The Fundamentals of Halide Perovskite Instability: CsPbBr3 as a Model System.

Halide perovskites (HPs) exhibit outstanding optoelectronic properties, making them highly attractive for advanced photonic and electronic applications. Among them, cesium lead bromide (CsPbBr3) exhibits high photoluminescence efficiency, narrow emission linewidths, and excellent color purity, making it suitable for light-emitting diodes, photodetectors, scintillators, and lasers. However, its practical application remains limited by intrinsic instability and environmental stressors such as moisture, heat, oxygen, and illumination. This review discusses the fundamental degradation mechanisms in HPs, using CsPbBr3 as a representative system, primarily in nanocrystals (NCs), while also considering thin-film and bulk forms. Intrinsic instability arises from the soft ionic lattice, a relatively low tolerance factor, and anharmonic lattice dynamics, which promote defect formation, ion migration, and structural distortion. Low activation energies for ionic transport enable migration of halide (Br-) and A-site (Cs+) ions, leading to defect accumulation, non-radiative recombination, and phase instability. In NCs, surface-dominated effects further accelerate degradation, driven by high defect densities and ligand desorption. Extrinsic factors promote degradation through hydrolysis, photoinduced defect formation, radical-driven reactions, and transformation into secondary phases. These processes can be intensified when multiple environmental stressors act simultaneously, reducing operational stability. These processes form a coupled, condition-dependent degradation network. Their interactions and relative contributions depend on the material form and environmental or operational conditions. Overall, this review highlights the interconnected roles of lattice dynamics, defect formation, ionic transport, and environmental interactions in governing the instability of CsPbBr3-based HPs.