Bulletin of the American Physical Society
APS March Meeting 2013
Volume 58, Number 1
Monday–Friday, March 18–22, 2013; Baltimore, Maryland
Session C33: Focus Session: Organic Electronics and Photonics - Interfaces and Contacts |
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Sponsoring Units: FIAP Chair: Xinran Zhang, Georgetown University Room: 341 |
Monday, March 18, 2013 2:30PM - 3:06PM |
C33.00001: Interface Charge Transport in Organic Transistors as Investigated by Field-Induced Electron Spin Resonance Invited Speaker: Tatsuo Hasegawa Most of high-performance organic thin-film transistors (OTFTs) as recently developed is attainable with non-doped, single-component $\pi $-conjugated materials that exhibit high layer crystallinity both for small-molecules and polymers. The layer crystallinity is quite suitable to compose channel transport layers of the OTFTs, although the main origin to hinder the charge transport or the intrinsic carrier mobility is still controversial; intra- or intermolecular electron-phonon coupling, polarization effects by the gate-dielectrics, or thermal or extrinsic disorder effects. Here we discuss the interface charge transport in the OTFTs, as investigated by field-induced electron spin resonance (FESR) technique that probes 1/2 spin of carriers induced by gate voltage. It is shown that the FESR technique is extremely useful especially for OTFTs, because of the fairly small spin-orbit interactions in organic materials as well as of the high layer crystallinity and the anisotropy. The following important aspects of the interface charge transport are presented and discussed: (1) Carrier motion in OTFTs can be understood in terms of the multiple trap-and-release (MTR) transport. The analyses of the motional narrowing effects allow us to estimate the average trap residence time that reaches about 1 ns [1]. (2) Carriers are frozen at the respective trap sites at low temperature. The low-temperature spectral analyses allow us to obtain the distribution of trapped carriers over their degree of localization [2, 3]. (3) We also developed a unique technique to investigate the intra- and inter-domain transport in polycrystalline OTFTs by using anisotropic FESR measurements. The method allows us to evaluate the potential barrier height at the domain boundaries within the films [4].\\[4pt] [1] Phys. Rev. Lett. 100, 126601 (2008).\\[0pt] [2] Phys. Rev. Lett. 104, 056602 (2010).\\[0pt] [3] Phys. Rev. B 85.085211 (2012).\\[0pt] [4] Phys. Rev. B 85.035308 (2012). [Preview Abstract] |
Monday, March 18, 2013 3:06PM - 3:18PM |
C33.00002: Gold contacts for rubrene SC--FETs: the older, the better Tino Zimmerling, Bertram Batlogg Excellent charge injection, device stability, and high reproducibility in the device fabrication are key to investigate intrinsic properties of organic semiconductors in e.g. diodes and field-effect transistors (FETs). In this systematic study we show that these requirements can be met by properly conditioning the gold electrodes in a rubrene flip crystal FET --- a setup which is frequently used to explore fundamental organic electronic physics. Gold electrodes have been evaporated under HV conditions on Cytop as gate insulator. The electrodes had been exposed to air for 15~min to 1000~min before rubrene crystals have been laminated. We evaluated the efficiency of charge injection by calculating the contact resistance at the gold--rubrene interface. We observe a systematic decrease of the contact resistance by factors of up to 10$^3$ and improved device stability in terms of contact resistance and mobility after long-term air exposure. From these findings we deduce a simple recipe to fabricate non-contact-limited FETs employing rubrene crystals and gold electrodes. These observations in a typical laboratory environment are in line with the view that charge injection is not simply determined by the ideal metal workfunctions and the HOMO/LUMO levels measured under UHV conditions. [Preview Abstract] |
Monday, March 18, 2013 3:18PM - 3:30PM |
C33.00003: Indium Free Transparent Electrodes with a Tungsten Oxide Hole Blocking Layer for Organic Photovoltaic Devices Roy Murray, Patrick Reinecke, Nopporn Rujisamphan, Uli W\"urfel, S. Ismat Shah Indium Tin Oxide (ITO), the standard transparent electrode used in organic photovoltaic (OPV) devices, is expensive and cannot be deposited well on flexible plastic substrates due to its high temperature post deposition annealing. As a replacement for ITO, we used a sputtered Al-ZnO/Ag/WOx film as the transparent electrode. The work function of this electrode was found using a Kelvin Probe to be between 5 and 5.4 eV, depending on thickness. We tested several OPV materials of varying LUMO and HOMO levels on the WOx layer and found that a difference of greater than 0.2 eV between the HOMO of the donor and the conduction band of the WOx resulted in poor device performance. We further investigated the alteration of the WOx work function through doping and altering the thickness. Device analysis and cross sectional transmission electron microscope (TEM) pictures using a focused ion beam were performed. [Preview Abstract] |
Monday, March 18, 2013 3:30PM - 3:42PM |
C33.00004: Poly(3-hexylthiophene) Band Alignment With SiO2 Determined By Internal Photoemission Wei Li, Xuelei Liang, James Basham, Kun Xu, Qin Zhang, Oleg Kirillov, Rusen Yan, Curt Richter, Thomas Jackson, N.V. Nguyen, David Gundlach We report band alignment for the widely studied organic semiconductor, Poly(3-hexylthiophene) (P3HT), by using internal photoemission (IPE). P3HT solution was spin coated onto 280 nm thick SiO$_{2}$ on heavily doped P-type silicon. A 10 nm thick aluminum (Al) electrode with adjoining 70 nm thick Al contact pad were deposited onto the P3HT film through aligned shadow masks. Photocurrent in the IPE measurement was generated using a monochromator with photon energy ranging from 1.5eV to 6.0eV (0.05 eV steps) and with a DC voltage which ranged from 20V to -20V (-2V steps) applied between the silicon backside and the thick Al contact. Both positive photocurrent and negative photocurrent were observed. For the IPE measurement, the yield (Y) is defined as the ratio of the carriers contributing to the photocurrent to the incident photon flux, and the threshold at each applied voltage is obtained by extrapolating Y$^{1/3}$(h$\nu )$ to zero. The barrier height is determined from Schottky plots extrapolated to zero field. By using this established method we extract a barrier height of 4.2 eV $\pm$ 0.1 eV for the Si:SiO2 interface and 4.0 eV $\pm$ 0.1 eV for the P3HT:SiO$_{2}$ interface, respectively. [Preview Abstract] |
Monday, March 18, 2013 3:42PM - 3:54PM |
C33.00005: Electron Injection to Control Self-Assembly and Disassembly of Phenylacetylene on Gold Arthur P. Baddorf, Qing Li, Chengbo Han, J. Bernholc, Humberto Terrones, Bobby Sumpter, Miguel Fuentes-Cabrera, Jieyu Yi, Zheng Gai, Peter Maksymovych, Minghu Pan The power of two-dimensional organic molecular systems for applications including electronics, functionalization and nanolithography is enabled by our ability to produce structures through self-assembly on a surface. Unfortunately, relying on thermal fluctuations to drive the surface attachment reactions has limited self-assembled molecules (SAMs) to little beyond alkanethiols on gold. We demonstrate a seminal example of non-thermal control over molecular self-assembly, where hot-electron injection rather than thermal fluctuations transform a disordered layer of weakly bonded hydrocarbon molecules into an ordered, dense monolayer. The process is reversible, in that injection of holes reverts to a disordered state. Since electron and hole injection is accomplished with a STM, unprecedented local control over ordered and disordered domains is achieved. STM imaging and correlated density functional calculations reveal that ordered domains consist of molecules vertically aligned and more strongly attached to the gold substrate through the acetylene tail, while disordered domains contain weakly bound molecules lying flat. [Preview Abstract] |
Monday, March 18, 2013 3:54PM - 4:06PM |
C33.00006: Scanning Tunneling Microscopy and Spectroscopy of Thin Films of the Organic Semiconductor Picene Simon Kelly, Geoffrey Rojas, Petro Maksymovych Characterizing organic semiconductors at the single molecule scale has greatly enhanced our understanding of intermolecular interactions, revealing new approaches to controlling film structure, while probing the electronic properties of organic interfaces. Pentacene has long been a model system for such studies. Here we study monolayer and bilayer films of picene, a structural isomer of pentacene. We grow these films on Ag(111) by thermal evaporation in UHV and measure them in-situ using a low-temperature STM at $\sim$ 77 K. Topographic STM measurements were used to establish the film structure. Much like pentacene, picene bonds with its molecular plane parallel to the surface, but unlike pentacene, picene forms dimers. Moreover, the work-function shift amounts to almost 1 eV (up to 2x the value for pentacene), suggesting that the molecule-surface distance is closer in this case. At the same time, the splitting of the LUMO, LUMO$+$1, and LUMO$+$2 molecular orbitals is somewhat larger than even semiempirically calculated values for the gas-phase. These measurements will be compared to first principles calculations made with the HSE functional to understand changes to the electronic structure with adsorption and the role of van-der-Waals interactions between flat-lying picene molecules. [Preview Abstract] |
Monday, March 18, 2013 4:06PM - 4:18PM |
C33.00007: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 4:18PM - 4:30PM |
C33.00008: Computational Study of Phenylacetylene Self-Assembly on Au(111) Surface Chengbo Han, Wenchang Lu, Jerry Bernholc, Qing Li, Miguel Fuentes-Cabrera, Humberto Terrones, Bobby Sumpter, Jieyu Yi, Zheng Gai, Arthur Baddorf, Petro Maksymovych, Minghu Pan The direct control over Phenyl-Acetylene (PA) self-assembly/disassembly on gold, achieved by carrier injection through an STM tip, is unprecedented. We discuss theoretical interpretation of PA structures observed on Au(111) by direct STM imaging in the preceding talk. We have examined 20 different adsorption geometries through first-principles calculations, simulated their STM images and compared them to the experimentally observed patterns. While weakly adsorbed PA prefers flat orientation on Au(111), the self-assembly leads to significant rearrangement of its adsorption structure. We obtain excellent agreement with the experimental data for a densely packed, ordered monolayer of vertically aligned but tilted styrene-derivative adsorption geometry, in which the acetylene tail is directly bonded to the surface. All the major features of the experimental STM image are reproduced by calculations, leading to unambiguous determination of the self-assembled structure [1]. We will also discuss the changes in adsorption energetics and molecular level alignment induced by the self-assembly process.\\[4pt] [1] Q. Li et al., ACS Nano, 6, 9267(2012) [Preview Abstract] |
Monday, March 18, 2013 4:30PM - 4:42PM |
C33.00009: Electronic Structure of CoPc Adsorbed onto Ag(100): Evidence for Molecule-Substrate Interaction Mediated by Co-3d Orbitals Eric Salomon, Patrick Amsalem, Noa Marom, Martin Vondracek, Leeor Kronik, Norbert Koch, Thierry Angot The electronic structure of cobalt-phthalocyanine (CoPc) molecules adsorbed on Ag(100) is investigated by photoemission spectroscopy. The results are compared to first principles electronic structure calculations, based on many-body perturbation theory in the GW approximation. The photoemission data, obtained from both multilayer and monolayer films of CoPc, showthat charge-transfer occurs between the first molecular layer and the metal surface. Varying the photon energy, to tune the photoionization cross sections, reveals that the charge-transfer related interface states mainly involve the Co-3d atomic orbitals of the Co central atom. GW calculations for the neutral CoPc molecule and its anion compare well with the experimental observations for a multilayer and a monolayer CoPc fillm, respectively. They confirm the major role played by the Co atom in the charge transfer process and elucidate the complex energy rearrangement of the molecular electronic levels upon adsorption on the metal. [Preview Abstract] |
Monday, March 18, 2013 4:42PM - 4:54PM |
C33.00010: The role of micro-shorts and electrode-film interface in the electrical transport of ultra-thin metallophthalocyanine capacitive devices Carlos Monton, Ilya Valmianski, Ivan K. Schuller The transport properties of metallophthalocyanine thin films are of much basic interest and are important ingredients in many technological applications. Ohmic conductance Co-phthalocyanine (CoPc) of thin film (15 nm to 90 nm) capacitive devices has been investigated in the 40K to 300 K temperature range. For Pd and V electrodes, the electrode-film (E-F) interface and metallic micro-shorts contribute substantially to the conductance with decrease CoPc layer thickness. A quantitative model which describes the E-F interface, CoPc roughness, micro-shorts, and the exponential temperature and thickness dependence of conductance was developed. Parameters obtained from this model are in good, quantitative agreement with independent measurements. The model predicts a 15-20 nm lower limit for capacitive device thickness, below which the conduction is mainly controlled by shorts. In this regime, small changes in mean CoPc thickness result in drastic variation in device conductance. [Preview Abstract] |
Monday, March 18, 2013 4:54PM - 5:06PM |
C33.00011: ABSTRACT WITHDRAWN |
Monday, March 18, 2013 5:06PM - 5:18PM |
C33.00012: Phase separation-driven stratification in conventional and inverted P3HT:PCBM organic solar cells Eleni Pavlopoulou, Guillaume Fleury, Dargie Deribew, Fabrice Cousin, Mark Geoghegan, Georges Hadziioannou We have used neutron reflectivity to investigate the stratification of poly(3-hexylthiophene) (P3HT) and phenyl-C$_{61}$-butyric acid methyl ester (PCBM) blend films. Films were spun-cast on poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and titanium oxide (TiOx) layers to mimic the procedures followed for the fabrication of conventional and inverted organic photovoltaics respectively. A 60{\%} and 75{\%} PCBM enrichment of the active layer at the interface with PEDOT:PSS and TiOx respectively has been revealed, as well as a PCBM depletion at the free surface of the film which is driven by the lower surface energy of P3HT. PCBM segregation close to the substrate is further enhanced by annealing. In case of the films cast on PEDOT:PSS, this stratification could be detrimental for conventional solar cell performance, since the electron-acceptor material enriches the interface with the hole-collecting electrode. The agglomeration of PCBM at the TiOx interface could, however, be favorable for an enhanced charge collection, thus improving device performance. [Preview Abstract] |
Monday, March 18, 2013 5:18PM - 5:30PM |
C33.00013: Atomic and Electronic Structure of the P3HT/PCBM Interface From First-Principle Calculations Longhua Li, Oleg Kontsevoi, Arthur J. Freeman Fundamental research on donor/acceptor (D/A) interfaces of organic photovoltaics (OPV) have drawn immense interest because of their crucial roles in charge separation (CS), charge transfer (CT) and charge recombination (CR). The blend system consisting of regioregular poly(3-hexylthiophene) (rr-P3HT) and fullerene derivative [6,6]-phenyl C$_{61}$ butyric acid methyl ester (PCBM) is a widely investigated binary system. Despite significant efforts that have been done to optimize the OPV, such as the D/A ratio, detailed information on their structure, interfaces, and morphology are far from complete. Additionally, fewer investigations have focused on the elementary charge transfer processes. In this work, such a hetero-interface was carried out by annealing simulation; and then interfacial electronic structure and charge transfer were studied by DFT calculations. The process of PCBM assembly on the P3HT surface were shown and the carrier mobilities could be tuned by PCBM orientations.Our calculations provide an important understanding on the assembly of PCBM and charge transfer at the binary interface. [Preview Abstract] |
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