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 discuss our ongoing efforts to further understand this interesting polymer.First,we discovered that all these interesting properties shown by PBnDT-FTAZ can only be obtained with polymers of a quite narrow range of molecular weight.Second,molecular engineering of PBnDT-FTAZ by substituting alkylthiol and selenophene in PBnDT-FTAZ give mixed results: Voc seems fixed around 0.8 V; however,Jsc can be further increased.