Tuning molecular aggregation to enhance photovoltaic performance of polymers by isomerizing benzodithiophene moiety

Benzo[1,2-b:4,5-b']dithiophene derivatives (BDT) have been extensively utilized as electron-donating (D) units for the synthesis of polymer donor materials. However, its isomers, benzo[2,1-b:3,4-b']dithiophene derivatives (iBDT), are rarely employed for this application. In this research, three polymer donors, namely PTz-BDT, PTz-iBDT-L, and PTz-iBDT-H, were designed and synthesized using BDT and iBDT as the D moieties, respectively. Notably, compared to PTz-BDT, the iBDT-based polymers exhibit superior solubility and more favorable aggregation properties. When combined with the non-fullerene acceptor Y6, the iBDT-based polymers exhibit a more advantageous phase morphology, tighter packing, higher charge mobilities, more balanced charge transport, and less charge recombination in the devices. Consequently, the power conversion efficiency (PCE) of the PTz-iBDT-H:Y6 PSC reaches 11.04 %, significantly surpassing those of the PSCs based on PTz-BDT (7.15 %) and the iBDT-based polymers reported in literature. This study introduces an effective approach to enhance the performance of polymeric donors by isomerizing its backbone and synchronously increasing its molecular weight.