Hybrid perovskites have emerged as promising materials for optoelectronic applications due to their excellent photovoltaic properties, tunable bandgaps, and low-cost fabrication. However, their environmental instability remains a critical challenge for commercialization. Degradation mechanisms triggered by moisture, oxygen, light, and heat significantly limit their operational lifetime. Understanding these pathways and developing mitigation strategies are essential for advancing perovskite-based technologies.
Moisture-induced degradation is one of the most significant threats to hybrid perovskites. Water molecules interact with the perovskite lattice, leading to irreversible decomposition. In methylammonium lead iodide (MAPbI3), for example, hydrolysis occurs in the presence of moisture, producing volatile methylamine (CH3NH2), hydrogen iodide (HI), and lead iodide (PbI2). The reaction proceeds as follows:
CH3NH3PbI3 + H2O → CH3NH2 + HI + PbI2.
This process is accelerated in humid environments, causing rapid deterioration of the material’s optoelectronic properties. Formamidinium-based perovskites (FAPbI3) exhibit slightly better moisture resistance but still degrade under prolonged exposure. The hygroscopic nature of organic cations, such as MA+ and FA+, exacerbates water infiltration, leading to phase segregation and loss of crystallinity.
Oxygen degradation primarily involves the oxidation of iodide ions within the perovskite lattice. Under ambient conditions, oxygen reacts with photoexcited carriers, forming superoxide species (O2−) that attack the organic cations and iodide ions. This results in the formation of iodine vacancies and metallic lead (Pb0), which act as recombination centers, degrading device performance. Oxygen-induced degradation is particularly pronounced under illumination, where photogenerated electrons facilitate superoxide formation. Mixed-halide perovskites, such as MAPb(I1-xBrx)3, show varying susceptibility to oxygen, with higher bromide content improving stability but often at the cost of reduced efficiency.
Light-induced degradation is another critical factor, especially under continuous illumination or ultraviolet (UV) exposure. Photoexcitation generates charge carriers that can induce ion migration, phase segregation, and defect formation. In mixed-halide perovskites, light exposure causes halide segregation, leading to the formation of iodide-rich and bromide-rich domains, which alters the bandgap and reduces photovoltaic efficiency. UV light also accelerates decomposition by breaking organic-inorganic bonds, releasing volatile organic components. Additionally, light and electric fields synergistically enhance ion migration, further destabilizing the perovskite structure.
Thermal degradation arises due to the low formation energy of hybrid perovskites, making them susceptible to heat-induced phase transitions and decomposition. At elevated temperatures, organic cations such as MA+ and FA+ become volatile, leading to the collapse of the perovskite lattice. MAPbI3 undergoes a phase transition from tetragonal to cubic at around 60°C, but prolonged heating above 85°C causes irreversible decomposition into PbI2 and gaseous byproducts. Inorganic perovskites like CsPbI3 exhibit better thermal stability but face challenges in phase stability at room temperature. Thermal cycling between high and low temperatures also induces mechanical stress, leading to crack formation and delamination.
Encapsulation strategies are widely employed to mitigate environmental degradation. Thin-film barriers made of metal oxides (e.g., Al2O3, SiO2) or polymers (e.g., PMMA, Parylene) are deposited on perovskite layers to block moisture and oxygen permeation. Atomic layer deposition (ALD) is particularly effective in creating dense, pinhole-free oxide layers that provide long-term protection. Multi-layer encapsulation combining organic and inorganic materials further enhances stability by compensating for the weaknesses of individual layers. However, achieving complete impermeability remains challenging due to defects in encapsulation layers and interfacial diffusion.
Material modifications offer another route to improving stability. Incorporating hydrophobic organic cations, such as phenylethylammonium (PEA+) or butylammonium (BA+), into the perovskite structure enhances moisture resistance. 2D/3D hybrid perovskites, where 2D layers act as protective barriers around 3D perovskite grains, demonstrate superior environmental stability. The bulky organic cations in 2D perovskites repel water molecules while maintaining charge transport through the 3D regions. Additionally, partial substitution of organic cations with inorganic cesium (Cs+) improves thermal and phase stability.
Doping and defect passivation also play a crucial role in mitigating degradation. Incorporating small amounts of additives like potassium (K+) or rubidium (Rb+) suppresses ion migration and reduces defect density. Surface passivation with Lewis bases (e.g., thiophene, pyridine) binds to undercoordinated lead atoms, preventing moisture ingress and oxygen interaction.
Despite these advancements, achieving long-term stability under real-world conditions remains an ongoing challenge. Future research must focus on understanding degradation kinetics at the atomic level and developing scalable stabilization techniques. Combining encapsulation, compositional engineering, and defect passivation will be key to unlocking the full potential of hybrid perovskites in practical applications.
The degradation pathways of hybrid perovskites are complex and interdependent, often accelerating when multiple environmental factors coexist. Addressing these challenges requires a multidisciplinary approach, blending materials science, chemistry, and engineering to develop robust solutions for next-generation optoelectronic devices.