Anharmonicity and Disorder in the Black Phases of CsPbI3 used for Stable Inorganic Perovskite Solar Cells

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Abstract
Hybrid perovskite materials have emerged over the past five years as absorber layers for new high-efficiency yet low-cost solar cells (PSCs) that combine the advantages of organic and inorganic semiconductors. Here, employing the linear response (DFPT) approach of Density Functional Theory (DFT) and frozen phonon calculations, we reveal strong anharmonic effects in the inorganic CsPbI3 perovskite. We also 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. Among the four phases of CsPbI3, we found that this phase is the only one to behave harmonically around equilibrium. Last, we explain the Rashba effect in these materials as a consequence of the symmetry breaking induced by the double-well instabilities.

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