Stability of CsPbI3 with divalent cations incorporated via mechanochemical alloying

Abstract

Cubic CsPbI₃ is a promising perovskite material for optoelectronic applications. This material possesses an energy band gap of 1.7 eV and an optical absorption coefficient of 105 cm⁻¹, and can be synthesized at high temperatures using various methods. However, cubic CsPbI₃ faces a significant challenge with degradation reported under ambient conditions. This degradation results in the formation of a delta phase, which comprises a non-perovskite structure with poor optical and electrical properties at room temperature. This paper proposes the partial substitution of lead in CsPbI₃ with divalent cations, such as Sn²⁺, Mn²⁺, Ni²⁺, and Ca²⁺, to improve the overall stability. These cations were selected because their ionic radii meet the Goldschmidt factor and favor the formation of cubic halide perovskites. For the synthesis of the materials, a solid-state mechanochemical method is used that allows for the incorporation of the above-mentioned cations into the CsPbI₃ matrix in a single step at room temperature. Replacing Pb²⁺ with Sn²⁺ produced the most stable material and resulted in a cubic perovskite structure with similar optical properties to CsPbI₃. In particular, a composition of CsPb_0.6Sn_0.4I₃ resulted in cubic perovskite and did not show degradation when exposed to room temperature, ambient humidity, and atmospheric pressure for up to 25 days. On the other hand, even though Ca²⁺ possesses a smaller ionic radius than Pb²⁺, replacing Pb²⁺ with Ca²⁺ was not effective in stabilizing CsPbI₃ due to the hygroscopic nature of CaI₂. Replacing Pb²⁺ with Mn²⁺ and Ni²⁺ produces stable alloys in a controlled environment (glovebox) at room temperature but quickly decomposes to non-perovskite δ-CsPbI₃, iodine, and metal oxides when exposed to air. The degradation mechanism of these materials was studied in detail using XPS techniques, revealing potential alternatives to produce stable Sn²⁺ containing perovskites with properties similar to those of cubic CsPbI₃ at room temperature without solvents and increased stability under ambient conditions when Pb²⁺ was partially replaced with Sn²⁺.