Effect of fluorine substitution on photovoltaic performance of DPP-based copolymer

•Copolymers composed of DPP and benzene with/without fluorination were synthesized.•Fluorination lowers the frontier energy levels of polymers.•Fluorination leads to smaller size of fibril in polyemr:PC71BM blends.•The copolymer with two fluorine atoms shows a promising PCE of 5.63%.

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 (−5.30 eV), leading to higher VOC (0.72 V), and also enhanced crystallinity (polymer chain packing), leading to higher JSC (12.4 mA cm−2) as compared to mono- and non-fluorinated ones. The copolymer with two fluorine atoms shows a promising power conversion efficiency of 5.63%.

Graphical abstractFluorination leads to deeper HOMO energy level, higher SCLC hole mobility and finer fibril size of polymer and therefore p2FPhDPP with two fluorine atoms exhibits a promising PCE of 5.63%.Figure optionsDownload full-size imageDownload as PowerPoint slide