(Invited) Influence of Y6 Aggregation on Energetics and Charge Generation in High Performance OPV Blends

Abstract

We investigated the origin of the high ratio of open circuit voltage to charge transfer state energy (eVOC/ECT) of OPV polymer blends consisting of the electron donating polymer PM6 and the non-fullerene electron acceptor Y6. PM6-Y6 and related donor-acceptor OPV blends exhibit remarkable optoelectronic properties and record power conversion efficiencies in part because of their unusually high eVOC/ECT ratios, which is believed to be related to the small energetic offset of the HOMO levels of PM6 and Y6. In this study, we varied the amount of Y6 in PM6 blends as a means to control the aggregation of Y6 molecules. We examined the corresponding influence that Y6 aggregation has on the energetic offsets and the charge generation efficiency of the materials by probing the appearance of polaron absorption signals in the mid-infrared using time-resolved infrared spectroscopy. Furthermore, we examined the optical absorption and emission spectra of the PM6-Y6 blends over the range of compositions to correlate the efficiency of charge generation with the signatures of Y6 aggregation in the optical spectra. We observed an abrupt transition around 30 mass% Y6 content at which polarons were efficiently generated by charge separation from PM6 to Y6. Comparison to composition dependent GIWAXS studies of PM6-Y6 blends revealed the formation of Y6 aggregates around the same 30 mass% threshold. These observations demonstrate that aggregation of Y6 molecules is required for charge separation to occur from PM6 to Y6 as measured through the mid-infrared absorption of polarons in the TRIR spectra. Although charges were efficiently generated in blends with only 30 mass% Y6 content, OPV device studies revealed that 50-60 mass% Y6 was needed to reach optimized short-circuit current and OPV device efficiency because these measurements convolve charge generation with charge transport. These findings clarify the optoelectronic properties of the novel class of Y6 acceptors and emphasize the importance of molecular aggregation for tuning the energetics of high performance systems that minimize energy offsets for maximum OPV power conversion efficiencies.