Effect of Halide Composition on the Photochemical Stability of   Perovskite Photovoltaic Materials
  Photochemical stability of encapsulated films of mixed halide perovskites with a range of MAPb(I1-xBrx)3 compositions (solid solutions) was investigated under accelerated stressing using concentrated sunlight. Evolution of light absorption and the corresponding structural modifications in the films were recorded by UV-vis spectroscopy and X-ray diffraction (XRD), respectively. MAPbBr3 film exhibited no degradation. In MAPbI3 and mixed halide MAPb(I1-xBrx)3 films, decomposition of the perovskite material with crystallization of inorganic PbI2, and the corresponding degradation of light absorption, were recorded. Introduction of bromine into the solid solution was found to stress its structure and accelerate the degradation. The relevance of accelerated testing to standard operational conditions of solar cells was confirmed by comparison to degradation experiments under outdoor sunlight exposure. Reasons for the reduced stability of the mixed halide compositions are discussed. 
Ravi K. Misra, Laura Ciammaruchi, Sigalit Aharon, Dmitry Mogilyansky,   Lioz Etgar, Iris Visoly-Fisher and Eugene A. Katz
cond-mat.mtrl-sci