Charge Generation in Non-Fullerene Donor-Acceptor Blends for Organic Solar Cells

The power conversion efficiencies of solar cells incorporating non-fullerene donor-acceptor now approach those of fullerene-based devices in the best performing examples. However, the lack of clear structure-property relationships means that the poor device performance of many novel non-fullerene systems cannot be readily explained. We report a series of non-fullerene acceptors, comprising essentially the same chromophore, that differ in terms of their shape and number of chromophores. To understand the impact of these structural differences on charge generation, we have employed transient absorption spectroscopy to investigate the photophysical properties of the acceptors in blends with the conjugated polymer PTB7. To minimize the impact of morphology, we employed a broad range of acceptor concentrations and compared the results with those of blends containing the fullerene derivative PC70BM. In terms of singlet exciton harvesting, the non-fullerene acceptors exhibited similar performance to PC70BM in blends with PTB7. The rate of singlet exciton quenching as a function of acceptor concentration was consistent with exciton diffusion mediated quenching with a diffusion length of 4-5 nm for the PTB7 singlet exciton. The polaron generation efficiency of the non-fullerene acceptors was comparable to that of PC70BM although the fraction of polarons that subsequently underwent geminate recombination was much greater in the non-fullerene blends. Furthermore, the photophysical properties of the non-fullerene acceptors were not influenced by the shape of the acceptor, the chromophore number, or the donor-acceptor ratio, which indicates that the higher geminate recombination is related to the structure of the acceptor chromophore and the interface formed with the donor. The implication of these results is that despite appropriate energetics and optical absorption, the performance of some non-fullerene donor-acceptor blends will be intrinsically limited by the choice of acceptor chromophore and the nature of the interface it forms with the donor material.