Thiophene fluorination to enhance photovoltaic performance in low band gap donor-acceptor polymers.

We report the first synthesis of a tetrafluorinated 4,7-bis(3,4-difluorothiophen-2-yl)-2,1,3-benzothiadiazole monomer and its polymerisation with dithieno[3,2-b:2',3'-d]germole by Stille coupling to afford a low band gap polymer with a high ionisation potential. Direct comparison to the non-fluorinated analogue demonstrates that fluorination results in an increase in ionisation potential with no change in optical band gap, and enhanced aggregation over the non-fluorinated polymer. These desirable properties result in a significant enhancement in OPV device performance in blends with PC(71)BM.