Illumination‐Dependent Interfacial Recombination in Perovskite Photovoltaics Driven by Trap States in Metal Oxide Electron Transport Layers

Perovskite solar cells (PSCs) now exceed 27% power conversion efficiency (PCE); however their performance under varying illumination conditions remains strongly influenced by interfacial charge carrier dynamics. Although bulk defects have been substantially reduced through perovskite compositional engineering, trap states in metal oxide electron transport layers (ETLs) can still capture carriers and promote non‐radiative recombination, with effects that depend strongly on photon flux. Here, we investigate trap‐assisted interfacial recombination in metal oxide ETLs using a Low‐High‐Low (LHL) illumination protocol, which probes device behavior from low‐light (1000 lux) to 1 Sun and back. We find that interfacial trap states have only a minor effect under low‐light but become highly detrimental under 1 Sun illumination, where they dominate interfacial recombination and reduce the open‐circuit voltage (V OC ). This effect is particularly pronounced for metal oxides with higher densities of surface trap states, such as TiO 2 . Upon returning to low‐light after 1 Sun exposure, devices show a marked V OC decrease followed by slow recovery, consistent with gradual charge release from interfacial traps and the subsequent suppression of recombination. These findings reveal the strongly illumination‐dependent role of metal oxide trap states and provide design guidance for optimizing PSCs for both outdoor and indoor photovoltaic applications.