Ab initio study of the role of iodine in the degradation of CH3NH3PbI3

Abstract

As one of the degradation products, molecular I₂ was recently found to rapidly decompose the CH₃NH₃PbI₃ (MAPbI₃) perovskite to form PbI₂. However, the microscopic details of the degradation process are unclear. In this work, the reactions between MAI- and PbI₂-terminated MAPbI₃ (110) surfaces and I₂ molecules were investigated by density functional theory (DFT) calculations. Our study shows that the I₂ molecule is readily adsorbed on the surfaces, and preferentially forms a triiodide (I₃⁻) ion with a surface iodide ion. This then facilitates the formation of a split interstitial iodide defect with a small activation energy. On the other hand, I˙ radicals, generated in photolysis of molecular I₂, were found to react more readily with the perovskite surfaces, again forming split interstitial iodide defects and surface I₃⁻ ions. We also studied the properties of photo-generated electrons and holes on the MAPbI₃ surfaces, and their roles in the degradation process. While the excess electrons tend to delocalize on the perovskite surfaces, the excess holes can be strongly trapped around several surface iodides. The presence of the excess electrons or holes was found to enhance I˙ radical adsorption and its reaction with the MAPbI₃ surfaces. Besides, we identified the mechanism of the formation of a surface I₃⁻ ion from the surface-trapped holes without the addition of molecular I₂. This provides a key understanding of photo-induced MAPbI₃ degradation in a vacuum. Finally, we discuss how the formation of the split interstitial iodide defects induces the decrease in the distance between two neighboring Pb ions which leads to the formation of the local PbI₂ structure.