Impact of the Humidity on the Structural and Optoelectronic Properties of Triple-cation Halide Perovskite Thin Films

Halide perovskite solar cells (PSCs) offer exceptional power conversion efficiencies and low-cost fabrication potential, but their sensitivity to environmental humidity remains a major barrier to scalable production. This study investigates the ambient fabrication of triple-cation perovskite thin films—(FA₀.₇₉MA₀.₁₆Cs₀.₀₅)Pb(I₀.₈₅Br₀.₁₅)₃—under controlled relative humidity (RH) levels ranging from near 0% to 60%. Comprehensive structural, morphological, and optoelectronic characterizations reveal that 15% RH yields the most favorable film properties, including a photoluminescence (PL) lifetime of ~1100 ns, grain size of ~317 nm, and minimal PbI₂ residue. In contrast, films fabricated at 60% RH exhibit increased grain size (~430 nm), higher surface roughness (Ra ≈ 24.1 nm), reduced thickness (~222 nm), and a void ratio exceeding 1.4%, all of which degrade optoelectronic performance. Ultraviolet–visible spectroscopy and time-resolved photoluminescence results confirm that higher humidity introduces mid-gap states and band tailing in perovskite thin films, reducing light absorption and increasing non-radiative recombination. These findings quantitatively demonstrate the critical role of humidity in perovskite film formation and identify optimal processing conditions for reproducible, high-quality PSC fabrication in ambient environments.