Bulletin of the American Physical Society
APS March Meeting 2011
Volume 56, Number 1
Monday–Friday, March 21–25, 2011; Dallas, Texas
Session W38: Focus Session: Organic Electronics and Photonics -- Charge transport |
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Sponsoring Units: DMP DPOLY Chair: Almantas Pivrikas, Johannes Kepler University Linz Room: A130/131 |
Thursday, March 24, 2011 11:15AM - 11:27AM |
W38.00001: Charge Transport in Functionalized Fluorinated Pentacenes Oana Jurchescu, Daniel David, Claire McLellan, Balaji Purushothaman, Shubin Liu, Veaceslav Coropceanu, John Anthony, Laurie McNeil We report on charge transport in fluorinated functionalized pentacenes and discuss the effect of trialkylsilyl and the number of fluorine atoms. We show that modifications in the chemical composition influence the molecular packing, crystal formation and electrical properties, allowing us to measure mobilities from 10$^{-5}$ to 1.7 cm$^{2}$/Vs. The mobilities correlate with the packing, demonstrating that tuning the solid-state order to induce pi-stacking improves electrical properties. By combining Raman measurements with theoretical calculations predicting the vibrational spectrum, we explore the vibrational modes of the crystals, providing information about the intermolecular coupling and electron-phonon interactions governing charge transport. We calculate the intermolecular electronic couplings and band structures by using density functional theory, and study the effect of fluorination and trialkylsilyl substitution on crystal packing and the electronic properties. [Preview Abstract] |
Thursday, March 24, 2011 11:27AM - 11:39AM |
W38.00002: Semicrystalline high performance poly (thienothiophene) thin films: crystallites and defects Chenchen Wang, Javier Dacu\~na, Bj\"orn Br\"auer, Dan Daranciang, Alberto Salleo Effects of liquid crystalline temperature annealing and surface treatment on Poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophenes) (PBTTT) thin films were studied. Time resolved terahertz spectroscopy (TRTS), which measures the local carrier mobility, suggests that the mobility improvement of annealed PBTTT on octadecyltrichlorosilane (OTS), compared with as cast film, is mainly due to superior local carrier mobility. Scanning transmission x-ray microscopy (STXM), which measures the in-plane molecular orientation with 30nm spatial resolution, shows similar domain size of annealed films on both OTS and $SiO_{2}$, and implies that the higher mobility of film on OTS cannot be accounted for by domain size. These results are also supported by the mobility edge model, which extracts trap density and quasi-free carrier mobility in crystallites from transistor characterization. Annealing film on OTS improves mobility in crystallites, but has little effects of reducing trap density. The modeling shows great elimination of interface trap density of film on OTS, compared with film on $SiO_{2}$, which might be the main reason for its higher mobility. [Preview Abstract] |
Thursday, March 24, 2011 11:39AM - 11:51AM |
W38.00003: First-principles study of polythiophene and polyselenophene crystals for organic electronics Takao Tsumuraya, Jung-Hwan Song, A.J. Freeman Semiconducting polymers, like regioregular poly(3-hexylthiophene)/poly(3-hexylselenophene) (rr-P3HT/rr-P3HS) are currently the most widely studied materials in a variety of applications for polymer based bulk-heterojunction (BHJ) solar cells and organic field-effect transistors (OFET). [1,2] For both applications, the performance of devices has been attributed to thin film structures of rr-P3HT/rr-P3HS on substrates. To understand their mechanisms, the crystal structure has been extensively investigated by using various experimental techniques. However the crystal structure has yet to be unambiguously characterized. Here, we proposed several possible structures and investigated their stabilities from first-principles density functional calculations based on the all-electron FLAPW method. [3] We found that two base-centered monoclinic structures belonging to space group $A$2 are in the degenerate lowest energy structures. The electronic and transport properties are also discussed. Lastly, we report on the differences in electronic and crystal structure between rr-P3HT and rr-P3HS.\\[0pt] [1] G. Li $et$ $al$., Nature Mater. \textbf{4}, 864 (2005).\\[0pt] [2] H. Sirringhaus $et$ $al$., Nature \textbf{401}, 685 (1999).\\[0pt] [3] E. Wimmer $et$ $al$., Phys. Rev. B \textbf{24}, 864 (1981). [Preview Abstract] |
Thursday, March 24, 2011 11:51AM - 12:03PM |
W38.00004: Analysis of Metallic Conduction at the Interface of TTF and TCNQ Crystals Viktor Atalla, Mina Yoon, Matthias Scheffler Organic materials are promising candidates for a next generation of electronic devices, since they offer a variety of new intriguing electronic phenomena while being environmentally friendly, low cost, and mechanically flexible. Here we study the donor/acceptor interface of TTF and TCNQ organic molecular crystals which was found to exhibit metallic conduction whereas the individual crystals are large band-gap semiconductors. Using density functional theory (DFT) we first compare the performance of different exchange-correlation (XC) functionals for TTF and TCNQ dimers. All employed XC functionals consistently give electron transfer from TTF to TCNQ and the van der Waals (vdW) corrected molecular binding distances are within $\approx 0.1$~{\AA} of the MP2 value, indicating that within DFT the system can be qualitatively described by semilocal functionals. We construct interfaces between the two types of crystals and calculate their electronic structures. On a PBE + vdW level we find indications for metallic conduction at the interface, due to metallic bands that are exclusively induced from the interface layers of TTF and TCNQ molecules. [Preview Abstract] |
Thursday, March 24, 2011 12:03PM - 12:15PM |
W38.00005: Charge Trapping in Organic Thin-Film Transistors Claire McLellan, Jack Owen, Marsha Grimminger, John Anthony, Oana Jurchescu Charge trapping in the bulk of the organic semiconductors and at interface with the gate dielectric and/or contacts is one of the determining factors governing charge transport in organic thin-film transistors (OTFTs). We explore the current-voltage characteristics in different charge density regimes and extract the field-effect mobility. The dependence of mobility on gate/drain voltage give us valuable insight into the mechanism of charge transport and the relevance of trapping states. We perform measurements on devices fabricated using different methods, such as spin-coating, drop-casting or spray-coating, on a silicon gate electrode, silicon dioxide gate dielectric, and gold source and drain contacts. We demonstrate that the performance of OTFTs is strongly dependent on processing details. We show that even when using the same processing method, we are able to systematically tune the charge trapping states by chemically modifying the contact and dielectric surfaces with self-assembly monolayers. [Preview Abstract] |
Thursday, March 24, 2011 12:15PM - 12:27PM |
W38.00006: Organic electrical double layer transistors gated with ionic liquids Wei Xie, C. Daniel Frisbie Transport in organic semiconductors gated with several types of ionic liquids has been systematically studied at charge densities larger than 10$^{13}$ cm$^{-2}$. We observe a pronounced maximum in channel conductance for both p-type and n-type organic single crystals which is attributed to carrier localization at the semiconductor-electrolyte interface. Carrier mobility, as well as charge density and dielectric capacitance are determined through displacement current measurement and capacitance-voltage measurement. By using a larger-sized and spherical anion, tris(pentafluoroethyl)trifluorophosphate (FAP), effective carrier mobility in rubrene can be enhanced substantially up to 3.2 cm$^{2}$V$^{-1}$s$^{-1}$. Efforts have been made to maximize the charge density in rubrene single crystals, and at low temperature when higher gate bias can be applied, charge density can more than double the amount of that at room temperature, reaching 8*10$^{13}$ cm$^{-2}$ holes (0.4 holes per rubrene molecule). [Preview Abstract] |
Thursday, March 24, 2011 12:27PM - 12:39PM |
W38.00007: Hopping transport in electrolyte-gated P3HT organic field effect transistors Shun Wang, Mingjing Ha, Michael Manno, C. Daniel Frisbie, C. Leighton Using ion-gel-gated poly (3-hexylthiophene) (P3HT), we successfully fabricated p-type organic field effect transistors (OFET) with on/off ratios of 10$^{5}$ and mobility of the order of 1 cm$^{2}$/V s at room temperature. We studied charge transport in the electrochemically doped P3HT as a function of gate voltage, temperature, magnetic field, film thickness, and roughness. Carrier concentrations were obtained from both gate charging current and Hall effect measurements. The resistance vs. temperature (down to 5K for large gate voltages) characteristics indicate 2D hopping transport. Large positive magnetoresistance at temperatures lower than 50 K was observed, but with anomalously low anisotropy due to high roughness. [Preview Abstract] |
Thursday, March 24, 2011 12:39PM - 12:51PM |
W38.00008: Charge transport and velocity distribution in ambipolar organic thin film Transistors based on a diketopyrrolopyrrole-benzothiadiazole copolymer Tae-jun Ha, Prashant Sonar, Samarendra Pratap Singh, Ananth Dodabalapur There have been reports of charge transport mechanisms in organic thin film transistors (OTFTs) focusing on steady-state characteristics but these measurements provide limited information. Time-resolved measurements can provide additional information in understanding transport mechanisms but existing reports have focused on unipolar organic characteristics. No previous reports on ambipolar organic devices have involved entire velocity distribution and charge transport mechanisms. Recently, we have fabricated ambipolar OTFTs based on a diketopyrrolopyrrole-benzothiadiazole copolymer (PDPP-TBT) with a field-effect mobility of more than 0.2 cm$^2$ V$^-1$ s$^-1$. Velocity distributions are measured by performing specialized dynamic measurements while keeping the RC-time constant of the measurement circuit small. This yields a distribution in arrival times of charge carriers from source to drain which can be converted to velocity distributions. We will also describe dynamic transport measurements on high-k-dielectric PDPP-TBT OTFTs. [Preview Abstract] |
Thursday, March 24, 2011 12:51PM - 1:03PM |
W38.00009: Organic field effect transistor fabricated by directly grown poly (3 hexylthiophene) crystalline nanowires on solution processed carbon nanotube aligned array electrodes Biddut Sarker, Jianhua Liu, Lei Zhai, Saiful Khondaker We demonstrate convenient and highly reproducible approach to fabricate organic field effect transistors (OFETs) using the direct growth of crystalline P3HT nanowires on aligned array SWNT interdigitated electrodes. Compared to the OFETs with metal electrodes, the OFETs with SWNT electrodes show high mobility and high current on-off ratio with a maximum of 0.13 cm$^{2}$/Vs and 3.1$\times $10$^{5}$, respectively. The improved device characteristics are also demonstrated by the absence of short channel effect which is dominant in gold electrode OFETs. Such remarkable improvement of the device performance as high mobility, high current on-off ratio, absence of short channel effect and better charge carrier injection can be attributed to the improved contact via strong \textit{$\pi -\pi $} interaction SWNT electrodes with the crystalline P3HT nanowires as well as the improved morphology of P3HT due to one dimensional crystalline structure. . [Preview Abstract] |
Thursday, March 24, 2011 1:03PM - 1:15PM |
W38.00010: Interfacial Width Measurements of Dielectric/P(NDI2OD-T2) Using Resonant Soft X-ray Reflectivity Hongping Yan, Ziran Gu, Eliot Gann, Brian Collins, Sufal Swaraj, Cheng Wang, Torben Schuettfort, Chris McNeill, Harald Ade Interfaces between a conjugated polymer and a dielectric play a critical role in organic thin-film transistors, yet it's difficult to measure. Resonant Soft X-ray Reflectivity (R-SoXR) is a unique and relatively simple method to investigate such interfaces. By tuning the soft X-ray energies, we are able to selectively and quantitatively characterize the interfacial width and thicknesses of the films. In an effort to relate performance to interface structure, we have used R-SoXR to investigate polystyrene (PS) or poly(methyl methacrylate) (PMMA) as the top layer and poly{\{}[N,N9-bis(2-octyldodecyl)-naphtha-lene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5, 59-(2,29-bithiophene){\}} (P(NDI2OD-T2)) as bottom layer supported on a Si substrate. We found that the device with PS as dielectric has a higher threshold voltage, which correlates to the increased interfacial trapping due to increased interfacial roughness. The extension of R-SoXR to the energy of fluorine K absorption edge is also demonstrated. [Preview Abstract] |
Thursday, March 24, 2011 1:15PM - 1:27PM |
W38.00011: Temperature Induced Structure Evolution of Regioregular Poly (3-hexylthiophene) in Dilute Solution and its Influence on Thin Film Morphology Charles Han, Ye Huang, He Cheng The structure evolution of regioregular Poly (3-hexylthiophene) (P3HT) in THF dilute solution, and its influence on thin film morphology were studied. A thermal treatment at high temperature effectively re-disperses P3HT micro-sized aggregates, and introduces two modes in DLS measurements. The structures of these 2 modes are identified, and the two structures of P3HT in dilute solution can greatly influence the morphology on subsequent thin films produced. Since the P3HT structures are carried into the film morphology, proper understanding and control of the structures in solution are important and can enhance electronic and opto-physical properties of the final devices. [Preview Abstract] |
Thursday, March 24, 2011 1:27PM - 1:39PM |
W38.00012: Effect of Cooling Rate on Microstructure and Charge Transport in Semiconducting Polymer Thin Films Evan Kang, Eunseong Kim Thermal annealing of polymer thin films often enhances charge carrier mobility which can be attributed to self-healing of the film morphology. We have investigated the effect of cooling rate following the annealing treatment on the thin film microstructure and the charge transport properties using a high performance semiconducting polymer, poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-b]thiophene) (PBTTT). The cooling rate plays a key role in determining the microstructure and performance of polymer thin films. Differential scanning calorimeter measurement shows that fast cooling suppresses the crystallization process. The microstructure of thin films is investigated by using 2D X-ray diffraction and atomic force microscopy. Slow cooling results in well-connected large domains with enhanced three dimensional ordering whereas fast cooling leads to misalignment of small domains with relatively rough surface. Transport characteristics at various temperatures show increase in the charge carrier mobility and decrease in the activation energy when the cooling rate is slowed. This change in the mobility and activation energy becomes saturated with cooling rate below 15 \r{ }C/min. [Preview Abstract] |
Thursday, March 24, 2011 1:39PM - 1:51PM |
W38.00013: Exponential behavior of the Ohmic transport in organic films Corneliu Colesniuc, Rudro Biswas, Samuel Hevia, Alexander Balatsky, Ivan Schuller An exponential dependence of conductance on thickness and temperature was found in the low voltage, Ohmic regime of copper (CuPc) and cobalt (CoPc) phthalocyanine, sandwiched between palladium and gold electrodes, unlike ever claimed in organic materials. A comparison with conventional models fails to explain all the data with a single set of parameters. On the other hand, a model which incorporates tunneling between localized states with thermally-induced overlap agrees with the data, and decouples the contributions to conductance from the electrode-film interface and the bulk of the film. Work supported by AFOSR, DOE and the UCOP program on carbon nanostructures. [Preview Abstract] |
Thursday, March 24, 2011 1:51PM - 2:03PM |
W38.00014: Atomistic Molecular Dynamics Simulation of the Surface Properties of P3HT Films Yeneneh Yimer, Sima Mofakham, Ali Dhinojwala, Mesfin Tsige In recent years P3HT has attracted much interest mainly because of its potential applications in solar cells, light emitting diodes and field effect transistors. The performance of these devices is strongly dependent on the structural packing, morphology and interfacial properties of the P3HT. In order to improve the devices efficiency, understanding the structural and dynamical properties of P3HT at the atomic level is important. Most studies on P3HT have mainly focused on understanding its bulk properties. However, the orientation of P3HT chains at the polymer/air interface has not been well investigated. Using molecular dynamics simulations we have studied the interfacial properties of free-standing P3HT films. The simulation results show that at the air/polymer interface the alkane side groups of the P3HT chains orient mainly to the interface in qualitatively good agreement with SFG experimental results. The surface tension of P3HT in its melt state shows strong dependence on temperature and chain length and is directly related to the roughness of the P3HT surface. [Preview Abstract] |
Thursday, March 24, 2011 2:03PM - 2:15PM |
W38.00015: Switching or triggering by light organic materials in the 100 nm size range Vina Faramarzi, Jean Francois Dayen, Bernard Doudin We investigate optoelectronic fabrication and characterization of organic electronics devices in the 100 nm range. This intermediate size has advantages in simplicity of device fabrication and robustness of observed properties. For this aim high aspect ratio lateral electrodes separated by a sub 100nm gap were produced by means of simple optical lithography techniques [1]. The electrical measurements set-up is integrated with an inverted optical microscope, allowing simultaneous optical and electrical measurements followed by temperature and magnetic field studies. We demonstrate that electrical contacts are suitable for a wide range of current measurements going from 10$^{-13}$ to 10$^{-2}$ A. This versatility makes the nanotrench design compatible for studying a broad variety of nanoparticles and molecular systems. Electrical transport properties of different devices are presented, e.g molecular switches, Iron based spin-transition nanoparticles, Conductive molecular chains and 2D nanoparticle networks. The promising reproducible results reveal novel intrinsic transport properties and confirm the high interest and reliability of this approach for further studies in the field of molecular electronics and spin dependent transport in molecular structures. \\[0pt] [1] J-F Dayen, V Faramarzi et al, \textit{Nanotechnology} \textbf{21}(33), 335303 (2010) [Preview Abstract] |
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