Minimizing voltage losses in organic solar cells with low energy offset

Abstract

Recent advances in organic solar cells (OSCs) based on non-fullerene acceptors (NFAs) come along with reduced non-radiative voltage losses. We show that the non-radiative voltage losses in these state-of-the-art donor:NFA OSCs show no correlation with the energies of charge-transfer electronic states at donor:acceptor interfaces, different from conventional fullerene-based OSCs. Based on a combined temperature-dependent electroluminescence experiments and dynamic vibronic simulations, we have been able to rationalize the low voltage losses in these devices, where we highlight the critical role of the thermal population of local exciton states in decreasing the non-radiative losses. An important finding is that the molecular photoluminescence properties of the pristine materials define the limit of non-radiative voltage losses in OSCs, indicating that it is critical to design high-luminescence-efficiency donor and acceptor materials with complementary optical absorption bands extending into the near-infrared region. We further demonstrate that there is no intrinsic limit for efficient charge separation in OSCs with small non-radiative voltage losses.