The effect of oxygen molecule adsorption on lead iodide perovskite surface by first-principles calculation

Highlights: • The reason why work function increases when O₂ is adsorbed on the surface of MAPbI₃ is elucidated. • The formation of $O_{2^{-}}$ is easy because unoccupied 2π* orbitals of O₂ are located between VB and CB of the MAPbI₃ perovskite. • Physisorption of O₂ can slightly affect the band structure of MAPbI₃, thus reducing the PCE of MAPbI₃ perovskite solar cell. Oxygen molecule has a negative effect on perovskite solar cells, which has been investigated experimentally. However, detailed theoretical research is still rare. This study presents a microscopic view to reveal the interaction mechanism between O₂ and perovskite based on the first-principles calculation. The results show that O₂ is adsorbed on the (100) surface of MAPbI₃ perovskite mainly by Van der Waals force. O₂ adsorption makes the MAPbI₃ surface generate a small number of positive charges, which leads to the increase of the work function of the MAPbI₃ surface. This is in agreement with the experimental measurement. And increased work function of MAPbI₃ surface is not beneficial to electron transfer from perovskite to electronic extraction layer (such as TiO₂). Comparison of the density of states (DOS) of the clean (100) surface and the adsorbed system shows that an in-gap state belonging to O₂ appears, which can explain the phenomenon observed from experiments that electron transfers from the surface of perovskite to O₂ to form superoxide. The theoretical power conversion efficiency of the system with and without O₂ adsorption is evaluated, and it turns out that the power conversion efficiency of the system with O₂ adsorption is slightly lower than that of the system without O₂ adsorption. This result indicates that avoiding the introduction of O₂ molecules between perovskite and electronic extraction layer is beneficial to the perovskite solar cell.