Cascaded energy landscape as a key driver for slow yet efficient charge separation with small energy offset in organic solar cells

Recent studies have shown that efficient free carrier (FC) generation with a small voltage loss can be achieved in organic solar cells (OSCs); however, the photophysical insights underpinning this remain unclear. Herein, we examined the mechanisms underlying the FC generation in a state-of-the-art OSC consisting of PM6 and Y6 as an electron donor and electron acceptor, respectively, wherein the energy offset between the lowest excited singlet state and the charge transfer state is as small as ~0.1 eV. We used transient absorption spectroscopy to track the time evolution of electroabsorption caused by electron–hole pairs generated at donor/acceptor interfaces. Upon photoexcitation of the lower-bandgap Y6, we observed slow yet efficient spatial charge dissociation on a time scale of picoseconds. Based on temperature dependence measurements, we found that this slow yet efficient FC generation is driven by downhill energy relaxation of charges through the energy cascade generated near the interfaces.