Structure Evolution of Fluorinated Conjugated Polymers Based on Benzodithiophene and Benzothiadiazole for Photovoltaics

Two novel copolymer PTBDTffDC(8)TBT and PTBDTffDC(8)TBT-FB based on alkylthienyl benzodithiophene (TBDT) and fluorinated benzothiadiazole (ffDTBT) blocks were synthesized, and the effects of n-octyl side chains (C-8) on thiophene bridge and pentafluorobenzene (FB) end group on properties of the copolymers has been investigated. With the similar properties of good thermal stability, wide spectral absorption (band gap of 1.68 eV), the molecular energy levels, and the arrangement of the polymer chains, the best pristine power conversion efficiency of PTBDTffDC(8)TBT-FB:PC61BM (1:1) was higher than that of PTBDTffDC(8)TBT, 3.6% versus 2.1%. A more optimal microphase separation morphology of PTBDTffDC(8)TBT-FB:PC61BM was one of the driving force to enhance the short-circuit current density (J(sc)) and fill factor (FF), consequently resulting in the improved performance. Comparison of the five analogous polymers was implemented from both experiment measurement and density functional theory simulation in detail. It was demonstrated that the FB end group contributed to promote the molecule dipole moment, which benefit the exciton separation for improvement in the performance.