The high-temperature, all-inorganic CsPbI perovskite black phase is metastable relative to its yellow, nonperovskite phase at room temperature. Because only the black phase is optically active, this represents an impediment for the use of CsPbI in optoelectronic devices. We report the use of substrate clamping and biaxial strain to render black-phase CsPbI thin films stable at room temperature. We used synchrotron-based, grazing incidence, wide-angle x-ray scattering to track the introduction of crystal distortions and strain-driven texture formation within black CsPbI thin films when they were cooled after annealing at 330°C. The thermal stability of black CsPbI thin films is vastly improved by the strained interface, a response verified by ab initio thermodynamic modeling.
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