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

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 (T₁) and/or the excited state (S₁), 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 S₁ state, together with excellent charge mobility and light absorption, required for higher performance OPV. But their low S₁ state causes several non-radiative recombinations due to strong S₁-S₀ 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 S₁, and the ideal scenario would be to elevate the T₁ state, thereby mitigating the energy gap law.