One of the key areas of study in organic photovoltaics is the development of so-called 'non-fullerene acceptors' (NFAs), which enjoy several benefits over older, fullerene-based acceptors, such as low cost, high absorptivity, and tuneability. A recent report Fei et. al. demonstrated conversion efficiencies of 13% in donor-acceptor blends comprising a fluorinated derivative of the common donor PBDB-T and an alkylated derivative of the ITIC (C8-ITIC) acceptor species. Understanding the underlying dynamics of this material is therefore important for the rational design of new NFAs.
In order to understand the photophysical processes in C8-ITIC, we performed ultrafast transient absorption studies of four donor-acceptor blends, containing various combinations of C8-ITIC, PFBDB-T, and their unmodified predecessors. Long-lived excitons form at the acceptor regardless of the excitation frequency, suggestive of rapid energy transfer from the donor to the acceptor. Exciton decay at early times was more rapid in C8-ITIC compared to non-alkylated ITIC. A distinct change in exciton decay characteristics was observed at longer timescales in tandem with spectral drift in the acceptor’s excitonic peak. We use global analysis and a broader array of ultrafast spectroscopic techniques to elucidate the identity and mechanism behind this feature. Our results will help to shed light on the efficiency of this material and aid the development of more efficient and effective non-fullerene acceptors.