  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.
