Symposium Organizers
Marc Baldo Massachusetts Institute of Technology
Antoine Kahn Princeton University
Paul W. M. Blom University of Groningen
Peter Peumans Stanford University
H3: Organic Transitors
Session Chairs
Wednesday AM, December 03, 2008
Room 311 (Hynes)
9:30 AM - **H3.1
Self-Assembled Monolayer Modified Organic Thin-Film Transistors.
Ian Hill 1
1 Physics, Dalhousie University, Halifax, Nova Scotia, Canada
Show AbstractOrganic light emitting devices (OLEDs) have reached the early stages of commercialization. There are several up-and-coming technologies, including organic thin film transistors (OTFTs) and organic photovoltaic (OPV) devices that are still in the early stages of research and development. Much of the recent success of organic electronic devices has been due to the close synergy between applied electronic device researchers, organic chemists, and those studying the fundamental electronic properties of organic thin films and the interfaces formed between these films and metal contacts.For the past few years we have been studying the use of a self-assembled monolayer (SAM) modified gate dielectrics in OTFTs. The original motivation was to replace the commonly used octadecyl trichlorosilane (OTS) with a more stable and easily applied monolayer based on a phosphonic acid linkage rather than the silane. This has given us the opportunity to study the use of monolayers that share the same linkage, but with various organic moieties based on aliphatic, heterocyclic aromatic and aromatic structures, the latter resulting in OTFTs with the lowest reported trap density [1,2]. Most recently, we have studied the influence of the alkyl chain length on the electronic properties of the pentacene thin-film in n-alkyl phosphonic acid modified OTFTs [3]. Surprising results have been found, with a clear maximum in all device metrics at a chain length of 8-10 carbon atoms, which is much shorter than the octadecyl chain length typically used. Origins of the improved performance and the resulting charge trap densities will be discussed. [1] M. McDowell, I. G. Hill, J. E. McDermott, S. L. Bernasek and J. Schwartz, "Improved Organic Thin-Film Transistor Performance Using Self-Assembled Monolayers", Applied Physics Letters 88, 073505 (2006).[2] I. G. Hill, J. Hwang, A. Kahn, C. Huang, J. E. McDermott and J. Schwartz, “Energy Level Alignment between 9-Phosphonoanthracene Self Assembled Monolayers and Pentacene”, Applied Physics Letters 90, 012109 (2007).[3] I. G. Hill, C. M. Weinert, L. Kreplak and B. P. van Zyl, “Influence of self-assembled monolayer chain length on modified gate dielectric pentacene thin-film transistors”, submitted to Journal of Applied Physics A: Materials Science and Processing (2008).
10:00 AM - H3.2
Direct Comparison of the Field Effect Mobility – Carrier Density Relationship in Polymer and Single Crystal Organic Semiconductors using FETs with Liquid Dielectrics.
Yu Xia 1 , JeongHo Cho 1 , Jiyoul Lee 1 , P. Ruden 2 , C. Frisbie 1
1 Department of Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Department of Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe previously reported “air-gap” transistor architecture based on molded poly(dimethylsiloxane) (PDMS) stamps provides a convenient testbed for comparing field effect transport in organic single crystals and polymer semiconductors. We have modified the air-gap testbed by filling the gap with liquids of different dielectric constants. Using ionic liquid 1-ethyl-3- methylimidazoliumbis(trifluoromethylsulfonyl)imide ([EMIM][TFSI]), silicone oil or vacuum as the gate dielectric material, we are able to vary the specific capacitance of the gate/dielectric stack from 10-5 F/cm2 to 10-10 F/cm2. This allows investigation of charge transport in organic semiconductors as a function of controlled carrier densities varying over four orders of magnitude (from 1010 cm-2 to 1014 cm-2). Importantly, the mobility of polymer transistors was found to increase monotonically with increasing carrier density, whereas the mobility of single crystal transistors decreased with increasing carrier density. Such damatically different behavior for single crystals versus polymer films in the same transistor testbed can be rationalized by the different levels of structural order in these two systems. While our essential conclusions about carrier mobility in crystals and polymers have been reported before, the liquid-gap transistors provide a particularly elegant demonstration of the different transport behaviors of these two types of organic semiconductors. In general, the liquid dielectric FETs we employ open opportunities to carefully examine the role of capacitance and dielectric polarizability on carrier transport in organic semiconductors.
10:15 AM - H3.3
Organic Three-dimensional Field-effect Transistors.
M. Uno 1 2 , Jun Takeya 1
1 , Osaka University, Toyonaka Japan, 2 , Technology Research Institute of Osaka Prefecture, Izumi Japan
Show AbstractOrganic field-effect transistors (OFETs) have attracted much attention due to their applicability to flexible, low-cost, low power-consumption devices. So far, however, their performances are much inferior to those of commercial inorganic transistors; as typical carrier mobility of organic transistors remains lower than or comparable to 1 cm2/Vs except for single-crystal devices, continuous challenges are being made to elevate their performance by seeking for new materials. On the other hand, admitting it as a material property, another approach is to decrease the channel length L and increase the channel width W, so that the carrier traveling efficiency is improved. Along this line, we propose a three-dimmentional (3D) OFET to achieve both high on-current and low off-current, which is required for matrix-controlling devices to realize high contrast ratio. In our devices, micro-pillar arrays are built to sustain semiconductor films longitudinal to the substrate, so that the total channel width is given multiplied by large number of the micro-pillars. Ionic liquid is introduced as a gate insulator between the pillars, which is advantageous in injecting high-density carriers with low gate voltage. We note that a vertical-type transistor, referring as a static induction transistor (SIT) was formally reported to enhance the on-current by dispersing the current paths around embedded comb-shaped gate electrodes, though the disadvantage was their high off-current due to difficulty to deplete the whole carriers in principle.We fabricated a prototype device with 400 square-shaped micro-pillars of SU8 (Kayaku Microchem. Corp.) epoxy resin for one pixel area on a gold-coated surface (source electrode). The length and the height of the pillar are both 20 μm. Pentacene film was deposited on the micro-pillar array from diagonal direction to cover the sidewalls of the pillars, so that the channel area is formed from the bottom source electrode to the top of the pillars. Then, 10-nm-thick gold film was formed on top of the pillars, and the pillars were capped with gold film on glass substrate as an upper drain electrode. Ionic liquid as a gate insulator was inserted into the micro-pillar array and filled in the channel area, to complete the device fabrication process. Since each pillar has 2 channel faces and the number of the pillars is 400 for each pixel, W/L ratio of the device is 800. As the FET characteristics were evaluated, the drain current is induced up to 6.1×10-7 A with very high ratio of 1.0×10-6 A/V when gate voltage is applied. The result of the high enhancement ratio is due to the high W/L ratio and efficient carrier injection due to only 1-nm thick electric double layers that hold the gate electric field. Although we still need to optimize the condition of the pentacene deposition to reduce the off-current, the present results suggest promising performance of the new organic 3D transistors for the matrix-controlling devices of flexible displays.
10:30 AM - H3.4
Insulator Polarization Mechanism in Polyelectrolyte-Gated Organic Field-Effect Transistors.
Oscar Larsson 1 , Elias Said 1 , Lars Herlogsson 1 , Xavier Crispin 1 , Magnus Berggren 1
1 Dept. of Science and Technology, Linkoping University, Norrköping Sweden
Show AbstractThe extensive and ongoing research on the topic of organic field-effect transistors (OFETs) promises for low-cost and flexible electronics. A major drawback with OFETs is that they commonly need high operational voltages; making them excessive power consumers and thus impractical in low-end applications in which the available voltages in general are low. One route to lower the operational voltages is to increase the effective capacitance per area of the gate-insulating material. For this purpose, we use an electrolyte material as gate-insulator, more specifically the solid-state polyanionic electrolyte poly(styrenesulfonic acid) (PSSH). When a bias is applied to the electrodes sandwiching an electrolyte; cations (anions) migrates towards the negatively (positively) biased electrode and forms electric double-layer capacitors (EDLCs) characterized by a short distance (a few Å) between the ions and the charged electrode surface. When polyanions, such as PSSH, are used as gate-insulators; the OFETs can be operated at voltages less than 1 V due to the high capacitance of the EDLCs (~ 20 μF/cm2) (Herlogsson et.al. Adv. Mater. 2007, 19 and Said et.al Appl. Phys. Lett. 2006, 89). Here, we investigate the mechanism of formation of the channel and show that it is limited by the polarization of the polyelectrolyte for transistors with short channel lengths. The time needed for the ions to be transported towards the organic semiconductor/PSSH and PSSH/gate interface and the time required to form the electric double-layers are investigated by impedance spectroscopy. The effect of the ionic mobility is investigated by varying the relative humidity of the environment during the measurements. A physical model of the polarization of the polyelectrolyte explains the frequency dependence of the impedance of the EDLCs and the time response of the OFETs.
10:45 AM - H3.5
Device Physics of Organic Electrochemical Transistors for Sensor Applications.
Fabio Cicoira 1 2 , Sang Yoon Yang 1 , John DeFranco 1 , George Malliaras 1
1 Materials Science and Engineering, Cornell UNiversity, Ithaca, New York, United States, 2 Istituto di Fotonica e Nanotecnologie, CNR, Trento Italy
Show AbstractThe considerable research efforts in organic electronics during the last three decades have led to the development of a number of devices like organic light emitting diodes, solar cells and field effect transistors that are nowadays in production or prototype stage. Along with these well-established fields exciting emerging applications are taking advantage of the mixed ionic electronic conductive character of conjugated organic materials [1].In this context the application of organic semiconductor devices to chemical and biological sensors seems to be a great fit. Organics offer facile processing, which can result into low fabrication costs - a requirement for sensor proliferation. Chemical synthesis can be used to tune their electronic properties and to attach chemical and biological receptor sites, leading to high sensitivity and specificity. Low-end performance, which prohibits organics semiconductors from competing with silicon in some technology areas, is not a limitation for sensors. Also, lifetime issues, which usually plague organics, are not relevant for disposable sensors. A promising approach towards organic-based sensors involves the use of conducting polymer (or electrochemical) transistors. These devices can be operated in aqueous environment as efficient ion-to-electron converters, thus providing an interface between the worlds of biology and electronics. Organic electrochemical transistors have received little attention in the scientific community until the recent resurgence due to their application in biosensors. Therefore little is known about the fundamental processes that take place in these devices, essential for their use in sensing applications. In this work we intend to address this important issue. We developed a technique to fabricate arrays of electrochemical transistors on large surface using photolithography and surface engineering. This allowed us to study the electronic properties of devices using different geometries and to analyze their mechanism of operation [2] [3]. In particular we focused on the effect of gate/channel are ratio and the gate material (metal or polymer) on the modulation of the transistor current. The detection limit of the devices of different size to hydrogen peroxide, a species involved in glucose sensing, is analyzed for different device geometries.[1]J. M. Leger, Ad. Mater. 2008, 20, 837.[2]D. A. Bernards, G. G. Malliaras, Adv. Funct. Mater., 2007, 17, 3538.[3]D.A. Bernards, G. G., Malliaras, D. J. Macaya, M. Nikolu, J. A. DeFranco, S. Takamatsu, G. G. Malliaras, J. Mater. Chem. 2008, 18, 116.
11:30 AM - **H3.6
Microstructure and Charge Transport in Semicrystalline Polymeric Semiconductors.
Alberto Salleo 1
1 Materials Science and Engineering, Stanford University, Stanford, California, United States
Show Abstract12:00 PM - H3.7
Crystalline Dielectric Surface-modification Layer: A General Strategy for High Performance Organic Thin-film Transistors.
Zhenan Bao 1 , Ajay Virkar 1 , Stefan Mannsfeld 1 , Michael Toney 2
1 , Stanford University, Stanford, California, United States, 2 , Stanford Synchrotron Radiation Lab , Menlo Park, California, United States
Show AbstractIn organic thin film transistors (OTFTs), charge transport occurs in the first few monolayers of the semiconductor near the semiconductor/dielectric interface. Previous work has investigated the roles of dielectric surface energy, roughness and chemical functionality on performance. However, large discrepancies in performance, even with apparently identical surface treatments, indicate that additional surface parameters must be identified and controlled in order to optimize OTFTs. We found a crystalline, dense octadecylsilane (OTS) surface modification layer promotes two-dimensional growth in a variety of organic semiconductors. Higher mobility is consistently achieved for films deposited on crystalline OTS compared to on disordered OTS, with mobilities as high as 5.3 cm2/Vs and 2.2 cm2/Vs for C60 and pentacene, respectively. We also developed a simple, scalable spin-coating method to produce crystalline OTS. This is a significant step towards a general approach for morphological control of organic semiconductors which is directly linked to their thin film charge carrier transport.
12:15 PM - H3.8
High-Performance Solution Deposited Ambipolar Organic Transistors Based on Terrylene Derivative.
Chuan Liu 1 , Ronald Naber 1 , Henning Sirringhaus 1 , Zhihong Liu 2 , Hoi Tsao 2 , Klaus Müllen 2 , Henrik Lemke 3 , Martin Nielsen 3
1 Physics, University of Cambridge, Cambridge United Kingdom, 2 Physics, Max Planck Institute, Mainz Germany, 3 Niels Bohr Institute, University of Copenhagen, Copenhagen Denmark
Show AbstractThe thin film transistor characteristics of soluble organic oligomer semiconductor terrylene tetracarboxdiimide (TDI), which is homologues of perylene tetracarboxdiimides (PDI), are described. In bottom gate structure, while the transistors with as spun TDI showed poor performance in n-type region, the transistors after simple thermal annealing showed much enhanced n-type performance with FET mobility of 1.1E-2cm^2/(Vs), comparable to the highest mobility reported on soluble oligomers. When applied to top gate structure with special gate dielectric, ambipolar performance was observed with FET mobility of 7.2E-3cm^2/(Vs) and 2.2E-3cm^2/(Vs) for n-type and p-type respectively. The mobilities were highly enhanced by three orders of magnitude compared to that of the samples without annealing. To understand the change of the microstructure causing the enhancement by annealing effect, investigation including AFM, XRD and Raman spectroscopy were studied. It was observed that after annealing, TDI film was crystallized in terrace structure and the molecules packed hexagonally in discoid with axis perpendicular to the substrate, forming closed packing favorable by charge transport between the neighboring terrylene main bones. From these results, we suggest molecular modification on the alkyl chain of the end group of TDI can probably assist charge transport between columns of the packing and thus higher mobilities would be obtained. Also, TDI was the first homologues of perylene that was reported with ambipolar performance in FET. The special dielectric used here was proved to be trap-free for assisting ambipolar charge transport and was suitable for most semiconducting materials.
12:30 PM - H3.9
Device Operation of Polymer Dual-Gate Field-effect Transistors.
Francesco Maddalena 1 , Mark-Jan Spijkman 1 2 , Johan Brondijk 1 , Peter Fonteijn 1 , Frank Brouwer 1 , Jan Hummelen 1 , Dago de Leeuw 1 2 , Paul W. Blom 1 , Bert de Boer 1
1 Molecular Electronics, University of Groningen, Groningen Netherlands, 2 , Philips, Eindhoven Netherlands
Show AbstractA very important issue in organic field-effect transistors (OFETs) is the control of the threshold voltage (VTH), which becomes crucial for proper operation and low power consumption, especially in complicated circuitry. In a dual-gate OFET the additional top gate can be used to accurately control the threshold voltage. As a result in dual-gate transistors the on/off current ratio and subthreshold slope can be increased [1]. Furthermore, dual-gate transistors are reported to operate as single AND gates[2]. So far, dual-gate OFETs have mainly been realized with evaporated pentacene as semiconductor. Here we report on dual-gate OFETs fabricated by solution processing using different p-type polymer semiconductors and polymer top-dielectric materials on prefabricated substrates. These polymeric dual-gate OFETs were characterized in double gate mode by sweeping the bottom gate bias from 40 to –50 V while fixing the top gate potentials, and vice versa. We demonstrate that the change in the threshold voltage of the bottom gate depends on the top gate bias with two linear relationships for two different regimes: If one of the gate potentials is positive and the channel is in depletion, while the other channel is in accumulation, then both gate potentials will influence the active channel.If both channels are in accumulation, the gate potentials are screened by the accumulated charge carriers closest to that gate and both channels operate individually: no mutual influences are observed. For a dual-gate OFET with its top channel in accumulation, we demonstrate a drop in the transconductance when the bottom gate potential becomes negative. This transition regime between both linear regimes is marked by a drop in the transconductance, where the bottom channel depends on the bottom gate only and the top channel will depend on both gates. The transition regime results from the fact that the charges accumulated in the bottom channel will start to screen the influence of the bottom gate potential on the top channel and the change in overall drain current will depend only on the change of the current of the bottom channel.[1]G.H. Gelinck, E. van Veenendaal, and R. Coehoorn, Appl. Phys. Lett, 87, 073508, (2005).[2]L. Chua, P. K. H. Ho, and R. H. Friend, Appl. Phys. Lett, 87, 253512, (2005).
12:45 PM - H3.10
Measuring Electrical Resistance of Aromatic Molecules as a Function of Molecular Length and Contact Type.
BongSoo Kim 1 , SeongHo Choi 1 , Moon Sung Kang 2 , Xiaoyang Zhu 1 , C. Daniel Frisbie 2 1
1 Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States, 2 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractUnderstanding of charge transport at the interface of metal/organic molecules is of great importance for the advancement of organic electronic devices. Critical factors that determine conductivity include the nature of metal-molecule contacts and the electronic structure of the molecular backbone. We have studied the electronic properties of self-assembled monolayers (SAMs) of aromatic acenes (benzene, naphthalene, anthracene, tetracene) monothiols and dithiols by conducting probe atomic force microscopy (CP-AFM). Nanoscopic metal-molecule-metal junctions were formed by contacting Au-, Pt-, or Ag-coated AFM tips with the SAMs of the molecules on Au, Pt, or Ag substrates. At low biases, resistance (R) increases exponentially with molecular length in both cases, which is consistent with the well known relationship, R = R0 exp (β●s) where R0 is the contact resistance, β is the tunneling attenuation factor, and s is molecular length. The tunneling attenuation factor (β) is ca. 0.5/Å for aromatic monothiols and ca. 0.2/Å for dithiols, while the contact resistance of aromatic dithiols is much less than that of monothiols. Moreover, the contact resistance decreases with increasing metal work function. Ultraviolet photoelectron spectroscopy measurements revealed the electronic structure of these interfaces. In general, we find good correlation between HOMO levels and the measured resistance.
H4: Organic Light Emitting Devices
Session Chairs
Wednesday PM, December 03, 2008
Room 311 (Hynes)
2:30 PM - **H4.1
Free Radicals in Operational Degradation of OLED Devices.
Denis Kondakov 1
1 , Eastman Kodak Company, Rochester, New York, United States
Show Abstract3:00 PM - H4.2
White Phosphorescent OLEDs.
Brian D'Andrade 1 , James Esler 1 , Chun Lin 1 , Vadim Adamovich 1 , Sean Xia 1 , Michael Weaver 1 , Raymond Kwong 1 , Julie Brown 1
1 , Universal Display Corporation, Ewing, New Jersey, United States
Show Abstract3:15 PM - H4.3
High-efficiency Phosphorescent White Organic Light Emitting Devices with Multiple Exciton Generation Regions.
Yiru Sun 1 , Stephen Forrest 2
1 Dept. of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Departments of Electrical Engineering and Computer Science, Physics, and Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractWe demonstrate a white electrophosphorescent organic light emitting device (WOLED) with a three-section emission layer (EML) where excitons are formed in multiple regions across the EML. The three sections in the EML, separately doped with corresponding red, green, and blue (R,G and B) phosphors, also act as a hole or electron injection layer to the adjacent emission section. The three ambipolar hosts form a stepped progression of highest occupied and lowest unoccupied molecular orbital energies. Analysis shows that (36±6)% of the excitons form in the blue emitting region, while (64±6)% form in the green emitting region at 100mA/cm2. The dopant-host energy alignment in each EML section allows for efficient utilization of excitons formed in these multiple regions. Based on this architecture, the WOLED exhibits an internal quantum efficiency close to unity. The WOLED has peak total external quantum and power efficiencies of ηext, t =(26±1)% and ηp,t = (63±3)lm/W [REF1]. When an undoped electron transport layer is used, the peak efficiency increases to ηext,t = (28±1)%. Due to the distributed exciton formation in the EML, the WOLED exhibits a total efficiency higher than monochromatic devices employing the same R, G, and B dopant-host combinations, and >2.5 times higher than the efficiencies of a WOLED that uses the same R, G, and B phosphors but has a local accumulation of excitons.Reference:REF1: Yiru Sun, and S.R. Forrest, APPLIED PHYSICS LETTERS, 91, 263503 (2007)
3:30 PM - H4.4
Origins of the Quantum Efficiency Roll-off in Fluorescent and Phosphorescent Organic Light Emitting Diodes.
Noel Giebink 1 2 , Stephen Forrest 2
1 Dept. of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 2 Depts. of Physics, Electrical Engineering & Computer Science, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe external quantum (EQE) and power efficiencies of both fluorescent and phosphorescent organic light emitting devices (OLEDs) commonly show a monotonic decrease with increasing current density after peaking in the range 0.1-10mA/cm2. Monitoring the photoluminescence transients from an optically pumped, electrically driven device allows for separately quantifying the component contributions to the roll off from quenching due to bimolecular and field-induced processes, and loss of charge balance. We study four fluorescent and phosphorescent OLEDs based on the following emitting molecules: 2,3,7,8,12,13,17,18-octaethylporphine platinum (PtOEP), fac tris-2-phenylpyridine iridium [Ir(ppy)3], 4-dicyanmethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM), and neat tris(8-hydroxyquinoline) aluminum (Alq3). In the case of PtOEP, the EQE roll-off is due solely to quenching, however, for Ir(ppy)3, loss of charge balance is the dominant source at low current densities, and quenching only becomes appreciable at J>1A/cm2. No quenching is observed in the case of DCM-doped Alq3, even at J>25A/cm2 (i.e. all roll-off observed is due to loss of charge balance). We find that the magnitude of quenching scales with emitter lifetime, and discuss our results in the context of designing ultra-bright fluorescent and phosphorescent OLEDs.
3:45 PM - H4.5
Photolithographic Micropatterning of Organic Electronic Materials.
H. H. Fong 1 , Alexander Zakhidov 1 , Jin-Kyun Lee 1 , John A. DeFranco 1 , Ha Soo Hwang 1 , Magarita Chatzichristidi 1 , Christopher K. Ober 1 , George G. Malliaras 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractWe demonstrate a critical technique of patterning multi-layer organic devices by different orthogonal development media including supercritical carbon dioxide and hydrofluoroethers. Small molecules and polymer materials can be patterned on top of different substrates such as metal, transparent conducting oxide, and even polymer, with a spatial resolution down to a micron. Using supercritical carbon dioxide, we fabricated a patterned polymer light-emitting diode. A molecularly-doped polymer light emitting diode is patterned on top of Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS). This green-emitting device exhibits an efficiency of 22 cd/A and 8 lm/W at 100 cd/m2, which is compatible to the typical device performance. Using hydrofluoroethers as development media, we developed a novel photosensitive material that allows us to pattern traditional organic electronic materials without any damage. A stack structure of patterned multi-layers is demonstrated. Furthermore, P3HT and pentacene transistors with patterned Au top contacts using lift-off method is demonstrated.
4:30 PM - **H4.6
Towards a Second Generation OLED Model.
Reinder Coehoorn 1 2 , Peter Bobbert 3
1 Photonic Materials and Devices, Philips Research Labs, Eindhoven Netherlands, 2 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands, 3 Theoretical and Polymer Physics, Eindhoven University of Technology, Eindhoven Netherlands
Show AbstractOLEDs can be highly efficient large-area light sources, with a potential for lighting applications in hitherto unprecedented ways, such as light-emitting flexible foils. In the past five years, the luminous efficacy of prototype white OLEDs has shown a very fast increase, towards a present world-record value of approximately 100 lm/W. This has been made possible by developing OLEDs with ever-increasing complexity of the layer structure. For the further development of OLEDs with a high efficiency, long lifetime and good color stability, the availability of an experimentally validated opto-electronic device model will be crucial. At present, such a model is not available. It has recently become clear that today's "first generation" models, based on conventional understanding of transport and photo-physical processes, are insufficient for realistic OLED materials. Such models neglect several consequences of the disordered nature of the organic semiconductors used. In the talk, the present status of advanced device simulations of transport and recombination in OLEDs will be discussed, as well as experiments which validate this work. The advanced device simulations include three-dimensional (3D) Master Equation studies of steady state transport, through organic semiconductors with realistic energetic disorder. We term such more advanced 3D-OLED models “second generation models”. It will be shown that the current density in OLEDs is expected to be highly filamentary, instead of uniform, and that this gives rise to an enhancement of the current density as compared to that expected from one-dimensional continuum models, in the space-charge limited as well as in the injection limited current regimes. Disorder has also distinct effects on the small signal a.c. response (admittance), leading even to a negative capacitance in double carrier devices, as has been observed indeed for some OLED systems, and on the large signal (step) response (“dark injection”). For obtaining a more full understanding of transient effects, a novel model is developed for calculating the complex a.c. component of the mobility function (which describes relaxation effects). These theoretical and modeling developments are used to analyze the d.c. and a.c. electron and hole transport in blue-emitting polymer OLEDs and in several small-molecule materials which are used frequently in OLEDs. The experimental data are analyzed assuming different types of disorder (with and without correlations between the site energies), allowing us to indicate the possible role of correlations. It is shown how a proper inclusion of the effects of disorder modifies the conclusions about the mobilities and injection barriers in OLEDs obtained using conventional transport models which neglect the effects of disorder.
5:00 PM - H4.7
Direct Measurements of Energetic Disorder.
Priya Jadhav 1 , Kemal Celebi 1 , Kaveh Milaninia 1 , Mihai Bora 1 , Marc Baldo 1
1 Department of Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States
Show AbstractEnergetic disorder in organic semiconductors creates important differences between the electronic properties of conventional and organic semiconductors. Consequently, charge transport models for organic semiconductors depend on knowledge of the density of states (DOS) as a function of energy. Recently, Tal et al [1] have extracted high resolution DOS measurements using Kelvin probe force microscopy (KPFM). They examined the hole transport material N,N’-diphenyl-N,N’-bis(1-naphthyl)-1,1’-biphenyl–4,4’-diamine (α-NPD) and reported both a central Gaussian DOS and an exponential tail. In this work, we adopt the approach of Tal, et al. and apply it to the common hole transport material copper phthalocyanine (CuPC). We combine synchronous measurements and new electrostatic models of non-uniformities in the channel potential to probe the limits of the KPFM technique. In contrast to Tal, et al., we find only an exponential DOS with a characteristic energy of 0.11eV over at least a 0.5eV range of energies in the tail of the CuPC HOMO. The range of data is sufficient to confirm that, in CuPC at least, the DOS is not Gaussian, as is commonly expected. The range of data is limited for large DOS by non-uniform channel potentials and hysteresis for small DOS. We find, however, that both limits are alleviated for thin channel potentials. 1.Tal, O., Rosenwaks, Y., Preezant, Y., Tessler, N., Chan, C. K. & Kahn, A. Direct determination of the hole density of states in undoped and doped amorphous organic films with high lateral resolution. Physical Review Letters 95, 256405 (2005).
5:15 PM - H4.8
Full-wave Simulation of Enhanced Outcoupling of Organic Light Emitting Devices with an Embedded Low-index Grid.
Michael Slootsky 1 , Yiru Sun 2 3 , Stephen Forrest 1 3 4
1 Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 4 Material Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractEnhancement of light outcoupling into substrate modes by a previously demonstrated rectangular grid of low-refractive-index material embedded into the organic layer[1] of an OLED is analyzed using full-wave simulations. The low-index grid (LIG) allows modes normally trapped within the high-index organic and ITO layers (waveguide modes) to be redirected into the substrate where they can be further extracted to the free space using such methods as microlens arrays or roughened surfaces[1,2]. This significantly increases the external efficiency of the device since more than 45% of light emission is confined in waveguided modes in conventional OLEDs on flat glass substrates[2]. For the simulation, the light source is assumed to be a randomly aligned dipole that is decomposed into vertical (perpendicular to the plane) and horizontal (in-plane) components whose waveguiding properties are separately considered. Our results indicate that the vertical dipole couples strongly to waveguided modes, and hence the LIG increases the outcoupling to the substrate by a factor of >3, while for the weakly-waveguided horizontal dipole the glass modes are increased by 5-10%. When averaged over all dipole orientations, the LIG reduces the amount of light trapped in the waveguided modes by >50%. These results are consistent with experimental data indicating that high-external-efficiency OLEDs can be achieved by using a LIG in conjunction with microlens arrays that efficiently outcouple the glass modes from the substrate into the air[1]. In particular, this method can be used in white OLEDs for low-cost and efficient displays and indoor lighting since the improvement is not wavelength-dependent nor strongly directional. Optimized LIG geometries and materials will be discussed in this talk.References:1. Yiru Sun, and Stephen R. Forrest, “Enhanced Light Outcoupling of Organic Light Emitting Devices using Embedded Low-Index Grids,” Nature Photonics (accepted, 2008).2. Toshitaka Nakamura, Naoto Tsutsumi, Noriyuki Juni, and Hironaka Fujii, “Thin-film waveguiding mode light extraction in organic electroluminescent device using high refractive index substrate,” J. Appl. Phys. 97, 054505 (2005).
5:30 PM - H4.9
Light Extraction and Optical Loss Mechanisms in Organic Light-emitting Diodes.
Wolfgang Bruetting 1 , Stefan Nowy 1 , Joerg Frischeisen 1 , Benjamin Krummacher 2
1 Institute of Physics, University of Augsburg, Augsburg Germany, 2 , Osram Opto Semiconductors GmbH, Regensburg Germany
Show AbstractThe internal quantum efficiency of organic light-emitting diodes (OLEDs) can reach values close to 100% if phosphorescent emitters to harvest triplet excitons are used, however the fraction of light that is actually leaving the device is considerably less. Loss mechanisms are for example waveguiding in the organic layers and the substrate as well as the excitation of surface plasmon polaritons at metallic electrodes. We have used numerical simulations based on a model of emissive dipoles, which are embedded in the multilayer stack of an OLED, to identify and quantify different loss mechanisms. Changing various simulation parameters, for example the distance of the emissive layer to the cathode or the thicknesses of the surrounding layers, enables us to study their influence on the fraction of light leaving the device. An important parameter in these simulations and for the actual device is the intrinsic quantum efficiency of the emitter material, which is defined as the radiative quantum efficiency in an unbounded space filled by the dye and its matrix. The simulations show that due to weak microcavity effects the radiative decay channel can be significantly enhanced in an OLED as compared to free space emission of a dipole. As an example, we present simulations of bottom-emitting OLEDs based on the well-known green emitter tris-(8-hydroxyquinoline) aluminum (Alq3) with transparent indium tin oxide anode and a calcium/aluminum cathode. We will show that the simulations comply very well with measured angular and polarization dependent emission spectra. Furthermore, we will demonstrate the influence of microcavity effects on the internal quantum efficiency and quantify the influence of various device parameters on the amount of radiation going into different optical channels. Additionally, we use our simulation tool on a series of thickness variations of a phosphorescent OLED stack in order to extract the actual internal quantum efficiency of the emitter material, which in turn is crucial for the optimization of the devices.
5:45 PM - H4.10
Fiber Based OLEDs for Solid-State Lighting Applications.
Denis Nothern 1 , Brendan O'Connor 2 , Max Shtein 1
1 Materials Science & Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractThe performance of organic light emitting devices (OLEDs) has increased markedly in the last decade, with white OLED power efficiency exceeding 40-60 lm/W.1,2 The increased power efficiency, coupled with the ability to deposit OLEDs on low-cost substrates suggests their application to solid-state lighting.3 However, several challenges remain to be addressed, including the high cost of ITO and low light outcoupling efficiency. Recently, we have demonstrated ITO-free OLEDs deposited on fiber substrates, showing the performance of a single OLED fiber to be comparable to an analogous structure deposited on a planar substrate.4 In this work, we consider arrays of fiber OLEDs and predict interesting performance gains over the conventional planar device architecture. We calculate the emission spectra of planar and fiber-based OLEDs for metal-organic-metal layer structures, and devices that utilize optimized dielectric coating structures to enhance broadband light outcoupling efficiency. In the case of fiber-shaped devices, the substrate geometry results in uniform emission intensity distribution with angle, leading to a device with simultaneously optimized broadband and broad-angle emission. Furthermore, bundles of red-, green-, and blue-emitting fibers have been simulated; reflections off neighboring fibers produce white light in the far-field, for a broad range of angles. These results suggest that fiber-based OLEDs may offer an avenue for realizing efficient and cost-effective, ITO-free OLEDs for solid-state lighting applications.[1] Y. Sun et al. Appl. Phys. Lett. 91, 263503 (2007).[2] J. H. Seo et al. Appl. Phys. Lett. 90, 203507 (2007).[3] B. W. D’Andrade and S. R. Forrest, Adv. Mater. 16, No. 18 (2004).[4] B. O’Conner et al. Adv. Mater. 19, 3897 (2007).
H5: Poster Session
Session Chairs
Thursday AM, December 04, 2008
Exhibition Hall D (Hynes)
9:00 PM - H5.1
Doping Poly[2-methoxy-5-(2'-ethylhexyloxy)-1, 4-phenylenevinylene] (MEH-PPV) with Nanoparticles.
Zhengzheng Zhang 1 , David Chambers 2 , Orhan Kizilkaya 3 , Yaroslav Losovyj 3 , Sandra Selmic 2 , Fauzia Khatkhatay 2 , Srikanth Karanam 2
1 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States, 2 Electrical Engineering Program and Institute for Micromanufacturing, Louisiana Tech University, Ruston, Louisiana, United States, 3 Center for Advanced Microstructures and Devices, Louisiana State University, Baton Rouge, Louisiana, United States
Show Abstract9:00 PM - H5.10
Air-Stable n-Channel Organic Single-Crystal Field-Effect Transistors.
Takafumi Uemura 1 , Masakazu Yamagishi 1 , Yukihiro Tominari 1 , Jun Takeya 1 2
1 , Osaka University, Toyonaka Japan, 2 , JST-PRESTO, Kawaguchi Japan
Show AbstractHigh-performance and air-stable organic field-effect transistors (OFETs) of both p- and n-types are indispensable to develop organic complementary circuits for practical use. In particular, it is crucial to figure out proper prescriptions to build high-mobility n-type OFETs which operate stably in air, because such devices are still very limited even in recent reports. So far, several possible reasons are argued for their relatively low carrier mobility and/or poor chemical stability in air. With the use of noble metals for carrier-injecting electrodes, energy-level mismatch between their Fermi levels and the lowest unoccupied molecular orbital (LUMO) levels of organic semiconductors results in inefficient electron injection. Moreover, influence of the atmospheric oxidants O2 and H2O absorbed in grain boundaries of organic semiconductors or OH groups at the surface of SiO2 gate dielectric may also harm electron transport, acting as electron-trapping centers [1,2]. In this study, OFETs based on n-type organic single crystals are employed to enable more fundamental evaluation of the intrinsic device properties, eliminating influences of the grain boundaries, as have been successfully done for p-type crystalline OFETs. Indeed, the highest mobility in air exceeds 40 cm2/Vs for rubrene single crystal transistors [3].In order to prevent the energy mismatch between the organic semiconductor and metal electrodes, we chose molecular compounds with relatively large electron-affinity to build air-stable n-type single-crystal OFETs. We grew single crystals of the TCNQ molecules, using the technique of physical vapor transport. The bottom-contact OFETs were fabricated by laminating thin TCNQ single crystals onto Au electrodes patterned on SiO2/doped silicon substrate and were evaluated in the air. All the five transistors prepared exhibited text-book like n-type characteristics even under ambient conditions with very small threshold and negligible hysteresis. Typical mobility is evaluated to be approximately 0.2 cm2/Vs, which is two orders of magnitude higher than the reported value of TCNQ thin-film transistors measured in the air [4]. In particular, we emphasize that the absence of the threshold voltage VG is contrasting to the fact that almost all the n-type transistors in the literature show large VG exceeding 10 V. The results indicate that the electron-trapping tendency can be reduced in the TCNQ single-crystal OFETs without influence of harmful adsorbates at grain boundaries. As long as the electron-affinity of the organic semiconductors is large enough, no essential obstacles appear to prevent the air-stable n-type field-effect operation, providing a prescription of high-performance and air-stable organic complementary circuits.[1] Lay-Lay Chua et al., Nature, 434, 194 (2005).[2] T. J. Marks et al., J. Am. Chem. Soc., 129, 15259 (2007).[3] J. Takeya et al., Appl. Phys. Lett., 90, 102120 (2007).[4] A. R. Brown et al., Synth. Met., 66, 257 (1994).
9:00 PM - H5.11
Influence of Hierarchical Crystal Structure on Carrier Transport in Pentacene Polycrystalline Thin Films: Morphological Grain—crystalline Domain—crystallite.
Ryousuke Matsubara 1 , Yuki Sakai 1 , Noboru Ohashi 1 , Masatoshi Sakai 1 , Kazuhiro Kudo 1 , Atsuhumi Kumagai 2 , Noriyuki Yoshimoto 2 , Masakazu Nakamura 1
1 Department of Electrical and Electronic Engineering, Chiba university, Chiba-shi, Chiba, Japan, 2 Department of Materials Science and Engineering, Iwate university, Morioka-shi, Iwate, Japan
Show AbstractSemiconducting organic polycrystalline films with high carrier mobility such as pentacene are important for the application to organic thin-film transistors (OTFTs). Their detailed electric characters and its relation with the film structure are therefore expected to be clarified. However, the carrier transport in polycrystalline semiconductors is not straightforward to analyze because of their structural complexity. Even for the widely studied pentacene, the carrier transport has been often analyzed by simply applying a physical model for amorphous materials or by applying a model for conventional single-crystal semiconductors with an empirical parameter including various physical phenomena. The essential description as a polycrystalline semiconductor is, therefore, still an open question. We have been comprehensively studying the carrier transport mechanism in pentacene polycrystalline films using four-point-probe field-effect (FPP-FET) measurements that can eliminate the influences of electrodes and atomic-force-microscope potentiometry (AFMP) that can quantitatively visualize the transport bottlenecks in working OTFTs. We have already demonstrated that there exist bottlenecks of carrier transport not only at morphological grain boundaries but also at crystalline domain boundaries in the grain [1], and that small HOMO band fluctuation exists even in the crystalline domain [2]. In addition to these studies, we have recently performed a crystallographic analysis with glazing incidence X-ray diffraction (GIXD) using synchrotron orbital radiation, and have found that the crystallite, that is a domain having the same structure as a single crystal, of the pentacene films is much smaller than the domain size. The relation between the electrical ununiformity and the hierarchical crystal structure, morphological grain—crystalline domain—crystallite, will be discussed.[1] R. Matsubara et al., Appl. Phys. Lett. 92 (2008) 242108.[2] N. Ohashi et al., Appl. Phys. Lett. 91 (2007) 162105.
9:00 PM - H5.12
Large Polaron in an Anharmonic Oligotheophenes Molecular Crystals (Organic Field Effect Transistors).
Adil-Gerai Kussow 1
1 Dept. of Physics, Univ. of Massachusetts Lowell, Lowell, Massachusetts, United States
Show Abstract9:00 PM - H5.13
The Study of Surface Modification of Indium-Tin-Oxide and Resist Surfaces with CF4/O2 Plasma for Manufacturing Organic Light-Emitting Diodes by Ink Jet Printing.
Masakuni Ikagawa 1 3 , Ichiro Tohno 1 , Tadashi Shinmura 1 , Shigeyuki Takagi 1 , Yoshinori Kataoka 2 , Masamichi Fujihira 3
1 Corporate Manufacturing Engineering Center, Toshiba Corporation, Yokohama, Kanagawa, Japan, 3 Department of Biomolecular Engineering, Tokyo Institute of Technology, Yokohama, Kanagawa, Japan, 2 , SED Incorporated, Hiratsuka, Kanagawa, Japan
Show AbstractWe studied a surface modification technique of indium-tin-oxide (ITO) anodes and resist banks for manufacturing organic light-emitting diodes (OLEDs) by inkjet printing. The ITO surface modified by inductively coupled plasma (ICP) with an optimized CF4/O2 (7:3) gas mixture was compatible in improvement in both of hydrophilicity and its work function, while the resist surface treated by the plasma became hydrophobic. The resist and the ITO surfaces treated by plasmas of various gas mixtures (i.e. CF4, CF4/Ar (1:2), CF4/O2 (x:1; x=1, 7/3, 4, and 9) were analyzed by X-ray photoelectron spectroscopy (XPS) of the C 1s, F 1s, O 1s, and In 3d5/2 core levels. On the ITO surfaces modified by CF4/O2 plasmas, organic contaminants were removed more efficiently and deposition of CFx decreased with an increase of oxygen. The amount of F in the form of InFx increased about 10 % by the CF4/O2 (7:3) plasma in comparison with that by the CF4/Ar plasma. We investigated species in the CF4 plasma and in the CF4/O2 plasmas by mass spectrometry. In the CF4/O2 (7:3) plasma, the sum of the peak intensities of F+, HF+ and F2+ increased by 2.6 times and that of O+ and O2+ by 9.8 times in comparison with those in the CF4 plasma. The results suggest that In2O3 was generated more by oxidation reaction of indium with O, and InFx by fluoridation reaction of indium with HF. By introducing InFx on ITO surfaces by the CF4/O2 plasma, a hole-injection energy barrier could be reduced.
9:00 PM - H5.14
Low Contact Resistance of O2 Plasma Treated Molybdenum Metal Electrodes for an Organic Thin-film Transistor.
SeYoung Park 1 , Min-Joo Kim 1 , Jaeseok Heo 1 , Young-Hoon Hoh 1 , Sul Lee 1 , Ho-Cheol Kang 1 , Jae-Yeal Kim 1 , In-Byeong Kang 1
1 , LG Display, Anyang-si Korea (the Republic of)
Show AbstractIn this work, molybdenum (Mo) is used for source and drain electrodes of an organic thin-film transistor (OTFT). Mo electrodes are treated by O2 plasma to form Mo oxide at the metal surface before the organic semiconductor is spin-coated. This metal oxide reduces the contact resistance between organic semiconductor (OSC) and metal electrodes. Mo oxide very thin-film, directly contacting the organic semiconducting layer, acts as a charge-injection layer. We found that the very thin metal oxide layer formed by O2 plasma played an important role in improving the Ohmic-contact charac-teristics of OTFTs. Devices with the Mo oxide elec-trodes showed enhanced hole-injection compared to those with only metal electrode. High field-effect mo-bility of 0.15 cm2/Vs and on/off current ratios of 105 were obtained in the soluble TIPS pentacene derivative based TFTs using the Mo oxide S-D electrodes at a gate bias of –30 V. The contact resistance between O2 plasma treated Mo S-D electrodes and OSC is obtained about 20 MΩ compared to that of Au S-D electrodes, about 10 MΩ.
9:00 PM - H5.15
Examination of Oxide/organic Injection Contact by Dark Injection Space-charged-limited-current Technique.
Chi Hang Cheung 1 , Shing Chi Tse 2 , Shu Kong So 1
1 , Department of Physics and Centre for Advanced Luminescence Materials, Hong Kong Baptist University, Hong Kong, N/A, China, 2 , Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada
Show AbstractTransition metal oxides such as molybdenum oxide (MoOx) and vanadium oxide (V2Ox), were demonstrated to improve the device performance in the field of organic electronics by facilitating charge injection from electrodes. However, reports on the examination for their injection quality are lacking. In this contribution, several metal oxides are used as hole injection layers in current-voltage (J-V) and dark injection space-charged-limited-current (DI-SCLC). The organic material under investigation is N, N’-diphenyl-N,N’-bis(1-naphthyl)(1,1’biphenyl)-4,4’diamine (NPB), which is a common hole transporter for organic light emitting diodes (OLEDs). With a large value (~ 5.4eV) of highest occupied molecular orbital (HOMO), NPB is an ideal candidate for hole injection behavior examination. In J-V experiment, the measured currents of NPB are in satisfactory agreement with the theoretical space-charge-limited currents. Besides, typical DI-transients can be clearly observed in DI-SCLC experiment over an entire range of electric fields. The extracted hole mobilities from DI experiment are in good agreement with those obtained from independent time-of-flight (TOF) experiment. Therefore, it can be concluded that the transition metal oxides can form a nearly Ohmic injection contact with NPB. The applications of these metal oxides to organic thin film transistors will be presented.
9:00 PM - H5.16
Effect of Temperature and Pressure on the Organic Bistable Device with Negative Differential Resistance (NDR).
Yue Shao 1 , Wei Lek Kwan 1 , Liping Ma 1 , Guanwen Yang 1 , Bao Lei 1 , Yang Yang 1
1 Materials Science and Engineering, UCLA, Los Angeles, California, United States
Show AbstractStudieshave shown detrimental effects of pressure and temperature on the current-voltage characteristic including negative differential resistance (NDR) in metal-organic-metal structures. Devices exposed to atmospheric pressure, oxygen and air, lost their conductivity with the application of voltage but showed no significant degradation if they are returned to vacuum without application of voltage. Devices operating under 1atm pressure Ar showed no degradation while those under 1atm N2 showed slight degradation. Peak current of formed devices showed pressure and temperature dependence. Filamentary conduction model was used to explain the conduction process for formed devices.
9:00 PM - H5.17
Modification of Charge Transport in Triphenyldiamine Films using Acid Oxidized Single-walled Carbon Nanotube Interlayers.
Li Wei Tan 1 , Ross Hatton 2 , Gianluca Latini 1 , John Shannon 1 , Ravi Silva 1
1 Faculty of Engineering and Physical Science, Advanced Technology Institute, Guildford, Surrey, United Kingdom, 2 Department of Chemistry, University of Warwick, Coventry United Kingdom
Show AbstractUnderstanding how carbon nanotubes (CNTs) interface with conjugated organic molecules is crucial for the utilization of CNTs in organic electronics. We report the modification of the transport, optical, morphological and electrical properties of N, N’-diphenyl-N,N’-bis(3-methylphenyl) 1,1’biphenyl-4,4’diamine (TPD) films on indium-tin oxide (ITO) glass when a low density of acid oxidized single-walled CNTs (o-SWCNTs) are present at the ITO/TPD interface. Comparing the current-voltage characteristic of ITO/TPD/Al and ITO/o-SWCNT/TPD/Al devices, the results show compelling evidence for a change from contact to bulk limited behavior after the insertion of o-SWCNT. Based on the morphology changes of TPD observed from AFM measurement, we have shown that the changes in transport properties of the device is correlated with the changes of the morphology of TPD by the presence of o-SWCNTs at the ITO/TPD interface. Furthermore, electrical and morphology results based on conducting AFM measurement together with optical absorption measurements provide compelling evidence for the existence of a high density of trap states extending throughout the TPD film, which are not present in the absence of o-SWCNTs. We postulate that these observations result from a change in the structure of the TPD film seeded by the presence of o-SWCNTs at the ITO surface. This result shows how SWCNTs confined at interfaces can modify the bulk properties of an overlaying organic semiconductor films, having important implications for the utility of SWCNTs as electrode materials in organic optoelectronic devices.
9:00 PM - H5.18
Structural Properties and Electronic States of Germole Compounds.
Yongmin Kim 1 , Young Hwan Park 1 , Yong Ho Shin 1 , Seung Jung Noh 1 , Honglae Sohn 2
1 Applied physics, Dankook University, Yongin Korea (the Republic of), 2 Chemistry, Chosun Univeristy, Gwangju Korea (the Republic of)
Show AbstractComposed of thin multilayers of hole transporting, emissive, and electron transporting (ET) materials sandwiched between two electrodes, are a great deal of interest because of their possible application for large-area flat panel displays. One of the major subjects in this field is the development of efficient ET materials. The efficiency of the electron injection for the devices reported so far seems to be inferior to that of the hole injection, due mainly to the large differences between the Fermi levels of cathode metals and the LUMO levels of the ET materials. High electron affinity may thus be the first requisite for the design of ET materials. Silicon-containing cyclic π-electron system, silole (silacyclopentadiene), can be a good example as a novel core component for efficient ET materials. Recently, it has been theoretically demonstrated that the low-lying LUMO energy level of the silole ring is ascribed the σ*- π* conjugation between the σ* orbital of the exocyclic σ bond on silicon and the π* orbital of the butadiene moiety in the ring. Silole ring has the lowest LUMO energy level among them. It was thus anticipated that new efficient ET materials would be realized by using the silole ring as a core component. Germanium can be a substitute for silicon in a silole compound called germole. We carried out molecular orbital and electronic structures (HOMO and LUMO energy levels) calculations for germole compounds on the restricted Density Functional Theory (DFT) level using a B3LYP/6-31G(d) basis set, employing the Gaussian 98 suit of programs. Optical properties investigation using the photoluminescence (PL) at room temperature. We report on highly efficient organic light-emitting diodes (OLEDs) using two novel germole derivatives as emissive and electron transport materials.
9:00 PM - H5.2
Color Tunable and Signal Transmission Utilizing White Light Organic Light-Emitting-Diodes Fabricated by Solution and Vacuum Process.
Yutaka Ohmori 1 , Toshikuni Kimpara 1 , Noriyoshi Takahota 1 , Hirotake Kajii 1
1 Ctr for Adv Sci & Innov (CASI), Osaka University, Suita, Osaka, Japan
Show Abstract9:00 PM - H5.20
Injection Barriers, the AC Mobility and the Recombination Rate Constant in Disordered Organic Semiconductor Devices, Studied from Admittance Measurements.
Siebe van Mensfoort 1 2 , Wijnand Germs 2 , René Janssen 1 , Reinder Coehoorn 1 2
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands, 2 , Philips Research Laboratories, Eindhoven Netherlands
Show AbstractFor a proper design of high-efficiency, long-lived organic light-emitting diodes (OLEDs) accurate knowledge of injection barriers, mobility functions and recombination rate constants is essential. However, precise determination of these quantities from steady-state transport, studied using single-carrier diodes and full (double carrier) OLEDs, is non-trivial as a result of a lack of understanding of the role played by energetic disorder. Previously, it has been shown that studies of transient transport in OLEDs, such as admittance measurements, can be used to independently obtain the electron and hole mobility. So far, these analyses did not include the full effects of disorder. In this contribution, we show that admittance experiments can be used to independently study injection barriers, the recombination rate constant and energetic relaxation in OLEDs. For that purpose, we present a novel AC drift-diffusion transport model for OLEDs, which is based on the Extended Gaussian Disorder Model (EGDM) introduced by Pasveer et al. [1], and apply our results to blue-emitting polymer OLEDs [2].First, we show how admittance experiments can be used to independently measure the injection and extraction barriers at the anode and cathode interfaces, using a distinct peak in the voltage-dependent differential capacitance, measured at low frequencies [3]. It is shown that the peak becomes more pronounced with increasing energetic disorder. We envisage that studies of the peak height and peak voltage will provide direct information about interface-related processes, e.g. about changes of the injection barriers during lifetime.Second, we find from our model that for double-carrier OLEDs the low-frequency capacitance can become very small or even negative for strong but still realistic Gaussian disorder. This effect has been observed experimentally for various systems, and has been attributed earlier to deviations from the ‘standard’ Langevin recombination rate constant, or to the possible role of monomolecular recombination. Our results emphasize the important role of energetic disorder.Finally, we show that it is possible to properly include in the AC transport model the effects of energetic relaxation of charge carriers in a Gaussian density of states, by developing a comprehensive approach for calculating the frequency, disorder and charge carrier density dependence of the AC mobility. Application of this novel ab initio model to our blue polymer OLEDs leads to an excellent description of the frequency dependent shape of the capacitance-voltage curves.[1]. W.F. Pasveer et al., Phys. Rev. Lett. 94, 206601 (2005). [2]. S.L.M. van Mensfoort et al., Phys. Rev. B (accepted). [3]. S.L.M. van Mensfoort and R. Coehoorn, Phys. Rev. Lett. 100, 086802 (2008).
9:00 PM - H5.21
Blue-light-emitting Ambipolar Organic Single Crystal Field-effect Transistors.
Hajime Nakanotani 1 , Ryota Kabe 1 , Masayuki Yahiro 1 , Taishi Takenobu 2 , Yoshihiro Iwasa 2 , Chihaya Adachi 1
1 Center for Future Chemistry, Kyushu University, Fukuoka Japan, 2 Institute for Materials Research, Tohoku University, Sendai Japan
Show AbstractRecently, light-emitting organic field-effect transistors (LE-OFETs) have been widely studied due to their unique applications for optoelectronics. In particular, some LE-OFETs have shown excellent ambipolar operation with intense electroluminescence under strictly controlled environmental conditions. In this study, we developed an ambipolar LE-OFET based on a wide-band-gap 1,4-Bis(4-methylstyryl)benzene (BSP-Me) single crystal. Single crystals of BSP-Me were grown by physical vapor transport with a flow of pure argon gas (50 cm3/min.) at temperature of 170°C. The BSP-Me single crystal has very high photoluminescence quantum efficiency (ΦPL) of 89±2%, while ΦPL of the BSP-Me vapor-deposited film is limited to a much lower value of 54±2%. Moreover, we found that BSP-Me single crystals show an amplified spontaneous emission (ASE) with rather low ASE threshold of 25 ± 5 μJ/cm2 under optical excitation. From X-ray structural analysis, we found that the BSP-Me crystals have a herringbone packing and that the long axis of the molecules is perpendicularly ordered with the wide crystal face. This characteristic feature of the BSP-Me single crystal represents a significant advantage for achieving high optical gain. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of a BSP-Me crystal were estimated to be 5.6 eV and 2.7 eV by ultraviolet photoelectron spectroscopy and UV-Visible absorption edge from the HOMO level. To inject both holes and electrons equally from the electrodes into the single crystal, asymmetric gold-calcium contacts were used as for both the source and drain electrodes. By taking advantage of the work functions of gold (4.5 eV) and calcium (2.9 eV), the BSP-Me single crystal FET is expected to provide efficient double injection at both interfaces. Ambipolar operation with successive blue electroluminescence from the FETs based on the BSP-Me single crystals was demonstrated by realizing nearly equal electron and hole mobilities (about 0.005 cm2/Vs). However, we note the edge of BSP-Me single crystal showed strong emission compared with that from the surface of the crystal, indicating that the crystal formed an optical-slab waveguide and the light is well propagated in the crystal. Since BSP-Me single crystals can perform light amplification, the BSP-Me-based ambipolar LE-OFET is a promising candidate for future electrically driven organic blue-emitting solid-state lasers.
9:00 PM - H5.22
Stacked White Organic Light Emitting Devices Consisting of Separate Red, Green and Blue Subelements.
Xiangfei Qi 1 , Michael Slootsky 1 , Stephen Forrest 2
1 Department of Physics, University of Michigan, Ann Arbor, Michigan, United States, 2 Departments of Electrical Engineering and Computer Science, Physics, and Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractConventional WOLEDs introduce red, green, and blue (R, G, and B) phosphorescent dopants in either a single emission layer (EML)(1), or multiple doped regions that allow for exciton formation in an expanded EML. Although the latter structure shows external quantum efficiency (EQE) approaching 20%(2), finding a suitable combination of hosts and phosphorescent dopants can be difficult due to the multiple constraints that are placed on the relative energies of the constituents in these architectures. Here we introduce a stacked WOLED (SWOLED) where individual R, G and B subelement OLEDs are vertically stacked and are electrically connected by MoO3, which works as the transparent charge generating layer (CGL)(3). As an early demonstration, a SWOLED with CGLs interposed between R, G and B EMLs, were achieved with an EQE of (9.5±0.6)%, CIE coordinates of (0.37, 0.45), and a color rendering index of 66 at J=1 mA/cm2. Besides the novelty of this structure in achieving white light, the stacked structure with CGLs sandwiched between the EMLs can be used as a means for understanding the nature and efficiency of the charge generation process by monitoring the shift in spectral output and operating voltage as a function of injection current density. In this talk, we will discuss optimized device characteristics, along with the processes of charge generation for MoO3 as well as other charge generation layer compositions. (1) B. W. D’Andrade, R. J. Holmes, and S. R. Forrest, Adv. Mater. 16, 624 (2004).(2) Y. Sun, and S. R. Forrest. Appl. Phys. Lett. 91, 263503 (2007)(3) H. Kanno, R. J. Holmes, Y. Sun, S. Kena-Cohen, and S. R. Forrest, Adv. Mater. 18, 339, (2006)
9:00 PM - H5.23
Photophysical and Electroluminescent Properties of SnF2-OEP having Thermally Activated Delayed Fluorescence.
Ayataka Endo 1 , Mai Ogasawara 2 , Atsushi Takahashi 3 , Yoshimine Kato 2 , Chihaya Adachi 1
1 Center for Future Chemistry, Kyushu University, 744, Motooka, Fukuoka, fukuoka, Japan, 2 Department of Materials Science and Engineering, Kyushu University, 744, Motooka, Fukuoka, fukuoka, Japan, 3 , Sogo Pharmaceutical Co., Ltd., Hebinami 28-3, Johban Shimofunao, Iwaki, Fukushima, Japan
Show AbstractRecent efforts to develop organic light-emitting diodes with high electroluminescence efficiency have focused on phosphorescent metal complexes such as iridium (Ir) and platinum (Pt) complexes because these emitters can harvest both singlet and triplet electrically generated excitons. In contrast, while OLEDs using phosphorescence materials such as iridium complexes achieved almost ηexciton~100%, the pronounced roll-off characteristics of electroluminescence efficiency (ηEL) were observed with an increase of current density due to the occurrence of the triplet-triplet (T-T) annihilation. Therefore, in order to avoid these shortcomings, development of novel light emitting mechanisms has been anticipated.One of the potential mechanisms to achieve compatibility of harvesting both singlet and triplet excitons and to avoid of triplet annihilation is the use of the thermally activated delayed fluorescence (TADF). In TADF materials, heat accelerates reverse intersystem-crossing (ISC) from a triplet excited state (T1) to a singlet excited state (S1) and thus lead to an increase of the fluorescence intensity. Therefore, when the TADF materials are used in OLEDs, heating the device would accelerate the reverse ISC and thus make it possible to fabricate OLEDs with a high EL efficiency without any roll-off characteristics. We searched for highly efficient TADF materials and found highly efficient tin (IV) fluoride-octaethylporphine (SnF2-OEP) showing rather intense TADF. The unique TADF characteristics are anticipated to the dopant in the OLED architecture, and we fabricated polymer dispersed films which contain SnF2-OEP for use as a dopant in the emitting layer. We prepared a polymer dispersion film of 2mol%-SnF2-OEP:PVCz. Under optical and electrical excitation, intense fluorescence and TADF were observed at 570nm while the weak room temperature phosphorescence was also observed at 703nm. The fluorescence intensity significantly increased with an increase of temperature, which indicated that acceleration of the reverse intersystem crossing was caused by heat activation. The S1-T1 thermal energy gap, which is necessary in order to induce the reverse intersystem crossing, was estimated to be 37-40 kJ/mol from the fluorescence and phosphorescence spectra. The proposed TADF-OLED system will open new way for enhancing OLED efficiency.
9:00 PM - H5.24
OFETs with Organic-metal Electrodes.
Takehiko Mori 1 , Koji Shibata 1 , Hiroshi Wada 1 , Jun-ichi Inoue 1
1 Department of Chemistry and Materials Science, Tokyo Institute of Technology, Tokyo Japan
Show AbstractUnderstanding charge injection from a metal electrode to an organic semiconductor is an important aspect of device physics of organic field-effect transistors (OFET). The authors have demonstrated in OFETs using pentacene and dibenzotetrathiafulvalene (DBTTF) that the use of a highly conducting organic charge-transfer salt (tetrathiafulvalene)(tetracyanodimethane) (TTF)(TCNQ) as source and drain contact materials reduces the contact resistance largely, and improve the performance of bottom-contact OFETs as high as that of top-contact ones. This is due to continuous morphology of the active layer to the organic contact materials, as well as the reduced interfacial potential at the organic/organic interface. The use of (TTF)(TCNQ) electrodes has been extended to solution-processed OFETs using P3HT, and n-channel OFETs using F16CuPc and dicyanoquinonediimine (DCNQI). When the contact metals are changed from Au to Cu and Ag, the contact resistance of p-channel OFETs is basically interpreted by the order of the metal work functions, but that of n-channel devices shows complicated behavior. Thanks to the "null" of the interfacial potential in the (TTF)(TCNQ) electrodes, such a behavior is analyzed in view of the unusual potential shift at the interface between the organic acceptor and a metal. Since (TTF)(TCNQ) is useful both for p-channel and n-channel devices, ambipolar OFETs are fabricated by using a F16CuPc/CuPc heterojunction. The n-channel device of DCNQI is air stable, but the stability considerably depends on the electrode materials, indicating that stability of not only the active layer material but also the organic/electrode interface is important.
9:00 PM - H5.25
Synthesis, Characterization and Photophysical Properties of Iridium Complexes with a 2-(Diphenylamino)pyridine and 2-(Carbozolyl)pyridine Framework and Their Application in Organic Light-Emitting Diodes.
Hao-Chun Li 1
1 , Industrial Technology Research Institute, Hsinchu Taiwan
Show AbstractIridium complexes 2a-2d bearing six-membered 2-(diphenylamino) pyridine or 2-(carbozolyl)pyridine ligands were synthesized and characterized to attain highly efficient, room-temperature UV-Vis phosphorescence in the range of 520-595 nm. An increase of the energy gaps of these iridium complexes can be achieved via the substitution of 2-(diphenylamino)pyridinyl ligand, 2a: 567 nm (in CH2Cl2) with 2-(carbozolyl)pyridine, 2c: 529 nm. The HOMO-LUMO energy levels of complexes 2a-2d were also characterized by electrochemistry and UV-spectra to firmly support the effects of carbozolyl and/or trifluoromethyl groups on the relative energy levels. Our results, on the basis of steady-state, relaxation dynamics, and temperature dependent studies, lead to a conclusion that, for complexes 2a-2d, the large amplitude motion involving the six-membered chelate fragments in the T1 state may play a crucial role for the fast radiationless deactivation. In the OLED devices, one representative complex 2b shows a external quantum efficiency value of 2.60 % at J = 20 mA/cm2 and the maximum brightness is 11,045 cd/m2 (x = 0.51, y = 0.48) with a full width at half maximum (FWHM) 95 nm at 15 V, demonstrating the six-membered chelated iridium complexes suited for the application in OLED devices.
9:00 PM - H5.27
Improvement of Carrier Balance in Polymer Light-Emitting Diodes having Multilayer Structure Prepaerd by Evaporative Spray Deposition using Ultradilute Solution.
Masato Shakutsui 1 , Tetsuo Tsutsui 1 3 , Katsuhiko Fujita 1 2
1 Department of Applied Science for Electronics and Materials, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, Japan, 3 , Kyushu University, Fukuoka Japan, 2 , Institute for Materials Chemistry and Engineering, Kyushu University, Kasuga Japan
Show AbstractThe multilayer structure is useful to improve the carrier balance even in polymer light-emitting diodes (PLEDs). We have found that drastic improvement of the carrier balance brought about when hole transport layer (HTL) is formed without thermal treatment above the glass transition temperature (Tg).It was reported that introducing an interlayer of fluorene-triarylamine copolymers between a hole injecting layer, poly(3,4-ethylenedioxythiophen) doped with poly (styrenesulfonate) (PEDOT:PSS), and an emitting layer (EML) improve the device efficiency as well as the device lifetime. The interlayer is needed to undergo thermal annealing above the Tg to prevent dissolution during the spin process of the second layer. The thermal annealing is indispensable as far as using spin-coat process. However, new polymer thin film preparation technique, evaporative spray deposition using ultradilute solution (ESDUS) enables forming polymer layered structure using multiple polymer semiconductors which are soluble in a same solvent. We investigated the influence of the thermal treatment and the thickness of fluorene-triarylamine copolymer layer as a HTL on the device performance of bilayer (HTL/EML) PLEDs using ESDUS. For bilayer devices, a poly(9,9-dioctyl- fluorene)-co-N-(4-butylphenyl)diphenylamine) (TFB) layer (40 nm) was spin-coated as a HTL onto the glass/ITO/PEDOT:PSS substrate and then annealed at 130 oC or 180 oC or 230 oC (device A, B and C, respectively) and a thinner TFB layer (20 nm) was spin-coated onto the substrate and annealed at 130 oC (device D). A poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) layer (65 nm) was deposited as an EML onto the TFB layer by ESDUS using tetrahydrofuran solution. A cathode electrode (LiF/Al) was thermally deposited in a vacuum evaporator.The external quantum efficiency (EQE) of device A is 2 % (the maximum EQE is 2.3 %) throughout a current region as wide as four orders of magnitude, whereas those of device B and C are lower than that of device A in all current regions and show a gradual increase with current density. The EQE of device D is lower than that of device A in the low-current region and shows a gradual increase with current density (the maximum EQE is 2.2 %). We consider that the annealing above Tg (156.0 oC) of the TFB layer would induce the degradation of the hole injection and that the thinner TFB layer would have insufficient electron blocking.Comparison with characteristics of PLEDs having the TFB layer prepared in various conditions reveals that preparation process of the interlayer, which needs the thermal treatment above Tg, hinder the hole injection. The interlayer is used, which is annealed above Tg of the polymer and rinsed with solvent, therefore, the insoluble residue of the polymer remains as the interlayer. It is needed that the degradation of polymers by annealing above those Tg is taken into consideration in case of constructing polymer multilayer structure for PLEDs.
9:00 PM - H5.28
Controlled P-type Doping of MEH-PPV with a Solution-processed Method.
Yuan Zhang 1 , Mingtao Lu 1 , Bert de Boer 1 , Paul Blom 1
1 , Molecular Electronics, Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands
Show AbstractIn organic light-emitting diodes (OLEDs) based on evaporated small molecules it has been demonstrated that doping of the hole- and electron transport layers strongly reduces their operation voltage [1]. Furthermore, the use of doped layers makes the OLEDs less sensitive to the workfunctions of the anode and cathode. For solution-processed LEDs based on conjugated polymers, however, doped charge transport layers are hardly applied. A major bottleneck is that already in solution charge transfer between the host polymer and the dopant occurs, leading to aggregation and precipitation of the active components. Here we present controlled p-type doping of poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) deposited from solution. As a dopant tetrafluoro-tetracyanoquinodimethane (F4-TCNQ) is used. By adjusting the polarity of the solution aggregation can be prevented and doped films can be deposited. Upon doping the low voltage part of the J-V characteristics of MEH-PPV based hole-only devices is increased by several orders of magnitude and a clear Ohmic behavior appears. Due to the density dependence of the charge carrier mobility [2,3] also the space-charge limited current at higher voltages is increased. Taking the density dependence into account the free carrier concentration due to doping can directly be derived from the J-V characteristics. We find that a doping concentration of 1.0wt.% leads to a free carrier density of 2×1022 m-3. Furthermore, the activation energy of the mobility is reduced from 0.45 eV for undoped films towards 0.27 eV for the 1.0wt.% doped films. The free carrier densities are further confirmed by impedance measurements on Schottky diodes based on F4-TCNQ doped MEH-PPV and a Ag electrode. Neglecting the density dependence of the mobility would have led to an overestimation the charge carrier density as obtained from the J-V measurements by an order of magnitude. Finally, we demonstrate the first working example of a polymer light-emitting diode (PLED) including a doped hole transport layer (HTL). With a total thickness of 100 nm, including a doped HTL of 40 nm, the PLED behaves electrically identical as a 60 nm thick undoped PLED, but is far less sensitive to shorts.References:[1] M. Pfeiffer et al., Organic Electronics, 4, 89-103 (2003).[2] C. Tanase, E. J. Meijer, P. W. M. Blom, and D. M. de Leeuw, Phys. Rev. Lett. 91, 216601 (2003).[3] C. Tanase, P. W. M. Blom, D. M. de Leeuw, Phys. Rev. 70, 193202 (2004).
9:00 PM - H5.29
Hybrid Inorganic-organic Light Emitting Diodes.
Michele Sessolo 1 , Hendrick Bolink 1 , Eugenio Coronado 1
1 Institute of Molecular Science, University of Valencia, Paterna Spain
Show AbstractA hybrid approach to organic light emitting diodes (OLEDs) that uses metal oxides as the electron injecting material will be presented. HyLEDs have an inverted structure compared to OLEDs, with the cathode on the substrate and the anode as the top contact of the device. The use of air-stable anodes like gold make HyLEDs interesting candidates for low cost large area electroluminescent applications as they are less sensitivity to atmospheric conditions and hence require less stringent encapsulation. HyLEDs are prepared in a simple two step process in which the metal oxide cathode (TiO2, ZnO) is deposited as a thin layer on top of a transparent conductor (up to now indium tin oxide, ITO) by spray pyrolysis. The light emitting polymer (LEP) is then processed by conventional spin-coating methods. Subsequently, the gold anode is thermally evaporated on top of the polymer. Up to now the only LEP employed in HyLEDs is poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) as its LUMO is among the lowest in commercially available LEPs. This choice comes from the need of decreasing the energy gap between the conduction band of the metal oxide and the LUMO of the polymer and thus to help electron injection. Promising luminance values, 6500 cd/m2, at low voltages have been reported for F8BT based HyLEDs.[1,2] Unfortunately the efficiency is rather low caused by the high current density. We will present a model that explains the results and provides avenues for further optimization of these devices. It is based on the idea that the barrier for electron injection is lowered by the space charge field over the metal oxide-LEP interface, due to the build up of holes in the polymer.[3] Furthermore, we present compelling evidence for the proposed model consisting of HyLEDs with further improved performances. These HyLEDs additionally, are no longer restricted to the use of low LUMO LEPs, but also give high brightness and efficiencies (>20000 cd/m^2 and >7 cd/A, respectively) when the well know PPV based light emitting polymer, “super yellow” is used as LEP. Finally, an outlook for the use of HyLEDs for phosphorescent type OLEDs will be presented.[1] K. Morii, M. Ishida, T. Takashima, T. Shimoda, Q. Wang, M. K. Nazeeruddin, M. Graetzel, Appl. Phys. Lett. 2006, 89, 183510.[2] H. J. Bolink, E. Coronado, D. Repetto, M. Sessolo, Appl. Phys. Lett. 2007, 91, 223501.[3] H. J. Bolink, E. Coronado, D. Repetto, M. Sessolo, E. Barea, J. Bisquert, G. Garcia-Belmonte, J. Prochazka, L. Kavan, Adv. Funct. Mater. 2008, 18, 145.
9:00 PM - H5.3
Measurement and Modeling of Hole Transport in Blends of N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPB) and the Hole-conducting Material HIL.
Christoph Zimmermann 1 , Manuel Boesing 1 , Alaa Abdellah 1 , Philipp van Gemmern 2 , Hans-Peter Loebl 3 , Michael Heuken 1 4 , Holger Kalisch 1 , Rolf Jansen 1
1 Chair of Elelctromagnetic Theory, RWTH Aachen University, Aachen Germany, 2 , Philips Technologie GmbH, Aachen Germany, 3 , Philips Technologie GmbH Forschungslaboratorien - Philips Research Laboratories, Aachen Germany, 4 , AIXTRON AG, Aachen Germany
Show Abstract9:00 PM - H5.30
Carrier Transport Mobility of Light Emitting Layers and Recombination Region; Effects on the Electrical Properties and Efficiency of Dye-Doped Electrophosphorescent OLED.
Byung Doo Chin 1 , Yuri Choi 1 , Nam Su Kang 1 , Heume-Il Baik 2 , Changhee Lee 2
1 Material Sci. and Eng. Research Division, Korea Institute of Science and Technology, Seoul Korea (the Republic of), 2 School of Electrical Engineering & Computer Science, Seoul National University, Seoul Korea (the Republic of)
Show AbstractThe light emitting efficiency and spectral characteristics of the organic electrophosphorescent devices, whose emission characteristics are strongly dominated by the charge trapping, are investigated in this work. The carrier mobility and electrical properties of dye-doped light emitting layers are given, along with a relative estimation of a trap-depth for both hole and electron. On the basis of our finding based on the steady state J-V behavior and transient photocurrent measurement data by time-of-flight (TOF), we have suggested a simple model illustrating the possible shift of the exciton recombination region for different energy level of emissive dopants. Not only by the difference of dopant-induced trap formation, but also the affinity of host materials between hole and electron transport govern the charge balance and recombination region of devices, while its indirect evaluation is also conducted by the change of the interfacial states adjacent to the light emitting layers (for example, use of different hole/electron transport layers with various triplet energy levels). By the fabrication of the devices with regionally-doped sensitizing layers, precise shifting of the exciton recombination zone and its influence on the efficiency roll-off are characterized. The approach shown here will provide more detailed insight into the physical interpretation and systematic design scheme for phosphorescent OLEDs with higher efficiency and improved operational stability.
9:00 PM - H5.31
Study on Triplet Exciton Emission and Quenching Processes by Low-Temperature Photo- and Electroluminescence Spectroscopy.
Nils A. Kaufmann 1 , Frank Jessen 1 , Herbert Boerner 2 , Michael Heuken 1 3 , Holger Kalisch 1 , Rolf Jansen 1
1 Chair of Elelctromagnetic Theory, RWTH Aachen University, Aachen Germany, 2 , Philips Technologie GmbH Forschungslaboratorien - Philips Research Laboratories, Aachen Germany, 3 , AIXTRON AG, Aachen Germany
Show AbstractOrganic light emitting diodes (OLEDs) are efficient light sources based on organic semiconductors. Unlike inorganic LEDs which are small-area light sources, OLEDs can be planar with up to 1 m2 in area. By using organic materials, they are cheap to produce and economical to use. The determination of triplet exciton energy levels is of interest for the development of efficient OLEDs, based on the fact that electrical excitation usually creates three times as much triplets as singlets. Additionally, the knowledge of these energy levels is crucial for the design and choice of emitter matrix materials and exciton blocking layers. These energies are normally determined by photoluminescence (PL) measurement in solutions for materials which show intersystem crossing (ISC) between singlet and triplet states. For some materials, the triplet levels cannot be measured this way because some materials prohibit ISC.In this work, a method is presented which allows the determination of the energy levels using low-temperature electroluminescence (EL) spectroscopy. The dependence on ISC is avoided by creating triplets directly by electrical excitation and this allows measuring all types of organic materials.Current-resolved low-temperature EL spectra have been recorded for N,N'-bis(3-methylphenyl)-N,N'-diphenyl-[1,1'-biphenyl]-4,4'-diamine (TPD) in a 3-phenyl-4-(1'-naphthyl)-5-phenyl-1,2,4-triazole (TAZ) matrix (TPD/TAZ 1:3) at 77 K. Triplet emission is only observed at very low charge carrier density (0.5 μA/mm2). Quenching processes such as field-induced exciton dissociation and carrier-exciton interaction are analyzed using combined EL and PL measurements as well as unipolar devices. Two factors can be the cause of the quenching: A strong quenching based on a low concentration of electrical activated impurities could explain the dependency. The other explanation points to a quenching based on electrons in the emitting layer.This might be explained with triplet-polaron quenching (TPQ). TPQ is proportional to the charge carrier density and could contribute the dominant part to the quenching at low current densities.
9:00 PM - H5.32
Coalescence of Pentacene Domains with Au Electrodes Observed by Real-time PEEM Measurement During Growth Process.
Yuki Tsuruma 1 , Abdullah Mahboob 2 , Susumu Ikeda 1 , Genki Yoshikawa 2 , Jerzy Sadowski 2 , Yasunori Fujikawa 2 , Toshio Sakurai 2 , Koichiro Saiki 1
1 Department of Complexity Science and Engineering, The University of Tokyo, Kashiwa Japan, 2 Institute for Materials Research , Tohoku University, Sendai Japan
Show Abstract Organic electronic devices have been studied extensively in the past decades because of advantages such as low cost processing, less weight, mechanical flexibility, and ease of large-scale manufacturing. Organic field-effect transistor (OFET) is one of the most important elements in the field of organic electronics. There are two types of OFETs; so-called top-contact configuration and bottom-contact configuration. The latter has a merit that small-size OFETs can easily be fabricated on the source/drain (S/D) electrodes prepared by well-developed lithography techniques. However, its field-effect mobility is rather low compared with that of the top-contact structure. This is due to the appearance of transition region at the boundary between the channel and the electrode in the organic thin film. This transition region is caused by different orientations of growing organic molecules between on a gate insulator and on a metal electrode. Thus, removal of the transition region is a key issue for increasing FET mobility. Surface modification of the S/D electrodes using self-assembled monolayer (SAMs) films has been known as a possible way for improving the FET mobility. Although the change of molecular orientation on the SAM-treated electrode is thought to bring about the improvement, the detailed mechanism of film growth and the resultant film morphology has not yet been understood completely. In the present work, we observed growing feature of pentacene film by photoemission electron microscope (PEEM) and atomic force microscope (AFM), especially focusing on the transition region between the channel (on SiO2) and the S/D electrode (on Au). Evolution of PEEM image during pentacene growth indicates that pentacene nuclei seemed to be formed avoiding the periphery of the Au electrodes and consequently there remained grooves at the boundary just after completion of the first wetting layer. For the thiol-SAMs treated gold electrodes, on the other hand, pentacene nuclei were formed ignoring the existence of gold electrodes and there remained no gap in the structure of the pentacene film between the channel region and the electrode. AFM observation showed that less pentacene molecules were piled up on the thiol-SAMs treated Au electrode than on the bare one after the same amount of pentacene deposition and thus reduction of absorption by gold helped improvement of wettability between pentacene and Au. We measured FET characteristics of pentacene films using Au electrodes with or without the SAMs treatment as a function of film thickness. The current flow was observed within 1 ML thickness for the thiol-SAMs electrode, while 10 ML was necessary to flow the current for the bare electrode. It clearly indicates that the formation site of nuclei dominates continuity of pentacene film at the channel-electrode boundary and final FET characteristics.
9:00 PM - H5.33
Highly Efficient, Charge Balanced Blue Phosphorescent Organic Light-emitting Device with High Triplet Energy and High Mobility Electron Transport Layer.
Neetu Chopra 1 , Jaewon Lee 1 , Franky So 1
1 Dept of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractOrganic light-emitting diode (OLED) is very promising for flat panel display and solid state lighting applications. With the recent progress being made in this field along with the ease of processing and promise of low manufacturing cost this field is attracting even more attention. For these applications a high efficiency blue emitting OLED is indispensable.Hereby, with improve carrier balance and exciton confinement, we have demonstrated a substantial efficiency enhancement in blue phosphorescent organic light-emitting devices (PHOLEDs) using iridium (III) bis[(4,6-di-fluorophenyl)-pyridinate-N,C2`]picolinate (FIrpic) as an emitter with peak emission wavelength of 472 nm. The PHOLEDs with conventional electron transport materials such as 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) have been demonstrated to be hole dominant as most electron transport materials have lower electron mobility. Hence the recombination zone is found to be located at the interface between the emitting layer (EML) and the electron transport layer (ETL). Since the triplet energy of most electron transport materials is lower than that of FIrpic (T1= 2.72 eV), these lower triplet energy electron transport materials can quench the luminescence at the EML/ETL interface. Hence it is imperative to have a high mobility electron transport material with high triplet energy level to maximize the efficiency of blue PHOLEDs.We report the use of tris[3-(3-pyridyl)-mesityl]borane (3TPYMB) as the ETL in FIrpic based blue PHOLEDs. 3TPYMB has very high triplet energy (T1) of 2.98eV, which is much higher than that of conventionally used ETLs. Hence 3TPYMB would be able to provide better triplet exciton confinement. Also the electron mobility of 3TPYMB is an order of magnitude higher than that of many electron transporting materials such as tris (8-quinolinolato) aluminum (Alq3). Hence, the use of 3TPYMB as ETL would provide better charge balance in the device. With the use of 3TPYMB as an ETL we were able to achieve a high external quantum efficiency of ~23%, which is a 50% increase compared with devices using conventional electron transport materials. The device shows 49.4 Cd/A current efficiency and 29.9 lm/W luminous efficacy at a brightness of 100 Cd/m2. We also studied how the high triplet energy and high mobility affect the performance of the OLED device and the results indicate that both high triplet energy and high electron mobility are necessary for high efficiencies.
9:00 PM - H5.34
Influence of Substrate Temperature on Multilayer Thin Film Growth, Charge Carrier Injection and Efficiency of OVPD-processed Organic Light Emitting Diodes.
Manuel Bosing 1 , Alaa Abdellah 1 , Christoph Zimmermann 1 , Frank Jessen 1 , Klaus Meerholz 2 , Philipp van Gemmern 3 , Dietrich Bertram 3 , Dietmar Keiper 4 , Nico Meyer 4 , Michael Heuken 1 4 , Holger Kalisch 1 , Rolf Jansen 1
1 Chair of Electromagnetic Theory, RWTH Aachen University, Aachen Germany, 2 Institute of Physical Chemistry, University of Cologne, Köln Germany, 3 , Philips Technologie GmbH, Aachen Germany, 4 , AIXTRON AG, Aachen Germany
Show Abstract9:00 PM - H5.35
Orthogonal Micropatterning Technique for Organic Electronic Devices.
Alexander Zakhidov 1 , Jin-Kyun Lee 1 , John Defranco 1 , Hon Hang Fong 1 , Priscilla Taylor 1 , Christopher Ober 1 , George Malliaras 1
1 , Cornell University, Ithaca, New York, United States
Show AbstractMicro-patterning and processing of organic materials for electronic and optoelectronic systems remain among the most challenging issues to be addressed in the field of organic electronics. We have developed photolithographical micropatterning approach with very high compatibility with the majority of materials used in organic electronic devices as well as with materials used in hybrid and inorganic electronics. The approached is based on the orthogonality of processing solvents and photoresist to active organic materials. We demonstrate the potential of developed approach by fabrication the prototypes of flexible organic light emitting displays.
9:00 PM - H5.36
Acid-sensitive Semi-perfluoroalkyl Resorcinarene: An Imaging Material for Organic Electronics.
Jin-Kyun Lee 1 , Margarita Chatzichristidi 1 , Alexander Zakhidov 1 , John DeFranco 1 , Priscilla Taylor 1 , Hon Hang Fong 1 , Ha Soo Hwang 1 , Andrew Holmes 2 , George Malliaras 1 , Christopher Ober 1
1 Materials Science & Engineering, Cornell University, Ithaca, New York, United States, 2 School of Chemistry, BIO21 Institute, University of Melbourne, Melbourne, Victoria, Australia
Show Abstract9:00 PM - H5.37
Determining the Density of States for Interface Hole Traps in MIS Devices Based on P3HT.
David Taylor 1
1 School of Electronic Engineering, Bangor University, Bangor United Kingdom
Show AbstractIdentifying the mechanism(s) responsible for threshold voltage instability in organic thin film transistors is key to their commercialisation in electronic circuits. Likely sources of instability are electronic states located at the semiconductor-insulator interface in these devices. In recent years we have demonstrated [1,2] that small-signal measurements of capacitance and conductance reveal the presence of fast interface states that interact with majority hole carriers in devices based on poly(3-hexylthiophene), P3HT, as the active semiconductor. In this contribution we show that the model developed for investigating interface states in crystalline silicon are applicable also where the semiconductor is characterised by an exponential density of localised bulk states albeit with a different temperature behaviour. Thus by measuring the voltage-dependence of capacitance and loss (conductance/angular frequency) in a metal-insulator-semiconductor (MIS) capacitor over a wide frequency range it becomes possible to extract the density of states for interface hole traps. The measurements were made on MIS capacitors formed from P3HT spin-coated onto a thermally-cured film of the spin-on-glass, polysilsesquioxane and show the classic signatures expected for a distribution of interface states, i.e. stretching of the C-V plot and frequency-dependent maxima in the loss-voltage plots moving to more positive gate voltages as the frequency decreased. By combining the two sets of data we recently [3] reported that the density of states, Dit, of the interface hole traps responsible for these features in a well-annealed device is approximately linear. The trap densities were estimated from the maximum in loss at a given frequency after correcting for parasitic effect. The trap energies were estimated from C-V plots by extracting the depletion capacitance and calculating the potential drop across the depletion region in the P3HT. Hence the trap energies were quoted relative to the bulk Fermi level of the semiconductor which probably lies about 0.2 to 0.3 eV above the HOMO level in P3HT. The measurements are now being extended to a study of environmental factors such as oxygen and moisture on the density of states. References[1] I.Torres and D.M.Taylor, J. Appl. Phys., 98, 073710 (2005).[2] D.M.Taylor and N.Alves, J. Appl. Phys., 103, 0544509 (2008). [3] N.Alves and D.M.Taylor, Appl. Phys. Lett., 92, 10331 (2008).
9:00 PM - H5.38
Synthesis and Application of a Non-Chemically Amplified Photoresist Processible in Hydrofluoroether Solvents.
Priscilla Taylor 1 , Jin-Kyun Lee 1 , Alexander Zakhidov 1 , Margarita Chatzichristidi 1 , John DeFranco 1 , Hon Hang Fong 1 , Eisuke Murotani 1 , George Malliaras 1 , Christopher Ober 1
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States
Show AbstractOrganic electronics has attracted great attention as a technology to enable large-scale, flexible electronic devices through solution processing of organic materials. While inorganic semiconductors rely on photolithographic patterning techniques, organic electronic devices have difficulty in adopting those robust, high-resolution and high-throughput patterning methods because of chemical compatibility issues between organic materials and patterning chemicals. This challenge has led us to develop alternative lithography materials and to adopt non-damaging processing solvents.In this research, a new non-chemically amplified negative-tone photoresist has been synthesized, which can be processible in segregated hydrofluoroethers (HFEs). This photoresist can be patterned under 365 nm and 254nm and 193 nm conditions and is shown to give submicron-sized features on silicon, glass and PSS:PEDOT substrates. The new photoresist and processing solvents have been applied to the patterning of PSS:PEDOT and pentacene in the fabrication of a bottom-contact transistor.Acknowledgement. This research was accomplished with the support of the National Science Foundation (Materials World Network DMR-0602821), NYSTAR and IGERT fellowship. We also appreciate 3M for valuable discussions on Novec™ Engineered Fluids and the Cornell Nanofabrication Facility for use of their facilities.
9:00 PM - H5.39
Polymer Light-emitting Electrochemical Cells: Mechanism, Materials, Devices, and New Developments.
Zhibin Yu 1 , Qibing Pei 1
1 Department of Materials Science and Engineering, UCLA, Los Angeles, California, United States
Show AbstractPolymer light-emitting electrochemical cells (LEC) consisting of a conjugated polymer admixed with a solid electrolyte were introduced to obtain efficient electroluminescence at low operating voltages1,2. The ideal LEC model calls for electrochemical p- and n-doping on the anode and cathode, respectively, and an intermediate p-i-n junction where electroluminescence is generated. The doping effectively removes the charge injection barriers at both electrodes and enables the use of air-stable electrode materials and thick polymer films. We will describe the various experimental observations that support this model. Results that deviate from the ideal model will also be discussed. Developing suitable conjugated polymers and polymer-electrolyte systems have been extensively explored. Polymers with dual electronic and ionic conductivities are shown to produce high EL quantum and power efficiencies. Additives that boost ionic conductivity increase the response speed of the LECs, but may reduce the operation stability.We have demonstrated polymer p-i-n junction diodes, wherein the junction was formed during initial charging in a similar fashion as in the LECs except that the mobiles ions are subsequently immobilized. The static junction is being utilized for electroluminescence with fast response, high efficiency, and long lifetime.1 Q. Pei, et al, Science,1995,269:1086.2 Q. Pei, et al. JACS, 1996, 118:3922.
9:00 PM - H5.40
Transient Current Analysis of Pentacene FET with Polymer Dielectrics for Stable Device Operation.
Kouji Suemori 1 , Misuzu Taniguchi 1 , Toshihide Kamata 1 2
1 , AIST, Tsukuba, Ibaraki, Japan, 2 , Tsukuba Univ., Tsukuba, Ibaraki, Japan
Show AbstractReliability and stability of device operation are one of the most important issues for the development of organic transistors. In order to know the factors for stable device operation of organic transistors, transient current analysis is an effective technique. In this study, we have measured the transient current flow under the DC and AC gate bias stress for pentacene transistor. In order to modify the interface condition of the pentacene transistor, we used several kinds of polymer dielectrics. From the detailed analysis of the transient current, we have found that two types of instable current flow appeared under the constant gate bias stress. One appeared at msec order in the transient current. Drain current was reduced gradually at constant applied drain and gate voltages. This phenomenon is well related with the interface species between organic semiconductor and polymer dielectric layers. In case that large dipole moment was located at the interface between the organic semiconductor and polymer dielectric layers, drain current was decreased under the constant applied voltage. On the other hand, the other type of instable current flow appeared at more than 10sec order. It was rather long term phenomenon. In order to reveal the origins of these instable current flows, relationship between the instable current and detailed molecular structure was studied. The former type of instable current was depending on the surface dipoles. On the other hand, the long range instable current was independent of the surface dipole but surface roughness and thermal motion of the surface layer. Stretched exponential model was applied for the analysis of these trap site. From the obtained results, we have suggested the viblational motion of surface species is one of the main reason for these instable current flows.
9:00 PM - H5.41
Enhancement of Light Outcoupling in Organic Light Emitting Devices via Photoinduced AutoStructuration of Azobenzene Polymer.
Juhwan Kim 1 , Hong-Ju Park 1 , Jae-Chul Hong 1 , Seok-In Na 1 , Dong-Yu Kim 1
1 Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of)
Show Abstract In the recent years, organic light emitting devices (OLEDs) have attracted attention for use in flat panel display because of advantage of high contrast ratio, high speed and low power consumption. However, they efficiencies are typically limited by poor external efficiencies that arise from the photon trapping inside the device due to Snell’s law. Actually, although internal quantum efficiency can almost reach to 100 %, the external quantum efficiency of OLEDs constructed to date is typically about 20 % of internal efficiency. To overcome this problem, various patterning processes have been studied, such as microlenses, surface roughening, insertion of scattering medium, and so on. They are mostly based on so called lithography, and improve the light outcoupling but they have some drawback including complicated processes, such as etching, removal of residual materials and fabrication of master mold for pattern transfer. In contrast, pattering process by using azobenzene surface relief gratings (SRGs) is an attractive approach in order to easily fabricate regular structures. This process is only single step all-optical process, and thus fabrication of master pattern and after-processes are unnecessary. Here, in order to fabricate OLEDs to enhanced light outcoupling, we introduce an easy, simple, controllable and reproducible pattern fabrication process via photoinduced SRGs of azobenzene material to OLEDs.
9:00 PM - H5.42
The Modification of Gate Dielectric of Organic Field Effect Transistor using the Ambipolar Semiconductor Organic Molecule.
Mi-Hee Jung 1 , Kyu Ho Song 2 , Hyoyoung Lee* 1
1 National Creative Research Initiative Center for Smart Molecule Memory, Electronics and Telecommumications Research Institute, Daejeon Korea (the Republic of), 2 BioMEMS Team, Convergence & Technology Research Laboratory, Electronics and Telecommumications Research Institute, Daejeon Korea (the Republic of)
Show AbstractOrganic thin film transistors (OTFT) have attracted much attention because of their potential application in low cost, flexible, and large area electronics. High performance n-type semiconductors are an essential component in OTFT applications, such as ambipolar transistors and complementary metal-oxide semiconductor (CMOS) circuits. To improve the performance of OTFT further, the role of the organic semiconductor/gate dielectric interface is very important. In this research, we report on the effect modifying the SiO2/Si substrate using the ambipolar semiconductor organic molecule. Push-pull typed organic molecules which were composed of electron donating group (EDG) and electron withdrawing group (EWG) in molecular backbone have been interested in the field of organic electronics, such as OTFT, DSSC, and Organic Solar Cell. We proposed push-pulled molecule containing triarylamine as an EDG, thiophene as a spacer, and malononitrile as an EWG. Thiophene spacer controls the charge-separation properties from triarylamine to malononitrile. The charge-separation will be affected by distortion angle derived from steric hindrance of methyl side chain. In these structure, the highly electron deficient, malononitrile provide ambient stability to the gate field induced electron carriers. Furthermore, the donor-acceptor backbone enhances core rigidity and affords the low band gap semiconductor. Bottom contact OTFT with organic molecule were constructed on the heavily n-doped Si substrates with 300 nm SiO2 layer. A 10 nm organic molecules was vacuum deposited onto Si/SiO2 substrate. Gold electrodes as source and drain contacts were deposited onto the organic layer through the shadow mask. The channel length and width were 100 μm and 10000 μm, respectively. 50 nm thick pentacene films were vacuum deposited on organic molecule/SiO2 substrates held at 30 oC, under a deposition rate of 0.1 Å/s and 10-7 torr. All electrical measurements for the devices were carried out in air and the mobility was extracted from the saturation regime. For the device on the bare SiO2/Si substrate, a field-effect electron mobility up to 0.002 cm2V-1s-1 was obtained at the optimized substrate temperature of 30 oC. The mobility could be further enhanced to 0.1 ~ 0.2 cm2V-1s-1 with a current on/off ratio of 105 for devices based on push-pull typed organic molecules modified substrate.
9:00 PM - H5.43
Ambipolar Characteristics of Solution Processed Bilayer Field Effect Transistors.
Takeomi Morita 1 , Syuichi Nagamatsu 3 , Shinya Oku 1 , Wataru Takashima 2 , Keiichi Kaneto 1
1 Graduate School of Life Science and Systems Engineering, Kyushu Institute of Technology, Kitakyushu Japan, 3 Departmen of Computer science and Electronics, Kyushu Institute of Technology, Kitakyushu Japan, 2 Research Center for Advantage Eco-Fitting Technology, Kyushu Institute of Technology, Kitakyushu Japan
Show AbstractAmbipolar organic field effect transistors (OFET) are attractive for future ubiquities electronics, because of the simple structure, high performance as well as low cost fabrication.The organic ambipolar materials are commonly obtained by blending of p- and n-type organic materials. However, the blending usually results in reduction of carrier mobilities because of carrier scattering by impurities introduced by blending. In high electron mobility transistor (HEMT) is consisting of layered structures with electron donating (n-type semiconductor) and carrier transport (pure semiconductor) layers. In this structure, carriers supplied from donor layer migrate in pure semiconductor layers without suffering from impurity scattering. In this report, we show how to prepare the OFET with the layered structure, and the transfer and current-voltage characteristics. The device was fabricated on Si/SiO2 substrate. Thin film of [6, 6]-Phenyl C61 butyric acid methyl ester (PCBM) was spread over a poor solvent, then the film was lifted onto a Si/SiO2 substrate. Thin poly(3-hexylthiophene) (P3HT) film was lied down on them. Source and drain gold electrodes were vacuum deposited between them. The hole mobility of 3.1×10-5 cm2/Vs and electron mobility of 7.4×10-4 cm2/Vs, which should be compared with the blended ambipolar OFETs with 5.0×10-5 cm2/Vs and 2.5×10-4 cm2/Vs, respectively. These should be also compared with those of single layer OFET with 2.7×10-3 and 2.8×10-3 cm2/Vs as hole and electron mobilities. The reversed bilayer structure of OFETs also was fabricated and showed the similar ambipolar characteristics. Grazing incidence X-ray diffraction (GIXD) profiles of out-of-plane represents a strong lamella structure of PHT, which hinder weak diffraction patterns from PCBM. In-plane profiles, however, succeeded to show superimpose of both of the PHT and PCBM diffraction pattern, which can be confirmed from the single layer samples. The findings indicate that the procedure proposed in this study forms a multilayer of thin organic semiconductor conserving the stacking structure.
9:00 PM - H5.45
Electrical Characteristics and Air-Stability of Pentacene Based MOS Diodes.
Md. Akhtar Uzzaman 1 , Shun-ichiro Ohmi 1 , Hiroshi Ishiwara 1
1 Dept. of Electronics and Applied Physics , Tokyo Institute of Technology, Yokohama Japan
Show AbstractThe electrical characteristics of pentacene based MOS diodes were investigated. In order to realize the low voltage operations, we deposited pentacene films (30~60 nm) directly on SiO2 (12 nm) / n+-Si (100) substrate. The layer formed by 850οC wet oxidation and the pentacene film was grown by the vacuum evaporation with deposition rate of 0.1~0.3 nm/s at room temperature. Au for pentacene and Al for n+-Si as electrical contacts were ex-situ evaporated. The C-V characteristics were measured. The stability of the diode was also evaluated up to 30-days in air. The obtained C-V for as-deposited film shows a little hysteresis (36 mV) without any surface treatment by HMDS or OTS. This result suggested that the excellent interface properties were obtained for the fabricated MOS diode. However, after 30-days, the hysteresis width was further increased up to 100 mV and flat-band voltages shifted towards positive voltage as well which indicates that the electron trapping effects become stronger either by moisture or oxygen and thus the stability of pentacene based MOS diode characteristics degraded gradually. The ferroelectric gate field-effect transistors based on pentacene will also be discussed.
This work was partially supported by a Grand-in-Aid for Scientific Research Priority Areas (A) (No. 19206039) from the Ministry of Education, Culture, Sports, Science and Technology.
9:00 PM - H5.46
The Origin of Hole Injection Improvement in Pentacene Thin-Film Transistors with Molybdenum Oxide/aluminum Bilayer Electrode.
Pyung Eun Jeon 1 , Kyul Han 1 , Hyunbok Lee 1 , Hyun Sung Kim 1 , Chung-Nam Whang 1 , Kwangho Jeong 1 , Kwanghee Cho 2 , Sang Wan Cho 1 , Yeonjin Yi 3
1 Physics, Yonsei University , Seoul Korea (the Republic of), 2 , Hynix semiconductor Inc., , Icheon-Si GyeungGi-Do Korea (the Republic of), 3 , Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of)
Show AbstractThe insertion of a thin film metal oxide interlayer between the organic semiconductor and metal elctrode has shown improvement of carrier injection of the organic thin-film transitors(OTFTs). The electronic structures of pentacene/molybdenum oxide/aluminum and pentacene/aluminum were studied using in situ ultraviolet photoelectron spectroscopy and x-ray photoelectron spectroscopy. The ultraviolet and x-ray spectra allowed us to evaluate the complete energy level diagrams and to analyze the chemical interactions at the interface. We found that inserting molybdenum oxide between pentacene and aluminum led to the highest occupied molecular orbital of pentacene shifting to a lower binding energy compared to that without molybdenum oxide. The hole injection barrier was reduced by 0.72 eV compared to the structure without molybdenum oxide.
9:00 PM - H5.47
Infrared and Raman Studies of Pentacene Thin Films Doped with Acceptors.
Yoshinobu Hosoi 1 , Koya Osuka 1 , Yukio Furukawa 1
1 Department of Chemistry and Biochemistry, School of Advanced Science and Engineering, Waseda University, Tokyo Japan
Show AbstractChemical doping in organic semiconductors is one of the key techniques for development of organic electronic devices, for example, lowering operation voltage in organic light emitting diodes (OLED). Upon doping, charge transfer between an organic compound and a dopant is expected to occur. In this study, we have investigated the structures of pentacene thin films doped with acceptors such as FeCl3 and 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4TCNQ) by infrared and Raman spectroscopy. The 633-nm excited Raman spectrum of a pentacene thin film doped with FeCl3 shows several bands assigned to neutral pentacene. In the 830-nm excited Raman spectrum, however, the observed bands have been assigned to the radical cation of pentacene. Thus, radical cations are formed upon FeCl3 doping. The radical cation gives rise to the electronic transitions in the region from 800 to 1000 nm. The intensities of the Raman bands due to the radical cation are enhanced with 830-nm by the resonance Raman effect. In the infrared spectrum of a FeCl3-doped pentacene film, some bands have been attributed to the neutral and radical cation of pentacene and some bands are possibly attributed to the oxidized species of pentacene.
9:00 PM - H5.48
Carrier Blocking Nature at α-NPD/Alq3 Interface Studied by Displacement Current Measurement: Is Orientation Polarization of Alq3 film an Origin of the Interface Charge?
Yutaka Noguchi 3 1 , Naoki Sato 1 , Yukimasa Miyazaki 2 , Yuya Tanaka 1 , Yasuo Nakayama 3 , Hisao Ishii 3 1
3 Center for Frontier Science, Chiba University, Chiba-shi Japan, 1 Graduate School of Advanced Integration Science, Chiba University, Chiba-shi Japan, 2 Department of Electronics and Mechanical Engineering, Chiba University, Chiba-shi Japan
Show Abstract In organic light emitting diodes (OLEDs), carrier blocking at organic-organic interfaces plays a key role to device performance and operation mechanism. For example, such a carrier blocking can make the recombination zone narrow to increase the efficiency. The carrier blocking nature depends on the interfacial properties such as conductance mismatch and injection barrier at the interface. Besides these factors, the formation of interfacial charge has been proposed in literature. Concerning ITO/α-NPD/Alq3/Cathode type device, which is one of the most studied OLEDs, Brütting et al. revealed that there exists negative fixed-charge at the α-NPD/Alq3 interface. The charge governs the hole blocking property and the device performance. The origin of the charge is, however, not understood. On the other hand, E. Ito et al. reported that Alq3 film formed under the dark spontaneously built a giant surface potential (SP), as much as 28 V (at a film thickness of 560 nm), due to ordering of dipole moment of Alq3, and that the giant SP was removed by a photo irradiation. Although several research groups have investigated the mechanism of the giant SP, little has been known how the giant SP affects on the properties of actual OLEDs containing the Alq3 film. In this paper, we demonstrate that these two findings are closely-linked with each other. We examined the effects of ambient light during the fabrication of an OLED (ITO/α-NPD/Alq3/Al) on the device properties by a displacement current measurement (DCM). We found the device properties which can be attributed to the giant SP.The organic layers and Al were successively formed on an ITO coated glass substrate under a white light. The DCM was performed by applying triangular wave biases within ±5.0 V at a sweep speed of 1 V/s. We found that the photo irradiation during the device significantly reduced the density of the interfacial charge (σ0) existing at the α-NPD/Alq3 interface to a half of that in the control devices fabricated under the dark. The σ0 of the control device was estimated to be 1.21 mC/m2. This agrees well with the one that obtained from the giant SP (1.55 mC/m2). It is plausible that the origin of the interfacial charge is the dipole moment of Alq3. DCM also revealed that light irradiation induces the amount of traps in Alq3 layer probably due to dipolar disorder effect. Similar results was also obtained for the device including 1,3,5-tri(phenyl-2-benzimidazole)-benzene (TPBi). These results strongly demonstrate that we should pay attention to possible orientation polarization to discuss OLEDs.
9:00 PM - H5.49
Optoelectronic Characteristics of an Array of Sub-micron OLEDs Deposited on Pyramids.
Yiying Zhao 1 , Kwang-Hyup An 2 , Kevin PIpe 2 , Max Shtein 1
1 Department of materials science and engineering, the University of Michigan, Ann Arbor, Michigan, United States, 2 department of mechanical engineering, the University of Michigan, Ann Arbor, Michigan, United States
Show AbstractNanoscale light sources show promise for applications in quantum communications, near-field scanning optical microscopy, nanoscale photolithography, and light sensing.[1,2] In our previous work, an OLED-based sub-micrometer scale light source has been integrated with a single AFM cantilever for application to scanning probe microscopy.[3] In this work, we describe the fabrication of arrays of such small emitters and characterize their optical properties. In particular, we discuss the electroluminescence (EL) spectra of these micro- and nano-OLEDs, which evolve with voltage as a consequence of the non-planar substrate geometry. These devices provide a vehicle for more precise characterization of electrically pumped optical probes and, with regard to low-cost, large-area applications of OLEDs, these device architectures provide an opportunity to study in a controlled fashion how substrate non-uniformities affects electrical performance and degradation. References:[1] Betzig, E., and Trautman, J.K., Science 1992, 257(5067), 189-195.[2] Minh, P. N.; Ono, T.; Fsashi, M.; Fabrication of silicon microprobes for optical near field applications, CRC Press: Boca Raton, FL, 2002.[3] Zhao, Y., An, K., Chen S., O’Connor, B., Shtein. M., and Pipe. K. P., Nano Lett. 2007, 7(12), 3645-3649.
9:00 PM - H5.5
Determination of Internal Quantum Efficiency of OLEDs with the Consideration of the Purcell Effect.
Wallace Choy 1 , X. Chen 1 2 , Sailing He 2 , P. Chui 1
1 Department of Electrical & Electronic Engineering, the University of Hong Kong, Hong Kong China, 2 Centre for Optical and Electromagnetic Research, Zhejiang University, Hangzhou China
Show Abstract9:00 PM - H5.50
Electronic Structures of Rubrene Single Crystal and its Interface Studied by Photoemission Yield Spectroscopy.
Shinichi Machida 1 , Yasuo Nakayama 2 , Takeo Minari 3 , Kazuhito Tsukagoshi 3 , Yutaka Noguchi 1 2 , Hisao Ishii 1 2
1 Graduate School of Advanced Integration Science, Chiba University, Chiba Japan, 2 Center for Frontier Science, Chiba University, Chiba Japan, 3 , RIKEN, Wako Japan
Show Abstract9:00 PM - H5.51
Low-Voltage Operation of Inverted OLED using Low-Injection Barrier Electrodes, CuxSe and C12A7:e-.
Hiroshi Yanagi 1 , Ikue Koizumi 1 , Ki-Beom Kim 1 , Hidenori Hiramatsu 2 , Masashi Miyakawa 3 , Toshio Kamiya 1 2 , Masahiro Hirano 2 3 , Hideo Hosono 1 2 3
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 2 ERATO-SORST, Japan Science and Technology Agency, Yokohama Japan, 3 Frontier Research Center, Tokyo Institute of Technology, Yokohama Japan
Show AbstractCarrier injection barriers at electrodes / organic semiconductors interfaces in organic semiconductor based devices such as organic light emitting diodes (OLEDs) are generally larger than 1 eV and a serious obstruction for stable and low power consumption device operation. To reduce an electron injection barrier (EEIB), a low work function material is suitable for a cathode because electron affinities of organic semiconductors are smaller than the work functions of typical cathode materials. 12CaO7Al2O3 electride (C12A7:e-) [Matsuishi et al. Science 301 (2003) 626] is one of the ideal materials for an efficient cathode for organic semiconductors because C12A7:e- has a high electrical conductivity of 1500 S/cm [Kim et al. Nano Lett. 7 (2007) 5], a low work function of ∼2.4 eV and good chemical stability at room temperature (RT) in air [Toda et al. Adv Mater. 19 (2007) 3564]. Recently, we have reported a low EEIB of ∼0.6 eV for a C12A7:e- / Alq3 interface [Kim et al. J. Phys. Chem. C, 111 (2007) 8403]. However, the present technique to fabricate C12A7:e- films require a high process temperature ∼1100 oC and therefore an inverted structure, which employs a bottom cathode and a top anode, is required to apply C12A7:e- to a cathode of an OLED. For an anode, Sn doped In2O3 (ITO) is usually employed; however depositing ITO as a top anode layer has several technical issues: sputtering deposition, which is usual deposition technique for ITO films, damages the organic layers due to ion bombardment. Although a surface treatment such as oxygen plasma and UV-ozone is required for realizing high efficient hole injection between ITO and an organic semiconductor, such a treatment is impossible when ITO is deposited on organic layers. In this study, we employed CuxSe (x ≈ 2) as a candidate for a high-efficient top anode for inverted OLEDs: low hole injection barrier (EHIB) of ∼0.6 eV for NPB and ∼0.5 eV for CuPc were achieved without surface treatments. CuxSe films were prepared by a PLD method at RT. The deposited CuxSe films were transferred from the PLD chamber to our vacuum system after exposure to air. Our apparatus can carry out depositions of organic and metal layers and measurements of UPS-XPS without breaking vacuum. To remove atmospheric contaminants, oxygen plasma treatment was carried out on some CuxSe films. The obtained EHIB values without surface treatment are comparable to those at the interface with surface treated ITO. The oxygen plasma treatment reduced the EHIB by ∼0.2 eV. The hole-only devices with Al/NPB/CuxSe and Al/CuPc/CuxSe stacking structures showed low threshold voltages (<0.5 V) and high current densities (>1.5 A/cm2 at 1 V). Prototype inverted top-emitting OLEDs (ITOLEDs) with C12A7:e- cathodes and CuxSe anodes were compared with ITOLEDs with conventional electrodes such as an Al cathode and an ITO anode. The C12A7:e- cathode and the CuxSe anode were effective to achieve a lower threshold voltage and higher luminance efficiency.
9:00 PM - H5.52
Enhanced Stability of Poly(3-hexylthiophene) Field-Effect Transistors by Solution-Processed Polymer Passivating Films.
Yu Fu 1 , Feng Tsai 1
1 Materials Science and Engineering , National Taiwan University, Taipei Taiwan
Show AbstractThin-film transistors (TFTs) with poly(3-hexylthiophene) (P3HT) as the semiconductor require passivation to achieve adequate lifetime, but direct application of many types of polymer films onto the P3HT layer as passivation causes serious degradation to the device characteristics, most notably drastically increased off-state current and disappearance of the field effect property. The passivation-caused degradation is due to the polar functional groups of the passivating polymers, which induce charge carriers in the P3HT layer to increase the off-state current. The passivation-caused degradation can be minimized by using polymers with nonpolar or low-polarity functional groups, of which poly(methyl methacrylate) (PMMA) is demonstrated as a satisfactory passivating material. By examining solvents with different polarity for the PMMA passivation process, this study also discovers that the solvent, regardless of its polarity, has no effect on the TFTs. With a PMMA film as the first passivation layer, additional layers of more polar polymers can be applied to reinforce passivation without degrading the TFTs. TFTs passivated with a bilayer PMMA/poly(vinylidene chloride-co-acrylonitrile) film show high hole mobility (0.02 cm2/ V s) and on/off ratio (10E4) that remain unchanged upon 1000 hr of air exposure.
9:00 PM - H5.53
Effective Heat Dissipation of OLEDs Fabricated on Stainless Steel Substrate.
Seungjun Chung 1 , Jae-Hyun Lee 2 , Jang-Joo Kim 2 , Yongtaek Hong 1
1 Electrical Engineering and Computer Science, Seoul National University, Seoul Korea (the Republic of), 2 Materials Science and Engineering, Seoul National University, Seoul Korea (the Republic of)
Show AbstractThermal stress of organic light-emitting diodes (OLEDs) from heat localization around light-emitting area has been considered one of the factors causing OLED degradation such as indium-tin-oxide (ITO) decomposition and acceleration of indium electro-migration, or crystallization of organic layers. As device temperature increases with operation time, OLED current increases at a give voltage, resulting in increased electrical input power, most of which will be converted into thermal energy, accelerating thermal stress to OLEDs. Therefore, it is important to effectively dissipate accumulated thermal stress of OLEDs for longer device lifetime. To further investigate thermal stress of OLEDs, we have studied heat dissipation behavior of top-emission OLEDs fabricated on silicon, glass, and stainless steel substrates, which have thermal conductivities of 150, 1, and 16 W/m●K, respectively. In all cases, OLED performance was verisimilar and its luminous and power efficiencies were 9.3±0.4 cd/A and 3.8±0.1 lm/W, respectively. As expected, glass and silicon substrates showed the worst and best heat dissipation, respectively. For stainless steel substrates, although there is an organic planarization layer (0.3 W/m●K), it showed much better heat dissipation than glass substrate. Peak temperature was measured on backside of the substrates after OLEDs were operated at luminance of 10,000 cd/m2 for 3 minutes. Infrared thermal image camera from FLIR systems were used to accurately measure heat distribution over the substrate. For silicon, glass, and stainless steel substrates, the peak temperatures were 21.4, 64.3, and 40.5°C, respectively. From the uniform thermal distribution images obtained by the Infrared thermal image camera, we found that heat generated in OLEDs was effectively dissipated over the silicon and stainless steel substrates. However, OLEDs on glass substrate showed heat localization at device area and outward thermal gradient. OLED lifetime measured at time of 90% luminance reduction from initial value increased by 213% for SS substrate in comparison with glass substrate. Since OLED degradation is also closely associated with surface roughness of the bottom electrode, we measured surface roughness of the bottom reflective electrodes by using AFM method. In all cases, surface roughness was 2.7±1.2 nm in root-mean-square and 23±5 nm in peak-to-valley. Therefore, it can be concluded that the lifetime improvement in our cases is from effective heat dissipation of OLEDs through the stainless steel substrate rather than from surface roughness difference for each substrate. Since organic planarization layer has a poor thermal conductivity, we expect better heat dissipation can be obtained if appropriate via structures are formed to provide direct thermal contact between one of OLED electrodes and stainless steel substrate. Details of our thermal stress and lifetime test results will be presented at conference.
9:00 PM - H5.54
Experimental Determination of Energy Level Alignment at All Interfaces in a Complete OLED Structure.
Selina Olthof 1 , Rico Meerheim 1 , Karl Leo 1
1 TU Dresden, IAPP, Dresden Germany
Show AbstractWednesday, 12/3WITHDRAWN posterH5.54
9:00 PM - H5.56
Molecular Orbital Study of the Interface Alq3 Oxidation at the Organic Semiconductor / Metal Contact.
Kyul Han 1 , Pyung Eun Jeon 1 , Hyunbok Lee 1 , Hyun Sung Kim 1 , Chung-Nam Whang 1 , Kwangho Jeong 1 , Sang Wan Cho 2 , Yeonjin Yi 3
1 Institute of Physics and Applied Physics, Yonsei University , Seoul Korea (the Republic of), 2 Department of Physics, Boston University, Boston, Massachusetts, United States, 3 Division of Advanced Technology, Korea Research Institute of Standards and Science, Daejeon Korea (the Republic of)
Show AbstractAluminum tris-(8-hydroxyquinoline), (Alq3) is used in organic light-emitting devices (OLEDs) as an electron-transport material and emitting layer. A failure mode of an OLED at the Alq3 / Al interface has been observed using the in-situ x-ray and ultra-violet photoelectron spectroscopy (XPS and UPS). In the experiment, with the trace amount O2 exposure, the thin Alq3 near the Al cathode shows a degradation mechanism by formation of the Alq3 oxidation. Density functional calculation has been carried out to characterize the oxidation of Alq3 phenomena. Using Becke-style 3 parameter calculation using the Lee-Yang-Parr correlation functional with the 6-31G(d) basis set, the Alq3 single molecule, the Alq3 cation and the Alq3 molecule with an O2 adsorbate molecule are geometrically optimized in vacuo and the structural and electronic properties are presented. The simulated UPS valence and XPS core-level spectra have been successfully generated and compared with the experimental spectra. In support of the experimental conclusions, the interface Alq3 cation explains why the oxidation phenomena occur at the interface rather than at the neutral bulk Alq3. Our proposed degradation mechanism of the electron injection barrier increase by the interface Alq3 oxidation at the organic semiconductor / metal contact has been confirmed.
9:00 PM - H5.57
Temperature Dependence of Electrolabsorption (EA) and Photocurrent (PC) Spectra for Two Different Classes of Conjugated Light Emitting Polymers : Possible Role of Density of States.
Awnish Tripathi 1 2 , Yashowanta Mohapatra 1 2
1 Department of Physics, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India, 2 Samtel Centre for Display Technologies, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India
Show AbstractAt the present state of development of polymer electronics, the evaluation of electronic quality of polymer thin films on the basis density of states (DOS) within the HOMO-LUMO gap is becoming increasingly important. The presence of subgap energy levels and its impact on opto-electronic properties of conjugated polymers is not well understood. In this paper we present a correlated study of temperature dependence of Electroabsoprtion spectra (EA), photocurrent spectra (PC), and voltage dependent electro-absorption transients(t-EA) for two different polymers in order to isolate the mechanisms involved in these charge processes and the occupancy within the density of states.We have carried out temperature dependence (10K-300K) of EA and PC spectra on two different class of polymers viz. poly [2-methoxy-5-(2'-ethyl-hexyloxy)-1, 4-phenylene vinylene] (MEH-PPV) and Arylenevinylene-co-pyrrolenevinylenes (AVPV) in a sandwiched device configuration. We demonstrate the relationship between PC and EA spectra both of which mirror joint DOS participating in these processes. PC involves many processes including generation of excitons within the material, conversion into charge transfer (CT) states, dissociation of CT into free carrier, and transport followed by final collection at the electrodes. All these processes involved in PC generation can in principle be temperature dependent. In order to understand the cross-sensitive parameters, temperature dependence of EA spectra is helpful. A careful comparison of changes with temperature in onset energies, peak position and peak width of EA spectra for the two polymers and their corresponding features in PC are carried out. Both onset and peak energies increase with increase in temperature for both polymers displaying parabolic dependence. The variation in magnitudes of the peak has distinctly different behavior and seems to originate from occupancy of states. We have also performed Electroabsorption transient (t-EA) measurement in slow time scale at room temperature for both the polymers. The t-EA shows voltage dependent time constants in reaching saturation and is indicative of involvement of subgap energy levels in the opto-electronic process of conjugated polymers. In summary, we use three experimental tools to demonstrate the influence of subgap energy levels in conjugated polymers, and have shown the impact of these levels on opto-electronic property of conjugated polymeric systems.
9:00 PM - H5.58
Continuous Tuning of Organic Transistor Operation from Enhancement to Depletion Mode by Means of Organic Bulk Heterojunctions.
Piero Cosseddu 1 2 , Joern-Oliver Vogel 3 , Beatrice Fraboni 4 , Jurgen P. Rabe 3 , Norbert Koch 3 , Annalisa Bonfiglio 1 2
1 Dept. of Electrical and Electronic Engineering, University of Cagliari, Cagliari Italy, 2 S3 nanoStructures and bioSystems at Surfaces, CNR-INFM, Modena Italy, 3 Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany, 4 Department of Physics, University of Bologna, Bologna Italy
Show AbstractIn this paper we report on the possibility to use organic bulk heterojunctions of two derivatives of a conjugated molecule for tuning and controlling charge carrier population and transport within the active layer. In particular, we report on the fabrication and operation of a series of OFETs, which exhibit (i) accumulation and depletion mode operation, (ii) wide tunability of threshold voltage, (iii) mechanical flexibility. We fabricated OFETs both on SiO2, as reference substrate, and on plastic substrates (Mylar®), acting at the same time as gate dielectric, by co-depositing two organic semiconductor materials, sexithiophene (6T) and α,ω-dihexylsexithiophene (DH6T), at various ratios. OFETs comprising pure 6T channels exhibited a slightly negative threshold voltage (VT), thus working in p-type accumulation mode. When only DH6T formed the channel, large positive VT was observed, giving evidence that an accumulation of p-type charge carriers was already present without applying any gate bias and that these OFETs work in the depletion regime. Photocurrent spectroscopy measurements confirmed that a larger concentration of charge carriers is present in DH6T OFETs. Despite the identical conjugated moiety, DH6T has a 0.15 eV lower ionization energy than 6T, consequently, DH6T is more easily p-doped by atmospheric oxygen than 6T, explaining why pure DH6T OFETs exhibit depletion mode operation (heavily doped) and pure 6T ones are accumulation type (intrinsic). As a result, For OFETs with mixed 6T:DH6T channels we found a linear dependence of VT on the 6T: DH6T ratio.Moreover, the hole mobility was essentially constant for all mixing ratios (≥ 5x10^-3 cm^2/Vs), and even higher than for pure 6T flexible OFETs (3x10^-3 cm^2/Vs) [1]. X-ray diffraction and atomic force microscopy studies showed that 6T and DH6T co-deposited films (on Mylar® and Si-oxide) have very similar structural and morphological properties as the pure materials films, and that the two molecules form intercalation compounds. This is particularly interesting because it demonstrates the possibility to adjust the device working point and tune its operational mode without negatively affecting charge carriers transport across the channel. Our work shows that extremely wide OFET function tunability can be achieved by combining molecules with appropriately adjusted properties in molecularly mixed films. A rational approach to precisely control OFET performance has thus been established. Moreover, this approach is not influenced by the particular substrate employed as gate dielectric, and it is therefore suitable for tuning the electrical properties in flexible plastic devices, providing a considerable extension of the application-potential of organic electronics.[1] P. Cosseddu, J.-O. Vogel, B. Fraboni, J. P. Rabe, N. Koch, A. Bonfiglio, Adv. Mater. in press
9:00 PM - H5.59
Electron Induced Current Instability in Hole-dominated, Organic Field-effect Transistors.
Christopher Siol 1 , Niels Benson 1 , Christian Melzer 1 , Heinz von Seggern 1
1 Electronic Materials Division, Institute of Materials Science, Technische Universität Darmstadt, Darmstadt, Hessen, Germany
Show AbstractIn the last years the performance of organic field effect transistors (OFETs) has improved substantially. Today hole conducting OFETs with performances comparable to amorphous inorganic silicon devices can be realized. Nevertheless drain current hysteresis and long term stability is still an issue for OFETs. While hole traps are known to cause threshold voltage instabilities in p-type OFETs the impact of electron traps is usually neglected due to the absence of significant electron current. Since, however, in logic elements p-type OFETs are frequently biased in ambipolar mode, sufficient electrons may enter the channel and an electron-induced device instability is possible. To investigate this issue, the Kelvin probe force microscopy was applied. A combination of classical current voltage measurements and simultaneous detection of the localized surface potential allows one to separate trapped from mobile charges in the transistor channel and thus to investigate hysteresis effects in OFETs in detail. Furthermore, unintended electron injection into the channel in depletion mode can be detected.Kelvin measurements on typical p-type OFET structures show that injection of electrons is possible in all devices investigated, even so no steady state electron current was observed. The injected electrons distribute over the entire transistor channel where they are partially trapped. It will be demonstrated that those trapped negative charges codetermine the effective gate potential and thus the channel conductivity. Depending on their stability these negative charge densities result in a substantial drain current hysteresis and cause long term changes in the channel conductivity which even outlast hole accumulation proving that trapped electrons are not only affecting n-type but also p-type devices.
9:00 PM - H5.6
Independently Controllable Stacked OLEDs with High Efficiency by using Semitransparent Al/oxide/Ag(Au) Intermediate Connecting Layer.
Wallace Choy 1 , H. Zhang 1 2 , Y. Dai 2
1 Department of Electrical & Electronic Engineering, the University of Hong Kong, Hong Kong China, 2 State Key Laboratory of Polymer Physics and Chemistry, Changchun Institute of Applied Chemistry, Chinese Academy of Sciences, Changchun China
Show Abstract9:00 PM - H5.60
Evolution of Single-Walled Carbon Nanotube Based All Printed Thin Film Transistors for Printed Electronics.
Gyoujin Cho 1 2 , Minhoon Jung 1 2 , Jaeyoung Kim 2 , Kyunghwan Jung 2 , Junghye Kim 1 , Youngkwan Song 1
1 School of Applied Materials, Sunchon National University, Sunchon, Jeonnam, Korea (the Republic of), 2 Printed Electronics Research Institute, PARU Co., Sunchon, Jeonnam, Korea (the Republic of)
Show AbstractPrinted electronics is an emerging new technology to replace current Si technology in the field of inexpensive and flexible macroelectronics, especially for RFID and display. For successfully edging the printed electronics in markets, the most priority in works of the printed electronics is to provide all printed thin film transistors (TFTs) comparable to the amorphous Si based TFTs not only for electrical properties but also for practical production. Therefore, for last decades, a lot of efforts have been poured in developing fully printed TFTs on flexible plastic foils. However, up to present, materials and processes for fully printed TFTs comparable to the amorphous Si based TFTs have not been demonstrated yet. Although printable TFTs on Si wafer have shown the similar performance to the amorphous Si based TFTs, there was no investigation how electrical properties of those TFTs on Si wafer will be changed as they are transferred into fully printed process on the plastic foils. As a consequence of developing fully printed TFTs on plastic foils to substitute the amorphous Si based TFTs, we anticipated in developing simple and inexpensive materials for gate electrode, dielectric layer, drain-source electrodes, and semiconducting layer to show how their performance will be altered from printing on Si wafer to printing on plastic foils using single-walled carbon nanotube as a semiconductor.
9:00 PM - H5.61
Control of the Work Function of Transparent Electrodes using Organic Surface Modifiers: Effects on Charge Injection in Organic Electronic Devices.
Asha Sharma 1 , Andreas Haldi 1 , Peter Hotchkiss 2 , Seth Marder 2 , Bernard Kippelen 1
1 School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 School of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractWe report on means to control the work function of indium tin oxide (ITO) electrodes using organic surface modifiers based on phosphonic acid anchoring groups and fluorinated benzyl functional decorating groups. The effects on charge injection of the changes in work function due to dipolar surface modifiers are studied in single layer diodes fabricated from organic semiconductors. The measured current density-voltage (J-V) characteristics as a function of temperature are fitted using an equivalent circuit model in which carrier injection is modeled by a thermionic emission process. Injection barrier heights are extracted from the fits and are correlated with the energy barrier heights predicted from the difference in work function of the modified ITO and the HOMO energy of the organic semiconductors. It is found that the charge injection barrier height is nearly constant despite of large variation in the work function of the modified ITO anode.
9:00 PM - H5.62
Impact of Residual Benzyl Halide Groups on Performance of Poly(phenylenevinylene) Based Organic Light Emitting Diodes.
Arne Fleissner 1 , Thorsten Schwalm 2 , Katja Stegmaier 1 , Christian Melzer 1 , Matthias Rehahn 2 , Heinz von Seggern 1
1 Electronic Materials, Technische Universitaet Darmstadt, Darmstadt Germany, 2 Macromolecular Science, Technische Universitaet Darmstadt, Darmstadt Germany
Show AbstractDerivatives of the organic semiconductor and electroluminescent polymer poly(para-phenylenevinylene) (PPV) are frequently employed in organic light emitting diodes (OLEDs). For the synthesis of PPVs, the Gilch route is the most prominent polymerization method involving polymerization and dehydrohalogenation in solution. The latter is a base induced elimination of halogen groups resulting finally in the pi-conjugation of the polymer. Up to now the main defect is the tolane-bisbenzyl (TBB) structure which is hold responsible for a reduced lifetime of PPV-based OLEDs.In this study we show that besides the TBB defects benzyl halide groups have a large impact on OLED device performance. Halogen remains in the polymer backbone of the PPV due to an incomplete dehydrohalogenation during synthesis. It will be shown that upon a decreased amount of benzyl halide defects in the PPV, respective OLEDs exhibit an increased luminance efficiency and a markedly extended lifetime. Influences can be observed if less than 1% of the regular vinyl bonds are replaced by the defect structure. This is demonstrated by preparing PPVs by prolonged synthesis times resulting in a decreased amount of benzyl halide groups with an otherwise similar TBB defect concentration. The influence of the benzyl halide group on the device and material parameters, such as current-voltage characteristic, device lifetime and charge carrier mobility is discussed.
9:00 PM - H5.63
The Influences of Gap States and Energy Levels on the Carrier Injections in Organic Light Emitting Devices.
Chih-I Wu 1 2 , Chan-Tin Lin 2 , Mei-Hsin Chen 2
1 Electrical Engineering, National Taiwan University , Taipei, Taiwan, Taiwan, 2 Graduate Institute of Photonics and Optoelectronics, National Taiwan University, Taipei Taiwan
Show AbstractEffective carrier injections are the keys to the success of the organic light emitting diodes (OLEDs). In this paper, we will present a systematic study of the effects of gap states and energy level alignments on the carrier injection mechanisms in the organic light emitting devices. The performances of devices with various combinations of anode and cathode structures were compared. To find out the key factors affecting carrier injection efficiency, the interfacial electronic structures and chemical properties were then studied with using X-ray and ultra violet photoemission spectroscopy (XPS and UPS). For electron injection, we compared the effects of LiF and CsF as electron injection layers on two different electron transport layers, including Alq3 and oligofluorenes. We found that devices with CsF in the anode structures have greater performance than those with LiF. From the UPS data, however, both LiF and CsF can significantly reduce the electron injection barrier heights. The better performance of devices with CsF is attributed to the gap states found at the interfaces of anode and electron transport layers. As for hole injection, devices with MoOx in the anode structures show excellent current injection characteristics, regardless the metallic anodes used. XPS and UPS data indicate that the reasons lead to these anode insensitive devices might be the Fermi level pinning due to the gap states at the interfaces. Although energy alignments are still the keys to the efficient carrier injections, we found that the gap states, in these cases, play more important roles to help the carriers move over the barriers and inject into the organic materials. The origins of the gap states will also be discussed
9:00 PM - H5.64
Progress in Organic Crystal Transistors for High-Performance Organic Electronics.
Jun Takeya 1 , T. Uemura 1 , M. Uno 1 2 , M. Yamagishi 1 , Y. Tominari 1
1 , Osaka University, Toyonaka Japan, 2 , Technology Research Institute of Osaka Prefecture, Izumi Japan
Show Abstract Developing high-performance organic field-effect transistors (OFETs) is a key technology for an extensive market of organic electronics, providing matrix-controlling devices for flexible displays, as well as easy-to-fabricate switching components in logic circuits, for examples. In this presentation, we report our recent progresses for the maximum field-effect performances with the use of organic crystal semiconductors, including air-stable high-mobility n-type transistors, ionic-liquid based transistors with the highest transconductance, and three-dimensional (3D) transistors enabling very high current density per pixel area. In order to have prescriptions of high-performance n-type organic transistors, we built highly electron-affine TCNQ crystal transistors. We fabricated them in air and measured their performance also in air. All the measured transistors exhibited text-book like characteristics with negligible threshold, very small hysteresis and relatively high mobility of 0.5 cm2/Vs, all of which are essential in designing high-performance complementary logic devices. In particular, the absence of the threshold voltage VG is contrasting to reported n-type transistors with large VG exceeding 10 V, indicating reduction of electron traps in the single-crystal OFETs without influence of harmful adsorbates at grain boundaries. As long as the electron-affinity of the organic semiconductor is large enough, no essential obstacles appear to prevent the air-stable n-type field-effect operation, providing a prescription of high-performance organic complementary circuits. For p-type crystal transistors, since high mobility exceeding 10 cm2/Vs is achieved as an intrinsic material performance, the next step is to reflect the excellent charge transport property in real current-enhancement function. Combination of rubrene single crystals and room-temperature ionic liquids realizes the highest current enhancement per VG ever achieved for organic transistors (sheet transconductance of 20 μS with drain voltage 1 V). Since the transconductance is a product of mobility and carrier density per VG, the result comes from high mobility of the rubrene crystal, efficient carrier accumulation with the use of the electrolyte, and most importantly, minimized damage at the crystal surface interfacing the liquid. Moreover, high ionic conductance of the ionic liquids leads to fast switching up to 1 MHz for the devices functionalizing the solid-to-liquid interfaces. Finally, we have developed a construction of 3D multi-pillar structures of organic semiconductors, so that the above-mentioned high transconductance can be applied to highly contrasting pixel-controlling devices. To realize the gating function, the liquid easily immerses to the complicated surface shape of the semiconductor pillars. In our first prototype device of pentacene 3D transistors, the field-effect current reaches ~ 1 μA per pixel, which is required for high-contrast organic displays.
9:00 PM - H5.65
Molecular Assembly of Organic Semiconductors Controlled by Functionalized Surfaces.
Takeo Minari 1 , Masataka Kano 2 , Tetsuhiko Miyadera 3 , Sui-Dong Wang 3 , Yoshinobu Aoyagi 1 , Kazuhito Tsukagoshi 3 4
1 Advanced Research Institute, RIKEN, Wako, Saitama, Japan, 2 , Dai Nippon Printing, Kashiwa, Chiba, Japan, 3 Nanotechnology Research Institute, AIST, Tsukuba, Ibaraki, Japan, 4 , JST-CREST, Kawaguchi, Saitama, Japan
Show AbstractSince the performance of organic field-effect transistors (OFETs) has now reached that of amorphous silicon based transistors, practical applications are seriously being considered. The applications may stem from flexible and light-weight features of organic devices fabricated by a solution or printing technique. On the other hand, solution processed formation of OFETs involves some difficulties, especially, in coating and patterning the organic semiconductor layers. We have developed a selective organization technique that allows us simultaneous formation of organic transistor arrays from solution phase. This technique is based on patterned functionalities on surface; difference in wettability given by the surface-modified materials leads the selective crystallization of soluble organic semiconductors with desired geometry. The self-organized organic films are extended to channels of high performance OFETs. [1]. The self-organized formation of organic layers has been achieved by patterning self-assembled monolayers (SAMs) on the surface of the insulating layer. Both silicon wafers with oxide layers and plastic substrates with polymeric gate insulators can be used for the technique. The insulator surface was coated with a SAM having an alkyl group, providing uniform hydrophobicity over the entire substrate surface. The area that had been selected to be the channel region of the OFETs was then irradiated with ultraviolet light through a shadow mask for removal of the alkyl SAM. This area was modified again with a SAM containing a phenyl group. The Phenyl modified surface is wettable for organic solvents. As a result, regions are modified to become wettable and unwettable, by Phenyl and alkyl modifications, respectively. The organic semiconductor solution was then dropped onto the substrate with patterned wettability. Due to the difference in wettability on the surface, the solution is likely to diffuse into the wettable area, which results in organic semiconductor films patterned in the desired geometry, and the self-organized organic films are extended to channels of high performance OFETs. [1] T. Minari et al., Appl. Phys. Lett. 92, 173301 (2008).
9:00 PM - H5.67
Energy Transfer Effect on Magnetic Response in Organic Semiconducting Materials.
Liang Yan 1 , Bin Hu 1
1 , University of Tennessee, Knoxville, Tennessee, United States
Show AbstractAn external magnetic field can change electroluminescence and photocurrent, leading to magnetic field effects in organic semiconductor devices. In general, the magnetic field effects can be generated if (i) an external magnetic field can change singlet and triplet ratios in excited states and (ii) singlet and triplet excited states have different involvements in light emission and photocurrent due to their largely different lifetimes and binding energies. The magnetic field effects form a new mechanism to integrate magnetic, electronic, and optical properties in the development of organic spintronics. It has been observed that fluorescent and phosphorescent organic semiconductors exhibit significant and unappreciable magnetic field effects, respectively, due to their weak and strong spin-orbital coupling strengths. Especially, the unappreciable magnetic field effects give rise to a significant obstacle to use phosphorescent organic semiconductors for magnetic field-based organic spintronics. This presentation will demonstrate a new mechanism to introduce magnetic field effects in phosphorescent organic semiconductors by using energy transfer in excited states. Specifically, when phosphorescent molecules are dispersed into fluorescent semiconducting polymer matrix, the efficient energy transfer in excited states can carry the magnetic field effects occurring in fluorescent polymer matrix over to phosphorescent molecules. As a consequence, the energy transfer in excited states can lead to significant magnetic field effects in phosphorescent molecules. This energy transfer-based magnetic field effects form a new mechanism to amplify magnetic field effects in organic semiconducting materials. This presentation will discuss how an external magnetic field changes singlet and triplet excited processes and how energy transfer amplifies the magnetic field effects in organic semiconducting materials.
9:00 PM - H5.68
Critical Comparision of Simulation & Experimental Capacitance Based Characterization of Organic Semiconductor Diode Structures.
Dheeraj Mohata 1 3 , Durgesh Tripathi 2 3 , Yashowanta Mohapatra 2 3
1 Department of Electrical Engineering, IIT Kanpur, Kanpur, Uttar Pradesh, India, 3 Samtel Center for Display Technologies, IIT Kanpur, Kanpur, Uttar Pradesh, India, 2 Department of Physics, IIT Kanpur, Kanpur, Uttar Pradesh, India
Show AbstractThe characterization of organic semiconductor based devices is crucially dependent on a thorough understanding of electrical characteristics of which capacitance based techniques form an integral part. However, for organic devices, capacitance based studies are hindered by an inadequate understanding of manifestation of basic processes in such characteristics, and a lack of clear interpretation of features observed experimentally. In this paper, we focus on a critical comparison between simulation and experiment of Capacitance-Voltage (C-V), Capacitance-frequency (C-f) characteristics and impedance spectroscopic studies of simple Single Layer and Metal-Insulating–P-doped (MIP) structures commonly used in practice. We demonstrate significant features in simulation as observed experimentally without invoking traps or problems with contacts. We also show that C-f characteristics are one of the simplest and useful ways of obtaining mobility of both carriers.The simulations are carried out using standard ATLAS (SILVACO) platform with suitably modified parameters for organic semiconductors. The comparison of simulations with experiments is carried out for well controlled polymer and small molecule based single layer devices, and m-MTDATA based doped layer (MIP) devices. The low frequency C-V shows a typical maximum in capacitance around built-in voltage. We quantitatively account for the occurrence of this peak, and its usefulness in experimental characterization for various cathode-anode combinations. The C-f characteristics for various forward bias voltages are also compared leading to clear identification of regions of capacitance change with bias in the low frequency regime including the observation of so-called negative capacitance (NC). We demonstrate extraction of mobility of carriers using frequency scaled differential capacitance of the C-f characteristics both from simulation and experiment. The Poole-Frenkel mobility dependence on electric field for single layer hole only devices are shown to be easily recovered from the simulated C-f characteristics using the suggested analytic strategy. The role of Langevin recombination in determining NC region in low frequency regime is explored through simulation, and its impact on interpretation of experimental characteristics is discussed.
9:00 PM - H5.69
Direct Patterning of Metals and Oxides on Organic Molecular Crystals with PLD.
Peter de Veen 1 , Guus Rijnders 1 , Dave HA Blank 1
1 MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands
Show AbstractThe study of intrinsic electronic properties of organic semiconducting materials (e.g. pentacene, rubrene) requires a reproducible device fabrication process that prevents degradation of the organic materials. In general, two approaches are used in the preparation of field-effect devices based on organic single-crystals. Mostly encountered is the ‘flip-crystal’ method, where a crystal is placed on a pre-fabricated transistor structure. The other method is depositing the contacts and gate insulator directly on the crystal surface. Difficulties arise as most deposition techniques damage the crystal surface, thus destroying the interface. In both methods, the nature and quality of the interfaces remain unclear.Objective of our research is to deposit metallic contacts and an oxide dielectric layer on an organic single crystal, without destroying the interfaces, enabling the fabrication of field-effect devices. For this, Pulsed Laser Deposition (PLD) at room temperature, combined with patterning via stencils as shadow masks, is used. As previously reported, this direct patterning technique can be used to fabricate nano-structures on fragile substrates (e.g. Au, Pt, Pd and Cu electrodes on organic SAMs) [1,2]. It should therefore also be suitable for use on fragile organic molecular crystals.Experimental results show that controlled deposition as well as patterning of 5 to 100 micrometer sized features can be obtained, with a height up to 75 nm, of various metals (Au, Pt, Ni and Co) and various high-k oxide dielectric materials (CeO2, Al2O3 and HfO2) on the surface of pentacene single-crystals. The typical pentacene substrate terrace steps (the inter-monolayer distance d(001) of 1.41 nm) are fully duplicated on top of the -up to 75 nm high- deposited features, indicating a high-quality growth. This substrate morphology duplication has been observed for all investigated materials, as long as rather mild deposition parameters are used. We also found that controlled patterning of metals like Ni and Co is critically dependent on the applied background gas pressure.In this contribution we will focus on our results on the controlled growth and patterning through stencils of oxide dielectrics and metallic contacts on the surface of pentacene single-crystals by pulsed laser deposition. Besides that, the electrical characterization of the fabricated devices will be presented.[1] E.A. Speets et al., Nano Letters, 4, 841-844 (2004)[2] E.A. Speets et al., Advanced Functional Materials, 16, 1337-1342 (2006)
9:00 PM - H5.7
Electrical and Optical Limitations of Stacked OLEDs and their Interconnecting Units.
Sami Hamwi 1 , Thomas Winkler 1 , Marco Witte 1 , Jens Meyer 1 , Thomas Riedl 1 , Wolfgang Kowalsky 1
1 , Institute for High-Frequency Technology, TU Braunschweig, Braunschweig Germany
Show AbstractThe concept of stacking n organic light emitting diodes (OLEDs) on-top of each other ideally allows for an n-fold increase of the quantum efficiency in the stack device as opposed to a single OLED. This enables longer lifetimes since each light emitting unit is stressed significantly less to achieve a given luminance level. In order to optimize efficiency of the stack device one has to take electrical as well as optical considerations into account. Not only the position of the light emitting layers but even more importantly the dimensions of the interconnecting units that serve as charge generation layers (CGL) require attention. In optimized devices, we will demonstrate that linear increase of the current efficiency with the number of stacked OLEDs is in general limited by fundamental optical characteristics. Typically, a sub linear increase is obtained which can not be explained by an eventually increased optical absorption due to the increasing number of layers involved. Rather the sub-linearity stems from suboptimal outcoupling of light from each light emitting unit. In a series of stacked OLEDs with one, two and three light emitting units the current efficiency is 46 cd/A, 83 cd/A and 126 cd/A, respectively. The results are in excellent agreement with simulation of the dielectric stack. The first requirement is a fully functional charge generation structure. In the present case it consists of a sharp heterointerface between the electrochemically n-doped layer Cs2CO3 (10% vol.) doped 4,7-diphenyl-1,10-phenanthroline and WO3 at the p-side. The minimum thickness of this p-/n-layer structure that still supports a functional CGL mechanism is one of the key parameters in the layout of stacked devices. Luminance I-V measurements revealed the minimal thickness of the charge generation layers used within a tandem OLED are 2.5 nm for WO3 and 5 nm for BPhen:Cs2CO3. For thinner layers no charge generation can be observed. This is supported by angular resolved analysis of the spectral emission characteristics and concomitant device simulation.
9:00 PM - H5.70
Influence of Surface Modifications on the Complex Impedance of Metal-polyaniline-metal Films.
Helder da Cunha 2 , Carlos L. Constantino 3 , Carla Eiras 2 , Rodrigo Bianchi 1
2 Departamento de Física, Universidade Federal do Piauí, Teresina, PI, Brazil, 3 Faculdade de Ciências e Tecnologia, Unesp, Presidente Prudente, SP, Brazil, 1 Departamento de Física, Universidade Federal de Ouro Preto, Ouro Preto, MG, Brazil
Show Abstract9:00 PM - H5.71
Impact of Gate Metal Workfunction on Device Performance of Organic Thin Film Transistor.
Wudyalew Wondmagegn 1 , Ron Pieper 1
1 Electrical Engineering, University of Texas at Tyler, Tyler, Texas, United States
Show AbstractIn this paper we have presented the simulation and analysis of the impact of the work function of the gate metal on the parameters and performance of the organic thin film transistor (OTFT). The bottom contact transistor, which is made from pentacene active material, paryelene dielectric and gold source/drain electrodes, has been used for our simulation. The simulations have been performed using Silvaco’s Atlas device simulator. The Poole-Frenkel transport model was used in the pentacene active material. To simulate the gate leakage current we have used the Fowler–Nordheim and the hot carrier tunneling models. The results of the simulation have shown an impact of the gate metal work function on the threshold voltage, the gate leakage current and the on/off ratio of the device. The simulations were done for different values of the gate electrode work function, ranging from 4.0 eV to 5.4 eV. The threshold voltage has changed from -3.5 V to -2.3 V by changing the work function in the indicated range. The simulation has shown an increase in both the gate leakage and the drain currents with increasing gate work function. In addition there was an observed decrease in an ON/OFF ratio of the devices as gate work functions increased. The probed electric field at the interface between pentacene and parylene has also shown a slight increase as the gate electrode workfunction increased. Physical arguments supporting the trends observed have been discussed.
9:00 PM - H5.72
Effect of Self Assembled Monolayer Preparation on the Microstructure and Carrier Mobility of an N-type Organic Semiconductor.
Parul Dhagat 1 , Hanna Haverinen 2 , R. Kline 3 , Y. Jung 3 , Daniel Fischer 3 , Dean DeLongchamp 3 , Ghassan Jabbour 1
1 School of Material, Advanced Photovoltaic Center and Flexible Display Center, Arizona State University, Tempe, Arizona, United States, 2 Department of Electrical and Information Engineering, University of Oulu, Oulu Finland, 3 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States
Show Abstract9:00 PM - H5.73
Simultaneous Lateral and Vertical Patterning of Molecular Organic Thin Films via Microcontact Lift-off Applied to Fabrication of Organic Light Emitting Device Arrays.
Jennifer Yu 1 , Vladimir Bulovic 1
1 Jennifer Yu, MIT, Cambridge, Massachusetts, United States
Show AbstractWe introduce a simple, subtractive patterning technique to define micron-sized features of nanometer range thickness on molecular organic thin films and apply it to fabrication of high resolution multi-color OLEDs. The procedure involves placing a poly(dimethylsiloxane) relief stamp in conformal contact with an organic thin film, and subsequent release of the stamp from the substrate removes organic material in contact with the stamp, patterning the organic thin film. The procedure is performed without applied pressure or heat and can be done in ambient environment although a nitrogen environment is preferred for fabrication. Lateral patterning occurs with good yield if film is patternable. Vertical patterning is dependent on organic material, with a trend that lower sublimation temperature materials tend toward complete lift-off thickness of original film. This technique is applied in fabrication of 13-micron-sized patterned features of a two-color organic LED structure.
9:00 PM - H5.74
Effect of Pentacene Film Formation on Detection Behavior of Organic Transistor DNA Microarrays.
Lakshmi Jagannathan 1 , Vivek Subramanian 1
1 Electrical Engineering, University of California, Berkeley, Berkeley, California, United States
Show AbstractPentacene transistors have been previously demonstrated to be usable as sensing elements for detecting DNA in low-cost DNA microarrays. The charged nature of DNA has been postulated to induce carriers in the exposed channel of such devices, causing a threshold voltage shift in the same. It has been previously shown that DNA immobilizes on the surface of the pentacene through hydrophilic-hydrophobic immobilization. Therefore, it is expected that that the surface morphology of the pentacene will strongly influence the sensitivity of DNA microarrays based on these devices. Here, the optimum substrate condition for the most sensitive immobilization of DNA is studied. The effect of pentacene thickness, deposition temperature, and evaporation conditions are related to channel grain structure, transistor characteristics, and DNA sensor sensitivity to establish a detailed mechanistic explanation for DNA sensing using organic transistors, and to provide an optimization path for sensitivity enhancement in these devices. We have found that roughness of the pentacene film depends strongly on substrate temperature and evaporation conditions. This has been related to deposition rate. Since the roughness determines the concentration of sites for DNA immobilization, this in turn is used to optimize sensor sensitivity.
9:00 PM - H5.75
Pentacene Organic Field-effect Transistors Patterned by Surface Modulation Technique.
Sung-Jin Kim 1 , Ioannis Kymissis 1
1 Electrical Engineering, Columbia University, New York, New York, United States
Show AbstractA new pattering technique to integrate disparate functional elements of organic materials in advanced organic-based electronic systems needs to be developed for fabricating a large-area format organic field-effect transistors (OFETs) so that it can meet the requirements of low-cost, simplicity, and high resolution. In this work, we present a new patterning technique for fabricating surface modulation-patterned OFETs by an ultraviolet/ozone process. The technique relies on the photochemical process of conversion of a poly(dimethylsiloxane) (PDMS) film to a silicon oxide (SiO2) film which provides the selective growth of pentacene (PC) thin-films on the hydrophobic PDMS and hydrophilic SiO2. The selective patterned PC thin-films were used for the active layers in the OFETs and the devices fabricated by this technique exhibited FET behaviors. In addition, a gate leakage current was reduced by more than 82.4 % at VD = 0 V and VG = -40 V. Our technology presented here would be applicable for devising advanced organic-based systems, such as flexible integrated circuitries, with functional organic elements on a variety of substrates.
9:00 PM - H5.76
A Theoretical/Experimental Characterization of the Charge Injection Process in Polymer Light-Emitting Diodes (PLEDs).
Giovani Gozzi 1 , Edivaldo Queiros 1 , Roberto Faria 1 , Valtencir Zucolotto 1
1 , IFSC-USP, São Carlos Brazil
Show Abstract9:00 PM - H5.77
Tuning the Performance of Organic Thin Film Transistors.
Aram Amassian 1 , Vladimir Pozdin 1 , Alexios Papadimitratos 1 , Stefan Kowarik 4 , Sukwon Hong 2 , Tushar Desai 2 , Arthur Woll 3 , Detlef Smilgies 3 , Aravind Killampalli 2 , Jared Mack 2 , Frank Schreiber 4 , George Malliaras 1 , James Engstrom 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 4 Institut für Angewandte Physik, Universität Tübingen, Tübingen Germany, 2 School of Chemical and Biomolecular Engineering, Cornell University, Ithaca, New York, United States, 3 Cornell High Energy Synchrotron Source, Cornell University, Ithaca, New York, United States
Show AbstractWe have fabricated top-contact organic thin film transistors (OTFTs) of pentacene and diindenoperylene (DIP) by supersonic molecular beam deposition on clean SiO2 and SiO2 treated with hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS). Supersonic molecular beams make it possible to control the flux and kinetic energy of incident molecules (∼1 to 13 eV) independently. In the case of pentacene, the field-effect mobility of OTFTs was found to increase systematically with increasing kinetic energy of the molecular beam. The improvements observed at increasing energy are more important on surfaces treated with HMDS (factor of 3) and OTS (factor of 5) as compared to clean SiO2 (factor of 2). In the case of DIP, the field-effect mobility of DIP TFTs fabricated at high energy on HMDS-treated SiO2 was found to be an order of magnitude higher (0.12 cm2-V-1-s-1) than previous reports of field effect mobility of OTFTs fabricated on clean SiO2 from vacuum sublimation (<0.01 cm2-V-1-s-1). The field-effect mobility of OTFTs was found to increase with increasing energy of molecules when SiO2 was treated with HMDS (factor of 3), and - to our surprise - was found to decrease with increasing energy on clean SiO2 (factor of 3). The structure, morphology, and growth of thin films of pentacene and DIP were characterized by a combination of in situ time-resolved X-ray reflectivity, in situ grazing incidence X-ray diffraction, and ex situ atomic force microscopy (AFM). AFM images reveal that pentacene thin films deposited at high kinetic energy form with significantly larger grains – independent of surface treatment – than films deposited using low energy beams. A similar effect is observed in the case of DIP, but it is much less pronounced. Analysis of time-resolved X-ray data reveals that energetic molecules of DIP have a lower probability of adsorption on HMDS-treated SiO2 than either on clean SiO2 or on top of a film of DIP, suggesting that the kinetics of growth of the first few monolayers vary with the energy of molecules and chemistry of the surface. In addition, it appears that films deposited on HMDS-treated SiO2 tend to form in a more layer-by-layer mode than on clean SiO2. While the changes in the kinetics and mode of growth of the initial monolayers explain some of the trends observed, they do not entirely explain why energetic deposition improves the performance of DIP TFTs so dramatically.
9:00 PM - H5.78
Investigation of Temperature Dependence of the Hole Mobility in Poly(3-hexylthiophene) Thin Film Transistor.
Jung Jin Yang 1 2 , Hyun Duck Cho 1 2 , Seung Uk Noh 1 2 , Yong Taek Hong 1 2 , Chang Hee Lee 1 2
1 , Seoul National University, Seoul Korea (the Republic of), 2 , Inter-university Semiconductor Research Center, Seoul Korea (the Republic of)
Show AbstractThe temperature dependence of the field effective mobility was investigated with poly(3-hexylthiophene) (P3HT) thin film transistor. The hexamethyldisilazane (HMDS) treated 300 nm-thick SiO2 were used as a gate insulator on heavily doped Si wafer. P3HT layer with the thickness of 150 nm was spun at 2000 rpm for 30 sec from solution of P3HT (3 wt. %) in chlorobenzene. The Au source and drain electrodes with the thickness of 80 nm were deposited on P3HT film through a shadow mask at the deposition rate of 1 Å/s. The channel length and width of the P3HT TFT were 40 μm and 1800 μm, respectively. The capacitance of SiO2 insulator was 11.8 nF/cm2. The mobility of the P3HT TFT was calculated from the transfer characteristics (drain current vs. gate voltage at drain voltage VD = -80 V) in the saturation regime. All measurements were performed in an N2 dry box. At room temperature the mobility was about 4.4x10-4 cm2/Vs. The mobility increased gradually from the room temperature to 110 °C. At 110 °C, the mobility was about 3.3x10-3 cm2/Vs, 7.5 times higher than the mobility at room temperature. Above 110 °C, the mobility was decreased to 2.3x10-4 cm2/Vs at 150 °C. After measuring the characteristics of the P3HT TFT at various temperatures from room temperature to 150 °C, the mobility at room temperature was increased to 6.5x10-4 cm2/Vs. And the on/off current ratio of the transistor was increased from 4.4x102 at room temperature to 5.6x103 at 110 °C. After temperature varying measurement from room temperature to 150 °C, the on/off ratio of P3HT TFT was 6.5x103 at room temperature. The increase of the field effect mobility and the on/off current ratio was attributed to the thermal annealing effect which improved the crystallinity of P3HT film.
9:00 PM - H5.80
Study on Semiconductor/dielectric Interface Dependent Properties in Pentacene-based Transistors.
Emanuele Orgiu 1 2 , Francesco Arca 1 , Beatrice Fraboni 3 , Erika Scavetta 4 , Giovanni Maria Lelli 4 , Annalisa Bonfiglio 1 2
1 DIEE, department of Electrical and Electronic engineering, University of Cagliari, Cagliari Italy, 2 Centre S3 nanoStructures and bioSystems at Surfaces, INFM-CNR, Modena Italy, 3 department of Physics, University of Bologna, Bologna Italy, 4 Department of Chemical and Inorganic Chemistry, University of Bologna, Bologna Italy
Show Abstract9:00 PM - H5.81
Charge Transport in Organic Materials with Small Energetic Disorder.
Akira Ohno 1 2 , Jun-ichi Hanna 1 2
1 Imaging Science and Engineering Lab., Tokyo Intstitute of Technology, Yokohama, Kanagawa, Japan, 2 , JST-CREST, Yokohama Japan
Show Abstract We have investigated charge carrier transport properties in organic materials with a small energetic disorder such as molecular crystals and liquid crystals using Monte Carlo simulation. In the case of charge transport for organic amorphous semiconductors, whose charge carrier transport is described often by Gaussian disorder model or its derivative models, the energetic disorder in the hopping sites is larger than ~100 meV and their charge transport is governed by Poole-Frenkel law in a wide range of temperature. However, the molecularly ordered semiconductors that have attracted high attention for OFETs recently such as molecular crystals and liquid crystals have a small energetic disorder, e.g., less than 50meV. Thus the energetic reorganization of molecules associated with a charge transfer or ionization cannot be neglected in the event of charge transport. In this study, we modeled the charge carrier transport of molecularly ordered semiconductors and simulated them by Monte-Carlo simulation, which are often dressed by these extrinsic regimes such as defects and device configurations and cannot be illustrated detail in experiments. In our simulation, we directly adopted the Marcus's charge transfer regime to describe a charge hopping process in neighboring molecules without approximation. In addition, we adopted the reorganization energy in the model because of the reason mentioned above. Consequently, they show varieties of field dependence of mobilities for each lattice structure and no field or negative field dependence are obtained for high temperature T > sigma/kB, although the results are in good agreement with some theoretical models. It should be noted that these behaviors are different from the model which is widely used and known as Gaussian disorder model whose charge transfer process is often described by Miller-Abraham regime and energetic disorder is large (~100meV). This model is valid for two-dimensional hopping systems such as the bulk transport of smectic liquid crystals and the channels of organic transistors. Our model is useful for analyzing the charge transport in high performance organic semi-conductive materials including OFET materials including liquid crystals.
9:00 PM - H5.82
Cathode Metal Diffusion and Doping in Organic Light Emitting Devices.
Priya Jadhav 1 , Benjie Limketkai 1 , Marc Baldo 1
1 , MIT, Cambridge, Massachusetts, United States
Show Abstract9:00 PM - H5.83
Tetrathienoacene Copolymers as High Mobility, Soluble Organic Semiconductors.
H. H. Fong 1 , Vladimir Pozdin 1 , Aram Amassian 1 , George G. Malliaras 1 , Detlef-M. Smilgies 1 , Mingqian He 2 , Susan Gasper 2 , Feixia Zhang 2 , Michael Sorensen 2
1 Materials Science and Engineering, Cornell University, Ithaca, New York, United States, 2 , Corning Incorporated, Elmira, 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.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 - H5.84
Ambipolar Charge Transport in Air-stable Organic Heterostructure Field-effect Transistors.
Shun-Wei Liu 1 , Jia-Cing Huang 2 , Wei-Cheng Su 2 , Chin-Chien Lee 2 , Chin-Ti Chen 1 , Juen-Kai Wang 3 4
1 Institute of Chemistry, Academia Sinica, Taipei Taiwan, 2 Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei Taiwan, 3 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 4 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show Abstract9:00 PM - H5.85
Growth of Rubrene Crystals in a Limited Space.
Jizheng Wang 1 , Hanna Haverinen 1 3 , Parul Dhagat 1 2 , Ghassan Jabbour 1 2 4
1 Flexible Display Center, Arizona State University, Tempe, Arizona, United States, 3 Department of Electrical and Inforamtion Engineering , University of Oulu, Oulu Finland, 2 School of Materials, Arizona State University, Tempe, Arizona, United States, 4 Advanced Photovoltaics Center, Arizona State University, Tempe, Arizona, United States
Show Abstract9:00 PM - H5.86
Synthesis and Characterization of Organic Thin Film Transistor based on Naphthalenetetracarboxylic Diimide Derivatives.
Sung Yang 1 , Xiaoliang Li 1 , Seon Ho Kim 1 , Tielong Gao 1
1 Chemistry, Kyung Hee Univ., Yongin Korea (the Republic of)
Show Abstract9:00 PM - H5.87
Laser Action in Half Lambda Thick Organic Semiconductor Microcavities.
Jonathan Tischler 1 , Elizabeth Young 2 , Vladimir Bulovic 1 , Daniel Nocera 2
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, 2 Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show Abstract9:00 PM - H5.88
High Efficiency White Organic Light-emitting Devices with Dual Emissive Layers.
Sang-Hyun Eom 1 , Ying Zheng 1 , Jiangeng Xue 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show Abstract9:00 PM - H5.89
Nano-Island Based Surface Enhanced Fluorescence of Conjugated Polymer Thin Films.
Michael Griffo 1 , Sue Carter 1
1 Physics, UC Santa Cruz, Santa Cruz, California, United States
Show AbstractPhotoluminescence (PL) of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) in the presence of surface plasmons is studied. A layered structure is constructed where the MEH-PPV and a silver nano-island layer is separated by a transparent and insulating Lithium Fluoride layer. While quenching is minimized by polymer-metal separation, resonance wavelengths of the surface plasmons are matched to the absorption peak of MEH-PPV and a correlation between the plasmon resonance scattering wavelength and photoluminescence enhancement is found with a maximum ratio of enhancement over 5, for wavelengths near the peak absorption of MEH. The PL spectral response is found to increase over a broad range within the visible spectrum. Effects from varying the nano-island particle size are investigated in addition to plasmon to polymer distance dependence.
9:00 PM - H5.90
Predictions of the Temperature Dependence of Hole Mobility in Organic Molecular Crystals.
Matthew Rossi 1 , Karl Sohlberg 1
1 Chemistry, Drexel University, Philadelphia, Pennsylvania, United States
Show AbstractOrganic molecular solids are receiving steadily increasing attention for potential use in data storage, optoelectronics and electronic devices. For these applications, specific materials are chosen based on desirable charge transfer properties, one especially useful metric of which is hole mobility, μ. A popular existing protocol, due to Deng and Goddard, utilizes Marcus-Hush theory to calculate hole mobility. We have extended the method to account for thermal effects, including thermal vibrations of the molecular monomers and thermal expansion of the material. We also derive an estimate of the magnitude of the influence of phonon modes. Our method does not assume an isotropic interaction potential between molecular monomers. We circumvent this approximation by applying Boltzmann statistics to a discretized representation of the potential energy surface felt by a representative molecular monomer to approximate the monomer distribution function. We have further considered an extension of our method to compute hole mobility for molecular thin-films. Using our method, predicted hole mobilities are in agreement with published experimental measurements and theoretically calculated values. Our results show that in general, thermal expansion of the crystal lattice has the largest overall impact on hole mobility, decreasing mobility with increasing temperature. We have constructed a characteristic curve that is independent of molecular species and use insight obtained from our calculations to demonstrate the conditions under which hole mobility is higher for thin-films than their molecular crystal counterparts. To serve as a screening tool for materials selection it is desirable to improve computational efficiency. We show that this can be achieved by employing a composite of DFT and AM1 electronic structure data, using DFT to obtain information about the representative molecular monomer and AM1 for dimers (monomer-monomer interactions). The hybrid method is very efficient and our calculated values of hole mobility still fall within the observed range of experimental values.
Symposium Organizers
Marc Baldo Massachusetts Institute of Technology
Antoine Kahn Princeton University
Paul W. M. Blom University of Groningen
Peter Peumans Stanford University
H6: Organic Photovoltaics I
Session Chairs
Thursday AM, December 04, 2008
Room 311 (Hynes)
9:30 AM - **H6.1
Device Design and Designer Polymers for Tandem Solar Cells.
Olle Inganas 1 2
1 Biomolecular and organic electronics, Linkoping University, Linköping Sweden, 2 Center of organic electronics (COE), Linköping University, Linköping Sweden
Show Abstract10:00 AM - H6.2
High-efficiency Organic Solar Cells for Low-illumination Applications.
Tayebeh Ameri 1 , Pavel Schilinsky 1 , Markus Biele 1 , Claudia Hoth 1 , Christoph Waldauf 1 , Hamed Azimi 1 , Patrick Denk 1 , Karen Forberich 1 , Markus Scharber 1 , Gilles Dennler 1 , Christoph Brabec 1
1 , Konarka, Linz Austria
Show AbstractIn order to enter the market for consumer electronics, organic photovoltaic devices have to provide reasonable efficiencies under indoor light conditions for which luminous flux and intensity can be up to 1000 times lower than natural sunlight. This brings the low light efficiency of photovoltaic devices into the spot of interest.This study shows how the performances under different lighting conditions can be predicted for photovoltaic systems from jV-data in the dark and under illumination. From this pair of data the internal loss mechanisms of the device can be estimated. Therefore, together with the spectrum of the provided light source, short circuit current, fill factor and open circuit voltage and hence device performance for any special light condition can be predicted. Comparison of measured and calculated data proofs the quality of this approach.The presented investigation enabled fast and easy optimisation of organic photovoltaic devices for low light purposes which resulted in similar or even higher efficiencies under indoor light conditions with respect to already commercial products.
10:15 AM - H6.3
Highly Efficient Inverted Polymer Solar Cell by Low Temperature of Cs2CO3 Interlayer.
Li-Min Chen 1 , Hua-Hsien Liao 2 , Zheng Xu 1 , Gang Li 3 , Yang Yang 1
1 Materials Science and Engineering, University of California, Los Angeles, Los Angeles, California, United States, 2 Electric Engineering, National Tsing Hua University , Hsinchu Taiwan, 3 , Solarmer Energy, Inc., El Monte, California, United States
Show AbstractWe demonstrate a highly efficient inverted bulk heterojunction polymer solar cell based on regioregular poly(3-hexylthiophene) [6,6] -phenyl C61 butyric acid methyl ester with a low temperature annealed interfacial buffer layer, cesium carbonate (Cs2CO3). This approach improves the power conversion efficiency of the inverted cell from 2.3% to 4.2%, with short-circuit current of 11.17 mA/cm2, open-circuit voltage of 0.59 V, and fill factor of 63% under AM1.5G 100 mW/cm2 irradiation. This result is the highest reported PCE for inverted structure polymer solar cell so far and is comparable to the previous regular structure device on the same system. Ultraviolet photoelectron spectroscopy (UPS) shows that the work function of annealed Cs2CO3 layer decreases from 3.45 to 3.06 eV. X-ray photoelectron spectroscopy (XPS) results reveal that Cs2CO3 can decompose into low work function, doped cesium oxide Cs2O upon annealing, which is accountable for the device efficiency improvement due to a lower interface resistance.
10:30 AM - H6.4
Bulk Heterojunction Solar Cells using High Mobility, Low Band Gap Donor Materials Processed with Solvent Additives.
Jeffrey Peet 1 , Nam Sung Cho 2 , Michelle Senatore 1 , Sang Kyu Lee 2 , Martin Drees 3 , Guillermo Bazan 1 2
1 Materials, University of California, Santa Barbara, California, United States, 2 Chemistry and Biochemistry, University of California, Santa Barbara, California, United States, 3 Luna Nanoworks, Luna Innovations, Danville, Virginia, United States
Show Abstract11:15 AM - **H6.5
Increased Exciton Diffusion Length in Organic Solar Cells via Sensitized Phosphorescence.
Barry Rand 1
1 , IMEC vzw, Leuven Belgium
Show AbstractRecently, the performance of organic solar cells has improved rapidly due to the development of new materials and a better control over film morphology. In order to continue this trend, new device architectures should also be considered. One issue requiring attention is that of the typically low exciton diffusion lengths (LD) of organic semiconductors, given that solar cell performance relies on the ability of excitons to diffuse to a donor-acceptor interface. Therefore, the major bottleneck for achieving high efficiency organic solar cells is in balancing the low LD with thick enough layers to absorb most of the incident light. In fact, the bulk heterojunction device concept, that of an interpenetrated donor-acceptor network, was created for exactly this reason. However, it is difficult to achieve a very fine phase separation on the order of a few LD, while also preserving the good charge transport required for low resistance devices. If LD can be increased, ultimate device efficiencies will also increase, and the strict requirements for phase separation of blends will be relaxed.One method that should be able to yield larger LD is to use molecules which have efficient intersystem crossing, in order to convert singlet excitons into longer lived triplet excitons. However, because phosphorescent organic semiconductors are typically characterized by low mobilities and absorption that do not overlap well with the solar spectrum, their direct use as active, absorbing layers may be difficult to realize for the best performing devices. Here, we demonstrate enhanced photocurrent from a fluorescent material via the process of sensitized phosphorescence, whereby a phosphorescent dye converts initially generated host singlet excitons into triplet excitons. By doing so, LD was more than doubled in a polyphenylene vinylene polymer host, from 4 to 9 nm, and the photocurrent originating from that layer was increased by 40%. In contrast, a similar dopant which lacks the ability to convert host singlet excitons into triplet excitons was shown to reduce the photocurrent from the host material.
11:45 AM - **H6.6
The Role of Ground and Excited Charge Transfer States in Polymer:Fullerene Solar Cells.
Rene Janssen 1 , Dirk Veldman 1 , Stefan Meskers 1
1 Molecular Materials and Nanosystems, Eindhoven University of Technology, Eindhoven Netherlands
Show Abstract12:15 PM - H6.7
Formation of a Charge Transfer State at the Heterojunction of Polymer/fullerene Solar Cells.
Markus Hallermann 1 , Stephan Haneder 1 , Enrico Da Como 1
1 Department of Physics and CeNS, Phog, Muenchen Germany
Show AbstractThe conjugated polymer:fullerene solar cell is one of the most promising candidates for the development of organic semiconductor based photovoltaics. In this donor:acceptor material combination the fundamental processes involved in the cell operation are exciton generation, electron-hole separation and free-charge carrier diffusion. Here we focus our attention on the charge separation process taking place at the heterojunction between the conjugated polymer poly(2-methoxy-5-(3,7-dimethyloctyloxy)-1,4-phenylene vinylene) (MDMO-PPV) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM). Because of the large electronegativity of the fullerene acceptor, the charge separation from the light absorbing polymer MDMO-PPV to PCBM is very efficient. However, the formation of intermediate states, where electron and holes are still electrostatically bound, is considered as one of the intrinsic limits for achieving higher efficiencies [1, 2]. Previous studies have excluded the presence of intermediate states originating from a ground-state electrostatic interaction, taking place at the heterojunction. In this communication we demonstrate the presence of a ground-state interaction at the MDMO-PPV:PCBM heterojunction. Evidence for such interaction is provided by photoluminescence (PL) spectroscopy, where a red-shifted emission (1.5 eV) is detected in the blend film, in addition to the MDMO-PPV excitonic emission at 2.22 eV. A comparison with the spectra of the pristine polymer or PCBM indicates that the emission at 1.5 eV originates exclusively from a charge transfer interaction between the two materials in the blend. By performing a PL excitation spectrum we observe the direct excitation of the charge transfer state below the MDMO-PPV absorption onset, providing evidence for an interaction in the ground state. By measuring the electric field induced quenching of the PL, we estimate the exciton binding energy for the electron hole pairs occupying the charge transfer state. The implications of our findings for polymer:fullerene solar cells are discussed. 1V. D. Mihailetchi, L. J. A. Koster, J. C. Hummelen, et al., Physical Review Letters 93, 216601 (2004).2J. G. Müller, J. M. Lupton, J. Feldmann, et al., Physical Review B 72, 195208 (2005).
12:30 PM - H6.8
Tandem Polymer Solar Cells Processed from Solution.
Jan Gilot 1 , Martijn Wienk 1 , Mathieu Turbiez 2 , René Janssen 1
1 , Eindhoven University of Technology, Eindhoven Netherlands, 2 , Ciba SC, Basel Switzerland
Show AbstractA transparent intermediate layer, positioned between the two active layers is required to construct an organic tandem solar cell. This layer should provide electrical contact between the two cells via efficient recombination of the holes and electrons created in the different sub-cells, without voltage loss. Here we present tandem solar cells incorporating polymer:fullerene bulk heterojunctions with complementary absorption spectra as active layers and solution processed electron and hole transport layers. The recombination layer between the active layers is deposited by spin coating ZnO nanoparticles from acetone, followed by spin coating neutral pH PEDOT from water, and short UV illumination of the completed device. The key advantage of this procedure is that each step does not affect the integrity of previously deposited layers. Previously we have shown that the open-circuit voltage (Voc) for double and triple junction solar cells built from well known polymer:fullerene combinations (PPV:PCBM, P3HT:PCBM) is close to the sum of the Voc’s of individual cells [1]. This simple procedure to create multi junction devices will enable increasing the overall efficiency, when active materials with different optical band gaps are used. By combining a green-absorbing active layer with a red-absorbing active layer, the complete visible spectrum is covered. On top of that the different band-gaps of the polymers used allow taking profit of the higher voltage obtainable from a wide band-gap polymer compared to a small band-gap polymer. For improving the efficiency of tandem solar cells, the current generated in the maximal power point in both active layers must be equal to minimise the losses originating from the excess of charges in one active layer. By calculation of the absorption in every single layer with optical modeling and thereby determining the current generation of each individual active layer, the optimal layer thicknesses of different layers become predictable. Hereby more efficient solution processed tandem solar cells, with active layers generating matching currents, become in reach.A promising output of 4.0 mW/cm2, under a simulated solar light illumination (~100 mW/cm2) with masking, was obtained using two complementary materials. The current (5.3 mA/cm2) was close to the calculated optimal value (5.8 mA/cm2) for the thicknesses used. The voltage was the sum of both single layer cells (1.62 V = 0.64 + 0.98 V). [1] J. Gilot, M.M. Wienk, R.A.J. Janssen, Appl. Phys. Lett. (2007) 90, 143512.
12:45 PM - H6.9
Optical and Electrical Design of Low Band Gap Bulk Hetero-junction Polymer:Fullerene Solar Cells.
Jan Kotlarski 1 , Paul Blom 1
1 Molecular Electronics, University of Groningen, Groningen Netherlands
Show AbstractOne of the main problems in polymer:fullerene bulk heterojunction (BHJ) solar cells is the poor overlap between the solar spectrum and the absorption of the materials used. In order to increase the photon harvesting smaller bandgap polymers are needed in order to absorb longer wavelengths. Since a BHJ solar cell consists of a thin film layer stack of different materials on a transparent substrate it has been recognized that optical interference effects play an important role in the performance of solar cells. An important question therefore is whether these interference effects still allow the efficient absorption of longer wavelengths, or that the layer stack has to be adjusted. Here we use a combined electrical and optical model to theoretically investigate the performance of low bandgap BHJ solar cells. As a reference solar cell poly(2-methoxy-5-(3‘,7‘-dimethyloctyloxy)-p-phenylene vinylene) (MDMO-PPV) as electron donor and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM) as electron acceptor in a 1:4 wt. ratio is used. Subsequently, by systematically red-shifting the optical parameters of MDMO-PPV the influence of the bandgap of the donor on the solar cell performance simulated. For thin solar cells with thicknesses smaller than 60 nm there is no performance improvement with decreasing bandgap. The increased overlap with the solar spectrum is canceled by interference effects. For larger thicknesses a clear improvement is observed: For a red-shift of 0.8 eV the efficiency of the solar cell is nearly doubled, the efficiency increases from 2.3 % for a 70 nm thick active layer to 4.0 % for a 100 nm thick active layer. For thicker solar cells an unbalanced charge transport leads to a buildup of space-charges, thereby reducing the fill factor (FF) [1]. For MDMO-PPV:PCBM cells, with the hole mobility one order of magnitude lower than the electron mobility, space-charge formation starts to limit the performance for thicknesses larger than 100 nm. For low bandgap cells, however, the charge carrier densities are higher due to the improved absorption. As a result these solar cells are even more sensitive to space-charge effects. For a one order difference in electron- and hole mobility the FF drops to about 0.48 in regular MDMO-PPV solar cells with 300 nm thickness, whereas for a low bandgap cell with equal mobilities it would drop to 0.38. As a result the balancing of charge transport is a key design aspect for low bandgap BHJ solar cells. [1] M. Lenes, L. J. A. Koster, V. D. Mihailetchi and P.W.M. Blom, Appl. Phys. Lett. 88, 243502 (2006).
H7: Organic Photovoltaics II
Session Chairs
Thursday PM, December 04, 2008
Room 311 (Hynes)
2:30 PM - **H7.1
Controling Morphology in Organic Bulk-Heterojunction Solar Cells.
Klaus Meerholz 1 , Adam Moule 1 2
1 Physical Chemistry, University of Cologne, Cologne Germany, 2 Chemical Engineering and Materials Science, UC Davies, Davies, California, United States
Show Abstract3:00 PM - H7.2
Towards Efficient Solar Cells Based on Ternary Polymer/fullerene Blends Nanostructured and Thermodynamically Stabilised with Block Copolymers.
Georges Hadziioannou 1 , Cyril Brochon 1 , Thomas Heiser 2 , Nicolas Leclerc 1 , Patrick Leveque 2 , Fanny Richard 1 , Rony Bechara 2
1 , University Louis Pasteur/CNRS, Strasbourg France, 2 , InESS/CNRS, Strasbourg France
Show AbstractSemiconducting conjugated polymers are promising materials for the development of low cost optoelectronic applications. Binary blends of semiconducting polymers with molecular materials such as fullerenes having electron donor and electron acceptor properties respectively, as well as bipolar charge transport properties, have shown promising results as photovoltaic materials. The control of the blend morphology in terms of nanostructuring and crystalline order as well as the blend thermodynamic stability influence to a great extend the power conversion efficiency and the long life-term performance of the photovoltaic devices. In order to produce efficient and thermodynamically stable active polymer photovoltaic materials rod-coil block copolymers (regioregular poly-3-hexylethiophene-b-poly(butylacrylate-stat-C60 methylstyrene)) with electron donor, electron acceptor blocks have been designed, synthesized and used as compatibilizers to the binary blends of regioregular poly-3-hexylthiophene(rr-P3HT)/[6,6]-phenyl-C61-butyric acid methyl ester (PCBM). It was found that a threshold macromolecular size and composition in the blend, as low as 2.5 weight percent, of the copolymer is enough to considerably modify the thin film morphology with a control of the nanostructures and crystalline order, giving rise to ribbon like micelles that form a bicontinuous microemulsion. The latter nanostructured morphology has a positive effect on the photovoltaic response that results in a more than two fold increase in the energy conversion efficiency, in comparison to block copolymer-free devices.
3:15 PM - H7.3
Plasmonic Nanocavity Arrays for Enhanced Efficiency in Organic Photovoltaic Cells.
Wade Luhman 1 , Nathan Lindquist 2 , Sang-Hyun Oh 2 , Russell Holmes 1
1 Chemical Engineering and Materials Science, University of Minnesota, Minneapolis, Minnesota, United States, 2 Electrical and Computer Engineering, University of Minnesota, Minneapolis, Minnesota, United States
Show AbstractThe performance of organic photovoltaic cells (OPVs) has increased significantly in recent years. The power conversion efficiency of these devices continues to be limited by low optical absorption efficiency. The absorption of light in an OPV leads to the creation of an exciton, which must be dissociated in order to generate a photocurrent. Since dissociation is realized only at a heterojunction between electron donating and accepting materials, the exciton diffusion length strongly impacts active layer thickness. In commonly used OPV materials, the exciton diffusion length (LD~10 nm) is much shorter than the optical absorption length (LA~100 nm). This tradeoff results in the use of very thin active layers which have limited absorption. Multiple device architectures have been demonstrated to overcome this bottleneck, including bulk and planar-mixed heterojunctions. The implementation of long diffusion length phosphorescent materials has also been demonstrated. This work describes an alternate approach to overcome the diffusion bottleneck that instead relies on a subwavelength plasmonic nanocavity array for optical energy confinement in the OPV active layers. The conventional indium tin oxide (ITO) anode is replaced with a patterned silver film consisting of slits having a period of 409±5 nm and a width of 120±15 nm. The use of a metallic anode leads to the excitation of surface plasmons at the metal-organic interface. The evanescent character of surface plasmons permits the trapping of light well below the diffraction limit, leading to increased absorption. Here, the integration of a plasmonic nanocavity array with a copper phthalocyanine (CuPc)/C60 OPV leads to a 3.2±0.4 fold increase in power conversion efficiency relative to the case of a device with unpatterned metallic contacts under simulated solar illumination.
3:30 PM - H7.4
Enhanced Efficiency of Polymer Solar Cells by Controlled Surface Plasmon of Gold Nanorods.
Ji Hwang Lee 1 , Jong Hwan Park 2 , Jong Soo Kim 1 , Dong Yun Lee 2 , Kilwon Cho 1 2
1 School of Environmental Science and Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of), 2 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractThursday, 12/4New PresenterH7.4 @ 2:30 PMEnhanced Efficiency of Polymer Solar Cells by Controlled Surface Plasmon of Gold Nanorods. Kilwon Cho
3:45 PM - H7.5
Thin Film Encapsulation of Small Molecule and Conjugated Polymer Based Organic Photovotaics with ALD and PECVD Processing.
Namsu Kim 1 , Willliam Potscavage 2 , Benoit Domercq 2 , Bernard Kippelen 2 , Bernard Kippelen 2 , Samuel Graham 1
1 Mechanical Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States, 2 Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States
Show AbstractContinued advancements of organic small molecule and conjugated polymer materials have led to organic solar cells that are thin, flexible, lightweight, and potentially low-cost energy conversion devices. While the main research focus has been towards the achievement of high-power conversion efficiency, which has recently reached 6.5% in tandem solar cells, the reliability of organic solar cells remains as one of the greatest challenges which must be addressed prior to widespread commercialization. It is well known that environmentally-induced degradation of the active materials and low work function electrodes remains a valid concern, thus bringing about much research on the development of thin film encapsulation layers to increase their lifetime. Many vacuum deposition processes have been developed to address the development of low permeation barrier coatings including both plasma enhanced chemical vapor deposition (PECVD) and atomic layer deposition (ALD). However, few studies have focused on the challenges of integration of such layers with actual devices and linking a known encapsulation barrier performance to an actual device lifetime.In this work, we will present results on the development of our thin film encapsulation layers produced by PECVD and ALD. Effective water vapor transmission rates (WVTR) were measured by calcium corrosion tests. Results show that permeation rates less than 5 × 10-6 g/m2/day and 5 × 10-5 g/m2/day could be achieved with SiNx/parylene and SiOx/parylene multilayer encapsulation films grown by PECVD at low temperatures (<110°C), respectively, and the permeation rate could be tailored by the number of layers. On the other hand, single layer ALD-deposited Al2O3 films produced at 110°C have shown WVTR values as low as 1.7x10-5 g/m2/day. Integration of these encapsulation layers were demonstrated by encapsulating pentacene/C60 solar cells and blend solar cells of poly 3-hexylthiophene and a soluble C60 derivative, [6,6]-phenyl C61 butyric acid methyl ester on glass substrates. Measurements of the power conversion efficiency (η), field factor (FF), and short-circuit current density (JSC) as a function of the time have been preceded. Lifetime results were compared between small-molecule based and conjugated-polymer based solar cells encapsulated with barriers of similar performance, which were stored and tested in an environmental chamber at 20 °C and 50% relative humidity [R.H.]. The optimized encapsulation structure for successful integration with solar cells will be discussed.
4:30 PM - H7.6
High Efficiency Organic Solar Concentrators.
Michael Currie 1 , Jonathan Mapel 1 , Timothy Heidel 1 , Shalom Goffri 1 , Marc Baldo 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractMono-crystalline III-V and silicon solar cells are efficient but often cost prohibitive without optical concentration. Organic solar concentrators (OSC) enable the cost effective utilization of these solar cells by achieving high optical concentration in a non-tracking format. The OSC is comprised of a thin film of luminescent chromophores that emit into an adjacent planar waveguide with light-collecting photovoltaic cells at its edges. OSC system performance is limited by the self-absorption of emitted light in the waveguide due to the spectral overlap of absorption and emission amongst chromophores. We have reduced self-absorption in OSCs by exploiting solid-state solvation, resonant energy transfer, dipole alignment and phosphorescence. We demonstrate solid state solvation by preparing OSCs comprised of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran(10), DCJTB, as a guest molecule within a tris(8-hydroxyquinoline) aluminum, AlQ3, host. The highly polar excited state of DCJTB is stabilized by its local environment resulting in bathochromically-shifted emission and weaker spectral overlap. Resonant energy transfer is used to minimize the spectral overlap by reducing the molecular concentration of the final (guest) dye in an OSC. We prepare OSCs comprised of AlQ3 and rubrene hosts along with a DCJTB guest where energy is optimally transferred from high density hosts to the low density guests. Finally, we prepare OSCs comprised of the phosphorescent system platinum tetraphenyltetrabenzoporphyrin, Pt(TPBP). The heavy metal platinum atom at the core of the molecule promotes intersystem crossing of the singlet excited state to a lower-energy triplet excited state. The shift associated with emission from this triplet excited state lowers self-absorption dramatically. Through judicious and optimal use of these molecular properties and interactions, we report optical quantum efficiencies exceeding 50% and projected power conversion efficiencies of 6.8% when coupled to III-V solar cells. The vast reduction in self-absorption allows optical concentrations that lower the cost per watt of solar cells by a factor of ten.References:High-efficiency Organic Solar Concentrators for Photovoltaics, M.J. Currie,J.K. Mapel, T.D. Heidel, S. Goffri and M.A. Baldo. Science. In press.
4:45 PM - H7.7
Luminescent Solar Concentrators Employing Phycobilisomes.
Carlijn Mulder 1 , L. Theogarajan 1 , M. Currie 1 , J. Mapel 1 , M. Baldo 1 , M. Vaughn 2 , A. Godman 3 , P. Willard 2 , B. Bruce 2 3 4 , M. Moss 5 , C. McLain 5 , J. Morseman 5
1 Department of Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States, 2 Department of Biochemistry, Cellular and Molecular Biology, University of Tennessee at Knoxville, Knoxville, Tennessee, United States, 3 Department of Microbiology, University of Tennessee at Knoxville, Knoxville, Tennessee, United States, 4 Sustainable Energy & Education Research Center, University of Tennessee at Knoxville, Knoxville, Tennessee, United States, 5 , Columbia Biosciences Corporation, Columbia, Maryland, United States
Show AbstractAt current manufacturing growth rates, it is expected to take at least 20 years to produce enough Si-based solar cells to make a significant impact on the world energy supply. It may be possible, however, to alleviate production constraints by employing optical concentrators to collect light over larger areas and focus it on solar cells. Luminescent solar concentrators (LSCs) are especially promising because they do not need to track the sun to obtain high optical concentration factors. However, loss mechanisms that are associated with optical self-absorption decrease the efficiency of LSCs at increasing values of the geometric gain, G, which is defined as the ratio of the facial area to the edge area. In this work, we demonstrate LSCs based on phycobilisomes – water-soluble photosynthetic antenna complexes that collect light in red algae and cyanobacteria. The phycobilisomes are cast in a solid state matrix that preserves their internal energy transfer pathways and hence their optical properties. The unique nanostructure of the phycobilisomes facilitates cascading Forster energy transfer from multiple short wavelength chromophores to a handful of long wavelength emissive chromophores. By comparing the performance of structurally complete phycobilisomes with partially coupled and fully decoupled complexes, we establish that energy transfer within the intact complexes reduces self absorption losses in LSCs. The efficiency of the fully coupled complex decreases by 18% as G increases from 1.4 to 9.3. In comparison, the efficiencies of the partially coupled and decoupled complexes decreased by 33% and 41%, respectively. These results demonstrate that phycobilisomes are the model dye system for a new generation of cast solar concentrators with improved efficiency at high optical concentrations.
5:00 PM - H7.8
Interdigitated Organic Photodetectors.
Brian Crone 1 , Alyson Niemeyer 1 2 , Ian Campbell 1 , Kevin See 3 , Byung Jung 3 , Howard Katz 3 , Franky So 2
1 , Los Alamos National Laboratory, Los Alamos, New Mexico, United States, 2 Department of Materials Science and Engineering, University of Florida, Gainesville, Florida, United States, 3 Department of Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States
Show AbstractOrganic semiconductors are promising materials for photovoltaics and photodetectors. There is currently a great deal of research aimed at improving the efficiency of these devices. In this work, we investigate organic photodetectors in which the organic semiconductors are deposited on a substrate pre-patterned with interdigitated electrodes. The organic layer can be spun cast from a solution to form bulk polymer heterojunctions (such as MEH-PPV:PCBM), or evaporated to form electron transporting:hole transporting multilayer devices. In ideal multilayer devices, photons are absorbed to form excitons which diffuse vertically to a heterojunction, and the charges are then transported laterally to the electrodes. We consider NTCDI derivatives synthesized by H.E. Katz, NTCDI, PTCDI and C60 as electron transporting materials, and pentacene and other acenes as hole transporting materials. Organic materials typically have large exciton binding energies (> 0.5 eV), so that dissociation only occurs at heterojunctions, contacts, or extrinsic defects. By varying the number of layers, layer thickness, and choice and deposition conditions of evaporated materials, we are able to study exciton dissociation and charge collection in these devices. By varying the layer thicknesses we determine the exciton diffusion lengths in these materials. By varying the materials used in the devices we determine the importance of energy offsets at the heterojuctions in exciton dissociation. Carrier mobility is determined by material choice and deposition conditions, and affects separation of charges after excitons dissociate. Carrier mobility is assessed by transient photocurrent measurements. Charge trapping in the devices can lead to reduction of photocurrent. Photocurrent scaling with interdigitated finger spacing and applied field is used to study charge carrier trapping. We have demonstrated quantum efficiencies of 80% in bulk heterojunction polymer devices, and over 10% in single heterojunction bilayer NTCDI derivative: Pentacene devices. We will discuss the influence of heterojunction energy offsets, exciton diffusion length, carrier mobility, and charge trapping on the performance of interdigitated organic photodetectors.
5:15 PM - H7.9
Microcavity Tunable Organic Photodetector.
Kwang Hyup An 1 , Brendan O'Connor 1 , Kevin Pipe 1 , Max Shtein 2
1 Mechanical Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractOrganic materials are promising for optoelectronic applications for reasons that include strong absorption that can be spectrally tuned through chemistry. Additionally, because organic devices are very thin (less than the wavelength of light), optical microcavity effects (e.g. Fabry-Perot resonances) can be engineered with significant influence on the optical response of the device. For example, microcavity tuning of organic light-emitting diodes has been demonstrated extensively. However, relatively little research1 has examined microcavity tuning of organic photodetectors (OPDs). Part of the challenge is due to a dearth of candidate materials, which should have the properties of low optical absorption, high electrical conductivity, and appropriate HOMO and LUMO energy levels.In this study, we use two vacuum-deposited optical spacer materials – MoO3 and Ruthenium(III) acetylacetonate (Ru(acac)3) – to demonstrate a wide range of tunability of the quantum efficiency spectrum of an OPD across the visible spectrum. Using designs that incorporate either one or both optical spacer layers, we position the active region of the device in a precise location with respect to a microcavity resonant mode. The multi-layer OPD is composed of two thick metal mirrors that function as anode and cathode, sandwiching conductive spacer(s), active organic layers, and an exciton blocking layer. The measured external quantum efficiency (EQE) spectrum exhibits a peak with a full width half maximum (FWHM) of 60 nm and a center wavelength that can be tuned by varying the optical spacer thickness. The EQE spectrum matches well with calculations based on transfer matrix optical modeling combined with exciton diffusion; the peak originates from the second Fabry-Perot resonance mode of the cavity. The effects of interface roughness on FWHM are included in optical modeling, using topographical data taken by atomic force microscopy. Potential applications of tunable OPDs in high spatial resolution fluorescence detection using electrically pumped scanning probes will be further discussed.[1] J. Lupton et al., Adv. Mat. 15, 1471 (2003).
5:30 PM - H7.10
Tunneling Organic Photodiodes Exhibiting Large Photomultiplication under Low Bias.
William Hammond 1 , Jiangeng Xue 1
1 Materials Science & Engineering, University of Florida, Gainesville, Florida, United States
Show AbstractSemiconductor devices that exhibit photomultiplication are essential for weak signal detection. Avalanche photodiodes are functional in this regard, yet large area devices are difficult and costly to fabricate. For low cost and large area applications, organic semiconductor devices exhibiting photomultiplication could prove ideal. The energetic disorder of molecular materials prevents these materials from exhibiting impact ionization; yet, interestingly, this same disorder has been shown to give rise to an alternate form of photomultiplication, wherein localized fields associated with trapped carriers modulate tunneling through an injection barrier (Reynaert et al., Adv. Funct. Mater. 16, 784-790). Previous studies have demonstrated devices that rely on photogenerated carriers trapped in the bulk of active layers to perform this function. Here, we introduce a new device structure that uses an insulating layer coupled with a hole-blocking layer (HBL) to selectively trap photogenerated holes and multiply electron injection. This structure allows more precise control and optimization of the photomultiplication effect and device performance. It furthermore allows the use of a variety of absorbing organic semiconductor layers, without regard to their degree of energetic disorder. Various device structures were tested, using either Al2O3 or tetratetracontane (TTC) as insulating layer and 4,4′-bis(9-carbazolyl)-2,2′-biphenyl (CBP) or 1,4,5,8-naphthalene-tetracaboxylic-dianhydride (NTCDA) as HBL. Active absorbing layers tested include C60, copper phthalocyanine (CuPc), perylene-3,4,9,10-tetracarboxylic-dianhydride (PTCDA), and 3,4,9,10 perylenetetracarboxylic bisbenzimidazole (PTCBI). It was found that photomultiplication is enhanced when the injecting electrode work function and the lowest unoccupied molecular orbital LUMO energies of both the HBL and adjacent absorbing semiconductor layer are similar. For example, a device with electrons injected from an aluminum electrode through a 3 nm TTC layer into a 4 nm NTCDA HBL and 40 nm absorbing C60 layer exhibits photomultiplication of 3000% under 0.2 mW/cm2 450 nm monochromatic illumination and 2 V applied bias.
5:45 PM - H7.11
Lateral J-aggregate Hybrid Photoconductor.
Yasuhiro Shirasaki 1 , John Ho 1 , M. Bradley 1 , Jonathan Tischler 1 , Gleb Akselrod 1 , Vladimir Bulovic 1
1 , Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractH8: Poster Session
Session Chairs
Friday AM, December 05, 2008
Exhibition Hall D (Hynes)
9:00 PM - H8.10
Improved Contact Performance in Organic Photovoltaic Devices Based on Surface-modified Metal Anode.
Chi-Feng Lin 1 2 3 , Wei-Feng Hsu 1 , Shun-Wei Liu 1 , Juin-Haw Lee 1 , Kuei-Hsien Chen 2 3 , Li-Chyong Chen 3 , Juen-Kai Wang 2 3
1 Graduate Institute of Photonics and Optoelectronics and Department of Electrical Engineering, National Taiwan University , Taipei Taiwan, 2 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan, 3 Center for Condensed Matter Science, National Taiwan University, Taipei Taiwan
Show AbstractIn this report, we present a copper phthalocyanine (CuPc) (20 nm)/fullerene (C60) (40 nm)/bathocuproine (BCP) (7 nm) hetero-junction photovoltaic device wherein a thick layer of Ag acts as the anode and the optical radiation is incident from the 15-nm half-transparent Ag cathode. To remedy the possible decrease in the open-circuit voltage, an additional 4-fluorobenzylmercaptan self-assembled monolayer (SAM) was introduced between the Ag anode and the CuPc layer. The SAM decreases the work function of Ag from 4.3 eV to 5.1 eV, which was determined by a photoelectron spectrometer at atmospheric pressure (AC-2, RKI Instruments). Under the illumination of an AM 1.5G 1-sun solar simulator, the device exhibits a decrease of series resistance by nine times and an increase of shunt resistance by one fold, leading to a 20% enhancement in fill factor. The measured short-circuit current density is lowered by 30%, compared with that of the previously reported device which is composed of a PEDOT:PSS-coated ITO electrode, while the open-circuit voltage remains approximately. The decreased short-circuit current density may be due to the un-optimized thickness of the semi-transparent metal cathode. The greatly improved series resistance is also reflected by the high injection current density (300 mA/cm2 at a bias of 1 V) that is about 7.5 times of that of the conventional structure. This can be attributed to the greatly improved conductivity of the metallic anode. Beside the improvement in the electrical properties, the optical cavity effect of this metal/organic/metal structure is also evident from its absorption spectrum and wavelength dependence of external quantum efficiency, providing extra benefit to control light field distribution in the device. The utilization of a SAM-modified metal anode in organic solar cells thus offers another design approach to improve the electrical contact performance in the anode side that is commonly compromised.
9:00 PM - H8.11
Fluoropolymer Anode Buffer Layers in Organic Photovoltaics.
Li Wei Tan 1 , Bonan Kang 2 , Ravi Silva 1
1 Faculty of Engineering and Physical Science, Advanced Technology Institute, Guildford, Surrey, United Kingdom, 2 Electronics Engineering Department, Jilin University, Changchun China
Show AbstractWe investigate the effects of polytetrafluoroethylene (PTFE) on device performance in poly(3-hexylthiophene):phenyl-C61¬-butyric acid methyl ester (P3HT:PCBM) based bulk-heterojunction organic photovoltaic (OPV) devices. OPV with thermally evaporated PTFE films as buffer layers deposited in-between the indium-tin oxide (ITO) anode and the active organic layer shows significantly improved open circuit voltage (Voc), short circuit current (Isc), fill factor(FF) and the expected commensurate increase in power conversion efficiency (PCE) when compared to a device with only poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS). The effect of the PTFE thickness towards the device performance is also investigated. The enhancement of the overall performance of the OPV is attributed to the formation of an artificial dipolar layer resulting from a layer of rich, negatively charged fluorine that helps to enhance the hole tunnelling injection. Compared to OPVs without buffer layer, the current device obtained with the ITO/PTFE anode shows that the Voc increases from 0.44 to 0.54 V while the Isc increases from 4.5 to 7.2 mA/cm2, and finally the PCE has improved from 1.2 to 2.4 %, without any post-annealing processes. This result shows that PTFE has great potential for a low cost and environmentally stable ITO buffer layer for use in organic electronic device.
9:00 PM - H8.13
Polymer Infrared Photo-detector and Proximity Sensor.
Hsin-Fei Meng 1 , Shin-Rong Tseng 1 , En-Chen Chen 2
1 Institute of Physics, National Chiao Tung University, Hsinchu Taiwan, 2 Department of Electrical Engineering , National Tsing Hua University, Hsinchu Taiwan
Show AbstractWe study the near infrared photocurrent response in solution-processed bulk heterojunction of poly(3-hexylthiophene) (P3HT) donor and (6,6)-phenyl-C61-butyric acid methyl ester (PCBM) acceptor. While the exciton absorption exists only for wavelength below 650 nm, direct generation of charge-transfer exciton extends the absorption wavelength to 950 nm. For films with micrometer thickness the photon-to-electron conversion efficiency is about 60 % at 750 nm wavelength under reverse voltage bias. Photo-detector with high sensitivity covering exclusively the 650 nm to 1000 nm near infrared region can therefore be made without a low bandgap material. The charge-transfer exciton absorption coefficient and photocurrent sensitivity depends on the annealing condition which controls the donor-acceptor morphology. A polymer proximity sensor is made based on such P3HT-PCBM infrared photo-detector in combination with an infrared polymer light emitting diode. The detection distance is up to 19 cm for objects with various colors and roughness under ambient indoor lighting. Such proximity sensor array can be used for robot skin and touch panel.
9:00 PM - H8.14
Influence of the Alkyl Side Chain Length on the Photophysics of poly(3-alkylthiophene) Polymer Blend Photovoltaic Devices.
Bettina Friedel 1 , Christopher McNeill 1 , Neil Greenham 1
1 Physics, University of Cambridge, Cambridge United Kingdom
Show AbstractRegioregular poly(3-alkylthiophenes) (P3ATs) are semicrystalline polymers which are commonly used in several types of optoelectronic devices, especially because of their good transport properties, easy processability and their tendency to form ordered structures. The effects of alkyl chain length and processing conditions on crystallinity, charge mobility and energy levels are complex, and not yet fully understood, particularly in blends with other materials. Here we use polythiophenes with butyl (P3BT), hexyl (P3HT), octyl (P3OT) and decyl (P3DT) side chains as the donor material in photovoltaic devices with the polyfluorene copolymer poly(9,9-dioctyl-fluorene-co-4,7-di-thiophen-2-yl-benzothiadiazole) (F8TBT) as the electron acceptor. We find that good device performance can be obtained from both P3HT:F8TBT and P3OT:F8TBT devices. Annealing is required in order to improve the crystallinity of the P3AT component, and we study the effect of annealing on exciton dissociation and charge transport properties. P3OT has a lower glass transition temperature than P3HT, and we find that annealing the P3OT blends as high as the P3HT blends improves the fill factor, compared to lower annealing temperatures, but decreases the short-circuit current, which we attribute to a drop in exciton dissociation efficiency due to a more evolved phase separation.
9:00 PM - H8.15
Carrier Mobility Dynamics in a Conjugated Polymer.
Dirk Hertel 1 , Almis Serbenta 2 , Andrius Devizis 2 , Klaus Meerholz 1 , Vidmantas Gulbinas 2
1 Chemistry, University of Cologne, Cologne Germany, 2 , Institue of Physics, Vilnius Lithuania
Show Abstract9:00 PM - H8.16
Mechanism of Exciton Dissociation in OrganometallicPhosphorescent Emitters.
Jakub Mezyk 1 , Francesco Meinardi 1 , Massimo Cocchi 2 , Riccardo Tubino 1
1 CNR-INFM and Department of Material Sciences, University of Milano-Bicocca, Milano Italy, 2 Institute of Organic Synthesis and Photoreactivity, National Research Council of Italy, Bologna Italy
Show AbstractPhosphorescent organic complexes find applications as emitters for highly efficient organic light emitting devices (OLEDs) [1]. These devices operate with maximal quantum yields at relatively low electric fields. At large driving current densities (high electric fields), however, the electroluminescence quantum efficiency is strongly reduced. The roll off is thought to be a result of at least three processes quenching the excited states: (i) exciton-exciton annihilation [2], (ii) exciton-charge carrier annihilation [3,4], and (iii) electric field-enhanced dissociation of excited states [3-5].In this communication we investigate the influence of electric field on excited states in organometallic phosphorescent emitters by measuring the electric field-dependent time-resolved photoluminescence (PL) for thin vacuum-evaporated films of a model compound - fac tris(2-phenylpyridine) iridium (III) [Ir(ppy)3]. The results prove that excitons in this material are effectively quenched due to their dissociation in external electric fields above 1 MV/cm. By resolving temporally the electric field-induced changes in the evolution of the concentration of excited states, we have shown that the dissociation occurs from higher lying spin-mixed states (|SMn*>) before their relaxation to the lowest emissive levels (|SM1>). The lifetime of the later is not significantly affected by external electric field. The vibronically hot |SMn*> sates dissociate into correlated electron-hole pairs (e-h), where charge carriers are separated by the distance of about 1.4 nm and bounded by coulombic interaction. The (e-h) pairs recombine geminately regenerating the emissive states or dissociate into free and/or trapped charge carriers. The probability of final dissociation is effectively modified by the external electric field and the PL quenching of the order of 30 % is observed. Knowledge about the mechanism of exciton dissociation in this class of materials is relevant for theoretical simulations of exciton kinetics in phosphorescent OLEDs as well as for optimization of the performances of these devices. In particular, in the host-guest electrophosphorescent systems, where the dopant molecules are excited by the triplet energy transfer, the dissociation effect can be reduced by a suitable choice of the matrix with the triplet level matching well the lowest emissive state of the phosphorescent complex.[1] M.A. Baldo, D.F. O’Brien, Y. You, A. Shoustikov, S. Silbey, M.E. Thompson, and S.R. Forrest, Nature 395, 151 (1998), [2] M.A. Baldo, C. Adachi, and S.R. Forrest, Phys. Rev. B 62, 10967 (2000), [3] J. Kalinowski, J. Mezyk, F. Meinardi, R. Tubino, M. Cocchi, D. Virgili, Chem. Phys. Lett. 453, 82 (2008), [4] J. Kalinowski, J. Mezyk, F. Meinardi, R. Tubino, M. Cocchi, D. Virgili, J. Chem. Phys. 128, 124712 (2008), [5] J. Kalinowski, W. Stampor, J. Mezyk, M. Cocchi, D. Virgili, and P. Di Marco, Phys. Rev. B 66, 235321 (2002).
9:00 PM - H8.17
Characterization of Organic/organic/substrate Interfaces of Thin p-n Heterostructures.
Ewelina Andrzejewska 1 , Jesus Alvarez 1 , Maria Jose Capitan 1 2 , Rodolfo Miranda 1 3
1 , Universidad Autónoma de Madrid, Madrid Spain, 2 , Consejo Superior de Investigaciones Científicas, Madrid Spain, 3 , Instituto Madrileño de Estudios Avanzados - Nanociencia, Madrid Spain
Show Abstract9:00 PM - H8.19
Organic Photovoltaic Cell Employing Continuously-tuned Sub-cells in Lateral-tandem Configuration.
Changsoon Kim 1 , Jungsang Kim 1
1 Electrical and Computer Engineering, Duke University, Durham, North Carolina, United States
Show AbstractWe report a systematic solution to the efficiency limitation of organic PV cells by employing continuously-tuned, series-connected sub-cells arranged in lateral-tandem configuration. The proposed system, which we refer to as the lateral tandem cell, consists of a one-dimensional periodic array of dispersive-focusing elements and resonant sub-cells. The dispersive-focusing element spectrally re-distributes the incoming solar photons to establish a wavelength distribution [λ(x)] on the sub-cells, where x is the position coordinate along the sub-cells. The device geometry of the sub-cells is continuously varied along the x-direction to allow photons incident on x to resonantly excite the cell at that location, thereby optimizing the external quantum efficiency over a broad spectrum. The donor-acceptor pair and charge transport layers of each sub-cell are chosen so that the open-circuit voltage of each sub-cell is maximized. The lateral tandem configuration allows the short-circuit currents of the sub-cells to be balanced by varying the widths of the sub-cells, providing a straightforward platform for the cell design. We perform electromagnetic and exciton diffusion analyses of a planar microcavity device to be employed in a lateral tandem cell, consisting of substrate / 150 nm Ag / organic multilayer / 10 nm Ag. The organic multilayer consists of a 10-nm-thick absorption layer located between two transport layers with equal thickness. The optical resonance is tuned by varying the thickness of the transport layers. Assuming that the optical absorption and exciton diffusion lengths in the absorption layer are 100 nm and 20 nm, respectively, we show that the external quantum efficiency larger than 70 % can be obtained over a wide spectral region (490 nm to 1000 nm). Based on the results, we estimate the power-conversion efficiency of the lateral tandem cell employing the microcavity sub-cells, assuming that an ideal dispersive-focusing element is available. Our estimation shows that a power-conversion efficiency approaching 20 % can be obtained. We also discuss the performance of a lateral tandem cell employing surface-plasmon-polariton-enhanced sub-cells [1].[1] C. Kim, J.-Y. Lee, P. Peumans, and J. Kim, Proceedings of NSTI-Nanotech 2008, 1, p533 (2008).
9:00 PM - H8.2
Force-field with Charge Transfer and Classical Molecular Dynamics Study for Metal-/Metal Oxide/Polyimide Interfaces.
Yoko Saito 1 , Mariko Miyazaki 1 , Tomio Iwasaki 1 , Naoya Sasaki 1 , Hong Mei Jin 2 , Michael Sullivan 2 , Ping Wu 2
1 Mechanical Engineering Research Laboratory, Hitachi, Ltd., Hitachinaka, Ibaraki, Japan, 2 Computational Materials Science and Engineering, Institute of High Performance Computing, The Capricorn Singapore
Show AbstractIn the electronic devices, good adhesion of polymers with metals is an important subject. To carry out classical molecular dynamics (MD) simulation for this system, we need the force-field that can well describe the interfacial properties between them. However, the appropriate force-field for polymer/metal interface has not been well known, because charge transfer could happen at the interface. In this study, we have developed the methodology of deriving force-field and carried out classical MD simulation. The systems including polyimide (PI) and different substrate (Cu, Ni and CuO) were used. New force-field was optimized to fit to the ab initio calculation for each system, which has a PMDA-ODA monomer unit and periodically repeated slabs. Proposed force-field is taking into account the effect of charge transfer by including electron negativity and atomic hardness. We found that obtained force-field can well represent the property of benzene rings, which attract electrons from metal surface. By using optimized force-field, the adhesion energy per unit area of PI thin film to metal surface was calculated. The adhesion energy of PI/Ni was larger than that of PI/Cu, because of charge transfer effect. On the other hand, the adhesion energy of PI/CuO was larger than that of PI/Cu, because the polarization of CuO is larger. These results are consistent with ab initio simulation. From these calculations, it is said that proposed force-field can well describe the interfacial property for polymer/metal system and make it possible to perform large scale MD simulation, considering charge transfer.
9:00 PM - H8.20
Photophysical and Charge Transfer Properties of Phenyl Cored Thiophene Dendrimers.
Muhammet Kose 1 , Kwiseon Kim 1 , Peter Graf 1 , Nikos Kopidakis 1 , Sean Shaheen 2 , Garry Rumbles 1
1 , NREL, Golden, Colorado, United States, 2 , University of Denver, Denver, Colorado, United States
Show AbstractWe studied a group of pi-conjugated dendrimers, which are soluble organic molecules consisting of a core group to which branched arms (dendrons) are attached. These dendrimers have a phenyl core with 3 or 4 arms, i.e., 3 or 4 thiophene dendrons. To gain detailed understanding of the electronic properties of these materials, we used several ab initio and semiempirical methods to predict the optical properties, charge transport, and degree of excitonic localization in these relatively large molecules [1]. The theoretical findings on the optical properties agree well with the experiment. We studied the size and the location of excitons in these dendrimers obtained through 3D transition density plots (TDs) and correlated electron-hole pair diagrams (CEHPDs). The relative positions of arms in the dendrimers determine the nature of excitonic behavior; in particular, meta substitutions inhibit interaction of arms between each other. Steric hindrance also causes excitonic localization due to weak electronic coupling between nonplanar units. Our results suggest better delocalization of hot excitons in the dendrimers with four arms as compared to the dendrimers with three. Delocalized excitons are needed for efficient electronic coupling between the molecules in charge-transfer reactions. Substitution schemes and size choices of the dendrimer can further improve the charge mobility and device efficiencies as well as reducing the band gap. We also present the results of charge transfer calculations on a large ensemble of bimolecular complexes at various intermolecular separation and orientations. A Monte Carlo method is exploited to simulate the carrier mobilities by using the charge hopping rates as defined in semi-classical Marcus theory. Overall, we obtain good agreement between the experiment and the theory [1] Kose, Mitchell, Kopidakis, Chang, Shaheen, Kim, and Rumbles, J. Am. Chem. Soc. 129, 14257 (2007).
9:00 PM - H8.22
Harvesting Triplet Excitons for Application in Polymer Solar Cells.
M. Arif 1 , K. Yang 1 , M. Foerster 2 , U. Scherf 2 , Suchi Guha 1
1 Physics, University of Missouri, Columbia, Missouri, United States, 2 Makromolekulare Chemie , Bergische Universität , Wuppertal Germany
Show Abstract9:00 PM - H8.23
Functionalized Zinc Oxide for Improved Organic Photovoltaic Systems.
Darick Baker 1 , C. Allen 1 , J. Albin 1 , H. Oertli 1 , M. Bergren 1 , T. Berman 1 , T. Furtak 1 , R. Collins 1 , D. Olson 2 , M. White 2 , D. Ginley 2 , C. Ladam 3 , C. Weigand 3 , K. Valset 3 , J. Grepstad 3 , H. Weman 3
1 Physics, Colorado School of Mines, Golden, Colorado, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , Norwegian University of Science and Technology/SINTEF, Trondheim Norway
Show AbstractRecent studies of polymer solar cells using ZnO nanofiber electron acceptors have underscored the importance of controlling electronic and structural properties at the ZnO/polymer interface. Formation of molecular layers on the ZnO surface may allow this manner of control. We have explored a siloxane attachment strategy for the functionalization of ZnO surfaces. This approach uses the family of triethoxysilane-based molecules in conjunction with an amine catalyst to ensure covalent attachment to the ZnO surface without bulk polymerization in solution as often occurs with more reactive silanes. By changing the end group of the triethoxysilane molecule, the nature of the functionalized ZnO surface can be modified and controlled. Using octadecyltriethoxysilane (OTES) and decyltriethoxysilane (DTES), infrared, XPS, and water contact angle studies confirm formation of an alkyl-terminated surface. Contact angle and XPS studies of phenyltriethoxysilane (PTES) treated ZnO films show a phenyl-terminated molecular layer. Surface coverage, however, is often incomplete with the triethoxysilane chemistry, and we discuss AFM and infrared results in terms of island formation and growth of the molecular layer on the ZnO surface. The surface conformation of siloxane molecular layers is also discussed and compared to layers coupled to ZnO utilizing other attachment schemes. When OTES and DTES treated ZnO films are spin-coated with poly(3-hexylthiophene) (P3HT), visible wavelength absorption studies confirm improved ordering of the P3HT at the ZnO surface as compared to control samples. Bilayer organic PV devices show, however, reduced short circuit current when the molecular layers are present. Our investigation probes whether the alkyl end group or siloxane attachment is responsible for limiting charge separation and transfer at the interface. Recent results with thiol attached alkyl layers that exhibit increased current suggest it may be the latter. PTES treatment of ZnO improves the wetting of P3HT and the intercalation of the polymer into ZnO nanowire arrays. This is potentially important for improved performance in hybrid polymer/ZnO nanofiber array PV devices. Additional work is progressing with alternative end groups and attachment schemes, as well as with molecular layers that may result in more favorable energy level alignment between polymer LUMO and ZnO conduction band levels. This work was supported by the National Science Foundation under Grant No. DMR-0606054.
9:00 PM - H8.24
Charge Carrier Injection Across Organic Heterojunctions: Impact of Interface Energy Alignment and Deep Trap States.
Sai Wing Tsang 1 , Mike. W. Denhoff 2 , Ye Tao 2 , Zheng Hong Lu 1
1 Material Science and Engineering, University of Toronto, Toronto , Ontario, Canada, 2 Institute for Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada
Show Abstract9:00 PM - H8.25
Managing Device Architectures for High Efficiency Organic Photovoltaic Solar Cells.
Jong Soo Kim 1 , Ji Hwang Lee 1 , Jong Hwan Park 1 , Kilwon Cho 1
1 Chemical Engineering, Pohang University of Science and Technology, Pohang Korea (the Republic of)
Show AbstractRecently there have been a few challenging reports on photovoltaic devices structurally designed using nanorod, nanowires and ordered bulkheterojunction structures. However, it is still difficult to design and optimize the device structures. In particular, multilayered photovoltaic devices with polymeric materials have received only limited attention to date because it is not easy to control the layer thickness on a scale of nanometers and to find selective solvents multilayer deposition, mainly because the undercoated layer is damaged during the following overcoating. In this presentation, we choose selective solvent of poly(3-hexylthiophene) (P3HT) / PCBM, and can diversify the multilayered device structures by using solution process. Especially, in this process using selective solvent, P3HT nanofibrillar network structures which can enhance the charge transport and connectivity were obtained from solution recrystallization. With P3HT nanofibrils, we fabricated the multilayered cells such as the device which has a donor/acceptor composition gradient all through the film. Furthermore, by using inverted solar cell geometry with P3HT nanowires, we could obtain the enhanced solar cell performance. Acknowledgement. This work was supported by a grant from the ERC Program of MOST/KOSEF (R11-2003-006-06004-0) and RTI04-01-04 from the Regional Technology Innovation Program of the MOCIE of Korea.
9:00 PM - H8.26
First-principles Study of Charge Transport through Fullerene-metal Interfaces for Nanodevice Applications.
Yong-Hoon Kim 1 , Gain Lee 2 , Jeung Ku Kang 2
1 , University of Seoul, Seoul Korea (the Republic of), 2 , KAIST, Daejeon Korea (the Republic of)
Show AbstractWe employ a first-principles computational approach [1] to explore the potential of polymerized one-dimensional [60]fullerene chains as the channel material for improved nanoelectronics applications [2]. Coherent electron transmissions of the fullerene wires obtained from [2+2] cycloaddition are calculated at different numbers of fullerene units, electrode materials, and contact configurations. We find that metal-induced gap states are localized within the first side fullerenes in contact with the electrodes, so conclude that polymerized fullerene wires including more than three units should show a robust device characteristic irrespective of the type of electrode metals and contact configurations. We also explore the possibility of utilizing fullerene polymerization in the contact engineering for the carbon nanotube-based device applications.Acknowledgments: This work was supported by the Korea Research Foundation Grant No. KRF-2007-331-C00077.References: [1] Y.-H. Kim, J. Tahir-Kheli, P. A. Schultz, and W. A. Goddard III, "First-principles approach for the charge transport characteristics of monolayer molecular electronic devices: Application to hexanedithiolate devices", Phys. Rev. B 73, 235419 (2006)[2] G. I. Lee, J. K. Kang, and Y.-H. Kim, "Metal-independent coherent electron tunneling through polymerized fullerene chains", J. Phys. Chem. C 112, 7029 (2008).
9:00 PM - H8.27
Development of Inverse Photoemission Spectrometer for Organic Materials.
Kiichirou Koyasu 1 , Toshio Nishi 1 , Tatsuhiko Nishi 1 , Kouki Akaike 1 , Kentaro Sakai 1 , Tadahisa Nishida 1 , Yukio Ouchi 1 , Kazuhiko Seki 1 , Kaname Kanai 2
1 Department of Chemistry, Division of Material Science, Graduate School of Science, Nagoya University, Nagoya Japan, 2 Research Center for Materials Science, Nagoya University, Nagoya Japan
Show AbstractRecently, organic semiconductors have attracted considerable interest owing to the possibility for application to various types of electronic devices such as organic light emitting diodes [1], organic field effect transistors [2], and organic photo-voltaic cells [3]. In order to understand the operation mechanism and improve the performance of such organic devices, it is important to obtain information of the electronic structures below and above the Fermi level of the organic semiconductors. The inverse photoemission spectroscopy (IPES) is a powerful tool to elucidate the unoccupied electronic structure of materials. However, the irradiation-damage on the sample with the accelerated electron-beam during IPES measurement has prevented us from applying IPES to organic materials. It has been consequently a serious issue that the organic thin film can be damaged even with the low energy electron-beam [4].To investigate organic materials, especially organic semiconducting thin films related to organic devices, we have developed IPES measurement system for organic materials with reduction of irradiation damage by changing irradiated area on the sample during the measurement.In the presentation, we will discuss the electronic structure of several thin films of organic molecules such as [6,6]-phenyl-C61 butylic acid methyl ester (PCBM) [5], poly(9,9-dioctylfluorene) and poly(3-hexylthiophene) polymer thin films as well as the development of the measurement system. [1] C. W. Tang and S. A. VanSlyke, Appl. Phys. Lett., 1987, 51, 913.[2] S. Allard, M. Forster, et al., Angew. Chem. Int. Ed., 2008, 47, 4070.[3] Dieter Wöhrle and Dieter Meissner, Adv. Mater., 1991, 3, 129.[4] K. Tsutsumi, H. Yoshida and N. Sato, Chem. Phys. Lett., 2002, 361, 367.[5] K. Akaike et al., J. Appl. Phys., in press.
9:00 PM - H8.28
Electronic Structures of a Low Work Function Oxide C12A7:e- / Organic Semiconductors Interfaces Studied by Photoelectron Spectroscopy.
Hiroshi Yanagi 1 , Toshifumi Kuroda 1 , Ki-Beom Kim 1 , Yoshitake Toda 2 , Masashi Miyakawa 2 , Toshio Kamiya 1 3 , Masahiro Hirano 2 3 , Hideo Hosono 1 2 3
1 Materials and Structures Laboratory, Tokyo Institute of Technology, Yokohama Japan, 2 Frontier Research Center, Tokyo Institute of Technology, Yokohama Japan, 3 ERATO-SORST, Japan Science and Technology Agency, Yokohama Japan
Show AbstractThe compound 12CaO7Al2O3 (C12A7) is a typical oxide, which is used as, for example, a cement constituent. However, C12A7 has interesting physical and chemical properties originating in its unique crystal structure composed of cage structures. The cage is positively charged and thus negatively charged anions are entrapped therein. Usually O2- is entrapped in the cages, whereas electrons are introduced in the cages in place of the O2- ions. The resulting material is C12A7 electride (C12A7:e-) [Matsuishi et al. Science 301 (2003) 626]. The electrons are weakly confined in the cages and migrate to neighboring cages, giving electrical conductivity as high as 1500 S/cm at room temperature [Kim et al. Nano Lett. 7 (2007) 5]. Furthermore, the electride has a low work function of ∼2.4 eV and chemical stability [Toda et al. Adv Mater. 19 (2007) 3564]. This work function is lower than those of Ca and Mg. Thus C12A7:e- is one of the ideal materials for high-efficient cathode for organic semiconductors. Recently, we have prepared C12A7:e- / Alq3 interface and realized a low electron injection barrier height (∼0.6 eV) [Kim et al. J. Phys. Chem. C, 111 (2007) 8403]. In this study, interfacial electric structures between C12A7:e- and organic semiconductors, pentacene, CuPc and C60, were investigated using UPS and XPS measurements. In preliminary study, we found it was difficult to get atomically-flat low-work function surface of C12A7:e- with good reproducibility by simple chemical mechanical polishing (CMP). Then, a combined process of CMP, chemical etching, and vacuum annealing was developed to obtain smooth and low work function surface of C12A7:e-. First, single crystals of C12A7 were grown by the CZ method. The C12A7 were converted to C12A7:e- by a Ti reduction treatment. The surface of the single crystal C12A7:e- was polished by CMP. Then, polished C12A7:e- was dipped in a 20% H3PO4 solution for removing a surface insulating layer. After that, the chemically etched C12A7:e- was transferred into our vacuum system, in which we can carry out organic and metal layer depositions, and UPS-XPS measurements without breaking vacuum (the base pressure is ∼10-7 Pa). The chemically etched C12A7:e- was annealed at 1000 oC for 40 min. The obtained C12A7:e- surface reproducibly shows a low work function of ∼3.0 eV. Pentacene, CuPc and C60 thin films were deposited on the C12A7:e- surface by thermal evaporation under the pressure of ∼3×10-6 Pa. Interfacial electronic structures were observed by the in-situ UPS measurements by varying the thickness of the organic over-layer on the C12A7:e-. Electron injection barriers at surface-modified C12A7:e- / pentacene, CuPc, and C60 were estimated to be ∼0.8 eV, ∼1.0 eV, and ∼0.4 eV, respectively. These values are lower than those at the interface with conventional cathodes such as Al but comparable to or larger than those with unstable cathodes such as Mg and Ca.
9:00 PM - H8.29
Organic Photovoltaic Devices with Semitransparent Cathode.
Wei Feng Hsu 1 , Chi-Feng Lin 1 , Shun-Wei Liu 1 , Juin-Haw Lee 1
1 , Graduate Institute of Photonics and Optoelectronics and Department of Engineering, Taipei Taiwan
Show AbstractIn this report, we demonstrated a transparent organic photovoltaic (OPV) device based on a copper phthalocyanine (CuPc) (20 nm)/fullerene (C60) (40 nm)/bathocuproine (BCP) (7 nm)/Ag (x nm) hetero-junction structure with the thin metal as the semi-transparent cathode. Ag thickness is varied from 5, 10, 15, 25, 35, 45, 55, and 100 nm. When Ag thickness is less than 15 nm, transmittance spectra show a dip between 400 and 500nm, corresponding to the Ag plasmonic absorption which results from Ag aggregation. Current-voltage measurement shows a dramatic increase in conductivity at Ag> 15 when it forms a flat thin-film. With increasing the Ag thickness from 15 to 100 nm, the efficiency of OPV decreases from 0.29 to 0.07%, measured from the cathode (top) side. At the same time, the efficiency from anode (bottom) side increases from 0.91 to 1.23%, respectively. By putting the OPV in the integral sphere, the optimized transparent device (Ag=15nm) exhibits a 7.4 % increase in short circuit current compared to the control device (Ag=100 nm). Moreover, the introduction of additional index matching layer such as N,N′-di(naphthaleneyl)-N,N′-diphenylbenzidine (NPB) after the semi-transparent cathode will increase the incident light from the top side.
9:00 PM - H8.3
Laboratory Teaching Modules on Organic Electronics and Liquid Crystal Displays for Undergraduate and Graduate Education.
Brandon Conover 1 , Michael Escuti 1 , David Carroll 2 , Ananth Dodabalapur 3 , Gregory Crawford 4
1 Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States, 2 , Wake Forest University, Winston-Salem, North Carolina, United States, 3 , University of Texas, Austin, Texas, United States, 4 , University of Notre Dame, Notre Dame, Indiana, United States
Show AbstractWe have developed of a sequence of laboratory teaching modules that give hands-on experience with organic semiconductor devices and liquid crystal display technologies. Each lab module guides participants through the process of fabricating (from scratch) and characterizing the following four devices: (i) an organic light-emitting-diode (OLED), (ii) an organic photo-voltaic (OPV) solar cell, (iii) an organic thin-film-transistor (OTFT), and (iv) a twisted nematic liquid crystal display (LCD) pixel. Supported by a CCLI grant from the National Science Foundation, these modules aim to broadly support the education of undergraduate and graduate students in organic materials and devices, which are at the forefront of research and the core technology of an expanding set of consumer products (e.g. displays, lighting, flexible electronics, renewable energy devices). Our target audience are students with at least some introductory solid-state electronics background (e.g. those from electrical engineering, physics, or materials science curricula). In the comprehensive lab manuals, we aim to expand on this traditional foundation to address key concepts pertinent to organic devices, including thin-film coating methods, electrical/optical characterization, charge injection, charge transport, self-assembly, light emission, absorption, polarization, and basic molecular orbitals and structure-property relationships. A unique aspect these lab modules is that the entire sequence may be offered in a traditional classroom laboratory (i.e., using a custom built N2 glove box and a ductless, charcoal-based, air filter), with a minimal capital investment. Neither a clean room nor a fume hood are required, in such as way as to be inherently transferable to other universities. In this talk, we will introduce our devices, fabrication methods, materials, infrastructure, pedagogical principles, and dissemination plan.
9:00 PM - H8.30
Sub-band Gap Photocurrent Spectra and Gap States Distribution of Pentacene Thin-film Transistors.
Manabu Ohtomo 1 , Toshihiro Shimada 1 , Tetsuya Hasegawa 1
1 Department of Chemistry, School of Science, The University of Tokyo, Tokyo Japan
Show AbstractWe investigated the photocurrent spectra of pentacene field effect transistors in order to study various device characteristics including gap states distribution. We compared the photocurrent spectra of devices with two structures: top-contact FETs and bottom-contact ones. In top-contact devices, the photocurrent spectrum versus photon energy exhibited the typical Urbach tail, while such a tail was not observed in bottom-contact devices. The formation of this Urbach tail can be explained by assuming that diffused gold nanoparticles at the gold/pentacene interface create microfields and stabilize the excitons. We suggest that metal contacts without diffused nanoparticles are necessary for the intrinsic photocurrent measurement; that is, the top-contact geometry should be avoided and the bottom-contact devices with “clean” metal/organic interface should be employed. By employing the bottom-contact geometry, we investigated the intrinsic photocurrent spectrum of the polycrystalline pentacene films and found a flat distribution of energy levels in the sub-band-gap region. We also report the correlation between sub-band-gap photocurrent and various device parameters of polycrystalline pentacene field-effect transistors. It was established that the photocurrent-VG characteristics can reveal the origins of the photo carriers, i.e., the interband transition or excitation of electrons in the localized gap states, by changing the gate bias and drain voltage during the photocurrent measurement. In our experiment, carrier mobility was not related to photocurrent in the sub-band-gap region, even though a large photocurrent implies a large number of trapping states. On the other hand, the off-current obviously correlates with the photocurrent, suggesting that the origin of the off-currents is the gap states widely distributed near the valence band edge.Reference [1] M. Ohtomo, T. Shimada, T. Saiki, and T. Hasegawa, J. Appl. Phys. 102, 064510 (2007). [2] M. Ohtomo, T. Shimada, and T. Hasegawa, Jpn. J. Appl. Phys. 46 L817 (2007).
9:00 PM - H8.31
Adhesion in Organic Electronic Structures.
Tiffany Tong 1 2 , Juan Meng 1 3 , Nima Rahbar 1 3 , Shimelis Admassie 1 4 , Babaniyi Babatope 1 5 , Winston Soboyejo 1 3
1 Princeton Institute for the Science and Technology of Materials, Princeton University, Princeton, New Jersey, United States, 2 Electrical Engineering, Princeton University, Princeton, New Jersey, United States, 3 Mechanical and Aerospace Engineering, Princeton University, Princeton, New Jersey, United States, 4 , Princeton University, Princeton Ethiopia, 5 , Engineering Materials Development Institute, Akure Nigeria
Show AbstractThis paper presents the results of experimental and theoretical studies of adhesion in organic electronic structures. These include biomaterial couples that are relevant to organic solar cells and organic light emitting devices. Atomic force microscopy techniques are used to measure the adhesive interactions between surfaces in these layered structures. The measured pull-off forces are then integrated into adhesion theories that are used to compute the surface energies. The estimated surface energies are shown to be in good agreement with the interfacial fracture energies obtained from Brazil disk fracture mechanics experiments. The mode mixity dependence of fracture toughness is also predicted using a contact shielding model.
9:00 PM - H8.32
Thin Films of Perfluoroalkyl- Substituted Phthalocyanines, a New Group of Easily n-doped Robust Materials.
Christopher Keil 1 , Martin Lener 1 , Robert Gerdes 2 , Lukasz Lapok 2 , Sergiu Gorun 2 , Cameliu Himcinschi 3 , Dietrich R. Zahn 3 , Derck Schlettwein 1
1 Institute of Applied Physics, Justus-Liebig-University Giessen, Giessen Germany, 2 Department of Chemistry and Environmental Science, New Jersey Institute of Technology, Newark, New Jersey, United States, 3 Institute of Physics, Technical University Chemnitz, Chemnitz Germany
Show AbstractThin (0.5-100 nm) films of 2,2’,2’’,2’’’,3,3’,3’’,3’’’-octa(perfluoropropyl) 1,1’,1’’,1’’’,4,4’,4’’,4’’’ – octafluoro phthalocyanine complexes (F64Pc) with different central metal groups (Cu, Zn, VO) could be prepared as vapor- deposited thin films on glass substrates despite their large molecular mass and bulky substituents. The optical absorbance and electronic conduction were measured in-situ during film growth in order to reveal the formation of conducting pathways, the mechanism of film growth as well as potential microscopic intermolecular electronic coupling. Subsequent to deposition spectral ellipsometry was performed at different incident angles to analyze in detail the average orientation of the chromophores. Potential applications of this new class of molecules to semiconductor materials and devices will be discussed.
9:00 PM - H8.33
Effect of Heterojunction Structure on the Bias Dependence of Photocurrent in Organic Photovoltaic Devices.
Jason Myers 1 , Jiangeng Xue 1
1 Materials Science and Engineering, University of Florida, Gainesville, Florida, United States
Show Abstract9:00 PM - H8.34
Improved Spectral Uniformity and High Open-Circuit Voltage Organic Tandem Solar Cells Based on the Subphthalocyanines.
Brian Lassiter 1 , Guodan Wei 1 , Zhi Li 2 , Stephen Forrest 1 2 3
1 Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan, United States, 2 Department of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, 3 Department of Physics, University of Michigan, Ann Arbor, Michigan, United States
Show AbstractA high open circuit voltage of 1.5 V has been obtained for organic tandem solar cells consisting of two subcells containing of the donors [1], chloro[Subphthalocyanine]boron(III) (SubPc) and copper phthalocyanine (CuPc), along with a fullerene acceptor. Deposition of the materials was either by vacuum thermal evaporation or organic vapor phase deposition. The complementary absorption ranges of SubPc and CuPc, 500-600 nm and 600-700 nm respectively, result in improved uniformity of the spectral response across the visible region compared to that of the individual subcell responses. An ultra thin Ag nanocluster layer separating the subcells serves as a charge recombination region. The resulting photocurrent of the tandem cell exceeds that of the individual front and back cells. Optical field modeling has been utilized to determine the optimum device structure (i.e. layer thicknesses and cell positions) based on the complex indices of refraction, exciton diffusion lengths, and series resistances of the materials employed. Several different cell structures, including planar heterojunction and mixed heterojunction cells, have been fabricated. A comparison of analytical and experimental results will be discussed in this presentation, and the ultimate efficiency limits expected for this materials combination will be considered.
[1] K. L. Mutolo, E. I. Mayo, B. P. Rand, S. R. Forrest and M. E. Thompson, J. Am. Chem. Soc., 128, 8108 (2006).
9:00 PM - H8.35
Energy Level Alignment at the Interface Between La0.7Sr0.3MnO3, Modified by Acridine Orange Base, and Organic Semiconductors.
Fenghong Li 1 , Michel de Jong 1 , Yiqiang Zhan 1 , Mats Fahlman 1 , William R. Salaneck 1
1 IFM, Linköping University, Linköping Sweden
Show AbstractOrganic spintronics is a new branch of molecular electronics. It deals with the injection and detection of spin polarized carriers in organic semiconductors by means of, e.g., spin valve devices. Injection and detection of spins may be achieved using a pair of ferromagnetic electrodes, for example La0.7Sr0.3MnO3 (LSMO) and Co. LSMO is potentially a spin injection contact in spintronic devices based on organic semiconductors as has been shown for the spin valve LSMO/Alq3/Al2O3/Co.1 The electronic structure of thin films of LSMO has been extensively characterized with photoelectron spectroscopy, X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichriosm (XMCD), by de Jong et al.2 3 In order to understand spin injection and detection in spin valve devices, it is crucial to study the appropriate organic-ferromagnetic interfaces. Zhan, et al, have shown that barriers for hole and electron injection between LSMO and Alq3 are 1.7 eV and 1.2 eV, respectively.4 Therefore it is necessary to increase or decrease of work function (WF) of LSMO in order to facilitate hole or electron injection from LSMO into organic semiconductors. Based on our experience in tuning the WF of substrates by adsorbing donor or acceptor molecules, it is straightforward to use a novel n-type dopant acridine orange base (AOB)5 to modify the WF of LSMO, and thereby decrease the barrier of electron injection from LSMO into the organic semiconductors. Here we report the results of a study of the energy level alignment at the interface between Alq3 and LSMO modified by AOB. Upon the deposition of a monolayer (0.7 nm) AOB, the WF of LSMO changes from 4.8 eV to 4.1 eV (±0.1 eV) corresponding to the generation of a dipole of 0.7 eV. Further deposition of Alq3 on the molecularly modified substrate leads to a gradual decrease in the WF decrease, eventually reaching to a steady state value equal of 3.7 eV. Obviously a dipole of 0.4 eV is induced by the Alq3 itself. Valence band spectra have shown that HOMO versus Fermi level of Alq3 shifts from 1.9 eV to 2.8 eV when Alq3 thickness changes from 1.4 nm (~ monolayer) to 7 nm. A shift of the Fermi level away from HOMO is an indication of n-type doping. We assume that an energy level bending of 0.9 eV in the depletion region results from an increase of free electron density at the Alq3/AOB/LSMO interface. X-ray photoelectron spectra and XAS indicate that there exist some redox interactions at Alq3/AOB/LSMO interface. XMCD demonstrates that the ferromagnetic property of LSMO is not significantly influenced by the presence of the AOB thin film, even though 5 nm AOB is deposited on LSMO surface. Consequences of indicated by these observations will be presented.1 Z. H. Xiong et al., Nature 427, 821 (2004).2 M. P. de Jong et al., J. Appl. Phys. 94, 7292 (2003).3 M. P. de Jong et al., Phys. Rev. B 71, 014434 (2005).4 Y. Q. Zhan et al., Phys. Rev. B 76, 045406 (2007).5 K. Harada et al., Appl. Phys. Lett. 91, 092118 (2007).
9:00 PM - H8.36
Inverted Small Molecule Organic Photovoltaic Cells.
Stephen Forrest 1 , Rhonda Bailey-Salzman 2 1 , Xiaoran Tong 1
1 , University of Michigan, Ann Arbor, Michigan, United States, 2 Electrical Engineering, Princeton University, Princeton, New Jersey, United States
Show Abstract9:00 PM - H8.38
High-performance Solution Processed Organic Solar Concentrators.
Shalom Goffri 1 , Michael Currie 1 , Jonathan Mapel 1 , Marc Baldo 1
1 Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States
Show AbstractSolar concentrators are promising for reducing the cost of renewable electricity. Solar concentrators maximize power output of small photovoltaic cells by increasing the sunlight incident on them. Conventional solar concentrators, however, require tracking of the sun as it transits the sky and cooling of the solar cells increasing maintenance and installation costs. Luminescent solar concentrators (LSCs) concentrate sunlight without the need to track the sun or cool the active solar cells, thereby reducing the cost-per-watt of the module. Historically LSC performance has been limited by self-absorption of the emitted light due to the spectral overlap of absorption and emission. Recently Currie et al. have demonstrated a new variant of the LSC, the organic solar concentrator (OSC), where self-absorption losses are significantly reduced [1]. This is achieved by utilizing resonant energy transfer, solid-state solvation and phosphorescence. Solution processed OSCs have potential to significantly reduce cost-per-watt due to low cost manufacturing. Here we demonstrate solution processed OSCs with power conversion efficiency exceeding 4% via an energy transfer mechanism. By using resonant energy transfer between soluble dye, we significantly reduce self-absorption losses enabling us to increase and maintain performance for larger geometric concentration factors, G. We report a multi-component perylene-diimide-based system with optimized dye composition in the film. The spectral overlap between the guest emission and OCS absorption is reduced by lowering the molecular concentration of the guest dye in the OSC leading to external quantum efficiencies (EQE) exceeding 46%.Loss mechanisms for OSCs are studied by measuring changes in photoluminescence (PL) spectra and EQE with increasing G. We observe significant red-shifting of the guest dye PL with increasing G due to re-absorption and re-emission events. This red-shift leads to reduced spectral overlap between absorption and emission and enables high G factors that are not limited by self-absorption.References:1.High-efficiency Organic Solar Concentrators for Photovoltaics, M.J. Currie, J.K. Mapel, T.D. Heidel, S. Goffri and M.A. Baldo. Science. In press.
9:00 PM - H8.39
Optical Constants of C60 Thin Films for Solar Cell Device Applications.
Debjit Datta 1 2 , Satyendra Kumar 1 2
1 Department of Physics, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India, 2 Samtel Centre for Display Technology, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India
Show Abstract9:00 PM - H8.40
Quasi-Static Modeling Approach for Metal-Organic Semiconductor-Metal Devices.
Vasu Puttagunta 1 , Wudyalew Wondmagegn 1 , Ron Pieper 1
1 Electrical Engineering, University of Texas at Tyler, Tyler, Texas, United States
Show AbstractWe present a comprehensive quasi-static device model for a tri-layer metal-organic semiconductor-metal device. The metal -organic interfaces are evaluated from material constants corresponding to metal work function, organic semiconductor electron affinity, organic semiconductor material HOMO and LUMO levels and effective doping. The overall device is viewed as a series concatenation of up to three nonlinear devices. There are the two metal organic interfaces which selectively, pending material constants, can comply with either ohmic or rectifying (Schottky) behavior. If the conditions at either end produce an ohmic metal semiconductor contact then that specific interface is treated with a simple linear low-value contact resistance. On the other hand either side with right combination of material constants can produce a rectifying contact and are treated with the Schottky thermionic emission/diffusion models. This model includes field-dependent barrier lowering associated with image charge effects . The third sandwiched layer is an organic material which is tested with effective nonlinear resistance predicted from both Hopping Model and Poole-Frenkle Model mobility rules. In most practical cases one of the two metal organic interfaces will be ohmic although this is not required in simulation tests. The MATLAB model to test this physical series configuration required the constraints of matching currents with a consistent Kirchoff voltage rule for the three regions. Physical models provide the basis for nonlinear IV characterization for each region. The organic semiconductor transport region length is diminished by the space charge regions lengths from either or both of two sides of the metal semiconductor interfaces. The nonlinearity of the problem is treated, within the program, by iterative fashion solving for the crossing condition for matching currents in two adjacent metal-organic semiconductor regions. The two region solution is polynomial fitted to provide nonlinear IV model for the composite of first two regions analyzed. The third remaining metal-organic interface region is again treated as a two region problem where the other region IV characteristic is taken as the composite IV generated in the first step. The MATLAB model tests were compared with 2D finite element type analysis using commercial software ATLAS package of Silvaco International. The three region composite IV MATLAB characterizations were in close agreement with Silvaco simulations for both temperature and total applied voltage variations.
9:00 PM - H8.41
Effect of Annealing on Device Parameters of CuPc-C60 Based Organic Solar Cells.
Debjit Datta 1 2 , Anirban Bagui 1 2 , Vibha Tripathi 2 3 , Satyendra Kumar 1 2
1 Department of Physics, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India, 2 Samtel Centre for Display Technology, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India, 3 Department of Electrical Engineering, Indian Institute of Technology Kanpur, Kanpur, Uttar Pradesh, India
Show Abstract9:00 PM - H8.42
Photoinduced Charge Transfer Between Poly(3-hexylthiophene) and Single-walled Carbon Nanotubes Studied by Time-resolved Microwave Conductivity.
Andrew Ferguson 1 , Jeffrey Blackburn 1 , Chaiwat Engtrakul 1 , Michael Heben 1 , Nikos Kopidakis 1 , Garry Rumbles 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show Abstract9:00 PM - H8.43
Tunable Acceptor Materials for Improved Efficiencies in Organic Solar Cells.
Martin Drees 1 , Claudia Cardona 1 , Brian Holloway 1 , Russel Ross 2 , Edward Van Keuren 2 , Dirk Guldi 3 , Jeff Peet 4 , Guillermo Bazan 4
1 , Luna Innovations Incorporated, Danville, Virginia, United States, 2 , Georgetown University, Washington, District of Columbia, United States, 3 , Friedrich-Alexander-Universität Erlangen-Nürnberg, Erlangen Germany, 4 , University of California, Santa Barbara, Santa Barbara, California, United States
Show AbstractOrganic solar cells are a promising technology that will allow a low-cost roll-to-roll manufacturing process on flexible, light-weight substrates. To improve the efficiency of organic solar cells beyond the current state-of-the-art (~5%), novel donor and acceptor materials need to be developed that have better matching molecular orbitals and reduce energy losses during charge transfer.We present a novel acceptor material based on TRIMETASPHERE® carbon nanomaterials (TMS). TMS are endohedral metallofullerenes that consist of a tri-metal nitride cluster enclosed in a C80 fullerene. These novel acceptor materials have molecular orbitals that are better matched to common donor materials than C60-PCBM, the most common acceptor, and can be tuned by the choice of metal enclosed. Electrochemical measurements show a shift of the lowest unoccupied molecular orbital (LUMO) of up to 280 meV compared to C60-PCBM, which has resulted in a 280 mV enhancement of the open circuit voltage. The unique electrochemical behavior of TMS acceptors has been shown to be influenced by the choice of linker used for the side group attachment. The solubility and miscibility of the material has been affected by changing the side group, however the side group exchange does not influence the electrochemical behavior and therefore the LUMO level advantage. Photophysical studies in thin films with the donor polymer poly(3-hexylthiophene) (P3HT) show an efficient and stable charge separation with a lifetime of 2.2 milliseconds. The charge carrier mobility has been found to be similar to C60-PCBM. The advantages of TMS acceptors found by these fundamental studies have been confirmed in organic solar cell devices with P3HT, achieving a maximum voltage increase of 280 mV over C60-PCBM-based devices and efficiencies exceeding the P3HT/C60-PCBM reference devices.
9:00 PM - H8.44
Inkjet Fabrication of Tandem Dye-Sensitized Bulk Heterojunction Polymer Solar Cells.
Annick Anctil 1 , Roberta DiLeo 1 , Andrew Merrill 1 , Abbey Weaver 1 , Kyle Rugg 1 , Joshua Kolev 1 , Brian Landi 1 , Ryne Raffaelle 1
1 , RIT - NanoPower Research Labs, Rochester, New York, United States
Show AbstractThere are several challenges restricting the commercial viability of state-of-the-art polymer solar cells involving derivatized fullerene-polymer blends: (1) device efficiency, (2) environmental stability of active materials, and (3) large area processing. Currently, the theoretical efficiency in an optimized device is limited by the poor mismatch to the solar spectrum. Empirical modeling has been performed using device parameters from current-voltage and spectral response measurements. The maximum efficiency for PCBM[70]-MEH-PPV and PCBM[60]-P3HT systems is calculated to be between 6 and 8%, under 1 sun AM1.5 illumination. However, incorporation of molecular dyes which absorb between 800-900 nm could theoretically increase the power conversion efficiency to 16% under AM1.5 illumination. In the present work, soluble organic dyes (i.e., cyanine, phthalocyanine, benzoquinone) with bandgaps in the near-infrared (NIR) are utilized in a bulk heterojunction polymer solar cell to extend the spectral response. Electrochemical cyclic voltammetry was employed to investigate the suitability of these NIR absorbing dyes for this application, based on their energy level alignment with the conducting polymer (e.g., MEH-PPV, P3HT, MDMO-PPV, etc.) and the PCBM [60] and [70] fullerene derivatives. The deposition of active layers was performed using a Dimatix printer which allowed for precise control over film thickness, uniformity, and deposition morphology. The prospect of coupling a NIR-enhanced device with a conventional PCBM-polymer device to fabricate a multi-junction device is described. Optical profilometry and spectroscopic ellipsometry has been used to develop an optical model for layer thickness optimization to ensure proper absorption and charge transport in this tandem cells. Finally, a passivation scheme involving a UV-curable photopolymer to maintain performance under ambient environmental conditions (humidity and oxygen) has been developed. The optical characterization and current-voltage measurements under simulated AM1.5 illumination for an inkjet deposited dye-sensitized tandem organic device which demonstrates open-air stability will be presented.
9:00 PM - H8.45
Poly(2,7-Carbazole) Derivatives for High Efficiency Organic Solar Cells.
Salem Wakim 1 , Nicolas Blouin 2 , Serge Beaupre 2 , Ye Tao 1 , Mario Leclerc 2
1 Institute of Microstructural Sciences, National Research Council of Canada, Ottawa, Ontario, Canada, 2 Canada Research Chair on Electroactive and Photoactive Polymers, Université Laval, Québec, Quebec, Canada
Show Abstract9:00 PM - H8.46
Morphological Impacts of C80 Endohedral Fullerene Acceptor Materials for Bulk-heterojunction Organic Solar Cells.
Russel Ross 1 , Edward Van Keuren 1 , Nikos Kopidakis 2 , Matthew Reese 2 , Claudia Cardona 3 , Holloway Brian 3 , Martin Drees 3
1 Physics, Georgetown University, Washington, District of Columbia, United States, 2 , National Renewable Energy Laboratory, Golden, Colorado, United States, 3 , Luna Innovations Incorporated, Danville, Virginia, United States
Show AbstractNew materials are needed for Organic Photovoltaics to be competitive in the renewable energy market. The current architectures employed have been optimized around the C60PCBM acceptor, yet these efforts have failed to gain market acceptance due to their low efficiencies. Recent results have shown that C80 trimetallic nitride endohedral fullerene acceptors have an increased lowest unoccupied molecular orbital energy which leads to higher open circuit voltages ( 280 mV increase) and higher efficiencies (3.6% at abstract submission) compared to C60PCBM reference devices (reference efficiency of 3.4%). The morphological impact these endohedral fullerenes have on the formation of highly ordered bulk-heterojunction devices was investigated with respect to fullerene functionalization, blend concentration and film annealing procedure. Polymer fullerene blend films with the donor polymer poly(3-hexylthiophene) were studied and optimized using absorption spectroscopy, grazing incident x-ray diffraction, and modulated differential scanning calorimetry analysis. Results indicate that a bulk-heterojunction network based on these novel acceptor materials will have efficiencies >6% in poly(3-hexylthiophene) and >10% in known low band-gap polymer systems.
9:00 PM - H8.48
Alternative Hole Transport Layers for Organic Photovoltaics.
Dana Olson 1 , Matthew White 1 , Matthew Reese 1 , Joseph Berry 1 , David Ginley 1
1 NCPV, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractImprovements in the lifetime and efficiency of organic photovoltaics are critical to its development as a viable technology. One of the important interfaces in OPV devices is between the transparent electrode and the organic active material, where an interfacial material can aid in charge extraction. In our typical OPV devices we employ a glass/indium tin oxide (ITO) substrate with an interstitial PEDOT:PSS to modulate the interface between the ITO and active P3HT:PCBM bulk heterojunction (BHJ) region. The PEDOT:PSS material serves as the hole transport layer (HTL) in the devices, but has numerous drawbacks with respect to morphology, chemistry, stability and scalability. Here we report on our use of alternative HTLs based on mixed metal oxides and commercially available organic HTLs. We compare the results from P3HT:PCBM BHJ devices using these alternative HTLs with traditional devices employing PEDOT:PSS. Changes in device efficiency, as well as work function, pH, and conductivities are quantified as a function of the HTL employed. In addition, we will employ transparent electrodes other than ITO that are not accessible when using PEDOT:PSS as the HTL. We also report on initial device degradation studies, which examine the correlation between the HTL used and time dependent changes in device efficiency.
9:00 PM - H8.5
Polymer Solar Cells with Poly(3,4-ethylenedioxythiophene) as Transparent Anode.
Leeyih Wang 1 , Yi-Ming Chang 1 , Wei-Fang Su 1
1 Center for Condensed Matter Sciences, National Taiwan University , Taipei Taiwan
Show Abstract9:00 PM - H8.50
Excitonic Surface Plasmon Resonance Biosensor.
Mihail Bora 1 , Kemal Celebi 1 , Marc Baldo 2
1 Physics, MIT, Cambridge, Massachusetts, United States, 2 Electrical Engineering and Computer Science, MIT, Cambridge, Massachusetts, United States
Show Abstract9:00 PM - H8.51
Measuring Charge Transfer at Organic-Inorganic Semiconductor Interfaces Using Field-Effect Transistors.
Josef Spalenka 1 , Byoungnam Park 1 , Peerasak Paoprasert 2 , Ryan Franking 2 , Robert Hamers 2 , Padma Gopalan 3 , Paul Evans 3
1 Materials Science Program, University of Wisconsin, Madison, Wisconsin, United States, 2 Department of Chemistry, University of Wisconsin, Madison, Wisconsin, United States, 3 Department of Materials Science and Engineering, University of Wisconsin, Madison, Wisconsin, United States
Show AbstractField-effect transistors (FETs) are sensitive to the electrical properties and morphology of the first few molecular layers near the gate dielectric and have well understood current-voltage characteristics. FETs, therefore, can be used as test structures to characterize electrical interfaces between semiconducting materials. This work explores photoinduced charge transfer for semiconductor interfaces including 1) the interface between pentacene and a C60 self assembled monolayer (SAM) and 2) between pentacene and ZnO. Under illumination electrons are transferred from pentacene to C60, creating a conducting channel at zero gate voltage and shifting the measured threshold voltage. The threshold voltage was observed to shift by as much as 84 volts, the magnitude of this threshold voltage shift can be used to quantify the number of electrons transferred from the pentacene. Morphology of the deposited pentacene was examined using atomic force microscopy. Using an FET device structure isolates the charge transfer effect at the interface from other effects by minimizing bulk conduction and complex interface geometry and offers a simple route for characterizing photoinduced charge transfer between organic donor and acceptor materials and between organic and inorganic semiconductors.
9:00 PM - H8.52
Single-Walled Carbon Nanotube Network Contacts for Organic Photovoltaics.
Jeremy Bergeson 1 , Jeffrey Blackburn 1 , Robert Tenent 1 , Dana Olson 1 , Teresa Barnes 1
1 , National Renewable Energy Laboratory, Golden, Colorado, United States
Show Abstract9:00 PM - H8.53
The Influence of Solvents and Thermal Annealing on the Morphology and Photoconductivity in Films of P3HT and PCBM Blended with P3HT.
Alexandre Nardes 1 , Brian Appleby 1 , Sean Shaheen 1 , Nikos Kopidakis 2 , Davor Balzar 1
1 Dep of Physics and Astronomy, University of Denver, Denver, Colorado, United States, 2 Basic Sciences, National Renewable Energy Laboratory, Golden, Colorado, United States
Show AbstractDevice performance in organic solar cells, as well as other molecular electronic devices, is to a large extent determined by the mobility of charge carriers, which strongly depends on material morphology. Thus, an understanding of the relation between the mechanism of charge transport and the morphology of the active organic material is essential for improvement of device performance.In this work, the influence of different solvents and thermal annealing on the morphology and photoconductivity properties of drop-cast thick films (4-6 μm) of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM), with 1:1 ratio of P3HT:PCBM in weight, has been investigated. The photoconductivity of the blends, in different solvents (chloroform, chlorobenzene and dichlorobenzene), and pure P3HT films, in dichlorobenzene, was measured with the contactless time-resolved microwave conductivity technique (TRMC), before and after annealing, allowing the quantum yield of free carrier production times the mobility (Σφμ) to be determined as a function of absorbed light intensity. It is observed that upon addition of 50% (by weight) of PCBM the end-of-pulse photoconductivities are enhanced by more than one order of magnitude when compared to that of pure P3HT films. Amongst the blend films, however, no significant changes in Σφμ with different solvents are observed, indicating that the type of solvent used it is not a sine qua non condition for the photogenerated charges. On the other hand, from the photoconductivity transients, a fast decay of the photogenerated charge carriers is observed for the blend in chloroform as the absorbed light intensity is decreased, being nearly 2x faster than the other samples at 10^11 photons/cm^2/pulse. Samples in chlorobenzene and dichlorobenzene exhibit roughly the same time decays. After the thermal annealing the time decays are slightly enhanced for all samples and this effect is more pronounced at high light intensities, ~ 10^15 – 10^13 photons/cm^2/pulse.The transient photoconductivity results are in agreement with topographic atomic force microscopy (AFM) images and x-ray diffraction (XRD) measurements. After annealing, the 60-80 nm sized-clusters of the films made from chlorobenzene and dichlorobenzene show closer packing of the polymer chains, which is expected to create a better conductive pathway for the charge carriers as well as reduced carrier recombination due to higher purity of the donor-acceptor domains. Finally, the XRD measurements showed that the annealing promotes the ordering of P3HT. The peak at around 2θ value of 6°, which is attributed to crystalline ordering of P3HT, becomes narrower and shifts towards higher 2θ values upon annealing for all samples.Implications of our results on the operation of organic photovoltaic devices based on P3HT:PCBM bulk heterojunctions are discussed.
9:00 PM - H8.55
Effect of Including Surface-Plasmon Active Materials on the Electronic Properties of Bulk Heterojunction Organic Photovoltaics.
Anthony Morfa 1 2 , Jao van de Lagemaat 2
1 , University of Colorado at Boulder, Boulder, Colorado, United States, 2 , NREL, Golden, Colorado, United States
Show Abstract9:00 PM - H8.56
Carrier Transport of Poly(3-hexylthiophene)/[6,6]-phenyl C61-butyric Acid Methyl Ester Blends: Solvent Dependence.
Shun-Wei Liu 1 , Chin-Han Wang 2 , Yu-Jui Huang 1 , Chin-Chien Lee 2 , Ping-Tsung Huang 3 , Chin-Ti Chen 1 , Juen-Kai Wang 4 5
1 Institute of Chemistry, Academia Sinica, Taipei Taiwan, 2 Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei Taiwan, 3 Department of Chemistry, Fu Jen Catholic University, Taipei Taiwan, 4 Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, 5 Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan
Show Abstract9:00 PM - H8.6
Charge Transfer between Metal (II) (Co, Ni, Cu) Phthalocyanines and Au (111) Substrate.
Jie Xiao 1 , Peter Dowben 1
1 Department of Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States
Show AbstractPhotoemission and inverse photoemission spectra are shown for different coverages of metal (II) (Co, Ni, Cu) phthalocynaines (MPc) deposited on Au (111). While thick films show similar features for all three metal (II) (Co, Ni, Cu) phthalocynaines, thin films behave differently for these three molecules on gold. Charge transfer occurs at the interface between metal phthalocynaines and the gold substrate, resulting in difference in the molecular orbital offsets. Different 3d orbital configurations or the 3d filling (Co d7, Ni d8, Cu d9) appear to determine the direction of charge transfer at the interface. From the combined photoemission and inverse photoemission spectra of these three different molecules on Au, we found that electrons move from gold to CoPc to fill up the empty cobalt d orbitals, while for CuPc, electrons depeletion appears to occur, all consistent with prior STM studies [1]. As a result of charge transfer at interface, the Fermi level of CoPc is closer to conductance band, while the Fermi level of CuPc closer to valence band. The interface interactions and electron filling and depletion of the metal phthalocynaines explain the different STM images of CoPc and CuPc [1].[1]Xing Lu, K. W. Hipps, X. D. Wang and Ursula Mazur, J. Am. Chem. Soc. 118, 7197-7202 (1996)
9:00 PM - H8.7
Photoinduced Charge Transfer in Blends of PbSe Quantum Dots and Semiconducting Polymers.
Kevin Noone 1 , Nick Anderson 1 , David Ginger 1
1 Chemistry, University of Washington, Seattle, Washington, United States
Show AbstractWe use quasi-steady state photoinduced absorption spectroscopy to study photoinduced charge transfer in bulk heterojunction blends of PbSe quantum dots and two conjugated polymers, poly-3-hexylthiophene (P3HT) and poly[2-methoxy-5-(3,7-dimethyloctyloxy)]-1,4-phenylene vinylene (MDMO-PPV). In contrast to blends made of these polymers with PCBM and with CdSe quantum dots, we find no polaron absorption in semiconducting polymer blends with PbSe QDs. The lack of polymer polarons in the presence of PbSe indicates that photoinduced charge transfer is inefficient or non existent in these blends. These results explain the low efficiencies observed in bulk heterojunction blends of PbSe nanocrystals and PPV and P3HT polymers. We explore alternative materials that might yield different results.
9:00 PM - H8.8
Exciton Transport Simulations in Phenyl-Cored Thiophene Dendrimers.
Muhammet Kose 1 , Peter Graf 1 , Kwiseon Kim 1 , Nikos Kopidakis 1 , Garry Rumbles 1 , Sean Shaheen 2
1 , National Renewable Energy Laboratory, Golden, Colorado, United States, 2 , University of Denver, Denver, Colorado, United States
Show AbstractPhenyl cored three-arm and four-arm thiophene dendrimers are promising donor materials for use in photovoltaic devices. It is important to understand the energy transfer mechanisms in these molecules in order to guide the experimental efforts for the synthesis of novel dendrimers with improved photovoltaic activity. A theoretical methodology is developed to estimate the exciton diffusion lengths for these dendrimers and similar chromophores in amorphous films. The theoretical approach exploits Fermi’s Golden Rule to estimate the energy transfer rates for a large ensemble of bimolecular complexes in random relative orientations. Utilization of Poisson’s equation in the evaluation of Coulomb integral led to very efficient calculation of excitonic couplings between the donor and the acceptor transition densities. Electronic coupling calculations with delocalized transition densities revealed efficient coupling pathways in the bulk of relatively large dendrimers, however failed to result in strong couplings between the chromophores which were observed for more localized transition densities in small dendrimers. Planar chromophores are expected to have larger excitonic coupling due to more favorable arrangement of chromophores in space. Monte Carlo simulations were performed to reveal the dynamics of energy transport in these dendrimers. Experimental exciton diffusion lengths of the dendrimers vary between 10 and 20 nm, increasing with the size of the dendrimer. Simulated diffusion lengths correlate well with the experimental data. The chemical structure of the chromophore, the shape of the transition densities and the exciton lifetime are found to be the most important factors which determine the size of the exciton diffusion length in amorphous films of conjugated materials.
9:00 PM - H8.9
Interfacial Electronic Structure Related to [6,6]-Phenyl-C61-Butyric Acid Methyl Ester (PCBM): PCBM/Ag substrate and PCBM/Metal-Phthalocyanine.
Kouki Akaike 1 , Kaname Kanai 2 , Yukio Ouchi 1 , Kazuhiko Seki 1
1 Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya Japan, 2 Research Center for Materials Science, Nagoya University, Nagoya Japan
Show Abstract Fullerene (C60) has been used as the n-type semiconductor for the organic field transistors and solar cells. Recently, the soluble C60-derivatives have been synthesized for the device fabrication using solution-processes. [6,6]-phenyl-C61 butyric acid methyl ester (PCBM) is one of them and often used as the n-type semiconductors for organic solar cells. Previously, we reported that the ionization energy and electron affinity of PCBM are 0.4 - 0.6 eV smaller than those of C60. Since the charge injection, extraction and separation occur at organic/metal and organic/organic interfaces, understanding the electronic structure at interfaces related to PCBM is desired in addition to the information of PCBM itself as described above. However, the interfacial electronic structure related to PCBM has not been studied yet. In this study, we investigated the electronic structure of PCBM/Ag substrate, PCBM/copper phthalocyanine (CuPc) and zinc phthalocyanine (ZnPc) interfaces with ultraviolet photoelectron spectroscopy (UPS), x-ray photoelectron spectroscopy (XPS) and inverse photoemission spectroscopy (IPES), respectively. The UPS and IPES spectra showed the formation of the interfacial states at PCBM/Ag substrate, which can be derived from the hybridization between the lowest unoccupied molecular orbital (LUMO) and d-bands of Ag. The vacuum level (VL) shifts toward high binding energy (BE) with the PCBM molecules charged positively at PCBM/Ag, while the VL shifts toward low BE at C60/Ag interfaces. The C1s and O1s XPS spectra showed that the charge transfer occurs from the oxygen atoms of the side chain of PCBM to Ag, which overcomes the charge transfer from Ag to the carbon atoms of the C60-backbone at the interface, leading the positive charge on PCBM molecule. These findings indicate that the introduction of the substituent affects on the electronic structure of not only molecules and thin film but also interface. At PCBM/CuPc and PCBM/ZnPc interfaces, the valence and core levels of CuPc shift toward low BE while those of PCBM shift toward high BE. The VL shifts toward low BE with increasing the thickness of PCBM. This suggests that the energy level alignment at these interfaces is clearly different from that at the typical pn-junction know in inorganic semiconductors, which means that the depletion layer is not formed at these interfaces. We also found that the ionization energies of PCBM, CuPc and ZnPc change at the interfaces, compared to the bulk region. This is due to the different polarization energy between interface and bulk region. Therefore, we consider that the energy level alignment at organic pn-junctions is achieved by the following two phenomena; (1) the charge transfer from CuPc or ZnPc to PCBM, (2) the changes of the ionization energies of PCBM, CuPc and ZnPc.