Passivating CsPbI3 Surface Iodine Vacancy Defects with Carboxylic Functional Group and the Impact of Carbon Chain Length

Cesium lead iodide (CsPbI3) has attracted significant attention as a promising material for solar photovoltaic applications, owing to its excellent stability. In this study, five carboxylate ions (carboxylic acid, acetic acid, propanoic acid, butanoic acid, and benzoic acid) with different carbon chain lengths are used to modify the PbI2 and CsI surfaces of CsPbI3 perovskite. The iodine vacancy defects are designed at the perovskite surface. The results reveal that the PbI2-terminated surface exhibits greater stability than the CsI-terminated surface. The long-chain molecules such as benzoic acid, butanoic acid, and propanoic acid contribute to defect reduction in CsPbI3 and make a strong covalent bond. Among these carboxylate ions, long-chain carboxylate ions exhibit a better passivation effect on iodine vacancy defects. This effect can be confirmed to their projected density of states (PDOS), negative formation energy, negative grand surface energy, and higher work function. Structural stability was also examined, and electron-phonon coupling was analyzed via vibrational frequency calculations. It is observed that the light absorption coefficient of the perovskite degrade with increasing carbon chain length due to wide band gap. In conclusion, these results demonstrate that carboxylate ions with longer carbon chain lengths have a better effect on the electronic and optical properties of CsPbI3.