Effects of Rb Incorporation and Water Degradation on the Stability of the Cubic Formamidinium Lead Iodide Perovskite Surface: A First Principles Study

Despite the major improvements in device efficiency, perovskite solar cells (PSCs) are limited by their poor stability due to the degradation toward moisture. Recently studies have demonstrated that the perovskite materials can be stabilized by adding more inorganic elements such as Cs and Rb. However, the specific mechanism is not yet fully established. In this paper, first-principles calculations were employed to investigate the effects of Rb incorporation and water degradation on cubic formamidinium lead iodide (FAPbI(3)) perovskite. The termination dependence of the structural stability of the representative (100), (010), and (001) surfaces was characterized by surface energies. The PbI2-terminated (001) surface was found to be the most stable surface structure. The Rb dopants were found to be energetically favorable, existing on the surface more than staying in the inner atomic layers. Furthermore, the degradation mechanism was investigated by water adsorption on the Rb-incorporated FAPbI(3) (001) surface. According to our calculations, the water molecules tend to interact with the surface on the Pb-top sites with the largest adsorption energy. The orientation of the surface FA cations exerted a crucial influence on the adsorption energy as well as the surface energy. The moisture-induced decrease in optical absorption in the visible region could be observed in absorption spectra. It indicates that the adsorbed water molecules subsequently reduce the conversion efficiency. More importantly, we find that Rb incorporation at the surface region with the largest adsorption energy will destroy the surface structure leading to the obvious drop in the absorption spectra. These findings further illustrate the initial degradation process of Rb-incorporated FAPbI(3) under the humid condition, which should be helpful to improve the stability of Rb-doped FAPbI(3) perovskites in the presence of moisture.