Chemistry
The Chemistry of Perovskite Solar Cells and Their Degradation Pathways
Quick fact
Perovskite solar cells have achieved power conversion efficiencies above 25% in just over a decade, yet they can degrade within hours when exposed to humid air, highlighting their extreme sensitivity to environmental conditions.
Why this is interesting
Solar cells that break efficiency records are made of a crystal that dissolves in water—how can that be?
Read the full explanation
Understanding The Chemistry of Perovskite Solar Cells and Their Degradation Pathways
Perovskite solar cells are built around a crystal structure known as ABX3. In the most common type, methylammonium lead iodide (CH3NH3PbI3), A is an organic molecule (methylammonium), B is lead, and X is iodine. This structure allows the material to absorb light very efficiently and generate charge carriers, which is why it is so promising for solar energy. However, these same chemical components make the material fragile: the organic cation is volatile, hydrogen bonding between the organic and inorganic parts is weak, and the lead-iodide framework is soft and easily distorted. When exposed to moisture, oxygen, heat, or light, these weaknesses become pathways for chemical reactions that break the material apart, reducing its ability to produce electricity.
A deeper explanation
Degradation happens through several interconnected chemical pathways. Moisture triggers a hydration reaction: water molecules break the hydrogen bonds, forming a hydrated intermediate that eventually decomposes into lead iodide and methylammonium iodide. Oxygen can combine with photogenerated electrons to create superoxide ions that attack the organic cation, releasing volatile methylamine. Heat accelerates these reactions, and also drives off the organic components directly. UV light can cause trap states in the titanium dioxide electron transport layer, generating reactive radicals that degrade the perovskite. Additionally, ions like iodine can migrate under the electric field, creating defects and halide segregation. These degradation mechanisms are interconnected and often synergistic, meaning that a combination of stressors can cause faster failure. Understanding these pathways allows researchers to design strategies: replacing the volatile organic cation with cesium or formamidinium, using hydrophobic charge transport layers, incorporating additives to passivate defects, or developing robust encapsulants that block moisture and oxygen.