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