Using time- and frequency- resolved photoluminescence to probe exciton dynamics in organic photovoltaic materials

Non-fullerene acceptors (NFAs) are gaining increasing attention recently because organic photovoltaics (OPVs) made with NFAs significantly outperform those made with traditional fullerene acceptors. Despite its outstanding performance, it remains unclear how NFAs can enable free-carrier generation from bound-excitons to occur with a much lower energy loss as compared to fullerene acceptors. This work is focused on investigating the exciton dynamics in bulk heterojunctions made with PM6 donor polymer and Y6 NFA as a function of temperature. Time- and frequency- resolved photoluminescence spectroscopy is used to probe both the dynamics and energy relaxation of excitons. We found that significant energy relaxation occurs at a very low temperature (~ 77 K), but this relaxation process is absence at higher temperatures (> 150 K). The result suggests that free-carrier generation is a thermally assisted process with a very small energy barrier that is likely lowered by entropy. This work is supported by US DOE under Award Number DE-SC0024525.