Bulletin of the American Physical Society
2007 APS March Meeting
Volume 52, Number 1
Monday–Friday, March 5–9, 2007; Denver, Colorado
Session U25: Organic Field-Effect Transistors |
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Sponsoring Units: DPOLY DMP Chair: Lynn Loo, University of Texas at Austin Room: Colorado Convention Center 203 |
Thursday, March 8, 2007 8:00AM - 8:12AM |
U25.00001: Intrinsic transport anisotropy in single-crystal FETs on new rubrene derivatives A.F. Stassen, W. Kalb, S. Haas, U. Berens, H.J. Kirner, B. Batlogg For the charge transport in a field-effect transistor (FETs), both the spectral density of gap states and the intrinsic mobility play a critical role. The latter is closely related to the arrangement of the molecules in the solid. We present measurements on FETs fabricated on single crystals of new rubrene derivatives. One of them crystallizes in two polymorphic forms: One polymorph shows a very high field-effect mobility ($>$10 cm$^2$/Vs) and a transport anisotropy which can be directly related to the crystal packing. In the second polymorph, the same molecules are arranged in a different structure with minimal $\pi$-overlap. No charge transport could be induced, highlighting the crucial role played by the wave-function overlap associated with the packing. [Preview Abstract] |
Thursday, March 8, 2007 8:12AM - 8:24AM |
U25.00002: RC Transmission Line Characterization of Organic Thin Film Transistors Daniel Lenski, Adrian Southard, Michael S. Fuhrer The transport properties of organic semiconductors are typically measured in a field-effect transistor geometry with DC gate and drain bias. We have developed a novel method of characterizing organic thin films using a 2- or 3-contact transmission line configuration, in which an AC voltage is applied to the thin film and the phase and magnitude of the current are measured. This method can determine both sheet resistance and contact resistance. Additionally, this method may reveal inherent frequency-dependent transport properties of organic semiconductors, which may be significant for future applications of these materials. We present the results of transmission line measurements of pentacene thin films, and compare them to transport data obtained from FET characterization. This work has been supported by the Laboratory for Physical Sciences. [Preview Abstract] |
Thursday, March 8, 2007 8:24AM - 8:36AM |
U25.00003: Eliminating gate bias stress effects in organic field-effect transistors Wolfgang L. Kalb, Thomas Mathis, Simon Haas, Arno F. Stassen, Bertram Batlogg We report on organic field-effect transistors with unprecedented resistance against gate bias stress. The single crystal and thin-film transistors combine small molecule organic semiconductors and an organic gate dielectric with a remarkable electrical breakdown strength. The single crystal devices have no current hysteresis. Extended gate bias stress leads to almost unmeasurable changes in the transfer characteristics: the induced interface state density is of order 10$^{9}$/cm$^{2}$. In contrast, stress-induced trap densities of order 10$^{12}$/cm$^{2 }$ have been identified previously in devices with SiO$_{2}$ or OTS-treated SiO$_{2}$ gate dielectrics. Therefore, adverse gate bias stress effects are not generic to oligocene organic semiconductors, and there is no conceptual limitation for the stability of organic-based transistors in contrast to hydrogenated amorphous silicon. [Preview Abstract] |
Thursday, March 8, 2007 8:36AM - 8:48AM |
U25.00004: Field dependent hole transport mobility studies on a select group of conjugated polymers N. C. Heston, B. Wilson, E. M. Galand, D. B. Tanner, J. R. Reynolds The thin film hole transport properties of a set of cyanovinylene-linked 3,4-propylenedioxythiophene (ProDOT)/dialkoxyphenylene polymers, and regioregular poly(3-hexyl-thiophene), were measured by fabricating hole dominated devices and measuring the room-temperature I-V characteristics. The data were fitted to a model of field-dependent space-charge-limited current. By carrying out both the fabrication and the measurements inside an inert atmosphere we attained significant improvements to the reproducibility of our results. We also found that exposing the devices to heating increased the field dependent hole mobilities of the polymers with values ranging from 5.2 x 10$^{-7}$ (cm$^{2}$/Vs) to 3.1 x 10$^{-6}$ (cm$^{2}$/Vs). [Preview Abstract] |
Thursday, March 8, 2007 8:48AM - 9:00AM |
U25.00005: Electric Field Induced Conductivity of Disorder Driven Anderson Insulator Vladimir Prigodin, Arthur Epstein The effect of an electric field on the Anderson localization is considered. In the 3d case the field mixing the localized and extended states leads to delocalization. In the 2d case, the localization exponentially weakens with increasing kinetic energy so that in principle any field leads also to total delocalization. The field induced IMT occurs in the 1d case [1]. However, the delocalization effect of field on the Anderson insulator hardly is observable because it is masked by phonon assisted hopping. In the localized phase for each localized state along the field there are remote empty states whose energies are lower and electrons hop to those states by emission of phonons. Therefore the electric field induces the crossover in temperature dependence of hopping conductivity from activation (variable range hopping conductivity) to emission (metallic like conductivity) regime. The results have implications for the recent studies of field effects in polymer based transistors [2]. \begin{enumerate} \item V.N. Prigodin, Sov. Phys. JETP\textbf{ 52}, 1185 (1980 \item F.C. Hsu, \textit{et al}., Phys. Rev. B, in press (2006); A.S. Dhoot \textit{et al.}, Phys. Rev. Lett. \textbf{96, }46403(2006). \end{enumerate} [Preview Abstract] |
Thursday, March 8, 2007 9:00AM - 9:12AM |
U25.00006: Device Model for Organic Semiconductor Light-Emitting Field-Effect Transistors Darryl Smith, P. Paul Ruden Recent experiments demonstrate ambipolar channel conduction and light generation in polymer field effect transistors (FETs).$^{1,2}$ In the ambipolar mode of operation, the gate potential lies between those of the source and drain contacts, hence electrons are injected from one of these contacts and holes from the other. The carriers recombine in channel regions where both types of carriers are present, and the location of the resulting light emission is controlled by the voltages applied to the terminals. We describe a device model for ambipolar organic FETs based on the gradual channel approximation. Trapping of injected carriers in localized states within the polymer energy gap is shown to be important. A non-linear differential equation for the channel potential is derived and solved numerically. Carrier density and recombination profiles are determined. The calculations are in good agreement with experimental data$^{1,2}$. \newline \newline 1) J.S. Swensen, C. Soci, and A.J. Heeger, Appl. Phys. Lett. 87, 253511 (2005). \newline 2) J. Zaumseil, R.H. Friend, and H. Sirringhaus, Nature Materials 5, 69 (2006). [Preview Abstract] |
Thursday, March 8, 2007 9:12AM - 9:24AM |
U25.00007: Electrostatic Injection of Very Large 2D Charge Carrier Densities to Obtain Metallic Conductivities in Organic Semiconductors Matthew Panzer, C. Daniel Frisbie The traditional choice of SiO$_{2}$ for the gate dielectric material in organic field-effect transistors (OFETs) limits the amount of charge that one can induce via the field effect due to its relatively weak dielectric strength. In fact, the maximum 2D charge density achievable (near SiO$_{2}$ breakdown, typically $>$100 V applied) is only $\sim $10$^{13}$ charges/cm$^{2}$, while the 2D molecular packing density of many common organic semiconductors is on the order of 5 $\times $ 10$^{14}$ molecules/cm$^{2}$. In order to achieve a higher fraction of charged semiconductor molecules in the 2D OFET channel, a dielectric layer with a higher capacitance is required. We have used a solid polymer electrolyte as an OFET dielectric in order to obtain 2D charge densities exceeding 10$^{14}$ charges/cm2 at operating voltages under 3 V in a variety of organic semiconductors. We have observed metallic conductivity values ($\sim $1000 S/cm) and nearly temperature-independent resistance ratios in poly(3-hexylthiophene) films using a polymer electrolyte-gated OFET. In addition, conductivity maxima at carrier densities approaching 1 charge/molecule were observed in oligomeric, polymeric, and single-crystal organic semiconductors alike. This phenomenon may be caused by carrier correlations or a complete emptying of the semiconductor transport band at very high charge densities. [Preview Abstract] |
Thursday, March 8, 2007 9:24AM - 9:36AM |
U25.00008: Charge mobility of discotic mesophases of hexabenzocoronene derivatives: a multiscale quantum/classical study of the effects of side chain substitution Denis Andrienko, Valentina Marcon, Kurt Kremer, James Kirkpatrick, Jenny Nelson Discotic liquid crystals form columnar phases, where the molecules stack on top of each other and the columns arrange in a regular lattice. The self-organization into stacks results in the one-dimensional charge transport along the columns. Using atomistic molecular dynamics (MD) simulations we study the solid and liquid crystalline columnar discotic phases formed by the alkyl-substituted hexabenzocoronene mesogens. Correlations between the molecular structure, packing, and dynamical properties of these materials are established. Combining Kinetic Monte Carlo and MD trajectories a correlation between the material morphology and charge mobility is then obtained. We are able to reproduce the trends and magnitudes of mobilities as measured by pulse-radiolysis time-resolved microwave conductivity technique. [Preview Abstract] |
Thursday, March 8, 2007 9:36AM - 9:48AM |
U25.00009: The electronic structure and charge carrier dynamics in organic molecular crystals Na Sai, Zhiqiang Li, Vitaly Podzorov, Michael Martin, Michael Gershenson, Dimitri Basov, Massimiliano Di Ventra Organic molecular crystal semiconductors have been receiving intense attention recently. The much higher carrier mobility and intrinsic physical properties uncovered in these materials offer many fundamental advantages over their polycrystalline counterpart. Combined with infrared absorption studies of the rubrene based field effect transistor, we study the intrinsic anisotropy in the basal a-b plane of the rubrene crystal. We report the structure and electron structure of the rubrene crystals using density-functional theory. We find fairly light effective masses of the order of the free electron mass, in agreement with those extracted from infrared measurements. In light of these results, we discuss possible mechanisms of charge transport in these crystals. [Preview Abstract] |
Thursday, March 8, 2007 9:48AM - 10:00AM |
U25.00010: Analysis of the Injection Efficiency Saturation in Polyfluorene Copolymers David Dunlap, Tianjian Lu, Hon Hang Fong, George Malliaras Recent experiments of the transport of holes across a 1 - 5 $\mu$m layers of the copolymer poly [9,9-dioctylfluorenyl-2,7-diyl)- co-(4,4'-(N-(4-sec-butylphenyl))diphenylamine)] (TFB) reveal an injection efficiency that saturates at high voltages, which is indicative of space-charge-limited current. The injection efficiency is on the order of 10$^{-3},$ three orders of magnitude smaller than what would be expected if the current followed the Mott-Gurney law. It is difficult to explain this behavior as arising from trapping phenomena, for time of flight transients show clear plateaus, and the extracted mobility is only weakly dependent on temperature[1]. We propose that the behavior may be accounted for by an interface dipole at the metal/organic injecting contact which is dependent on the applied voltage. Quantitative support to for this mechanism is obtained through numerical simulations of the convection-diffusion equation. \newline [1] H. H. Fong, A. Papadimitratos, and G. G. Malliaras, Appl. Phys. Lett. 89 (2006) 172116 [Preview Abstract] |
Thursday, March 8, 2007 10:00AM - 10:12AM |
U25.00011: High-resolution electrical characterization of polyaniline/p-type organic semiconductor interfaces in thin-film transistors Kwang Seok Lee, Timothy J. Smith, Chris Zangmeister, Joung Eun Yoo, Keith J. Stevenson, Yueh-Lin (Lynn) Loo While the work functions of polyaniline (PANI) and gold are known to be similar (ca. 5eV), we found the electrical properties of PANI/ and gold/p-type organic semiconductor interfaces in dihexylthiophene (DHT-Ant) and pentacene thin-film transistors (TFTs) to be significantly different. Specifically, the current-voltage (IV) characteristics of DHT-Ant TFTs with PANI electrodes show current crowding in the saturation regime. Such current crowding is absent in DHT-Ant TFTs with gold electrodes. Surface potential measurements reveal reduced potential drops at the gold/DHT-Ant interface, relative to the PANI/DHT-Ant interface suggesting limited hole injection into DHT-Ant from PANI. In contrast, pentacene TFTs with PANI electrodes show near-ideal IV characteristics and negligible surface potential drops at the electrode/channel interface, whereas pentacene TFTs with gold electrodes exhibit significant potential drops at the electrode/channel. Hole injection thus appears to be more efficient from PANI than gold in pentacene TFTs, which can be directly correlated with the similar pentacene grain size and the identical fused ring orientation across the PANI/channel interface. [Preview Abstract] |
Thursday, March 8, 2007 10:12AM - 10:24AM |
U25.00012: Polymer LED interfaces studied with resonant soft x-ray reflectivity. Cheng Wang, B. Watts, T. Araki, H. Ade, A. Hexemer, A. Garcia, T.-Q. Nguyen, G.C. Bazan, K.E. Sohn, E.J. Kramer Multilayered polymer structures produced by alternate spin casting from polar and non-polar solvents are promising for polymer light emitting diodes (PLEDs). The structure of the interface between the active layers most likely significantly affects the device efficiency, yet little is known about the interfacial structure and how it affects properties of such devices. Recently, it has been shown that Resonant Soft X-ray Reflectivity (RSoXR) is an excellent tool to study polymeric thin films without the need for deuteration. RSoXR can enhance the sensitivity to a particular interface by using specific photon energies. We have used RSoXR and characterized the interfacial width of bilayers of poly[2-methoxy-5-(2'-ethylhexyloxy)-p-phenylene vinylene] (MEH-PPV) and poly[9,9-bis(6'-N,N,N,-trimethylammoniumhexyl)fluorene-co-alt-1,4 phenylene bromide] (PFNBr), materials relevant to PLED devices. MEH-PPV is a neutral conjugated polymer spun-cast from toluene (non-polar solvent) and PFNBr is a charged conjugated polymer (conjugated polyelectrolyte) spun-cast from methanol (polar solvent). Bilayers nominally 20 nm/80 nm PFNBr/MEH-PPV and 80 nm/20 nm MEH-PPV/PFNBr were investigated and their interfacial widths were determined to be 1.3 and 1.4 nm, respectively. [Preview Abstract] |
Thursday, March 8, 2007 10:24AM - 10:36AM |
U25.00013: Enhanced transport in metallic particle modulated organic field-effect transistors Yu Chen, Masaya Nishioka, Allen Goldman, Yu Xia, Daniel Frisbie Modulated organic field-effect transistors (FET) of rubrene were made by laminating organic single crystals on the top of two-dimension quantum dots arrays. By introducing the single crystal under-layer dots arrays, we were bale to effectively separate organic crystals into mesoscopic-sized grains and reduced the density of trapping sites. Comparing with the conventional organic FETs, these structures show an enhanced mobility at low temperature. This was exhibited as a sharp increase in motilities when temperature decreased below a characteristic temperature. [Preview Abstract] |
Thursday, March 8, 2007 10:36AM - 10:48AM |
U25.00014: Viewing density of states of the contact in organic thin-film transistors Takeo Minari, Kazuhito Tsukagoshi, Tetsuhiko Miyadera, Hiromi Ito, Yoshinobu Aoyagi The large contact resistance is one of the most critical issues in the research and development of organic thin-film transistors (OTFTs). In this study, we developed a method for evaluating the injection process of OTFTs through the effective use of the gate voltage dependence as energetic spectroscopy of charge injection. In this spectroscopy, quasi-Fermi level tuning in contact by scanning the gate voltage provides essential clues to the determination of the energy state of the contact. Based on the concept that the contact resistance is composed of the resistance of a barrier at metal/organic interface and bulk resistance of organic film itself, we attempted to reduce the interface barrier height by inserting a thin doping layer of charge-transfer molecule (CTM) into the metal/organic interface. A suppression of the interfacial barrier height unveiled energetic distribution of the density of states (DOS) localized at the contact. We also found that the bulk transport clearly obeys the Meyer-Neldel rule, according to which the exponential density of states near the band edge limits the charge injection. [Preview Abstract] |
Thursday, March 8, 2007 10:48AM - 11:00AM |
U25.00015: ABSTRACT WITHDRAWN |
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