Performance Optimization of Air-Processed Perovskite Solar Cells via Compositional Engineering and Residual Stress Analysis

Compositionally engineered multication/anion perovskites offer improved stability against heat and moisture-induced degradation in perovskite solar cells (PSCs). However, such mixed-ion systems often suffer from material inhomogeneity and internal stresses arising from local lattice mismatch, and thermal expansion coefficient (TEC) differences with the underlying substrate, which can deteriorate the device stability and performance. In this study, we systematically investigate the mechanical properties of FA1–xCsxPbI3 perovskites using nanoindentation, varying the Cs concentration from 5% to 25%. X-ray diffraction analyses reveal a tunable transition in internal stress profiles: from tensile to compressive at the film surface, whereas compressive to tensile in the bulk with increasing Cs concentration. Further material characterizations show that an optimal incorporation of 10% Cs (x = 0.10) yields a favorable balance between tensile and compressive stresses, which not only enhances the structural integrity but also suppresses nonradiative recombination and ion migration in the solar cell device. The best performing PSC fabricated in ambient air (T = 27 °C, RH = 25% ± 5%) with this optimized composition, employing TiO2 as the electron transport layer (ETL) and TOP-3 as the hole transport layer (HTL), demonstrates a promising power conversion efficiency (PCE) of 17.13% with minimal hysteresis, while retaining 85% of their initial performance after 800 h of operation, measured periodically at intervals of 100 h. This showcases that careful tuning of internal stresses by targeted compositional engineering and material selection can achieve a meaningful trade-off, balancing high efficiency with long-term stability in perovskite devices.