Why does Y6 with bulk charge photogeneration and bipolar charge transport properties still fail in non-heterojunction organic photovoltaics?

Abstract

It is widely believed that organic solar cells (OSCs) must operate based on the donor/acceptor (D/A) heterojunction concept, although it introduces inevitable photovoltaic loss. Motivated by the spontaneous charge photogeneration and bipolar charge transport in non-fullerene acceptors (NFAs), the development of high-performance non-heterojunction (NHJ) devices is expected to overcome the intrinsic limitations of OSCs. Despite the good short-circuit current density (J_SC) predicted from film absorbance, NHJ OSCs generally demonstrate ultra-low J_SC. Herein, we try to figure out the underlying cause behind this large gap and give a quantitative estimation. Taking the milestone NFA Y6 as an example, we clarify that activation energy is required for exciton splitting in Y6, which means that spontaneous charge photogeneration at room temperature is unavailable. Estimated from the activation energy, the theoretical J_SC limit drops to 3.74 mA cm⁻² from 25.49 mA cm⁻² according to Y6 absorbance. The inefficient bipolar charge transport further decreases the J_SC to merely 0.28 mA cm⁻², and we discover that achieving balanced mobility close to unity can improve the J_SC by 2.26 times. These findings rectify the traditional views on spontaneous charge photogeneration and utilization and highlight future directions for NHJ OSCs.