Effects of Side Chains on Thiazolothiazole-Based Copolymer Semiconductors for High Performance Solar Cells

New thiazolothiazole-dithienosilole copolymer semiconductors bearing side chains of different type, size, and topology were synthesized and used to demonstrate the influence of side chains on morphology, charge transport and photovoltaic properties. The field effect mobility of holes varied from 0.01-0.03 cm(2)V(-1)s(-1) in PSOTT and PSEHTT to 0.12 cm(2)V(-1)s(-1) in PSOxTT. The average power conversion efficiency of solar cells under 1.0 sun illumination could be varied from 2.1% in PSOxTT and 4.1% in PSOTT to 5.0% in PSEHTT. The highest photovoltaic efficiency achieved in PSEHTT, that has all-branched alkyl side chains and face-on pi-stacking orientation, was corroborated by its enhanced charge photogeneration observed by transient absorption spectroscopy.