (DFT) Calculations.
Michael Korn 1 2 , Siva Adusumilli 1 2 , Ron Pieper 1 2
1 Electrical Engineering, The University of Texas at Tyler, Tyler, Texas, United States, 2 Center For Organic Semiconductor Modeling and Simulation (COSMOS), The University of Texas at Tyler, Tyler, Texas, United States
Show AbstractThe synthesis of 3,4:3’,4’-bibenzo[b]thiophene (BBT) (CAS# 70160-67-7) was first reported by Wudl et al. some 40 years ago [1]. Complexes of this molecule with iodine conducted electricity [1]. Whereas several compounds of the general class of benzothiophenes have been synthesized and employed as materials for organic semiconductors in recent years [2], [3], little research has been pursued for BBT-based molecules. Only very recently, derivatives of BBT were reported as materials for organic transistors [4]. The measured properties for several of those compounds resulted in mobilities of approximately 0.35 cm^2/Vsec, on/off ratios of 2×10^5, and in good shelf life [4]. To better understand the performance of those BBT-based molecules we performed DFT calculations on the B3LYP-6-31* level for selected derivatives. HOMO and LUMO levels, as well as ionization potentials and electron affinities of these compounds were calculated. We found that compounds for which mobilities were reported in ref. [4] all had bandgaps of around 3.5 eV, not much different from the non-substituted parent BBT molecule. However, in the course of our modeling, we discovered that significantly lower bandgaps, as low as 1.5 eV, can be achieved when following a systematic design pattern. We also found that this pattern not only holds true for BBT-based molecules but also resulted in low bandgaps for other benzothiophenes. We will report the structures of these novel low bandgap materials along with their calculated energies. [1] F. Wudl, R. C. Haddon, E. T. Zellers, F. B. Bramwell, J. Org. Chem. 44 (1979) 2491. [2] A. R. Murphy, J. M. J. Fréchet, Chem. Rev. 107 (2007) 1066. [3] J. E. Anthony, Chem. Rev. 106 (2006) 5028. [4] F. Takahiro, T. Yoshiyuki, N. Masakatsu, JP 2008140989 (A). This work was supported by a grant from the U.S. Army Research Lab.
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Microscopy and Spectroscopy Study of Poly(3-hexylthiopene) Coiled Carbon Nanotube for Photovoltaic Applications.
Michele Giulianini 1 , Eric Waclawik 1 , John Bell 1 , Manuela Scarselli 2 , Paola Castrucci 2 , Maurizio De Crescenzi 2 , Nunzio Motta 1
1 , Queensland University of Technology, Brisbane, Queensland, Australia, 2 , Universita' di Roma Tor Vergata, Roma Italy
Show AbstractThis microscopy and spectroscopy study is focused on determining the causes of low power conversion efficiencies of solar cells based on carbon nanotubes included in Poly(3-hexyl-thiophene) matrix. By combining Scanning Tunnelling Microscopy (STM) and Spectroscopy (STS), and High Resolution Transmission Electron Microscopy (HR-TEM) it has been possible to investigate at atomic resolution the adhesion and the self organisation of the polymer on the nanotube surface. STM and HR-TEM analysis evidenced the tendency of P3HT to coil the nanotube by self-assembling on a helical structure that can result regular and ordered over several tenths of nanometres. The stacking distance evidences a non-covalent bonding between the π orbitals of the thiophene ring of the polymer backbone and the structure of the nanotube. Polymer structures oriented along well defined directions confirm the role of the nanotube chirality on the final assembly. STS measurements on the polymer-free sections of the wrapped nanotube demonstrate the good agreement between the measured nanotube density of states and an independently developed theoretical model. Remarkably, no evidence of the influence of the adjacent polymer wrapping can be observed when analysing the bare nanotube section, while on the polymer coiled sections, asymmetric I-V curves show a rectifying behaviour that is affected by the charge migration from the polymer thiophene ring to the nanotube. This phenomenon could increase the probability of recombination of the light-generated exciton and hence, the solar cell is expected to perform inefficiently during the photovoltaic conversion process. HR-TEM of the same blends also evidenced that in some cases the polymer wrapping leads to mechanical deformation of nanotubes along the sidewalls. As a consequence of the polymer coiling, the nanotube external radius presents a variability of more than 5%. Further Raman Spectroscopy analysis confirmed that the characteristic nanotube G-band peak upshifts increasingly as the polymer content is raised, reaching a maximum of 11 cm-1. Although this shift is consistent, it is limited by the charge transfer from the polymer that produces a downshift of the same peak in the direction opposite to the compression.
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Fully Functionalized Block Copolymers for Organic Electronic Applications.
Sven Huettner 1 2 , Michael Sommer 2 , Justin Hodgkiss 1 3 , Peter Kohn 4 , Thomas Thurn-Albrecht 4 , Richard Friend 1 , Ullrich Steiner 1 , Mukundan Thelakkat 2
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Applied Functional Polymers, University of Bayreuth, Bayreuth Germany, 3 MacDiarmid Institute for Advanced Materials and Nanotechnology, and School of Chemical and Physical Sciences, Victoria University of Wellington, Wellington New Zealand, 4 Experimentelle Polymerphysik, University of Halle-Wittenberg, Halle Germany
Show AbstractIn conventional approaches bulk heterojunction solar cells consist of a donor (p-type) and an acceptor (n-type) material that are blended together to form the active layer. We use a block copolymer instead, where a donor and an acceptor polymer are covalently linked. Block copolymers are well known to phase separate in highly ordered nanostructures on length scales commensurate with the exciton diffusion length. The acceptor block consists of a polyacrylate backbone with pendant perylene bisimide moieties and the donor block consists of poly(3-hexylthiophene) (P3HT). We demonstrate photovoltaic devices with high external quantum efficiencies up to 30% - the highest of any block copolymer based system, whereby we observe a strong dependence on the molecular weight. We combine temperature dependent small angle and wide-angle X-ray scattering measurements to investigate the block copolymer phase separation as well as the influence of the crystallisation kinetics of the acceptor and donor block. Intermolecular and intramolecular interactions drive the self-assembly of structures from molecular lengthscales to larger mesostructures of some nanometers to microphase separation of some tens of nanometers in size. The investigation of the morphology is accompanied by steady state spectroscopy and transient absorption spectroscopy resolving the exciton motion, charge separation and recombination processes on a time scale from ps to ms. The interfacial crystallinity of P3HT plays a significant role for the device performance in bulk heterojunction devices and we show that it can be significantly increased by the use of block copolymers. Furthermore organic thin film transistors are used to characterize the transport properties in these novel materials which are found to exhibit unique properties such as the tunability between n-type, p-type or ambipolar transport. This offers potential applications in ambipolar transistors and complementary logic devices from a single material.
9:00 PM - II6.8
Study of Electronic Properties of Vanadyl-phthalocyanine Molecular Layers on HOPG by Scanning Probe Microscopy.
Weiguang Xie 1 , Jin An 1 , Kun Xue 1 , Jun Du 1 , Jianbing Xu 1
1 Department of Electronic Engineering and Materials Science and Technology Research Center, Chinese University of Hong Kong, Hong Kong China
Show AbstractMetallo-phthalocyanine (MPc) molecules have attractive considerable attention for years due to their promising application in electronics. Devices based on non-planer MPc, such as vanadyl-phthalocyanine (VOPc) may have a better performance than that of the planer MPc. However, the growth and properties at the interface of non-planar MPc are relatively unexplored, in comparison with those of planer MPc. In this work, multiple layers of VOPc molecules deposited on highly ordered pyrolytic graphite (HOPG) are studied by scanning probe microscopy (SPM). The as-deposited VOPc molecules form a full layer on HOPG, and then individual VOPc islands continue to grow at room temperature. Each layer of VOPc molecules is found to be bilayer. Band-gap on different layers of VOPc molecules is acquired by scanning tunneling spectroscopy (STS). The surface potential changes in stratified layers are measured by exploiting scanning Kelvin probe microscopy (SKFM). Surface potential shifting of tens of millivolts is observed between each bilayer. The as-grown VOPc thin film on HOPG is annealed at different temperatures in ultra-high vacuum, and dissociation of the bilayer is observed. Morphology multiplicity on layers of VOPc molecules and charge transfer effect at the interface are discussed.
9:00 PM - II6.80
A Study of 3,12-Dimethoxy-7,8-Dicyano-5,6,9,10-Tetrahydro[5]Helicene as a New Emitter for Organic Light-emitting Diodes.
Somboon Sahasithiwat 1 , Thanasat Sooksimuang 1 , Tipwan Mophuang 1 , Laongdao Menbangpung 1 , Siriporn Kamtonwong 1
1 , National Metal and Materials Technology Center (MTEC), Klong Luang, Pathumthani, Thailand
Show Abstract3,12-Dimethoxy-7,8-dicyano-5,6,9,10-tetrahydro[5]helicene (DMDCTH) was introduced as a new emitter for organic light-emitting diodes (OLEDs). The lowest unoccupied molecular orbital (LUMO, -2.9eV), the highest occupied molecular orbital (HOMO, -5.8eV) and energy band gap (2.9eV) of this compound were determined using cyclic voltammetry and spectroscopy technique. The fluorescence quantum yield of DMDCTH in chloroform is 0.71. OLEDs with a configuration of ITO/ PEDOT:PSS(35nm)/CBP:DMDCTH/BCP/Alq3/LiF(1nm)/Al(100nm) were fabricated. The OLEDs were optimized for the highest maximum current efficiency by varying the amount of DMDCTH in CBP, and layer thicknesses of CBP:DMDCTH, BCP, and Alq3. As a result, the best OLED possessed a configuration of ITO/PEDOT:PSS(35nm)/CBP:3%DMDCTH(30nm)/ BCP(10nm)/Alq3(40nm)/LiF(1nm)/Al(100nm) and exhibited a maximum current efficiency of 1.31 cd/A, a turn-on voltage at 5.4 V, and a maximum brightness at 3413 cd/m2. The OLED emitted bluish green light with CIE coordinates of (x = 0.18, y = 0.23).
9:00 PM - II6.82
Properties and Applications of Gravure Printed Organic Diodes.
Kaisa Lilja 1 , Sampo Tuukkanen 1 , Timo Joutsenoja 1
1 Department of Electronics, Tampere University of Technology, Tampere Finland
Show AbstractOrganic thin film electronics offers the intriguing possibility to fabricate large-area, flexible electronic products such as thin displays, solar cells, identification tags and smart labels with cost effective manufacturing methods. We report the electronic characteristics and applications of gravure printed organic diodes that are fabricated using a process that is scalable to high-volume production. The diodes consist of a layer of poly(triarylamine) (PTAA) sandwiched between copper and silver electrodes. A wet etching process where the etch resist was printed by rotary screen printing was used to pattern the copper cathode. The PTAA and silver layers were printed using a laboratory scale automatic gravure printing press. One possible application for printed organic diodes is the rectifier in an organic RFID tag. Although organic rectifiers have been demonstrated before, the work has concentrated on evaporated semiconductors and metal electrodes patterned by shadow masking or photolithography. We report gravure printed organic diodes that deliver excellent DC output at AC input frequencies close to the RFID HF standard (13.56 MHz).[1] The diodes exhibit current densities of over 1 mA/cm2 at 5 V and rectification ratios over 10 000. The HF standard frequency can be reached by further optimising the device geometry, electrodes, printing parameters and semiconductor properties. The diode fabrication and characterization was performed in ambient laboratory conditions. The diode characteristics showed no significant degradation after months of storage.We also report a flexible organic diode backplane for driving an electrophoretic display.[2] Currently, TFTs are used to control the pixels in organic active matrix displays. However, the cost-effective fabrication of organic TFTs is challenging due to high resolution patterning required for the transistor channel. One possible solution is to return to the concept of organic thin film diode based active matrix displays. As vertical, two terminal structures, organic thin film diodes are less demanding with regard to feature size and alignment. This enables the backplane circuit to be manufactured using cost effective printing processes. In the display, two printed diodes are used to control a pixel in a lateral 4 by 4 matrix structure. By developing the backplane geometry, operating voltage and material properties, the pixel size, aperture ratio and contrast ratio can be optimized for large, low resolution display applications.References[1] K. E. Lilja, T. G. Bäcklund, D. Lupo, T. Hassinen, T. Joutsenoja, Org. Electr. 10 (2009) 1011.[2] K. E. Lilja, T. G. Bäcklund, D. Lupo, J. Virtanen, E. Hämäläinen, T. Joutsenoja, Printed organic diode backplane for matrix addressing an electrophoretic display, submitted to Thin Solid Films.
9:00 PM - II6.83
High Efficiency Blue Phosphorescent Organic Light Emitting Diodes (PHOLEDs) Without any Electron Injection Layer Using a Phosphine Oxide Derivative as a Host Material.
Sang Eok Jang 1 , Chul Woong Joo 1 , Oh Young Kim 1 , Jun Yeob Lee 1
1 polymer science and engineering, Dankook University, Yongin-si Korea (the Republic of)
Show AbstractWe have designed and synthesized a new blue phosphorescent host material based on the phosphine oxide moiety. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the 2,7-bis(diphenylphosphoryl)-9,9'-spirobi[fluorene] (SPPO13) were 6.56 eV and 2.91 eV, respectively. The triplet energy level of SPPO13 was calculated to be 2.73 eV. The blue phosphorescent organic light-emitting diodes (PHOLEDs) were fabricated using the SPPO13 based emitting layer doped with bis(4',6'-difluorophenylpyridinato)iridium (FIrpic). A maximum quantum efficiency of 19.9 % and a current efficiency of 39.7 cd/A were obtained in the SPPO13 blue device. In addition, a high quantum efficiency of 19.0 % with a current efficiency of 37.5 cd/A was achieved in the SPPO13 blue PHOLEDs even without any electron injection layer due to good electron injection properties of SPPO13 from Al electrode .
9:00 PM - II6.85
A Novel Benzobisthiazole based Semiconductor Material For OFET Applications.
Greg McEntee 1 , Peter Skabara 1 , Filipe Vilela 1 , Steven Tierney 2 , Thomas Anthopoulos 3 , William Clegg 4
1 Pure and Applied Chemistry, University of Strathclyde, Glasgow United Kingdom, 2 , Merck Chemicals, Southampton United Kingdom, 3 Department of Physics, Imperial College London, London United Kingdom, 4 School of Chemistry, Newcastle University, Newcastle Upon Tyne United Kingdom
Show AbstractThe synthesis of a novel benzobisthiazole based molecule is presented. Previous work on these systems has shown three dimensional molecular self assembly, resulting in close intermolecular contacts.[1] This novel system, 2,6-bis(3,6-dihexylthieno[3,2-b]thiophen-2-yl)benzo[1,2-d:4,5-d']bis(thiazole), has been designed to include thienothiophene end units to improve charge injection whilst keeping the molecule planar by utilizing non–covalent interactions. The compound has been characterized by cyclic voltammetry, UV-Vis spectroscopy and X-ray crystallography. Vacuum deposited films of the molecule have demonstrated hole mobilities as high as 0.1 cm2 V-1 s-1 in bottom contact devices. Mobilities have been shown to vary depending on annealing temperature which causes the formation of differing crystalline phases. X-ray measurements show close π-π stacking in the solid state, which acts to enhance mobility. Polymer films were grown electrochemically onto indium tin oxide coated glass. The optical and electrochemical properties of the polymer are reported.1.Pang, H.; Vilela, F.; Skabara, P. J.; McDouall, J. J. W.; Crouch, D. J.; Anthopoulos, T. D.; Bradley, D. D. C.; De Leeuw, D. M.; Horton, P. N.; Hursthouse, M. B., Advantageous 3D ordering of pi-conjugated systems: A new approach towards efficient charge transport any direction. Advanced Materials 2007, 19, 4438-4442.
9:00 PM - II6.86
Direct Observation of Pentacene-thiol Interaction Using x-ray Spectroscopy.
Zhang Jia 1 , Vincent Lee 1 , Luca Floreano 2 , Alberto Verdini 2 , Albano Cossaro 2 , Alberto Morgante 2 , Ioannis Kymissis 1
1 Electrical Engineering, Columbia University, New York, New York, United States, 2 , CNR-INFM Laboratorio Nazionale TASC, Basovizza Italy
Show AbstractThere has been an intense interest in the surface modification of the source and drain electrodes for organic field effect transistors (OFETs) to improve transistor performance. A number of thiol based self assembled monolayers (SAMs) on the source and drain metal of OFETs have demonstrated to improvements to the contact resistance and channel transistor performance. Morphological improvements at the contacts, a change in the effective work function, and charge transfer between the thiols and the semiconductor have all been credited with the observed performance improvements. Using in-situ semiconductor deposition together with x-ray photoelectron spectroscopy (XPS) and near-edge x-ray absorption fine structure (NEXAFS), we are able to directly probe two technologically relevant organic field effect transistor device stacks. This work directly measures the interaction between pentacene and two thiols which have been associated to contact improvement in pentacene-based OFETs: an electroneutral thiol (1-hexadecanethiol) and an electronegative thiol (pentafluorobenzenethiol). Based on our results we observe no chemical interaction between pentacene and the thiol. The electrical improvements to transistor performance, based on these thiol systems, can be attributed to work function shifts of the contacts and morphological improvements of the organic semiconductor.
9:00 PM - II6.87
Method for Using (previously) Incompatible Treatments for Dielectric and Bottom Contact Surface Modification in Organic Thin-film Field-effect Transistors with Lithographic Patterned Gold-free Bottom Contacts.
Robert Mueller 1 , Steve Smout 1 , Jan Genoe 1 , Paul Heremans 1 2
1 Large Area Electronics, IMEC v.z.w., Leuven Belgium, 2 ESAT, KULeuven, Leuven Belgium
Show AbstractOrganic field-effect transistors (OFETs) are attracting considerable attention due to their potential applications in flexible organic integrated circuits [1]. Corresponding transistors with short channel lengths (required for high frequencies) are typically made with photolithographic and lift-off patterned gold bottom contacts. Both the dielectric (SiOx or AlOx) and the source/drain (Au) electrodes are furthermore modified by self-assembled monolayers (SAMs) for achieving large pentacene grain size on the contacts, the dielectric AND the interface between the contact and the dielectric, resulting in good charge injection and mobility [2].The high cost of the bottom contact metal (gold) itself however precludes mass-market applications as for example RFID tags. Therefore alternative metals [3] – as well as chemical surface treatments of metals [4] - are currently investigated as bottom contacts in pentacene OFETs. For these approaches, with a photolithographic/lift-off made bottom contact – processing incompatibilities however typically preclude simultaneous surface modification of the dielectric AND the bottom contact (for example by a SAM or a surface reaction) . As a consequence, good charge injection at the dielectric/bottom contact interface is difficult to obtain using alternative metals for gold.In this contribution, we present a new way circumventing this issue. This allows preparing gold-free pentacene bottom contact OFETs with short channel length (using a lift-off process) and a surface modification layer on both the dielectric and the bottom contact metals. These surface modification layers are incompatible when using traditional processing.We apply this method to pentacene OFETs with by lift-off patterned – and afterward chemically surface modified – silver bottom contacts and a silane modified SiO2 dielectric layer. Prototypes of such OFETs currently achieve saturation mobilities of 0.044 cm2/(V.s) and a threshold voltage of -0.3 V.In addition we present examples illustrating the usage of our processing method to OFETs with SAM modified dielectrics and bottom contacts modified chemically by processing sensitive materials. This work was performed in a collaboration between IMEC and TNO in the frame of the HOLST Centre.[1] (a) K. Myny et al., ISSCC Dig. Tech. Papers, 2009, 206; (b) K. Myny et al., Sol. State Electr. (in press, doi:10.1016/j.sse.2009.10.010)[2] I. Kymissis, "Organic Field Effect Transistors. Theory, Fabrication and Characterization", Springer, 2009[3] H. Klauk et al., IEEE Trans. Elect. Dev. 46 (1999) 1258[4] (a) C.-a. Di et al., J. Am. Chem. Soc. 128 (2008) 16418; (b) J.-W. Park et al., Electrochem. Sol. State Lett. 10 (2007) H340; (c) D.-J. Yun et al., J. Electrochem. Soc. 156 (2009) H634.
9:00 PM - II6.88
Negative Differential Resistance of Photocurrent in Metal/ Insulator/Metal Junctions.
Fuming Wang 1 , Michael Preiner 1 , Nick Melosh 1
1 Geballe Laboratory for Advanced Materials, Stanford University, Stanford, California, United States
Show AbstractMetal-molecule band alignment is a critical parameter for designing contacts for organic and molecular electronics. Significant charge exchange between metals and molecules can profoundly change the band alignment of the interface relative to the bulk, yet uncertainties in the electrode to molecule coupling strength and competing contributions from HOMO vs LUMO states complicate electrical analysis. To address this issue, we have developed an optical technique based on hot-electron injection for measuring the band offsets at near-zero applied bias. While investigating a series of Au/ alumina/ molecule/ Au metal-insulator-metal (MIM) devices, we discovered a dramatic negative differential resistance (NDR) feature in the photocurrent, strong enough to change the sign of the photocurrent at higher voltages. This effect has not been observed previously outside of GaAs multi-quantum well structures, and was wholly unexpected. Here we present new results demonstrating the NDR is wavelength independent for photons energies from 1.26 to 2.4 eV, and is linearly power dependent. We explain the phenomena in terms of a surface-trap model for hot-carrier tunneling, and discuss how this measurement can be used to elucidate the electronic structure of a metal-molecule interface.
9:00 PM - II6.89
Silicon-based Wide Band-gap Bipolar Organic Materials With Balanced Carrier Injection and Transport Properties.
Soonnam Kwon 1 , Kyung Wee 1 , Jeong Kim 2 , Sang Kang 1
1 , Korea University, Chochiwon, Chungnam Korea (the Republic of), 2 , Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of)
Show AbstractDevelopment of wide band-gap organic semiconducting materials with balanced charge injection and transport properties is of special interest in achieving high performance organic electronic devices. However, there appears a dearth of bipolar wide band-gap materials, due to the synthetic difficulty arising from the limited scope of carbon-based chemistry.In this contribution, a new silicon-based bipolar material namely, CBPSiTAZ, will be introduced, featuring a high triplet energy at 2.84 eV and high electron and hole mobilities in the range between 10-5 and 10-4 cm2V-1s-1. Spectroscopic data from ultraviolet photoemission spectroscopy (UPS), low temperature phosphorescence, and transient photoluminescence corroborated well to the occupied electron density of states, a wide triplet energy-gap, and an efficient energy transfer, respectively. Furthermore, the role of silicon was manifested by the observation of close-correlation between UPS and density functional theory (DFT) calculation data on CBPSiTAZ. Charge transport properties will be discussed using the time of flight method, and further evaluated by characterization of hole and electron only devices. And finally, the possibility of application to deep blue PHOLEDs will be discussed.
9:00 PM - II6.9
Temperature-resolved Local and Macroscopic Charge Carrier Transport and Optical Absorption in Thin P3HT Layers.
Patrick Pingel 1 , Achmad Zen 2 , Ruben D. Abellon 3 , Ferdinand C. Grozema 3 , Laurens D.A. Siebbeles 3 , Dieter Neher 1
1 Institute of Physics and Astronomy, University of Potsdam, Potsdam Germany, 2 Research and Technology Center Asia Pacific, Robert Bosch Pte Ltd, Singapore Singapore, 3 Department of Chemical Engineering, Delft University of Technology, Delft Netherlands
Show AbstractRecent investigations of the morphology and charge transport in poly(3-hexylthiophene) (P3HT) layers revealed a strong molecular weight (Mw) dependence of the field-effect mobility, ranging from 10-6 cm2/(Vs) in low-Mw samples up to 10-2 cm2/(Vs) in high-Mw samples [1,2]. This surprising effect was shown to be closely related to the layer morphology. It has been found that these regioregular P3HT samples are semicrystalline, with nanocrystallites separated by amorphous regions. Current investigations refer to the question, if the macroscopic charge transport, e.g. in a field-effect device, is limited by grain boundaries or by the presence of the low-mobility amorphous phase within the transport path of the charges.To gain an insight into the semi-crystalline morphology of P3HT layers, we performed in-situ optical studies of the temperature dependence of absorption. Using a recent analytic model by Spano [3] allowed us to identify and analyze the absorption of the amorphous phase and the crystalline aggregates. Evaluation of the aggregate absorption in terms of the exciton band width reveals that the aggregates in medium-Mw P3HT (Mn=7.2 kDa) undergo a “pre-melting” starting from the crystallite edges already well below the DSC melting transition. In contrast, aggregates in the high-Mw P3HT (Mn=27 kDa) sample appeared to be little affected by heating, probably because chain-folding impedes the aggregate “pre-melting”. Most importantly, we found that the layers of both fractions contained considerable amounts of non-aggregated (amorphous) chains, the portion of which is higher in the medium-Mw P3HT sample.We link our spectroscopic results to the temperature- and Mw-dependence of the local and macroscopic charge transport properties as deduced from pulse-radiolysis time-resolved microwave conductivity (PR-TRMC) and OFET measurements. We found that the local intragrain transport is thermally activated for both Mw-fractions, irrespective of the changes in aggregate size and quality with temperature. In contrast, the macroscopic OFET mobility, being determined by transport through both amorphous and crystalline domains, clearly decreased with increasing temperature in medium-Mw P3HT. Also, the macroscopic field-effect mobilities measured for this fraction were significantly lower than the corresponding intragrain mobilities. We conclude that charge transport in these medium-Mw P3HT samples is strongly determined by the amorphous matrix which embeds the crystalline domains. Interestingly, the temperature dependence and magnitude of the OFET mobility in high-Mw samples closely follows that of the local transport, which is in accordance with the preceding proposal that the long chains are able to interconnect crystalline regions in high-Mw P3HT [4].[1]A. Zen et al., Adv. Funct. Mater. 14 (2004) 757[2]R.J. Kline et al., Adv. Mater. 15 (2003) 1519[3]F.C. Spano, J. Chem. Phys. 122 (2005) 15[4]M. Brinkmann et al., Adv. Funct. Mater. 17 (2007) 101
9:00 PM - II6.90
Synthesis of Novel Smectic Glassy Liquid Crystals and Their Characterization of Charge Carrier Transport Properties.
Jiang Wu 1 , Takayuki Usui 1 2 , Jun-ichi Hanna 1 2
1 , Tokyo Institute of Technology, Tokyo Japan, 2 , JST-CREST, Kawaguchi, Saitama, Japan
Show AbstractIn recent years, liquid crystals have been recognized as a new type of quality organic semiconductors because of the electronic conduction characterized by a high mobility up to 1cm2/Vs. A new material having both self-organized molecular alignment in liquid crystal and solid nature in amorphous materials, i.e., a liquid crystalline glass, is of interest both for understanding the charge carrier transport properties in liquid crystals and for device applications. Because of these, we synthesized a series of novel smecitc glassy liquid crystals, one of these model compounds, 1,3,5-benzenetricarboxylic acid tris{12-[6’-(4”-octylphenyl)-2’-naphthyloxy]-1-dodecyl ester, was fully studied in terms of glassy forming ability and charge transport property. We found that this novel smectic glassy liquid crystal shows smectic A (SmA) and smectic B (SmB) liquid crystalline phases from 125°C to 101°C and from 101°C to 67°C, respectively, and SmB glassy phase with a high Tg of 53°C when cooled rapidly from SmB liquid crystalline phase; the time-of-flight (TOF) measurement revealed that the charge carrier transport mobility in the SmB glassy phase was ambipolar and exhibited 4×10-4 cm2/Vs and 5×10-4 cm2/Vs for holes and electrons, respectively, which is comparable to those in the SmB phase. In addition, the mobility in SmB glassy phase depends on temperature, but does not on electric field, which is quite different from amorphous materials. These properties indicate that the smecitic glassy liquid crystal provides us with solid self-organized molecular aggregates while keeping feasibility of easy control of molecular alignment in smectic liquid crystals. We compared and discussed the charge carrier transport properties in the smectic glassy phase with those in smectic liquid crystalline phases, and show how promising the glassy liquid crystal is as a new type of organic semiconductors.
9:00 PM - II6.91
Directing the Heavy Atom Effect: A Design Principle to Activate Efficient Triplet Emission in Metal-free Organic Materials.
Onas Bolton 1 , Kangwon Lee 1 , Hyong-Jun Kim 1 , Jeff Kampf 3 , Kevin Lin 2 , Jinsang Kim 1 4 5
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 Chemistry, University of Michigan, Ann Arbor, Michigan, United States, 2 Chemical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 Chemical Engineering, Macromolecular Engineering, and Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 5 WCU invited professor, Hybrid Materials for Sustainability, Seoul National University, Seoul Korea (the Republic of)
Show AbstractThough the parallel fields of organic materials and phosphorescent materials are each enjoying rapid expansion, only the relatively limited family of organometallic compounds unites them. Metal-free organic triplet emitters are extremely rare. Of those known none have proven particularly attractive for optical applications as they are very inefficient unless in impractically strict conditions, exhibiting quantum yields well below one percent. To open the door to a wider array of organic phosphor we have discovered a design principle to activate high efficiency phosphorescence from metal-free organic materials that is both robust and tunable to various organic structures. Simple halogenated aromatic aldehydes undergo halogen bonding in crystalline states directing a non-covalent interaction between the halogen and neighboring aldehyde. In this close position the heavy atom effect is amplified, promoting intersystem crossing by enhancing spin-orbit coupling at the aldehyde. Tight solid-state packing, also afforded by the halogen bond, suppresses vibrational dissipation and the net effect is phosphorescent emission with ambient, solid-state room temperature quantum yields measured over 60%. Structural changes to the compounds allow for coarse and fine color tuning as well as designed alteration of other physical material characteristics such as thermal properties and crystallinity.
9:00 PM - II6.92
Enhancement of Metal-free Organic Phosphors by Polymer Entanglement.
Onas Bolton 1 , Sunghyun Jeon 2 , Jinsang Kim 1 3 4
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Visiting scholar of Chemical Engineering, Hanyang University, Seoul Korea (the Republic of), 3 Chemical Engineering, Macromolecular Engineering, and Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 4 WCU invited professor, Hybrid Materials for Sustainability, Seoul National University, Seoul Korea (the Republic of)
Show AbstractWe recently devised novel organic phosphorescent materials through directed and confined heavy atom effects. In order to achieve bright phosphorescence, vibrational freedoms must be reduced to ensure that non-emissive thermal relaxation is not competitive with quantum relaxation. Thus these, like all phosphors, typically operate in rigid solid states. Metal-free aromatic aldehydes produce bright phosphorescence in crystal states, but their crystals require tight packing and strong halogen bonding, both of which may be difficult to design into the crystals of larger chromophores. We will present an alternative to crystal packing through polymer entanglement. As presented, a liquid organic small molecule becomes brightly phosphorescence when doped into films of common polymers. Our study reveals that specific structural similarity between the polymer and the small molecule greatly affects the efficiency of the emission and that small variations to the polymer backbone and pendant group play a major role in the quality of entanglement. From this we present the idea of entanglement and use the phosphorescent emission intensity of the small molecule as a measurement of its efficiency.
9:00 PM - II6.93
Poly(3-hexylthiophene) Nanofibers Fabricated by Electrospinning and their Optical Properties.
Surawut Chuangchote 1 , Michiyasu Fujita 1 , Takashi Sagawa 1 , Susumu Yoshikawa 1
1 Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan
Show AbstractPolymer nanofibers have a diameter that ranges from an order of a few nanometers to several micrometers and are remarkable for their small pore size and very high porosity, their very high surface area per unit mass, and a low basis weight. Optimal nanofibers can be prepared by controlling electrospinning conditions, such as properties of the polymer solution, strength of the applied electric field, and deposition distance. Conductive nanofibers with the average diameters ranging in nanometers to sub-micrometers (60 nm - 2 µm) were fabricated by electrospinning of a mixture of poly(3-hexylthiophene) (P3HT) and polyvinylpyrrolidone (PVP) in a mixed solvent of chlorobenzene and methanol. Beaded fibers and/or uniform, smooth-surface fibers were successfully fabricated. The average diameter of the as-spun fibers decreased and the color of as-spun fibers changed with decreasing the concentration of P3HT or PVP. As-spun fibers showed relatively higher crystallinity, higher conjugation length, and a remarkable blue shift of photoluminescence (PL) peak was observed, in comparison with the cast film. Optical properties, including UV absorption and photoluminescence (PL) of fibers were investigated. For the UV absorption, the increase in P3HT composition resulted in the blue-shift peak, indicating that there were the changes in chain conformation of conductive polymer within each fiber and affected to delocalized π-conjugation and extended conformation. In case of PL, the emission peak of higher composition of P3HT showed the red-shift in position. It is well known that the peak position of PL spectra of conducting polymers is much influenced by the distance between their molecules. Therefore, it is reasonable that the distance between the molecules of P3HT in the P3HT/PVP composite fibers increased with increasing in the composition of P3HT. After the removal of PVP from as-spun fibers by Soxhlet extraction, pure P3HT fibers were obtained as a groove-like morphological appearance which may be widely applicable for some specific applications, such as photovoltaic cells, thin film transistors, and light emitting diodes. Some applications will be carried out and reported.
Symposium Organizers
Zhenan Bao Stanford University
Alejandro L. Briseno University of Massachusetts
Vitaly Podzorov Rutgers University
Iain McCulloch Imperial College London
II7: Devices II
Session Chairs
Thursday AM, April 08, 2010
Room 3001 (Moscone West)
9:00 AM - II7.1
Synthesis of Non–fullerene Based Electron Acceptors for Applications in Organic Photovoltaics.
Xu Han 1 , Alan Sellinger 1
1 Material Science and Engineering, Stanford University, Palo Alto, California, United States
Show AbstractSolution-processed bulk heterojunction (BHJ) solar cells have attracted much attention during the last decade as a promising photovoltaic (PV) technology for flexible, semi-transparent, and low-cost power production. With regard to materials, fullerene compounds have been the dominating electron acceptor material in organic BHJ solar cell research. However, fullerenes have some disadvantages, such as weak absorption in the visible spectrum, high-cost production and purification, and a low lying LUMO which generally results in low open circuit voltages (Voc). To solve these problems, reported here is a family of new electron acceptor materials from simple, minimal step, high yield, and inexpensive synthetic processes for application in organic photovoltaics. In this presentation, the synthesis and characterization of a series of new electron accepting molecules based on imide and dicyanoimidazole moieties to form electron deficient π-conjugated systems will be discussed. The extended π-conjugated systems of these molecules help contribute to significant absorption of the visible spectrum, and can be systematically tuned to vary HOMO and LUMO levels. Solution processed OPV device results using these new materials with selected electron donor materials will be reported.
9:15 AM - II7.2
Polymer Thin Film Transistors and Photovoltaic Devices Prepared by Contact Film Transfer Method.
Keisuke Tajima 1 , Qingshuo Wei 2 , Shoji Miyanishi 1 2 , Akira Tada 1 , Kazuhito Hashimoto 1 2
1 Applied Chemistry, The University of Tokyo, Tokyo Japan, 2 , JST-ERATO HASHIMOTO Light Energy Conversion Project, Tokyo Japan
Show AbstractCharge carrier transports in the structures formed at the surface of various conjugated polymer films were investigated by constructing organic thin film transistors using a novel and simple contact film transfer method. By using this method, organic films can be transferred onto various target substrates with the sizes more than several square centimeters without damage. This film transfer process inverts the film geometry, and therefore the bottom-gate thin film transistors (TFTs) fabricated from the transferred films have transport layers originally formed at polymer/air interface. This fabrication process has advantages over the top-gate TFTs to preserve the interfacial structures during the transfer since there are no external forces or thermal treatment needed. As the result, TFTs prepared by the transfer process showed higher field effect mobility values compared with the conventional spin-coated devices for all the polymers studied. In contrast to the previous reports, the hole mobility in regioregular poly(3-alkylthiophene)s (C4 ~ C12) did not depend on the alkyl chain length when the contact film transfer was applied (from 0.05 cm2/Vs to 0.09 cm2/Vs) [2]. These results suggest that higher order out-of-plane orientation of the thiophene rings and strong interchain interaction are spontaneously formed at the polymer/air interfaces during the spin-coating.1 Furthermore, contact film transfer can be easily applied to prepare the layered structures free from the constraints of the conventional solution process. To demonstrate this, ambipolar OTFTs with a bilayer structure of poly(3-hexylthiophene) and [6,6]-phenyl C61 butyric acid methyl ester were also fabricated by using this method. The transistors showed high electron and hole mobilities of 2.1×10-2 cm2 V-1 s-1 and 1.1×10-2 cm2 V-1 s-1, respectively [1]. Complementary inverters based on two identical ambipolar transistors showed good performance with a gain of 14.2 These results suggest that the contact film transfer method could provide a facile and general method to evaluate the electronic properties of semiconducting organic materials and fabricate multilayer structure of different organic materials. The bias-stress stability of the transistors prepared by the contact film transfer and interfacial property control by using surface-segregated monolayer will be also presented. Also, the application of the film transfer to bilayer type organic solar cells combined with the spontaneously formed surface segregated monolayer will be presented [3,4]. [1] Wei, Q.S.; Miyanishi, S., Tajima, K. and Hashimoto, K., ACS Appl. Mater. Interfaces. in press . [2] Wei, Q.S.; Tajima, K. and Hashimoto, K., ACS Appl. Mater. Interfaces. 2009, 1, 1865-1868. [3] Wei, Q.S.; Nishizawa, T.; Tajima, K. and Hashimoto, K., Adv. Mater. 2008, 20, 2211-2216. [4] Q.S. Wei, K. Tajima, Y.J. Tong, S. Ye and K. Hashimoto, J. Am. Chem. Soc. 2009, in press.
9:30 AM - II7.3
Inkjet Printed P3HT/PCBM Solar Cells: A New Solvent System Approach.
Alexander Lange 1 , Michael Wegener 1 , Christine Boeffel 1 , Bert Fischer 1 , Armin Wedel 1
1 Functional Materials and Devices, Fraunhofer Institute for Applied Polymer Research, Potsdam Germany
Show AbstractBulk heterojunction organic solar cells represent a main research direction within the field of organic electronics. Among possible candidates for OSC devices, poly(3-hexylthiophene) (P3HT) and 1-(3-methoxycarbonyl)propyl-1-phenyl[6,6]C61 (PCBM) are often used as the active photovoltaic materials because of the absorption of P3HT in the visible range and the optimal energy level alignment of P3HT and PCBM. Usually organic solar cells are prepared by spin coating because this technique is considered a standard thin film processing method. Currently, different methods and techniques such as inkjet printing, gravure printing and slot die coating are being developed for the processing of thin functional films. Inkjet printing is often viewed as the more practical printing technique, because it is economical due to little material waste and it offers the long term possibility of role-to-role processing. The preparation of organic solar cells by inkjet printing has been successfully demonstrated. Usually, solvent systems such as dichlorobenzene with mesitylene and pure tetralin are used to generate films which effectively split exitons and transport holes and electrons to the anode and cathode, respectively. At the same time, the high boiling point component of the prior mentioned solvent systems functions to prevent print head nozzle clogging via solvent evaporation. However, the solubility of P3HT/PCBM in high boiling solvents such as mesitylene may not be good relative to the solubility of P3HT/PCBM in chlorinated solvents. This was the motivation to investigate the formulation of a P3HT/PCBM ink which is based on chlorinated solvents.Here, we demonstrate two aspects regarding the inkjet printing of P3HT/PCBM solar cells. First, a solvent combination based on chlorobenzene and trichlorobenzene is used for the processing of the active P3HT/PCBM layer. Based on the solvent system, the pre- and post-annealing procedures are adapted in order to optimize the photovoltaic properties. The evaluation of the solvent combinations is performed by taking into account the UV-vis spectra as well as the final solar cell properties such as the efficiency and the fill factor. Second, going one step ahead in direction of fully printed solar cells, the inkjet printing of the passive PEDOT:PSS layer is also demonstrated in combination with a inkjet printed active P3HT/PCBM layer and the device properties are discussed. In summary, it is demonstrated that a chlorobenzene and trichlorobenzene solvent mixture presents an alternative solvent system for the inkjet processing of P3HT/PCBM solar cells.
9:45 AM - II7.4
Electronic Properties of Cu-phthalocyanine-fullerene Planar and Bulk Hetereojunctions on PEDOT:PSS.
Andreas Wilke 1 , Toshiko Mizokuro 1 , Johannis Frisch 1 , Ralph Peter Blum 1 , Juergen P. Rabe 1 , Norbert Koch 1
1 Physics, Humboldt Universität, Berlin, Berlin, Germany
Show AbstractIn organic photovoltaic cells (OPVCs) typically two organic materials with electron-acceptor and electron-donor properties are placed between anode and cathode, forming either a layered planar or mixed bulk heterojunction where charge separation should occur. To improve the efficiency of this central process it is important to know and understand the energy levels at such heterojunctions, and how they may depend on heterojunction morphology. We report ultraviolet and X-ray photoelectron spectroscopy studies on layered planar and mixed bulk heterojunctions of Cu-phthalocyanine (CuPc) and C60, a prototypical material pair for OPVCs. The respective heterojunctions were formed on poly(ethylene-dioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) substrates, in order to achieve morphologies comparable to those in actual OPVCs. As a result of the CuPc-to-substrate electron transfer the work function of pristine PEDOT:PSS is reduced from 5.4 eV to 4.4 eV. The deposition of C60 onto CuPc leads, however, to a work function increase of 0.2 eV. The co-deposition of C60 and CuPc to form mixed bulk heterojunctions resulted in an effective anode work function of 4.6 eV. In all cases, the low energy onset of the highest occupied electronic level of CuPc on PEDOT:PSS is pinned at 0.40 eV below the Fermi-level, regardless of the initial work function of PEDOT:PSS. The energy offset between the highest occupied levels of CuPc and C60 was determined to 1.3 eV for both the layered and mixed bulk heterojunction. With reported values of the charge transport gap of C60 we estimate the upper limit of the open circuit voltage to be 1.0 eV for both types of heterojunctions. Our results demonstrate that the energy levels that determined the efficiency of charge separation in OPVCs made of CuPC and C60 are independent of particular interface morphology, and that differences in device efficiency are due to other effects.
10:00 AM - II7.5
Distribution of Localized States from Fine Analysis of Electron Spin Resonance Spectra in Organic Transistors.
Hiroyuki Matsui 1 2 , Andrei Mishchenko 3 , Tatsuo Hasegawa 1
1 Photonics Research Institute, AIST, Tsukuba Japan, 2 Dept. Advanced Materials Science, Univ. of Tokyo, Kashiwa Japan, 3 CMRG-ASI, Riken, Wako Japan
Show AbstractRecently, the field-induced electron spin resonance (FESR) technique was utilized to demonstrate the delocalized nature of field-induced charges that accumulate through applied gate voltages in organic transistors.[1-3] Typical pentacene thin-film transistors (TFTs) [1,2] and rubrene single-crystal transistors [3] exhibit fairly sharp FESR spectra whose temperature-dependent single-Lorentzian linewidth presents motional narrowing effects with increase of the temperature. Important finding from the observation is that the estimated long autocorrelation time can be ascribed to the residence time of carriers at respective trap sites and is well consistent with the multiple trap-and-release (MTR) carrier transport processes in organic transistors. However, the features of the FESR spectra are not well understood at low enough temperature where all the carrier motion should be frozen.Here we report a novel method for obtaining the distribution of trapped carriers over their degree of localization in organic transistors, based on the fine analyses of FESR spectra at low enough temperatures.[4] We first demonstrate through continuous wave (cw) saturation experiments that the Lorentzian-like FESR spectra at T < 50 K are clearly understood in terms of the inhomogeneous broadening due to the charge localization. Analyzing the experimental FESR spectrum at 20 K, where all carriers are localized, we show that the signal can be split into the sum of multiple Gaussian components, each of which correspond to different spatial extensions. It is found that the major trap levels comprise localized wave functions spanning around 1.5 and 5 molecules and a broad feature at N = 6–20 molecules. Using the diagrammatic Monte Carlo (DMC) calculations, the spatial extension is mapped into the binding energies at 140 ± 50 meV and 22 ± 3 meV, respectively, with the broad feature distributed between 5 and 15 meV. The filling of shallow traps between 5 and 15 meV indicates that the Fermi energy is very close to the valence band edge. These shallow in-gap states should be crucially important in understanding and improving the device performance of organic TFTs.[1] H. Matsui, T. Hasegawa, Y. Tokura, M. Hiraoka, and T. Yamada, Phys. Rev. Lett. 100, 126601 (2008).[2] H.Matsui and T.Hasegawa, Jpn. J. Appl. Phys. 48, 04C175-1-4 (2009).[3] H.Matsui and T.Hasegawa, Mater. Res. Soc. Symp. Proc. 1154, B08-04 (2009).[4] H. Matsui, A. S. Mishchenko, and T. Hasegawa, submitted.
10:15 AM - II7.6
Oxygen Effects on Rubrene Photoconductivity.
Ashok Maliakal 1 , Judy Chen 2 , Woo-Young So 3 , Steffen Jockusch 2 , Bumjung Kim 2 , Maria Ottaviani 5 , Alberto Modelli 4 , Nicholas Turro 2 , Colin Nuckolls 2 , Arthur Ramirez 1
1 , LGS Innovations/Bell Labs, Florham Park, New Jersey, United States, 2 Chemistry, Columbia University, New York, New York, United States, 3 Applied Physics, Columbia University, New York, New York, United States, 5 Dept. of Geological Sciences, Chemical and Environmental Technologies, Scientific Campus Loc. Crocicchia, Urbino Italy, 4 Chemistry, University of Bologna, Bologna Italy
Show AbstractOxygen is known to have a pronounced effect on photoconductivity and also dark conductivity in rubrene single crystal devices, however the detailed mechanism of this effect has to date not been established. We have performed electron paramagnetic resonance (EPR) spectroscopy and steady-state and time dependent photoconductivity (PC) measurements in order to identify the mechanism of oxygen enhanced photoconductivity. The EPR data indicate the presence of rubrene radical cation and oxygen radical anion pairs formed within the crystalline structure when rubrene is irradiated in the presence of oxygen. Radical lifetimes determined using EPR spectroscopy correlate well with transient PC data and provide strong evidence that the rubrene radical cation is the charge carrier responsible for enhanced conduction. This process is reversible, although photo-degradation is also observed. The oxygen-enhanced PC of rubrene is thus explained by an electron transfer mechanism that generates radical cation “hole” carriers within the crystal via the oxygen acceptor.
10:30 AM - II7: Devices II
BREAK
11:00 AM - **II7.7
Flexible Organic Floating Gate Transistors for Non-volatile Memory.
Takao Someya 1 2 3 , Tsuyoshi Sekitani 1 , Tomoyuki Yokota 2 , Ute Zschieschang 4 , Hagen Klauk 4 , Siegfried Bauer 5 , Ken Takeuchi 1 , Makoto Takamiya 6 , Takayasu Sakurai 6
1 Electrical and Electronic Engineering and Information Systems, University of Tokyo, Tokyo Japan, 2 Applied Physics, University of Tokyo, Tokyo Japan, 3 Institute for Nano Quantum Information Electronics (INQIE), University of Tokyo, Tokyo Japan, 4 , Max Planck Institute for Solid State Research, Tokyo Germany, 5 Soft Matter Physics (SOMAP), Johannes Kepler University, Linz Austria, 6 Institute of Industrial Science, University of Tokyo, Tokyo Japan
Show AbstractIn this talk, we report organic non-volatile memory arrays on flexible plastic substrates. The organic transistors with a floating gate embedded in hybrid dielectrics that comprise a 2 nm thick molecular self-assembled monolayer and a 4 nm thick plasma-grown metal oxide, we have realized non-volatile memory arrays on flexible plastic substrates. Thanks to the small thickness of the dielectrics, program and erase voltages to produce a large non-volatile, reversible threshold-voltage shift are 6V or less. The transistors endure more than 104 program/erase cycles. Employing a flexible array of organic floating-gate transistors that are integrated with a pressure-sensitive rubber sheet, we have demonstrated a sensor matrix that detects the spatial distribution of applied mechanical pressure and stores the analog sensor input as a two-dimensional image.
11:30 AM - II7.8
The Dynamic Organic p-n Junction.
Piotr Matyba 1 , Klara Maturova 2 , Martijn Kemerink 2 , Nathaniel Robinson 3 , Ludvig Edman 1
1 Department of Physics, Umeå University, Umeå Sweden, 2 Department of Applied Physics , Eindhoven University of Technology, Eindhoven Netherlands, 3 Department of Physics, Chemisty and Biology, Linköping University, Linköping Sweden
Show AbstractThe functionality of a number of ubiquitous electronic devices, such as bipolar transistors and light-emitting diodes, is based on the existence of an interface between a p-type doped and an n-type doped inorganic semiconductor, a so-called p-n junction. Recently, organic semiconductors have began to attract great interest, since they promise to allow low-cost fabrication of electronic devices via wet processing methods, such as printing and/or roll-to-roll techniques.[1] Here, we demonstrate that it is possible to form a p-n junction within an organic semiconductor in-situ under applied voltage bias, and examine some of the properties of this fascinating dynamic organic p-n junction structure.We have employed a planar light-emitting electrochemical cell (LEC) structure with a conjugated polymer (the organic semiconductor) intermixed with an electrolyte and with two identical gold electrodes, which define the inter-electrode gap of 120 μm. We used a combination of optical probing and scanning Kelvin probe microscopy (SKPM) for a detailed in-situ study on the formation and evolution of the dynamic p-n junction within the organic semiconductor.[2] We demonstrate that the conjugated polymer can be (electrochemically) p- and n-type doped, and that a light-emitting p-n junction can form in the bulk of the active material. We also demonstrate that in such a device, biased with a voltage of V = 5 V, a significant part of the potential drops between the electrodes within a thin region, which coincides with the light-emitting p-n junction positioned ca. 30 μm from the negative electrode.Further, in analogy with the well studied inorganic p-n junction, we measure a built-in potential over the junction region. However, in sharp contrast to the situation in conventional static p-n junction structures, the doping profile in the herein probed organic semiconductor is dynamic and can be formed or changed following a shift in the applied bias. Following the application of a reverse voltage bias of V = -5 V, the shape of the potential profile evolves with time. The profile eventually reaches a steady state, which resembles the mirror image of the initial steady-state profile. Significantly, the new location of the largest potential drop again coincides with the newly formed light emission p-n junction, reinforcing the fact that these are indeed reversible dynamic p-n junction structures.[1] Forrest, S. R. "The path to ubiquitous and low-cost organic electronic appliances on plastic". Nature 428, 911-918 (2004).[2] Matyba, P., Maturova, K., Kemerink, M., Robinson, N. D. & Edman, L. "The dynamic organic p-n junction." Nature Materials 8, 672-676, (2009).
11:45 AM - II7.9
Low-voltage Operating Organic Field-effect Transistors and Circuits on Flexible Substrates.
Yoonyoung Chung 1 , Boris Murmann 1 , Zhenan Bao 2
1 Electrical Engineering, Stanford University, Stanford, California, United States, 2 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractGate dielectric is one of the most important components in organic field-effect transistors (OFETs). Since the conducting channel is formed at the interface between the gate dielectric and organic layer, surface roughness and trap density of the dielectric significantly affect the performance of OFETs. Considering the fact that high-temperature annealing of gate dielectric, which is commonly used in silicon transistors, is not possible with plastic substrates, it is important to deposit high-quality gate dielectric at low temperatures for flexible devices and circuits.Atomic layer deposition (ALD) of aluminum oxide has several advantages in practical applications of OFETs and circuits on flexible substrates. The maximum processing temperature can be lower than 100 C, and no further annealing step is required to make high-quality dielectric with an optimized process. The thickness of deposited dielectric can be precisely controlled, so the operating voltage of OFETs can be tuned in accordance with power requirements. In addition, the dielectric can be deposited on a variety of substrates, so the choice of gate metals is not limited by the process of gate-dielectric deposition.We used ALD process to deposit nanometer-scale aluminum oxide as the gate dielectric of bottom-gate OFETs. The thin dielectric resulted in high capacitance, on the order of hundreds of nanofarads (nF) per square centimeter. The highest measured field-effect mobilities were 1.3 cm2/Vs for p-type and 0.8 cm2/Vs for n-type transistors. The OFETs were operated at less than 3 V, which is low enough for battery-powered portable applications.In this talk, we present our low-voltage operating OFETs and circuits on flexible substrates.
12:00 PM - II7.10
Organic Complementary MOSFET Technology Operating at Stage Delays Below 1µs.
Dieter Bode 1 2 , Kris Myny 1 2 , Bregt Verreet 1 2 , Bas van der Putten 3 , Petar Bakalov 1 4 , Steve Smout 1 , Peter Vicca 1 , Soeren Steudel 1 , Jan Genoe 1 , Paul Heremans 1 2
1 SSET/PMELAE, IMEC, Leuven Belgium, 2 ESAT, KULeuven, Leuven Belgium, 3 , TNO, Eindhoven Netherlands, 4 , Jacobs University, Bremen Germany
Show AbstractRecently shown large scale circuits [1] with more than 1000 organic thin film transistors, demonstrate the increasing maturity of organic electronics. Remaining restraints on the market potential of organic circuit applications are partly related to the limited circuit speed as well as to the narrow parameter spread required by unipolar logic to obtain sufficient yield [2]. Organic complementary MOSFET technology (OCMOS) improves circuit speed and reduces sensitivity to parameter spread. Here, we present an OCMOS technology based on two widely used organic semiconductors, pentacene and C60. Both n- and p-type transistors have a bottom-gate bottom-contact device structure with a high-k dielectric, aluminum oxide. Consequently short transistor channels (design rule of L=5µm) can be achieved.We describe the process development of bottom-gate bottom-contact n-type transistors that balance pentacene-based p-type transistors, using evaporated C60 for the organic semiconductor. We furthermore discuss the effective, close integration of these n-type transistors in a patterned-pentacene p-type baseline process. In the resulting OCMOS technology, we have realized, amongst others, ring-oscillators of 5 to 19 stages. The good matching of n-type and p-type transistors is reflected in working oscillators at inverter footprints down to the minimum, i.e. Wn=Wp. Oscillation starts at supply-voltages VDD of only 5V. At a supply voltage of 20V, a 5 stage ring oscillator achieves a frequency above 100kHz translating into a record stage delay below 1μs for an output swing that is over 70% of VDD. These measurements demonstrate that OCMOS technologies offer performance clearly exceeding unipolar organic technologies reported in literature.AcknowledgementThis work was performed in a collaboration between IMEC and TNO in the frame of the HOLST Centre.[1] K. Myny, S. Steudel, P. Vicca, M. J. Beenhakkers, N. A.J.M. van Aerle, G. H. Gelinck, J. Genoe, W. Dehaene and P. Heremans, Plastic circuits and tags for 13.56 MHz radio-frequency communication, Solid State Electron., in press (2009).[2] S. De Vusser, J. Genoe, and P. Heremans, Influence of Transistor Parameters on the Noise Margin of Organic Digital Circuits, IEEE T. Electron. Dev. 53, 601 (2006).
12:15 PM - II7.11
Scalable, High Resolution Patterning of Organic Small Molecule Films.
Corinne Packard 1 , Katherine Aidala 2 , Jennifer Yu 1 , Vladimir Bulovic 1
1 Department of Electrical Engineering & Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Physics, Mount Holyoke College, South Hadley, Massachusetts, United States
Show AbstractIn working with organic small molecules for optoelectronic display and energy applications, patterning technologies which are dimensionally scalable, capable of high resolution, and compatible with flexible substrates are highly desired. We have developed a scalable stamping technique which does not require elevated temperatures, pressure, wet chemicals, or plasma etching to pattern films of organic small molecules. In this process, a molded elastomeric stamp is brought into contact with a thermally evaporated organic small molecule thin film, then removed to subtractively pattern the film. We demonstrate the ability of this process to pattern a wide range of materials employed as electron transport or hole transport layers in OLEDS and photovoltaic cells. Patterning in the lateral dimension is controlled by the stamp features, while the extent of lift-off in the vertical dimension is time-dependent and results in complete removal for film thicknesses below a material-specific critical value. Additionally, we characterize the surface morphology of patterned regions and demonstrate devices formed using this method including multicolor OLEDs.
12:30 PM - II7.12
Flexible Low-voltage Organic Transistors and Circuits Based on an Air-stable, High-mobility Conjugated Semiconductor.
Ute Zschieschang 1 , Tatsuya Yamamoto 2 , Kazuo Takimiya 2 , Hirokazu Kuwabara 3 , Masaaki Ikeda 3 , Tsuyoshi Sekitani 4 , Takao Someya 4 , Hagen Klauk 1
1 , Max Planck Institute for Solid State Research, Stuttgart Germany, 2 , Hiroshima University, Higashi-Hiroshima Japan, 3 , Nippon Kayaku Co., Ltd., Tokyo Japan, 4 , University of Tokyo, Tokyo Japan
Show AbstractPentacene is one of most popular small-molecule conjugated semiconductors for organic thin-film transistors (TFTs), because it consistently provides carrier mobilities well above 1 cm2/Vs [1-3]. The large field-effect mobility results from the fact that the crystal structure of the thin-film polymorph of pentacene is characterized by large orbital overlap in the (001) lattice plane, and from the fact that the grain boundaries in polycrystalline pentacene films do not cause significant impediment of the carrier transport through the film. However, pentacene is very susceptible to oxidation at the 6 and 13 positions of the molecule, so the carrier mobility of pentacene TFTs degrades rapidly when the devices are exposed to air [4,5]. Recently, a six-ring fused heteroarene, dinaphtho-[2,3-b:2',3'-f]thieno[3,2-b]thiophene (DNTT) has been synthesized that has a crystal structure and a thin-film morphology that are very similar to those of pentacene, but which is much less susceptible to oxidation [6]. Here we report on the performance and stability of DNTT TFTs and ring oscillators on flexible polyethylene naphthalate (PEN) substrates. The TFTs use an inverted staggered (bottom-gate, top-contact) device structure with patterned, evaporated aluminum gates, a thin gate dielectric based on an oxygen-plasma-grown aluminum oxide layer (3.6 nm thick) in combination with a solution-processed self-assembled monolayer (SAM) of an aliphatic phosphonic acid (1.7 nm thick), a thermally evaporated DNTT layer, and evaporated gold source/drain contacts. Gates, semiconductor, and source/drain contacts were patterned using shadow masks [7]. Owing to the large capacitance of the AlOx / SAM gate dielectric (800 nF/cm2), the TFTs and circuits can be operated with low voltages of about 3 V. The flexible low-voltage DNTT TFTs have a carrier mobility as large as 2.2 cm2/Vs, an on/off ratio as large as 108 and a subthreshold swing of 100 mV/decade. The mobility of the flexible DNTT TFTs is several times larger than that of pentacene TFTs manufactured with the same technology [7]. More importantly, the carrier mobility of the DNTT TFTs does not degrade during several months of continuous exposure to ambient air, despite the lack of encapsulation, which represents a significant advantage over pentacene TFTs. Unipolar ring oscillators based on DNTT TFTs with a channel length of 10 μm oscillate with a signal delay of 10 μsec per stage at a supply voltage of 5 V. This is the smallest signal delay reported for an organic circuit operated with a supply voltage of less than 30 V [8] and substantially faster compared with low-voltage pentacene ring oscillators [7]. [1] T. W. Kelley, Chem. Mater. 16 (2004) 4413. [2] M. E. Roberts, PNAS 105 (2008) 12134. [3] Y. Ito, JACS 131 (2009) 9396. [4] W. J. Kim, JVSTB 23 (2005) 2357. [5] H. Jung, APL 92 (2008) 163504. [6] T. Yamamoto JACS 129 (2007) 2224. [7] H. Klauk, JAP 102 (2007) 074514. [8] D. J. Gundlach, Nature Mater. 7 (2008) 216.
12:45 PM - II7.13
Direct Spectroscopic Evidence for a Photo-doping Mechanism in Polythiophene and Poly(bithiophene-alt-thienothiophene) Organic Polymer Semiconductor Thin Films Involving Sorbed Moisture.
Jingmei Zhuo 2 1 , Lihong Zhao 1 , Rui-Qi Png 1 , Loke-Yuen Wong 1 , Jie-Cong Tang 1 , Eric C.-W. Ou 3 , Soo-Jin Chua 3 , Wee-Sun Sim 2 , Lay-Lay Chua 2 1 , Peter K.-H. Ho 1
2 Chemistry, National University of Singapore, Singapore Singapore, 1 Physics, National University of Singapore, Singapore Singapore, 3 , Institute of Materials Research and Engineering, Singapore Singapore
Show AbstractFourier-transform infrared spectroscopy of two prototypical high-mobility polymer organic semiconductors (OSCs), regioregular poly(3-hexylthiophene) (rr-P3HT) and poly[2,5-bis(3-tetradecylthiophen-2-yl)thieno(3,2-b)thiophene] (PBTTT), reveals a photoinduced electron transfer pathway to water molecules dissolved in the polymer matrices. About 2 x 1019 cm–3 of water molecules are present in these films primarily as monomers, dimers and small clusters at room temperature and humidity (RH=60%). Exposure to light then generates a polaron density of a few 1017 cm–3 together with the charge-compensating hydroxide ion. Oxygen plays a secondary role by promoting water sorption. This mechanism accounts for the degradation of both the saturation and on–off characteristics of their organic field-effect transistors (FETs). The greater stability of PBTTT is found to be largely kinetic in origin and related to exclusion of moisture from its crystalline domains.
II8: Charge Transport I
Session Chairs
Alex Briseno
Vitaly Podzorov
Thursday PM, April 08, 2010
Room 3001 (Moscone West)
2:30 PM - **II8.1
Employing `Liquid Gap’ Transistors to Examine the Mobility-carrier Density Relation in Polymer and Single Crystal Organic Semiconductors.
Daniel Frisbie 1
1 , University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractIt is generally known that the carrier mobility in organic semiconductors can depend on carrier density, but the precise relationship hinges on the degree of structural order and the dielectric polarizability at the organic/dielectric interface. We have fabricated both single crystal and polymer transistors using the PDMS stamp approach pioneered by Podzorov and Rogers [1], where we have replaced the usual ‘air gap’ in these structures with liquids having different dielectric constants. This structure allows us to examine transport in single crystals and polymer semiconductors as a function of tunable dielectric constant and also charge density. We find striking differences in transport behavior for organic single crystals versus polymer semiconductor films using these liquid dielectric transistors. For organic single crystals such as rubrene, the carrier mobility does not seem to be a function of charge density but does strongly depend on the liquid dielectric constant, in keeping with previous results reported by Morpurgo [2] on the effects of dielectric polarizability. For polymer semiconductors, the effect of charge density is overwhelming; there is a strong increase in charge mobility with increasing carrier concentration, following a power law. These results are already largely known, but the ‘liquid gap’ transistors provide a convenient testbed for examining these effects side-by-side for different materials in the same device. We will describe the device fabrication and the nature of our results, as well as discuss the origins of the very different behavior for single crystals versus polymer semiconductor films. 1) Sundar, V.C., et al. Science 303 (2004) 1643.2) Hulea, I. N., et al. Nature Mater. 5 (2006) 982.
3:00 PM - II8.2
High Performance Photo-patternable Polyelectrolyte-gated Transistors from Electrospun P3HT Nanofibers.
Sung Won Lee 1 , Unyong Jeong 1
1 Material engineering, Yonsei university, Seoul Korea (the Republic of)
Show AbstractHigh performance OFETs based on polyelectrolyte insulator and electrospun P3HT nanofibers were fabricated on a flexible PET polymer substrate. The use of UV crosslinkable hydrogel for the insulating layer allowed fast and large-area fabrication of the transistors. The one dimensional structure of electrospun nanofibers directly deposited on the substrate allowed covalent bonds between the hydrogel layer and the substrate, which provides mechanical stability with the devices. The average field effect mobility of the OFETs was ~ 2 cm^2/Vs at saturation regime and the on/off ratio was ~ 10^5. The devices could be operated at -1 V in air.
3:15 PM - II8.3
Low Voltage, Polyelectrolyte Gated Organic Field Effect Transistors With Vertical Channel.
Jiang Liu 1 , Lars Herlogsson 1 , Anurak Sawatdee 2 , Isak Engquist 1 , Mats Sandberg 2 , Magnus Berggren 1
1 Dept. of Science and Technology, Linköping University, Norrköping Sweden, 2 , Acreo AB, Norrköping Sweden
Show AbstractShort-channel, vertically structured organic transistors with polyelectrolyte as gate insulator are demonstrated. The devices are fabricated using low-resolution, self-aligned and mask-free photolithography. The channel length is controlled by varying the thickness of the insulating photoresist sandwiched between the drain and source electrodes. Owing to the use of polyelectrolyte, very high capacitance is created at the semiconductor/electrolyte interface, resulting in an operating voltage below one volt. Functional vertical electrolyte gated organic field effect transistors (VEGOFETs) with channel length between 2.2 µm and 0.7 µm have been fabricated using poly(3-hexylthiophene) (P3HT) as organic semiconductor and characterized morphologically by transmission electron microscopy. Electrical characterization shows that the VEGOFETs display saturation characteristics, but have a moderate on-off ratio of 22, probably due to bulk punch-through in the P3HT. Transient response measurements indicate turn-on and turn-off times of 200 µs and 5 µs, respectively. Due to the vertical geometry and the use of polyelectrolyte, these transistors are promising candidates for printed short channel devices with low operating voltage.
3:30 PM - II8.4
Improved Injection in n-Type Organic Transistors With Conjugated Polyelectrolytes.
Jung Hwa Seo 1 , Andrea Gutacker 1 , Bright Walker 1 , Shinuk Cho 1 , Renqiang Yang 1 , Thuc-Quyen Nguyen 1 , Alan J. Heeger 1 , Guillermo C. Bazan 1
1 , UC Santa Barbara, Santa Barbara, California, United States
Show AbstractContact resistances at metal/semiconductor interfaces can limit the performance of organic thin-film transistors (OTFTs). Optimal devices should display a linear dependence of the channel potential profile and negligible voltage drops at the electrodes. These conditions allow one to accurately determine charge carrier mobilities from device characteristics and lead to low turn-on threshold voltages. In the case of n-channel OTFTs, and in the absence of interfacial effects, the electron injection barrier can be estimated from the difference in the energies of the work function (WF) of the metal and the lowest unoccupied molecular orbital (LUMO) of the semiconductor. High contact resistances are therefore obtained under these conditions when using stable high WF metals, such as Au. To improve injection in n-type organic thin film transistors, a thin conjugated polyelectrolyte (CPE) layer was interposed between electrodes and semiconductor. OTFTs were fabricated with [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) and Au source and drain contacts. We demonstrate that the insertion of CPEs beneath top-contact Au source/drain electrodes can be a very effective strategy for improving the carrier injection and reducing turn-on threshold voltages of n-channel OTFTs. Ultraviolet photoemission spectroscopy (UPS) shows a decrease of the electron injection barrier with strong interfacial dipoles at the Au/CPE interface.
3:45 PM - II8.5
Conduction Anisotropy and Carrier Mobility/Structure Relationship in Oligothiophene Monolayers Studied by CS-AFM.
Florent Martin 1 2 , Bas Hendriksen 3 , Clayton Mauldin 4 , Jean Frechet 4 2 , Nenad Vukmirovic 5 , Lin-Wang Wang 5 , Miquel Salmeron 1 2
1 Materials Science and Engineering, UC Berkeley, Berkeley, California, United States, 2 Materials Science Division, LBNL, Berkeley, California, United States, 3 Institute for molecules and Materials, Radoud University, Nijmegen Netherlands, 4 Chemistry, UC Berkeley, Berkeley, California, United States, 5 Computational Research Div, LBNL, Berkeley, California, United States
Show AbstractOrganic compounds are currently considered as the main building block of electronic devices that could lead to new commercial applications, which include flexible electronics as well as organic photovoltaics. Correlating structural and electrical properties is a key requirement to understand charge transport in organic thin films.We investigated the effect of crystallinity and grain boundaries on the conductivity of the Langmuir-Blodgett oligothiophene monolayers using Current-Sensing Atomic Force Microscopy (CS-AFM).We found that the native oxide insulating barrier in our AFM tip/Monolayer/SiO2/p+Si junction induced lateral conduction in the plane of the oligothiophene monolayer. Combining these CS-AFM results with molecular resolution AFM, we found evidence for conduction anisotropy caused by the asymmetry of the herringbone crystal structure of the monolayer. These results, as well as the crystal structure resolved in molecular resolution AFM, were confirmed by molecular dynamics simulations.We used the AFM tip as a tool to inject charges and manipulate the crystalline monolayer. We found that passing electrical current locally from the conductive AFM tip led to reversible charging of the native SiO2 layer supporting the film as far as microns away from the charge injection point. This effect, due to charge spreading through the crystalline monolayer, was used to image conduction pathways and study the effect of grain boundaries on the resistivity of the monolayer. In addition, we observed that scanning at loads in the order of 100nN led to a five fold decrease of the monolayer conductivity in CS-AFM while no noticeable changes were observed in the monolayer morphology. Subsequent molecular resolution AFM revealed that the degree of crystalline order in manipulated regions of the monolayer had strongly decreased, offering a direct proof of the correlation between order and conductivity in organic monolayers. We believe that such AFM manipulation of organic monolayers could be used to correlate crystalline order with carrier mobility in field-effect transistor experiments.
4:30 PM - **II8.6
Electric Field-effect in Solution-crystallized Organic Semiconductors.
Jun Takeya 1
1 Dept. of Chemistry, Osaka University, Toyonaka Japan
Show AbstractThe development of organic thin-film transistors (TFTs) has been intensively driven by the fact that they can be fabricated by easy and potentially low-cost processes. Furthermore, near-room-temperature processes enable semiconductor technologies to be realized on plastic substrates, raising expectation for post-silicon semiconductor industry. Solution-based processes are the most desired for producing high-performance organic TFTs in this regard. However, the transistor performances of such devices are not yet sufficiently high to replace amorphous silicon TFTs in practical applications. On the other hand, research into intrinsic charge transport properties of organic semiconductors with maximized ordering has been conducted rather independently in academia by developing organic single-crystal transistors. Excluding extrinsic effects caused by grain boundaries, it turned out that the intrinsic carrier mobility can exceed 20 cm2/Vs. In addition, it has been evidenced by the Hall-effect measurement that the transport is based on a band-like mechanism at room temperature. The high intrinsic mobility has set a tentative and realistic goal to reach in solution-processed TFTs by improving their molecular ordering.In this presentation, I report high-mobility charge transport in organic TFTs produced from solutions, employing a method of oriented crystal growth on a substrate for a derivative of 2,7-dioctylbenzothieno[3,2-b]benzothiophene (C8-BTBT). This method was also applied to fabricating top-contact transistor arrays, in which typical mobility values are more than a few cm2/Vs. The value is already significantly higher than those of typical amorphous silicon TFTs.Crystallinity of the solution-processed films was characterized by transmission X-ray-diffraction measurements. Well-defined Bragg spots appeared corresponding to the components in the conducting plane consistently to the reported crystal structure of the material, giving evidence that the semiconductor film is indeed crystallized. With the help of Hall-effect measurement, it turned out that diffusive band-like transport is achieved in the solution-crystalized organic transistor. The above observations are qualitatively same as that of rubrene transistors of vapour-grown single crystals, suggesting that organic single-crystal transistors are coming out from academia to the industry.I acknowledge K. Takimiya and Nihon Kayaku Co., ltd. for supplying the materials. The X-ray diffraction experiment was carried out at BL-4C of the Photon Factory, KEK, Japan.
5:00 PM - II8.7
High-power Organic Field-effect Transistors Using a Three-dimensional Structure.
M. Uno 1 2 , Y. Hirose 1 , K. Nakayama 1 , T. Uemura 1 , Jun Takeya 1
1 Dept. of Chemistry, Osaka University, Toyonaka Japan, 2 , TRI-Osaka, Izumi Japan
Show AbstractIn order to meet the growing demand for “low-carbon society”, organic field-effect transistors (OFETs) based on organic-semiconductor channels comprised of π-conjugated molecules are increasingly gaining attention for key switching devices such as active-matrix elements in flexible displays, owing to their attractiveness of environmental-friendly fabrication processes and of flexible or light-weight properties. Although many researches are made on organic materials, carrier mobility μ of typical organic semiconductors are still less than or comparable to 1 cm2/Vs except for that of single-crystal devices. Rather small values of the carrier mobility in organic semiconductors often result in unsatisfactory current amplification and operating speed for broad applications, restricting their possibility to the use in relatively slow devices. To compensate their insufficient material mobility, we have previously reported three-dimensional organic field-effect transistors (3D-OFETs) comprising multiplied vertical channels to enlarge the current amplification per unit area [1]. We achieved high output current of approximately 1 A/cm2 per unit area and high on-off ratio exceeding 106 using the configuration of the 3D-OFETs. The performance already exceeded requirements for active-matrix elements to drive organic light-emitting diodes in flat-panel displaysIn this presentation, we report a new construction adopting highly-perpendicular vertical channels, where it became possible to build both top-contact and bottom-contact devices even with sub-micrometer-length channels by dry etching very easily, without using electron-beam lithography technique. To realize extremely large ratio of channel width W and channel length L, multiplied vertical channels are fabricated with a shorter pitch of 10 μm, building densely-arranged semiconductor active layers. In order to investigate dependences on channel length, we prepared such devices with various L from 0.3 μm to 4.0 μm by changing the height of the 3D structures. High-performance field-effects are demonstrated using the devices in which dinaphtho[2,3-b:2',3'-f]thieno [3,2-b]thiophene (DNTT) is deposited to form air-stable semiconductor channels. Qualitatively, the output current increases with decreasing channel length, while on-off ratios are tend to be deteriorated. The device with the channel length of 0.8 μm exhibits an output current density of 8.0 A/cm2 and a high on-off ratio of about 105 with the applications of a drain voltage of -10 V and a gate voltage of -20 V. The carrier mobilities of the DNTT films deposited on the vertical walls are calculated to be typically 0.5-0.8 cm2/Vs. Future application to RF power transistors, for example, can be expected because of the achieved high-power amplification and low internal resistance. [1] M. Uno et al., Appl. Phys. Lett. 93, 173301 (2008); M. Uno et al., Appl. Phys. Lett. 94, 103307 (2009).
5:15 PM - II8.8
High Effiency Ambipolar Organic Light Emitting Transistors on Transparent Substrates.
Stefano Toffanin 1 , Raffaella Capelli 1 , Gianluca Generali 1 , Antonio Facchetti 2 3 , Michele Muccini 1
1 Istituto per lo Studio dei Materiali Nanostrutturati, Consiglio Nazionale delle Ricerche, Bologna Italy, 2 , Polyera Corporation, Skokie, Illinois, United States, 3 Department of Chemistry and the Materials Reserach Center, Northwestern University, Evanston, Illinois, United States
Show AbstractOrganic light-emitting transistors (OLETs) are emerging light sources combining, in the same architecture, the switching mechanism of a thin-film transistor and an electroluminescent device. OLETs may serve as testbeds to address fundamental general issues related to exciton-charge quenching and photon losses in organic light emitting devices [1].The photon losses at the metal electrodes are avoided by controlling the location of the light-emitting region well inside the device active area, and the exciton-charge quenching is reduced by a better current balance within the device. Furthermore, the most advanced OLETs encompass a huge technological potential for the realization of intense nanoscale light sources and highly integrated optoelectronic systems, including the long time searched electrically pumped organic laser.The main drawback that prevents this technology to be fully exploited as a viable route towards practical organic light emitting devices is the limited device efficiency. In the most impressive demonstration to date, the external quantum efficiency (EQE) does not exceed 0.5% for OLETs based on truly ambipolar single layers. Indeed, in these single layer devices the charge carrier accumulation and the exciton formation zones largely coincide, leading to severe exciton-charge quenching.One of the most promising approach for obtaining balanced charge transport and efficient light emission is to realize a heterojunction using different materials each having a specific function. Clearly, matching the overall device characteristics with the functional properties of the single materials composing the active region of the OLET, is a great challenge that requires a deep investigation of the morphological, optical and electrical features of the system [2].Here we introduce the novel concept of using a tri-layer organic heterostructure in OLETs to enhance light-emitting transistor efficiency and opto-electronic performances. In these devices the active region is composed by a central emitting layer sandwiched between an electron and a hole transport layer. So each layer is devoted to a single functionality and each relevant interface (organic/organic, organic/contact and organic/dielectric) can be independently optimised by controlling the film growth parameters . The specificity of the presented tri-layer based OLET is the intrinsic separation of the charge transport region from the exciton formation region thus preventing intrinsically the exciton-carrier quenching. Moreover the location of the light emission area multi-micron away from the electrodes eliminates electrode-induced photon losses.Our devices are 100x more efficient than the equivalent OLED fabricated using the same heterostructure and 10x more efficient than any other reported OLETs.[1]Muccini, M. Nat. Mater. 5, 605-613 (2006).[2]Dinelli, F., Capelli, R., Loi, M.A., Murgia, M.,Muccini, M., Facchetti, A., Marks, T. J. Adv. Mater. 18, 1416–1420 (2006).
5:30 PM - II8.9
Monolithic Complementary Inverters Based on Intrinsic Semiconductors of Organic Single Crystals.
Takafumi Uemura 1 , Jun Takeya 1
1 , Osaka University, Toyonaka, Osaka Japan
Show AbstractOrganic complementary inverters are regarded as key components to develop high-performance organic logic circuits, where both p- and n-type OFETs are indispensable. Most of organic complementary inverters are usually composed of different chemical compounds for p- and n-type semiconductors, which are contrasting to Si-based inverters. Here we disclose a monolithic complementary inverter fabricated directly on an organic single crystal, using high- and low-work function metals suitable to inject hole and electron, respectively. The performance itself manifests that organic semiconductors are intrinsic semiconductors by nature. The monolithic devices provide benefits of minimizing fabrication steps, shrinking a circuit scale, and accelerating a clock rate without unnecessary transmission delays.In this study, a rubrene single crystal was used as a monolithic substrate. The single crystal was grown by physical vapor transport in a glass tube and transferred into a glove-box without exposing it to air. The single crystal was laminated onto a thin dielectric layer of PMMA (~200 nm) to form p- and n-type top-contact transistors. In order to control the type of injection carriers, Au was deposited for p-channel and Ca was deposited for n-channel onto the single crystal. The device fabrication and evaluation were carried out in an Ar-filled glove-box because the atmospheric oxygen and water cause a fatal damage to n-channel operation of rubrene FETs.The device characteristics showed the hole and electron mobility were 1.2 and 0.2 cm2/Vs, respectively. The channel dimensions of complementary inverter were designed based on the difference of mobility. The thin dielectric layer of PMMA enables these both channels were driven within the gate voltages of ±10 V, as a result, the complementary inverter could be operated under 10 V, which is relatively small voltage among reported organic-based inverters. In addition, the subthreshold slopes showed small values where p- and n-channel were 0.6 and 0.5 V/decade, respectively. These small values realize a high dc gain of 150, which is owing to the high-performance of single-crystal FETs.In conclusion, we demonstrate the operation of complementary inverter fabricated on a monolithic substrate of organic semiconductor. The complementary inverter could be fabricated only by patterning of metal electrodes and minimizing a circuit scale, in addition, the device shows the high-performance inverter switching because the device based on the high-performance of single-crystal FETs.
5:45 PM - II8.10
Modelling Charge Transport in Poly-crystalline Molecular Films.
Joe Kwiatkowski 1 , Andrew Spakowitz 1 , Alberto Salleo 1 , Jenny Nelson 2 , Jarvist Frost 2
1 , Stanford University, Palo Alto, California, United States, 2 Physics, Imperial College London, London United Kingdom
Show AbstractWe present three different models of charge transport in molecular films.At varying levels of detail, each links the macroscopic electronic properties of the films to their microscopic properties.We show how these models can be used to determine film properties that aren't experimentally accessible.In the most detailed scheme, we simulate the physical vapour deposition of C60 films and use semi-classical Marcus theory to calculate charge transfer rates between molecules.We explicitly calculate all quantum chemical parameters with Density Functional Theory.We reproduce experimentally observed field-effect transistor (FET) characteristics, including electrical characteristics, electrochemical potentials, and charge mobilities.We show that, due to the high symmetry of the molecule, mobilities in C60 FETs are relatively insensitive to the film morphology, with the mobility varying by only a factor of two between the most disordered morphology and the single crystal.In a coarser scheme, we simulate polycrystalline Poly(3-hexylthiophene) (P3HT) films with a 'Boltzmann-weighted hop' approach in which the distribution of charges is assumed to quasi-equilibrate within crystallites before hopping across grain boundaries.This approach allows us to consider experimentally relevant sample sizes.In tandem with experimental data from directionally crystallised P3HT films, we use this model to calculate electronic film properties such as grain boundary hopping rates and activation energies.We also show how this approach can be used in conjunction with atomic force microscopy to quantify the density and type of film defects.Finally, we present a 'Steepest descent' method in which charges are presumed to travel along the field gradient in between grain boundaries, with minor adjustments made for diffusion.With experimental data from directionally crystallised films, we quantify the electronic properties of the perylenediimide (PDI8–CN2) films, and show how the model can complement existing understanding of structure-property relationships. We find that experimentally observed anisotropy in mobilities is strongly dependent on the exact density and distribution of grain boundaries.
II9: Poster Session: Organic Materials for OFETs, OLEDs and OPV
Session Chairs
Friday AM, April 09, 2010
Salon Level (Marriott)
9:00 PM - II8.15
Anisotropic Carrier Transport Properties of Stretch-oriented Regioregular Poly (3-Hexylthiophene) in Organic Field-effect Transistors.
Takeshi Yasuda 1 2 , Tetsuo Tsutsui 3
1 , National Institute for Materials Science, Ibaraki Japan, 2 , PRESTO, Japan Science and Technology (JST) Agency, Tokyo Japan, 3 , Kyushu University, Fukuoka Japan
Show AbstractIn organic semiconductors, π-conjugated polymers, being quasi-one-dimensional macromolecular electronic systems, offer a number of unique properties. Processing can be performed from solutions, which allows organic field-effect transistors (OFETs) to be manufactured using printing techniques. Also, by orienting the π-conjugated polymer chain, devices exhibiting polarized optoelectronic and electrical characteristics have been demonstrated. To obtain highly oriented polymers or small molecules in the preferred direction, various techniques have been applied1,2). Among these techniques for obtaining a highly oriented polymer film, the stretching technique dates back a long time. As a well-known example, by stretching a film of classical conductive polymers such as doped polyacetylene, polyarylenevinylene or polyaniline, conductivity along the stretching direction is significantly increased. In this way, the stretching technique can become a powerful tool for making a film of uniaxially aligned π-conjugated polymers and for measuring the anisotropic and enhanced carrier transport properties in OFETs using the film. Hence, in the present study, we fabricated OFETs based on a stretch-oriented polymer film from a relatively modern π-conjugated polymer, regioregular poly(3-hexylthiophene) (rr-P3HT) and investigated the anisotropic carrier transport properties in the OFETs3).OFETs were prepared in one of two orientations: either with the current flow direction parallel to the stretching direction or with the current flow direction perpendicular to the stretching direction. The field-effect hole mobility was calculated to be 2.7×10-3 cm2V-1s-1 for OFETs with the current flow paralle to the stretching direction. On the other hand, the field-effect hole mobility was calculated to be 8.0×10-4 cm2V-1s-1 for OFETs with the current flow perpendicular to the stretching direction. The field-effect mobility in a current flow parallel to the stretching direction is larger by a factor of 3.4 compared to that perpendicular to the stretching direction. These results indicate that charge transport via the π-conjugated rr-P3HT chains is advantageous in the stretch-oriented rr-P3HT film.References: 1) T. Yasuda, K. Fujita, T. Tsutsui, Y. Geng, S. W. Culligan, and S. H. Chen, Chem. Mater., 17, 264-268 (2005), 2) T. Yasuda, M. Saito, H. Nakamura, and T. Tsutsui, Appl. Phys. A, 95, 179-183 (2009), 3) T. Yasuda, L. Han, and T. Tsutsui, J. Photopolym. Sci. Technol. (in press)
9:00 PM - II9.1
Modification of Distyrylbithiophene Structure: Impact on the Mobility in Field Effect Transistors.
Yahia Didane 1 , Christine Videlot-Ackermann 1 , Philippe Marsal 1 , Frederic Fages 1 , Hugues Brisset 1
1 Centre Interdisciplinaire de Nanoscience de Marseille, CINaM CNRS UPR 3118, Faculty of Sciences of Luminy, Marseille cedex 9 France
Show AbstractThe research for new p and\or n type organic semiconductors stable in the air for the thin film transistors (OTFTs) applications are a big challenge for development organic electronics [1,2]. We showed that distyryloligothiophenes constitutes a new class of organic semiconductors which lead to OTFTs with high mobilities as p-type semiconductors along with exceptional stability under ambient conditions [3,4] confirmed recently by Fréchet and coll.[5]. Our researches on this new class of semiconductors continue according to two directions: 1) the research for n-type semiconductors, 2) the increase of the electrical performances of p-type semiconductor.In the first case, our strategy was based on the grafting of cyano groups in various positions on the vinyl bond connections of the distyryl-bithiophene (CN-DS2T) [6]. The behaviour in thin films reveals a p-type character or isolating, according to the position of cyano. Besides the experimental results, a theoretical study by DFT to understand the effects of these groups on the DS2T will be presented.In the second case, the strategy to improve the hole mobility of the distyryl-bithiophene (DS2T) was based on the rigification of the bithiophene core. The XR structure of the bridge compound (kite-DS2T) reveals a totally unexpected shape in the oligothiophene series, which we likened to a kitesurf [7]. Initially perceived to be undesirable, Kite-DS2T-based OTFTs exhibit rather surprisingly in contrast to the generally held view, a combination of excellent performances in air higher by a factor 5 than the parent unbridged semiconductor DS2T. The synthesis, the XR structure of this new compound as well as the electric properties in thin layers will be presented.[1] A.R. Murphy and J.M.J. Frechet, Chem. Rev., 2007, 107, 1066.[2] Z. Bao, J. Locklin, Organic Field-Effect Transistors (Optical Science and Engineering Series); CRC Press: Boca Raton, FL, 2007[3] C. Videlot-Ackermann, J. Ackermann, H. Brisset, K. Kawamura, N. Yoshimoto, P. Raynal, A. El Kassmi, F. Fages, JACS, 2005, 147, 16346.[4] C. Videlot-Ackermann, J. Ackermann, K. Kawamura, N. Yoshimoto, H. Brisset, P. Raynal, A. El Kassmi, F. Fages, Organic Electronics, 2006, 7, 465.[5] C.E. Mauldin, K. Puntambekar, A.R. Murphy, F. Liao, V. Subramanian, J.M.J. Fréchet, D.M. DeLongchamp, D.A. Fischer, M.F Toney, Chem. Mater., 2009, 21, 1927.[6] Y. Didane, P. Marsal, F. Fages, A. Kumagai, N. Yoshimoto, H. Brisset, C. Videlot-Ackermann, submitted.[7] Y. Didane, G.H. Mehl, A. Kumagai, N. Yoshimoto, C. Videlot-Ackermann, H. Brisset, JACS, 2008, 130, 17681.
9:00 PM - II9.10
Solid-state Dye-sensitized Solar Cells Using Novel Hole-conducting Materials based on Cyclic Phosphazenes.
Jinhee Park 1 , I-Kang Ding 1 , Brian E. Hardin 1 , Michael D McGehee 1 , Alan Sellinger 1
1 Materials Science and Engineering and the Center for Advanced Molecular Photovoltaics (CAMP), Stanford University, Stanford, California, United States
Show AbstractThe top efficiencies of liquid electrolyte dye sensitized solar cells (DSSC) exceed those of solid state cells (ssDSSCs) 11.5 to 5.1% respectively. However, as liquid electrolyte cells may have complications with sealing and leakage when produced at high volumes, ssDSSCs are a major topic of interest for this R&D community. For the past 10 years, most research on ssDSSCs have focused on the optimization of devices using 2,2’7,7’-tetrakis(N,N-di-p-methoxyphenyl-amine)-9,9’-spirobifluorene (spiro-OMeTAD) as the hole conductor material (HC). Surprisingly, very few HC materials have come close to achieving the efficiencies obtained with spiro-OMeTAD. In an effort to move towards higher ssDSSCs efficiencies, we have developed new HC materials based on cyclic phosphazene (CP) cores. These materials can be easily tuned with HC dendrons of interest, are amorphous, and have good film forming and pore filling properties. In this study we report three novel HCs that have varying number of methoxy-triphenyl-amine groups (zero-, mono-, di-methoxy) attached to the CP cores. We first studied the effect of the number of methoxy groups on chemical and electrical properties of the new HCs. All three HCs showed similar LUMO levels to that of spiro-OMeTAD, but have HOMO levels that become deeper with decreasing numbers of methoxy groups. For example, the HOMO levels of these HCs for 0, 1 and 2 methoxy groups were -5.69, -5.49 and -5.36 eV respectively. All new HCs were transparent in the visible wavelength range with peak absorption in the range of 290-310 nm, which was lower than that of spiro-OMeTAD (420 nm). The new HCs did not show any melting transitions (Tm) from differential scanning calorimeter (DSC) results, and glass transition temperatures (Tg) were in 80-90 oC range. With lower Tg compared to spiro-OMeTAD (121 oC), we carried out thermal infiltration that resulted in improved pore filling based on cross sectional SEM images. Thermal infiltration with spiro-OMeTAD is not possible because of its high Tg and tendency to crystallize at elevated temperatures. For standard solution-based pore infiltration, the solubility of the di-methoxy HC material was enhanced over that of spiro-OMeTAD in chlorobenzene which allows for more concentrated solutions and better HC infiltration. Lastly, ssDSSCs device results will be reported for the new HC materials and compared with spiro-OMeTAD.
9:00 PM - II9.11
Pure White-light-emitting Diodes from Phosphorescent Single Polymer Systems.
Po-I Lee 1 , Steve Hsu 1 , Jung-Feng Lee 1
1 , Department of Materials Science & Engineering, National Cheng-Kung University, Tainan Taiwan
Show AbstractRecently, white polymeric light-emitting diodes (WPLEDs) have drawn a lot of attention because of their potential applications in back-lighting for liquid-crystal displays (LCDs), full-color light-emitting diodes (LEDs) with color filters and lighting sources. The general approach to obtain WPLED is to blend fluorescent or phosphorescent dyes into a polymer matrix 5-13 or to use polymer blending systems, such as blending red-, green-, and blue-light-emitting polymers or blue- and orange-light-emitting polymers. However, a phase separation problem may occur in the blending systems, so white light emitting diodes from using a single polymer system has been regarded as a better way to overcome this problem.New white polymeric light-emitting diodes (WPLEDs) from phosphorescent single polymer systems have been developed using a blue-light-emitting fluorene monomer copolymerized with a red-light-emitting phosphorescent dye, and end-capped with a green-light- emission dye. The copolymerization reactions were carried out via a Yamamoto polycondensation reaction with Ni(0) as the catalyst. The molar ratio of brNTI and Ir(brpbt)2macac in the copolymers is in the range of 1-5 %. The weight-average molecular weights (Mw) of these copolymers range from 15000 to 49000 g/mol. All of the copolymers have good thermal stability with 5 % weight loss temperatures at 380-413°C and glass transition temperatures at 75-137°C. The UV/vis spectra of the copolymers are dominated by an intense peak at around 390 nm, which is from the fluorene unit in the polymer backbone. The absorption peaks of naphthalimide (NTI) segment and iridium complex cannot be observed from the spectra, because their contents in the polymer chain are too low. The PL spectra of copolymers indicate the incomplete energy transfer. According to the results, the energy transfer from fluorene segment to NTI is more efficient than to the iridium complex. We find that the emission peak of 600 nm is small when the iridium complex content is small (1-2 %), but the contribution of the NTI unit at 460 nm is large even though its content is only 1 %. We obtained white-light-emission devices by adjusting the molar ratio of the co-monomers with a structure of indium tin oxide /poly(3,4-ethylenedioxythiophene): poly(styrene sulfonic acid)/polyvinylcarbazole (PVK)/emission layer/Ca/Ag. The highest brightness in such a device configuration is 300 cd/m2 at a current density of 2900 A/m2 with Commission Internationale de l’Eclairage (CIE) coordinates of (0.33, 0.34). The EL spectra changed slightly with the applied voltage, and the red-light emission decreased with the increasing current density. This effect is due to a saturation of emission capability of iridium complex at higher current densities, which leads to changes in the CIE coordinates of EL spectra.
9:00 PM - II9.12
Efficient White Polymer-light-emitting-diodes Based on Polyfluorene End-capped With Yellowish-Green Fluorescent Dye and Blended With a Red Phosphorescent Iridium Complex.
Jung-Feng Lee 1 , Steve Hsu 1
1 , Department of Materials Science & Engineering, National Cheng-Kung University, Tainan Taiwan
Show AbstractA novel series of blue and yellowish-green light emitting single polymers were prepared by end-capping of low contents of 4-bromo-7H-benzo [de]naphtha [2',3':4,5]imidazo[2,1-a]iso-quinolin-7-one (M1) into polyfluorene. The amount of M1 added to the polymerization were 5 mol-% (PFNAP-5), 1 mol-% (PFNAP-1), 0.5 mol-% (PFNAP-0.5), 0.25 mol-% (PFNAP-0.25), 0.125 mol-% (PFNAP-0.125), and 0.06 mol-% (PFNAP-0.06). The weight-average molecular weights (Mw) of these polymers are from 22500 to 31600 g/mole, and the number-average molecular weights (Mn) are from 13200 to 22500 g/mole. Electroluminescence (EL) spectra of these polymers exhibit blue emission (λmax = 430 /460 nm) from the fluorene segments and yellowish-green emission (λmax = 510 /530 nm) from the M1 units. The turn-on voltage, maximum brightness, and maximum luminous efficiency of polymers are: (7V, 6534 cd/m2, 0.56 cd/A) for PFNAP-5, (6V, 13300 cd/m2, 1.1 cd/A) for PFNAP-1, (4.5V, 26200 cd/m2, 1.49 cd/A) for PFNAP-0.5, (4V, 11400 cd/m2, 1.5 cd/A) for PFNAP-0.25, (4V, 11600 cd/m2, 1.93 cd/A) for PFNAP-0.125, and (4V, 6704 cd/m2, 0.87 cd/A) for PFNAP-0.06, respectively. PFNAP-5 has a higher turn-on voltage, a lower maximum brightness and a maximum luminous efficiency, which is due to the charge trapping effect and concentration quenching effect. The CIE coordinates of polymers are: (0.36, 0.56) for PFNAP-5, (0.36, 0.56) for PFNAP-1, (0.33, 0.54) for PFNAP-0.5, (0.32, 0.51) for PFNAP-0.25, (0.29, 0.47) for PFNAP-0.125, and (0.25, 0.34) for PFNAP-0.06, respectively. All the EL devices of polymers have good color stabilities. When M1 unit’s content is 0.06 mol-% (PFNAP-0.06), the EL intensities of blue emission (λmax = 430/460 nm) and yellowish-green emission (λmax = 510/530 nm) are the same. A new white polymer-light-emitting-diode (WPLED) can be developed from the single polymer (PFNAP-0.06) system blended with a red phosphorescent iridium complex [Bis(2-[2'- benzothienyl)-pyridinato-N,C3'] iridium (acetylacetonate) (BtpIr)]. We were able to obtain a white-light-emission device by adjusting the molar ratio of BtpIr to PFNAP-0.06 with a structure of indium tin oxide (ITO) / poly (3,4- ethylenedioxythiophene) : poly (styrene sulfonic acid) [PEDOT:PSS] / PVK / emission layer / Ca / Ag. When PFNAP-0.06 is blended with 1 mol-% red phosphorescent iridium complex (BtpIr), we could balance the light colors and make a WPLED. At different applied voltages on the devices, the CIE coordinates and brightness of the WPLED device are 8V [(0.34, 0.35 ), 2066 cd/m2], 9V [(0.32, 0.34 ), 4030 cd/m2], 10V [(0.31, 0.34 ), 4729 cd/m2], 11V [(0.30, 0.36 ), 5578 cd/m2], and 12V [(0.29, 0.37 ), 7050 cd/m2], respectively. The WPLED device exhibits blue emission (λmax = 430/460 nm) from the fluorene segments, yellowish-green emission (λmax = 510/530 nm) from the M1 units, and red emission (λmax = 615/670 nm) from the BtpIr. We could observe a pure white emission from the device during operation.
9:00 PM - II9.13
Synthesis, Structural and Electronic Characterization of Azine Derived Semiconductors; Implementation in Organic Field-effect Transistors.
Rocio Ponce Ortiz 1 2 , Antonio Facchetti 1 , Tobin Marks 1 , Raul Blanco 3 , Helena Herrera 3 , Jose Segura 3
1 Department of Chemistry, Northwestern University, Evanston, Illinois, United States, 2 Department of Physical Chemistry, University of Malaga, Malaga Spain, 3 Deparment of Organic Chemistry, Complutense University of Madrid, Madrid Spain
Show AbstractIn this contribution, we present the synthesis of a new series of azine derived semiconductors for organic electronics. The structural and electronic properties of these materials were characterized by means of UV/Vis spectroscopy, fluorescence spectroscopy, cyclic voltammetry and DFT theoretical calculations. Field-effect transistors were fabricated by thermal evaporation and/or by solution processes and the corresponding thin-films were characterized by X-Ray diffraction (XRD) and Atomic Force Microscope (AFM). According to these results, the trend between crystallinity, morphology and electrical performance is analyzed. Furthermore, the estimation of the intramolecular reorganization energy, which considers the structural reorganization needed to accommodate charge as a prerequisite for efficient transport, is used to rationalize the electrical behaviour of the molecules.
9:00 PM - II9.14
Experimental Study and Time Dependent Modeling of OFETs With Solid Electrolyte Gate Dielectrics.
Katharina Schaetzler 1 , Klaus Schmidt 1 , Walter Fix 1 , Gottfried Doehler 1 , Heiko Weber 2
1 , PolyIC GmbH & Co. KG, Fuerth Germany, 2 Lehrstuhl für Angewandte Physik, Friedrich-Alexander-Universitaet, Erlangen-Nuernberg Germany
Show AbstractA major goal in printed organic electronics is the development and understanding of new materials compatible to present set-up and high volume fabrication that enable improved device performance and open new possibilities for applications. Crucial tasks for improvement are to increase mobility or charge carrier density in the channel region of the semiconductor which can be achieved by modifying either the semiconductor or the insulator. In the latter decreasing thickness or increasing dielectric constant is potential. It is already well known that electrolytes as gate dielectrics in organic field effect transistors (OFETs) offer low operating voltages by means of a high capacitance and hence get the best out of the organic semiconductor. That is why we successfully investigated and simulated OFETs with a solid organic insulator containing ionic salt. OFETs are built up in top gate geometry on a flexible plastic substrate (PET foil) with metallic electrodes, an organic insulator blend and polythiophene (PHT) as semiconductor. For VDS=-20V and VGS =-20V these transistors are driven in saturation and achieve an ION/IOFF ratio of 200. Neither the output nor the transfer characteristics show a hysteretic behaviour. The addition of a small amount of ionic salt into the organic insulator matrix strongly enhances the effect of the applied voltage through accumulation of ions at the interface to the semiconductor. As a consequence the number of charge carriers accumulated in the channel region is increased dramatically. This fact allows transistor operation between 0V and -5V at similar ON currents but strongly reduced OFF currents compared to the salt free transistor. Hence, an increase of ION/IOFF ratio by two orders of magnitude can be achieved. As expected, the drain-source current IDS shows a strong time dependency because of the limited mobility of the ions in the solid organic insulator matrix. However, the time dependency of the system is the key point of whether OFETs with electrolyte gated insulators can be successfully applied for any kind of circuitry or not. Consequently we developed a time dependent model for the spatial distribution of the ionic current within the insulator matrix. The ionic current splits in a diffusion and a drift process whereas the diffusion takes the gradient of concentration and the drift the electric force into account. Based on this model we are able to simulate the ionic current of both cation and anion and their effect on IDS of the transistor for switching gate voltage. The model very precisely reproduces the time as well as the ion concentration dependency of IDS. Parameters like ion and semiconductor mobility are extracted and are in good agreement with parameters extracted from the OFET and capacitance characteristics. Furthermore the understanding of it let us purposely improve OFETs with electrolyte insulators and therefore find an appropriate candidate for new applications.
9:00 PM - II9.16
Novel Silylethynyl Substituted Pentacenes With High-temperature Thermal Transitions.
David Redinger 1 , Robert Clough 1 , James Novack 1 , Gregg Caldwell 1 , John Anthony 2 , Marcia Payne 2
1 Corporate Research Materials Lab, 3M, St. Paul, Minnesota, United States, 2 , Outrider Technologies LLC, Lexington, Kentucky, United States
Show AbstractOver the past several years triisopropylsilylethynyl pentacene (TIPS-Pentacene) has been extensively studied for use in organic thin-film transistors, with display backplanes as a leading application. The soluble nature of TIPS-Pentacene enables solution-processing techniques which may result in a lower cost of fabrication. TIPS-Pentacene has a well-known solid-solid thermal transition at approximately 124 oC, which has the potential to limit its use in applications that require high-temperature processing such as photoresist baking or color filter lamination. Elimination of the solid-solid thermal transition would allow small molecule organic semiconductors to be compatible with a wide range of higher temperature processes, and ultimately make the glass-transition temperature of a flexible polymeric substrate the limiting factor in device fabrication.Small modifications to the TIPS-Pentacene molecule can result in significant changes in semiconductor properties. Substitution of only one of the three isopropyl groups with another group can lead to large changes in solubility, mobility, crystal packing (e.g. 1-D versus 2-D) and thermal transition characteristics. In fact, solid-solid thermal transitions can be eliminated altogether, leaving the melting point as the lowest temperature transition. However, the increased thermal stability cannot come at the price of lower mobility. We present two novel p-type small-molecule organic semiconductors, based on modifications of TIPS-Pentacene, which show not only elimination of the solid-solid thermal transition, but high mobility as well. The first material has a melting point of 199 oC and mobility greater than 1 cm2/Vs, and the second melts at 237 oC and has mobility greater than 3 cm2/Vs.
9:00 PM - II9.17
Efficient Single Layer Polymer Light Emitting Diodes.
Li Lu 1 , Dinesh Kabra 1 , Myoung Song 1 3 , Henry Snaith 2 , Richard Friend 1
1 Optoelectronic Group, Physics, University of Cambridge, Cambridge United Kingdom, 3 School of Mechanical and Advanced Materials Engineering, Ulsan National Institute of Science and Technology, Ulsan (UNIST), Ulsan Korea (the Republic of), 2 Clarendon Laboratory, Physics, University of Oxford, Oxford United Kingdom
Show AbstractWe present high efficiency and brightness from metaloxide-polymer composite light emitting diode. Recently, we have shown these are potential candidate for future organic light emitting diode technology.1,2 In present studies, we have fabricated single layer polymer light emitting diodes (PLEDs) using poly (9, 9’-dioctylfluorene)-co- benzothiadiazole (F8BT). The F8BT layer is sandwiched between a solution-processed compact ZnO/Cs2CO3 electron-injecting layer and a thermally evaporated MoO3 hole-injecting layer. In spite of the high ionization potential (5.8 eV) of F8BT, MoO3 provides ohmic injection of holes, allowing operation of diodes with a relatively thick polymer layer at low voltages. Optimum luminous efficiency was achieved for 1 μm polymer layers; this is an order of magnitude thicker than optimized conventional PLEDs and offers significant advantages for manufacturing. A further investigation of single carrier device, photo-luminance quantum efficiency and integrated electro-luminance measurement were also carried out to explain the enhanced efficiency in hybrid PLED architecture.
9:00 PM - II9.19
Sensitizer Effects on the Photo Carriers in PF6:TPD Polymer Semiconductor.
Karina Aleman 1 , Svetlana Mansurova 1 , Andrey Kosarev 1 , Ponciano Rodriguez 1 , Klaus Meerholz 2 , Sebastian Koeber 2
1 Optics, National Institute for Astrophysics, Optics and Electronics , Puebla, Puebla, Mexico, 2 Insitut fuer Physikalische Chemie, Universitaet zu Koeln Luxemburgerstr, Koeln, Koeln, Germany
Show AbstractRecently, a considerable amount of works has been devoted to the characterization of charge transport in organic semiconductors based on blend of conjugated polymer and small molecule sensitizer. However, there is no sufficient study of sensitizer effect on electronic properties even for widely used PCBM sensitizer. In this work we studied the effect of sensitizer concentration on a mobility-lifetime product μτ of the majority carriers, and on photoconductivity response time τ and drift mobility μ by the photo-EMF technique. When periodically oscillating interference pattern induces alternating current (ac) through the short-circuited photoconductive sample. In addition to photo-EMF experiments the ac photocurrent measurements were performed.The material studied consists of a mixture of the hole-conducting polymer PF6:TPD (poly-hexyle triophene:N,N-bis(4-methylphenyl)-N,N-bis-(phenyl)-benzidine) and sensitized with the highly soluble C60 derivative PCBM (phenyl-C61-butyric acid methyl ester) (1, 5, 10, 15 wt%). The experiment was performed using He-Ne laser (λ=633 nm) with the thin polymer film sandwiched between two glasses and illuminated by two coherent beams from the opposite sides through transparent ITO electrode. It was experimentally observed that as the sensitizer concentration growed the ac photocurrent amplitude increased. Mobility-lifetime product of the majority carrier (holes) showed, however, the opposite behavior, μτ decreased with sensitizer concentration to the lowest value μτ= 1.3E-11 cm2/V. As far as concerned to the majority carriers response time, it is only slightly affected by the sensitizer concentration resulting in holes drift mobility evaluated from the above data decays significantly with growing sensitizer concentration.
9:00 PM - II9.2
Copolymers Based on 2,7-Carbazole With Alkylated and Perfluoroalkylated Thiophene Derivatives.
Pierre-Luc Boudreault 1 , Badrou Aich 2 , Anne-Catherine Breton 1 , Ye Tao 2 , Mario Leclerc 1
1 Chemistry, Universite Laval, Québec, Quebec, Canada, 2 Institute of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada
Show AbstractOrganic field-effect transistors (OFETs) have been greatly studied over the last few decades. Usually the best performances are obtained with small molecules because of a better organization in thin films. However, polymeric materials have much better processability, lower cost of device fabrication and also better mechanical properties which make them very promising. Recently, some new polymers have shown performances close to evaporated organic semi-conductors. A cyclopentadithiophene-based copolymers reached a maximum hole mobility of 1.7 cm2/(V.s). This breakthrough leads to think that by optimizing the chemical structure, the processing of the films as well as the morphology, polymers can reach performances that are similar to small molecules.Recent developments have shown that by copolymerizing electron donor units with different thiophene derivatives, good performances can be achieved even though the x-ray diffraction (XRD) analyses of these polymers do not show a high degree of organization. In parallel, new thiophene derivatives have been obtained by adding perfluoroalkyl side chains on the thiophene backbone leading to good organization and therefore good OFET performances. We decided to follow these two strategies to synthesize new carbazole-based materials copolymerized with thiophene derivatives containing either regular alkyl chain or perfluoroalkylated side chains. We will present the synthesis of such thiophene-based comonomers along with the Suzuki cross-coupling copolymerization of each derivatives. The addition of alkyl and perfluoroalkyl chains will help understanding effect of such side chains when added into a polymer. The study has been pursued by comparing the UV-vis spectroscopy, the differential scanning calorimetry (DSC) and the XRD. We will be able to show some long range organization within the copolymers and also some π stacking interactions which are enhanced when a perfluoroalkyl chain is used. We then fabricated field-effect transistors from these materials to observe if the characterization performed allows to qualitatively predict the performances of each materials. The preliminary performances indicate that the hole mobility of one of the new synthesized material is around 10E-05 cm2/(V.s). Right now, we only have one material tested on the five new materials that will be presented and hope to obtain better performances with the other materials. Complete optimization of the devices will also be presented with different thermal treatments and by using different processing techniques such as spin-coating and drop-casting.
9:00 PM - II9.20
Synthesis and Study of Blue Emitting Polymers Based on Dibenzophosphole.
Inam ul Haq Raja 1 , Wallace Wong 1 , Aedan Cosgriff 1 , Scott Watkins 2 , David Jones 1 , Andrew Holmes 1 2
1 Chemistry, University of Melbourne, Melbourne, Victoria, Australia, 2 , CSIRO Molecular and Health Technologies, Ian Wark Laboratory, Clayton South, Melbourne, Victoria, Australia
Show AbstractSolution processed polymer OLEDs provide an attractive alternative to small molecule devices, mainly due to their significantly reduced production cost. Although all the three primary colors have been demonstrated in PLEDs, only red (orange) and green PLEDs have sufficient efficiencies and lifetimes to be of commercial value. Thus, to achieve full color polymer emissive displays, there is a need for stable blue-emitting conjugated polymers with high photoluminescence efficiency. We report here synthesis and photoluminescence of some novel deep blue and blue emitting polymers. The synthesis of new homopolymer and alternating copolymers from novel and reported dibenzophosphole monomers is discussed. Introduction of the electronegative phosphorus can lower the LUMO of the polymer and aid electron transport which is an important requirement for efficient polymeric OLEDs. We demonstrated the physical and photophysical properties of polymers which show that these polymers are blue fluorescent. Device fabrication has been carried out with some of the polymers and their electroluminescence has been discussed. Effect of substitution on band-gap and solubility has been demonstrated.These studies show that the polymers synthesized during this project are soluble in organic solvents and are thermally stable. Polymers show blue to deep blue fluorescence and electroluminescence in the solid state which is important for application to full-color flat-panel displays because blue is the highest energy emission. Electroluminescence of these polymers will be tuned by optimizing the device parameters.
9:00 PM - II9.21
Effect of Fabrication Parameters on Three-dimensional Nanostructures and Device Efficiency of Polymer Light Emitting Diodes.
Chia-Yi Liu 1 , Bonnie Yu 1 , Wei-Chun Lin 1 , Wei-Ben Wang 3 , I-Ming Lai 3 , Che-Hung Kuo 1 2 , Szu-Hsian Lee 1 2 , Wei-Lun Kao 1 2 , Guo-Ji Yen 1 2 , Yun-Wen You 1 , Hsun-Yun Chang 1 , Chi-Ping Liu 1 3 , Jwo-Huei Jou 1 3 , Jing-Jong Shyue 1 2
1 Research Center for Applied Science, Academia sinica, Taipei Taiwan, 3 Dept. of Materials Science and Engineering, National Tsing Hua University, HsinChu Taiwan, 2 Dept. of Materials Science and Engineering, National Taiwan University, Taipei Taiwan
Show AbstractComparing with silicon-based opto-electronic devices, organic electronics gain popularity due to the simple fabrication process involved and the relatively low cost. However, while it is well known that the nanostructure affect the device performance significantly, it is difficult to examine such a relationship from organic electronics because of the difficulties in analyzing amorphous materials with electron microscopy. Based on cluster ion (C60+) depth profiling, the vertical nanostructure of organic opto-electronic devices have been studied with x-ray photoelectron spectrometry (XPS) because it causes insignificant alteration to the remaining organic surface. Nevertheless, the spatial resolution is limited with XPS. In this work, the cluster ion sputtering is combined with force modulation microscopy to examine the 3D nanostructure inside PLED fabricated with different parameter. It is found significant phase separation occurred when the PVK:FIrpic blend is spin-coated at 30°C; on the other hand, PVK:FIrpic coated at 60°C is more homogeneous. It is found that the device fabricated at 60°C has 30% higher power conversion efficiency than the 30°C counter-part. It is proposed that the nanostructure formed at 30°C provide a route for electron to pass through the luminance layer without efficient energy transfer while the nanostructure with 60°C process can trap charge carriers on PVK and allow exitons to diffuse to FIrpic guest hence the higher efficiency.
9:00 PM - II9.22
Highly Stable Organic Light-emitting Diodes by Using High Stereo-hindrance Host Material as Blockade.
Chi-Ping Liu 1 2 , Wei-Chun Lin 1 , Wei-Ben Wang 2 , Cheng-Wei Lin 2 , Jwo-Huei Jou 2 , Jing-Jong Shyue 1 3
1 Research Center for Applied Sciences, Academia Sinica, Taipei Taiwan, 2 Department of Materials Science and Engineering, National Tsing Hua University, Hsinchu Taiwan, 3 Department of Materials Science and Engineering, National Taiwan University, Taipei Taiwan
Show AbstractOver the past years, organic light-emitting diodes (OLEDs) are increasingly attracting interest because of their great potential as high-quality flat-panel displays and solid-state lighting. One of the basic requirements in any emissive device is providing adequate stability to ensure a sufficiently long lifetime. There are many accepted degradation mechanisms proposed for OLEDs. Recently, it is observed that small molecules migrate towards the ITO anode under a direct driving voltage while retaining its original structure. In order to prevent the electron-migration of small molecules, a chemical structure with higher stereo hindrance could be introduced as a blockade, thus molecular migration could be suppressed and the device lifetime could increase. In this work, OLED devices with different light luminance hosts including CBP, mCP, SimCP, and SimCP2 with increasing stereo hindrance are fabricated. The molecular distribution after operation is examined with X-ray photoelectron spectroscopy with in situ high-energy C60+ and low-energy Ar+ co-sputtering for depth profiling. It is found the electron migration is suppressed and lifetime prolonged with increasing the stereo hindrance.
9:00 PM - II9.23
Photophysical and Electrochemical Investigation of Donor-acceptor Perylene Diimide-thiophene Copolymers for Solar Cells.
Erika Kozma 1 , Dariusz Kotowski 1 , Filomena Munno 1 , Marinella Catellani 1 , Silvia Luzzati 1
1 Istituto per lo Studio delle Macromolecole, Consiglio Nazionale delle Ricerche, Milan Italy
Show AbstractPerylene diimide (PDI) are a well known class of thermo-stable dyes with semiconduction n-typeproperties and good charge mobility that have received much attention as building blocks to design active materials for organic electronic devices. PDI molecules have been tested as acceptor materials in bulk heterojunction solar cells with several electron-donor conjugated polymers because of their large molar absorption, high electron affinity and low cost. Despite their quite good solar harvesting properties, the devices made with these acceptor molecules have shown poor power conversion efficiencies. The alternation of a perylene diimide with a donor moiety in bay position, has been demonstrated to be a good approach to design new n-type materials for bulk heterojunction solar cells. Such acceptor copolymers led to significant improvements of the device performance compared to devices made using PDI molecules as acceptor. The reasons for such improvements are not yet fully understood as, from one side, passing from molecular to polymeric acceptors materials has implications in the active blend morphology, but the insertion of donor moieties in bay position is also affecting the electronic structure of the materials. Therefore, there is the need to get further insight in the relation between the molecular design of these n-type materials and their functionality as active components in bulk heterojunction solar cells. With this in mind, we have prepared a series of donor-acceptor copolymers, based on the alternation of N,N’-bis(10-nonadecyl)perylene-3,4,9,10-tetracarboxylic diimide with different donor moieties in bay position (single-bond and triple bond linked oligothiophenes). In this work, to assess the changes imparted by the different donor moieties to the copolymer electronic structure, we have investigated the redox and spectroscopical properties of these materials by cyclovoltammetry, by Raman, UV-VIS absorption and fluorescence and cw photoinduced absorption spectroscopy. Their use as photoactive acceptor materials in blends with poly-3-hexylthiophene is discussed in terms of photoinduced charge transfer, probed by photoluminescence and photoinduced absorption measurements.
9:00 PM - II9.24
Study of Ion-pairing Iridium Complexes and Their Application in OLED.
Chao Wu 2 , Hsiao-Fan Chen 3 , Mark Thompson 1 2
2 Department of Chemical Engineering and Materials Science, University of Southeren California, Los Angeles, California, United States, 3 Department of Chemistry, National Taiwan University, Taipei Taiwan, 1 Department of Chemistry, University of Southeren California, Los Angeles, California, United States
Show Abstract Luminescent cyclometalated Ir complexes have been widely studied for their display and lighting applications. Neutral Ir complexes play a significant role in the development of OLEDs, while cationic complexes have been applied to LECs successfully. However, Ir-based ion-pairing complexes (soft salts) and their application in light-emitting devices have not been reported before. In this paper, we report the synthesis, characterizations and device data of the soft salts that are composed of opposite-charged ionic Ir complexes. Using soft salts in OLED has several advantages such as the ambipolarity and the flexibility in tuning energy levels, due to two independent functional ions. The energy levels of the soft salts are found to be decided by the shallower (less negative) HOMO and the deeper (more negative) LUMO between two ions. By carefully choosing the ligands to optimize the alignment of the HOMO/LUMO between the ions, the device can achieve the external quantum efficiency up to the theoretical maximum. The device consists of three layers, i.e. PVK/soft salt/BCP, where PVK and BCP serves as charge injection layers and they also help to confine the charge inside the soft salt layer, which benefits the exciton generation and leads to the high quantum efficiency.
9:00 PM - II9.25
Electrical Characterization of Long Conjugated Molecular Wires: Probing the Physical Organic Chemistry of Hopping Conduction.
Seong Ho Choi 1 , BongSoo Kim 2 , C. Daniel Frisbie 1
1 Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States, 2 Chemistry, University of California at Berkeley, Berkeley, California, United States
Show AbstractFundamental understanding of the connection between molecular structure and charge transport is a central issue in the field of molecular electronics. In particular, variable rates of hopping conduction upon a change of structural entities should be addressed to provide guidance in designing highly conductive molecular wires as well as understanding transport in bulk organic devices. In this work, we provide direct evidence not only for a change in transport mechanism from tunneling to hopping in molecular junctions, but also for the large sensitivity of hopping conduction to wire architectures based on three series of long conjugated molecular wires: oligophenyleneimine (OPI, ranging in length from 1.5-7.3 nm), oligonaphthalenefluoreneimine (ONI, from 2.5-10.3 nm), and oligotetrathiafulvalenepyromelliticdiimideimine (OTPI, from 2.5-20.2 nm). Our experimental approach involves contacting molecular wires that are grown from one electrode using controlled aryl imine addition chemistry; a metal-coated atomic force microscope tip is used to make the second contact. We show that the addition chemistry works cleanly and that in some respects OTPI wires are a better system for transport studies than OPI and ONI wires. A change in transport mechanism from tunneling to hopping is apparent near 4-5 nm in length for all wires, as evidenced by striking changes in length, temperature and electric field dependence of the current-voltage (I-V) characteristics. For longer wires, we have been able to estimate the single wire conductivity and the hopping activation energy. Single-wire conductivity of OTPI wires is estimated to be 2×10-2 S/cm, two orders of magnitude greater than OPI and ONI wires and their hopping energy is reduced by 70- 160 meV as compared to OPI and ONI wires, which is attributed to the narrow band gap (~1.3 eV) in the OTPI architectures. These nanoscale transport measurements describe how precise control of wire architecture allows modulation of physical parameters in electrical hopping conduction.
9:00 PM - II9.26
Increased Power Efficiency in Phosphorescent Blue Organic Light Emitting Devices Using Solution Processible p-doped Hole Transport Layers.
James Swensen 1 , James Rainbolt 1 , Phillip Koech 1 , Evgueni Polikarpov 1 , Liang Wang 1 , Lelia Cosimbescu 1 , Asanga Padmaperuma 1 , Daniel Gaspar 1
1 , Pacific Northwest National Laboratory, Richland, Washington, United States
Show AbstractIn the push to develop organic light emitting devices (OLEDs) for application in flat panel displays and solid state lighting applications, obtaining low drive voltage is key to minimizing power consumption. One route to accomplish this is through doping the hole transport layer (HTL) with an electron acceptor molecule and/or the electron transport layer (ETL) with an electron donor molecule. Successful doping of the hole and electron transport layers can yield operating OLEDs with drive voltages very close to the bandgap of the emitting molecule.Reports of HTLs doped with organic dopants such as tetracyanoquinodimethane (F4TCNQ) or inorganic dopants such as ReO3 have typically been accomplished through co-deposition of the dopant with the HTL material via vacuum sublimation. We present experimental results demonstrating for the first time phosphorescent blue OLEDs fabricated using organic p-dopants in solution-processed HTLs. We compare two organic p-dopants synthesized in our laboratories, F6TNAP and F3TCNQ-adl, to the commercially available p-dopant F4TCNQ. The solution processible HTLs consist of a 4,4',4"-tris (3-methylphenylphenylamino) triphenylamine (MTDATA) and a p-dopant blended into a Poly(N-vinylcarbazole) (PVK) matrix.Our UV-Vis absorption results for PVK:MTDATA:p-dopants thin films confirm the formation of a charge transfer complex between the MTDATA and the p-dopant. Single carrier devices fabricated with ITO/PVK:MTDATA:p-dopants (150 nm)/ Au (20 nm)/Al (100 nm) show clear evidence of conductivity doping. Finally,OLEDs fabricated with our solution-processed, doped HTLs show increased power efficiency in phosphorescent blue OLEDs. This work demonstrates that organic p-dopants can be solution processed and don’t have to be co-deposited through vacuum sublimation. It opens up new molecular architectures for p-dopants as they no longer have to be stable to thermal sublimation.
9:00 PM - II9.28
All Ink-jet Printed Polyfluorene Photosensor for High Illuminance Detection.
Leah Lavery 1 , Gregory Whiting 1 , Ana Arias 1
1 Electronic and Materials Devices Laboratory, Palo Alto Research Center (PARC), Palo Alto, California, United States
Show AbstractHigh intensity light sensors are needed in order to monitor working environmental conditions and help with diagnosis of blast-related injuries which include traumatic brain injury (TBI). We have been developing a monitoring system that could be worn by soldiers to collect daily information that can be used as part of their medical record. This device is designed to be disposable, inexpensive and adhered directly to objects including non-planar surfaces. This requires that the development of light sensors with transparent top electrodes, high light intensity response and process compatibility with flexible substrates. All layers of the light sensor were fabricated by ink-jet printing. The all-printed light sensor is based on a blend of hole-accepting and electron-accepting derivatives of poly(9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB) and poly(9,9’-dioctylfluorene-co-benzothiadiazole) (F8BT), respectively. The surface of the PFB-F8BT organic photoactive layer is hydrophobic and represents a challenge when fabricating top light-absorbing devices from solution. We solved this fabrication challenge by using poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) (PEDOT:PSS) as the top electrode and modifying the PEDOT:PSS ink in order to print directly onto the hydrophobic surface of the PFB:F8BT blend layer, placing the transparent electrode at the top of the light sensor structure. The bottom electrode was printed from a Ag nanoparticle solution.The printed high light intensity sensors were tested under illumination provided by a Newport 1600W solar simulator. I-V characteristics were measured as well as testing of the all-printed PFB:F8BT photosensors under short illumination pulses (0.5 to 0.75 seconds) as it occurs in light exposure from a explosion or blast. The photosensors show good linearity over multiple measurements at high illuminance values of (100 to 400) klux. External quantum efficiency and atomic force microscopy results will also be presented.
9:00 PM - II9.29
Self-organized Conducting Polymer Anode With Tunable Work Function for Highly Efficient Flexible Organic Light-emitting Diodes.
Seong-Hoon Woo 1 , Tae-Hee Han 1 , Mi-Ri Choi 1 , Joo-Hyun Hwang 1 , Tae-Woo Lee 1
1 Department of Materials Science and Engineering, Pohang University of Science and Technology, Pohang, Gyeongbuk, Korea (the Republic of)
Show AbstractAs the importance of ultra-thin and flexible panel displays and solid-state lighting devices increases, flexible organic light-emitting diodes (FOLEDs) based on organic materials are attracting great attention among researchers. In order to embody highly efficient FOLEDs, highly conductive polymeric anode is indeed critically required to replace the conventional inorganic anode, indium tin oxide (ITO). However, the conventional polymeric anode still has low work function (ca. 5.0 eV) so that we still need a hole injection layer (HIL) on top of the anode to facilitate the hole injection from the anode. Therefore, we developed work-function tunable polymeric anodes which can be achieved by self-organization of conducting polymer compositions during the spin coating. The conducting compositions are composed of highly conductive poly(3,4-ethylenedioxythiophene)/poly(4-styrenesulfonate) (PEDOT:PSS), a perfluorinated ionomer (PFI), and dimethyl sulfoxide (DMSO), which we call AnoHIL. The small molecule green OLEDs using the AnoHIL without a HIL showed highly improved luminous efficiency, ca 18 cd/A, compared with conventional device using both the ITO anode and the HIL. (ca. 12 cd/A for ITO/2TNATA and ITO/PEDOT:PSS, ca 15 cd/A). We investigated the hole-injection characteristics of the device employing the AnoHIL at the interface between the AnoHIL and a hole-transport layer using dark Injection space-charge limited current measurement. We also investigated the electron–hole balance and recombination characteristics inside the devices by using transient electroluminescence measurements. Furthermore, we observed capacitance-voltage characteristics to analyze the efficient charge injection from the anode and electron-hole recombination inside the device. Finally, we demonstrate highly efficient flexible OLEDs using the AnoHIL which shows superior reliability to the ITO-based flexible devices. As a result, we can conclude that our work is a great step toward highly efficient FOLEDs with improved flexibility and simplified structures by overcoming the charge injection barrier from the anode.
9:00 PM - II9.30
A Thermally Stable Semiconducting Polymer.
Shinuk Cho 1 , Jung Hwa Seo 1 , Sung Heum Park 1 , Serge Beaupre 2 , Mario Leclerc 2 , Alan Heeger 1
1 Center for Polymers and Organic Solids, University of California at Santa Barbara, Santa Barbara, California, United States, 2 Départment de Chimie, Université Laval, Quebec City, Quebec, Canada
Show AbstractAlthough the opportunities associated with “printable plastic electronics” are well-known, the poor stability of semiconducting polymers when exposed to modest temperatures, to oxygen and/or to water vapor has been and continues to be a barrier to commercialization. We report here the example of an excellent thermal stable semiconducting polymer. UV-Visible absorption spectra and X-ray photoelectron spectroscopy (XPS) data demonstrate that the electronic structure of the poly(2,7-carbazole) derivative (PCDTBT) is stable in air at annealing temperatures up to 150 °C and in N2 even after exposure to temperatures up to 350 °C. Field-effect transistors (FETs) fabricated with PCDTBT are stable; the FET characteristics and the hole mobility are stable in air at temperatures up to 150 °C and in N2 after exposure to temperatures up to 350 °C.
9:00 PM - II9.31
Delayed Phosphorescence from Metal-free Organic Phosphors.
Onas Bolton 1 , Jinsang Kim 1 2 3
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Chemical, Macromolecular, and Biomedical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 3 WCU invited professor, Hybrid Materials for Sustainability, Seoul National University, Seoul Korea (the Republic of)
Show AbstractExcimers represent some of the most common and detrimental quenching pathways in emissive organic molecules and great efforts are undertaken to prevent their formation in optoelectronic devices. As an established norm, chromophores are strictly diluted into host materials to prevent these intermolecular interactions. In a rare example of emission reclaimed from these quenching states here we report delayed phosphorescence, the phenomenon of emission from triplets states that have been thermally populated from lower-lying triplet excimers. Analogous to established delayed fluorescence, emission is spectrally identical to prompt phosphorescence, yet much slower indicating its more complicated route. Also, this emission is thermally variable, with none detected at low temperatures. Presented here are crystals, the densest possible form of the molecule, capable of producing delayed phosphorescent quantum yields of 0.029 at 298 K. We believe the discovery of this phenomenon holds promise for the enhancement of phosphorescent organic device efficiencies by recapturing emission from normally quenching states.
9:00 PM - II9.32
Organically Functionalized DNAs and Their Application to BioLED and BioFET.
Youn Sun Kim 1 , Ki Hwa Jung 1 , U Ra Lee 1 , Kyung Hwan Kim 1 , Min Ju Cho 1 , Jung-Il Jin 1 , Dong Hoon Choi 1
1 Chemistry, Korea University, Seoul Korea (the Republic of)
Show Abstract Recently, DNA has been studied electronic, photonic, and magnetic properties. Those are utilizable in such advanced technologies as bio-organic field effect transistors (BioFET), bio-organic light emitting diodes (BioLED), nanowire, template, optomagnetic switching, and molecular magnetic information storage. The purified natural DNA can be dissolved only in water, but not soluble in any other organic solvent. In this study, we modified the structure of natural DNA and change its solubility by post-functionalization. We applied organically functionalized DNAs to electronics and optoelectronics.The organic soluble DNA contains chalcone moiety which is photoreactive was utilized as a gate insulator for BioFET. We fabricated TFT device using organic semiconductor material and the photocrosslinked DNA-based gate insulator by solution deposition method. The device performances will be demonstrated in this presentation. Another carbazole-substituted DNA is used as a triplet host material to study the BioLED performances. The preparation of carbazole-substituted DNA was carried out by precipitating the purified DNA in water with a carbazolyl cationic surfactant. A synthesized organic soluble DNA containing dodecylcarbazole side chain moieties is less hydrophilic and improve host properties such as carrier mobility, miscibility, and energy transfer effect to guest Ir(III) complex. Multilayered electrophosphorescent device was fabricated with Cz-DNA and Ir(Cz-ppy)3 for investigating their device performances.
9:00 PM - II9.33
Charge Transport and Photovoltaic Properties of Dithieno Pyrrole Oligomer and Polymer.
Manoj Parameswaran 1 , Ganapathy Balaji 1 , Suresh Valiyaveettil 1 , Tan Jin 2 , Chellappan Vijila 2
1 , National University of Singapore, Singapore Singapore, 2 , Institute of Materials Research and Engineering, Singapore Singapore
Show AbstractConjugated materials have received increased attention owing to its optical and electronic properties and potential applications in photovoltaic devices, light emitting diodes, and field effect transistors(1). Organic materials in electronic applications overcome many of the draw back of silicon technology such as expensive fabrication, difficulty in tuning the properties and processabilty(2). In general π conjugated low molecular weight oligomers or highly defined polymers are used for such applications. The charge transport properties of these conjugated materials play a key role in the design and fabrication of optoelectronic devices. A detailed charge transport study will provide information on the degree of disorders in the polymer/oligomer film and the transport efficiency of materials. In this aspect, we studied the charge mobility characteristics of a newly synthesized dithieno pyrrole oligomer(3) and polymer using time-of-flight photoconductivity measurement. The oligomer as well as polymer was found to be hole transporting in nature with hole mobility in the order of 10-6 cm2/Vs and 10-4 cm2/Vs respectively. The dependence of hole mobility with applied electric field and temperature is studied in detail by analyzing the experimental results using the Bassler’s Gaussian disorder model and Correlated disorder model. Also the bulk heterojunction solar cells were fabricated using a blend of this polymer with PCBM and tested its photovoltaic properties.Acknowledgement: The authors wish to acknowledge the funding support from Agency for Science and Technology Research (ASTAR) Singapore. References1.Günes, S; Neugebauer, H.; Sariciftci, N.S. Chem. Rev. 107, 2007, 1324.2.Sirringhaus, H.; Tessler, N.; Friend, R. H. Science 280, 1998, 1741.3.Parameswaran, M.; Balaji, G.; Mein Jin, T.; Vijila, C.; Vadukumpully, S.; Furong, Z.; Valiyaveettil, S. Org. Electron. 2009, doi:10.1016/j.orgel.2009.08.022.
9:00 PM - II9.34
Three Primary Color WOLED Using MADN as Host Material.
Mei Meng 1 , You-Hyun Kim 1 , Sang-Youn Lee 1 , Wook Song 1 , Woo Young Kim 1
1 School of Display Engineering, Hoseo University, Asan Korea (the Republic of)
Show AbstractThree primary colors white organic light emitting diodes (WOLEDs) using MADN as host material were fabricated. Layer structures of device A and device B were composed of NPB(700Å) / MADN:BCzVBi-5%(100Å) / MADN:DCJTB-0.6%(80Å) / MADN:C545T-1.0%(40Å) / Alq3(300Å) / Liq(20Å)/Al(1000Å) and NPB(700Å) / MADN:BCzVBi-5%(100Å) / MADN:C545T-1.0%(40Å) / MADN:DCJTB-0.6%(80Å) / Alq3(300Å) / Liq(20Å) / Al(1000Å), respectively. Current density, maximum luminescence and luminous efficiency of device A and device B were 83.123 mA/cm2 and 76.303 mA/cm2 at 6.0 V, 18230cd/m2 and 18120cd/m2 at 9.0V, and 3.11cd/A and 3.08 cd/A at 5.0 V, respectively. CIExy coordinates of device A was (0.33, 0.33) at 6.0 V and was changed from (0.35, 0.34) at 4.0V to (0.34,0.33) at 9.0V while that of Device B was (0.31, 0.33) at 6.0 V and was done from (0.32, 0.33) at 5.0V to (0.32,0.35) at 9.0V. Device A has little change in color coordinate with various voltages due to the high stable probability of electron-hole recombination in MADN used as a host of the three-colored emission layer. Device B has relatively lower red intensity compared with device A caused by the lower probability electron-hole recombination in red emissive layer than green emissive layer because blue emissive BCzVBi’s LUMO level is 0.2eV higher than that of MADN. Then, trapped electrons in the interface of MADN were used for recombination to generate green emission from C545T as similar results were happened to generate relatively higher red emission than green emission in device A. Device A showed better white CIExy coordinates than that of device B although current efficiency and color stability with various voltages were not significantly different from device B. WOLED from mixture of three primary colors will be applied perfectly for LCD backlight unit, light source, and full color OLEDs as well in near future.
9:00 PM - II9.35
The Interfacial Electronic Structures of N,N'-Bis-(1-naphthyl)-N,N'-Diphenyl-1,1'-Biphenyl-4,4'-Diamine (NPB)/1,4,5,8-Naphthalene-Tetracarboxylic-Dianhydride/ITO and NPB/ITO: A Comparative Study.
Pyungeun Jeon 1 , Hyunbok Lee 1 , Hyun Sung Kim 1 , Kwangho Jeong 1 , Yeonjin Yi 2
1 Physics, Yonsei University , Seoul Korea (the Republic of), 2 Division of Advanced Technology, Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of)
Show AbstractOrganic electronic devices are considered as a next generation device for their promising potentials such as low cost fabrication and flexibility. N,N'-bis-(1-naphthyl)-N,N'-diphenyl-1,1'-biphenyl-4,4'-diamine (NPB) is one of the most common hole transporting material in organic light emitting devices (OLEDs). However, it is still required to reduce the hole injection barrier from indium-tin-oxide (ITO) electrode to NPB. NTCDA insertion between NPB and ITO reduces hole injection barrier, thus it enhances device performance. The NTCDA, however, shows strong chemical interaction in contact with ITO, rendering difficulties to estimate interfacial electronic structures. Therefore, it is highly required to understand the origin of the interaction at the NTCDA interface. We studied NTCDA interface using photoemission spectroscopy in order to understand the interfacial electronic structures, so do the origin of the interaction. We analyzed the changes in secondary electron cutoff, highest occupied molecular orbital and core-level during the interface formation and evaluated the complete energy level diagrams. The detailed interactions and interfacial electronic structures will be presented.
9:00 PM - II9.36
Synthesis and Photovoltaic Properties of Low-bandgap Alternating Copolymer Consisting of 3-Hexylthiophene and [1,2,5]Thiadiazolo[3,4-g]Quinoxaline Derivatives.
Yoonkyoo Lee 1 , Thomas Russell 2 , Won Ho Jo 1
1 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of), 2 Polymer Science and Engineering, University of Massachusetts, Amherst, Massachusetts, United States
Show AbstractIn recent years, bulk heterojunction (BHJ) solar cells fabricated by simple blending of poly(3-hexylthiophene) (P3HT) as an electron-donating polymer and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), which serves as an electron acceptor, have reached the power conversion efficiency as high as 5 %. Although P3HT has several advantageous properties, it has a bandgap of 1.9 eV and thus harvests only 22 % of available photons. To absorb photons at longer wavelengths, where more photon flux is found from the emission of the sun, development of low-bandgap polymer is strongly required. Donor-acceptor conjugated alternating copolymers have great advantage because their electronic properties can be effectively manipulated by intramolecular charge transfer between the electron donor and acceptor units. We are particularly interested in the [1,2,5]thiadiazolo[3,4-g]quinoxaline (TQ) as an acceptor unit due to the strong electron-withdrawing property of the four imine groups in the TQ unit. Although some research groups already have synthesized TQ-based low bandgap copolymers and reported the photovoltaic properties of those copolymers, the molecular weights of the copolymers were very low due to the lack of solubility, which resulted in low power conversion efficiency below 0.7 %.In this study, we report the synthesis and characterization of new class of donor-acceptor alternating copolymers comprising 3-hexylthiophene (3HT) and TQ derivatives. We employed the palladium-catalyzed Stille coupling reaction to synthesize alternating copolymers of 3HT and TQ derivatives as electron-donating and electron-withdrawing units, respectively. The number average molecular weights of the polymers were 7,000 - 11,000, and the polymers were highly soluble in common organic solvents, such as chloroform, toluene, and THF at room temperature. The polymers showed broad absorption band from visible to infrared region. The bandgaps (1.0 eV - 1.3 eV) of the polymers were successfully tuned by changing the substituents on the acceptor unit. Furthermore, the HOMO and LUMO energy levels of the alternating copolymer were appropriate for the photoinduced charge transfer between the copolymer and PCBM. The BHJ polymer solar cells, which had the layered configuration of glass/ITO/ PEDOT:PSS/polymer:PCBM/LiF/Al, were fabricated. By optimizing the device parameters, such as the blend ratio of copolymer to PCBM and the choice of processing solvent, the power conversion efficiency as high as 1.27 % was achieved under standard solar radiation conditions (AM 1.5G, 100 mW/cm2).
9:00 PM - II9.37
Synthesis of C60-end Capped P3HT and Its Application for High Performance P3HT/PCBM Bulk Heterojunction Solar Cells.
Jea Uk Lee 1 , Jae Woong Jung 1 , Won Ho Jo 1
1 , Seoul National University, Seoul Korea (the Republic of)
Show AbstractFor efficient bulk heterojunctions (BHJ) solar cells, the three-dimensional bicontinuous and nanometer-scale morphology of the active layer is of utmost importance, because efficient photoinduced charge generation, transport, and collection at each electrode crucially depend on the morphology of the composite films. However, it is extremely difficult to control the donor/acceptor blend morphology and to optimize the device performance, since the nanometer-scale morphology of the active layer is not thermodynamically stable. One approach to control the nanometer-scale morphology and to improve the long-term thermal stability of conjugated polymer-fullerene BHJ solar cells is to use diblock copolymers, which have two different blocks consisting of conjugated polymer and fullerene, as a compatibilizer. However, the synthesis of diblock copolymers composing of poly(3-hexylthiophene) (P3HT) and fullerene blocks is very difficult, and moreover the presence of a substantial amount of insulating moieties inevitably required for introducing C60 in the second block can deteriorate the charge carrier transport in the BHJ devices.To overcome these limitations of diblock copolymer, here, we synthesized C60-end capped P3HT and used it as a compatibilizer for P3HT/PCBM BHJ solar cells. The C60-end capped P3HT (P3HT-C60) was synthesized via Grignard Metathesis (GRIM) polymerization method followed by directly linking a fullerene derivative at the chain end of the regioregular P3HT. The chemical structure of P3HT-C60 was identified by nuclear magnetic resonance, Fourier Transform Infrared spectroscopy, and gel permeation chromatography. We also examined the effect of P3HT-C60 compatibilizer on the morphology of P3HT/PCBM blend and its long-term stability of solar cell performance. When a small amount of the P3HT-C60 compatibilizer was added to P3HT/PCBM blend, the bicontinuous and nanometer-scale film morphologies were preserved for a long annealing time at 150 °C while the standard P3HT/PCBM BHJ solar cell exhibited macro-phase separation. This observation leads us to conclude that the P3HT-C60 compatibilizer controls the domain size of P3HT/PCBM blend and suppress the macro-phase separation during thermal treatment.As a result, the P3HT/PCBM/P3HT-C60 BHJ solar cells exhibited the excellent long-term thermal stability of device performance and uniform film morphology, whereas the power conversion efficiency of standard P3HT/PCBM solar cells drastically decreased after two days of annealing at 150 °C. In short, the enhancement of long-term thermal stability of P3HT/PCBM/P3HT-C60 solar cells can be explained by the fact that the P3HT-C60 reduces the interfacial tension between P3HT and PCBM phases and suppresses the phase segregation of P3HT/PCBM blend after prolonged annealing time.
9:00 PM - II9.38
New Polymer Semiconductors Based on Fused Heteroaromatic System: Synthesis, Structures, and Transistor Applications.
Itaru Osaka 1 , Toru Abe 1 , Shoji Shinamura 1 , Kazuo Takimiya 1
1 Chemistry, Hiroshima University, Higashi-Hiroshima, Hiroshima, Japan
Show AbstractPolymer semiconductors offer an advantage versus small molecular semiconductors in printable electronics from the viewpoint of processability and uniformity of thin films. Regioregular poly(3-alkylthiophene)s, rrP3ATs, have been the most common polymer semiconductor that provide very good carrier mobilities in field-effect transistors. In recent years, however, some polymer semiconductors have been reported to show even higher mobilities relative to rrP3ATs, which may originate from the enhanced π-π interactions between polymer chains promoted by an introduction of rigid fused rings. Here, we report new polymer semiconductor system based on a fused heteroaromatic ring with extended π-electron system, demonstrating great performance in solution-processed organic field-effect transistors. Polymers with different alkyl chain length and components were synthesized. Transistor devices with the polymers showed the highest mobility of 0.5 cm2/Vs and average mobility of >0.2 cm2/Vs, with on/off ratios of ~107. Structure-property relationships will also be discussed.
9:00 PM - II9.39
Organic and Inorganic Interlayers for Hole Injection in Inverted Organic Light-emitting Devices.
Jeong Won Kim 1 , Yoon Hak Kim 1 , Soon Mi Park 1
1 , Korea Research Insititute of Standards and Science, Daejon Korea (the Republic of)
Show AbstractCurrently, organic light-emitting diodes (OLEDs) have been proven of their readiness for commercialization in terms of lifetime and efficiency. In accordance with emerging new technologies, enhancement of light efficiency and extension of application fields are required. Particularly inverted structures, in which electron injection occurs at bottom and hole injection on top, show crucial advantages due to their easy integration with Si-based driving circuits for active matrix OLED as well as large open area for brighter illumination. In order to get better performance and process reliability, usually a proper buffer layer for carrier injection is needed. In inverted top emission OLED, the buffer layer should protect underlying organic materials against destructive particles during the electrode deposition, in addition to increasing their efficiency by reducing carrier injection barrier. As a buffer material, a few kinds of transition metal oxides for inverted OLED applications have been successfully utilized aiming at efficient hole injection properties. Among them, we chose 2 nm of WO3 between NPB [4,4'-bis(N-phenyl-1-naphthylamino)biphenyl] and Au (or Al) films. The interfacial energy-level alignment and chemical reaction as a function of film coverage have been measured by using in-situ ultraviolet and X-ray photoelectron spectroscopy. It turned out that the WO3 interlayer substantially reduces the hole injection barrier irrespective of the kind of electrode metals. It also avoids direct chemical interaction between NPB and metal atoms. This observation clearly validates the use of WO3 interlayer as hole injection for inverted OLED applications. Finally, instead of WO3, an organic hole injection layer, 1,4,5,8,9,11-hexaazatriphenylene-hexacarbonitrile (HAT-CN) was also applied and compared.
9:00 PM - II9.4
Interface Structure of Poly (3-alkylthiophene) FETs: Influence of Alkyl Side-chain Length and SiO2 Dielectric on FET Mobility.
Chris McNeill 1 , Wibren Oosterbaan 2 , Lars Thomsen 3 , Jean-Christophe Bolsee 2 , Veerle Vrindts 4 , Laurence Lutsen 4 , Jean Manca 2 4 , Dirk Vanderzande 2 4
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, 2 Institute for Materials Research, Hasselt University, Diepenbeek Belgium, 3 , Australian Synchrotron, Melbourne, Victoria, Australia, 4 , IMEC-IMOMEC, Diepenbeek Belgium
Show AbstractWe present a combined microstructure/charge transport study of poly(3-alkylthiophenes) (P3ATs) in organic field-effect transistors. Specifically, Near-Edge X-ray Absorption Fine-Structure (NEXAFS) spectroscopy is used to characterize the structure of the top and bottom interfaces of P3AT films with varying alkyl side-chain length spin-coated on silicon dioxide. Regarding the SiO2/P3AT interface exposed via HF etch, for different side-chain lengths we find a strong correlation between field-effect mobility and interfacial order of the P3AT films as characterised by the average tilt of the P3AT backbone to the substrate. For the P3AT/air interface, much higher tilt angles are measured indicative of a higher degree of ordering with no strong correlation of average tilt angle with alkyl side chain length. Furthermore, by detaching the film from the SiO2 substrate and reattaching to another transistor substrate air-side down in order to utilise this well ordered interface, we demonstrate field-effect mobilities of 0.1 cm2/Vs for P3ATs of side chain length n = 5 – 9 on bottom-gate, bottom-contact transistor geometries with SiO2 as the dielectric. Thus while surface treatment of SiO2 is typically used to achieve high hole mobilities in P3AT FETs with a SiO2 dielectric layer, poor hole mobilities with bare SiO2 dielectrics originate from poor interfacial ordering of the P3AT layer during solution processing rather than the ability of the bare SiO2 interface to facilitate good interfacial charge (hole) transport. Furthermore, the demonstration of 0.1 cm2/Vs FET mobilities for P3ATs independent of alkyl side chain length suggests that alkyl side-chain length does not determine the ultimate mobility of P3ATs, rather alkyl side-chain length influences mobility only through changes in interfacial ordering in solution processed films.
9:00 PM - II9.40
Microcapillary Lasers Based on Emissive π-Conjugated Polymers Under a Wet Condition and Their Spectral Modulation.
Naoyuki Yamasaki 1 , Kimihiro Masuyama 1 , Akihiko Fujii 1 , Masanori Ozaki 1
1 Electrical, Electronic and Information Engineering, Osaka University, Osaka Japan
Show AbstractThe use of π-conjugated conducting polymers as functional materials has attracted much attention from fundamental and practical viewpoints because of their high processability, stability, and promising electrical and optical properties. The light-emitting devices utilizing π-conjugated polymers, such as polymeric lasers, are attractive applications. In this study, the optical properties, such as absorption, photoluminescence(PL), lasing, and refractive index, of poly(p-phenylenevinylene) PPV and poly(2-methoxy-5-dodecyloxy-p- phenylenevinylrne) MDDOPPV films under a wet condition were investigated. We used cyclohexane, which is poor solvent for both PPV and MDDOPPV, to obtain the wet condition of the polymer films.The absorption peaks, which correspond to the π-π* transition, of the polymer films in a vacuum and immersed in cyclohexane are located at 376 and 378 nm for the PPV film, and at 480 and 476 nm for the MDDOPPV film, respectively. The PL peaks of the polymer films in vacuum and immersed in cyclohexane are located at 500 and 499 nm for the PPV film, and at 588 and 579 nm for the MDDOPPV film, respectively. Although the PPV film exhibits the same spectra with or without immersion, the MDDOPPV film shows marked spectral changes, such as a blue shift and a decrease in emission around 630 nm.A uniform polymer film was obtained on the inner surface of a quartz microcapillary tube by a capillary action of the polymer solution. We demonstrated multimode lasing actions from cylindrical microcavities with the polymer films fabricated onto the inner surface of a quartz microcapillary tube. Lasing actions under the wet condition were also investigated by injecting cyclohexane into the capillary tube. We succeeded in producing multimode laser emission under the wet condition from the cylindrical microcavities with the polymer films. In the microcapillary with the MDDOPPV film, a spectral modulation of the laser emission was demonstrated. The center of the lasing spectrum shifted to a 15 nm shorter wavelength and the peak interval in the lasing spectrum increased from 0.80 nm to 0.94 nm when the microcapillary with the MDDOPPV film is used under the wet condition. From the ellipsometry measurements, a change in the refractive index of the MDDOPPV film induced by immersion in cyclohexane was observed. We consider the lasing spectral modulation to be a result of the change in the refractive index of the MDDOPPV film due to cyclohexane infiltration.
9:00 PM - II9.41
Photoreactive Conjugated P-type Semiconducting Materials for Thin Film Transistors and Photovoltaic Cells.
Mai Ha Hoang 1 , U Ra Lee 1 , Min Ju Cho 1 , Kyung Hwan Kim 1 , Dong Hoon Choi 1
1 Department of Chemistry, Korea University, Seoul Korea (the Republic of)
Show AbstractA number of researchers have attempted to synthesize π-conjugated small molecules, dendrimers, oligomers, and polymers because of their strong potential applications to electronics and optoelectronics such as in organic light-emitting diodes (OLEDs), organic field effect transistors (OFETs), and organic photovoltaic cells (OPVs). The development of these devices has generated a great deal of interest in the availability of solution processible organic semiconductors, which can combine good semiconducting properties, stability and patternability.We recently reported the facile synthesis of new p-type photoreactive conjugated dendrimers and polymers incorporating photoreactive end groups, which can undergo a photopolymerization without using a photoinitiator. A good correlation between theoretical calculations performed on model compounds and the experimental HOMO, LUMO, and band gap energies of the serial materials has been obtained. The solubility of molecules was highly improved for solution processing. The patterning process of organic semiconductors was carried out by photopolymerization of the reactive dienyl end groups, creating a crosslinked network. These materials were characterized by UV-visible absorption spectroscopy, photoluminescence spectroscopy, cyclic voltammetry, thermogravimetric analysis and differential scanning calorimetry. They also display a p-type semiconducting behavior and their electrical properties are investigated in detail. We also investigated the performance and the stability of bulk heterojunction type photovoltaic device before and after irradiating ultraviolet light to induce photocrosslinking. This study can be contributed to design of photopatternable semiconductor systems for electronic/optoelectronic application.
9:00 PM - II9.42
Change of Interface Dipole Energy With Stoichiometry of Copper Oxide Buffer Layer in Metal/Organic Interface.
Sungjun Kim 1 , Kihyon Hong 1 , Kisoo Kim 1 , Illhwan Lee 1 , Jong-Lam Lee 1
1 Graduate Institute of Advanced Materials Science & Department of Materials Science and Engineering, POSTECH, Pohang Korea (the Republic of)
Show AbstractThe performance of most organic optoelectronic devices depends on the carrier injection process at their metal/organic interface. A number of attempts have been conducted to enhance the hole-injection property of anode, such as inserting a high work function metal oxide buffer layer between anode and organics as the hole-injection layers. The effects of thickness of oxide hole injection layer on formation of interface dipole between hole injection layer and hole transport material is reported. However, the role of stoichiometry of oxide interlayer in reducing an hole injection barrier and interface dipole energy is not reported. In this letter, we investigated the formation of interface dipole between hole transport layer of 4,4’-Bis[N-(1-naphthyl)-N-phenylamino]biphenyl(α-NPD) and stoichiometry-controlled copper oxide(CuOx) coated Ag. Silver (Ag) is deposited on cleaned glass.(“Ag”). The samples were loaded into a thermal evaporator and 20 Å CuOx was deposited on Ag by evaporating Cu2O powder with deposition rate of 0.1 Å/s and 1 Å/s(“0.1 Å/s – CuOx”, “1 Å/s – CuOx”). Then, α-NPD with a thickness of 70 nm, tris(8-hydroxyquinoline) aluminum (Alq3, 40 nm) doped with the fluorescent dye C545T (1%), undoped Alq3(20 nm), LiF (1 nm), calcium (10 nm) and Ag (10 nm) layers were deposited in sequence. The operation voltage of devices Ag at current density of 1 mA/cm2 was 6.2 V. It decreased to 5.0 V and 3.1 V when CuOx layer deposited by 0.1 Å/s and 1 Å/s were inserted. The operation voltage corresponding to 5000 cd/m2 was 14.6 V for device without CuOx, 12.1 V and 10.2 V for device with CuOx layer deposited by 0.1 Å/s and 1 Å/s. Cu2p SRPES spectra of two kinds of samples shows that the intensity of CuO satellite peaks was decreased as deposition rate increased. The Cu2p3/2 peak separated into two components, Cu2O (“Cu+”) and CuO(“Cu2+”). The peak intensity of “Cu2+” centered at 933.7 eV decreased, but that of “Cu+” centered at 932.1 eV increased as deposition rate increased. It seems that stoichiometric oxide, Cu2O, could be formed with high deposition rate. The work function of Ag, 0.1 Å/s – CuOx and 1 Å/s – CuOx were calculated to be 4.49 eV, 5.02 eV and 4.98 eV. The work function shifted by about 0.96 eV, 1.04 eV and 0.76 eV after deposition α-NPD. The corresponding interface dipole energies were calculated to be -0.93 eV for Ag, -0.88 eV for 0.1 Å/s – CuOx and -0.72 eV for 1 Å/s – CuOx. The hole injection barrier height (ΦBH) at the α-NPD / w/o CuOx interface was 1.7 eV. In α-NPD on 0.1 Å/s-CuOx and 1 Å/s-CuOx, ΦBH was 1.16 eV and 0.92 eV. In conclusion, we have reported the effect of stoichiometry of copper oxide interlayer in reducing interface dipole energy. The dipole energy decreased -0.88 to - 0.72 eV as stoichiometric copper oxide, Cu2O, substitute non-stoichiometric CuOx. Thus, the Cu2O layer lowered the potential barrier for hole injection from Ag to α-NPD, reducing the turn-on voltage of devices.
9:00 PM - II9.43
Photoinduced Charge Transfer and Threshold Voltage Shifts in Organic Field Effect Transistors With P3HT/PCBM Bulk Hetero-junction Interfaces.
Byoungnam Park 1 , Avishek Aiyar 1 , Elsa Reichmanis 1
1 , Georgia Institutue of Technology, Atlanta, Georgia, United States
Show AbstractPhotoinduced charge transfer at the electron donor/acceptor interface is a crucial step in realizing organic solar cell (OSC) devices. To optimize the performance of the OSCs, charge transfer efficiency at the interface should be considered separating from the nanoscale morphology at the interface and carrier collection efficiency. Here, we studied the photoinduced threshold voltage shift, which is a measure of photoinduced charge transfer, in organic field effect transistors with P3HT/PCBM bulk hetero-junction interfaces. The shift in the threshold voltage to a more positive value of up to 40 V, equivalent to the carrier density of 4.3×1012 cm-2, for hole conduction in the P3HT/PCBM composite was observed under illumination with the wavelength ranging from 400 nm to 650 nm. The large threshold voltage shift in the P3HT absorption region indicates that the change in the threshold voltage is mainly due to the holes left in the P3HT arising from the transfer of the photogenerated electrons to the PCBM. The threshold voltage for electron conduction in the PCBM was changed by more than 110 V as well. The density of holes transferred to the PCBM from the P3HT under illumination increased with increasing photon flux, while external quantum efficiency decreased. Through a comparison of the magnitude of the threshold voltage shift and a composition of the P3HT/PCBM composite film, the composition is believed to determine the interface area as well as the formation of the conducting path for charge carriers. The mechanism of the threshold voltage change under illumination is discussed based on the energy difference between the the Fermi energy and the higest occupied molecular orbital of the P3HT/PCBM composite film.
9:00 PM - II9.44
Electronic Structure and Charge Injection Properties at Organic Interfaces With Contaminated Molybdenum Trioxide Films.
Jens Meyer 1 , Andrew Shu 1 , Michael Kroeger 1 , Antoine Kahn 1
1 Department of Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show AbstractThe impact of ambient exposure of a vacuum-evaporated Molybdenum trioxide (MoO3) film on the electronic structure of its interface with a hole-transport material, e.g. N,N’-diphenyl-N,N’-bis (1-naphthyl)-1,1’-biphenyl-4,4’-diamine (α-NPD), is studied by ultraviolet and inverse photoelectron spectroscopy (UPS, IPES), and current-voltage (I-V) measurements of hole-only devices. UPS and IPES show that freshly evaporated MoO3 exhibit deep lying electronic states with an electron affinity (EA) of 6.7 eV and ionization energy (IE) of 9.7 eV.[1,2] In comparison, values for the ambient contaminated MoO3 (cMoO3) are reduced by about 1 eV, to 5.5 eV and 8.6 eV, respectively. Upon deposition of a few angstroms of α-NPD on cMoO3, the photoemission onset rapidly shifts toward higher binding energy, which is equivalent to a downward shift of the vacuum level and reduction of the work function. The interface dipole is 0.9 eV, which is about half of the correspond dipole at the interface with clean MoO3, yet still remarkably high compared to dipoles generally obtained on other air-exposed, standard high work-function electrodes like Au. Given that the work function of the contaminated MoO3 film remains as high as 5.7 eV, a significant dipole must form upon formation of an interface with a material like α-NPD, which has an ionization energy of 5.4 eV. We recently proposed a new interpretation of the hole-injection from MoO3 into hole-transport materials such as α-NPD.[2] Given the small energy barrier of ~ 0.8 eV between the conduction band minimum (CBM) of MoO3 and the highest occupied molecular orbital (HOMO) level of α-NPD, hole injection into α-NPD corresponds to an electron extraction from the organic HOMO level via the CBM of MoO3. One of the most important results of the current study is that the energy level alignment between contaminated MoO3 and α-NPD remains identical to that obtained with freshly evaporated MoO3. Indeed, the air-exposed transition metal oxide film remains n-doped (by oxygen vacancies) and its surface retains a large enough work function. The molecular level alignment determined by UPS was verified by device experiments. I-V – characteristics of hole-only devices, consisting of a Au/ MoO3 10nm/ α-NPD 100nm/ Au structure using either freshly evaporated MoO3 or cMoO3, were measured in-situ. Both devices showed nearly identical hole-injection properties. The results are of particular importance for low-cost manufacturing processes, since such devices will be built in low-vacuum environment or will eventually involve exposure of various surfaces to ambient conditions.[1] M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, Organic Electronics 10, 932-938 (2009).[2] M. Kröger, S. Hamwi, J. Meyer, T. Riedl, W. Kowalsky, and A. Kahn, Appl. Phys. Lett. 95, 123301 (2009).
9:00 PM - II9.45
Electrical Response of Highly-ordered Organic Thin Film Metal-insulator-semiconductor Devices.
Mujeeb Ullah 1 , D. Taylor 2 , Reinhard Schwoediauer 3 , Helmut Sitter 1 , Siegfried Bauer 3 , Neyazi Serdar Sariciftci 4 , Th. Singh 5
1 Institute for Semiconductors and Solid State Physics, Johannes Kepler University Linz, Austria, Linz, Upper Austria, Austria, 2 School of Electronic Engineering, Bangor University, Bangor United Kingdom, 3 Soft Matter Physics, Johannes Kepler University Linz, Austria, Linz Austria, 4 LIOS, Johannes Kepler University Linz, Austria, Linz Austria, 5 Ian Wark Laboratory, Molecular and Health Technologies, CSIRO, Clayton, Victoria, Australia
Show AbstractA detailed investigation has been undertaken of the electrical properties of organic field-effect transistors (OFETs) and metal-insulator-semiconductor (MIS) capacitors formed from highly ordered thin films of C60 as the active semiconductor and divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as the gate dielectric. Conventional current-voltage measurements show the OFETs to be n-channel devices characterized by a high electron mobility (~6 cm2/Vs) while standard capacitance-voltage (C-V) measurements at 1 kHz suggest the formation of a depletion region in the MIS capacitors as expected in n-doped semiconductors. However, admittance spectroscopy measurements undertaken on the capacitors over a wide frequency range show that the C-V behavior originates from an edge effect. Furthermore, by developing an equivalent circuit to describe the device and fitting the calculated response of the circuit to the experimentally obtained admittance spectra, we demonstrate enhanced injection of extrinsic charge carriers (electrons) into the C60 under increasing gate voltages, which is accompanied by a reduction in the insulation resistance of the BCB presumably by electron injection into the insulator. These measurements highlight the need for proper device design to ensure the unambiguous interpretation results obtained from such devices. References: 1- Th. B. Singh, N. Marjanovic, G. J. Matt, S. Günes, N. S. Sariciftci, A. Montaigne Ramil, A. Andreev, H. Sitter, R. Schwödiauer and S. Bauer, Organic Elec. 6, 105 (2005).2- I. Torres, D. M. Taylor, and E. Itoh, Appl. Phys. Lett. 85, 314 (2004).
9:00 PM - II9.46
Efficient Blue Light Emitting Solution Based Hetero-structure Polymer Devices.
Stefan Sax 1 , Nicole Rugen-Penkalla 2 , Alfred Neuhold 1 , Sebastian Schuh 3 , Egbert Zojer 3 , Emil J.W. List 1 3 , Klaus Muellen 2
1 , NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz Austria, 2 , Max Planck Institute for Polymer Research, Mainz Germany, 3 Institute of Solid State Physics, Graz University of Technology, Graz Austria
Show AbstractThe development of the first polymer light emitting device (PLED) by Burroughes et al. in1990 had a tremendous impact on industrial as well as scientific research due to the potential applications of PLEDs in large area display or solid state lighting applications.For both implementations, color stable and efficient electroluminescent devices in the red, green and blue spectral range are needed. To achieve the targeted lifetime beyond ten thousand of hours, several requirements must be met such as chemical stability of the active materials as well as very low concentrations of chemical defects and other impurities. In contrast to polymer based devices, such multilayer hetero-structures can simply be realized with small molecules by using various thermal vacuum deposition techniques. Within the scope of industrial mass production, organic light emitting devices based on small molecules suffer from the high vacuum deposition process steps, which clearly limit the size of the targeted application. Alternatively, large area applications can be easily realized from solution based processable organic semiconductors allowing for cost effective printing processing techniques and roll-to-roll fabrication.A basic design feature generally used in inorganic as well as organic solid state semiconductor devices is the so called hetero-structure, combining semiconductors with different band-gaps, band offsets and/or transport properties to yield enhanced device performance. So far various approaches toward hetero-structure device architectures enhancing the efficiency have been demonstrated for organic solid state devices. Auxiliary injection layers, doped interface regions or multiple tandem structures comprisingCs2CO3/MoO3 interconnecting layers have been used to enhance and balance charge injection and transport as well as exciton formation in the device. In this study a new methanol soluble polyfluorene with non-ionic side chains is reported, where different ethylene glycol oligomers are introduced at the 9-position of the fluorene. Due to the polar side chains this polymer can be coated from solution based on orthogonal solvents onto a polymer layer without a partial re-salvation. Using this new polymer in a ITO/PEDOT:PSS/PIF/PFpolar/CsF/Al heterostructure PLED configuration device efficiency could be enhanced by a factor of 3-5. From spectral measurements as well as quantum mechanical calculations the origin of the significant efficiency enhancement was identified as a consequence of enhanced exciton formation at the organic semiconductors interface in the device.
9:00 PM - II9.47
Synthesis and Nanostructure Control of All-conjugated Diblock Copolymer for the Application to Organic Electronic Devices.
Yue Zhang 1 , Keisuke Tajima 1 , Kazuhito Hashimoto 1 2
1 Applied Chemistry, the University of Tokyo, Tokyo, Tokyo, Japan, 2 HASHIMOTO Light Energy Conversion Project, ERATO, Japan Science and Technology Agency (JST), Kawaguchi, Saitama, Japan
Show AbstractControlling the orientation and the nanostructures of semiconducting polymers in solid state is of crucial importance for the development of high-performance polymer electronic devices such as organic field-effect transistors (OFETs) and photovoltaic devices (OPVs). To precisely control the nanostructure in polymer films, one interesting strategy is to use block copolymers (BCPs) containing two different semiconducting polymer blocks. BCPs are well known to self-organize into 3-D microphase-separated structures driven by the factors such as immiscibility or the crystallinity difference between the blocks. We have reported the synthesis of an all-conjugated BCP, poly(3-hexylthiphene-block-3-ethylhexylthiophene) (P(3HT-b-3EHT)) with well-controlled molecular weight and narrow PDI by a living GRIM polymerization [1]. In this presentation, we systematically studied the relationship between the block ratios of BCPs and their nanostructure formation. We used AFM and glazing angle X-ray diffraction to elucidate the film structures and orientation of the polymer chains in the nanopatterns. Compared with the corresponding random copolymers and the physical mixtures, it is clearly demonstrated that the difference between the crystalline natures of P3HT and P3EHT blocks in P(3HT-b-3EHT)s is an important factor for the formation of the microphase separated structures [2]. The bulk heterojunction photovoltaic devices using the BCP (83% of P3HT) and PCBM (1:0.8, w/w) showed the highest PCE of 3.42% after thermal annealing, while the reference P3HT:PCBM (1:0.8, w/w) device showed PCE of 3.39%. UV-vis spectra showed that better π-stacking of thiophene rings are formed in BCP compared to those in P3HT homopolymer, which can explain high performance in BCP:PCBM devices even though an amorphous P3EHT block is introduced in the polymer backbones. In OFET devices, although less crystalline P3EHT fractions were introduced, as-cast film of BCP (83% of P3HT) showed the highest hole mobility of 0.14 cm2 V-1s-1, which was twice higher than that of the OFET with P3HT homopolymers (0.07 cm2 V-1s-1, ). UV-vis spectra showed better π-stacking of the thiophene rings in the BCP as-cast films, which indicated amorphous P3EHT fractions enhanced the crystallinity of P3HT fractions probably during the spin-coating process. Temperature dependence of the hole mobility showed that the activation energy in BCP is higher than that in P3HT homopolymer, which could be attributed to the energy barrier between P3HT and P3EHT domains during the hopping process.Reference:[1] Yue Zhang, Keisuke Tajima, Kosuke Hirota and Kazuhito Hashimoto, J. Am. Chem. Soc., 130, 25, 7812 (2008).[2] Yue Zhang, Keisuke Tajima and Kazuhito Hashimoto, Macromolecules, 42, 18, 7008-7015 (2009).
9:00 PM - II9.48
Humidity Dependence Study on Electronic Property of PPy:PSS by Kelvin Probe Force Microscopy.
Ling Sun 1 , Jianjun Wang 1 , Gerhard Wegner 1 , Hans-Juergen Butt 1 , Elmar Bonaccurso 1
1 , Max Planck Institute for Polymer Research, Mainz Germany
Show AbstractA conjugated polymer blend is a mixture of a conductive polymer and a nonconductive matrix. The conductive polymer contributes to the electronic conductivity and the nonconductive polymer increases the processibility and the optical transparency of the blend. They can form conductive thin films via solution processing, i.e. spin-coating, ink-jet printing, or drop coating. Such properties make the conjugated polymer blends extremely interesting in the organic semiconducting device applications, i.e. polymer light-emitting diodes (PLEDs), solar cells, field effect transistors (FETs), etc. Due to the two-phase nature of conjugated polymer blends, their nanoscale morphology, e.g. the phase separation, domains, grain boundaries, etc. influences the opotoelectrical properties greatly. Therefore the study of nanoscopic structure of conjugated polymer blends is of great interest.Here we present a humidity dependence study of our newly synthesized conjugated polymer blend, Polypyrrole:Polystyrenesulfonate (PPy:PSS) by Kelvin probe force microscopy (KPFM). PSS is a polyelectrolyte which swells under high humidity, while PPy is an infusible and insoluble conjugated polymer in its conductive state. We stabilized PPy with PPS to obtain a water soluble conductive polymer. Under high humidity, i.e. above 40% relative humidity, water selectively adsorbs onto the PSS phase. This heterogeneous adsorption of water causes a change in electrostatic force between the sample and the cantilever tip. We measure simultaneously surface potential and the second harmonic of the electrostatic force during the KPFM measurement. The surface potential is related to the material work function and the second harmonic is proportional to the material dielectric property. They show different response with increasing relative humidity. We compare the change of both parameters from dry condition to up to 95% relative humidity. The results help us to analyze the phase separation of PPy and PSS and predict the nanoscopic structure of the blend.
9:00 PM - II9.49
Built-in Potential of a Pentacene Pin Homojunction Studied by Ultraviolet Photoemission Spectroscopy.
Selina Olthof 1 , Hans Kleemann 1 , Bjoern Luessem 1 , Karl Leo 1
1 Institut für Angewandte Photophysik, TU Dresden, Dresden, Sachsen, Germany
Show AbstractThe p-i-n homojunction is the archetype of inorganic semiconductor devices. Recently, it was shown by Harada et al. [1] that the concept can be realized in organic semiconductor devices as well: Diodes with excellent electrical properties and large built-in voltages could be realized.In this work, we investigate the energetic alignment in an organic p-i-n homojunction using ultraviolet photoelectron spectroscopy (UPS). The device is made of Pentacene and is doped by small molecules. The complete p-i-n structure is incrementally deposited on a silver substrate and at each step UPS is used to measure the energy levels which provide information on the interface dipoles and level bending effects present at the contacts and in the device. From the change in work function between the p- and n-doped layers, we can determine the built-in potential of this junction to be 2.1eV. However, the polarizability of the Pentacene layer changes upon doping, leading to a higher ionization potential for the p-doped layer and a lower one for the n-doped layer when compared to the intrinsic value. Due to this effect, the built in voltage in the transport layers is only 1.7eV. The results gained by UPS are verified with experiments performed by current-voltage measurements and impedance spectroscopy.[1] Harada, Kentaro and Riede, Moritz and Leo, Karl and Hild, Olaf R. and Elliott, C. Michael, Physical Review B, 77(2008) 195212
9:00 PM - II9.5
Influence of Dielectric Surface Modification on the Optical Properties of 2, 5-bis(4-biphenyl)Bithiophene Single Crystals.
Yan Wang 1 , Ryotaro Kumashiro 1 , Shunsuke Kono 2 , Hiroyuki Yokoyama 2 , Ryo Nouchi 1 , Katsumi Tanigaki 1
1 World Premier International Research Center and Department of Physics, Tohoku University, Sendai Japan, 2 New Industry Creation Hatchery Center, Tohoku University, Sendai, 980-8579, Japan
Show AbstractOrganic field-effect transistors (OFETs) based on organic single crystals are attracting much attention because they could offer the opportunity to investigate the physical mechanisms of carrier injection, accumulation and transport.1,2 Electrons and holes can be injected simultaneously into organic single crystals from source and drain electrodes, and then accumulated in the semiconducting layer so that excitonic states can be created to give light emission. The scientific community is pursuing an electrically-driven organic laser constructed from organic single crystal FETs because of their high current density and various prohibition processes away from non-radiative losses, such as exciton dissociations and/or annihilations near electrodes.1From the technological point of view, it is very important to observe spectrally narrowed emissions (SNE) on Si and SiO2 substrates because it is easy to fabricate actual photonic devices and electronic devices on Si substrates. However, the efficiency of electro-luminescence is not sufficiently high due to the trapping of carriers, especial electrons, on the surface of Si substrate. Therefore, it is necessary to modify the surface of Si substrate in order to maximize the effective exciton density between the channels of FET in the vicinity of a dielectric gate insulator surface. The modification layer would greatly influence the optical properties obtained in organic single crystal FETs. In the present study, we have investigated the influences of modification layer on the optical properties of organic single crystal by using optical pumping. We chose a 2,5-bis(4-biphenyl)bithiophene (BP2T) single crystal as the organic semiconductor because it possesses high luminescence efficiency and ambipolar electrical conducting behavior, and furthermore can provide self-waveguided propagation of the emission.3 The surface of dielectric SiO2 gate insulators is modified by PMMA, parylene C, parylene N and cytop thin films. Comparison of the optical properties of BP2T among these substrates will be presented. The investigation of organic gate insulators on optical properties of organic single crystal semiconductors would greatly help towards the realization of electrically-driven laser in the future technology.1T. Takenobu, S. Z. Bisri, T. Takahashi, M. Yahiro, C.Adachi, Y. Iwasa, Phys. Rev. Lett. 100, 066601 (2008).2V. Podzorov, S. E. Sysoev, E. Loginava, V. M. Pudalov, M. E. Gershenson, Appl. Phys. Lett. 83, 3504 (2003).3Y. Wang, R. Kumashiro, Z. Li, R. Nouchi, K. Tanigaki, Appl. Phys. Lett. 95, 103306 (2009)
9:00 PM - II9.50
Investigations on a Novel, Highly Emissive, Non-aggregating Pyrene-based Polymer for Blue Light Emitting Devices.
Roman Trattnig 1 2 3 , Pablo Del Rosso 4 , Teresa Figueira-Duarte 4 , Stefan Sax 2 , Emil J.W. List 1 2 , Klaus Muellen 4
1 Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 , NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz Austria, 3 Christian Doppler Laboratory for Nanocomposite Solar Cells, NanoTecCenter Weiz Forschungsgesellschaft mbH, Weiz Austria, 4 , Max Planck Institute for Polymer Research, Mainz Germany
Show AbstractIn the recent years organic light emitting diodes (OLEDs) and their applications have drawn a lot of attention within industrial and academic research. Up to now a lot of different organic chromophores, small molecules as well as conjugated polymers, were investigated which exhibit luminescent properties over a wide range in the visible spectrum. Sufficient lifetimes and efficiencies for commercial use could be demonstrated either for small molecules or polymers emitting green or red light. However, reports dealing with highly efficient and stable blue light emitting materials for electroluminescent applications are still rare. This fact mainly originates from high degradation tendencies resulting for example in undesirably altered emission colours or low quantum efficiencies.Pyrene is one of the most important and best investigated conjugated materials, thats attraction can be traced back to the fact that it exhibits exceptionally long fluorescence lifetime, a high affinity for forming excimers and a considerably good sensitivity of its excitation spectra to microenviroment changes. Over the last 50 years the excimer formation has been widely used in the investigation of water soluble polymers, which was turning pyrene into the most frequently used dye in fluorescent labelled polymers.On the other hand, exactly this propensity for the formation of excimers kept chemical stable and high quantum yield exhibiting pyrene out of applications in organic light emitting diodes.Here we present single layer OLEDs based on a novel, non-aggregating polypyrene which shows high solid state quantum yield exhibiting blue emission, excellent solubility in various solvents and high electro-optical performance in single-layer OLEDs as well as absolutely no excimer emission in the solid state.
9:00 PM - II9.52
Hybrid Polymer Light Emitting Diodes Using Carbon Nanotubes – A Study on Device Performance Correlated With a Photophysics Study.
Li Wei Tan 1 , Gianluca Latini 1 2 , Franco Cacialli 1 3 , Ravi Silva 1
1 Advanced Technology Institute, University of Surrey, Guildford United Kingdom, 2 Material Science and Engineering, Imperial College, London United Kingdom, 3 Department of Physics and Astronomy, University College of London, London United Kingdom
Show AbstractWe report on the detailed investigation of the effects of polymer light emitting diodes (PLEDs) in term of the optical properties and device characteristics using solution processable carbon nanotubes (including single walled and multiwalled CNT) interlayer in a hybrid system. Single layer and double layers PLED devices were fabricated using yellow/green emitting electron-transport polyfluorene, poly(9,9’-dioctylfluorene-alt-benzothiadiazole) (F8BT) and poly(2,7-(9,9-di-n-octylfluorene)-alt-(1,4-phenylene-((4-sec-butylphenyl)imino)-1,4-phenylene))(TFB) as the electron blocking layer. The external quantum efficiency (EQE) of the double layers of PLEDs with CNTs improve by up to two magnitudes of order, from in comparison to single layer PLED devices. A direct investigation on the influence of the CNT and ITO anode on the PL quantum efficiency of F8BT in time resolved and steady state were also studied using time correlated single photon counting (TCSPC). This data combined with the measure of the radiative singlet lifetime, by time resolved PL, the non-radiative decay rate (knr) is extrapolated. The PL efficiency of ITO/F8BT, o-SWCNTs/F8BT and o-MWCNTs/F8BT are 0.4, 0.34 and 0.31%, respectively. After the extrapolation, the knr for these three samples were found to be 0.825, 0.875 and 0.89 ns-1, respectively. The higher values of knr for CNTs system testify the presence of a non-radiative channel in this system. The non-radiative channel introduced by CNTs with large PL quenching were largely reduced by inserting an electron blocking layer, TFB between the CNT and F8BT layer. This result gives a clear explanation as to the reasons for the EQE improvement observed on double layer PLED hybrid system.
9:00 PM - II9.53
The Ultrafast Dynamics of Electronic Excitations in Pentacene Thin Films.
Henning Marciniak 1 , Bert Nickel 2 , Stefan Lochbrunner 1
1 Institut für Physik, Universität Rostock, Rostock Germany, 2 Fakultät für Physik and CeNS, Ludwig-Maximilians-Universität, München Germany
Show AbstractPentacene is deemed to be one of the most promising candidates for applications in organic electronics like transistors and OLEDs. In such optoelectronic devices the behavior of electronic excitations is crucial for the performance. Furthermore pentacene serves as model compound to understand the electronic properties of organic crystals. While the pentacene monomer fluoresces with a high quantum yield, pentacene crystals show only an extremely low photoluminescence pointing to the relevance of ultrafast relaxation processes. To investigate this dynamics, we apply pump-probe absorption spectroscopy with a time resolution of 30 fs to microcrystalline pentacene films. To discriminate between singlet and triplet signatures, measurements were performed with different angles of incidence. This is possible since the molecules are standing almost upright on the substrate and the transition dipole of the triplet-triplet absorption is aligned along the long molecular axis while the S1←S0 transition dipole is aligned along the short molecular axis.The presented measurements show that in microcrystalline pentacene films the primarily excited Frenkel excitons decay within 70 fs to a non fluorescing singlet species [1]. Contrary to expectations fission into triplets is only a minor channel. We propose that the ultrafast relaxation of the photoexcited excitons leads to a species very similar to excimers. Neighboring pentacene molecules form dimers which are bound stronger in the electronically excited state than in the ground state. The relaxation time probably reflects geometric rearrangements like a rotation around the long molecular axis and a reduction of the intermolecular distance to increase the overlap of the π-orbitales and the interaction strength. The resulting gain in energy should be in the order of a few 100 meV. In this case the excimer energy is smaller than two times the triplet energy and fission into triplets should be a thermally activated process. This results in a picosecond rate for the triplet formation directly after the excimers are generated and as long as the released energy has not yet dissipated. After this energy is distributed over a larger sample region the local temperature has dropped to such an extent that the fission rate becomes irrelevant. Accordingly, only a minor fraction of the excitons is transformed into triplets. Like in H-aggregates the radiative transition of the excimer to the ground state is electric dipole forbidden due to symmetry reasons. The excimer formation therefore provides an efficient mechanism to turn off the emission within 70 fs as it is observed in the experiment. [1] H. Marciniak, I. Pugliesi, B. Nickel, and S. Lochbrunner, Phys. Rev. B 79 (2009), 235318.
9:00 PM - II9.55
Quantum Dot Light-emitting Electrochemical Cells.
Amanda Norell Bader 1 2 , Janelle Leger 1 2
1 Physics & Astronomy, Western Washington University, Bellingham, Washington, United States, 2 Chemistry, Western Washington University, Bellingham, Washington, United States
Show AbstractSemiconducting polymers have enormous potential to expand the applications of electronic and optoelectronic technologies; they are solution-processable and can be deposited via screen printing or ink-jet printing, allowing inexpensive fabrication at low temperatures over large areas and on flexible substrates. While polymer optoelectronic devices have been successful, they typically suffer from poor performance and stability. To circumvent this difficulty, we are exploring a novel polymer light-emitting electrochemical cell (LEC) structure incorporating colloidal quantum dots as the active emitter. Quantum dots boast size-tunable band-gap energies and improved stability over organic emitters , but their efficacy in a typical hybrid LED structure is limited by an insulating surface ligand layer that increases the charge tunneling barrier. This often leads to significant unwanted emission from the polymer host material. Typically, hybrid LEDs are constructed with a monolayer of quantum dots in order to overcome this limitation with moderate success. An LEC structure, however, presents an alternative solution to this problem by limiting the thickness of the emissive region of the polymer/quantum dot film. Emission spectra of quantum dot LECs employing a commercially available green-emitting PPV/PF copolymer and CdSe/ZnS core-shell quantum dots show better color purity than polymer-only devices, as well as pure emission from the quantum dots, regardless of operation voltage. This device structure therefore has the potential to improve a number of issues currently limiting the performance of polymer based optoelectronic devices. We will discuss the potential of this approach for application to solid-state lighting and photovoltaics.
9:00 PM - II9.56
Non-volatile Organic Memory Elements Based on Carbon Nanotube Enabled Organic Vertical Field Effect Transistors.
Bo Liu 1 , Mitchell McCarthy 2 , Andrew Rinzler 1
1 Department of Physics, University of Florida, Gainesville, Florida, United States, 2 Department of Material Science & Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractHigh performance non-volatile organic memory elements based on carbon nanotube enabled organic vertical field effect transistors (CN-VFETs) are demonstrated. A thin cross-linking polymer layer on top of the gate dielectric acts as the charge storage layer. This results in a large, gate sweep tunable, hysteresis in the cyclic transfer curves exhibiting on/off ratios > 4 orders of magnitude. The carbon nanotube (CNT) random network source electrode facilitates charge injection into the organic charge storage layer, realizing the strong memory effect without sacrificing mobility in the vertical channel. The unique stack structure and vertical current pathway in the CN-VFET should allow for aggressive downsizing of the memory elements without the expense of high resolution patterning. Given their intrinsically simple fabrication and compact size CN-VFETs could provide a path to cost-effective, high density organic memory devices.
9:00 PM - II9.57
Controlling Charge Injection in Organic Infrared Up-conversion Devices.
Dong Woo Song 1 , Do Young Kim 1 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractIntegrating a near infrared absorbing organic photodetector and an organic light-emitting diode enables realization of infrared up-conversion devices with very low operating voltage. However, due to the similarity between the work function of indium-tin oxide (ITO) and the highest occupied molecular orbital (HOMO) level of IR absorbing materials such as tin (II) phthalocyanine (SnPc), hole injection from ITO starts even at low voltages, thus contributing to the device noise. In order to achieve high sensitivity, the dark current needs to be suppressed. In this presentation, we report the up-conversion devices with different hole injecting materials.We have fabricated a control device, devices with different IR sensitizing mixed layer of SnPc:C60, and devices with different hole blocking layers. The control device has the following structure: ITO/SnPc:C60 (1:1 ratio, 20 nm)/1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) (45 nm)/7% fac-tris(2-phenylpyridinato)iridium(III) (Ir(ppy)3):4,4-N,Ndicarbazole-biphenyl (CBP) (30 nm)/tris[3-(3-pyridyl)-mesityl]borane (3TPYMB) (45 nm)/LiF(1 nm)/Al (100 nm). In addition, we also used p-bis(triphenylsilyly)benzene (UGH2) and bathocuproine (BCP) layer as a hole blocker between the ITO and the IR absorbing layers.The luminance-current-voltage (LIV) characteristics were measured with and without 830 nm infrared light irradiation. Without infrared light irradiation, emission was not detected until 14.5 V in the up-conversion device with a 10 nm thick BCP layer, compared with the control device where the emission turn-on was at 6 V. The higher turn-on voltage shows that hole injection from ITO was effectively suppressed by the hole blocking layer. When the device was irradiated with infrared light, photo-generated holes were injected to the light-emitting layer and the device turned on at 3 V, resulting in an OLED current efficiency of 80cd/A.
9:00 PM - II9.58
Charge-modulation Spectroscopy of Trapped Polarons in Pentacene Thin-film Transistors.
Simon Haas 1 , Hiroyuki Matsui 1 2 , Tatsuo Hasegawa 1
1 Photonics Research Institute, AIST, Tsukuba Japan, 2 Dept. of Advanced Materials Science, The University of Tokyo, Kashiwa Japan
Show AbstractMarked progress in organic field-effect devices allows exploring fundamental aspects of semiconducting molecular solids. In this study, we use thin-film transistor (TFT) structures to independently detect applied-field and charge-induced effects on the optical absorbance of pentacene. We found that the obtained field- and charge-modulation (FM/CM) spectra give particular insights into the nature of electronic and polaronic excitations in pentacene. In the experiment, we used pentacene TFTs composed of a parylene gate insulator and semitransparent gold electrodes on a fused-silica or CaF2 substrate. By applying pulsed gate bias or, alternatively, dc bias modulated by a small ac voltage, modulated transmission spectra with a high sensitivity were recorded by using lock-in technique; field modulation is obtained by applying positive gate bias, while the charge density is modulated by negative gate bias.The observed FM and CM spectra show very different features: For FM, there is a homogeneous red-shift of the absorption of pentacene for all the UV/VIS spectral range up to 2.5 eV due to the Stark effect, as expressed by the close resemblance with the first derivative of the absorbance and the quadratic electric-field dependence. In sharp contrast, for hole injection in case of CM, the spectrum resembles bleaching (=decrease) of the absorption due to the depopulation of the HOMO level. The observed CM spectrum, however, is not as simple and shows additional, unusual features: 1) much stronger apparent bleaching of HOMO-LUMO transition than expected from the induced carrier number, 2) appearance of strongly enhanced absorption around 2 eV, and 3) appearance of weak enhanced absorption in the near-IR range around 0.8-1.0 eV. From these features it is clear that an induced hole causes an electronic and vibrational energy reconstruction for the surrounding molecules. Thus this energy reconstruction, rather than conventional bleaching, is responsible for the observed CM features. We conclude that the trapped polarons are extended over several molecules, which is compatible with shallow traps enabling high-mobility trap-and-release carrier transport in organic TFTs.
9:00 PM - II9.59
Light Emitting Electrochemical Cells Using Transparent Films of Single Walled Carbon Nanotubes as Electrodes.
Evan Donoghue 1 , Aubrey Dyer 2 , Justin Oberst 2 , Egle Puodziukynaite 2 , John Reynolds 2 , Andrew Rinzler 1
1 Department of Physics, University of Florida, Gainesville, Florida, United States, 2 The George and Josephine Butler Polymer Laboratories, Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractWe report on the fabrication of two types of light emitting electrochemical cells (LECs) using thin conductive films of single walled carbon nanotubes (SWNTs) as the transparent electrode. In the first architecture, devices are fabricated using a SWNT film and Au electrode sandwiching an emissive active layer consisting of poly[2-methoxy-5-(2'-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) admixed with the solid electrolyte poly(ethylene oxide) and lithium trifluoromethanesulfonate. The performance is compared to a conventional LEC in which tin-doped indium oxide was used as the transparent electrode. In the second architecture we exploit the insensitivity of LEC performance to the electrode work function by replacing the Au electrode with a second SWNT film such that both electrodes are transparent. This yields a device that emits light in both directions while otherwise transparent with the ability to be mechanically flexible.
9:00 PM - II9.6
Mechanism of Passivation-induced Degradations of Polymer Thin-film Transistors: Effects of Solvents and Passivating Polymers.
Yu Fu 1 , Tarng-Shiang Hu 2 , Feng-Yu Tsai 1
1 , National Taiwan University , Taipei Taiwan, 2 , Industrial Technology Research Institute, hsinchu Taiwan
Show AbstractThis study investigates the effects of the solvent and the chemical structure of encapsulating polymers on the encapsulation-induced degradations of poly(3-hexylthiophene) (P3HT) organic thin-film transistors (OTFTs). 9 solvents and 10 vinyl polymers with different side groups were tested on bottom-gate, bottom-contact P3HT OTFTs, and the following conclusions were obtained: (1) the solvents caused only temporary degradation to the OTFTs, which was fully reversible upon removal of the solvents; (2) the encapsulating polymers resulted in various degrees of irreversible doping effect on the OTFTs; (3) the encapsulating polymers with highly polar side groups, including -OH, halogens and nitrile, increased the off current by 2-4 orders of magnitude and rendered the OTFTs dysfunctional; (4) the encapsulating polymers with less polar side groups, including alkyl, phenyl, carbonyl, ether and ester, caused negligible doping effect. By taking into account these sources of degradation as well as the H2O/O2 permeability of encapsulating polymers, we designed and demonstrated an effective 2-layer encapsulating film, which caused no encapsulation-induced degradation and achieved > 2000 h of lifetime in the encapsulated OTFTs.
9:00 PM - II9.60
A Simple Model to Estimate the Recombination Efficiency of Organic Light-emitting Diodes.
Changgua Zhen 1 , Zhikuan Chen 2 , John Kieffer 1
1 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 , Institute of Materials Research and Engineering, Singapore Singapore
Show AbstractA simple analytical model is developed to estimate the recombination efficiency in organic light-emitting diodes (OLEDs). This model is established on Marcus’ electron transfer theory. Based on the assumption that current flow is transport-limited instead of injection limited, the recombination efficiency is mainly determined by the difference of the reorganization energies for electron and hole hopping between emitters. A comparison of the prediction of our model and the experimental data confirms its utility for materials design. Based on this model, we have designed and achieved very high efficient emitters for OLEDs.
9:00 PM - II9.61
Tuning the Optical Characteristics of Organic Light-emitting Diodes Based on Multilayer Transparent Electrodes.
Hyunsu Cho 1 , Changhun Yun 1 , Seunghyup Yoo 1
1 Electrical Engineering department, KAIST, Daejeon Korea (the Republic of)
Show AbstractMultilayer based on dielectric-metal-dielectric (DMD) structure has recently been demonstrated as an effective alternative transparent electrode in applications such as highly flexible organic light-emitting diodes (OLEDs) where conventional ITO electrodes are known to have a weakness[1,2]. Here we further demonstrate that the optical properties of these DMD-based transparent electrodes can be widely tuned to fulfill the requirements of a target OLED application. Upon careful control of the competition between Febry-Perot resonance condition and transmittance/ reflectance spectra of DMD electrodes, near-Lambertian emission and 2.2X improvement in forward luminous efficiency are demonstrated, respectively, for example. Optimization strategies oriented toward high luminous efficiency, ideal emission characteristics, or balancing between those two, are proposed and compared with experimental results.Reference[1] H. Cho, C. Yun, J.-W. Park, and S. Yoo, Org. Electron., 10, 1163 (2009) [2] C. Yun, H. Cho, H. Kang, Y. Lee, Y. Park, and S. Yoo, Appl. Phys. Lett., 90, 053301 (2009)
9:00 PM - II9.62
Using Diblock Copolymer Approach to Improving Performance of Organic Photovoltaic Cells.
Guoqiang Ren 1 2 , Pei-Tzu Wu 1 2 , Samson Jenekhe 1 2
1 Department of Chemical Engineering, University of Washington, Seattle, Washington, United States, 2 Department of Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractPolymer solar cells have drawn world-wide attention due to their great potential as a green, flexible and low-cost renewable energy source. Block copolymers have recently been proposed as a promising candidate due to their unique ability to self-assemble into nanoscale structures that is compatible with exciton diffusion in polymer solar cells. Meanwhile, by incorporating blocks with different functionalities into one block copolymer, both the electronic structures and morphology can be tuned towards achieving high power conversion efficiency in polymer solar cells. To demonstrate the advantageous applications of block copolymers, we have studied the photovoltaic properties of a series of diblock copoly(3-alkylthiophene) with different block compositions. By blending them with a fullerene derivative, we found that there is a strong dependence of photovoltaic properties on the block composition, and the maximum power conversion efficiency (PCE) of block copolymers can be increased by factors of 1.6 and 6 times of the PCE achieved from the parent homopolymers. These diblock copolymers are crystalline and form nanofibers in the blend thin films with nanoscale donor/acceptor phase separation. An interpenetrating, bicontinuous network for charge transport is obtained within the active layer. Space-charge limited current (SCLC) mobility of holes in diblock copolymers are one- and two-order of magnitude higher compared with the homopolymers. Our results showed block copolymer approach represents a promising way to enhancing the photovoltaic properties of known homopolymers.
9:00 PM - II9.63
Grafting of Poly(3-hexylthiophene) Brushes on Oxides Using Click Chemistry.
Peerasak Paoprasert 1 , Josef Spalenka 1 , Paul Evans 1 , Padma Gopalan 1
1 Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States
Show AbstractA challenging problem in organic-inorganic nanocomposite electronic devices is to ensure infiltration or complete wetting of the conjugated polymers onto high-surface-area inorganic semiconductors. Processing conditions such as choice of the spin-coating solvent and thermal history of the blend have been found to affect the overall device performance. Chemical modifications directly on the semiconductor surface by covalent anchoring of organic molecules can lead to a well defined interface with the conjugated polymer, but have not yet been explored. Covalent anchoring of the conjugated polymers directly onto the oxide surfaces could provide a means for controlling the critical conjugated polymer/inorganic interface relevant to many organic electronic applications. We have developed a versatile “grafting-to” approach based on Cu(I)-catalyzed Huisgen 1,3-dipolar cycloaddition between an azide and alkyne to form a triazole ring to anchor poly(3-hexylthiophene) (P3HT) chains directly onto silicon oxide substrate functionalized with a monolayer of azide. Regioregular ethynyl-terminated P3HT with molecular weight of 5900 g/mol and polydispersity of 1.2 was synthesized by catalyst-transfer polycondensation using Grignard metathesis mediated by a nickel-based catalyst. The azide SAM was prepared from bifunctional molecules containing azide and siloxane as click reaction precursor and surface linker, respectively. The grafted P3HT brushes were characterized by atomic force microscopy, ellipsometry, X-ray photoelectron spectroscopy, infrared reflection absorption spectroscopy, and UV-visible spectroscopy. The grafting of P3HT brushes was studied as a function of click reaction time and the growth of the brushes is governed by a diffusion-controlled process. P3HT brushes were prepared on the pre-fabricated field-effect transistor structures in order to probe the electrical properties of the brushes. The saturation hole mobility for P3HT brushes was 5 x 10-5 cm2/Vs, which is comparable to hole mobilities in FETs with monolayer thick dip-coated P3HT. We have further shown the generalization of this chemistry to a wide band gap semiconductor ZnO, which is especially challenging as it requires mild conditions to prevent etching of the substrate. Our work therefore provides the proof-of-concept for an approach where well-defined conjugated polymers can be combined with the efficient azido and alkynyl groups coupling reaction to graft conjugated polymers to a range of oxide substrates. The versatile synthetic methodology developed in this work can be generalized to prepare a wide variety of semiconducting conjugated polymer brushes on oxide surfaces relevant to organic electronic devices.
9:00 PM - II9.64
Polymerizable Ionic Liquids for Fixed Junction Light-emitting Electrochemical Cells.
Ilya Kosilkin 2 , Matt Martens 1 , Michael Murphy 1 , Janelle Leger 1
2 Chemistry, University of Washington, Seattle, Washington, United States, 1 Physics, Western Washington University, Bellingham, Washington, United States
Show AbstractFixed junction light-emitting electrochemical cells (LECs) have been shown to offer several advantages over traditional dynamic junction LECs such as faster turn on times, the ability to do photovoltaic work, and improved life-time. Recently we have reported fixed junction LECs that use ion-pair monomers (IPMs), ionic materials capable of covalent bond formation, in order to eliminate ionic mobility. These devices showed promising diode-like behavior and moderate overall performance. However, the materials presented in our previous work had several disadvantages, including poor miscibility of IPMs with electroluminescent polymers and poor ion dissociation/mobility. It has been shown previously that ionic liquids can avoid miscibility issues as well as poor dissociation in traditional dynamic junction LECs. Here we report on the synthesis and performance of polymerizable ionic liquids (PILs) as candidates for ionic charge carriers in LEC device structures. PILs are advantageous since they combine the improved materials properties of ionic liquids with the fixed-junction functionality of IPMs. Structurally the synthesized PILs have trialkyl allyl/vinyl ammonium cations with varying lengths of the alkyl groups and allyl sulfonate anions. We also discuss PILs with polymerizable imidazole cation and non-polymerizable ammonium cation. Devices employing PILs had uniform film morphologies and showed diode-like behavior. In addition, brightness and turn-on times were improved by an order of magnitude compared to the IPM based devices reported earlier. A method for creating high performance fixed junction LECs can lead to advances in low-cost and low-power consumption solid-state lighting and photovoltaic devices. In addition, the materials developed here may have further utility for additional applications in which control over ionic mobility or electrochemical doping is crucial.
9:00 PM - II9.65
Investigating the Loss Mechanisms in Polymer: Fullerene Bulk-heterojunction Solar Cells Through Temperature Dependence of Photovoltaic Performance.
Song Chen 1 , Kaushik Roy Choudhury 1 , Jegadesan Subbiah 1 , Chad Amb 2 , John Reynolds 2 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Department of Chemistry, Center for Macromolecular Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractThe efficiency of bulk hetero-junction (BHJ) polymer: fullerene solar cells has steadily improved over the last decade. In order to further improve the device efficiency, it is imperative to understand the factors limiting the key performance parameters. Since the devices are likely to be working at temperatures higher than those generally reported in laboratory experiments, an issue of concern is the effect of operating temperature on the performance. Temperature is a key factor in determining the loss of energy in the photon-electron conversion process. It also plays a crucial role in the transport of photogenerated carriers in organic BHJ polymer solar cells (PSCs). In the present work, we study the effect of temperature on the main photovoltaic parameters (Voc and Jsc), and on the loss mechanisms (generation, recombination) in a model high efficiency (η>5%) PSC fabricated from the low-bandgap donor-acceptor polymer PSBTBT and acceptor PC70BM. In this work, we report that the open-circuit voltages (Voc) of the solar cells are found to scale inversely with temperature. This behavior is explained by invoking an analytical model which correlates Voc with energy loss via interfacial recombination of excitons in organic photovoltaic devices. Our temperature dependent data, in conjunction with this model, establishes a maximum limit of Voc well in excess of the currently attained values. It also gives an estimate of Voc values to be expected under real operating conditions. Interestingly, the short-circuit current (Jsc) increases steeply at low temperatures, before attaining a saturation value. This is explained through a combined influence of temperature dependent carrier mobility and maximum photocurrent generation rate (Gmax). At temperatures below the saturation point, photocurrents are limited by carrier mobility. Beyond the saturation temperature, the photocurrent is dominated and limited by Gmax. Based on our model, the insight into the processes limiting the device performance offers new routes for further optimization of other device systems.
9:00 PM - II9.68
Charge Injection in ZnS-based Phosphorus AC EL Devices.
Jungho Park 1 2 , Corey Hewitt 1 , Corey Whitt 2 , Wes Mays 2 , Bob Summers 2 , David Carroll 1 2
1 Department of Physics, Wake Forest University, Winston-Salem, North Carolina, United States, 2 , PureLux Inc., Winston-Salem, North Carolina, United States
Show AbstractElectric field and temperature-dependent charge injection studies have been performed in ZnS based phosphorus AC electroluminescence (EL) devices. Richardson–Schottky model for thermionic emission accounts for the experimentally observed dependence of the injection current on various temperature. At low temperature of 150K, Fowler–Nordheim model with a field enhancement factor for tunneling injection is probed with field-dependent experiments, which can be understood as enhancing the local electrical field with a role of nanostructure.
9:00 PM - II9.69
Solution Processed High Mobility Tetrathienoacene Polymer Semiconductors.
H. H. Fong 1 , Vladimir Pozdin 1 , George Malliaras 1 , Mingqian He 2 , Jianfeng Li 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 , Corning, Corning, New York, United States
Show AbstractWe demonstrate that high performance, soluble polymeric semiconductors can be achieved by increasing the rigidity of the thiophene monomer through the use of an alkyl-substituted core that consists of four fused thiophene rings. We report on a member of the family of di-alkylated tetrathienoacene copolymers, namely poly(2,5-bis(thiophene-2-yl)-(3,7-ditridecanyltetrathienoacene) (P2TDC13FT4), that can be deposited from a 1,2-dichlorobenzene solution into highly ordered films with a field-effect hole mobility exceeding 0.3 cm2/V/s. Devices prepared on vapor phase deposition of silane based oxide gate oxide show a remarkably long life-time of two years in ambients, with no significant change on the performance.We show that di-alkylated tetrathienoacene copolymers represent a new class of organic semiconductors that can be easily processed from solution into ordered, high mobility thin films. The straightforward processing and the relatively low temperatures involved make them particularly attractive for the development of electronics on plastic substrates.
9:00 PM - II9.7
The Role of Hydroxyl Groups in Polymer Gate Dielectrics for n-channel Formation of Pentacene-based Organic Field-effect Transistors.
Jer-Wei Chang 1 , Wei-Lieh Hsu 1 , Chang-Yo Wu 1 , Tzung-Fang Guo 1 , Ten-Chin Wen 2 , Yao-Jane Hsu 3
1 Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan Taiwan, 2 Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan, 3 , National Synchrotron Radiation Research Center, Hsinchu Taiwan
Show AbstractPentacene-based organic field-effect transistors (OFETs) consisting of two polymers with hydroxyl groups such as polyvinyl alcohol (PVA) and poly 4-vinyl phenol (PVP) as gate dielectrics were prepared for studying the mechanism of electron channel formation. The output current and capacitance-voltage characteristics of OFET devices indicated that the formation of electron channel in the active layer was hindered by the electron traps at the contact interface between the pentacene and PVP dielectric layers, in which the hydroxyl groups of PVP are of higher dissociation constant than those of PVA. The dissociated protons at PVP dielectric layer form the electron traps and restrict the n-channel formation in the pentacene active layer. In comparison, devices applying PVA as the gate dielectric presents the decent n-type output characteristics. For fabricating the n-type pentacene-based FETs, one should consider the source-drain electrodes of appropriate work function and a trap-free dielectric layer. We demonstrated a pentacene-based FET device using Ca as electrodes and polystyrene as dielectrics, and its electron mobility could reach up to 0.08 cm2s-1V-1.
9:00 PM - II9.70
Photolithographic Micropatterning of Highly Fluorinated Organic Light Emitting Devices Using Ordinary Photoresists.
H. H. Fong 1 , Jin Kyun Lee 1 , Yee Fun Lim 1 , Alex Zakhidov 1 , Chris K Ober 1 , George Malliaras 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractSolution processed organic electronics is an emerging technology for low cost large area devices for many electronic applications, particularly printable polymer light emitting diodes (PLEDs) for display applications. Here, we present a new method of organic processing that allows patterning solution processed RGB using ordinary photoresists, without employing etching process. These light emitting polymers are highly fluorinated and chemically inert to ordinary solvents such as chloroform. Devices exhibit a current efficiency up to ~7cd/A. Patterned RGB EL pixels down to 10 micron size are demonstrated.
9:00 PM - II9.71
Spectroscopic Characterization of Three-dimensional Anisotropic Electronic Properties of Solution Grown Organic Single Crystals.
Beatrice Fraboni 1 , Anna Cavallini 1 , Alessandro Fraleoni-Morgera 2
1 Physics, University of Bologna, Bologna Italy, 2 , Sincrotrone ScpA, Trieste Italy
Show AbstractCharge transport processes in organic materials are highly sensitive to the molecular packing and interface defective states. It is has been shown that organic single crystals offer the possibility of studying the intrinsic properties of organic molecules thanks to their high purity and molecular order. The observation of anisotropic transport in single crystals, due to the anisotropic packing of the organic molecules, provides an assessment of the occurrence of band-like transport processes.We studied the three-dimensional anisotropic charge transport properties of millimiter-sized solution-grown organic single crystals based on a dipolar molecule 4HCB (4-hydroxy-cyanobenzene) by Space Charge Limited Current (SCLC), by spectral photocurrent (PC) and by X-ray diffraction analyses. We also carried out polarized FTIR analyses with polarized light to study the correlation between the crystalline lattice and the optical excitation. Most reports on organic single crystals deal with crystals grown by vacuum deposition methods, while only a few are available on solution-grown single crystals obtained [1,2]. Our results indicate that 4HCB crystals possess a clear and reproducible three-dimensional anisotropy in their main transport parameters: i) charge carrier mobility, ii) distribution of the electronic density of states (DOS) and iii) deep traps energy and concentration, and we report intrinsic-like three-dimensional mobility values for these crystals. The charge carrier mobility vales measured along the three crystallographic axes (i.e. 5x10-2 cm2/Vs for the main axis a, 3x10-3 for the axis b and 3x10-6 cm2/Vs for the axis c, along the crystal thickness), fully confirm the values we have previously obtained by using FET devices. In addition, a modelization of the obtained data allowed to evaluate the DOS distribution, the concentration and the activation energy of the dominant deep traps along each crystallographic direction.We observed how the exposure to visible light induced a different effect on the transport properties along the two directions, that we have attributed to the presence and alignment of the electron-attractor cyano group. We suggest that the presence of an intrinsic molecular dipole differently affects the flow of charge carriers along the two main planar crystal axes, thus altering the charge transport anisotropy induced by the molecular π-orbitals stacking. A band of electrically active deep traps was identified by PC analyses and we suggest that this lattice structure may be ascribed to the intrinsic 4HCB molecular electric dipole that may originate trapping centers that behave like deep donors and that are more efficient along one of the two main planar axes. [1] S. C. B. Mannsfeld, J. Locklin, C. Reese, M. E. Roberts, A. J. Lovinger, Z. Bao, Adv. Funct. Mater., 17, 1617 (2007)[2] B. Fraboni, C. Femoni I. Mencarelli, L. Setti R. DiPietro, A. Cavallini, A. Fraleoni-Morgera, Adv.Mater., 21, 1835 (2009)
9:00 PM - II9.72
New Ladder-type Organic Semiconductors: Synthesis, Properties, and Self-assembly of Nanowires.
Eilaf Ahmed 1 , Taeshik Earmme 2 , Guoqiang Ren 2 , Samson Jenekhe 1 2
1 Chemistry, University of Washington, Seattle, Washington, United States, 2 Chemical Engineering, University of Washington, Seattle, Washington, United States
Show AbstractThe design and synthesis of large polycyclic heteroaromatic compounds are of great interest for fundamental and technological interest due to their improved thermal properties, planar backbone framework, and enhanced intermolecular interactions as a result of extended π-conjugation. In this talk, we will present a new class of n-type heteroaromatic semiconductors that was synthesized via a one-step intermolecular cyclization, yielding highly fused heptacyclic π-conjugated framework. The photophysics, cyclic voltammetry, electron-transport, self-assembly and electroluminescence of a series of new n-type conjugated semiconductors will be compared and discussed. Single-crystal X-ray diffraction reveals that the heptacyclics are planar and leads to slipped face-to-face π-stacking with short intermolecular distances of 3.39-3.56 Å. Cyclic voltammetry measurements show that the new heptacyclics have high electron affinity with LUMO levels in the range of 3.65-3.72 eV. Charge transport of the new heptacyclics was evaluated by space-charge limited current and electron mobility as high as 3.84 x 10-4 cm2/V.s was measured in ambient air conditions. In addition, self-assembly of single-crystal nanowires was investigated by solution-phase and evaluated by SEM, TEM, and SAED techniques. OLEDs based on this new class of heteroarenes as the emissive layer showed high performance with maximum brightness of 7610 cd m-2 and maximum efficiency of 6.6 cd/A. The results demonstrate that this new class of n-type organic semiconductor is promising for electronics and optoelectronics applications.
9:00 PM - II9.73
Bulk Heterojunction Solar Cells Based on New Crystalline Donor-Acceptor Copolymers: Controlling Morphology for Achieving High Efficiency.
Hao Xin 1 , Xugang Guo 2 , Guoqiang Ren 1 , Felix Sunjoo Kim 1 , Mark D Watson 2 , Samson A Jenekhe 1
1 Department of Chemical Engineering, University of Washington, Seattle, Washington, United States, 2 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
Show AbstractPhthalimide based low band gap donor-acceptor copolymers, exemplified by poly(N-(dodecyl)-3,6-bis(4-dodecyloxythiophen-2-yl)phthalimide (PhBT12), showed high OFET hole mobility (~0.2 cm2/(Vs)) due to the strong π-π stacking and are promising candidates for bulk heterojunction solar cell application. However, the state-of-the-art thermal annealing approach resulted in micrometer scale phase separation and thus negligible photocurrent. By quickly drying the film in a 50 degree vacuum oven, severe phase separation was prevented and a moderate power conversion efficiency of 2.0% was obtained. To further control the morphology, a mixed solvent and warm solution was used for film processing, from which a finer phase separation is achieved and solar cell efficiency was almost doubled compared to that processed using room temperature single solvent solution. Absorption and space-charge limited current mobility measurements reveal similar light harvesting and hole mobilities in all the films, indicating morphology is the dominant factor determining phthalimide based donor-acceptor copolymer photovoltaic performance. Our results demonstrate that for highly crystalline and/or low-solubility polymers, finding a way to prevent polymer aggregation and large scale phase separation is critical for high performance solar cells.
9:00 PM - II9.74
Characterization of Channel/Dielectric Interfacial Trap States in Organic Field-effect Transistors by Novel Photo-excited Charge-collection Spectroscopy.
Kimoon Lee 1 , Byoung H. Lee 2 , Kwang H. Lee 1 , Ji Hoon Park 1 , Myung M. Sung 2 , Seongil Im 1
1 Institute of Physics and Applied Physics, Institute of Physics and Applied Physics in Yonsei University, Seoul, Seodaemun-gu, Korea (the Republic of), 2 Department of Chemistry, Department of Chemistry in Hanyang University, Seoul Korea (the Republic of)
Show Abstract We now introduce the photo-excited charge-collection spectroscopy as a novel direct measurement technique utilizing the photo-induced threshold voltage (Vth) response of a working organic field-effect transistor (OFET). Interface charges trapped at a certain energy level are liberated by the energetic photons and then electrically collected at the source/drain (S/D) electrodes. During this photo-electric process the Vth or the onset voltage of FETs is shifted. The magnitude of Vth shift provides us with a direct measure of the density-of-charge traps while the energy levels of those traps are simply scanned over by the photon energy. As a consequence, we can sensitively probe the fine density-of-states (DOS) profiles which can display detailed mid-gap states in the channel/dielectric interface of a FET device in operation for organic channel materials. To fabricate the pentacene-based OFETs, the indium-tin-oxide (ITO) on glass was patterned to be a bottom gate electrode by wet etching. For dielectric, we used atomic layer deposition (ALD)-grown 60 nm-thin Al2O3 as deposited it on patterned ITO glass. The surface of Al2O3 was functionalized with such a variety of self-assembled monolayers (SAMs) as Hexamethyldisilazane (HMDS), 7-octenyltrichlorosilane (7-OTS), and Trichloro-(1H,1H,2H,2H-perfluorooctyl)Silane (FTS). A 50 nm-thin pentacene active channel layer was patterned on the dielectric layer through a shadow mask by thermal evaporation, and then Au was evaporated onto the pentacene channel as S/D electrodes. When we measured the transfer characteristics of our pentacene-OFETs under an intense monochromatic illumination, the Vth starts to shift positively as the optical energy value (especially of ~1.32, ~1.38, and ~1.96 eV) increase. This means that the hole occupied trap level should be released by an optical illumination which corresponds to the energy value from the highest occupied molecular orbital (HOMO) level to the trap level (Et), thus the Vth shift as large as the number of DOS.(Detailed mechanism and analysis will be shown in the meeting.) From these spectral response characteristics of pentacene-OFETs with different SAM treatments, we could observe that the DOS peaks for traps (~1.32 and ~1.38 eV above HOMO) in pentacene channel/dielectric interface were dramatically decreased as the hydrophobicity of treatments increased. When we examined the electrical characteristics under the long-term bias-stress, we could also find out that the electrical stability was improved with the most hydrophobic SAM treatment of FTS, which had coherence with the result of DOS profiles. As a result, we can conclude that our photo-excited charge-collection spectroscopy is one of novel techniques to measure the channel/dielectric interface characteristics which can be adapted to most of field-effect device.
9:00 PM - II9.75
Photo-patternable Source/Drain Electrodes Using Multiwalled Carbon Nanotube/Polymer Nanocomposites for Organic Field-effect Transistors.
Kipyo Hong 1 , Chanwoo Yang 1 , Se Kim 1 , Chan Park 1
1 Department of Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractWe fabricated photo-patternable and conductive polymer/multiwalled carbon nanotube (MWNT) composites by dispersing MWNTs with poly(4-styrene sulfonic acid) (PSS) and poly(acrylic acid) (PAA) in water. PAA enables photo-crosslinking in the composite by adding ammonium dichromate, and PSS assists the dispersion of MWNTs in the composites, leading to higher conductivity. Composite films of PAA/PSS-MWNTs were characterized by conductivities of 1.4 to 210 S/cm and a work function of 4.46 eV, which could be increased to 4.76 eV during UV photo-crosslinking. By using PAA/PSS-MWNT composites as source/drain electrodes, 6,13-bis(triisopropylsilylethynyl) pentacene field-effect transistors (FET) exhibited a field-effect mobility of 0.101±0.034 cm2/Vs, which is nine times higher than that of FETs fabricated with gold as source/drain electrodes (0.012±0.003 cm2/Vs).
9:00 PM - II9.76
Thermal Stability of Organic Transistors With Self-assembled Monolayer Dielectrics.
Kazunori Kuribara 1 , Kenjiro Fukuda 1 , Tomoyuki Yokota 1 , Tsuyoshi Sekitani 1 , Ute Zschieschang 2 , Hagen Klauk 2 , Takao Someya 1
1 , The University of Tokyo, Tokyo Japan, 2 , Max Planck Institute for Solid State Research, Stuttgart Germany
Show AbstractWe have demonstrated to manufacture heat-resistant pentacene field-effect transistors (FETs) with self assembled monolayer (SAM) gate dielectric. The pentacene FETs could operate within 2 V, and exhibited high mobility of 0.81 cm^2/Vs and on/off ratio of 10^5 even at high temperatures up to 140 degC. For enhancing thermal stability, the pentacene FETs were encapsulated with polychloro-para-xyleylene passivation layer, which is hereafter referred to as the parylene layer. Although irreversible degradation began at 100 degC on FETs without encapsulation, the encapsulated FETs were not damaged electrically and mechanically at 100 degC and the decrease in currents was less than 12 % even at 140 degC. We also investigate molecule structure of thin-film pentacene using X-ray diffraction (XRD) spectrum, and found that on encapsulated pentacene, thin-film phase remain and did not change to bulk phase even after annealing at 160 degC. Organic transistors with SAM gate dielectrics were manufactured by vacuum evaporation and solution processes. A 25-nm-thick Al gate electrode was thermally evaporated through a shadow mask onto a heavily doped silicon wafer. The gate dielectric consists of a thin layer of aluminum oxide and a SAM of alkyl-phosphonic acid[1]. A thin aluminum oxide film with a large density of hydroxyl groups for molecular adsorption was formed by oxygen-plasma treatment (300 W, 30 min), and a SAM of n-tetradecylphosphonic acid was prepared from a 2-propanol solution at room temperature. Purified pentacene was deposited in vacuum through a shadow mask to form a 30-nm-thick patterned semiconductor layer on the gate dielectric. A 50-nm-thick Au layer was evaporated through a shadow mask to form the source/drain electrodes. The nominal length and width of the channel were 50 μm and 500 μm, respectively. Finally, some base substrates with transistors were uniformly encapsulated with a 2.5-μm-thick parylene layer, while other base substrates with transistors were kept as reference samples without the parylene layer. The mobility of FETs without parylene layer decreased from 0.55 to 0.25 cm^2/Vs after annealing at 100 degC. On the other hand, the mobility of passivated FETs decreased only 13% even after annealing at 140 degC. The as-grown thin-film pentacene has XRD peaks indexed as (001) with a periodicity of a lattice spacing d(001) of 15.4 Å, fundamentally. For pentacene without the parylene layer, diffraction peaks (001′) referred to as the bulk phase corresponding to 14.4 Å appeared after annealing at 100 degC, while for passivated pentacene, the bulk phase did not come out in annealing at below 160 degC. We have reported that the thin-film phase structure gradually change to the bulk phase with ambient temperature increasing, and bulk phase is not conductive in pentacene FETs[2].This study was supported by Kakenhi (WAKATE S) and NEDO.[1] H. Klauk, et al, Nature 445, 745 (2007). [2] K. Fukuda, et al, Appl. Phys. Lett. 95, 023302 (2009).
9:00 PM - II9.77
Printed Low-voltage Organic Transistors on Paper and Plastics.
Daniel Tobjoerk 1 2 , Nikolai Kaihovirta 1 2 , Roger Bollstrom 1 3 , Anni Maattanen 1 3 , Tapio Makela 1 , Petri Ihalainen 1 3 , Jouko Peltonen 1 3 , Martti Toivakka 1 3 , Ronald Osterbacka 1 2
1 Center for Functional Materials, Åbo Akademi University, Turku Finland, 2 Department of Physics, Åbo Akademi University, Turku Finland, 3 Laboratory of Paper Coating and Converting, Åbo Akademi University, Turku Finland
Show AbstractIn order for organic electronics to be used on a larger scale in every day life the devices should be fabricated by mass-printing techniques on low-cost flexible plastic and paper substrates and operate at low voltages in room atmosphere. This has, however, turned out to be rather challenging for organic field effect transistors (OFETs) that are usually fabricated with laboratory scale techniques on very smooth substrates and/or operating at high voltages.The problem can be solved by using a hygroscopic insulator OFET (HIFET) [1] that combines a low operation voltage with a rather thick dielectric layer, thanks to the ion motion in the polyanionic dielectric at normal room humidity. The large capacitance of the dielectric (and thus low-voltage operation) is due to the formation of an electric double layer capacitance at the interfaces. This operation principle also explains why the HIFET is less sensitive to insulator thickness variations and surface roughness [2] than traditional OFETs.These properties partly explain the successful fabrication of all-printed low-voltage organic transistors on a low-cost polyester substrate [3], as well as working transistors directly on the paper pigment surface of a recyclable multilayer-coated paper-based substrate [4].Ink-jet printing and reverse gravure coating was used when fabricating the transistors in this work. Poly(3,4-ethylene dioxythiophene):poly(styrene sulfonate) based inks was used for the gate electrode, while the source and drain contacts consisted of an inkjetted silver nanoparticle ink that was made conductive within 10 s by exposing the printed structures on paper to an infrared lamp. The semiconductor and dielectric consisted of regioregular poly(3-hexylthiophene) and poly(4-vinyl phenol), respectively.[1] H.G.O. Sandberg, T. G. Bäcklund, R. Österbacka, H. Stubb, Adv. Mater. 16, 1112 (2004).[2] N.J. Kaihovirta, D. Tobjörk, T. Mäkelä, and R. Österbacka, Appl. Phys. Lett. 93, 053302 (2008).[3] D. Tobjörk, N.J. Kaihovirta, T. Mäkelä, F.S. Pettersson, R. Österbacka, Org. Electron. 9, 931 (2008).[4] R. Bollström, A. Määttänen, D. Tobjörk, P. Ihalainen, N. Kaihovirta, R. Österbacka, J. Peltonen, M. Toivakka, Org. Electron. 10, 1020 (2009).
9:00 PM - II9.78
Solution Processable Copper Phthalocyanine For High Mobility Organic Thin Film Transistors.
N. Chaure 1 , I. Chambrier 2 , A. Cammidge 2 , M. Cook 2 , C. Murphy 3 , G. Hampton 4 , A. Magon 4 , M. Cain 3 , A. Ray 1
1 School of Engineering and Materials Science, Queen Mary, University of London, London United Kingdom, 2 School of Chemical Sciences and Pharmacy, University of East Anglia, Norwich United Kingdom, 3 , National Physical Laboratory, Teddington, Middlesex, United Kingdom, 4 , Qudos Technology, Didcot, Oxfordshire, United Kingdom
Show AbstractPhthalocyanine molecules are potential candidates for use as the organic semiconductor material in thin-film transistor structures. Inverted structure organic thin film transistors (OTFT) were fabricated using spin-coated films of a substituted copper phthalocyanine (CuPc) as the active semiconductor layer on pre-patterned octadecyltrichlorosilane (OTS) treated Si/SiO2 substrates. A plasma enhanced chemical vapour deposited 250nm thick SiO2 layer was used as the gate dielectric. Annealing temperature dependent electrical, morphological and structural properties were studied. OTFTs showed improved values of device parameters as the annealing temperature was increased to 100C. The devices which were heat treated at 100C were found to exhibit a high carrier mobility of 0.4cm2/Vs, an on/off current modulation ratio of ~10^7, a low threshold voltage of 2V and a sub-threshold swing of 1.1V/decade. The performance of the devices annealed at temperature >150C was reduced, probably due to the formation of very large crystalline grains (over 2microns) and associated layer discontinuity. The densities of trap states in the interfaces between the semiconductor and gate dielectric and the grain boundary region were evaluated by the method due to Levinson. The resulting values were found to be consistent with the observed annealing temperature dependent performance of the field effect transistors and with the measured surface morphology. Novel solution processable phthalocyanine materials, coupled with optimal processing regimes, show excellent promise for the active layer in flexible transistor circuitry.
9:00 PM - II9.79
Structure Analysis of Solution-crystallized 2,7-Dioctylbenzothieno [3,2-b]Benzothiophene Thin Films for Very High-mobility Transistors.
Junshi Soeda 1 , Masakazu Yamagishi 1 , Yuri Hirose 1 , Takafumi Uemura 1 , Akiko Nakao 2 , Shoji Shinamura 3 , Kazuo Takimiya 3 , Jun Takeya 1
1 Dept. of chemistry, Osaka University, Toyonaka Japan, 2 , High Energy Accelerator Research Organization, Tsukuba Japan, 3 Graduate school of Engineering, Hiroshima University, Higashi-Hiroshima Japan
Show AbstractOrganic field effect transistors (OFETs) have been considerably interested due to their ability of application to flexible, large area and low-cost electrical switching components. Although it is mostly argued to use them for relatively slow devices, further applications become possible with the achievement of higher mobility above ~10 cm2/Vs, which is demonstrated in single-crystal OFETs, in practically mass-producible transistors. Unfortunately, the conventional single-crystal OFETs have not been suited to fabricated large-scale devices. Very recently we developed a method to grow a crystalline film from a solution in an oriented way on a substrate and reported considerable mobility values exceeding a few cm2/Vs for devices with solution-crystallized 2,7-diocthylbenzoselenopheno[3,2-b]benzoselenophene (C8-BTBT) thin films [1]. Apparently, the film consists of crystalline domains spread to the whole channel length, so that molecular steps and terraces are observed in atomic-force-microscope view. In this study, we performed high-energy transmission X-ray diffraction measurement of the solution-crystallized C8-BTBT films on typically 2-μm-thick parylene substrates. The measurement was configured to elucidate crystallinity of the film, favored direction of the crystal growth, relationship between direction of the crystal and that of favorable carrier transport.Obvious Bragg peaks emerged corresponding to the components in the conducting a-b plane consistently to the reported crystal structure of the material. The peak patterns indicate that the C8-BTBT films consist of either one or a few crystal domains, within the sub-millimeter spot of the irradiated X-ray. It turned out that the c-axis is vertical to the substrate and that the a axis are inclined by approximately 30 degrees to the direction of crystal growth. The growth direction nearly corresponds to that of the highest average transfer integral. Note that the high-mobility transistor performance is measured in the same direction. We conclude that the high performance in the solution-processed C8-BTBT thin-film transistors are originated from extremely ordered molecular stacking due to the crystallization. [1] T. Uemura et al., Appl. Phys. Exp. in press.
9:00 PM - II9.8
How Semiconducting Molecules Self-assemble into SAMFETs.
Simon Mathijssen 1 2 , Edsger Smits 2 , Paul van Hal 2 , Tom Geuns 2 , Harry Wondergem 2 , Roland Resel 3 , Sergei Ponomarenko 4 , Martijn Kemerink 1 , Rene Janssen 1 , Dago de Leeuw 2
1 , Eindhoven University of Technology, Eindhoven Netherlands, 2 , Philips Research Laboratories, Eindhoven Netherlands, 3 , Institute of Solid State Physics, Graz Austria, 4 , Enikolopov Institute of Synthetic Polymer Materials of Russian Academy of Sciences, Moscow Russian Federation
Show AbstractSelf-assembly—the autonomous organization of components into patterns and structures— is a promising technology for the mass production of organic electronics. Making integrated circuits using a bottom-up approach involving self-assembling molecules was proposed in the 1970s. The basic building block of such an integrated circuit is the self-assembled-monolayer field-effect transistor (SAMFET), where the semiconductor is a monolayer spontaneously formed on the gate dielectric. The mobility of SAMFETs traditionally decreases dramatically with increasing channel length, severely restraining their applicability. However, we have recently demonstrated SAMFETs showing bulk-like mobilities that are virtually independent of channel length.[1] To elucidate this contradiction in the scaling of the mobility, we study SAMFETs with varying monolayer coverage and channel lengths. By assuring both proper charge carrier injection and an ordered SAM microstructure, we can systematically investigate the charge transport by local probes and transport measurements. We show that the charge transport in SAMFETs is determined entirely by the completeness of the monolayer between the source and drain, and is not affected by carrier injection, grain boundaries or conducting island size. The extracted device mobility depends exponentially on the channel length only when the monolayer is incomplete. This dependence is reproduced both numerically and analytically, rationalizing earlier findings.[2] At partial coverage, SAMFETs form a unique model system to study size-dependent conductance originating from charge percolation in two-dimensions.[1] E.C.P. Smits et al., Nature 455 956 (2008).[2] S.G.J. Mathijssen et al., Nature Nanotechnology 4 674 (2009).
9:00 PM - II9.80
Correlation Between Organic Field-effect Transistor Performance and Electronic Band Level Alignment.
Philipp Stadler 1 , Anna Track 2 , Mujeeb Ullah 3 , Helmut Sitter 3 , Gebhard Matt 3 , Georg Koller 2 , Thokchom Singh 4 1 , Helmut Neugebauer 1 , Niyazi Sariciftci 1 , Michael Ramsey 2
1 Johannes Kepler University, Institute for Organic Solarcells (LIOS) and Institute for Physical Chemistry, Linz, Upper Austria, Austria, 2 University of Graz, Institute for Physics, Graz, Styria, Austria, 3 Johannes Kepler University, Institute of Semiconductor- and Solid State Physics, Linz, Upper Austria, Austria, 4 Ian Wark Laboratory, CSIRO Molecular and Health Technologies, Clayton, Victoria, Australia
Show AbstractElectronic effects at organic dielectric-organic semiconductor interfaces influence the charge transport in organic field effect transistors (OFETs) and in many cases substantially enhance transport properties. Oxidic dielectric surfaces are modified by introducing thin polymeric layers on top, which change the overall device performance. Apart from a phenomenological description of the charge transport, in this work we consider the possibility of charge rearrangements at the interface with the organic semiconductor. In particular the introduction of a BCB divinyltetramethyldisiloxane-bis(benzocyclobutene) interlayer between the inorganic dielectric (Al2O3) and the organic semiconductor (C60) has been reported to yield low operating voltages and high mobilities [1]. We compare C60 on pure Al2O3 and C60 on BCB-interlayer modified Al2O3. With the BCB interlayer OFET performance is improved and the threshold voltage lowered by 0.8 V. A corresponding change is reflected in ultraviolet photoemission data for the same system, which show a 0.8 eV difference in the work function of C60 and a concomitant shift in the C60 valence band. The improvement is due to a dipole formed at the organic BCB – C60 interface. The photoemission investigations indicate that this dipole is the prime determinant for the threshold voltage in the organic transistor. Recently theoretical modelling has suggested that improvements in transfer characteristics and rigid shifts of the threshold voltage can result from the introduction of permanent space charge or dipole layers at the organic semiconductor / dielectric interface [2]. The magnitude and mechanism of this shift suggests detailed studies into tailoring organic-organic interfaces on the gate dielectric could be very promising for optimizing device performance [3]. [1] X.-H. Zhang, B. Kippelen, Appl. Phys. Lett. 93 (2008) 133305.[2] S. K. Possanner, K. Zojer, P. Pacher, E. Zojer, F. Schürrer, Adv. Funct. Mat. 19 (2009) 958 - 967[3] P. Stadler, A. M. Track, M. Ullah, H. Sitter, G. J. Matt, G. Koller, T. B. Singh, H. Neugebauer, N. S. Sariciftci, M. G. Ramsey, Org. Electron. 11 (2009) in press.
9:00 PM - II9.83
Flexible, Spin-coated Carbon Nanotube Electrodes for High-performance n- and p-type Organic Transistors.
Sondra Hellstrom 1 , Run zhi Jin 2 , Zhenan Bao 3
1 Applied Physics, Stanford University, Stanford, California, United States, 2 Materials Science, Stanford University, Stanford, California, United States, 3 Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractWe report simultaneous deposition and patterning of carbon nanotube/conjugated polymer composites from solution, with high nanotube density gradients and feature resolution down to 3 microns. We achieve this by exploiting the selective wetting and adhesion properties of these composites on surfaces functionalized with different self-assembled monolayers. The patterned nanocomposites are easily adaptable as flexible, semitransparent, spin-coated electrodes for large-area organic electronics. Towards this application, we demonstrate high-performance p- and n-type bottom contact organic transistors with pentacene and C60 as active layers respectively. The transistors consistently achieve on/off ratios of more than 10^5, bottom contact mobilities of > 0.5 cm^2/V*s, and linear behavior in the small-bias regime. This initial demonstration that high-mobility complementary transistors can both be driven with solution-deposited organic electrodes represents a clear step towards development of low-cost, high-performance all-organic circuits.
9:00 PM - II9.84
Nonvolatile Hybrid Memory-cell Embedded With Al Nanocrystals in Polystyrene.
Kyoung-Cheol Kwon 1 , Jong-Dae Lee 1 , Hyun-Min Seung 1 , Chang-Hwan Kim 1 , Jae-sung Lim 1 , Jea-Gun Park 1
1 Dept. of Electrical and Computer Engineering, Hanyang University, Seoul Korea (the Republic of)
Show AbstractWe studied the effect of Al nanocrystals surrounded by Al2O3 on nonvolatile memory characteristics via O2 plasma process. The device structure is triple layer; PS(polystyrene) / Al surrounded by Al2O3 / PS between top and bottom Al electrode. Al was deposited on SiO2 substrate by thermal evaporation and PS was dissolved toluene and fabricated with spin-coating process. Particularly, Al nanocrystals surrounded by Al2O3 in the middle metal layer were produced by O2 plasma process, which oxidized Al layer along with the Al grain boundary. A high vacuum thermal deposition method is needed for the formation of regular Al grain boundary that was not oxidized. It was found that the nonvolatile memory characteristics were shown at only memory cell treated O2 plasma process for forming Al nanocrystals. Al2O3 surrounding the Al nanocrystals played a role as the tunneling barrier, resulting in the memory characteristics. Nonvolatile hybrid memory cell showed an electrical nonvolatile memory characteristics, i.e., Vth of ~3 V, Vp (program) of ~3.7 V, NDR (negative differential region) of 3.7~7 V, Ve (erase) of 10 V, in particular, electrical characteristics at reverse bias was symmetrical. In addition, it showed Ion/Ioff ratio of 102 at Vr (read) of 1 V, the retention time of more than 1×105 sec and more than 100 endurance cycles.Acknowledgement* This research was supported by "The National Research Program for Terabit Nonvolatile Memory Development” sponsored by the Korean Ministry of Knowledge Economy.
9:00 PM - II9.85
Control of Morphology of Polythiophene-based Block Copolymers for Organic Electronic Applications.
Kyung-Youl Baek 1 , Hoichang Yang 2 , Seung Sang Hwang 1
1 Hybrid Materials Research Center, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 Department of Advanced Fiber Engineering, Inha University, Incheon, Korea, and 3Department of Chemical Engineering, Pohang Un, Inha University, Incheon Korea (the Republic of)
Show AbstractA series of well defined block copolymers of 3-hexylthiophene and methacrylate derivates were synthesized by combination of ATRP and GRIM. Obtained block copolymers with narrow polydispersity (PDI < 1.2) had various molecular weights with different weight fraction of P3HT. The block copolymers were then examined morphology changes by various solvents, which were selectively or both soluble for each block segments. Selective alignments of P3HT domains with a preferred conducting path applied for organic electronic applications such as organic thin film transistor and photovoltaic cell.
9:00 PM - II9.86
Interfacial Engineering With Multifunctional Organophosphonic Acid Monolayers in Low-voltage Organic Thin Film Transistors.
Orb Acton 1 , Hong Ma 1 2 , Tae-Wook Kim 1 , Guy Ting 3 , Daniel Hutchins 1 , Hin-Lap Yip 1 2 , Alex K.-Y. Jen 1 2 3
1 Materials Science & Engineering, University of Washington, Seattle, Washington, United States, 2 Institute of Advanced Materials and Technology, University of Washington, Seattle, Washington, United States, 3 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractOrganic thin film transistors (OTFTs) based on pi-conjugated materials are envisioned for use in ubiquitous low-cost flexible electronic devices, such as displays, sensors and electronic barcodes. A prerequisite for realizing practical applications lies on the development of stable gate dielectrics with low leakage current, low interface trap density, and high capacitance that afford low-voltage OTFT operation with high performance. However, standard OTFTs still require rather high operating voltages, often exceeding 20 V.We have developed a vacuum-free solution processed hybrid gate dielectric system compatible with plastic substrates which is composed of multifunctional organophosphonic acid self-assembled monolayers (SAMs) on high-k metal oxides. This hybrid dielectric system affords low-voltage, high performance OTFTs with low leakage currents and high charge carrier mobilities for both p- and n-channel devices. In the demonstrated OTFTs, the following device characteristics have been achieved: 1) low leakage current density - down to 1 nA/cm2; 2) large capacitance density - up to 630 nF/cm2; 3) low operating voltage (<2 V); 4) high charge carrier mobility - over 2.0 cm2/(Vs); 5) small subthreshold slope - down to 100 mV/decade; 6) low-hysteresis device operation. This is achieved by using well-packed and dense organophosphonic acid SAMs (<4 nm) on hafnium oxide (<10 nm) as ultrathin hybrid dielectrics and through interfacial engineering using an appropriate SAM to control the chemical, electrical, and morphological structure at the semiconductor/dielectric interface.Furthermore, this hybrid dielectric system is shown to be generally applicable for solution processed organic semiconductors by achieving stable device operation for polymer and soluble acene semiconductors with mobilities >0.1 cm2/(Vs). This work represents a major advancement towards developing large-area low-cost solution-processed/printed, flexible organic electronic devices.
9:00 PM - II9.9
A Microelectrostatic View on the Molecular Doping of Organic Semiconductors for Transistor Applications.
Alexander Mityashin 1 3 , Tanguy Van Regemorter 2 , Cedric Rolin 1 , Stijn Verlaak 1 , Yoann Olivier 2 , Nicolas Martinelli 2 , David Beljonne 2 , Jerome Cornil 2 , Jan Genoe 1 , Paul Heremans 1 3
1 Polymer and Molecular Electronics, IMEC, Leuven Belgium, 3 ESAT, Katholieke Universiteit Leuven, Leuven Belgium, 2 Laboratory for Chemistry of Novel Materials, University of Mons, Mons Belgium
Show AbstractDriven by the successful use of electrical dopant molecules in the transport layers of so-called p-i-n organic light emitting diodes, several attempts have been conducted recently to dope organic thin films using a small organic molecule (tetrafluorotetracyanoquinodimethane or F4TCNQ) to improve the characteristics of transistors based on these films [1,2]. It has been shown that such doping can lead to an enhanced transistor channel conductivity of up to six orders of magnitude and an improved charge injection from the source electrode into the transistor channel. Due to the low permittivity of an organic semiconductor (OSC), it is not expected that introducing an electrical dopant molecule in a matrix OSC should lead to a free carrier in that matrix, but rather to a Coulombically-bound charge pair. Therefore, the mechanism that leads to improved electrical conductivity upon electrical doping of an OSC is not trivial and requires further study.In this work, we compare experiments with microelectrostatical simulations [3] in order to gain a better understanding of the effect of electrical dopant molecules on charge transport within a small molecular weight organic semiconductor thin film. The studied system is a pentacene thin film doped with F4TCNQ in various concentrations. Microelectrostatics reveal the energetic landscape as seen by a charge moving across the crystal. We show how the details of the energetic landscape of the film strongly depend on the HOMO/LUMO energies of dopant molecules and their concentration. In particular, the simulations confirm that deep hole traps are introduced in the HOMO landscape of the pentacene matrix in the immediate vicinity of the dopant molecules. We infer that the conduction mechanism of charges in such doped systems is not simply hopping, but can involve tunneling between the deep traps at sufficient doping concentrations.In parallel to numerical simulations, control experiments have been conducted by fabricating and measuring transistors based on thin films of pentacene and F4TCNQ co-evaporated within a high vacuum chamber. A comparison with simulated results is established by studying the evolution of charge transport properties with temperature for different doping concentration. Dependences obtained for tunneling, hopping and combination of these two processes show an agreement with measured dependences of doped thin film transistors.[1] B. Maennig et al., Phys. Rev. B 64, 195208 (2001)[2] C. Vanoni et al., Appl. Phys. Lett. 94, 253306, (2009)[3] S. Verlaak et al., Phys. Rev. B 75, 115127 (2007)
Symposium Organizers
Zhenan Bao Stanford University
Alejandro L. Briseno University of Massachusetts
Vitaly Podzorov Rutgers University
Iain McCulloch Imperial College London
II10: Conjugated Polymers
Session Chairs
Friday AM, April 09, 2010
Room 3001 (Moscone West)
9:00 AM - **II10.1
Charge Transport Physics of Conjugated Polymer Semiconductors.
Henning Sirringhaus 1
1 , University of Cambridge, Cambridge United Kingdom
Show AbstractConjugated polymer semiconductors offer new opportunities for the controlled manufacturing of active electronic circuits by a combination of solution processing and direct printing. Organic semiconductors are complex materials with strong electron-electron and electron-phonon coupling. We will review current understanding of their charge transport and device physics with a particular emphasis on understanding the electronic structure at polymer - polymer heterointerfaces and the fundamental understanding of the charge transport mechanism in high mobility, solution-processed organic semiconductors with band-like temperature dependence of the mobility.
9:30 AM - **II10.2
The Role of Microstructure in Governing Charge Transport in Organic Semiconductors.
Alberto Salleo 1 , Jonathan Rivnay 1 , Chenchen Wang 1 , Leslie Jimison 1 , Ludwig Goris 1 , Michael Toney 2 , Antonio Facchetti 4 5 , Tobin Marks 4 , Martin Heeney 3 , Iain McCulloch 3
1 , Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States, 4 Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, 5 , Polyera Corp., Skokie, Illinois, United States, 3 Chemistry, Imperial College of Science and Technology, London United Kingdom
Show AbstractAs organic semiconductors approach commercialization, there is a need to better understand the relationship between charge transport and microstructure, in particular to identify the inherent bottlenecks to charge transport. P3HT is the most studied high-mobility polymeric semiconductor. Device modeling, x-ray characterization and sub-bandgap spectroscopy will be used to elucidate the effect of molecular weight, regio-regularity and processing parameters on transport as separated in contributions from crystallites and traps. PBTTT is the highest-performance p-type polymeric semiconductor to date. Its high mobility is attributed to the formation of large terraces. We will show that device modeling and THz spectroscopy on the other hand point towards the importance of local structure and crystalline perfection within the polymeric grains in controlling mobility. While for years their development trailed that of p-type materials, n-type semiconductors have recently progressed rapidly. PNDI2OD-T2 is a high-performance n-type polymeric semicoductor recently developed. The effect of structure and texture, as studied by x-ray diffraction, will be discussed. Furthermore, the role of the semiconductor/dielectric interface will be examined by making use of top and bottom-gate devices.
10:00 AM - II10.3
Polyselenophenes.
Michael Bendikov 1
1 Department of Organic Chemistry, Weizmann Institute of Science, Rehovot Israel
Show AbstractDespite the diverse applications of conducting polymers, only a limited number of conducting polymer families has been developed. We have developed a family of conducting polymers, namely polyselenophenes.1,2 In particular, we have developed new, general and efficient synthetic strategy for the preparation of poly(3,4-ethylenedioxyselenophene) (PEDOS), the first highly conductive polyselenophene.1 We have also obtained several other types of the polyselenophenes, some of which show very remarkable properties.2,3 Our extensive characterizations of PEDOS and other polyselenophenes reveal that they are highly conductive, electrochemically active, very stable and having bandgap, lower than their polythiophene analogs. PEDOS derivatives are excellent electrochromic materials which are very transparent in the doped state and are currently among the best electrochromic materials reported.2,4 In our initial work developing this area, we have taken advantage of the fact that parent polyselenophenes have a lower bandgap than parent polythiophenes to design polyselenophenes appropriate for application in organic solar cells and so synthesize a family of polyselenophenes, based on seleno[3,4-b]selenophenes, that includes members with a very low bandgap.1 A. Patra, Y. H. Wijsboom, S. S. Zade, M. Li, Y. Sheynin, G. Leitus and M. Bendikov, J. Am. Chem. Soc. 2008, 130, 6734-6736.2 For a review see: A. Patra and M. Bendikov, J. Mater. Chem. 2010, DOI: 10.1039/b908983g.3 Y. H. Wijsboom, A. Patra, S. S. Zade, M. Li, Y. Sheynin, L. J. W. Shimon and M. Bendikov, Angew. Chem., Int. Ed. Engl. 2009, 48, 5443-5447.4 M. Li, A. Patra, Y. Sheynin, M. Bendikov, Adv. Mater. 2009, 19, 1707-1711.
10:15 AM - II10.4
Soft-contact Lamination: A Modular Approach to Fabricating and Characterizing Polymer Solar Cells.
Jong Bok Kim 1 , Stephanie Lee 1 , Chang Su Kim 1 , Michael Toney 2 , Youn Sang Kim 3 , Yueh-Lin Loo 1
1 Chemical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Stanford Synchrotron Radiation Laboratory, Stanford Linear Accelerator Center, Menlo Park, California, United States, 3 Department of Nano Science and Technology, Seoul National University, Seoul Korea (the Republic of)
Show AbstractWe have successfully applied soft-contact lamination, a processing scheme that allows the modular construction of individual device components and these components brought together for electrical contact in the final step, to the fabrication of polymer solar cells. Specifically, we have separately fabricated the components of polymer solar cells on two separate substrates; these substrates are subsequently brought together after processing to complete the devices. The first substrate supports the top electrodes of inverted devices. Onto the second substrate we define the bottom electrodes and deposit the photoactive layer. Lamination of the two substrates completes the devices. Given the modularity of soft-contact lamination, the top electrodes can be readily removed and replaced so the polymer photoactive layer can be characterized post-device testing. This interface is otherwise inaccessible through conventional fabrication and processing routes. Grazing incidence x-ray diffraction (GIXD) on the once-buried photoactive layer surface of inverted polymer solar cells comprising bulk-heterojunctions of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) indicates that PCBM crystallinity increases significantly when the devices are annealed at higher temperatures. This enhanced crystallinity correlates with higher short circuit current densities during device testing. We also find soft-contact lamination to be extremely robust; replacing the existing gold top electrodes with fresh gold electrodes results in quantitatively similar device characteristics. Not surprisingly, contacting the same photoactive layer with silver electrodes also results in comparable device characteristics because oxidized silver has a similar work function to that of gold. In addition to bulk-heterojunction type devices, soft-contact lamination has also enabled the fabrication of polymer solar cells comprising bilayer structures. Bilayer devices are difficult to fabricate using conventional bottom-up, solution-deposition approaches because orthogonal solvents are required for the sequential deposition of different organic semiconductors. With soft-contact lamination, we have prepared the electron donor and electron acceptor independently on different substrates. Bringing the substrates together completes the bilayer devices. By isolating the deposition and processing of the electron donor and electron acceptor in such structures, we have been able to independently control the properties of the individual constituents. Systematic examination of such devices enables the decoupling of morphological transformations that take place in the individual layers; we have thus been able to map out structure-function relationships of the electron donor and electron acceptor using this platform.
11:00 AM - **II10.5
Design, Operation and Optimization of Polymer-fullerene Tandem Solar Cells.
Date Moet 1 , Paul de Bruyn 1 , Jan Kotlarski 1 , Paul Blom 1 2
1 , University of Groningen, Groningen Netherlands, 2 , Holst Centre, Eindhoven Netherlands
Show AbstractThe narrow absorption properties combined with low charge carrier mobilities limit the performance of the organic solar cells. One way to improve the absorption of organic solar cells is by using tandem (or multi-junction) structures. Because of the different band gap of the active layer each sub cell then absorbs light in a different part of the solar spectrum. In order to further optimize the performance of organic tandem solar cells, it is important to understand their operation. A generalized methodology using combined electrical and optical modeling is developed to predict the current–voltage characteristic of polymer tandem solar cells. The ability to predict the performance of tandem cells will strongly reduce the experimental work needed to reach the optimum device structure. The design of the middle electrode, normal and inverted tandem cell structures and the maximum efficiency that can be reached with organic tandem cells will be addressed.
11:30 AM - **II10.6
Self-assembled Interlayers for Organic Photovoltaic and Transistors Applications.
Antonio Facchetti 1
1 Chemistry and the Materials Research Center, Northwestern, Evanston, Illinois, United States
Show AbstractIn this presentation we will report the design, synthesis, and characterization of new molecular precursors for the fabrication of self-assembled (inter)layers on SiOx, metal oxide, metals, and organic films. These building blocks are designed to strongly adhere to the surfaces of these conductors altering charge injection, charge trapping, and light absorption. Several transistors and photovoltaic cell device architectures based on these interlayers are fabricated and shows that these materials enable improved performance or new functions.
12:00 PM - II10.7
A Conclusive Picture of the Color Shift in ``Single Polymer” White Light-emitting Diodes.
Dieter Neher 1 , Sebastian Bange 1
1 Institute of Physics and Astronomy, University of Potsdam, Potsdam Germany
Show AbstractDriven by the demand for efficient solid-state light sources, research on white organic light-emitting diodes has substantially intensified over the last few years. When regarding polymer-based devices, highly efficient diodes have been realized by incorporating red- green- and blue-emitting moieties into a single polymer chain. However, many of these “single copolymers” suffer from color shifts upon variations of the operating conditions. Recently, M.C. Gather et al. explained the spectral properties of a white-emitting copolymer utilizing a purely kinetic approach [1]. It was assumed that the red dye acts as an electron trap and proposed that the amount of red emission to be determined by the probability that electrons become captured by these chromophores before reaching the anode.Here, we present a detailed study of the emission properties of two white-emitting copolymers provided by Merck KGaA, Germany. We propose an alternative analytical model based on rate-equations for the trap occupation within the effective emission zone. In contrast to the approach outlined above, here the emission color is essentially determined by the recombination of holes either with free electrons or with electrons occupying the red-emitting dye. Model parameters were obtained by detailed measurements of the energy transfer, charge injection and charge transport properties [2]. Our model correctly reproduces the observed color shifts at varying drive voltages, upon temperature variation and after electrical conditioning of the device [3].In variance with the results from the kinetic model, we found that charge trapping by diffusion can be neglected at typical OLED driving conditions while drift-induced trapping dominates. Our experimental data is consistent with a strongly reduced density of hopping sites for the transport states. We discuss the results on the basis of an effective random walk trapping model and show that diffusive and drift trapping by hopping-type motion are consistent within this model only if the Einstein equation is not strongly violated and diffusion is restricted to reduced dimensionality, probably by motion being preferred along chains or effective percolation pathways. We finally note that the analysis of the emission color constitutes a powerful approach to “probe” the complex carrier dynamics in white-emitting devices.[1] M. C. Gather, R. Alle, H. Becker, and K. Meerholz, Advanced Materials 19, 4460 (2007)[2] S. Bange, A. Kuksov, D. Neher, A. Vollmer, N. Koch, A. Ludemann, and S. Heun, Journal of Applied Physics 104, 104506 (2008)[3] S. Bange and D. Neher, to be submitted to Adv. Funct. Mater.
12:15 PM - II10.8
Investigating the Magnetic Field Effect in Conjugated Polymers Using Electrical and Optical Probing Techniques.
Matthew Phillips 1 , Neil Greenham 1 , Richard Friend 1
1 Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom
Show AbstractWe report the effects of magnetic fields up to 10 T on the spectroscopic properties of polyfluorenes and their blends. In particular, we focus on the effect of magnetic field on the lifetime and yield of triplet excitons detected by quasi-steady-state photoinduced absorption. We find significant changes in the photoinduced absorption signal with magnetic field, and we discuss the role of the magnetic field in controlling the rates of intersystem crossing and triplet exciton decay over a broad range oftemperatures. With room temperature measurements using blends of the electron donor PFB, and the electron acceptor F8BT, we observe an enhancement of around 5% in the formation of charge transfer states at the interface, as well as an enhanced photoinduced absorption signal of around10% which is attributed to an increase in the intersystem crossing rate. We also compare the results with magnetic-field-dependent device measurements on the same materials systems.
12:30 PM - II10.9
Stable, Bright and Fast Solid State Light Emitting Electrochemical Cells.
Martijn Lenes 1 , Ruben Costa 1 , Antonio Pertegas 1 , Daniel Tordera 1 , Hendrik Bolink 1
1 Instituto de Ciencia Molecular, University of Valencia, Paterna Spain
Show AbstractLight-emitting electrochemical cells (LECs) are one of the simplest type of molecular electroluminescent devices. LECs have a simple architecture, are prepared from solution and operate with air-stable electrodes, which make them suitable for low cost/large area efficient lighting and signing applications.[1-4] In its simplest form it consists of a single active layer composed of an ionic transition-metal complex (iTMC) which supports all three processes of charge injection, charge transport and emissive recombination.[2-4] Until recently, the main drawback of these molecular devices was their short lifetimes. We showed that by using iridium(III) complexes capable of forming a supramolecularly-caged structure the lifetime of LECs can be increased to more than 3000 hours.[5-7] Remaining issues of these devices, for one, are there rather slow turn-on times. Additionally, there is a strong debate on the operation mechanism of these devices.[8] We will report on LECs using new complexes and device architectures that yield high luminances (>1000 cd/m2), short turn-on times (seconds) and high lifetimes (>1000 hours). Detailed analysis of carrier transport and injection is used to shed light on the operational mechanism of these devices. [1]Q. Pei, G. Yu, C. Zhang, Y. Yang, A. J. Heeger, Science 1995, 269, 1086.[2]E. S. Handy, A. J. Pal, M. F. Rubner, J. Am. Chem. Soc. 1999, 121, 3525.[3]F. G. Gao, A. J. Bard, J. Am. Chem. Soc. 2000, 122, 7426.[4]J. D. Slinker, J. Rivnay, J. S. Moskowitz, J. B. Parker, S. Bernhard, H. D. Abruña, G. G. Malliaras, J. Mat. Chem. 2007, 17, 2976.[5]H. J. Bolink, E. Coronado, R. D. Costa, E. Ortí, M. Sessolo, S. Graber, K. Doyle, M. Neuburger, C. E. Housecroft, E. C. Constable, Adv. Mat. 2008, 20, 3910.[6]S. Graber, K. Doyle, M. Neuburger, C. E. Housecroft, E. C. Constable, R. D. Costa, E. Ortí, D. Repetto, H. J. Bolink, J. Am. Chem. Soc. 2008, 130, 14944.[7]R. D. Costa, E. Ortí, H. J. Bolink, S. Graber, C. E. Housecroft, M. Neuburger, S. Schaffner, E. C. Constable, Chem. Commun. 2009, 2029.[8]Q. Pei, A. J. Heeger, G. G. Malliaras, J. D. Slinker, J. A. Defranco, M. J. Jaquith, W. R. Silveira, Y. Zhong, J. M. Moran-Mirabal, H. G. Graighead, H. D. Abruña, J. A. Marohn, Nature Mater. 2008, 7, 167.
12:45 PM - II10.10
Zone-Refinement Effects in Blends of Small Molecule Semiconductors With Polymers in Organic Thin Film Transistors.
Yeon Sook Chung 1 , Youngeun Jo 1 , Nayool Shin 1 , Jihoon Kang 1 , John Anthony 2 , Do Yoon 1
1 Department of Chemistry, Seoul National University, Seoul Korea (the Republic of), 2 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
Show AbstractThe blend films of small molecule semiconductors with polymers have been reported to exhibit better electrical characteristics of mobility, threshold voltage and uniformity in organic thin film transistors (OTFTs), as well as improved processability. In order to understand the mechanism of such improvements in blend semiconductors, we investigated 5,11-bis(triethylsilylethynyl) anthradithiophene (TES-ADT), a solution processable organic semiconductor which is unstable upon light-exposure, and its blends with poly(α-methylstyrene) (PαMS) as model systems. The generation of chemical defects in neat TES-ADT was found to significantly deteriorate the OTFT performance, but in the blends with PαMS the initial electrical characteristics of fresh TES-ADT were fully recovered. Such results on electrical properties were accompanied by corresponding structural characteristics as seen by DSC and X-ray diffraction experiments. Therefore, one major cause of improved electrical properties of blend semiconductors may arise from the zone-refinement effect of TES-ADT upon crystallization from miscible blends with PαMS.
II11:Interfaces and Morphology
Session Chairs
Friday PM, April 09, 2010
Room 3001 (Moscone West)
2:30 PM - **II11.1
Interface Doping for Reduction of Organic/Electrode Barriers.
Antoine Kahn 1
1 , Princeton University, Princeton, New Jersey, United States
Show AbstractOrganic / (metal) electrode barriers are central elements in the evaluation of device performance. The reduction of injection or extraction barriers is crucial for reducing drive voltages in OLEDs and increasing open circuit voltage and short circuit current in OPV cells. This talk provides a rapid review of current understanding of the electronic structure of organic/electrode interfaces, and presents an overview of the role of interface doping in reducing injection barriers. Interface doping, i.e. insertion of dopants in a 5-10 nm slice of the organic semiconductor, requires spatial control of the position of dopants, and relies on their stability vs. diffusion. We present recent results on three-dimensional organo-metallic molecular p-dopants that exhibit such stability, as determined by ion-scattering techniques. The electronic structure of these dopants, and of the hole transport materials doped with these molecules, is investigated via direct and inverse photoemission spectroscopy. The impact of interface doping, as opposed to bulk doping of the entire molecular film, is studied by comparing in-situ, temperature-dependent, current-voltage characteristics from undoped, interface doped and homogeneously (bulk) doped films. Comparison with electrode modification is discussed.
3:00 PM - **II11.2
Growth and Post-processing of Molecular Semiconductors for Organic Electronic Applications.
Aram Amassian 1
1 Materials Science and Engineering, Division of Physical Sciences and Engineering, King Abdullah University of Science and Technology, Thuwal Saudi Arabia
Show AbstractA key aspect of organic electronics research is the establishment of a structure-property-performance relationship for molecular materials used in device applications. Such insight is valuable, since it provides us with the design rules to improve the performance of materials, processes, and devices. In the case of most high performance molecular semiconductors, charge transport is believed to be limited by grain boundary defects along the channel of the organic field-effect transistor (OFET). The working assumption has been that additional improvements to the output characteristics of OFETs, including their field-effect mobility (μ), on-off ratio, and threshold voltage, require increased crystallite or grain size or - as a corollary - decreased density of grain boundaries. Here, we show that the quality of grain boundaries and the interconnectivity of grains near the semiconductor-dielectric interface may influence the output characteristics and performance of OFETs more profoundly than the size of the grains and the quantity of grain boundaries. Device-to-device variations of four orders of magnitude are reported for μ and the on-off ratio simply by introducing small changes in the degree of filling of molecular layers near the semiconductor-dielectric interface. These are made possible by tuning the state of incident molecules and the state of the dielectric surface. Next, we explore how post-processing can yield additional improvements in both insoluble and soluble molecular semiconductor thin films by performing solvent vapor annealing, a room-temperature alternative to thermal annealing. We provide new insight into this intriguing process by monitoring the solvent vapor annealing of molecular semiconductors in situ and in real time. This work shows that versatility and tunability of molecular thin film processing and post-processing strategies can yield improved device performance.
3:30 PM - **II11.3
Detailed Structure of Organic Semiconductor/Dielectric Interfaces by Grazing-incidence X-ray Diffraction.
Stefan Mannsfeld 1 , MingLee Tang 2 , Ajay Virkar 2 , Toshihiro Okamoto 2 , Anna Reichardt 2 , Zhenan Bao 2
1 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States, 2 Department of Chemical Engineering, Stanford University, Stanford, California, United States
Show AbstractIn an organic thin film transistor (OTFT) device in its saturated "on"-state, the electrical current predominantly flows in the first few molecular layers[1] and thus the packing and exact arrangement of the semiconductor molecules in these layers determine the material's intrinsic electrical resistance. Grazing incidence X-ray diffraction (GIXD) using a synchrotron light source can provide detailed information about the molecular packing in thin films as thin as a single monolayer[2]. GIXD can be utilized to harvest crystallographic properties across the whole semiconductor/dielectric interface, including self-assembled monolayer (SAM) dielectric surface modifications of the silicon oxide surface which are common to enhance the electrical performance of the TFT devices. We present crystallographic analyses of different crystalline alkane silane SAMs on SiO2 surfaces. Triisopropylsilyl-functionalized (TIPS) pentacene/thiotetracene derivatives[3] and pentacene exhibit very high performance in vaccum-evaporated OTFT devices (field effect mobility μ>1 cm2/Vs). Crystallographic refinement of the corresponding GIXD peak intensities provides, for the first time, direct structural information on the molecular packing in thin films of these materials on plain and SAM-modified SiO2 surfaces.
[1] A. Dodapalapur, L. Torsi, H.E. Katz, Organic Transistors, Science 268, 270-271 (1995).
[2] S.C.B. Mannsfeld, A. Virkar, C. Reese, M.F. Toney, Z.Bao, Adv. Mater. 21, 2294 (2009).
[3] M.-L. Tang, A.D. Reichardt, T. Siegrist, S.C.B. Mannsfeld, Z. Bao, Chem. Mater. 20, 4669 (2008).
4:00 PM - II11.4
Scanning Transmission X-ray Microscopy Imaging of the Grain Orientation in a Pentacene Field-effect Transistor.
Bjoern Braeuer 1 , Yves Acremann 2 , Ajay Virkar 1 , Stefan Mannsfeld 1 , David Bernstein 1 , Roopali Kukreja 1 , Zhenan Bao 1
1 , Stanford University, Stanford, California, United States, 2 , ETH Zürich, Zürich Switzerland
Show AbstractThe structural quality of organic semiconductors is a key parameter for achieving high field-effect mobility values for organic field effect transistors [1]. Polycrystalline films are composed of grains and grain boundaries. The hopping type transport is limited in such films due to significant charge-carrier trapping at the grain boundaries [2]. Detailed structural information about organic thin films can be obtained by X-ray diffraction, scanning probe techniques, and transverse shear microscopy [3,4].In this paper, scanning transmission X-ray microscopy (STXM) will be introduced as a complementary method to obtain orientation information about the molecules in bulk structures, which is of importance for organic solar cells and organic field-effect transistors (OFETs). In STXM, the X-ray beam coming from an elliptically polarized undulator (EPU) beamline, is focused by a zone plate to provide a lateral resolution of 30 nm. The sample is then raster scanned relative to the X-ray beam while the transmitted photon flux is detected. The imaging method is becoming increasingly popular owing to the following advantages: the relatively low radiation damage, the compositional information that is provided by near-edge X-ray absorption fine-structure spectroscopy (NEXAFS), the ability to image buried layers underneath several tens of nanometer thin films, and its sensitivity to the molecular orientation due to the linear dichroism effect [5]. We have built up pentacene-based OFET devices on SiNx membranes. The in-plane orientation of the pentacene molecules within the grains in the channel region and underneath the top conducting electrodes was derived from polarization dependent STXM investigations thus allowing the correlation of the electronic transport and structural properties on the nanometer length scale. It was shown that pentacene consists of 100 nm to several 1000 nm large domains made of several grains in which the pentacene molecules have a uniform orientation. The orientation is conserved for several neighbouring grains. Compared to other techniques the STXM investigations can be applied for studying the molecular orientation buried underneath metal films.[1] Bao, Z.; Lovinger, A.; Dodabalpur, A. Adv. Mater. 1997, 9, 42.[2] Horowitz, G.; Hajlaoui, M. E. Adv. Mater. 2000, 12, 1046. [3] Mannsfeld, S.C.B; Virkar, A.; Reese, C.; Toney, M.F.; Bao, Z. Adv. Mater. 2009, 21, 2294.[4] Kalihari, V.; Tadmor, E. B.; Haugstad, G. ; Frisbie, C. D. Adv. Mat. 2008, 20, 4033.[5] Stöhr, J.; Siegmann, H. C. Magnetism, Springer-Verlag, Berlin, Heidelberg, 2006.
4:15 PM - II11.5
Transverse Shear Microscopy: A Novel Microstructural Probe for Organic Interfaces.
Vivek Kalihari 1 , Greg Haugstad 2 , C. Daniel Frisbie 1
1 Chemical Engg and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Characterization Facility, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe microstructure of ultrathin organic semiconductor films (1-2nm) on gate dielectrics plays a pivotal role in the electrical transport performance of these films in organic field effect transistors. Similarly, organic/organic interfaces play a crucial role in organic solar cells and organic light emitting diodes. Therefore, it is important to study these critical organic interfaces in order to probe the relationship between thin film microstructure and electrical performance. Conventional characterization techniques such as SEM and TEM cannot be used to probe these interfaces because of the requirement of conducting substrates, the issue of beam damage, and the need for complex sample preparation. Here, we introduce a new contact mode variant of atomic force microscopy, termed transverse shear microscopy (TSM), which can be used to probe organic interfaces. The TSM produces striking, high contrast images of grain size, shape, and orientation in ultrathin films of polycrystalline organic materials, which are hard to visualize by any other method. The ability to image grain orientation substantially enhances the possibilities for quantitative analysis of microstructure.Using TSM, we have quantitatively analyzed the microstructure of pentacene (a benchmark organic semiconductor) monolayer on SiO2 gate dielectric and established that vapor-deposited pentacene layers on SiO2 exhibit coincidence-II epitaxy resulting in a 4 x 4 super-cell structure. This unique thin film homo-epitaxial growth in pentacene layers has not been reported previously for pentacene or for any other molecular system. Furthermore, we demonstrated using Kelvin probe microscopy that the non-epitaxial domains have a lower (more negative) surface potential, which provides a concrete connection between microstructure and electrical properties. We also investigated the fundamental physics behind TSM by comparing it with conventional friction force microscopy. We demonstrated that the TSM signal is different from the conventionally measured friction signal and originates from the tensor nature of the elastic modulus at the sample surface. In order to explain the TSM signal, we used the theory of linear elasticity and develop a model that agrees well with the experimental results and can predict the TSM signal based on the components of the in-plane elastic tensor of the sample. The TSM, with its ability to image elastic anisotropy at high resolution, can be very useful for microstructural characterization of soft materials, and for understanding bonding anisotropy that impacts a variety of physical properties in molecular systems.1) V. Kalihari, G. Haugstad, C. D. Frisbie, submitted to Physical Review Letters2) V. Kalihari, D. J. Ellison, G. Haugstad, C. D. Frisbie, Advanced Materials 21, 3092 (2009)3) P. R. Ribic, V. Kalihari, C. D. Frisbie, G. Bratina, Phys. Rev. B 80, 115307 (2009)4) V. Kalihari, E. B. Tadmor, G. Haugstad, C. D. Frisbie, Advanced Materials 20, 4033 (2008)
4:30 PM - II11.6
Charge Transport and the Effects of Morphology in High Performance Naphthalenedicarboximide-based Conjugated Polymer Films.
Jonathan Rivnay 1 , Michael Toney 2 , Isaac Kauvar 1 , Yan Zheng 3 , Zhihua Chen 3 , Antonio Facchetti 3 , Alberto Salleo 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Lightsource, Menlo Park, California, United States, 3 , Polyera Corporation, Skokie, Illinois, United States
Show AbstractThe achievement of a printable, highly stable, and high performance (up to 0.85 cm2/Vs) n-type polymer, poly{[N,N’-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6,-diyl]-alt-5,5’-(2,2’-bithiophene)}, P(NDI2OD-T2), has renewed interest in low cost complementary circuit and logic applications. The origins of P(NDI2OD-T2)’s high electron mobility, including the morphological chain packing in the bulk, as well as the specifics of backbone proximity and orientation with respect to the interface, are of great interest for further improving materials and device performance. We have shown, using synchrotron X-ray diffraction, that P(NDI2OD-T2) has enhanced ordering in the plane of the substrate as compared to out of plane; the opposite of what is usually expected in high-performance p-type semiconducting polymers such as poly(3-hexylthiophene). This n-type polymer, in fact, shows only weak π-stacking out of plane. In addition, we use advanced X-ray peak shape analysis to decouple grain size and disorder/strain contributions prevalent at the nanoscale of such materials. In order to fully understand the effect of such packing on charge transport we directly relate the morphology determined from X-ray analysis and AFM to in-plane field effect mobility and out-of-plane diode mobility for both spin cast and aligned films. Furthermore, we investigate the effects of device geometry and its relation to both morphology and processing history. These findings, we believe, will facilitate further improvement in the design of high performance polymeric semiconductors and their relevant devices.
4:45 PM - II11.7
Imaging of Electric Field and Charge Distribution With Sub-micron Resolution in a Polarized Organic Field Effect Transistor.
Calogero Sciascia 1 , Michele Celebrano 1 2 , Maddalena Binda 3 , Dario Natali 3 , Guglielmo Lanzani 4 , Juan Cabanillas-Gonzalez 1 5
1 Physics, Politecnico di Milano, Milano Italy, 2 Electronics, Politecnico di Milano, Milano Italy, 3 CNST-IIT@POLIMI, Politecnico di Milano, Milano Italy, 4 Laboratory of Physical Chemistry, ETH, Zurich Switzerland, 5 IMDEA Nanociencia, Madrid Institute of Advanced Studies, Madrid Spain
Show AbstractThe intrinsic chemical and morphological irregularity, typical of organic semiconductors, makes difficult to model the device internal distribution of electric field, charge and current.[1,2,3] Great help in understanding and thus designing new and better device comes from advanced device characterization.[4,5] Here we show how Stark spectroscopy, coupled with confocal microscopy, is able to directly map the electric field in a n-type Copper Fluorinated Phtalocyanine Transistor under different operative conditions. The signal that we detect, from a spot of about 300 nm diameter, is electro-absorption or electro-reflectance, directly proportional to the square of the electric field which is in the device under polarization. This novel technique of electric field imaging presents unique advantages in terms of sensitivity and spatial resolution, it is non-destructive and exploit low incident power compared with other optical techniques.[6,7] Combining the experimental data with numerical 2-D electro-static modelling, it is possible to extrapolate the space charge profile in the few-nanometer thick accumulation layer within the channel as well as the injected carriers at the metal-dye interface.1 H. Bassler, Phys. Status Solidi B 175, 15 (1993);2 M. C. J. M. Vissenberg, M. Matters, Phys. Rev. B 57, 12, 964 (1998);3 N. Tessler, Y. Roichman, Appl. Phys. Lett. 79, 2987 (2001);4 Burgi, H. Sirringhaus, R. H. Friend, Appl. Phys. Lett. 80, 2913-2915 (2002);5 D. Braga, G. Horowitz, Adv. Mater., 21, 1-14 (2009);6 Z. Q. Li, G. M. Wang, N. Sai, D. Moses, M. C. Martin, M. Di Ventra, A. J. Heeger, D. N. Basov, Nano Lett. 2, 224-228 (2006);7 T. Manaka, E. Lim, R. Tamura, M. Iwamoto, Nature Photonics, 1, 581-584 (2007);
5:00 PM - II11.8
Molecular-level Control of Oligothiophene-fullerene Self-organization in 2D.
Dmitrii Perepichka 1 , Jennifer MacLeod 2 , Oleksandr Ivasenko 1 , Chaoying Fu 1 , Tyler Taerum 1 , Federico Rosei 2
1 Chemistry, McGill University, Montreal, Quebec, Canada, 2 Energy, Materials and Telecommunications, INRS, Varennes, Quebec, Canada
Show AbstractThe single greatest advantage of organic molecules vs inorganic materials for electronic applications is their capacity to self-assemble into complex and, at times, functional architectures. In organic semiconductors, molecular order/packing defines the charge transport behavior, but our ability to rationally control this packing (and even understand) is extremely limited. This presentation will discuss application of Scanning Tunneling Microscopy (STM) to study complex self-organization properties of pi-functional organic material (p- and n-type molecular semiconductors) on surfaces. We will disclose our recent finding of striking long-range ordering in monolayers comprising functionalized oligothiophene and fullerene molecules,[1] and discuss details of their molecular-scale phase separation and ordering with potential implications for organic electronic devices, eg. bulk heterojunction solar cells. Prochiral terthienobenzenetricarboxylic acid (TTBTA) self-assembles at the solid/liquid interface into either a porous chicken wire network linked by dimeric hydrogen bonding associations of COOH groups, or a close-packed network linked in an unusual hexameric hydrogen bonding motif. The cavities of the chicken wire structure can efficiently host C60 molecules, which form ordered domains within the pores of the TTBTA lattice with either one, two or three fullerenes per cavity. The observed monodisperse filling and long-range co-alignment of fullerenes is described in terms of a combination of charge-transfer induced repulsions between fullerene molecules and the commensurability between the surface (HOPG) and molecular network.[1] J. M. MacLeod et al. J. Am. Chem. Soc. in press.
5:15 PM - II11.9
A Photoactive Insulated Organic Molecular Nanowire Comprised of Perylene Diimide Chomophores.
Maria Angelella 1 , Jonathan Baram 2 , Elijah Shirman 2 , Boris Rybtchinski 2 , Michael Tauber 1
1 Department of Chemistry and Biochemistry, University of California, San Diego, La Jolla, California, United States, 2 Department of Organic Chemistry, Weizmann Institute of Science, Rehovot Israel
Show AbstractRecently a novel amphiphilic derivative of perylene diimide (PDI) with polyethylene glycol substituents was synthesized and shown to self-assemble into robust micron-length fibers in a binary water/THF solution. (Baram et al. J. Am. Chem.Soc. 2008, 130, 14966–14967). We have investigated the photophysics and charge delocalization of the aggregates by femtosecond-to-millisecond transient absorption as well as EPR spectroscopy. The aggregates absorb broadly across the visible spectral region. Upon photoexcitation, we observe sub-picosecond charge separation. The charge separation persists into the microsecond time regime and is remarkably insensitive to the addition of external quenchers including oxygen. Furthermore, the EPR spectra of chemically reduced aggregates reveal charge-sharing over multiple PDI units on a timescale that exceeds 10 MHz. The insights from both optical and magnetic resonance spectroscopic methods reveal that the self-assembled PDI nanofibers act as an organic molecular wire, with a core that facilitates charge transport and an insulating exterior shield which protects the charges from the environment.
5:30 PM - II11.10
Laser Printing of p- and n-type Organic Semiconductors for Thin Film Transistors.
Abdou Karim Diallo 1 , Ludovic Rapp 2 , Sebastien Nenon 1 , Christine Videlot-Ackermann 1 , Frederic Fages 1 , Anne Patricia Alloncle 2 , Philippe Delaporte 2
1 Centre Interdisciplinaire de Nanoscience de Marseille, CINAM UPR-CNRS 3118, CNRS, Marseille France, 2 Laboratoire LP3 (Lasers, Plasma et Procédés Photoniques) - UMR 6182 CNRS , Université de la Méditerranée , Marseille France
Show AbstractThe study and the implementation of new techniques for electronic components manufacture on flexible supports are an important stage for the development of plastic micro-electronics. The development of the direct printing technologies associated with the use of new organic conducting and semiconductor materials would allow avoiding the use of complex and expensive techniques such as photolithography.Our objective is to develop a laser printing technique, the Laser-Induced Forward Transfer (LIFT) in plastic micro-electronics. The LIFT technique consists in removing a small piece of a thin layer previously deposited on a transparent substrate and transferring it on another substrate using a pulsed laser. This simple, single step, direct printing technique offers the ability to make surface micro patterning or localized deposition of material. This process is a promising alternative for fabrication of organic and metallic electronic components when complex architectures are needed or when usual techniques, such as inkjet printing cannot be considered. It can be applied to sensitive materials without altering their properties and allows to direct-write multilayer systems in a solvent-free single step, without requiring any shadowing mask or vacuum installation. Its opens the way to alternative manufacturing processes for the OTFT technology.Functional organic transistors were achieved using the LIFT technique in various configurations (BGTC, BGBC, TGTC, TGBC), i.e. Bottom Gate or Top Gate with for both Top and Bottom Contact. The LIFT technique reveals itself particularly effective for the precise micro deposition of a large variety of materials, such as metals, organic semiconductors and dielectric with a very high spatial resolution. The organic semiconductors are p-type copper phthalocyanine (CuPc) and n-type copper hexadecafluorophthalocyanine (F16CuPc). Operating devices have been characterized by current-voltage (I-V) measurements and the morphology and thickness of the deposits have been investigated by optical, scanning electronic and atomic force microscopy [1]. By a successive LIFT deposit of patterns to form bilayer-OTFTs, it is possible to switch from an unipolar approach to an ambipolar one where both p- and n-channels operate in a single transistor.[1] L. Rapp, A. K. Diallo, A.P. Alloncle, C. Videlot-Ackermann, F. Fages, P. Delaporte. Applied Physics Letter, 95 (2009) 171109.
5:45 PM - II11.11
Carbon Nanotube Enabled Vertical Organic FETs Using a High Mobility Active Layer.
Mitchell McCarthy 1 , Bo Liu 2 , Andrew Rinzler 2
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 2 Physics, University of Florida, Gainesville, Florida, United States
Show AbstractWe recently reported the development of a carbon nanotube enabled vertical organic field effect transistor (CN-VFET) in which a dilute (but well above percolation) network of carbon nanotubes deposited on top of a gate dielectric act as a source electrode for hole injection into a thin active organic semiconducting layer.[1] The devices operate based on a gate field modulation of the Schottky barrier between the nanotubes and the active layer. The vertical architecture provides the advantage of a readily produced short channel length without the need for high resolution patterning and a conducting channel that scales as the device area rather than a linear channel width as in a conventional TFT. These features have the potential to make the CN-VFET useful where high drive currents are required such as in active matrix organic light emitting diode displays. To date we have reported CN-VFETs with active layer mobilities lower than 10-3 cm2/Vs and operating voltages > 10 V. Here we demonstrate the use of a high hole mobility (> 1 cm2/Vs), high stability small molecule material (dinaphtho_2,3-b:2_,3_-f_thieno_3,2-b_-thiophene, abbreviated DNTT, developed by T. Yamamoto and K. Takimiya)[2] as the active layer. This leads to large output currents (> 10 mA/cm2) at low source-drain voltages (< 1.5 V). [1]B. Liu, M. A. McCarthy, Y. Yoon, D. Y. Kim, Z. Wu, F. So, P. H. Holloway, J. R. Reynolds, J. Guo, and A. G. Rinzler, "Carbon-Nanotube-Enabled Vertical Field Effect and Light-Emitting Transistors," Adv Mater 20 (2008).[2]T. Yamamoto and K. Takimiya, "Facile synthesis of highly pi-extended heteroarenes, dinaphtho[2,3-b : 2 ',3 '-f]chalcogenopheno[3,2-b]chalcogenophenes, and their application to field-effect transistors," Journal of the American Chemical Society 129 (8), 2224-+ (2007).