Acceptors Ionization Energy Mixing Enables to Continuously Tune the Quantum Efficiency of Ternary Solar Cells

Non-fullerene acceptor (NFA)-based ternary organic solar cells (TSC), maximizing the light absorption range, have demonstrated better photocurrent generation than their binary counterparts. In many instances, however, the 3^rd component improves the performances well beyond the broader absorption, with some quantum efficiencies enhanced compared to the corresponding binary cells. Here, we investigate the origin of this improvement and demonstrate that the ionization energy (IE) offset between the donor and the two NFAs controls the charge generation efficiency. If the IE offset between the donor and the acceptors is large, hole transfer from the NFA to the donor results in efficient free charge generation. Surprisingly, we observe that in ternary OSCs charge generation is controlled by the difference between the IE of the donor and the weight-averaged IE of both NFAs rather than the individual IE differences. Importantly, this enables efficient charge generation in ternary systems that are comprised of one low IE offset acceptor that, if used in a binary blend, would not generate charges. This work rationalizes the combination of NFA materials and their mixing ratio to maximize photocurrent generation.