Impact of Solvent-Induced Morphological Changes on Hole Transfer Dynamic during Charge Separation Process

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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 the conventional fullerene-based organic photovoltaics (OPVs), in which the excited electrons from the donor are transferred to the acceptor, an electron charge transfer state (eECT) that electrons pass through has a great influence on the device performance. In the bulk-heterojunction (BHJ) system based on low band-gap non-fullerene acceptor (NFA), however, a hole charge transfer state (hECT) from the acceptor to the donor has a greater influence on the device performance.In PM6:Y6 devices fabricated using chlorobenzene (PM6:Y6-CB), the reduced exciton dissociation probability is attributed to changes in hole transfer states 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 a 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. Figure 1