Extended-charge-transfer excitations in crystalline non-fullerene acceptors

In organic solar cells with non-fullerene acceptors (NFAs), an intra-moiety excited state, originating from intermolecular interactions in the acceptor domain, acts as a key immediate for charge separation. However, the nature of the intra-moiety state remains elusive. Here, we employ a model Hamiltonian with parameters derived from a model system of Y6 crystal to study the nature of low-lying excited states in NFAs. We find the intra-moiety excited state is mixed with local excitation and charge-transfer excitation (CTE) characters with nonnegligible contributions from extended CTEs with spatially-separated electrons and holes. The spatial extent of such a loosely bound state is susceptible to the intermolecular electronic interaction and electron–vibration interaction, which may be promoted by molecule engineering and morphology control. The findings provide an alternative strategy towards device optimization by manipulating the delocalization of intra-moiety state in organic photovoltaic materials.