Synergistic Effect of Dielectric Property and Energy Transfer on Charge Separation in Non-Fullerene-Based Solar Cells.

In non-fullerene-based photovoltaic devices, it still remains unclear how excitons efficiently dissociate into free charge carriers even under a tiny driving force (tens of meV) with reduced energy loss. Here, dielectric constants of different non-fullerene-based solar cells consisting of fluorinated-thienyl benzodithiophene (BDT-2F)-based polymer PM6 as the typical donor, and a selected series of non-fullerene acceptors were precisely measured by a newly developed method. It was found that most of the non-fullerene acceptors exhibited higher dielectric constants than fullerene derivatives (PC61BM and PC71BM). The corresponding photoactive films exhibited not only higher dielectric constants but also the larger dielectric constant differences between donor and non-fullerene acceptors. These would result in lower exciton binding energy and increased charge dissociation possibility with low geminate losses. Additionally, the overlap between the emission spectrum of donor and absorption spectra of non-fullerene acceptors would allow the resonance energy transfer from donor to the acceptor in these non-fullerene-based devices, which was confirmed by investigating the emission spectra of pristine donor (and acceptor) films and corresponding blend films. Such an energy transfer process enhanced the efficient exciton diffusion, promising improved device performance. Therefore, based on the synergistic effect of higher dielectric property and energy transfer on charge separation of selected non-fullerene-based photovoltaic devices, these results provided strong hints to interpret efficient charge separation for the high device performance with a tiny driving force. Our work paves another path to elucidate the intrinsic physical working mechanism on non-fullerene organic solar cells.