Resilience to Demixing and Phase Segregation in Perovskite Solar Cells under Light–Dark Cycles and Temperature

Light soaking impacts perovskite solar cells, causing cation rotation, octahedral distortion, and weakened hydrogen bonding. Using a unique <i>in-operando</i> setup for ISOS protocols, we monitor structural, optical, and electrical responses under prolonged light exposure, revealing progressive average changes without sample reloading uncertainties. Over 20 h intervals, light-induced lattice deformation causes progressive local demixing, partially reversible in dark, and residual amorphization that hinders electrical recovery. Lattice expansion and bandgap red-shift indicate increasing iodide local enrichment, while a bandgap blue-shift occurs under heating. FA-MA-Cs-perovskites resist to this ionic demixing more than FA-Cs. Sunlight is the primary trigger for that, surpassing the effects of bias or induced heating. Stress tests at 65 °C drive both formulations from demixing to irreversible phase segregation, with FA-Cs devices showing greater structural and electrical resilience than FA-MA-Cs. Since a demixing-remixing interplay governs the device operation, we recommend tracking it using <i>in-operando</i> protocols over 24-48 h of unaccelerated sunlight-dark testing.