Phase Behavior and Substitution Limit of Mixed Cesium-Formamidinium Lead TriIodide Perovskites

The mixed cation lead iodide perovskite photovoltaics show improved stability following site substitution of cesium ions (Cs ⁺) onto the formamidinium cation sites (FA ⁺) of (CH(NH ₂) ₂PbI ₃ (FAPbI ₃) and increased resistance to formation of the undesirable -phase. The structural phase behavior of Cs _0.1FA _0.9PbI ₃ has been investigated by neutron powder diffraction (NPD), complemented by single crystal and power X-ray diffraction and photoluminescence spectroscopy. The Cs-substitution limit has been determined to be less than 15%, and the cubic α-phase, Cs _0.1FA _0.9PbI ₃, is shown to be synthesizable in bulk and stable at 300 K. On cooling the cubic Cs _0.1FA _0.9PbI ₃, a slow, second-order cubic to tetragonal transition is observed close to 290 K, with variable temperature NPD indicating the presence of the tetragonal -phase, adopting the space group P4/mbm between 290 and 180 K. An orthorhombic phase or twinned tetragonal phase is formed below 180 K, and the temperature for further transition to a disordered state is lowered to 125 K compared to that seen in phase pure α-FAPbI ₃ (140 K). These results demonstrate the importance of understanding the effect of cation site substitution on structure-property relationships in perovskite materials.