Disentangling Exciton Dissociation in L8-BO Homojunction Films: Femtosecond Dynamics of Excited-State Evolution

Organic-semiconducting materials are of great importance in the exploration of next-generation photovoltaics. Among these materials, nonfullerene acceptors (NFAs) derived from the benchmark molecule Y6 have led the way in achieving power conversion efficiencies rivaling those of commercial silicon solar cells. L8-BO is a molecular electron acceptor with the same core structure as Y6 and has recently been in the spotlight of organic photovoltaic (OPV) research. Most of the improvement in PCE compared with Y6 can be attributed to more uniform solid-state packing. It has been reported that variation in the geometry of molecular dimers in the Y6 crystal structure is responsible for the energetic driving force for intrinsic charge-carrier generation. This observation raises the question of whether the improved structural uniformity in L8-BO impedes charge-carrier generation. Prior work has already questioned the ability of L8-BO, and other NFAs, to intrinsically form charge carriers. Additionally, the nature of charge-transfer (CT) states and the time scale of their generation has been ambiguous. In this work, we use various forms of ultrafast spectroscopy to present evidence of free charge-carrier generation in neat L8-BO films and uncover spectroscopic information that has been absent in previous literature. Broadband pump–probe spectroscopy shows that the majority of localized excitons are converted to intermolecular CT excitons within 100 fs of initial excitation. Furthermore, pump–push–probe spectroscopy shows that free charges dominate over CT excitons at 10 ps after excitation. This work provides insight into the photophysics of intrinsic charge generation in the highest performing Y-series acceptor, L8-BO, and adds clarity to the discussion on intrinsic charge generation by Y-series acceptors.