Shining a light on the photoluminescence behaviour of methylammonium lead iodide perovskite: investigating the competing photobrightening and photodarkening processes

Organic-inorganic perovskite materials, such as methylammonium lead iodide, are good candidates for the next generation of solar cells with an impressive certified 23.7% power conversion efficiency. However, their development is impeded by their poor stability to oxygen, moisture, light and combinations of these. The bulk photoluminescence of perovskite films is extremely sensitive to the environment (humidity, light intensity, temperature, atmosphere) and has been shown to undergo rapid photobrightening and photodarkening. Here, we study the changes in photoluminescence behaviour (intensity, wavelength and full-width-at-half-maximum) in both air and nitrogen and correlate this with changes in the crystal structure of the films under illumination. We show that photodarkening is caused by degradation of the perovskite and that both brightening and some degree of darkening are competitive processes occurring simultaneously. During photobrightening the fluorescence emission spectrum red-shifts which can be explained by the halide redistribution and passivation of trap states in the perovskite. We conclude that care is required when measuring fluorescence of perovskite due to the important variations in intensity, emission wavelength and full-width-at-half-maximum.