Perovskite materials have emerged as a promising class of semiconductors due to their exceptional optoelectronic properties, including high absorption coefficients, tunable bandgaps, and long charge carrier diffusion lengths. However, their practical application is hindered by intrinsic instability under environmental stressors such as moisture, heat, light, and oxygen. Understanding the degradation mechanisms at the material level is critical for developing stabilization strategies that enhance their longevity.

Moisture-induced degradation is one of the most significant challenges for perovskite stability. Water molecules interact with the perovskite lattice, leading to irreversible decomposition. In methylammonium lead iodide (MAPbI3), for example, hydrolysis occurs in a two-step process. First, water reacts with the organic cation (MA+) to form methylamine and hydroiodic acid. Subsequently, the hydroiodic acid further decomposes into gaseous HI and I2, while PbI2 precipitates as a yellow byproduct. The reaction proceeds as follows:
CH3NH3PbI3 + H2O → CH3NH2 + HI + PbI2
4HI + O2 → 2I2 + 2H2O

The presence of oxygen accelerates moisture-induced degradation by oxidizing iodide ions, exacerbating the breakdown of the perovskite lattice. This oxidative pathway is particularly detrimental in ambient conditions, where both moisture and oxygen are present. The formation of iodine vacancies further destabilizes the structure, creating defect sites that promote ion migration and phase segregation.

Thermal degradation is another critical factor, especially for hybrid organic-inorganic perovskites. At elevated temperatures, the organic cations (e.g., MA+, FA+) undergo thermal decomposition, releasing volatile species such as ammonia or hydrogen iodide. In MAPbI3, temperatures above 85°C induce the loss of MAI, leaving behind PbI2 and degraded intermediate phases. The thermal stability of formamidinium lead iodide (FAPbI3) is comparatively higher, but it still suffers from phase transitions from the photoactive α-phase to the non-perovskite δ-phase at temperatures above 150°C. Inorganic perovskites like CsPbI3 exhibit better thermal resilience but face challenges in maintaining their metastable perovskite phase under operational conditions.

Light-induced degradation is closely linked to ion migration and phase segregation. Under illumination, photogenerated carriers interact with the lattice, facilitating halide migration and the formation of iodide-rich and bromide-rich domains in mixed-halide perovskites. This phenomenon, known as halide segregation, leads to localized changes in the bandgap and deteriorates optoelectronic performance. Additionally, photoexcitation can generate reactive species that accelerate oxidative degradation, particularly in the presence of oxygen and moisture.

Oxygen-induced degradation occurs through the formation of superoxide species (O2−) when oxygen molecules interact with photoexcited electrons in the perovskite conduction band. These superoxides attack the organic cations, leading to deprotonation and the formation of volatile byproducts. Inorganic perovskites are less susceptible to this pathway but still suffer from surface oxidation, which creates trap states and reduces charge carrier mobility.

Chemical pathways such as halide migration and phase segregation are intrinsic to perovskite materials. Halide migration is driven by electric fields, strain gradients, and light exposure, leading to inhomogeneous distribution of ions and localized degradation. Phase segregation in mixed-halide perovskites (e.g., MAPb(I1−xBrx)3) results in the formation of iodide-rich and bromide-rich regions, which alters the optoelectronic properties and reduces device efficiency over time. The underlying mechanism involves the preferential migration of smaller halide ions (I−) under illumination, leaving behind bromide-enriched domains.

Stabilization strategies focus on mitigating these degradation pathways through material engineering and encapsulation. Compositional engineering involves substituting unstable organic cations with more robust alternatives, such as formamidinium (FA+) or cesium (Cs+). Mixed-cation and mixed-halide formulations (e.g., CsFAPbI3−xBrx) have demonstrated improved stability by suppressing phase transitions and halide segregation. Doping with larger cations (e.g., rubidium, potassium) can also inhibit ion migration by introducing steric hindrance within the lattice.

Encapsulation techniques aim to physically isolate perovskites from environmental stressors. Atomic layer deposition (ALD) of metal oxides (e.g., Al2O3, TiO2) provides dense, pinhole-free barriers against moisture and oxygen infiltration. Polymer-based encapsulants, such as polyisobutylene or ethylene vinyl acetate, offer flexible protection but require optimization to prevent delamination under thermal cycling. Multilayer encapsulation combining inorganic and organic layers has proven effective in extending perovskite lifetimes under accelerated aging tests.

Surface passivation is another critical strategy for reducing degradation. Treating perovskite films with bulky ammonium salts (e.g., phenethylammonium iodide) or Lewis bases (e.g., thiophene derivatives) can passivate surface defects and suppress ion migration. These passivation layers also act as barriers against environmental ingress, further enhancing stability.

In summary, perovskite degradation is a complex interplay of moisture, heat, light, and oxygen-induced mechanisms, each contributing to material breakdown through distinct chemical pathways. Stabilization efforts must address these factors holistically, leveraging compositional engineering, encapsulation, and surface passivation to achieve long-term stability. Advances in understanding degradation kinetics will enable the design of robust perovskite materials for next-generation optoelectronic applications.