Mixing of NFAs ionization energies: an additional tool to tune the quantum efficiency of ternary solar cells

Non-fullerene acceptor (NFA)-based ternary bulk heterojunction solar cells (TSC), represent the most efficient organic solar cells (OSCs) today due to their broader absorption, but also to quantum efficiencies (QE) often surpassing those of the corresponding binary blends. We study the relation between those QE and the energetics driving charge transfer at the electron donor:electron acceptor (D/A) interfaces in the blends of a donor (PBDB-T-2F) with several pairs of lower bandgap NFAs. Similar to binary blends, the QE increases with the ionization energy offset between donor and acceptor (ΔIE) due to the improved charge transfer efficiency, following an error function that maximizes and plateaus for ΔIE > 0.5 eV. Unexpectedly, photocurrent generation appears to proceed through a single channel, controlled by the difference between the IE of the donor and the weighted-average IE of both NFAs, rather than by individual D:NFA ΔIEs. As a proof of concept, we took a PBDB-T-2F:IEICO binary blend, having a ΔIE insufficient for charge generation, and significantly improved the photocurrent generation upon IEICO absorption by adding a deep IE third component: IT-4F. Those findings imply that combining two NFA can be used as an alternative to synthesizing new molecules, to optimize the D/A energy matching.