Synthesis and applications of difluorobenzothiadiazole based conjugated polymers for organic photovoltaics

A new electron deficient building block, difluorobenzothiadiazole (FBT) was designed and synthesized for the first time. Two low band gap polymers containing this unit or benzothiadiazole (BT) and an electron rich unit, benzo[1,2-b:4,5-b′]dithiophene (BDT) were synthesized by Stille coupling reaction. It was found that fluorine substitution significantly lowered the energy levels but did not affect the UV absorption properties of the polymer. Comparatively, the fluorinated polymer showed a higher melting point and a reduced solubility. The electrical and photovoltaic properties of these polymers were studied by fabricating thin film transistors and polymer solar cells. The solar cell based on FBT polymer and PC71BM gives power conversion efficiency up to 3.4% under AM 1.5 G illumination (100 mW cm−2).