Building Fast Proton‐Coupled Electron Transport Paths for Accelerating Reduction Reactions in Solar Cells Under Sunlight and Indoor Lighting: Modulating Energy Band Structure and Surface Reactions

Abstract The photovoltaic conversion efficiency of solar cells critically depends on electron transport and mass diffusion at electrode‐electrolyte interfaces. In this study, innovative BCN@Fe 3 C and PCN@Fe 3 C heterojunction catalysts are developed through tuning their energy band structure and surface reactions. The designed configurations utilize the metallic state of Fe 3 C to generate a unique localized electric field at the heterojunction interface, and the established proton‐coupled electron transport channel significantly improves the electron transport kinetics and mass diffusion for triiodide reduction (IRR) and copper redox reactions (CRR). Under standard AM 1.5G sunlight, a solar cell using the BCN@Fe 3 C counter electrode in the N719‐I 3 − /I − system achieves a power conversion efficiency (PCE) of 9.03%. Even under indoor lighting conditions of 500 lux, the BCN@Fe 3 C cell maintained a Voc of 0.76 V and PCE of 18.99%. PCN@Fe 3 C also achieves a PCE of 4.79% in the D35‐Cu 2+ /Cu + system. Theoretical calculations demonstrate that the Schottky junction‐induced space charge regions facilitate selective adsorption of Cu 2+ and I 3 − protons, while the interfacial built‐in electric field promotes directional electron migration. This study first reveals the transport mechanism of proton‐coupled electrons in photovoltaic systems and highlights the advantages of interfacial modulation in optimizing IRR and CRR kinetics.