Using photoluminescence to monitor the optoelectronic properties of methylammonium lead halide perovskites in light and dark over periods of days.

The degradation of methyl-ammonium lead iodide (MAPbI 3 ) upon exposure to air is a potentially limiting effect for large scale MAPbI 3 photovoltaic production. Here, we report a systematic study on effects of air-exposure on the structural and optical properties of MAPbI 3 thin films. The X-ray diffraction studies indicate a shrinking volume of MAPbI 3 upon air-exposure as the material decomposes back to its precursors, methyl amine (CH 3 NH 2 ) and lead iodide (PbI 2 ). However, the photoluminescence (PL) yield and carrier lifetime measured with time-resolved photoluminescence, show an increasing trend upon air-exposure. These phenomena can be explained by self-passivation of MAPbI 3 grains by PbI 2 layer that reduces the number of non-radiative recombination centres at the grain boundaries. However, this process is not self-limiting and it eventually leads to a film that has completely reverted back to its precursor state. It is shown that this conversion of MAPbI 3 film back to its precursors is also accelerated by exposure to laser illumination. Furthermore, we report unusual variation of PL intensity on a shorter time scale of a few seconds in all the films used for the experiment. The variations are found to follow different trends in the encapsulated samples as compared to the un-encapsulated samples. We propose that the decomposition followed by the ionic diffusion through film is responsible for such unusual behaviours. [ABSTRACT FROM AUTHOR]