Bulletin of the American Physical Society
2006 APS March Meeting
Monday–Friday, March 13–17, 2006; Baltimore, MD
Session D4: Organic Electronics |
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Sponsoring Units: DPOLY DMP Chair: Chang Yeol Ryu, Rensselaer Polytechnic Institute Room: Baltimore Convention Center 308 |
Monday, March 13, 2006 2:30PM - 3:06PM |
D4.00001: Excited states at polymer semiconductor heterojunctions. Invited Speaker: Excitons in bulk organic semiconductors are generally strongly bound due to the weakly-screened electrostatic electron-hole interaction, but can be de-stabilized at heterojunctions between semiconductors with substantial band-edge offsets. The charge-transfer states that then forms at the heterojunction (exciplexes) can still show radiative emission (with strongly increased radiative lifetimes and large red-shifts), and still require an external field to enable long-range charge separation as required in a photovoltaic diode. The effect of DC field on this process of charge separation, will be discussed, as measured by the suppression of exciplex luminescence and photocurrent. The strength of interchain interactions can be increased under pressure, and measurements of absorption and time-resolved luminescence under hydrostatic pressure will be reported. These reveal the changing character of the exciplex at the heterojunction. [Preview Abstract] |
Monday, March 13, 2006 3:06PM - 3:42PM |
D4.00002: Defects and Deformation in Organic Molecular Semiconductors Invited Speaker: We have been investigating microstructural defects in crystalline organic molecular semiconductors such as pentacene and soluble pentacene derivatives. We have examined the structure of grain boundaries, dislocations, and vacancies in these materials by polarized optical microscopy, X-ray diffraction, transmission electron microscopy, electron diffraction, low voltage electron microscopy, and low dose high resolution electron microscopy. We have quantified the nature and extent of local relaxations that occur near these structural defects. We have also studied the micromechanisms of plasticity using nanoindentation and scratching. Our microstructural studies provide detailed information about the defect-mediated mechanisms of plastic flow. We have also examined pentacene derivatives that can be processed from solution, and have found information about solid-state phase transformations that result in internal cracking within the organic semiconductor crystals. We have also examined the relationship between the type and density of these defects and the macroscopic properties in devices such as thin-film transistors. [Preview Abstract] |
Monday, March 13, 2006 3:42PM - 4:18PM |
D4.00003: Solution-Processable Organic Semiconductors and Conductors: Viable Materials for Functional Thin-Film Transistors Invited Speaker: Large-area displays based on organic materials promise low-cost fabrication, lightweight construction, mechanical flexibility and durability. To truly realize the low-cost aspects of organic electronics, however, conventional high-vacuum deposition technologies will have to be replaced by solution processing methodologies. This need has in turn driven the development of solution-processable organic semiconductors and conductors. We have focused on fabricating thin-film transistors with triethynylsilyl antradithiophene (TES ADT), a solution-processable p-type organic semiconductor. Subjecting the as-cast thin films of TES ADT to short solvent vapor annealing dramatically increases the device characteristics: we observe three orders of magnitude increase in carrier mobility and current on/off ratio, and a decrease in current hysteresis and threshold voltage. The improvement in the electrical characteristics can be directly correlated with morphological transformations during solvent vapor annealing. Our efforts in solution-processable organic conductors focus on water-dispersible polyaniline (PANI). We have fabricated bottom-contact thin-film transistors with PANI electrodes, which function as effectively as gold electrodes, when on-characteristics are concerned. Examination of the linear source-drain voltage regime suggests that PANI devices exhibit markedly less contact resistance than gold devices. [Preview Abstract] |
Monday, March 13, 2006 4:18PM - 4:54PM |
D4.00004: Structural Effects on the Performance of Polymeric Thin Film Transistors Invited Speaker: Semiconducting polymers are currently being researched for use in thin-film transistors (TFTs) in large-area electronic devices such as displays. Most high performance semiconducting polymers form semicrystalline thin films, but the details of the connection between their structure and the field-effect mobility of charge carriers in these materials in TFTs are poorly understood. We have used a variety of techniques to reveal the connection between the microstructure of semiconducting polymers and their electrical performance. We have used synchrotron x-ray scattering to determine the molecular ordering in thin films of several high-performance poly(thiophene)-based semiconducting polymers including poly[5,5'-bis(3-dodecyl-2-thienyl)-2,2'-bithiophene], known as PQT-12, and poly(2,5-bis(3-alkylthiophen-2-yl)thieno[3,2-$b$]thiophene). These materials show evidence of thermotropic liquid crystalline phases that can be used to improve the ordering in the films. We have used a delamination technique to transfer these films between substrates without substantially perturbing their structure to help disaggregate the contributions of film structure and of interfacial structure to device characteristics. Using films of constant morphology, we have found that chemically modified dielectrics can have a strong influence on the threshold voltage of TFTs, but have little impact on mobility. These results suggest that the field-effect mobility is mainly controlled by the structure of the first few molecular layers of the semiconducting films at the interface with the gate dielectric. [Preview Abstract] |
Monday, March 13, 2006 4:54PM - 5:30PM |
D4.00005: NEXAFS measurements of the development of interfacial order in organic semiconductor thin films. Invited Speaker: Interest in organic semiconductors has increased because of their potential use in new electronics applications such as radio frequency identification tags, biosensors, or photovoltaics. The development of solution processable organic semiconductors has made it possible to take advantage of low-cost processing methods such as spin coating, dip coating, or ink-jet printing onto flexible substrates. However, the performance of these materials in devices is difficult to control and new processing methods can deliver unexpected results. These deviations are often due to variability in film microstructure that leads to variability in carrier transport properties. The microstructure is sensitive to processing variables because they influence the dynamic assembly process of the material as it dries from a solution to a solid thin film that is crystalline or semicrystalline. To quantify structural changes, we employ synchrotron-based near-edge X-ray absorption fine structure (NEXAFS) spectroscopy, which measures the soft X-ray excitation of 1s electrons to unfilled molecular orbitals. For a polymer semiconductor, regioregular poly(3-hexylthiophene) (P3HT), we show that variations in the spin-coating process may be a source of differences in the P3HT microstructure because of changes in complex balance of inertial forces and solvent evaporation during the solidification of the film from solution. The morphology of semicrystalline polymers such as P3HT is affected by the rate of solidification; slower solvent removal rates result in materials closer to their equilibrium structure. For thermally convertible, oligomeric organic semiconductors, we show that the formation of interfacial order of the molecules is affected by the oligomer length. [Preview Abstract] |
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