On the Vegard’s law compliance for CsxFA(1-x)PbI3 perovskite thin films

Perovskite-based solar cells (PSCs) have demonstrated remarkable high power conversion efficiency in recent years. However, the use of mono-organic cations (such as Methylammonium or Formamidinium) limits the potential for large-scale development due to potential degradation under environmental conditions. The incorporation of multi-cations has emerged as a strategy to enhance both performance and stability. The cesium (Cs) cation represents a solid alternative for partial substitution of Formamidinium. However, the initial concentration of precursors in the solution is often reported without establishing the final concentration of the cation present in the thin films. Herein, the incorporation of Cs cations into the FAPbI3 structure to produce a CsxFA(1-x)PbI3 perovskite with different values of x using a one-step spin coating process is demonstrated. Assessing the structural and optical properties, it is demonstrated that CsxFA(1-x)PbI3 films behave according to Vegard's law for values of x between 0 and 0.66. In particular, CsxFA(1-x)PbI3, with an x concentration of 0.33 exhibits a cubic lattice parameter of 6.28 Å, lower than that for FAPbI3 but higher than that for CsPbI3. This concentration showed stability of the dark phase under ambient conditions for extended periods. In addition, this material has a bandgap of 1.5 eV, making it suitable for use in solar cells.