In the last decade metal halide (ABX₃) perovskite semiconductors have emerged as extraordinary materials for photovoltaic cells, light-emitting diodes and X-ray detectors because they are defect tolerant, can be deposited using inexpensive methods, and have direct band gaps that can be tuned from 1.2- 3 eV. However, perovskite semiconductors – particularly those materials with bandgaps greater than 1.7 eV – are far from reaching their full potential. Common wide bandgap perovskites such as MAPb(I_xBr_1-x)₃ suffer from photoinstabilities, environmental degradation, and poor optoelectronic performance that have limited their applicability in optoelectronic devices. In this talk I will share our work on using complementary in situ photoluminescence and X-ray diffraction techniques to reveal new insights into the instabilities and degradation pathways for these mixed-halide perovskites.

Under illumination, select mixed-halide perovskites undergo photoinduced halide segregation that drives the formation of a highly emissive lower bandgap phase. Using in situ grazing incidence wide angle X-ray scattering (GIWAXS), we track the structural changes that compliment these observations in optoelectronic property by monitoring perovskite samples under illumination and in subsequent relaxation in the dark [1]. Our results reveal a photoinduced halide segregation process that drives the formation of chemically distinct iodine- and bromine- rich phases for a range of mixed-halide perovskites. Additionally, we determine kinetic rate constants for the segregation and dissolution of these crystalline phases, and observe that this process, although greatly impacting the structure of the films, seems largely reversible and does not result in a change in crystalline orientation.

Beyond the largely reversible photoinduced halide segregation process, mixed halide perovskites are also subject to long-term and irreversible photodegregation processes. We monitor the structural and optical properties of mixed-halide perovskites over extended illumination and observe a selective expulsion of iodide from the perovskite lattice. Our results confirm that this is done while maintaining the perovskite structure, without the additional formation of substantial lead iodide or other crystalline decomposition products, and with the effect of substantial changing the optoelectronic properties and photostability of the remaining perovskite. Overall, this work highlights the utility of correlated structure-property measurements in understanding and ultimately addressing the instabilities observed for mixed-halide perovskites and perovskite semiconductors more broadly.