First-Principles Calculations of Graphene-Coated CH₃NH₃PbI₃ toward Stable Perovskite Solar Cells in Humid Environments

Despite the impressive development of CH3NH3PbI3(MAPbI3)-based perovskite solar cells (PSCs) in device efficiency, they are limited by their low stability due to the moisture-induced degradation of MAPbI3. Here, it is demonstrated by first-principles calculations incorporating a semiempirical dispersion–correction scheme that water molecules tend to combine with the Pb-top site on MAPbI3(001) surface in molecular form. In addition, the band gap of MAPbI3 increases to 1.85 eV when Pb atoms on surface are all adsorbed water molecules, which is not conducive to the visible light absorption. To solve the issue, the MAPbI3/graphene heterostructure was constructed and the corresponding role of graphene was investigated systematically in protecting MAPbI3 against humid environment. Both the adsorption energy and transition state search indicate evidently that on the whole, systems with water adsorption outside the interface are more likely than those in the interface. Further, it is confirmed that the water molecules have neglectable effect on the energy band structure and charge distribution of graphene-coated MAPbI3. The introduction of graphene can not only protect MAPbI3 from water erosion but also form a built-in electric field with MAPbI3 at the interface, which promotes the separation of photogenerated electron–hole pairs. These findings illustrate the protection mechanism of graphene under humid conditions to improve the stability of MAPbI3-based PSCs.