Studies of Organic Semiconductor Nanostructures and Their Photovoltaic Applications

Thesis (Ph.D.)--University of Washington, 2013Organic solar cells are promising by virtue of their low-cost production, mechanical flexibility of plastics, and the range of possible applications. Although progress has been made in developing organic solar cells in the past decade, the power conversion efficiency now about 8-10% is still substantially lower than silicon-based devices. It has been recognized that the photovoltaic conversion process in organic solar cells is dependent on the morphology of the photoactive layer which consists of a binary blend of donor and acceptor materials. This work explores different approaches to controlling the morphology of bulk heterojunction polymer solar cells towards improving the photovoltaic efficiency, including diblock copolymer assemblies, organic semiconductor nanowires, and the use of processing additives. In addition, we explore a new method of characterizing the nanoscale morphology of polymer solar cells. Investigation of the photovoltaic properties, charge transport, and morphology of a series of diblock conjugated copolymers as a function of block composition showed that the highest efficiency was achieved at the 50% block composition. Nanowires assembled from diblock copolythiophenes of different compositions showed a tunable average aspect ratio (length/width) of 50-260, which revealed an increase of efficiency with increasing aspect ratio. All-nanowire solar cells comprising a polymer nanowire donor and a small-molecule nanowire acceptor were found to have enhanced photovoltaic efficiency. The use of a processing additive was found to give optimum device performance in benzobisthiazole-based donor-acceptor copolymer/fullerene and poly(3-hexylthiophene)/non-fullerene photovoltaic blend systems. The performance of non-fullerene polymer solar cells was enhanced 10-fold by using only 0.2 vol% additive and the mechanism of enhancement in efficiency was explained in terms of the optimized nanoscale morphology. Scanning transmission electron microscopy coupled with energy dispersive X-ray spectroscopy was successfully used for the first time to image the nanoscale morphology of all-polymer bulk heterojunction solar cells, demonstrating high spatial resolution with chemical specificity