Heat- and Gas-Induced Transformation in CH3NH3PbI3 Perovskites and Its Effect on the Efficiency of Solar Cells

Following the remarkable success of the application of CH3NH3PbI3 perovskites in photovoltaics, a great focus has been placed on their stability to improve their optoelectronic properties and seek commercial production. To gain a better understanding of their thermal stability, we investigated the chemical, morphological, and photovoltaic transformations of CH3NH3PbI3 perovskites under elevated temperatures and various controlled atmospheric conditions (vacuum, 1 mbar O-2, and 1 mbar H2O). A temperature-dependent study showed that CH3NH3PbI3 decomposed to PM, with the release of CH3NH2 and HI under low-temperature annealing (25-150 degrees C). Further annealing resulted in the formation of metallic lead (Pb-0) under vacuum and Pb oxides and hydroxides under an O-2 or H2O pressure. Moreover, the sublimation of Pb-based compounds occurred at temperatures above 150 degrees C, causing structural changes, which resulted in a decrease in the power conversion efficiency of the solar cell. A time-dependent study showed that, compared with vacuum conditions, the addition of O-2 or H2O accelerated the degradation of the perovskite films.C