Donor-Acceptor Organic Semiconductors: Investigations of Optical, Electronic, and Morphological Properties

Organic semiconductors have drawn remarkable attention due to their light weight, feasible fabrication and flexibility in the field of organic photovoltaics and field effect transistors. The rapidly increasing world population and the accompanied with huge energy demands are becoming a big concern for the future, which makes harvesting unlimited solar power using photovoltaic devices extremely important. A remarkable amount of research has been done to improve the power conversion efficiency in terms of the materials design, morphology investigations and device engineering. Organic field effect transistors utilizing solution-processed conjugated polymers can be fabricated efficiently at a low cost roll-to-roll technique over a large area. Designing high performing conjugated polymers and investigating their morphology is essential. In this dissertation, the fundamentals and the recent developments of organic semiconductors are covered in Chapter 1. The basic operation mechanism of organic photovoltaics and field effect transistors are introduced. Semiconducting molecules and polymers that have been reported using benzo[1,2-b;4,5-b’]dithiophene (BDT), benzo[1,2-b;4,5-b’]difuran (BDF), 2,1,3-benzothiadiazole (BT), and diketopyrrolopyrrole (DPP), are discussed and summarized. Chapter 2 describes the isomeric effect of two small molecules containing BDT and BT units on the photovoltaic performance. The influence on UV-vis absorption, frontier molecular orbital energy level, and morphology due to the position of the BT unit was systematically investigated. The photovoltaic performance was studied in bulk heterojunction solar cells with [6, 6]-phenyl-C71-butyric acid methyl ester used as the acceptor. Chapter 3 describes the synthesis of novel conjugated polymers built from BDF and furan substituted DPP unit. Furan and its derivatives are regarded as green and renewable building units. This polymer was tested in bulk heterojunction solar cells with the highest power conversion efficiency of 5.55% and high fill factor of 0.73 achieved when 4% diphenyl ether was applied to optimize the phase separation. The morphology of the blend films was investigated by atomic force microscopy, grazing incident wide-angle X-ray scattering and transmission electron microscopy. In Chapter 4, an alternative copolymer built from furan substituted DPP and (E)-1,2-di(furan-2-yl)ethene was synthesized by Stille coupling and employed in organic field effect transistors. Hole mobility of 0.42 cm2 V-1 s-1 was achieved with current on/off ratio of 104 after annealing the thin film at 150 oC for 5 minutes. The higher mobility after thermal annealing was explained by the increased crystallinity, which was revealed by atomic force microscopy and grazing incident X-ray diffraction. At the end, the prospective and future work for the organic photovoltaics and field effect transistors are discussed in Chapter 5.