Bulletin of the American Physical Society
2005 APS March Meeting
Monday–Friday, March 21–25, 2005; Los Angeles, CA
Session D1: Crystalline Organic Semiconductors: Materials to Devices |
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Sponsoring Units: DCMP Chair: Mikhail Itkis, University of California, Riverside Room: LACC 152 |
Monday, March 21, 2005 2:30PM - 3:06PM |
D1.00001: Density of States and Trapping in Pentacene Invited Speaker: We show that optical and electrical measurements on pentacene single crystals can be used to extract the density of states in the HOMO-LUMO bandgap [1]. It is found that these highly purified crystals possess band tails broader than those typically observed in inorganic amorphous solids. Results on field effect transistors (FETs) fabricated from similar crystals imply that the gap state density is much larger within 5-10 nm of the gate dielectric. We also describe a defect generation phenomenon that is new to organic semiconductors [2]. A defect in pentacene single crystals can be created by bias stress and persists at room temperature for an hour in the dark but only seconds with 420nm illumination. The defect gives rise to a hole trap at E$_{v}$ + 0.38eV and causes metastable transport effects at room temperature. Creation and decay rates of the hole trap have a 0.67eV activation energy with a small (10$^{8}$ s$^{-1})$ prefactor, suggesting that atomic motion plays a key role in the generation and quenching process. [1] D. V. Lang, X. Chi, T. Siegrist, et al., Phys. Rev. Lett. 93, 086802 (2004); [2] D. V. Lang, X. Chi, T. Siegrist, et al., Phys. Rev. Lett. 93, 076601 (2004). [Preview Abstract] |
Monday, March 21, 2005 3:06PM - 3:42PM |
D1.00002: Ultrafast carrier dynamics in organic molecular crystals and conjugated polymers Invited Speaker: Organic semiconductors are being extensively studied by many research groups around the world for applications in electronic and photonic devices. For example, much work has focused on the development of organic thin film transistors based on thermally evaporated pentacene films, where the polycrystalline morphology typically results in a thermally-activated carrier mobility. On the other hand, more intrinsic bandlike transport, where the carrier mobility increases as the temperature decreases, has been observed in many organic single crystals. However, the nature of charge transport in organic molecular crystals is still not understood. Also, despite many advances in organic photonics, the nature of photocarrier generation in organic semiconductors is not completely understood and remains controversial even today. The generation of mobile charge carriers in photoexcited organic materials occurs over femtosecond to picosecond time scales, and so ultrafast pump-probe experiments are essential in order to improve our understanding of fundamental processes in these materials. Recently, time-resolved terahertz pulse spectroscopy has been used to directly probe transient photoconductivity in pentacene and functionalized pentacene thin films and single crystals [1,2], revealing photogeneration of mobile charge carriers over sub-picosecond time scales as well as bandlike carrier transport in both single crystal and thin film samples [1]. This talk will provide an overview of ultrafast carrier dynamics in organic semiconductors, and will emphasize how time-resolved terahertz pulse spectroscopy can be used to help understand the nature of photoexcitations and carrier transport in organic materials. (This work was supported by NSERC, CFI, CIPI, the Killam Trust, and ONR. Collaborators for this work are listed in Ref. 1.) [1] O. Ostroverkhova, D. G. Cooke, S. Shcherbyna, R. F. Egerton, F. A. Hegmann, R. R. Tykwinski, and J. E. Anthony, Phys. Rev. B., in press. [2] V. K. Thorsm{\o}lle, R. D. Averitt, X. Chi, D. J. Hilton, D. L. Smith, A. P. Ramirez, and A. J. Taylor, Appl. Phys. Lett. 84, 891 (2004). [Preview Abstract] |
Monday, March 21, 2005 3:42PM - 4:18PM |
D1.00003: High quality crystalline pentacene and rubrene FETs Invited Speaker: Molecular organic materials offer the promise of novel electronic devices but also present challenges for understanding charge transport in narrow band systems. We find that one of the most important intermolecular transport FET parameters, the effective channel mobility, is parameterized by two factors: (1) the degree of carrier trapping in localized DOS band-tail states, which are higher in concentration for FET structures than for bulk crystal, and (2) the free-carrier mobility, $\mu _{0}$. Our analysis shows crystalline devices possess $\mu _{0}\sim $70 cm$^{2}$/Vs, significantly greater than polycrystalline thin film devices where free-carrier mobility $\mu _{0}\sim $1 cm$^{2}$/Vs. Low temperature studies elucidate fundamental transport processes. We report the lowest temperature field effect transport results on a crystalline oligomeric organic material, rubrene. Gated transport shows a factor of $\sim $10 suppression of the thermal activation energy in 10-50 K range and nearly temperature independent resistivity below 10 K. Other examples of 2 dimensional charge carrier transport will also be discussed. [Preview Abstract] |
Monday, March 21, 2005 4:18PM - 4:54PM |
D1.00004: Intrinsic Charge Transport in Organic Field-Effect Transistors Invited Speaker: Organic field-effect transistors (OFETs) are essential components of modern electronics. Despite the rapid progress of organic electronics, understanding of fundamental aspects of the charge transport in organic devices is still lacking. Recently, the OFETs based on highly ordered organic crystals have been fabricated with innovative techniques that preserve the high quality of single-crystal organic surfaces. This technological progress facilitated the study of transport mechanisms in organic semiconductors [1-4]. It has been demonstrated that the intrinsic polaronic transport, not dominated by disorder, with a remarkably high mobility of ``holes'' $\mu$ = 20 cm$^2$/Vs can be achieved in these devices at room temperature [4]. The signatures of the intrinsic polaronic transport are the anisotropy of the carrier mobility and an increase of $\mu$ with cooling. These and other aspects of the charge transport in organic single-crystal FETs will be discussed. \newline \newline Co-authors are Etienne Menard, University of Illinois at Urbana Champaign; Valery Kiryukhin, Rutgers University; John Rogers, University of Illinois at Urbana Champaign; Michael Gershenson, Rutgers University. \newline \newline [1] V. Podzorov et al., Appl. Phys. Lett. 82, 1739 (2003); ibid. 83, 3504 (2003). \newline [2] V. C. Sundar et al., Science 303, 1644 (2004). \newline [3] R. W. I. de Boer et al., Phys. Stat. Sol. (a) 201, 1302 (2004). \newline [4] V. Podzorov et al., Phys. Rev. Lett. 93, 086602 (2004). [Preview Abstract] |
Monday, March 21, 2005 4:54PM - 5:30PM |
D1.00005: Gap states in organic semiconductors: Theory Invited Speaker: |
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