Synthesis of a low bandgap polymer based on a thiadiazolo-indolo[3,2-b] carbazole derivative for enhancement of open circuit voltage of polymer solar cells

For the purpose of synthesizing an electron donor polymer with a low-lying highest occupied molecular orbital (HOMO) energy level and thus to achieve high open circuit voltage (V_OC) of polymer solar cells, an indolocarbazole derivative, 3,8-bis-(4-octyl-thiophene-2-yl)-5,6-didodecyl-thiadiazolo- indolo[3,2-b]carbazole, was synthesized and used as a building block of a low bandgap alternating copolymer. An alternating copolymer, poly(3,8-bis-(4-octyl- thiophene-2-yl)-5,6-didodecyl-thiadiazolo-indolo[3,2-b]carbazole-alt-2,1, 3-benzothiadiazole) (PTICThBT), was synthesized via the Suzuki coupling reaction, and the copolymer exhibited a deep HOMO level of -5.4 eV. When a blend of PTICThBT and PC₇₁BM was used as an active layer in bulk heterojunction polymer solar cells, a high V_OC of 0.84 V was obtained with a power conversion efficiency of 2.62% under AM 1.5G condition.