Charge photogeneration dynamics in all-polymer solar cells based on polymerized small molecular acceptors with isomeric end-groups.

Isomerism of organic semiconductors is one of the critical factors contributing to the performance degradation of organic solar cells; however, the impact of isomerism on charge photogeneration dynamics remains poorly understood. In this work, we have investigated exciton properties as well as charge generation and recombination processes in organic solar cells based on the polymerized small molecular acceptors (PSMAs) with isomeric end-groups, PFBTz-T-γ (P1) and PFBTz-T-δ (P2), by using time-resolved spectroscopy. Our results show that while the exciton diffusion coefficient of P1 (8.7 × 10-3 cm2 s-1) is modestly lower than that of P2 (9.4 × 10-3 cm2 s-1), its exciton lifetime (∼514 ps) is significantly longer than that of P2 (∼162 ps). This results in a substantially higher exciton diffusion length for P1 (21.1 nm) compared to P2 (12.3 nm). Furthermore, charge transfer (CT) states in the blend film of donor/P1 dissociate more rapidly than those in the donor/P2 film. Additionally, we find that carrier recombination in donor/P1 and donor/P2 films is dominated by geminate recombination in ultrafast time scale, and the use of P1 can suppress carrier recombination. Optimized morphology, enhanced exciton diffusion, accelerated CT dissociation, and suppressed carrier recombination collectively contribute to a higher power conversion efficiency achieved by the donor/P1 solar cell.