Impact of solvent-induced morphological changes on the hole transfer dynamics during a charge separation process

Despite a clear elucidation that the change in PM6:Y6 morphology with different solvents affects photovoltaic performance, the charge dynamics during the charge separation process resulting from these morphological changes have not been extensively studied. However, studies on mobility and photocurrent have shown that holes play a crucial role in charge generation and separation. In PM6:Y6 devices fabricated using chlorobenzene (PM6:Y6-CB), the reduced exciton dissociation probability is attributed to changes in hole transfer state due to morphological variations. In PM6:Y6 devices fabricated using chloroform (PM6:Y6-CF), the hole transfer state (hECT) and Y6 highest occupied molecular orbital (HOMO) were almost degenerate. Consequently, the formation of lower effective hECT, which can interfere with hole transfer, was minimized. Conversely, in PM6:Y6-CB, the overlapping region of hECT and Y6 HOMO shifts to the lower energy side, creating a significantly lower effective hECT, which is energetically unfavorable for hole transfer. These findings from electroluminescence deconvolution analysis were validated using time-resolved photoinduced absorption spectroscopy. Consequently, the decrease in fill factor and current density in PM6:Y6-CB can be attributed to compromised hole transfer from Y6 HOMO to PM6 HOMO. This analysis underscores the importance of morphological changes in nonfullerene acceptor solar cells on hole transfer levels, ultimately affecting the charge separation efficiency.