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 cm² V⁻¹s⁻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.