Effects of fluorine substitution on photovoltaic properties of DPP-based copolymer for polymer solar cells

Abstract

New low bandgap conjugated copolymers composed of DPP as electron-accepting unit and benzene with/without fluorination as electron-donating unit were synthesized (as denoted by pPhDPP, p1FPhDPP and p2FPhDPP) to investigate the effect of the number of substituted fluorine on their photovoltaic properties. As the number of fluorine substitution increases, the HOMO and LUMO energy levels are lowered and the crystallinity of the copolymer is increased. Also, the fibril size of the copolymer:PC71BM blend becomes smaller as the number of fluorine substitution increases. As a result, deeper HOMO energy level (-5.32 eV) of p2FPhDPP (two fluorine substation) leads to higher VOC (0.72 V), and enhanced polymer packing leads to higher JSC (12.4 mA/cm2) as compared to non-fluorinated one, leading to a promising power conversion efficiency of polymer solar cell.