Julia W. P. Hsu Sandia National Laboratories
Leeor Kronik Weizmann Institute of Science
George G. Malliaras Cornell University
Nobuo Ueno Chiba University
F1: Charge Transport and Injection
Monday PM, November 26, 2007
Back Bay C (Sheraton)
9:30 AM - **F1.1
Polaron Formation and Charge Carrier Trapping in Organic Semiconductor Thin Films.
Xiaoyang Zhu 1 Show Abstract
1 Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States
Polaron formation is central to charge injection and transport in organic semiconductors. The flexibility of the organic molecule, the deformability of the van der Waals bonded lattice, and the narrowness of the electronic bands all favor the self-trapping of charge carriers and the formation of small polarons. This is in addition to the prevalence of structural and chemical defects that form the bulk of charge carrier traps in organic semiconductors. We take two spectroscopic approaches to probe charge carrier trapping and self-trapping in organic semiconductors. The first approach relies on in situ infrared absorption spectroscopy to directly monitor molecular vibrations and electronic transitions associated with charge carriers in gate-doped organic semiconductors. These experiments allow us to establish quantitatively the polaron-bipolaron equilibrium in gate-doped poly(3-hexylthiophene) (P3HT), the electron injection barrier and LUMO density-of-state distribution in N-N’dioctyl-3,4,9,10-perylene tetracarboxylic diimide (PTCDI-C8), and the de-trapping kinetics in these materials. The second approach relies on femtosecond time-resolved two-photon photoemission (TR-2PPE) spectroscopy to follow the formation and decay of small polarons in model systems. We photo-inject an electron from the metal or semimetal contact into the conduction band of a crystalline thin film and record the energy and parallel dispersion of the transient electron en route to localization in real time. These experiments provide unprecedented insight into charge carrier trapping and self-trapping in organic semiconductors.
10:00 AM - F1.2
Interfacial Hole Traps in Conjugated Polymers Thin Films.
Leonid Fradkin 1 , Kwang Jik Lee 1 , Paul Barbara 1 Show Abstract
1 , The University of Texas at Austin, Austin, Texas, United States
We studied the processes of charge injection and discharging of semiconductor conjugated polymer thin film devices using Fluorescence-Voltage Time Resolved (FV-TR) spectroscopy. The poly(9,9-dioctylfluorene-co-benzothiadiazole) (F8BT) was studied in current work. This polymer is in extensive research for its potential applications in Organic Light Emitting Diodes (OLED) and Photovoltaic devices. The structure of the capacitor-like, “holes only” device used in our study consists of the following layers, ITO/PEDOT:PSS/F8BT/SiO2/Al. The photogenerated carriers, originated from excitons dissociation, were found to play a dominant role in the fluorescence quenching mechanism. The evidence of relatively slow discharging rate suggests trapping of the holes in deep trap sites in studied polymer. These deep traps are formed at the interface of the polymer film and hole injection layer, PEDOT:PSS. We show that the presence of oxygen in the device strongly affects device characteristics. Aged samples demonstrate more fluorescence quenching at positive biases than freshly prepared. We believe that the candidate for the photobleached form of the polymer is F8BT+/anion complex. Such complex is formed by photoinduced electron transfer between F8BT molecules and oxygen forming F8BT+/O2-. Both processes of charging and discharging of the device were found to be bias dependant. However, we show that the presence of light (laser excitation) is required for more efficient hole-injection and discharging of the device.
10:15 AM - F1.3
Charge Transport Measurements on Anisotropic Polythiophene Thin Films Fabricated via Directional Crystallization.
Leslie Jimison 1 , Alberto Salleo 1 Show Abstract
1 Materials Science and Engineering, Stanford University, Stanford , California, United States
Recently, there has been considerable interest in the development of semiconducting polymers for use in printable electronic devices, such as transistors for display backplanes. These materials offer a cost effective alternative to conventional semiconductors, with key advantages of organic materials being their low processing temperature and possibility of solution processing in ambient, leading to roll-to-roll printing. Performance over the past few years has improved, but charge transport in these materials is not yet fully understood. These conjugated polymers generally form a semicrystalline microstructure, consisting of lamellar crystalline regions separated by amorphous grain boundaries. A mobility edge model has been proposed that suggests charge delocalization in the crystallite regions where mobile states exist, with the effective mobility limited by the localized states within the disordered grain boundaries and at defects. We have used a means of controlling the orientation and size of crystallites in the plane of the substrate to explore the relationship between trap density within grain boundaries and charge transport. Regioregular poly-3-hexylthiopene (P3HT) is the material under investigation. We have fabricated anisotropic films on glass and silicon substrates by taking advantage of the needle-like growth of 1,3,5-tricholorobenzene (TCB). We use TCB first as a solvent at an high temperature and then as a nucleating agent and substrate for epitaxy once cooled. When TCB is removed from the P3HT/TCB film, an oriented single-component polymer film is left behind, consisting of large (mm2) domains where the film extinguishes uniformly under crossed polarizers, suggesting long range orientation of the polymer chain axis. Characterization with an AFM reveals crystalline lamellae stacked along the fiber direction. Films were further characterized at the Stanford Synchrotron Radiation facility. We have collected 2D diffraction patterns, specular diffraction patterns, and grazing incidence diffraction patterns. Charge transport in the directionally crystallized films was probed by measuring in plane mobilities using thin film transistors with the oriented film as the active layer. Devices were made with different relative orientations between the channel and the polymer film. Transport measurements as a function of charge density and temperature for different orientations of our film were used to explore the effect of microstructure on trap distribution.
10:30 AM - F1.4
Anisotropic Transition in the Transport Mechanism in a Thin Film Phase Standing Pentacene Monolayer Studied by Angle-resolved Photoelectron Spectroscopy.
Toshihiro Shimada 1 Show Abstract
1 Department of Chemistry, University of Tokyo, Tokyo Japan
Understanding the conduction mechanism in organic semiconductors is essential for the materials development for plastic electronics. The role of lattice phonon scattering is particularly important for organic semiconductors because low energy intermolecular vibrations are heavily excited at room temperature. It sets the upper limit of the carrier mobility in highly ordered crystalline devices. We measured the in-plane band dispersion of standing ‘thin film phase” pentacene monolayer prepared by epitaxial growth on α-√3×√3 Bi-Si(111) with a nanoscale morphological template of bunched steps. Using this quasi-single crystalline monolayer film, the band dispersion along Γ-X, Γ-Y and Γ-M directions was obtained at different temperatures. Sinusoidal band dispersion of upper HOMO band observed at 140K was lost in Γ-Y and Γ-M directions while it remained in Γ-X direction at 300K. This result shows that the phonon scattering effect is strongly anisotropic, which provides a new guiding principle for designing high mobility organic semiconductors.
10:45 AM - F1.5
Hopping Mobility of Perfluoropentacene Films Measured with High-resolution UPS: Impacts of Intermolecular Interaction on the Hole-vibration Coupling.
Satoshi Kera 1 , Shunsuke Hosoumi 1 , Shin-ichi Nagamatsu 1 , Nobuo Ueno 1 Show Abstract
1 Graduate School of Advanced Integration Science, Chiba Univ, Chiba Japan
Pentacene (PEN) is currently the most potential conjugated organic molecule as active material in novel electronic devices, such as organic thin-film field effect transistors (OFFT). Therefore, extensive research efforts are being undertaken to investigate its electronic and electrical properties from both of fundamental and applied aspects. A newly synthesized molecule of perfluoropentacene (PFP) was reported to act as n-channel OFET and ambipolar transistors with pentacene was fabricated . When one discusses the hopping charge mobility, both the intramolecular charge reorganization energy (λ), which is related to the electron/hole-phonon coupling, and the transfer integral (t), which is related to the intermolecular electronic interaction, are the key parameters. That is, in principle, larger t and smaller λ are required in order to increase the charge mobility. However, very few detailed discussion on λ and t has been performed so far due to lack of high-resolution ultraviolet photoelectron spectra (UPS) of organic films. Recently, we have succeeded to assess λ and t directly from the fine features in high-resolution UPS of a well-ordered organic monolayer deposited on graphite [2,3]. In this paper, we will report determination of both λ and t with high-resolution UPS of a PFP bilayer on graphite. Furthermore, the electronic structures of PEN(monolayer)/graphite, PFP(monolayer)/graphite and PFP(bilayer)/graphite are compared. The UPS band derived from the highest occupied state (HOMO) for the two monolayer systems (PEN and PFP) clearly shows fine structures, and the band shape at 50 K is significantly different between the two systems due to changes in the vibration energy and the wave function spread of HOMO. The results of the monolayers show that λ of PFP is 2.4 times larger than PEN. Moreover, we observed a considerable difference in the HOMO band shape between the monolayer and bilayer of PFP. The HOMO of the bilayer splits into two prominent peaks with vibration satellites. The splitting could be explained by the bonding and antibonding states due to the strong HOMO-HOMO interorbital interaction . The vibration-satellite intensities in the bilayer are reduced to ca. 80 % of those in the PFP monolayer due to the interorbital interaction. We could estimate the hopping mobility (μ) of a PFP film to be about 10 cm2/Vs at 300 K using the energy separation (2t) of 0.43 eV and λ of 0.184 eV at 300K. Y. Inoue et al, Jpn, J. Appl. Phys. 44, 3663 (2005) and Y. Sakamoto et al, J. Am. Chem. Soc. 126, 8138 (2004). S. Kera et al, Chem. Phys. Lett. 364, 93 (2002) and H. Yamane et al, Phys. Rev. B 72, 153412 (2005).  S. Kera et al, Phys. Rev. B 75, 121305R (20
11:30 AM - **F1.6
Experimental Manifestation of Excess Electron Delocalization in Oligoacene Molecular Aggregates.
Atsushi Nakajima 1 2 , Masaaki Mitsui 1 Show Abstract
1 Department of Chemistry, Keio University, Yokohama Japan, 2 , JST-CREST, Yokohama Japan
Organic molecular clusters, consisting of 2-1000 molecules, are finite aggregates, providing a microscopic model to investigate the chemical and physical properties of molecular assemblies in organic nanomaterials or at interface. A unique nature of organic molecular clusters is originated from weak intermolecular van der Waals interactions, and thus organic molecules in a cluster fairly preserve their molecular identity. This feature results in narrow electronic bandwidths, and electron (or hole) charge carriers in organic molecular aggregates are often strongly localized on individual molecules, where molecular ions are formed. Such a molecular ion is instantaneously stabilized by the electronic polarization of its neutral neighbors, which significantly affects charge-transport energy levels in organic molecular aggregates. An intriguing subject of research on organic molecular clusters is to explore the evolution of the structural, electronic, thermodynamic, and chemical properties from molecular to condensed matter systems in a step-by-step manner. Since the number of constituent molecules can be tuned accurately in combination with mass spectrometry, size-selective investigations on a broad size range of organic molecular clusters can provide a deep understanding on the evolution of the electronic and geometric properties. In particular, oligoacenes such as tetracene and pentacene are classical examples for the study of charge carrier localization and transport. We have recently realized an efficient formation of large oligoacene molecular nanoclusters up to more than 200 constituent molecules; naphthalene (Nph), anthracene (Ac), tetracene (Tc), and so on. By adding a single excess electron, the corresponding nanocluster anions were produced in the gas-phase, and the size-selective properties could be revealed by photoelectron spectroscopy. In particular, two types of anion states are shown to coexist in nanometer-scale cluster anions of oligoacene of Nph, Ac, and Tc. The photoelectron spectra of size-selected cluster anions containing 2 to 100 molecules revealed that rigid “crystal-like” cluster anions emerge, greater than ~2 nanometers in size, and coexist with the “disordered” cluster anion in which the surrounding neutral molecules are reorganizing around the charge core. These two anion states appear to be correlated to negative polaronic states formed in the corresponding crystals. M. Mitsui, N. Ando, A. Nakajima, K. Kaya, et al., J. Chem. Phys. 121, 7553 (2004). M. Mitsui, A. Nakajima, K. Kaya, U. Even, J. Chem. Phys. 115, 5707 (2001). N. Ando, S. Kokubo, M. Mitsui, A. Nakajima, Chem. Phys. Lett. 389, 279 (2004). M. Mitsui, A. Nakajima, Bull. Chem. Soc. Jpn. (2007) in press M. Mitsui, N. Ando, A. Nakajima, submitted.
12:00 PM - **F1.7
Field Effect Transistors using Charge-transfer-complex Layer.
Kazuhiro Kudo 1 , Masatoshi Sakai 1 , Masakazu Nakamura 1 Show Abstract
1 Electrical and Electronics Eng., Chiba University, Chiba Japan
Charge transfer (CT) complexes are formed by partial transfer of electric charge from donor (D) molecules to acceptor (A) molecules. Several CT complexes show higher conductivity and metal-insulator transition (Mott transition). These properties attract much attention in the field of organic electronics. The conductivity of a CT complex film can be controlled if the degree of charge transfer is varied by applied external field such as electric field . We have fabricated new-type field-effect transistors (FETs) using CT complex layers consisting of several donor and acceptor molecules and have investigated the field-effect characteristics. The FET characteristics strongly depend on the molecular species and the condition of the film formation. The results indicate that the p- and n-type conduction, and the operational mode can be controlled by choosing molecular species and fabrication conditions. The CT complex wires are obtained by utilizing the electric-field assisted deposition method[2,3]. We have investigated the organic conductive wire and nano-transistor using co-evaporation method of tetrathiafulvalene (TTF) and tetracyanoquinodimethane (TCNQ) under the electric field. Needle-like crystals of TTF-TCNQ CT complex grow from the edge of source and drain electrodes along the electric field. Electric conductivity of the grown crystal changes by applying gate voltage. The current on/off ratio of the nano-transistor is 1/500. It is suggested that a tiny semiconductor area is formed at the connecting point of two conductive molecular wires. T.Sumimoto, Y.Shiratori, M.Iizuka, S.Kuniyoshi, K.Kudo and K.Tanaka, Synthetic Metals, 86, 2259(1997) N.A.Kato, M.Fujimura, S.Kuniyoshi, K.Kudo, M.Hara and K.Tanaka, Appl. Surf. Sci., 130, 658(1998) M.Sakai, M.Nakamura and K.Kudo, Appl.Phys.Lett., 90, 62101(2007)
12:30 PM - F1.8
Modeling of Charge Transport in an Hybrid Metal / Organic / Inorganic Device.
Henry Mendez 1 , Ilja Thurzo 2 , Dietrich Zahn 2 Show Abstract
1 Physics, Pontificia Universidad Javeriana, Bogota, Cundinamarca, Colombia, 2 Physics, Chemnitz University of Technology, Chemnitz, Sachsen, Germany
An hybrid Metal / organic / inorganic semiconductor heterostructure was built under ultrahigh vacuum conditions (UHV) and characterised in situ. The aim was to investigate the influence of thin film layers of the organic material Dimethyl–3,4,9,10–perylenetetracarboxylic diimide (DiMe–PTCDI) on the electrical response of organic–modified Ag / GaAs Schottky diodes. The device was tested by combining photoemission spectroscopy (PES) and electrical measurements (current–voltage I–V, and charge transient spectroscopy QTS). The energy level alignment through the heterostructure was deduced. This allows to consider electrons acting as majority carriers injected over a barrier by thermionic emission as a primary event in the charge transport. QTS measurements performed on the heterostructure showed the presence of two relaxations induced by deposition of the organic layer. The first one is attributed to a deep trap at the metal / organic interface, while the second one has very small activation energy (~ 20 meV) which is probably due to disorder at the organic film. With such information a fit of the I–V characteristics of DiMe–PTCDI organic modified diodes based on the analytical expressions of a trapped charge limited current regime (TCLC) was intended. The results of calculations performed with this model allowed a good agreement with the experimental data for organic films up to 60 nm thickness.
12:45 PM - F1.9
Interfacial Structure Modifying Interlayers: Impact of LiF Deposition on Highly Ordered Semiconducting Organic Thin Films.
Ayse Turak 1 , E. Barrena 1 , H. Dosch 1 Show Abstract
1 , Max Planck Institute for Metals Research, Stuttgart, Baden-Wuerttemberg, Germany
Though LiF is widely used in organic electronic devices, the effect of LiF deposition on the structural order of the underlying films is not well understood. With the move toward increasingly ordered films for small molecule organic photovoltaic devices, the interfacial structure at the cathode/organic interface has a critical impact on the photocurrent extraction efficiency. Using high resolution x-ray reflectivity, grazing incidence diffraction and atomic force microscopy, we examined the impact of LiF deposition on highly ordered multilayers of p-type planar molecule diindenoperylene (DIP). At thicknesses of LiF common to optoelectronic devices, incomplete coverage of the surface by LiF islands preserves the in-plane structure of the films. However, even very small amounts of LiF are sufficient to modify the electronic density of the DIP layers. These structural effects have significant implications for film conductivity and subsequent device performance.
F2: Organic Thin Film Transistors
Monday PM, November 26, 2007
Back Bay C (Sheraton)
2:30 PM - **F2.1
Organic Electronics: Towards the Realization of an Ideal Device.
Christof Woell 1 Show Abstract
1 Phys. Chemistry I, Univ. Bochum, Bochum Germany
After light emitting diodes made form organic materials (OLEDs) have already reached the market also organic field effect transistors (OFETs) using organic molecules as active semiconductors are reaching the stage where products come into sight, e.g. in “smart” identification tags. For transistors based on oligomers (rather than polymers) organic molecular beam deposition (OMBD) is a key requisite for the defined manufacturing of working devices. Since correlations of organic thin film structural quality with electrical properties have revealed that defects in the organic material severely limit the mobility of the charge carriers, a key parameter for the switching-speed of an OFET, the fabrication of high-performance devices will require the ability to grow highly ordered, epitaxial thin films on appropriate substrates. True epitaxy yielding structural qualities comparable to that of single crystals, however, has been realized only in a few cases . The reasons for these severe difficulties, including the effect of molecular conformations , will be discussed in this talk. Recent work has demonstrated that one of the most severe limitations results from pronounced dewetting [3,4].The last point to be presented in the talk is the fabrication of an “ideal” organic electronic device, where true intrinsic properties governing carrier injection and transport can be studied in the absence of any structural defects. The current-voltage characteristics of this device reveal a pronounced asymmetry. For negative polarity, the current characteristics is almost independent of the layerthickness. For positive polarity, the current onset is shifted significantly to larger voltages withincreasing layer thickness. Numerical simulations for a two-dimensional model system allow toidentify as reasons for this pronounced asymmetry the injection properties of the STM tip. Fornegative substrate bias the creation of holes in the valence band occurs by tunneling of electrons to the tip whereas in the opposite case holes have to be transported through the OSC-layer from the substrate. Thus, for low positive voltage the hole current limits the device current. Once the resulting voltage drop between layer and tip becomes larger than the barrier for electron injection, direct tunneling of electrons into the pentacene conduction band becomes possible and n-conduction begins to dominate . Reasons for the presence of n-conduction in pentacene, an organic semiconductor which normally only shows p-conductivity, will be discussed in the talk.References G. Witte and Ch. Wöll, J. Mater. Res., 19, 1889, (2004) D. Käfer, L.Ruppel, G. Witte, Ch. Wöll, Phys. Rev. Lett., 95, 166602, (2005) G. Beernink, T. Strunskus, G. Witte, Ch. Wöll, Appl. Phys. Lett., 85, 398, (2004) S. Lukas, S. Söhnchen, G. Witte, Ch. Wöll, Chem. Phys. Chem, 5, 266 (2004) L. Ruppel, A. Birkner, G. Witte, C. Busse, T. Lindner, G. Paasch, Ch. Wöll, submitted for publication
3:00 PM - F2.2
Surface Potential Imaging of Grain Boundary Effects in Solution Processable Acene-Based Thin Film Transistors.
Lucile Teague 1 , Behrang Hamadani 2 , Sankar Subramanian 3 , John Anthony 3 , David Gundlach 2 , James Kushmerick 2 Show Abstract
1 , Savannah River National Laboratory, Aiken, South Dakota, United States, 2 , National Institute of Standards and Technology, Gaithersburg, Maryland, United States, 3 Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States
The need to significantly reduce manufacturing costs is a driving force for expanding the applicability of active organic thin film electronic devices. With that, much attention has been directed toward the use of low cost, solution processable methods for creating organic thin film transistors (OTFTs). While significant improvements have been made in the electrical performance of OTFTs over recent years, surprisingly little is known about the fundamental mechanisms governing charge injection and transport in the transistor channel. Recent studies of soluble difluoro bis(triethylsilylethynyl) anthradithiophene (diF-TESADT) OTFTs have shown that chemically tailoring the source/drain contacts promotes the growth of highly ordered regions along opposing contact edges which extend into the transistor channel.1 We use scanning Kelvin probe microscopy (SKPM) to simultaneously probe the unique film microstructure and the potential distribution in these acene-based OTFTs. Our SKPM studies of electrically active devices explicitly show a correlation between organic thin film microstructure and large potential drops within the device channel. 2 This data suggests that charge transport is limited by the structure and interaction of the diF-TESADT grains within the active portion of the device rather than by the electrical contacts between the organic semiconductor and the source and drain electrodes.1Gundlach, D. J.; Royer, J. E.; Hamadani, B. H.; Teague, L. C.; Moad, A.; Jurchescu, O. D.; Kirillov, O.; Richter, C. A.; Kushmerick, J. G.; Richter, L. J.; Park, S.; Jackson, T. N.; Subramanian, S.; Anthony, J. E. 2007, submitted. 2 Teague, L. C.; Hamadani, B. H.,; Subramanian, S.; Anthony, J. E.; Gundlach, D. J.; Kushmerick, J. G., 2007, submitted.
3:15 PM - F2.3
Looking into the Interface Between Organic Semiconductor/organic Dielectric Interface in Pentacene Thin-film Transistors: Trapping Mechanisms Characterization, Modeling and Correlation with Transistor Performances.
Emanuele Orgiu 1 2 , Mohammad Taki 1 , Simone Locci 1 2 , Beatrice Fraboni 3 , Annalisa Bonfiglio 1 2 Show Abstract
1 DIEE, department of Electrical and Electronic engineering, University of Cagliari, Cagliari Italy, 2 Centre S3 nanoStructures and bioSystems at Surfaces, INFM, Modena Italy, 3 Dipartimento di Fisica, University of Bologna, Bologna Italy
The interface between an organic semiconductor/dielectric pair plays a decisive role in the functional performance of the Organic Thin-Film Transistors (OTFTs). Since it is well-known that the charge carriers flow through the channel in the first few nanometers of the channel, namely at the interface between the semiconducting and the insulating layer, a better understanding of which phenomena are involved in the conduction mechanism is due. Designed interface that promotes synergistic interactions between the semiconductor and dielectric is essential in achieving optimum FET performances.Surface properties, i.e. surface energy and surface roughness, of a dielectric layer represent distinctive factors which determine potential improvements in electric characteristics of OTFTs because they strongly affect the pentacene growth. In fact the carrier transport efficiency is increased through the presence of well-ordered crystalline structure and/or the large grain size. On the other hand comparative studies of pentacene grown both on polyvinyl alcohol (PVA) and polyvinylphenol (PVP) have shown that although the well-structured morphology of the pentacene film on PVP should result in better OTFTs performances in terms of mobility of the carriers the performances of OTFTs with a pentacene film grown on PVA were better than the former. In this work we aim at giving a clearer picture of the charge trapping, charge transport and the defects at the interface by means of either Capacitance-Voltage measurements with varying the frequency on Metal-Insulator-Semiconductor (MIS) structures and Ids-Vds/Ids-Vgs characterization on OTFTs devices. Different organic dielectrics have been tested as an insulating layer for the above-mentioned structures, namely PVA, PVP, polydimethylsiloxane (PDMS) and polymethylsilsesquioxane (pMSSQ). The semiconductor employed in all these structures was pentacene that exhibits a pretty higher mobility compared to many organic semiconductors. Variation of the OTFTs performances in terms of mobility, threshold voltage and contact resistance will be shown with only varying the dielectric. Pentacene films have been grown simoultaneously both on MIS and OTFTs structures and in the same environmental conditions (temperature, evaporation rate, light). By doing this we are sure that the only parameter ranging in the experiments is the interface between organic semiconductor and dielectric.Correlation between dielectric and field effect mobility will be provided together with modeling of the classic equation of the transistor that is often misused since it does take into account only the thickness and the permittivity constant of the dielectric. Dielectric/organic semiconductor interface properties rather affect both the mobility of the carriers that flow through the channel and the contact resistance as it can be shown by our investigations. The role of the dielectric in the physics of OTFTs saturation regime will be covered as well.
3:30 PM - F2.4
Current Injection in Top Contact Pentacene Thin Film Transistors with Copper Electrodes.
Sui-Dong Wang 1 2 , Kazuhito Tsukagoshi 1 2 , Takeo Minari 1 2 , Tetsuhiko Miyadera 1 2 , Yoshinobu Aoyagi 1 2 Show Abstract
1 , The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama Japan, 2 , CREST, JST, Honcho, Kawaguchi, Saitama Japan
The performance of organic thin film transistors (OTFTs) have been improved in the last decade, however, large contact resistance (RC) of OTFTs is one of the main problems for practical applications. Gold (Au) is now commonly used as source and drain electrodes in OTFTs due to its large work function. Other metals with lower cost, for example copper (Cu), are highly desirable in manufacturing. We found that copper (Cu) electrodes can produce lower RC in top contact pentacene TFTs than Au, though Cu is known to have smaller work function. The transfer line method (TLM)  was employed to derive channel mobility and RC in the pentacene TFTs. We define the local charge mobility in the contact region as contact mobility (μC), which can be simply calculated from RC. From the temperature dependence of μC, μC for both Cu and Au appear thermally activated with activation energies (EA) around several tens meV, and follow the Meyer-Neldel relation . Furthermore, the Meyer-Neldel energy (EMN) in the Cu contacts is much lower than that in the Au contacts, implying a narrower trap states distribution in the case of Cu. On the other hand, Cu contacts show relatively weak gate-bias (VGS) dependence of RC than Au, and the VGS dependence decrease with increasing temperature. These features can be well explained by the low EMN for the Cu contacts. We argue that RC in OTFTs is not dominated by the interfacial Schottky barrier, but by the access resistance to the channel. The reasons are that (1) If RC is dominated by Schottky barrier, the current should be limited by injection barrier and flow by either tunneling or thermal excitation processes across the barrier. The tunneling process is temperature independent, the thermal activation behavior of μC rule out this possibility. (2) The EA of μC are much smaller than the reported Schottky barrier height for pentacene/Au interface, and the Cu contacts show higher μC in spite of an expected higher hole injection barrier. Therefore, the thermal excitation process across injection barrier cannot interpret the results. (3) Both the tunneling and thermal excitation processes predict strong electric field dependence of the injection current, however, the derived electric field at the contacts are almost unchanged and decreased with the drain current for Cu and Au, respectively. This confirms our argument. The experimental results suggest that the conduction through the contact region in OTFTs is limited by exponentially distributed trap states, the effects of the trap distribution will be discussed in the presentation. T. Minari et al, Appl. Phys. Lett. 88 (2006) 083514. T. Minari et al, Appl. Phys. Lett. accepted.
3:45 PM - F2.5
Frequency Response Analysis of Pentacene Thin Film Transistors and the Effect of Contact.
Tetsuhiko Miyadera 1 2 , Takeo Mnari 1 2 , Sui-Dong Wang 1 2 , Kazuhito Tsukagoshi 1 2 , Hiromi Ito 1 , Yoshinobu Aoyagi 1 2 3 Show Abstract
1 , RIKEN, Wako, Saitama, Japan, 2 , JST-CREST, Kawaguchi, Saitama, Japan, 3 , Tokyo Inst. of Tec., Nagatsuda, Kanagawa, Japan
Frequency response analysis is one of the most significant issues of organic thin film transistors (OTFTs). However, only a few papers have been reported on this subject. In this report, we present frequency response analysis of pentacene OTFTs and metal - insulator - semiconductor (MIS) capacitors fabricated on Si/SiO2 substrates. Based on a simple CR equivalent circuit, the cutoff frequency (fc) can be expressed as fc ~ VgμL-2. Based on the model, channel length reduction and the increase of mobility improve the fc. However, the parasitic impedance distributed at various interfaces in an OTFT limits the response speed of the practical OTFT. In this work, the contact interface was found to limit the ac-characteristics of the OTFT. MIS analysis revealed the existence of parasitic impedance at the contact. The model for current transport at the contact was proposed. The parasitic impedance was analyzed quantitatively by means of Cole-Cole plot. The equivalent circuit model consists of charge injection barrier (Ci, Ri) and charge transport obstacle underneath the contact (Cb, Rb) well reproduced the measurement results. The parasitic impedance was effectively suppressed by local doping of F4TCNQ inserted at the interface between electrode and pentacene thin film. The ac-characteristics of OTFT were measured with different contact condition. The doped contact OTFT with short channel length (L = 100 um) reached a higher fc, up to 98 kHz, compared with the non-doped contact OTFT. The F4TCNQ doping effectively improves the ac-characteristics. The equivalent circuit model of an OTFT with parasitic impedance was proposed (T. Miyadera et. al., Appl. Phys. Lett., in press). The model well agrees with the frequency response results and the effect of the parasitic impedance at the contact was analyzed quantitatively. From the ac-response measurement and model analysis, the suppression of parasitic impedance at the contact by F4TCNQ doping was found to improve the fc. These results indicated that the F4TCNQ doping for short channel length OTFTs is a significant approach to achieve high-speed operation of OTFTs.
4:30 PM - F2.6
Using Self-assembled Monolayer to Improve the Charge Injection in Field Effect Transistors.
Saghar Khodabakhsh 1 2 , Ruth Rawcliffe 3 , Steve Francis 4 , Neville Richardson 4 , Alasdair Campbell 3 , Tim Jones 2 Show Abstract
1 Chemistry, University of Cambridge, Cambridge United Kingdom, 2 Chemistry, Imperial College London, Londn United Kingdom, 3 Physics, Imperial College London, Londn United Kingdom, 4 Chemistry, University of St. Andrews, St. Andrews United Kingdom
Self-assembled molecules containing aromatic molecules are of great interest because of their potential application in optoelectronic devices. Self-assembly of small aromatic thiols on gold substrates is studied using scanning tunnelling microscopy (STM), polarisation modulation reflection absorption infra-red spectroscopy (PM-IRRAS) and Goniometry is studied in this work. Aromatic thiols are of greater interest because of their conjugated nature as opposed to conventional aliphatic thiols which exhibit insulating properties due to them comprising long carbon chains. However, two aliphatic thiols were also studied in order to compare the insulating properties. The aromatic molecules are all thiophenol derivatives, with different Para substitutions, chlorine (CTP), fluorine (FTP), and methoxy (MOTP). 2,2,2-trifluoro ethne thiol (TFET) and 1h1h2h2h-perfluoro-1-decanethiol (MAX) were also studied as short and long aliphatic thiols, respectively. Direct evidence for chemisorption of thiols on Au substrate was obtained using PM-IRRAS. It was found that assembling a monolayer of aromatic thiols onto gold, results in formation of gold islands, which agrees well with literature. The formation of Au islands is attributed to the increased mobility of gold atoms as result of pi-pi interactions within the monolayer. Assembling a monolayer of molecules with large dipole moments result in creation of an electric field just outside the Au substrate which ultimately result in dramatic increase in the work function of Au. Au substrates coated with SAMs were then used as an electrode in polymer field effect transistors. We also demonstrated that the field-effect mobility is limited by injection and contact resistance originating from a work function mismatch between the metal and organic layer. This can be reduced or increased by modifying the metallic electrode using thiols to induce a dipole at the interface. The magnitude and direction of this dipole directly affects values derived for the contact resistance and mobility. Aromatic thiols performed best, as the short, conjugated spacer groups did not significantly inhibit injection, as compared to alkyl spacer groups. This is certainly a very useful means to combat the problem of non-ohmic injection in transistor structures, which is becoming more commonplace as air stable polymers tend to have ionisation potentials higher than gold.
4:45 PM - F2.7
Investigation of Carrier Modulation at the Semiconductor Channel in Organic Ferroelectric Field Effect Transistor.
Takuya Honjo 1 , Kei Noda 1 , Shuichiro Kuwajima 1 , Kenji Ishida 2 , Hirofumi Yamada 1 , Kazumi Matsushige 1 Show Abstract
1 Electric Science & Engineering, Kyoto University, Kyoto City Japan, 2 Chemical Science & Engineering, Kobe University, Kobe City Japan
Recently, much attention has been paid to a memory element based on ferroelectric field effect transistor (FeFET) because of its remarkable features such as non-volatile data retention and non-destructive-read-out. Operating principle of FeFET is based on modulation of the surface potential in a semiconducting channel by ferroelectric polarization, which is in intimate contact with the semiconducting layer. Therefore the transfer characteristics of FeFET are greatly affected by the electronic properties at the interface. In fact, inorganic FeFETs which require high temperature fabrication process have several problems such as charge trapping at the ferroelectric/semiconductor interface, due to the formation of silicon oxide. In this point, using organic ferroelectrics and semiconductors has an advantage since it is easy to fabricate intimate ferroelectric/semiconductor interfaces.In this study, we demonstrated organic FeFET consisting of ferroelectric molecule, vinylidene fluoride (VDF) oligomer, and studied mechanisms of carrier modulation at the interface between organic ferroelectrics and semiconductors caused by polarization switching of VDF oligomer. First, in order to clarify the fundamental correlation between ferroelectric dipole moments and carrier behavior in organic semiconductor, we investigated electrical properties of metal/VDF oligomer/pentacene/metal (MFSM) capacitor. Carrier modulation in organic semiconductor by polarization reversal of the ferroelectric layer was confirmed by the I-V measurement of the MFSM capacitor. Secondly, FeFET was fabricated by depositing VDF oligomer thin film as top gate insulator on pentacene OFET with bottom-contact structures. A clear hysteresis behavior was observed in Ids-Vgs characteristics and the on/off ratio larger than 20 was obtained at Vgs=0V. The correlation of the hysteresis behavior and the ferroelectric dipole switching will be discussed in detail.
5:00 PM - F2.8
Controlling the Characteristics of Organic Thin Film Transistors through Interface Modifications.
Peter Pacher 1 , Barbara Stadlober 2 , Alexandra Lex 3 , Veronika Proschek 1 , Ursula Haas 2 , Herbert Goled 2 , Anja Haase 2 , Norbert Koch 4 , Stephan Rentenberger 1 , Gregor Trimmel 3 , Christian Slugovc 3 , Egbert Zojer 1 Show Abstract
1 Institute of Solid State Physics, Graz University of Technology, Graz Austria, 2 Institute of Nanostructured Materials and Photonics, Joanneum Research, Weiz Austria, 3 Institute for Chemistry and Technology of Organic Materials, Graz University of Technology, Graz Austria, 4 Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany
In this contribution we discuss two conceptionally different approaches to controlling the characteristics of organic thin film transistors (OTFTs) by modifying the properties of the interface bet