Engineered Interfacial Control for Suppression of Phase Instability: Operando Visualization from Device to Module Scale.

Phase instability in wide-bandgap mixed-halide perovskites remains a critical challenge for achieving long-term stable perovskite solar cells. Here, real-time current density-voltage absorption spectroscopy (RTJAS) with a transparent indium tin oxide top contact enables operando, spatially resolved visualization of phase evolution across charge-extracting and charge-non-extracting regions under operational bias. Light-induced phase separation preferentially initiates in regions with locally insufficient carrier extraction, revealing spatial heterogeneity in phase stability. By modifying the interfacial configuration, device-level RTJAS measurements under identical operating conditions exhibit different phase evolution behaviors, motivating defect level analysis. Deep-level transient spectroscopy (DLTS) and density functional theory (DFT) modeling indicate that this contrast is associated with differences in interfacial defect landscapes and vacancy-related energetics. Importantly, operando RTJAS mapping at the module scale reveals that electrically inactive dead areas intrinsic to monolithic interconnect architectures act as preferential sites for the onset of phase instability. Together, these results establish that phase instability in mixed-halide perovskites is governed by localized charge extraction conditions and interfacial defect energetics. The spatially resolved operando framework presented here provides a diagnostic tool for identifying latent degradation pathways and offers design guidelines for achieving stable and scalable perovskite photovoltaics from device to module scale.