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.