Photoinduced Charge Transfer and Recombination Dynamics in Star Nonfullerene Organic Solar Cells.

Nonfullerene acceptors (NFAs) are regarded as star candidates for efficient organic solar cells with power conversion efficiency (PCE) over 18%. In contrast to the rapid development of NFA materials, however, the underlying excited-state dynamics which fundamentally govern the device performance remains unclear. In this Perspective, we discuss recent advances and provide our insights on photoinduced charge transfer and combination dynamics in NFA-based organic solar cells (OSCs), including the biphasic hole-transfer process and its correlation with morphology, the role of driving force and Marcus normal region behavior on interfacial hole-transfer properties, and charge recombination energy loss by NFA triplet formation. We also discuss our understanding of how to control the charge-transfer and recombination processes by phase morphology and molecular design to improve OSC performance. Finally, we suggest a few research directions, including the interfacial charge transfer and separation mechanism, the origin of low fill factor, and complex excited-state dynamics in multicomponent OSCs.