Revealing Light-Induced Changes in Mixed-Halide Perovskites Through Multimodal Microscopy

Stability remains a key concern in the development of lead halide perovskites for optoelectronic devices. While perovskites are extremely defect tolerant, a key to their success as solution processed semiconductors, the presence of ionic defects that can readily move through the lattice at room temperature have been attributed to both reversable changes in behavior (i.e. current-voltage hysteresis), and irreversible device degradation (i.e. electrode corrosion). To add to this complexity, excited-state carriers have been observed to change the number of and nature of defects in materials, exacerbating instabilities under illumination and bias. To develop stable photovoltaics requires understanding and control of these effects. Here, we provide insight into photo-induced instabilities in mixed-halide perovskites. Using multimodal microscopy with complimentary X-ray diffraction we track changes in composition and optoelectronic property at the microscale in MAPb(I_xBr_1-x)₃ perovskites. To understand the role of carrier-induced defects on heterogeneity we collect absorption and photoluminescence maps of encapsulated perovskite films before and after aging under one sun, allowing us to track local changes in optoelectronic property under illumination. With these measurements we observe that illumination drives both the well documented reversible halide-segregation process, and irreversible changes in the film structure, composition, and property. Over longer periods of time (>24 hours) we observe changes in structure and optical bandgap consistent with the selective expulsion of iodine from a mixed iodide-bromide perovskite composition. This is done without noticeable degradation of the perovskite, as the films are observed to have a higher bandgap, brighter luminescence, and more uniform luminescence after extended illumination. Overall, these advances improve our understanding of the complex photoinstabilities in mixed-halide perovskites, and the potential role tuning chemistry and microstructure can have on achieving long term stability.