The Photoexcited Mechanism of PM6:Y6 Organic Bulk Heterojunction

The efficiency of organic photovoltaic (OPV) devices has significantly improved, reaching ~20% with some recently developed non-fullerene acceptors (NFAs). The prototype of these NFAs is a molecule known as Y6, which exhibits strong absorption in the near-infrared region. While Y6-based systems demonstrate exceptional device performance, underlying mechanisms that enable efficient free-carrier generation from bound excitons but with very small energy loss remain insufficiently understood. To measure and understand mechanism; charge generation, chare separation and charge transfer we employed complementary Time- and frequency-resolved photoluminescence spectroscopy (TR-PL) and Time-resolved two photo photoemission spectroscopy (TR-2PPE) to investigate the exciton dynamics in bulk heterojunctions sample made with Y6 and a donor polymer known as PM6 while using pristine Y6 sample as a reference. TR-PL was specifically used to probe both the population dynamics and energy relaxation of excitons as a function of temperature. TR-2PPE was used to measure the excited state electron dynamics of both Bulk PM6:Y6 and the pristine Y6 at the surface level. The TR-PL reveals substantial energy relaxation occurs at a very low temperature (~77 K), whereas this relaxation process is absent at higher temperatures (> 150 K). This result indicates that more delocalized and less bound excitonic states are populated at temperatures as low as 150 K, which can be assisted by entropy. These more delocalized states are precursors for the charge separation process. The complimentary TR-2PPE showed a rapid decay of excited electrons within 20ps followed by energy shift indicating energy relaxation towards lower energy. This leads to a recombination process, where electrons lose energy and return to a more stable, lower-energy state.