Synergistic Additive Strategy Enables Sub-100 °C Fabrication of CsPbI ₃ Films for Efficient and Stable All-Inorganic Perovskite Solar Cells

All-inorganic CsPbI 3 perovskite holds promise for photovoltaics owing to its optoelectronic properties and thermal stability. However, its high nucleation barrier and thermodynamic instability hinder lowtemperature crystallization, limiting compatibility with flexible and tandem devices. Herein, a synergistic additive strategy combining lead acetate (PbAc 2) and ammonium benzenesulfonate (ABS) enables the fabrication of γ-CsPbI 3 below 100 °C. Acetate anions lower the nucleation energy barrier, while tailored ABS analogs stabilize the γ-phase via chelation and steric hindrance, suppressing octahedral tilting. This work establishes a mechanistic framework linking additive chemistry to the low-temperature crystallization of CsPbI 3 perovskite, providing guidance for the rational design of allinorganic perovskite formulations toward flexible and tandem photovoltaics. This approach achieves a record power conversion efficiency (PCE) of 16.33% for CsPbI 3 solar cells processed at temperatures below 100 °C, and retains 91% of the initial PCE after 600 h in dry air. Furthermore, flexible devices reach 13.53% PCE, demonstrating direct compatibility with flexible substrates.