A Comparative Study on Hole Transfer Inversely Correlated with Driving Force in Two Non-Fullerene Organic Solar Cells

We report a faster rate of hole transfer under a smaller Δ_HOMO in a comparative study of two group organic solar cells (OSCs) consisting of IT-4F as an acceptor and PBDBT and PBDBT-SF as donors. In the OSCs based on PBDBT-SF:IT-4F, a higher short-circuit current (J_SC) was observed with a Δ_HOMO of 0.31 eV compared to a lower J_SC in PBDBT:IT-4F OSCs with a larger Δ_HOMO (0.45 eV). Intensive investigation indicates that the rate of transfer of a hole from IT-4F to PBDBT-SF or PBDBT is inversely proportional to the Δ_HOMO between IT-4F and donors. The larger J_SC in the PBDBT-SF:IT-4F device is attributed to a synergy of faster hole transfer, slower recombination, and rapid charge extraction enabled by desired morphology and balanced charge carrier mobilities with PBDBT-SF, suggesting that under a sufficiently high Δ_HOMO, comprehensive considerations of the transport, film morphology, and energy levels are needed when designing new materials for high-performance OSCs.