Atomic and Molecular Hydrogen Impurities in Hybrid Perovskite Solar Cells

Hydrogen interstitials are expected to be important in organic–inorganic hybrid perovskites; however, the characteristics and behaviors of hydrogen in perovskites remain poorly understood. Here, on the basis of density functional theory calculations, we quantitatively reported that both atomic and molecular hydrogen interstitials can form in hybrid MAPbI3 and MASnI3 perovskites. Whereas molecular hydrogen interstitial, H2, is chemically inert, atomic hydrogen interstitial, Hi, serves as an electrically active negative-U defect. We identify high-density Hi+ as a significant origin of ionic conductivity in p-type MAPbI3 under the hydrogen-rich conditions, with the calculated activation energy being comparable to that measured in experiments. The highly diffusive Hi+ ions are expected to impact hysteresis, charge separation, device polarization, and photogenerated field-screening effect and consequently degrade the solar cell performance. We evaluated approaches for mitigating such detrimental effects and suggested that synthesizing the perovskites with slightly extra iodine addition or tin alloying can effectively suppress the concentration of Hi+. Our results are important to understand the fundamental aspects of hydrogen in perovskites in general and offer valuable insight for further improving the performance of perovskite solar cells and other optoelectronic devices via defect engineering.