Diselenogermole as a novel donor monomer for low band gap polymers

We report the synthesis of a new diseleno[3,2- b :2′,3′- d ]germole (DSG) monomer containing branched 2-ethylhexyl sidechains on the bridging germanium group. The utility of this new electron rich monomer is explored by co-polymerisation of a distannylated DSG monomer with N -octylthienopyrrolodione to afford a soluble low band gap polymer. The DSG containing polymer shows a broader and more red-shifted absorption spectrum compared its thiophene analogue. Polymer solar cells are fabricated from blends with PC 71 BM exhibiting initial power conversion efficiencies of 5.2% in inverted bulk heterojunction solar cells.