Molecular Engineering of D−π–A Copolymers Based on 4,8-Bis(4-chlorothiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene (BDT-T-Cl) for High-Performance Fullerene-Free Organic Solar Cells

Molecular conformation has a striking function in dominating the molecular orientation, crystallinity, charge mobility, film morphology, and photovoltaic performance for classic donor−π–acceptor (D−π–A) type copolymers. Herein, we systematically investigate this correlation by modulating the chemical structures of conjugated polymers composed of a new emerging electron-donating building block of 4,8-bis(4-chlorothiophen-2-yl)benzo[1,2- b:4,5- b ′]dithiophene (BDT-T-Cl). A D−π–A type copolymer PE31 is synthesized as the reference donor polymer, where BDT-T-Cl, benzotriazole (BTA), and thiophene (T) units are used as the D, A, and π-bridge, respectively. PE31 realizes a moderate power conversion efficiency (PCE) of 7.62% when paired with a nonfullerene acceptor Y6 . The methoxy substitutes in the BTA unit leads to a twisted backbone conformation of the final polymer PE32, which has negative effects on the charge transport and results in a slightly reduced PCE of 7.31%. The intra/interchain noncovalent interactions can be modulated by importing fluorine atoms on the BTA unit ( J52-Cl ) and further changing the π-bridge from thiophene to 3-hexylthieno[3,2- b ]thiophene ( PE4 ). The backbone of PE4 can be changed to a linear conformation, different from the zigzagged conformation of the other three polymers with thiophene as the π-bridge. PE4: Y6 shows the highest PCE of 14.02% with the highest fill factor (FF) of 0.75, superior to J52-Cl: Y6 combination (PCE = 12.31% and FF = 0.61). Our results clearly indicate that changing the backbone conformation by π-bridge engineering has an important significance of achieving the optimal active layer morphology and promising photovoltaic performance.