Stalking Halide Ion Migration in Lead Halide Perovskites
Halide ion mobility in metal halide perovskite films dictates the performance of solar cells. The mobility of halide ion can be induced both thermally and with bandgap excitation. The halide ion mobility is often dictated by the defects and halide ion composition. The excited state characterization using emission and transient absorption spectroscopy has allowed us to probe the halide ion migration and segregation in methylammonium lead iodide/bromide (CH3NH3PbBrxI3-x (x=0 to 3)). In addition to composition dependent absorption and emission properties they also undergo phase segregation to create Iodine-rich and Bromide- rich regions when subjected to visible irradiation. This intriguing aspect of halide ion movement in these mixed halide films can be tracked from the changes in the photoluminescence and absorption spectra. The photovoltaic performance decreases with continuous illumination even without external load. Upon storing these perovskite solar cells in dark, the photovoltaic performance recover indicating remixing of the halide ions. This recovery in photovoltaic performance is in line with the absorption recovery of the mixed halide band. Efforts to minimize photosegregation in mixed halide films will be discussed.