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

All-inorganic CsPbI3 perovskite holds promise for photovoltaics owing to its optoelectronic properties and thermal stability. However, its high nucleation barrier and thermodynamic instability hinder low-temperature crystallization, limiting compatibility with flexible and tandem devices. Herein, a synergistic additive strategy combining lead acetate (PbAc2) and ammonium benzenesulfonate (ABS) enables the fabrication of γ-CsPbI3 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 CsPbI3 perovskite, providing guidance for the rational design of all-inorganic perovskite formulations toward flexible and tandem photovoltaics. This approach achieves a record power conversion efficiency (PCE) of 16.33% for CsPbI3 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.