Nanoscale Structural Heterogeneity and Efficient Intergrain Charge Diffusion in a Series of Mixed MA/FA Halide Perovskite Films

Perovskite solar cells have made tremendous technological progress, but knowledge of the fundamental photophysical properties of perovskite materials is equally important for further advancement of the field. We use fluorescence microscopy to study the nanoscale properties of a series of mixed-cation perovskite MA1–xFAxPbI3 films, an important photovoltaic material. Measuring photoluminescence spectra on submicrometer scales reveals the compositional heterogeneity of the films. The heterogeneity is largest for the FA 50% fraction films which contain purely MA domains, purely FA domains, as well as domains composed of mixed MA/FA cations of varying ratios. The films also show photoluminescence intensity fluctuations (blinking), which reflects dynamic nonradiative quenching. The quenching is most suppressed for the FA 50% films which contain the truly mixed MA/FA domains. Further, the blinking is correlated between locations that are micrometers apart, indicating that the grain boundaries do not function as traps and are transparent toward efficient charge migration.