Remarkable Black-Phase Robustness of CsPbI₃ Nanocrystals Sealed in Solid SiO₂ /AlO_x Sub-Micron Particles

This work combines the high-temperature sintering method and atomic layer deposition (ALD) technique, and yields SiO₂ /AlO_x -sealed γ-CsPbI₃ nanocrystals (NCs). The black-phase CsPbI₃ NCs, scattered and encapsulated firmly in solid SiO₂ 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 SiO₂ encapsulations, and its condensed amorphous structures that prevent the permeation of H₂ O molecules. Nanoscale coating of Al₂ O₃ thin films, which are deposited on the surface of the CsPbI₃ -SiO₂ by ALD, enhances the protection against O₂ infiltration, greatly elevating the high-temperature stability of CsPbI₃ 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-CsPbI₃ under harsh environment, bringing the high-pressure preservation of black-phase CsPbI₃ from laboratory to industry toward potential applications in both photovoltaic and fluorescent areas.