Propionic acid-regulated one-step perovskite deposition for FAPbI3 solar cells under humid air

Air processing is essential for reducing the manufacturing cost and process complexity of perovskite solar cells (PSCs). However, one-step deposition under humid conditions remains challenging because moisture accelerates uncontrolled crystallization, induces pinholes, and generates deep-level defects in the perovskite layer. Here, we report a simple volatile-solvent/carboxylic-acid strategy for fabricating efficient PSCs by one-step spin-coating of the perovskite under approximately 70 % relative humidity, without using an air knife, gas-flow assistance, or substrate heating during film deposition. Acetone is introduced as a highly volatile co-solvent to create a transient solvent-rich microenvironment that suppresses direct moisture attack during crystallization, while propionic acid (PA) is incorporated to regulate phase conversion, mitigate tensile stress as supported by grazing-incidence X-ray diffraction and thickness-dependent photoluminescence analyses, and passivate undercoordinated Pb 2+ defects through carboxyl coordination. The optimized PA-containing perovskite films exhibit enlarged grains, reduced pinhole density, enhanced crystallinity, suppressed trap-state density, and extended carrier lifetime. Device-level analyses further reveal increased charge recombination resistance, reduced hysteresis, improved energy-level alignment, and better PCE retention in a representative ambient storage test. As a result, the optimized device achieves a J–V-derived champion power conversion efficiency of 21.01 %, demonstrating an effective route toward high-quality perovskite formation under high-humidity ambient conditions.