Remarkable Black‐Phase Robustness of CsPbI₃ Nanocrystals Sealed in Solid SiO₂/AlOx Sub‐Micron Particles

This work combines the high‐temperature sintering method and atomic layer deposition (ALD) technique, and yields SiO 2 /AlO x –sealed γ–CsPbI 3 nanocrystals (NCs). The black‐phase CsPbI 3 NCs, scattered and encapsulated firmly in solid SiO 2 sub‐micron particles, maintain in black phases against water soaking, ultraviolet irradiation, and heating, exhibiting remarkable phase stability. A new phase‐transition route, from γ via β to α phase without transferring into δ phase, has been discovered upon temperature increasing. The phase stability is ascribed to the high pressure exerted by the rigid SiO 2 encapsulations, and its condensed amorphous structures that prevent the permeation of H 2 O molecules. Nanoscale coating of Al 2 O 3 thin films, which are deposited on the surface of the CsPbI 3 ‐SiO 2 by ALD, enhances the protection against O 2 infiltration, greatly elevating the high‐temperature stability of CsPbI 3 NCs sealed inside, as the samples remain bright after 1‐h annealing in air at 400 ° C. These fabrication and encapsulation techniques effectively prevent the formation of δ–CsPbI 3 under harsh environment, bringing the high‐pressure preservation of black‐phase CsPbI 3 from laboratory to industry toward potential applications in both photovoltaic and fluorescent areas.