Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session R33: Focus Session: Organic Electronics and Photonics - Transport in Small Molecules |
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Sponsoring Units: DMP Chair: Jana Zaumseil, Institute of Polymer Materials, Friedrich-Alexander Universitaet Erlangen Room: 341 |
Wednesday, March 20, 2013 2:30PM - 3:06PM |
R33.00001: Intrinsic transport and photo-physical properties of high-mobility organic single crystals Invited Speaker: Vitaly Podzorov Small-molecule organic semiconductors form the basis for the emerging field of organic optoelectronics. In order to better understand the intrinsic photo-physical and transport phenomena in this important class of materials, it is necessary to study samples of very high structural order and chemical purity. Such materials exist in the form of molecular single crystals that can be used for fabrication of high-performance prototype devices, such as field-effect transistors, photo-conductors and photo-voltaic cells, in which intrinsic properties of organic semiconductors can be investigated without parasitic effects of disorder (see, e.g., [1,2]). This talk will cover the recent progress in organic single-crystal device electronics. In particular, several phenomena related to the previously discovered long-range triplet exciton diffusion and surface photocurrent generation (see, e.g., [3]) will be discussed.\\[4pt] [1] M. E. Gershenson, V. Podzorov, A. F. Morpurgo, ``\textit{Colloquium}: Electronic Transport in Single-Crystal Organic Transistors,'' invited review, \textbf{\textit{Rev. Mod. Phys.}}\textbf{ 78}, 973 (2006). \\[0pt] [2] V. Podzorov et al., ``Hall effect in the accumulation layers on the surface of organic semiconductors,'' \textbf{\textit{Phys. Rev. Lett.}}\textbf{ 95}, 226601 (2005). \\[0pt] [3] H. Najafov, B. Lee, Q. Zhou, L. C. Feldman and V. Podzorov, ``Observation of long-range exciton diffusion in highly ordered organic semiconductors,'' \textbf{\textit{Nature Mater.}} \textbf{9}, 938 (2010). [Preview Abstract] |
Wednesday, March 20, 2013 3:06PM - 3:18PM |
R33.00002: Variation in the Single-Molecule Conductance of Oligothiophenes Brian Capozzi, Emma Dell, Kateri DuBay, Jose Moreno, Timothy Berkelbach, David Reichman, Luis Campos, Latha Venkataraman Thiophenes are ubiquitous in organic electronic and photovoltaic applications; yet, they have received minimal attention in single molecule transport studies. Here, we carry out single molecule conductance measurements on a family of methyl sulfide-terminated oligothiophenes using the scanning tunneling microscope based break-junction technique. We find a non-exponential decay in conductance with the number of thiophene units (2 through 6) in the chain, which cannot be explained by a simple tunneling or hopping mechanism. We also find that the oligothiophenes exhibit a rather broad conductance distribution when compared to oligophenyls. Using a combination of experiment and molecular dynamics simulations, we show that this increased breadth is most likely due to different thiophene confomers sampled in the experiments, which do not necessarily maintain conjugation along the backbone. These measurements therefore reinforce the importance of conformation and conjugation effects in thiophene-based organic electronic devices where highly conducting molecular components are required. [Preview Abstract] |
Wednesday, March 20, 2013 3:18PM - 3:30PM |
R33.00003: X-ray Induced Trap States in the Organic Semiconductor Rubrene Tobias Morf, Tino Zimmerling, Simon Haas, Bertram Batlogg The charge transport in organic semiconductors and thus the device performance is broadly affected by localised electronic states capturing charge carriers. In a controlled irradiation experiment, the formation and microscopic origin of these trap states is studied quantitatively. Rubrene crystals with a low pristine trap density are irradiated with monochromatised CuK$\alpha$ radiation. The spectral density of trap states (DOS) is determined by the well-established SCLC method before and after each exposure step. After irradiation, a well defined additional DOS peak is measured. Its density grows linearly by approximately $10^{17}$ trap states per Joule of absorbed energy. These new states are closely peaked around 0.3\, eV above the HOMO (valence band) mobility level. The results are compared to those of the previous ion-irradiation study. Even though the ionic doses were higher by a factor of $10^{3}$ we find very similar changes in the DOS both with respect to quantity and energy of the trap states. This remarkable result suggests a much higher trap creation efficiency of X-rays as compared to ion radiation. Furthermore, the two different radiation methods seem to cause the same type of microscopic perturbation of the molecular crystal. [Preview Abstract] |
Wednesday, March 20, 2013 3:30PM - 3:42PM |
R33.00004: Trap effects in the analysis of conducting probe AFM current-voltage relations Kanokkorn Pimcharoen, Danial Olds, Jiebing Sun, Peng Peng Zhang, Phillip Duxbury Current-voltage relations of conducting probe AFM (CP-AFM) measurements demonstrate that trap effects are important in nanostructured P3HT thin films, particularly prior to thermal annealing. In order to analyze these measurements, we have developed fully three dimensional continuum device models incorporating the CP-AFM tip geometry and nanoscale morphology of the films. Results will be presented for a variety of trap systems in three dimensional model morphologies including nanofibrous structures and systems with inhomogeneous trap distributions. The simulation results will be compared with experimental data. [Preview Abstract] |
Wednesday, March 20, 2013 3:42PM - 3:54PM |
R33.00005: Wave-packet approach to thermal fluctuation effects on charge transport of organic semiconductors Hiroyuki Ishii, Nobuhiko Kobayashi, Kenji Hirose Organic materials are formed with weak van der Waals interactions between molecules. For typical organic semiconductors, the transfer integrals are small in the range of 10 - 100meV, which is comparable to the dynamical transfer integral modulations originating from the thermal fluctuations of molecular motions. Therefore the fluctuations provide important contributions to the understanding of the transport mechanism. To investigate such effects, we have developed a methodology to calculate the carrier transport coupled with inter- and the intramolecular vibrations of organic semiconductors based on the 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 evaluate the anisotropic mobility of organic semiconductors, such as pentacene crystals. We also clarify the change of temperature dependence of mobility from the thermal activated behavior to the power law behavior. I will talk about these results in my presentation. [1] H. Ishii, K. Honma, N. Kobayashi, K. Hirose, Phys. Rev. B, 85 (2012) 245206. [Preview Abstract] |
Wednesday, March 20, 2013 3:54PM - 4:06PM |
R33.00006: Computational Study of Electron-Phonon Coupling in Crystalline Organic Semiconductors Nenad Vukmirovic, Christoph Bruder, Vladimir M. Stojanovic Despite wide interest in organic molecular crystals and the recognition that electron-phonon (e-ph) coupling strength crucially determines the nature of charge carriers in these materials, ab-initio studies of e-ph coupling elements in these materials are still lacking. In this work [1], we calculated the e-ph coupling elements throughout the whole Brillouin zone in crystalline naphthalene using density functional perturbation theory within the generalized gradient approximation. Fourier-Wannier interpolation scheme [2] was then used to obtain the e-ph coupling constants on a fine k-point grid necessary for accurate evaluation of physical properties. Using the obtained e-ph coupling elements, we evaluated the quasiparticle residues for electrons and holes, obtaining the values of 0.74 and 0.78, respectively. These values suggest that e-ph coupling strength is insufficient for formation of small polarons in crystalline naphthalene and other oligoacene semiconductors. [1] N. Vukmirovic, C. Bruder, and V. M. Stojanovic, Phys. Rev. Lett. 109, 126407 (2012). [2] F. Giustino, M. L. Cohen, and S. G. Louie, Phys. Rev. B 76, 165108 (2007). [Preview Abstract] |
Wednesday, March 20, 2013 4:06PM - 4:18PM |
R33.00007: Van der Waals epitaxy of organic crystal films on hexagonal boron nitride layers for high-quality organic electronics Chul-Ho Lee, Theanne Schiros, Seok Ju Kang, Bumjung Kim, Kevin Yager, Colin Nuckolls, Philip Kim The charge transport in organic field-effect transistors (FETs) is strongly influenced by the dielectric and interface properties because crucial carrier processes including accumulation and transport take place at the interface between dielectric and organic materials. In this sense, hexagonal boron nitride (h-BN), which is a layered van der Waals (vdW) dielectric having atomically flat surface and no surface charge trap states, has great potential for both achieving high-quality organic FETs and investigating the intrinsic carrier transport properties in organic semiconductors. In this talk, we present the direct growth of rubrene crystal films on h-BN layers, demonstrating that there exists vdW epitaxial relation between rubrene and h-BN. Furthermore, charge transport properties in FETs using graphene electrodes will be discussed. [Preview Abstract] |
Wednesday, March 20, 2013 4:18PM - 4:30PM |
R33.00008: Voltage dependent capacitance -- a measure of energy level bending in naphthalene-tetra-carboxylic- di-imide based transistors Mathias Nyman, Oskar Sandberg, Josue Martinez Hardigree, Srinivas Kola, Howard Katz, Ronald Osterbacka We demonstrate transient capacitance measurements using charge extraction by a linearly increasing voltage (CELIV) on the small molecule naphthalene-tetra-carboxylic- di-imide (NTCDI) based organic transistors. The OFETs use Aluminum (Al) and Aluminum Oxide (AlO$_{x})$ as bottom gate and dielectric, with gold (Au) source and drain electrodes. The Al/AlO$_{x}$ gate is modified using two different self assembled monolayers, triethoxy(octyl)silane and perfluorooctyltriethoxysilane, in order to tune the turn-on voltage. We have clarified the voltage dependent capacitance in diode structures and found that when the transistor is in the fully on state a charge reservoir is formed at the AlO$_{x}$ interface and a saturation of the steady-state capacitance is seen, equaling the capacitance of the AlO$_{x}$ layer. When the transistor is in the fully off state the steady state capacitance saturates to the capacitance of the semiconductor bulk. We interpret this as a build-up of a charge reservoir in the semi conductor bulk when going from the off to the on state making it possible to charge the AlO$_{x}$ capacitance. By going from the on state towards the off state using a linearly increasing voltage pulse the dynamics of the depletion of the reservoir gives information about the energy level bending in the bulk. [Preview Abstract] |
Wednesday, March 20, 2013 4:30PM - 4:42PM |
R33.00009: Controlling Leakage Currents in Organic Field-Effect Transistors using Molecular Dipole Monolayers on Nanoscale Oxides Josue F. Martinez Hardigree, Thomas Dawidczyk, Robert Ireland, Gary Johns, Byung-Jun Jung, Nina Markovic, Howard Katz Self-assembled monolayers (SAM) have been explored as easily-processed, ultrathin interfacial layers in organic field-effect transistors (OFETs) for tuning the threshold voltage (Vt). We investigated the influence of Fermi-level pinning of the gate electrode by SAMs on leakage currents in OFETs fabricated on highly-doped n- and p-type Si gates with an intentionally marginal-quality, high leakage 8 nm SiO$_{2}$ dielectric. Two dipolar alkyl SAMs, octyltriethoxysilane (OTS) and its fluorinated analogue (FOTS), were employed under a 40 nm active layer of a naphthalenetetracarboxylic diimide (NTCDI) derivative. Transistors on nSi displayed more positive Vt for OTS ($+$0.23 V) and FOTS ($+$1.09 V) than bare oxide (-0.56 V), while OFETs on pSi showed a lower Vt for OTS ($+$0.26 V) and a higher Vt for FOTS ($+$1.25 V) devices relative to bare oxide ($+$1.15 V). Differences in gate and subthreshold leakage between bare and SAM-treated oxides match the trends in Vt. Scanning Kelvin-probe measurements were consistent with this trend, indicating FOTS made both nSi and pSi oxide surfaces more negative relative to bare oxide, while OTS treatment resulted in more positive surface potentials on pSi and more negative surface potentials on nSi. [Preview Abstract] |
Wednesday, March 20, 2013 4:42PM - 4:54PM |
R33.00010: Halogenated contorted hexabenzocoronene derivatives for electron transport in thin-film transistors and organic photovoltaics Anna Hiszpanski, Leo Shaw, Matthew Bruzek, Franziska Luettich, Antoine Kahn, John Anthony, Yueh-Lin Loo In investigating electron acceptor substitutes for fullerene derivatives in organic photovoltaic applications, we have modified a semiconductor, contorted hexabenzocoronene (HBC), with halogens to increase its oxidative stability and lower its lower unoccupied molecular orbital energy level relative to vacuum level. We synthesized a series of HBC derivatives with increasing fluorine substitution on the peripheral aromatic rings and elucidated the effect of chemical modification on electronic properties. Though we observe a 57 meV shift in both the highest occupied and lowest unoccupied energy levels of the molecules with each progressive addition of fluorine, none of the fluorinated HBC derivatives demonstrate electron transport in thin-film transistors. By substituting chlorine for four of the peripheral fluorines, however, this mixed-halogenated compound exhibits n-transport characteristics. Unoptimized thin-film transistors comprising 8F-8Cl-HBC have demonstrated electron mobilities as high as 0.01 cm$^{\mathrm{2}}$/Vs, and unoptimized bulk-heterojunction solar cells with poly(3-hexyl thiophene) as the polymer donor have yielded power conversion efficiencies as high as 0.9{\%}. [Preview Abstract] |
Wednesday, March 20, 2013 4:54PM - 5:06PM |
R33.00011: Tellurium-Organic Thin-Films in Hybrid Electronic Platforms Robert Ireland, Howard Katz Vacuum-deposited tellurium (Te) is considered with semiconducting, insulating, and charged organic films for hybrid electronic platforms. Specifically, polycrystalline films of Te and organic semiconductor (OSC) molecules are combined for the first time in bilayer field-effect transistors (FET). Although Te is not ideal for high dynamic range FETs, it serves as a useful test platform in inorganic-organic heterostructures because of high mobility ($\mu )$, defined composition, and amenable processing methods. Scanning Kelvin probe microscopy directly confirms the interfacial vacuum level offset for different Te-OSC junctions. By implanting electrostatic charges at the dielectric surface we demonstrate that interfacial fields determine the gate voltage range over which Te shows field-effect in heterostructured FETs. FETs are measured under both continuous and pulsed operation. Pulsed gating influences the measured $\mu $ by selectively concentrating charge carriers in semiconductor layers that are farther away from the gate dielectric. FETs comprising various Te-organic junctions gave consistent $\mu $ ranging from 0.001 to above 5 cm$^{2}$ V$^{-1}$ s$^{-1}$, compared to 2.7 cm$^{2}$ V$^{-1}$ s$^{-1}$ for Te deposited on bare silicon dioxide under the same conditions. [Preview Abstract] |
Wednesday, March 20, 2013 5:06PM - 5:18PM |
R33.00012: Spatial dependence of charge photogeneration and transport in an ordered, phase-separated liquid crystalline organic semiconductor Sanjoy Paul, Suvagata Tripathi, Brett Ellman, Robert Twieg Bulk heterojunction organic photovoltaic cells depend on charge and exciton physics within, and between, small regions of organic semiconductors. To probe the physics of charge generation and transport in these geometries, we have fabricated patterned, phase-separated mixtures of liquid crystalline (LC) organic semiconductors and photopolymerized polymers. To characterize transport in the LC regions, we have developed ``scanning time-of-flight microscopy'' (STOFm), whereby spatially resolved TOF and polarized microscopy data are acquired in parallel. We will discuss the technique, as well as our results on efficiency of charge generation, mobility, and trapping in these confined geometries. [Preview Abstract] |
Wednesday, March 20, 2013 5:18PM - 5:30PM |
R33.00013: Probing Polaron Dynamics and Transport in Multiporphyrin Conjugated Arrays by EPR and Optical Spectroscopy Paul Angiolillo, Jeff Rawson, Michael Therien The nature of mobile charge carriers and their transport in organic conducting and semiconducting materials is still an area requiring deeper understanding. Unlike in classical metals, charge carriers are not represented well by bare charges but rather as polarons. Hole and electron polarons were chemically generated in a systematic series of meso-to-meso ethyne-bridged (porphinato)zinc arrays (PZn$_{\mathrm{n}})$, spanning a linear dimension of 1.4 nm to 7.5 nm. Determination of the spin distribution through the nuclear hyperfine interaction suggest that both hole and electron polarons are extensively delocalized over the extent of the molecule at 298 K. Low temperature studies at 77 K further reveal that the polaronic states maintain their ability to explore the extent of the molecule. Concomitant optical absorption spectroscopy of the hole polaronic states in these oligomers further supports the delocalized nature of the excitation. Electron spin relaxation in organic materials devoid of heavy atoms is dominated by the nuclear hyperfine interaction. This decreased interaction manifest itself in a simultaneous decrease in the spin lattice relaxation rate (increase in spin lattice relaxation time T$_{\mathrm{1}})$ with oligomer size as determined through progressive microwave saturation with relaxation times on the order of 1$\mu $s at 298 K. These data demonstrate exceptional and unprecedented charge dynamics and polaron delocalization lengths. [Preview Abstract] |
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