PBnDT-FTAZ,a “weak donor-strong acceptor” polymer that combines benzodithiophene(BnDT)and fluorinated benzotriazole(FTAZ),has received significant amount of attention because it is a peculiar material with many outstanding features that need to be further explained.For example,with a medium band gap of 2.0 V,PBnDT-FTAZ can still generate over 7% power conversion efficiency with a noticeably high fill factor over 70% in its BHJ cells with a relatively thick film(200 – 300 nm).In this talk,I will try to summarize our further understanding and optimization of this interesting polymer.First,we discover that all these interesting properties shown by PBnDT-FTAZ can only be obtained with polymers of a quite narrow range of molecular weight.The molecular weight induced morphological changes in the polymer:fullerene blend provide the “missing link” that correlates the molecular weight with the photovoltaic device performance.Second,with a series of five copolymers with varying degrees of fluorination,we show that the photovoltaic properties of PBnDT-FTAZ vary depending upon the percentage of fluorine incorporation.We further reveal that increasing the fluorine content results in improved hole mobility,which appears to be the key factor for the “fluorine impact” in this series of polymers.