Effects of additional π-bridges on a terpolymer based on the second acceptor unit of DTBT and the performance of organic solar cells

Abstract

Fused electron acceptor units play a significant role in donor materials for organic solar cells (OSCs). This paper introduces the fused dithieno[3′,2':3,4;2″,3'':5,6]benzo[1,2-c][1,2,5]thiadiazole (DTBT) units into the benzo[1,2-b:4,5-b']dithiophene-benzo[1,2-c:4,5-c']dithiophene-4,8-dione (BDT-BDD) backbones to synthesize terpolymers. The photovoltaic performance of the terpolymers is investigated by adding the π-bridges on both sides of the DTBT. For Y6-based OSCs, the devices based on DTBT terpolymers without additional π-bridges can obtain high open-circuit voltage (V OC ) but low fill factor (FF). On the contrary, DTBT terpolymers with 3-octylthiophene π-bridges significantly enhance the FF but decrease the V OC in devices. By investigating terpolymers' photovoltaic and quantum chemistry properties, we find that the presence of π-bridge structures on both sides of the DTBT unit can effectively promote carrier collection and reduce the bimolecular recombination in the devices. However, its reduced charge-transfer state energy level results in high voltage losses. Therefore, the design of DTBT terpolymers for how to balance the voltage loss due to π-bridges brought to the optimization of polymer conformation and charge excitation and conversion will become an important subject. • A series of terpolymers based on dithienobenzothiadiazole are synthesized. • Additional π-bridges play non-negligible roles in terpolymers properties. • Balancing voltage loss and fill factor is vital in DTBT terpolymer-based OSC by π-bridges.