Tailoring intra-molecular coupling in BDT-based copolymers to enhance their performance in fullerene-free organic solar cells

Three copolymers based on a 4,8-bis(4,5-dioctylthiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDTT) donor unit coupled with 6-(2-ethylhexyl)-5H-[1,2,5]thiadiazolo[3,4-f]isoindole-5,7(6H)-dione (P1), 6-octyl-4,8-di(thiophen-2-yl)-5H-[1,2,5]thiadiazolo[3,4-f]isoindole-5,7(6H)-dione (P2) and 2-(2-ethylhexyl)-6-octyl-4,8-di(thiophen-2-yl)-[1,2,3]triazolo[4,5-f]isoindole-5,7(2H,6H)-dione (P3) acceptors were computationally designed and experimentally synthesized to tailor the intramolecular coupling in their backbone. A considerable decrease in distortion energy in P2 compared to P1 proved the major role of the π-spacer in the copolymer in releasing steric strain. In comparison to the [1,2,3]triazolo[4,5-f]isoindole-5,7(2H,6H)-dione-based copolymer, P3, the lower electrostatic potential (ESP) of the [1,2,5]thiadiazolo[3,4-f]isoindole-5,7(6H)-dione acceptor in P1 has been observed to shift its LUMO energy level by about 0.5 eV. Furthermore, the electron donating properties of the copolymers increased in the order of P1 < P2 < P3 due to the synergistic contribution of each unit rather than a single unit, confirming the importance of tailoring the intramolecular coupling to control the electro-optical properties of the copolymers. Finally, the copolymer with a poorer electron acceptor unit (P3) was found to exhibit complementary absorption with the non-fullerene acceptor, ITIC, yielding a PCE of 8.87% in solar cell devices, further demonstrating the relevance of each unit in the copolymer intramolecular coupling.