Abstract
Low bandgap conjugated copolymers composed of thiophene-capped diketopyrrolopyrrole and benzene with/without fluorination were synthesized to investigate the effect of fluorine substitution on their photovoltaic properties. With increasing the number of substituted fluorine atom on benzene, both HOMO and LUMO energy levels of the copolymer are lowered and its crystallinity is increased. The fibril size of copolymer:PC71BM blend becomes smaller as the number of substituted fluorine increases. As a result, the copolymer with two fluorine substitution exhibits deeper HOMO energy level (−5.30eV), leading to higher VOC (0.72V), and also enhanced crystallinity (polymer chain packing), leading to higher JSC (12.4mAcm−2) as compared to mono- and non-fluorinated ones. The copolymer with two fluorine atoms shows a promising power conversion efficiency of 5.63%.
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