Bulletin of the American Physical Society
2009 APS March Meeting
Volume 54, Number 1
Monday–Friday, March 16–20, 2009; Pittsburgh, Pennsylvania
Session J18: Charge Transport and Optical Properties of Organic Semiconductors |
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Sponsoring Units: DMP DPOLY Chair: Markus Wohlgenannt, University of Iowa Room: 319 |
Tuesday, March 17, 2009 11:15AM - 11:51AM |
J18.00001: Light Emitting Transistors of Organic Single Crystals Invited Speaker: Organic light emitting transistors (OLETs) are attracting considerable interest as a novel function of organic field effect transistors (OFETs). Besides a smallest integration of light source and current switching devices, OLETs offer a new opportunity in the fundamental research on organic light emitting devices. The OLET device structure allows us to use organic single crystals, in contrast to the organic light emitting diodes (OLEDs), the research of which have been conducted predominantly on polycrystalline or amorphous thin films. In the case of OFETs, use of single crystals have produced a significant amount of benefits in the studies of pursuit for the highest performance limit of FETs, intrinsic transport mechanism in organic semiconductors, and application of the single crystal transistors. The study on OLETs have been made predominantly on polycrystalline films or multicomponent heterojunctions, and single crystal study is still limited to tetracene [1] and rubrene [2], which are materials with relatively high mobility, but with low photoluminescence efficiency. In this paper, we report fabrication of single crystal OLETs of several kinds of highly luminescent molecules, emitting colorful light, ranging from blue to red. Our strategy is single crystallization of monomeric or oligomeric molecules, which are known to have a very high photoluminescence efficiency. Here we report the result on single crystal LETs of rubrene (red), 4,4'-bis(diphenylvinylenyl)-anthracene (green), 1,4-bis(5-phenylthiophene-2-yl)benzene (AC5) (green), and 1,3,6,8-tetraphenylpyrene (TPPy) (blue), all of which displayed ambipolar transport as well as peculiar movement of voltage controlled movement of recombination zone, not only from the surface of the crystal but also from the edges of the crystals, indicting light confinement inside the crystal. Realization of ambipolar OLET with variety of single crystals indicates that the fabrication method is quite versatile to various light emitting molecular solids, providing novel opportunities to get further insight on the intrinsic optoelectronic processes in organic semiconductors. \\[4pt] [1] T. Takahashi et al., Adv. Funct. Mater. 17, 1623 (2007).\\[0pt] [2] T. Takenobu et al., Phy. Rev. Lett. 100, 066601 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 11:51AM - 12:27PM |
J18.00002: ABSTRACT WITHDRAWN |
Tuesday, March 17, 2009 12:27PM - 12:39PM |
J18.00003: Modification of the electronic properties of rubrene crystals by extrinsic species Leonidas Tsetseris, Sokrates Pantelides The chemical stability of organic semiconductors is one of the most important factors for the performance of related electronic devices. Here, we report the results of first- principles calculations on the effect of some of the most typical defect culprits in the prototype system of rubrene, the current record-holder organic semiconductor in terms of carrier mobilities. We identify the most stable water and oxygen- related impurity structures, with species in either substitutional or interstitial configurations, and we analyze their complex role in changing the shape and profile of rubrene energy bands. In certain cases the impurities either give rise or help annihilate carrier traps. We discuss the relevance of our findings for the optimization of rubrene-based electronic systems, and, in particular, the possibilities for effective defect engineering. This work was supported in part by DOE Grant DEFG0203ER46096. [Preview Abstract] |
Tuesday, March 17, 2009 12:39PM - 12:51PM |
J18.00004: Charge carrier transport and optical properties of SAM-induced conducting channel in organic semiconductors. Vitaly Podzorov Certain types of self-assembled monolayers (SAM) grown directly at the surface of organic semiconductors can induce a high surface conductivity in these materials [1]. For example, the conductivity induced by perfluorinated alkyl silanes in organic molecular crystals approaches 10 to -5 Siemens per square. The observed large electronic effect opens new opportunities for nanoscale surface functionalization of organic semiconductors and provides experimental access to the regime of high carrier density. Here, we will discuss temperature variable measurements of SAM-induced conductivity in several types of organic semiconductors. [1]. M. F. Calhoun, J. Sanchez, D. Olaya, M. E. Gershenson and V. Podzorov, ``Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers'', Nature Mat. 7, 84 (2008). [Preview Abstract] |
Tuesday, March 17, 2009 12:51PM - 1:03PM |
J18.00005: Infrared spectroscopy of organic semiconductors modified by self-assembled monolayers O. Khatib, B. Lee, V. Podzorov, J. Yuen, A.J. Heeger, Z.Q. Li, M. Di Ventra, D.N. Basov Recently, self-assembled monolayers (SAMs) were used to modify electronic surface properties of organic single crystals, leading to several orders of magnitude increase in the electrical conductivity$^{1}$.~~Motivated by this discovery, the same technique was applied to polymers.~~Here we present a thorough spectroscopic investigation of organic semiconductors based on poly(3-hexlthiophene) (P3HT) that have been treated with a fluorinated trichlorosilane SAM.~~Infrared spectroscopy offers access to details of charge injection, electrostatic doping, and the electronic structure that are not always available from transport measurements, which can be dominated by defects and contact effects.~~In polymer films, the SAM molecules penetrate into the bulk, leading to a rich spectrum of electronic excitations in the mid-infrared energy range.~ $^{1}$~M. F.~Calhoun,~~J. Sanchez, D. Olaya, M. E. Gershenson, V. Podzorov\textit{,~Electronic functionalization of the surface of organic semiconductors with self-assembled monolayers},~Nature Mater$.$~\textbf{7}, 84--89 (2008)~ [Preview Abstract] |
Tuesday, March 17, 2009 1:03PM - 1:15PM |
J18.00006: Gate electric-field tuning of contact barriers between metals and organic semiconductor crystals J. Takeya, M. Yamagishi, K. Nakayama, T. Uemura Metal / organic semiconductor junctions are by all means essential in many organic electronics devices such as organic transistors, reasoning importance of understanding microscopic physics of carrier injection at the boundary. In this presentation, we focus on charge conductance through the metal / organic semiconductor contacts and their gate-electric field effects, fabricating structure of interfacing hole-rich rubrene single crystal crystals with metal electrodes. The hole-rich regions are formed at the top surface of the crystals either by field-effect accumulation using secondary gate electrodes or charge transfer from acceptor films of fluoro-silane molecular layers or F4-TCNQ layers. Application of gate voltage on the bottom surface of the crystals has given rise to a very sharp switching in the conductance through the contacts, because of very short (nanoscale) active length for the conductivity modulation. For the mechanism of the result, energy-level tuning between the metals and the hole-rich rubrene surface is suggested as the result of gradual band-bending in the direction of crystalline thickness. The minimum working length of the device is highly advantageous in high-frequency response and densities of the device integration. [Preview Abstract] |
Tuesday, March 17, 2009 1:15PM - 1:27PM |
J18.00007: Controlling Grain Size in Solution-Processed Organic Semiconductors for Thin-Film Transistors Stephanie Lee, Chang Su Kim, Enrique Gomez, Cheng Wang, Alexander Hexemer, Michael Toney, John Anthony, Yueh-Lin (Lynn) Loo We present a novel method for controlling the grain size in solution-processed triethylsilylethynyl anthradithiophene (TES-ADT) films through the addition of fractional amounts of fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (FTES-ADT). FTES-ADT can seed the crystallization of TES-ADT during solvent-vapor annealing. The grain size in these films follows an exponential dependence on the concentration of FTES-ADT; varying the FTES-ADT concentration by 2-fold induces a 3-order of magnitude change in the grain size. For channels in which the average grain size is 29 $\mu $m, device mobility of the organic thin-film transistors (OTFTs) is 0.05 cm$^{2}$/V-s. For channels in which the average grain size is 2700 $\mu $m, the device mobility is 0.35 cm$^{2}$/V-s. The relationship between device mobility and grain size is well described by a composite mobility model, which assumes a high intrinsic grain mobility and a low grain boundary mobility. Grazing incidence x-ray diffraction indicates that the crystal lattice of TES-ADT is preserved despite the addition of FTES-ADT. [Preview Abstract] |
Tuesday, March 17, 2009 1:27PM - 1:39PM |
J18.00008: Charge transport in crystalline organic semiconductors: using polymorphs to explore the effect of crystal packing Oana Jurchescu, Devin Mourey, Sankar Subramanian, Sean Parkin, Brandon Vogel, John Anthony, Thomas Jackson, David Gundlach Organic semiconductors are a fascinating class of materials, with a wealth of properties and diverse technological potential. For small-molecule organic semiconductors, charge transport is closely related to the crystal packing motif. Polymorphism is frequently encountered in these materials, given the weak intermolecular interaction energies. This represents a unique opportunity to explore phenomena related to the fundamental mechanism of charge transport in organic semiconductors, such as the influence of the crystal packing. For example, 5,11-bis(triethylsilylethynyl)anthradithiophene has two polymorphs inter-convertible through phase transition that occurs at $T $= 294 K. We report on their crystal structure, formation, and the effect of the different molecular packing on the electronic properties. We discuss the technological implications that a room-temperature phase transition has on the performance and stability of devices fabricated with this material. [Preview Abstract] |
Tuesday, March 17, 2009 1:39PM - 1:51PM |
J18.00009: A crystalline organic semiconductor grown from a mesophase: A test of polaron band theory Naresh Shakya, Chandra Pokhrel, Brett Ellman, Shin-Woong Kang, Satyan Kumar, Yulia Getmanenko, Robert Twieg We find that the hole mobility of the crystal smectic phases of the liquid crystal 1,4-di-(5-n- tridecylthien-2-yl)-benzene increases exponentially with decreasing temperature. While qualita- tively consistent with transport via polaron bands, we find that it is quantitatively difficult to explain the data with physically realistic parameters. In particular, the data demand either quite large typical optical phonon frequencies and/or phonon bandwidths. We also find evidence that an unusually highly ordered high temperature smectic-F phase templates the formation of crystalline smectic phases, which may have implications for device development. [Preview Abstract] |
Tuesday, March 17, 2009 1:51PM - 2:03PM |
J18.00010: First-principles study of charge injection and transport through pentacene multilayers Yong-Hoon Kim Applying a combined density-functional theory and matrix-Green's function approach [1,2], I study the coherent charge transport properties of pentacene nanowires sandwiched between Au(111) electrodes. Junction models based on pentacene trilayers in the ideal $\pi $-stacked and herringbone arrangements with the face-on and edge-on contact configurations at different contact distances are considered. I show that pentacene wires exhibit a robust p-type conductance behavior in agreement with experiments, and analyze the physical origin in terms of charge transfer between molecules and metal electrodes. \\[3pt] [1] Y.-H. Kim, S. S. Jang, Y. H. Jang, W. A. Goddard III, Phys. Rev. Lett. \textbf{94}, 156801 (2005). \\[0pt] [2] Y.-H. Kim, J. Tahir-Kheli, P. A. Schultz, and W. A. Goddard III, Phys. Rev. B \textbf{73}, 235419 (2006). [Preview Abstract] |
Tuesday, March 17, 2009 2:03PM - 2:15PM |
J18.00011: Low temperature, field-dependent mobility in pentacene thin-film transistors. Adrian Southard, Vinod Sangwan, Dan Lenski, Michael Fuhrer, Ellen Williams We measure the field-effect and saturation mobility of Au bottom contact thin-film polycrystalline pentacene field-effect transistors while varying temperature, channel length, and gate voltage. We utilize Au bottom contacts without a wetting layer, and achieve contact resistance as low as 1 k$\Omega $-cm despite disturbance of the pentacene morphology at the drain and source electrodes. By measuring multiple channel lengths, we extract a contact-resistance free mobility. We confirm this value using an alternative technique in which we short the source and drain electrodes and make two terminal measurements of the capacitance and loss between these electrodes and the gate as a function of frequency. We discuss the result of field-dependent mobility in the context of Poole-Frenkel theory to rationalize the non-linear dependence of drain current on drain voltage, and test the predictions of recently developed models for transport in such systems. [Preview Abstract] |
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