Study On Self-organization Of Active Layer And Performance Enhancement Mechanism Of Polymer RR-P3HT Organic Field-effect Transistors

Polymer organic field-effect transistors (OFET) are regarded as the most promising technology of microelectronics and organic electronics due toπ-conjugated polymer possessing excellent mechanical property, reasonable thermal stability, easy solution-processability, large coverage area, flexibility of device, and potential cost advantage. However, at present, the low mobility and deficient conduction of polymer OFET limit their applications. In order to improve their properties, the mechanism of microstructure self-organization of organic semiconductor that is regioregular poly(3-hexylthiophene) (RR-P3HT) of polymer OFET is investigated in this thesis. The conduction mechanisms of polymer are revealed, which provides a theoretical guidance to design and prepare polymer OFET with high performance in the future. Detail researches and results are as follows:Firstly, the crystallization action, the microstructure change of self-organization and the resulted conduction mechanisms of polymer semiconductor active thin layer in polymer OFET are investigated by synchrotron radiation grazing incident X-ray diffraction (GIXRD) for understanding the relationships between polymer self-organization and charge carrier mobility.The results indicate that self-assembled monolayers (SAMs) as the modified layer significantly improve the interface quality between the insulator layer and the organic semiconductor layer that is RR-P3HT. The change of the crystalline microstructure of RR-P3HT clarifies the effect of SAMs for improving the interface between the insulator layer and the organic semiconductor layer. The self-organiztion of RR-P3HT modified by SAMs improves the crystalliztion to pack form the thiophene rings along the perpendicular direction of substrate and results that theπ-πinterchains are stacked to parallel the substrate. The two-dimensional charge transport is improved. In addition, annealing under a suitable temperature can facilitate the process of self-organization of polymer thin film. Furthermore, we find that two-dimensional, conjugated, and self-organized crystalline lamellae are easier gained with slow grown film than with fast grown film.Secondly, the self-organization process is controlled by effective physical and chemical means to improve the performance of RR-P3HT OFET, especially the field-effect mobility. (1) Self-organization is improved by interface modification to enhance the performance of RR-P3HT OFET and the mechanism is investigated. Hexamethyldisilizane is used as the modification solution. The effective change of chemical processes and surface physical properties by SAMs modification, which favor for self-organized crystalline lamellae and molecule orientation, results in the enhancement of the performance of RR-P3HT OFET. In addition, the relationship between the morphology and charge transport properties of RR-P3HT is investigated in details. Charge transport properties of RR-P3HT deposited by different methods and the homogenization of drop-cast film are proved by measurements of surface morphology and crystallization. It is concluded that well-defined fibrillar morphology of drop-cast slow grown film is favored to charge transport, comparing worm morphology of spin-coat fast grown film.(2) Self-organization is improved by thermal annealing to enhance the performance of RR-P3HT OFET. We find that the crystal structure, the molecules interconnection, the surface morphology, and the charge carrier mobility of polymer films are affected by vacuum relaxation and annealing at suitable temperature (150℃) which facilitate the structure of RR-P3HT and result in the enhancement of field-effect mobility.(3) Self-organization is improved by low temperature solution-process to enhance the performance of RR-P3HT OFET. At low temperature, RR-P3HT solution properties are improved by optimizing concentration and non-solvent addition, and the quality of polymer film and the performance RR-P3HT OFET are improved. The results indicate that the performance of RR-P3HT OFET is improved drastically with the increase of RR-P3HT weight percentages in chloroform solution due to the formation of more microcrystalline lamellae and bigger nanoscale islands. On the basis of the results of surface morphologies and electrical properties, we presume that the charge carrier mobility depends on the morphology and thickness of organic active layer. Furthermore, we demonstrate that the thickness dependence of surface morphology and charge carrier mobility in polymer (RR-P3HT) OFET and small molecule (pentacene) OFET are different. In addition, an appropriate non-solvent addition (acetonitrile and ethanol) can improve the self-organization of polymer semiconductor layer, resulting in performance enhancement of polymer OFET.