One Interface, Two Driving Forces: Channel-Resolved Ultrafast Spectroscopy Separates Charge-Transfer Rate from Charge-Generation Yield in PTO2:IT-4F Organic Solar Cells

Abstract Efficient charge transfer at near-zero frontier-orbital offset is established; whether a fast transfer rate guarantees a high charge-generation yield is not. We examine this in PTO2:IT-4F (PCE 14.7%), whose HOMO (∼0.07 eV) and LUMO (∼0.52 eV) offsets are strongly asymmetric: 720 nm excitation opens only the near-zero-offset hole channel, whereas 530 nm excitation also opens the conventional-offset electron channel at the same junction. At 720 nm, a PTO2 hole-polaron bleach at 550 nm, free of acceptor features, reports hole transfer directly. Global target analysis in which all IT-4F excitons share one spectrum, requiring no extinction coefficients, shows that only about 45% of the excitons transfer in 0.57 ps; the rest transfer in 4.5 ps after diffusing to an interface, for a total injection yield of at least 60%. The yield is limited downstream: about 42% of the bound pairs recombine geminately (τ ≈ 140 ps). At 530 nm, energy transfer and direct electron-transfer quench PTO2 excitons within the instrument response in similar proportions (energy transfer ≈0.5, electron transfer ≈0.5). A delayed rise assigned to long-range separation persists in PTO2:IT-4F but is nearly absent in PTO2:PC71BM despite its larger HOMO offset, pointing to the donor/acceptor pair rather than energetic alignment as the origin of the high efficiency.