A low band gap co-polymer of dithienogermole and 2,1,3-benzothiadiazole by Suzuki polycondensation and its application in transistor and photovoltaic cells

We report the preparation of a 4,4'-bis(2-ethylhexyl) dithieno[3,2-b:2',3'-d]germole monomer and its polymerisation with 2,1,3-benzothiadiazole. Unlike the analogous dithienosilole, the dithienogermole is sufficiently stable under basic conditions to enable Suzuki polycondensation. The resulting high molecular weight polymer exhibits a high propensity to order in the solid state and demonstrates charge carrier mobilities up to 0.11 cm(2) V-1 s(-1) in field effect transistors. A PGeBTBT based BHJ solar cell device achieved a PCE of 4.5% with a high J(sc) of 18.6 mA cm(-2).