Improving Organic Thin-Film Transistor Performance by Controlling the Orientation of Solution-Processible Microcrystallites and Doping with Electron Acceptor.
| Autor: | Li, Ling-Ru, 李伶如 |
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| Rok vydání: | 2011 |
| Druh dokumentu: | 學位論文 ; thesis |
| Popis: | 99 In the thesis, we detail the progress made in improving the thin films of 2,8-difluoro-5,11-bis (triethylsilylethynyl) anthradithiophene (diF-TESADT) for high mobility OFET application. In part (I) experiment, different post-annealing methods were used to improve the organic thin film structure. Morphology, structure, and crystallinity of diF-TESADT films are of particular concern and investigated by combined techniques of atomic force microscopy (AFM), grazing-incidence x-ray diffraction (GIXD), and near-edge x-ray absorption fine structure (NEXAFS) spectroscopy. The diF-TESADT films produced from spin-coating exhibit wheat-like features and GIXD data reveal the presence of both (001)- and (111)-oriented crystallites on the surface. Despite the mixed orientation, as-grown diF-TESADT can still produce OFET with a best mobility of 0.37 cm2V-1 s-1. Improvement in film quality is achieved by employing solvent annealing followed by thermal annealing. The resultant diF-TESADT films exhibit smooth, plate-like features, and GIXD data show a complete structural transformation to (001)-oriented crystallite, a much favored structure for efficient hole transport. The highest OFET mobility reaches 2.70 cm-1V-1s-1, with the median mobility averaged over 15 devices equal to 1.4 cm-1V-1s-1. In part (II) experiment, we used the strong electron-acceptor 2,3,5,6-tetrafluoro- 7,7,8,8-tetracyanoquinodimethane (F4-TCNQ), as a dopant in diF-TESADT thin film to improve the charge transfer properties. We investigated how the energy levels of organic semiconducting thin films of diF-TESADT change with the F4-TCNQ dopant concentration with ultraviolet photoemission spectroscopy (UPS) and x-ray photoemission spectroscopy (XPS). The results showed that the shift the energy level of diF-TESADT -0.3 eV. The effect of solvent vapor annealing on the morphology and device performance of thin films was also investigated. When molar dopant ratio of reaches 0.37%, the unvanished plate-like features give a small degradation of mobility but the devices averaged threshold voltage shifts from 21.3 V to 1.47 V. We concluded the dramatic VTH variation is presumably due to the structure reorganization of thin film when post annealing. As a consequence, F4-TCNQ effectively neutralized the residual dipoles at the interface between semiconductor layer and the dielectric substrate. |
| Databáze: | Networked Digital Library of Theses & Dissertations |
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