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(NH2)2PbI3 (FAPbI3) and increased resistance to formation of the undesirable ∂-phase. The structural phase behavior of Cs0.1FA0.9PbI3 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, Cs0.1FA0.9PbI3, is shown to be synthesizable in bulk and stable at 300 K. On cooling the cubic Cs0.1FA0.9PbI3, a slow, second-order cubic to tetragonal transition is observed close to 290 K, with variable temperature NPD indicating the presence of the tetragonal β-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-FAPbI3 (140 K). These results demonstrate the importance of understanding the effect of cation site substitution on structure–property relationships in perovskite materials.

混合阳离子碘化铅钙钛矿光伏材料在将铯离子（Cs⁺）取代至(CH(NH₂)₂PbI₃（FAPbI₃）的甲脒阳离子（FA⁺）位点后，稳定性得到显著提升，且有效抑制了有害δ相的生成。本研究采用中子粉末衍射（neutron powder diffraction, NPD），结合单晶与粉末X射线衍射、光致发光光谱技术，对Cs₀.₁FA₀.₉PbI₃的结构相行为进行了系统研究。实验确定铯离子的取代极限低于15%，且立方α相Cs₀.₁FA₀.₉PbI₃可批量合成，并在300 K下保持稳定。对该立方相材料进行降温处理时，在近290 K处观测到缓慢的二级立方相向四方相转变；变温中子粉末衍射结果表明，在290 K至180 K温度区间内，材料呈现四方β相，其空间群为P4/mbm。当温度降至180 K以下时，材料转变为正交相或孪晶四方相；与纯相α-FAPbI₃中的无序转变温度（140 K）相比，该材料的无序转变温度被降至125 K。上述研究结果证实，阐明阳离子位点取代对钙钛矿材料结构-性能构效关系的影响，对该类材料的设计与优化具有重要指导意义。