Bulletin of the American Physical Society
APS March Meeting 2014
Volume 59, Number 1
Monday–Friday, March 3–7, 2014; Denver, Colorado
Session Q20: Focus Session: Organic Electronics and Photonics - Charge Transport |
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Sponsoring Units: DMP DPOLY Chair: Brian Collins, National Institute of Standards and Technology Room: 405 |
Wednesday, March 5, 2014 2:30PM - 2:42PM |
Q20.00001: Role of fluctuations on electron transport in soft materials Enrique Gomez Soft materials are characterized by weak intermolecular interactions and disorder. Although the extent of spatial fluctuations depends on the molecular structure, lattice fluctuations can be on the order of the unit cell dimensions in molecular crystals and soft materials. For example, our results from Quasi-elastic Neutron Scattering demonstrates that longer side chains in poly(3-alkylthiophene)s leads to an enhancement in motion of the thiophene rings. To ascertain the effect of dynamics on charge transport, we have developed a simple model to describe the roles of longitudinal and transverse modes of the intermolecular spacing between molecules on intrinsic electron mobilities. We demonstrate that the intrinsic mobility of soft materials appears thermally-activated by assuming fluctuations in a harmonic potential and an exponential decay in the charge transfer rate with intermolecular distance. For example, for poly(3-hexylthiophene), we can extract the characteristic decay as a function of separation distance from Density Functional Theory calculations and the extent of fluctuations from Molecular Dynamics simulations to predict the temperature dependence of the charge mobility. We find that the temperature dependence appears Arrhenius with activation energy of approximately 50 meV for a wide temperature range, in good agreement with experiments. This model suggests that fluctuations in the lattice spacing of soft materials lead to a significant intrinsic dependence of electron transport on temperature, regardless of the presence of band-tail states or traps. [Preview Abstract] |
Wednesday, March 5, 2014 2:42PM - 2:54PM |
Q20.00002: Temperature activated transport tuned by libration in the charge-transfer salt trans-stilbene -- 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (STB-F$_{4}$TCNQ) Katelyn P. Goetz, Derek Vermeulen, Margaret E. Payne, Jiang Hui, Hu Peng, Cynthia S. Day, Christian Kloc, Veaceslav Coropceanu, Laurie E. McNeil, Oana D. Jurchescu A common route to solubility in organic semiconductors is chemical functionalization. This adds librational modes to the molecules, which was theoretically predicted to impact charge transport. We discuss the effect of libration on charge transport in the charge-transfer complex trans-stilbene--2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (STB-F$_{4}$TCNQ). This material has a 300 K mobility of 0.3 cm$^{2}$V$^{-1}$s$^{-1}$ that decreases with an activation energy of 170 meV to 235 K, where it transitions to temperature independence. X-ray diffraction indicates that the cause of this is the freezing of the libration of the ethylene moiety within STB below 235 K. Above 235 K, it increases in amplitude with increasing temperature. Fourier difference maps suggest that the charge density unaccounted for by the STB and F$_{4}$TCNQ molecules is localized on the molecules at low temperature and more delocalized after the transition. This agrees with XRD and Raman spectroscopy estimates for the degree of the ground state donor to acceptor charge transfer, indicating zero transfer below and 0.1 electrons above the transition temperature, highlighting the strong coupling between molecular motion and charge transport. [Preview Abstract] |
Wednesday, March 5, 2014 2:54PM - 3:06PM |
Q20.00003: Incorporating Decoherence in the Dynamic Disorder Model of Organic Semiconductors Wei Si, Yao Yao, Chang-Qin Wu The transport phenomena in crystalline organic semiconductors, such as pentacene, have drawn much attention recently, where the electron-phonon interaction plays a crucial role. An important advance is the dynamic disorder model proposed by Troisi \textit{et. al.}, which is successful in determining the carrier mobility and explaining the optical conductivity measurements. In this work, we aim to incorporate the decoherence effects in the dynamic disorder model, which is essential for the self-consistent description of the carrier dynamics. The method is based on the energy-based decoherence correction widely used in the surface hopping algorithm. The resulting dynamics shows a diffusion process of wave packets with finite localization length, which scales with the decoherence time. In addition, the calculated mobility decreases with increasing temperature. Thus the method could describe a band-like transport based on localized states, which is the type of transport anticipated in these materials. [Preview Abstract] |
Wednesday, March 5, 2014 3:06PM - 3:18PM |
Q20.00004: Two extreme limits of carrier injection in organic semiconductor FETs Thangavel Kanagasekaran, Hidekazu Shimotani, Yoichi Tanabe, Satoshi Heguri, Katsumi Tanigaki The metal-semiconductor (MS) contact between a metal electrode and an organic semiconductor is generally in the Schottky limit, and the barrier height against carrier injection from the electrode is greatly dependent on the work function (?m) of the electrode. Consequently, air-unstable metals with low ?m's such as Ca are necessary for electron injection. Here, we report that the Schottky limit can be converted to the Bardeen limit and the carrier injection barrier height can become independent of the electrode work function. This is exemplified using tetratetracontane as a surface modification layer on an SiO2 dielectric gate insulator and the unambiguous evidences are given. Based on this finding we demonstrate an air-stable light-emitting organic field-effect transistor using Au electrodes for both hole and electron injection. A light emmitting FET stable in air is demonstrated using Au-Au electrodes. [Preview Abstract] |
Wednesday, March 5, 2014 3:18PM - 3:30PM |
Q20.00005: Gate Voltage Dependent Resistance across Interspherulite Boundaries in Solution-Processed Organic Semiconductor Thin Films Anna Hailey, Marcia Payne, John Anthony, Yueh-Lin Loo Grain boundaries formed by impinging spherulites in solution-processed organic semiconductor thin films limit charge transport in organic field-effect transistors that comprise these polycrystalline active layers. Thin films of triethylsilylethynyl anthradithiophene (TES ADT) exhibit limited order upon spin-coating; subsequent exposure to 1,2-dichloroethane vapor induces growth of TES ADT spherulites, with a mixture of low-angle (LA) and high-angle (HA) interspherulite boundaries (ISBs) defining neighboring spherulites. Our ability to control the directionality of TES ADT growth allows us to prescribe the formation of ISBs, forming exclusively LA [0$\pm$20$^{\circ}$] and HA ISBs [90$\pm$20$^{\circ}$] over macroscopic distances. Gated four-point probe transistor measurements allow us to quantify differences in resistance within spherulites and across these LA and HA ISBs as the devices are switched from ``off'' to ``on'' states. Surprisingly, for devices in the ``on'' state, the gate-independent resistances across LA [6$\pm$3M$\Omega$] and HA ISBs [16$\pm$6M$\Omega$] remain quantitatively different from that within an individual spherulite [1.9$\pm$0.7M$\Omega$], suggesting that the angle of mismatch of the ISB continues to affect charge transport, even after all traps are filled. [Preview Abstract] |
Wednesday, March 5, 2014 3:30PM - 3:42PM |
Q20.00006: Charge Energy Transport in Hopping Systems with Rapidly Decreasing Density of States Dan Mendels An accurate description of the carrier hopping topology in the energy domain of hopping systems incorporating a rapidly decreasing density of states and the subsequent energetic position of these systems' so called effective conduction band is crucial for rationalizing and quantifying these systems' thermo-electric properties, doping related phenomena and carrier gradient effects such as the emergence of the General Einstein Relation under degenerate conditions. Additionally, as will be shown, the 'mobile' carriers propagating through the system can have excess energies reaching 0.3eV above the system quasi-Fermi energy. Hence, since these mobile carriers are most prone to reach systems interfaces and interact with oppositely charged carriers, their excess energy should be considered in determining the efficiencies of energy dependent processes such as carrier recombination and exciton dissociation. In light of the stated motivations, a comprehensive numerical and analytical study of the topology of hopping in the energetic density of such systems (i.e. the statistics regarding which energy values carriers visit most and in what manner) was implemented and the main statistical features of the hopping process that determine the position in energy of the system's effective conduction band were distilled. The obtained results also help shed light on yet to be elucidated discrepancies between predictions given by the widely employed transport energy concept and Monte Carlo simulations. [Preview Abstract] |
Wednesday, March 5, 2014 3:42PM - 3:54PM |
Q20.00007: Field-induced low temperature transport in polythiophene thin films Evan Kang, Eunseong Kim Low temperature charge transport in poly(2,5-bis(3-tetradecylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT) field-effect transistors (FETs) was systematically investigated. The temperature dependent transport behavior was studied by varying drain-source electric field and gate bias. Thermally-assisted hopping is dominant at high temperatures. At low temperatures, the temperature dependence becomes weaker and tunneling becomes the prevailing transport mechanism. Under high drain-source electric field, the additional field-driven current leads to the non-ohmic current-voltage relations. The results will be discussed with previously suggested models, such as Poole-Frenkel-type hopping [1], Efros-Shklovskii hopping [2], multistep tunneling [3], and field emission [1, 4]. [1] J. H. Worne, J. E. Anthony, and D. Natelson, Appl. Phys. Lett. 96, 053308 (2010) [2] A. S. Dhoot, G. M. Wang, D. Moses, and A. J. Heeger, Phys. Lev. Lett. 96, 246403 (2006) [3] J. H. Wei, Y. L. Gao, and X. R. Wang, Appl. Phys. Lett. 94, 073301 (2009) [4] J. M. Beebe, B. Kim, J. W. Gadzuk, C. D. Frisbie, and J. G. Kushmerick, Phys. Rev. Lett. 97, 026801 (2006) [Preview Abstract] |
Wednesday, March 5, 2014 3:54PM - 4:06PM |
Q20.00008: Low trap density of states in solution-deposited organic semiconductors by Vibration Assisted Crystallization Peter Diemer, Christopher Lyle, Yaochuan Mei, Christopher Sutton, Marcia Payne, John Anthony, Veaceslav Coropceanu, Jean-Luc Bredas, Oana Jurchescu Solution-deposited organic thin-film transistors suffer from defects at the semiconductor/dielectric interface due to disorder. These defects act as trapping sites and lead to inferior performance compared to single crystals. During the evaporation of a solvent, the solute molecules are driven to minimize their configuration energy: molecules may settle into local energy minimum configurations, characterized by molecular displacements with respect to the global minimum. We demonstrate that applying gentle vibrations of 100Hz or less to the solution during film crystallization perturbs and partially re-dissolves the dislocated molecules, allowing them to escape the local energy minimum and crystallize into the global energy minimum. This results in markedly improved performance in devices based on several solution-cast organic semiconductors due to a decrease in trap density at the organic/dielectric interface. The performance of our Vibration Assisted Crystallization transistors approach that of the corresponding single-crystal devices, as shown in transistors made from 2,8-difluoro-5,11-bis(triethylsilylethynyl) anthradithiophene on SiO$_{\mathrm{2}}$ dielectric, with mobility of 3 cm$^{\mathrm{2}}$/Vs, subthreshold slope of 0.43 V/dec, and threshold voltage of 0.7 V. The low interfacial trap density of 6.76x10$^{\mathrm{11}}$ cm$^{\mathrm{-2}}$ eV$^{\mathrm{-1}}$ agrees well with the results of quantum mechanical calculations. [Preview Abstract] |
Wednesday, March 5, 2014 4:06PM - 4:18PM |
Q20.00009: Dynamical- and static-disorder effects on charge transport property of organic semiconductors Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose In comparison with inorganic materials, electron transfer energy of typical organic semiconductors is small in the range of 10 - 100meV, which is comparable to the magnitude of dynamical disorder of transfer energy originating from the thermal fluctuations of molecular motions. Furthermore, the static disorder inevitably exists in realistic organic devices and disturbs the transport of charge carrier. To clarify the influence of the dynamical and static disorders on the mobility, we employ a realistic static-disorder potential, which is deduced from the data obtained by electron-spin-resonance spectroscopy. We evaluate the carrier mobilities of pentacene and rubrene semiconductors under the realistic situation, using our time-dependent wave-packet diffusion method. [1] In this methodology, we carry out the quantum-mechanical time-evolution calculations of wave packets and the classical molecular dynamics simulations simultaneously. We clarify the relation between the charge transport property and these disorders. We will talk about these results in my presentation. [1] H. Ishii, K. Honma, N. Kobayashi, K. Hirose, Phys. Rev. B, 85 (2012) 245206. H. Ishii, N. Kobayashi, K. Hirose, Phys. Rev. B (to be published). [Preview Abstract] |
Wednesday, March 5, 2014 4:18PM - 4:54PM |
Q20.00010: A general relationship between disorder, aggregation and charge transport in conjugated polymers Invited Speaker: Rodrigo Noriega The large variety of semiconducting polymers is a result of the synthetic capabilities to tune the optoelectronic properties of materials. Such variability also leads to a wide range of microstructures when solid films of conjugated polymers are cast from solution. Indeed, the large number of degrees of conformational freedom of these macromolecules and their weak intermolecular interactions result in complex microstructures - displaying a coexistence of amorphous and ordered phases with varying degrees of order. Understanding the limitations of charge transport in conjugated polymers is difficult owing to the unusual range of disorder they exhibit. These microstructures highlight the necessity to study the contributions of electronic processes at various length scales. In this talk, I will describe X-ray diffraction studies that show a large amount of disorder in semiconducting polymers while optoelectronic measurements show charges stay in the semi-ordered regions. Considering a particular system, P3HT with variable molecular weight, very similar transitions are observed in the behavior of the lattice disorder parameter (paracrystallinity) and charge mobility. Additionally, a comprehensive comparison of structure-property data across a wide range of materials uncovers strong similarities between seemingly diverse families of conjugated polymers. These insights allow the grouping of materials and identification of the importance of short-range ordered aggregates in transport. This generalization explains the seemingly contradicting high performance of recently reported, poorly ordered polymers and suggests molecular design strategies to further improve the performance of future generations of organic electronic materials. Specifically, the key to designing high mobility polymers is not in increasing crystallinity but rather in increasing their tolerance to an inevitably large amount of disorder within the aggregates by allowing more efficient intra- and intermolecular charge transport/transfer at the segmental level. [Preview Abstract] |
Wednesday, March 5, 2014 4:54PM - 5:06PM |
Q20.00011: Low-Temperature Structural Phase Transition in a Soluble Oligoacene and Its Effect on Charge Transport Jeremy W. Ward, Abdulmalik Obaid, Cynthia S. Day, John E. Anthony, Oana D. Jurchescu Small-molecule organic semiconductors are of great interest to understanding fundamental properties of charge transport in organic semiconductors as they offer relatively structurally simple model systems. The crystal packing plays a crucial role in determining the electronic performance of a material, as we demonstrate for the case of fluorinated 5,11-bis(triethylsilylethynyl)anthradithiophene. Increased interest in this compound is driven by the recent demonstrations of its high stability and high performance in organic field-effect transistors. This material exhibits a structural phase transition around $T =$ 294 K, however properties below $T$ $=$ 230 K have not been investigated in detail. We identify an additional polymorph that forms below $T =$ 200 K and shows distinct properties compared to the previously reported polymorphs. We identity the phase transition generating the new polymorph using grazing incidence X-Ray diffraction, field-effect transistor electrical characterization and differential scanning calorimetry. The evolution of the field-effect mobility with temperature shows a one order of magnitude increase in value as the films transition from a pure phase to a co-existence of two phases. The structural changes in the film modify the injection picture in these devices, and irreversibly increase the contact resistance two orders of magnitude. [Preview Abstract] |
Wednesday, March 5, 2014 5:06PM - 5:18PM |
Q20.00012: Characterization of charge motion in Poly(3-hexylthiophene) field effect transistors with Scanning Probe Microscopy Jason P. Moscatello, Morgen Patterson, Andrew R. Davis, Kenneth R. Carter, Katherine E. Aidala Poly(3-hexylthiophene) (P3HT) is a promising conductive organic polymer for applications such as organic FETs and photovoltaics. Key to proper utilization of P3HT is the understanding of how charges move and are trapped in the polymer, which directly affects the mobility of the charges as well as device efficiency. Scanning probe techniques, such as Kelvin Probe Force Microscopy, offer the advantage of being able to observe charges and local potentials down to the nano-scale. We present our work using scanning probe techniques to study charge injection and flow through P3HT FETs. [Preview Abstract] |
Wednesday, March 5, 2014 5:18PM - 5:30PM |
Q20.00013: Photo-Patterned Ion Gel Electrolyte-Gated Thin Film Transistors Jae-Hong Choi, Yuanyan Gu, Kihyun Hong, C. Daniel Frisbie, Timothy P. Lodge We have developed a novel fabrication route to pattern electrolyte thin films in electrolyte-gated transistors (EGTs) using a chemically crosslinkable ABA-triblock copolymer ion gel. In the self-assembly of poly[(styrene-r-vinylbenzylazide)-b-ethylene oxide-b-(styrene-r-vinylbenzylazide)] (SOS-N$_{\mathrm{3}})$ triblock copolymer and the ionic liquid, 1-ethyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([EMI][TFSI]), the azide groups of poly(styrene-r-vinylbenzylazide) (PS-N$_{\mathrm{3}})$ end-blocks in the cores can be chemically cross-linked via UV irradiation ($\lambda =$ 254 nm). Impedance spectroscopy and small-angle X-ray scattering confirmed that ion transport and microstructure of the ion gel are not affected by UV cross-linking. Using this chemical cross-linking strategy, we demonstrate a photo-patterning of ion gels through a patterned mask and the fabricated electrolyte-gated thin film transistors with photo-patterned ion gels as high-capacitance gate insulators exhibited high device performance (low operation voltages and high on/off current ratios). [Preview Abstract] |
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