Anharmonicity and Disorder in the Black Phases of Cesium Lead Iodide Used for Stable Inorganic Perovskite Solar Cells

Hybrid organic–inorganic perovskites emerged as a new generation of absorber materials for high-efficiency low-cost solar cells in 2009. Very recently, fully inorganic perovskite quantum dots also led to promising efficiencies, making them a potentially stable and efficient alternative to their hybrid cousins. Currently, the record efficiency is obtained with CsPbI3, whose crystallographical characterization is still limited. Here, we show through high-resolution in situ synchrotron XRD measurements that CsPbI3 can be undercooled below its transition temperature and temporarily maintained in its perovskite structure down to room temperature, stabilizing a metastable perovskite polytype (black γ-phase) crucial for photovoltaic applications. Our analysis of the structural phase transitions reveals a highly anisotropic evolution of the individual lattice parameters versus temperature. Structural, vibrational, and electronic properties of all the experimentally observed black phases are further inspected based on several theoretical approaches. Whereas the black γ-phase is shown to behave harmonically around equilibrium, for the tetragonal phase, density functional theory reveals the same anharmonic behavior, with a Brillouin zone-centered double-well instability, as for the cubic phase. Using total energy and vibrational entropy calculations, we highlight the competition between all the low-temperature phases of CsPbI3 (γ, δ, β) and show that avoiding the order–disorder entropy term arising from double-well instabilities is key to preventing the formation of the yellow perovskitoid phase. A symmetry-based tight-binding model, validated by self-consistent GW calculations including spin–orbit coupling, affords further insight into their electronic properties, with evidence of Rashba effect for both cubic and tetragonal phases when using the symmetry-breaking structures obtained through frozen phonon calculations.有机-无机杂化钙钛矿自2009年起便成为高效低成本太阳能电池的新一代吸光材料。近年来，全无机钙钛矿量子点亦展现出优异的光电转换效率，使其成为杂化钙钛矿同类材料极具潜力的稳定高效替代方案。目前，最高效率的太阳能电池采用碘化铯铅（CsPbI3）制备，但其晶体学表征仍较为有限。本研究通过高分辨率原位同步辐射X射线衍射（XRD）测量发现，碘化铯铅可在其转变温度以下过冷，并能在室温下暂时保持钙钛矿结构，从而稳定了光伏应用中至关重要的亚稳态钙钛矿多型（黑相γ相）。我们对结构相变的分析表明，各晶格参数随温度的演化呈现极强的各向异性。研究团队进一步基于多种理论方法，对实验中观测到的所有黑相的结构、振动及电子特性展开了表征分析。尽管黑相γ相在平衡位置附近呈现简谐行为，但密度泛函理论分析显示，四方相与立方相一样，均表现出非简谐特性，且存在布里渊区中心双阱不稳定性。通过总能与振动熵计算，本研究阐明了碘化铯铅所有低温相（γ、δ、β相）之间的竞争机制，并揭示避免双阱不稳定性所引发的有序-无序熵项，是抑制黄色类钙钛矿相生成的关键所在。本研究采用包含自旋轨道耦合的自洽GW计算对基于对称性的紧束缚模型进行了验证，进一步揭示了其电子特性：通过冻结声子计算得到的破缺对称结构分析显示，立方相与四方相均存在拉什巴效应。