Regulate the Singlet–Triplet Energy Gap by Spatially Separating HOMO and LUMO for High Performance Organic Photovoltaic Acceptors

Abstract Reducing the single‐triplet energy gap (∆ E ST ) for organic photovoltaic (OPV) molecules has been proposed to be able to reduce the nonradiative recombination by tuning the low‐lying triplet state (T 1 ) and/or the excited state (S 1 ), thus reducing the energy loss ( E loss ) and increasing the open‐circuit voltage in their devices. However, how to design the non‐fullerene acceptor (NFA) with small ∆ E ST and high performance is challenging. Aiming to address this issue, YDF , YTF , and YTF‐H were synthesized. Among them, a device based on YDF with partially spatially separated highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) exhibits a much higher power conversion efficiency (PCE) of 20.04%, which is one of the most efficient efficiencies for binary systems. For YTF and YTF‐H , their completely spatially separated HOMO and LUMO indeed lead to a much reduced ∆ E ST caused by the low‐lying S 1 state, together with excellent charge mobility and light absorption, required for higher performance OPV. But their low S 1 state causes several non‐radiative recombinations due to strong S 1 ‐S 0 coupling (PCE < 1.5%). These results indicate that future designs to have high performance molecules with small ∆ E ST should avoid the sharp decrease in S 1 , and the ideal scenario would be to elevate the T 1 state, thereby mitigating the energy gap law.