In this study, we present a comprehensive investigation into the charge generation mechanism in bulk-heterojunction organic solar cells employing non-fullerene acceptors (NFAs) both with and without the presence of processing additives. While photovoltaic devices based on Y6 or BTP-eC9 have shown remarkable power conversion efficiencies, the underlying charge generation mechanism in polymer:NFA blends remains poorly understood. To shed light on this, we employ transient absorption (TA) spectroscopy to elucidate the charge transfer pathway within a blend of the donor polymer PM6 and NFAs. Interestingly, the charge carrier lifetimes of neat Y6 and BTP-eC9 are comparable, both reaching up to 20 ns. However, the PM6:BTP-eC9 blend exhibits substantially higher charge carrier generation and a longer carrier lifetime compared to PM6:Y6 blend films, leading to superior performance. By comparing TA data obtained from PM6:Y6 or PM6:BTP-eC9 blend films with and without processing additives, we observe significantly enhanced charge carrier generation and prolonged charge carrier lifetimes in the presence of these additives. These findings underscore the potential of manipulating excited species as a promising avenue for further enhancing the performance of organic solar cells. Moreover, this understanding contributes to the advancement of NFA-based systems and the optimization of charge transfer processes in polymer:NFA blends.
Keywords: charge carrier dynamics; non-fullerene acceptor; organic solar cell; processing additive; transient absorption spectroscopy.