Cesium cation additive in PbI2 layer regulates crystal growth and carrier dynamics in air processed MAPbI3 perovskites solar cells

The ambient processing of perovskite solar cells (PSCs) is crucial for scalable industrialization; however, moisture-induced degradation and non-uniform crystallization remain significant hurdles. In this work, we demonstrate a facile, ambient-processable two-step intercalation strategy that premixes cesium iodide (CsI) directly into the PbI2 precursor to regulate the crystallization and carrier dynamics of methylammonium lead iodide (MAPbI3) PSCs. By bypassing conventional solution-processed solubility limits, this pre-intercalation approach ensures a highly uniform Cs+ distribution. Mechanistically, this kinetically promotes Ostwald ripening during crystal growth and thermodynamically relieves native lattice microstrain. Consequently, the optimal CsI concentration (2 mg mL-1) significantly enlarged the perovskite grain size (from 161 nm to 255 nm), reduced dislocation density, and suppressed deep-level trap states. These structural refinements effectively mitigated trap-assisted recombination and improved charge extraction. The optimal ambient-processed device achieved a champion power conversion efficiency (PCE) of 23.17% (average 20.57 ± 1.28%) with minimized hysteresis. Furthermore, applying this CsI-intercalated architecture in a strictly controlled environment (glovebox) validated its fundamental optoelectronic potential, yielding a true PCE of 23.78% with an excellent fill factor of 80.9%. Finally, the unencapsulated devices demonstrated remarkable durability, retaining 97.7% of their initial efficiency under continuous operational illumination and 82% after 30 days of ambient storage. These findings establish controlled CsI precursor intercalation as a scalable and mechanistically sound pathway for highly efficient and stable perovskite photovoltaics.