Impact of Morphology Change on Hole Transfer Dynamic During Charge Separation Process

Although it was clearly elucidated that the change in the overall bulk-heterojunction (BHJ) morphology of PM6:Y6 induced by the difference in solvents results in a performance variation in the entire solar cell. The charge dynamics during the charge separation process induced by morphological changes have not yet been deeply studied. Based on mobility and photocurrent related studies, it was inferred that holes were deeply involved in these charge generation and separation processes. The decrease in exciton dissociation probability observed in the PM6:Y6 solar cell fabricated using CB (PM6:Y6-CB) was found to be due to the change in hole transfer state caused by morphological changes. For the PM6:Y6 solar cell fabricated using CF (PM6:Y6-CF), it was determined that the density of state (DOS) of effective hole transfer state (<i>^hE</i>_CT) was small, as <i>^hE</i>_CT and Y6 HOMO were almost degenerate, and the lower <i>^hE</i>_CT, which can interfere with hole transfer, was also formed minimally. However, for PM6:Y6-CB, the overlapping region of <i>^hE</i>_CT and highest occupied molecular orbital (HOMO) of Y6 shifted towards the lower energy side (upper <i>^hE</i>_CT) and a lower effective <i>^hE</i>_CT that can act as a defect was significantly formed. These facts obtained by EL deconvolution were clearly confirmed in time-resolved photoinduced absorption spectroscopy. As a result, it is concluded that the decrease in fill factor (FF) and current density (<i>J</i>_SC) in PM6:Y6-CB is due to the degradation of hole transfer from Y6 HOMO to PM6 HOMO, which is analyzed as the morphological changes in non-fullerene acceptor (NFA) solar cells affect the formation of hole transfer levels, which in turn affects the charge separation efficiency.