Significant influence of the benzothiophene ring substitution position on the photovoltaic performance of benzodithiophene-based donor polymers

In order to investigate the effects of the substitution position on the photovoltaic performance of donor polymers, two benzothiophene ring substituted benzo(1,2-b:4,5-b′)dithiophene (BDT)-based conjugated polymers (PBDTBTs-BDD and PBDTTBs-BDD) are designed and synthesized. The variation in the substitution position has small influences on the photophysical properties but has a great effect on the intramolecular π–π stack structure, charge transport and photovoltaic properties. PBDTBTs-BDD (with the 6-position of the benzothiophene substituent) exhibited a smaller π–π stacking distance of 3.67 A compared to 4.11 A seen for PBDTTBs-BDD (with the 2-position of the benzothiophene substituent). And the charge mobilities of PBDTBTs-BDD-based devices are higher and more balanced than those of PBDTTBs-BDD-based devices, which are highly beneficial for reducing recombination of free carriers and then lead to a higher short-circuit current density (JSC) and fill factor (FF) of devices. With ITIC or Y6 as non-fullerene acceptors, PBDTBTs-BDD-based devices exhibit power conversion efficiencies (PCE) of 7.76% and 12.07%, respectively, which are higher than those of PBDTTBs-BDD-based devices (5.04% and 5.81%). This work demonstrates that the photovoltaic properties of donor polymers can be highly tunable through slight modifications of their side chain structures.