Revisiting Open-Circuit Voltage in Heterojunction-Scarce Organic Solar Cells

Abstract

Organic solar cells (OSCs) have reached power conversion efficiencies (PCEs) above 21%, yet their market adoption is still limited by reliability issues rooted in the unstable bulk heterojunction (BHJ) architecture. Recent Y-series nonfullerene acceptors enable bulk photocarrier generation and open a pathway toward heterojunction-scarce active layers with improved uniformity and stability. However, the widely used charge-transfer state analysis and the simplified Schottky-junction model become insufficient to describe the open-circuit voltage (V_OC) in such systems. Here we develop a unified framework that couples composition-dependent density of state (DOS) redistribution with geminate recombination to explain V_OC when donor/acceptor (D/A) interfaces are scarce. DOS evolution governs Fermi-level (E_F) alignment and sets the upper limit of V_OC, whereas enhanced geminate recombination in weakly interfaced blends limits the achievable quasi-Fermi level splitting. This model reconciles the opposite V_OC trends and fill factor degradation observed in D- and A-poor PM6:Y6 and PCE10:Y6 devices. Guided by these insights, electrode work function engineering strengthens internal fields, suppresses geminate loss, and yields a record-low energy loss of 0.516 eV in PM6:Y6 cells. This framework clarifies voltage losses beyond the BHJ paradigm and provides design rules for reliable high-efficiency heterojunction-scarce OSCs.