Mixed Halide Perovskite Phase Stability

Halide perovskites have potential uses in next-generation printed optoelectronics with high performance demonstrated in photovoltaics and LEDs. These materials have ABX₃ stoichiometry and have properties that are broadly tunable through compositional changes of the A (1+ cation such as Cs¹⁺), B (2+ metal cation such as Pb²⁺ or Sn²⁺), and X-site (halide) components. Material stability is challenging, especially across the compositional range desired for varying applications. The degradation pathway in the crystal phase transformation from the perovskite phase to the non-perovskite delta phase is how structural stability can be measured. In this work, we changed the halogen site (X-site) of the all-inorganic halide perovskite CsPb(I_xBr_1-x)₃ and examined the structural stability. The change in halide composition directly affected the Goldschmidt Tolerance Factor of the structure, which is a common metric used for determining stability based on the ionic radii of the different sites in the structure. By using different iodine to bromine ratios, we investigated how changes in the tolerance factor affected phase change kinetics. To track the phase stability of the perovskites, films were fabricated using different halide compositions, and then the kinetics were tracked using absorption spectroscopy. As humid air was introduced, the perovskite structure began to undergo a phase transition. This was quantified, at the wavelength of each specific composition, to measure and quantify phase stability. As expected, higher bromine fractions resulted in greater phase stability. This can be understood as the result of changes to the Goldschmidt Tolerance Factor. In future, additional compositional changes will be made and the phase stability of the different alloys will be explored more fully.