Perovskite material appeared to be perspective and promising in photovoltaic application, such as solar cells, photodetectors, light emitting diodes and etc. due to outrageous properties like high carrier mobility, effective light absorption, mechanical flexibility, cheapness and simplicity of production makes them worthy opponent to crystalline silicon in creation less environment polluted world for our children. Although perovskites are already started to be used in solar cell fabrication on the market level, the extrinsic and intrinsic stability of them can be greatly improved. The most discussed issue nowadays is halide movement under continuous light illumination for I-rich and Br-rich domains. I-rich domain act as recombination centers in halide perovskites what impedes the carrier generation in the bromine-rich domain and blocks the electron flow through the device and reduces the efficiency of solar cells.

Although broad consensus exists that photoirradiation of mixed-halide lead perovskites leads to anion segregation, no model today fully rationalizes all aspects of this near ubiquitous phenomenon. In this work, we quantitatively compare experimentally the variety of dimensionality materials (such as bulk thin films, 2D Ruddlesden-Popper and 0D nanocrystals (NCs)) terminal anion photosegregation stoichiometries and excitation intensity thresholds to a band gap-based, thermodynamic model of mixed-halide perovskite photosegregation. Mixed-halide NCs offer strict tests of theory given physical sizes, which dictate carrier diffusion lengths. Further highlighting the importance of these studies are prior results, which suggest increased stabilities of mixed-anion perovskite NCs to irradiation. Observed qualitative and quantitative agreement with theory support a band gap-based model for anion photosegregation. More importantly, they suggest that mixed-halide perovskite photostabilities can be predicted using local gradients of (empirical) Vegard’s law expressions of composition-dependent band gaps. We have recently tested this alternative photostability metric on a mixed-cation/mixed-anion system, MA_0.5Cs_0.5Pb(I_1−xBr_x)₃ compared to MAPb(I_1−xBr_x)₃ and predicted that smaller local band gap gradients indeed correspond to improved anion photostabilities. Thus, not only is the developed band gap-based photosegregation model predictive but future extensions may rationalize remaining unexplained phenomena in lead halide perovskites such as halide remixing under large excitation intensities.