Photochemical Control of Perovskite Crystal Formation at Room Temperature

Abstract Lead halide perovskites combine outstanding optoelectronic performance with low‐cost and scalable manufacturing routes. However, their commercial success remains limited by a still‐evolving understanding of crystallization dynamics, a lack of sustainable, material‐informed processing techniques, and persistent challenges in fabricating blue‐emitting and 2D compositions with sterically‐impeded cations. Here, the interaction of light with organic–inorganic perovskite precursor solutions is uncovered by presenting a photochemically‐assisted crystallization control technique (PACCT). This low‐energy method leverages UV illumination to modulate reaction kinetics, allowing controlled crystal formation in both thin films and flexible perovskite‐polymer composites under ambient atmosphere at room temperature. The treatment's influence on the crystallization process either impedes (3D perovskites) or promotes (2D perovskites) crystal growth. In the latter case, this facilitates the growth of perovskites that can otherwise be challenging using thermal approaches, enabling a facile pathway toward blue emission and thin films with a photoluminescence stability exceeding 1000 h at 80 % relative humidity. For both reaction pathways, mechanistic descriptions are identified based on the UV‐induced deprotonation of the organic cation. Together, these results introduce a novel strategy for perovskite fabrication and provide new insights into their crystallization dynamics, offering a versatile and scalable route for advancing next‐generation optoelectronic materials with a reduced energy footprint.