Tailoring the Intermediate Phase to Control Formation of γ-CsPbI ₃ Films

Controlling the crystallization pathway of inorganic CsPbI 3 perovskite is essential for achieving high efficiency and stability in optoelectronic devices. Here, we report a solvent-engineering strategy that combines an antisolvent process with vacuum treatment (AVT) to modulate evaporation dynamics of the precursor, guiding the formation of highly oriented (CH 3 ) 2 NH 2 PbI 3 (DMAPbI 3 ) and Cs 4 PbI 6 intermediate phases. Synchrotron and in situ analyses revealed correlations between intermediate orientation and γ-CsPbI 3 crystallinity. This directional crystallization pathway promotes vertical alignment and grain enlargement in γ-CsPbI 3 films, resulting in fewer voids, lower defect densities, and reduced tensile strain. Photovoltaic devices based on AVT-processed films achieved a power conversion efficiency of 18.47% with a fill factor of 83.14% and retained 101.9% of their initial efficiency after 526 h without encapsulation. This study first reports that the quality of DMAPbI 3 and Cs 4 PbI 6 intermediates, controlled by combination of antisolvent and vacuum treatment, plays a crucial role in achieving high-quality γ-CsPbI 3 films.