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.