Double acceptor block-based copolymers for efficient organic solar cells: side-chain and π-bridge engineered high open-circuit voltage and small driving force

Abstract

Different from the traditional concept of donor–acceptor (D–A) structures, the newly reported design strategy of acceptor₁–acceptor₂ (A₁–A₂) type copolymers has been regarded as an efficient approach to boost the open circuit voltage (V_oc) of organic solar cells (OSCs). Herein, three A₁–A₂ type copolymers PBDD-BTZ-1, PBDD-BTZ-2 and PBDD-BTZ-3 with a 1,3-bis(thiophen-2-yl)-5,7-bis(2-ethylhexyl)benzo-[1,2-c:4,5-c′]dithiophene-4,8-dione (BDD) derivative as the A₁ unit and fluorine substituted benzotriazole (FTAZ) as the A₂ unit are prepared for OSCs. Subtle modification on the side chain and π-bridge is found to cause a big difference on the photoelectric properties and the resulting device performance of these new double A copolymers. A deep HOMO level of −5.63 eV with an extremely small driving force of 0.06 eV is found to be obtained by the PBDD-BTZ-3 copolymer. As a result, an impressive V_oc of 1.09 eV is obtained when it was blended with an ITIC-Th acceptor. On the other hand, the best device performance is achieved by the PBDD-BTZ-2 copolymer among these copolymers, due to its most favorable morphology. These results suggest that A₁–A₂ type copolymers can enrich the types of donor materials and will open a new possibility for developing efficient copolymers to boost the V_oc of OSCs with a small driving force by reasonable combination of different electron-deficient blocks.