Finite-temperature properties of CsPbI3 thin films

${\mathrm{CsPbI}}₃$ is an inorganic perovskite solar cell material with a band gap appropriate for solar cell applications and photo conversion efficiency greater than 18%. However, this material is also known to undergo many phase transformations within the perovskite structure, starting from a high-temperature cubic phase to a tetragonal phase, leading to a room-temperature orthorhombic phase, with each phase having a different photovoltaic response. The tetragonal phase has been shown to have the highest photoconversion efficiency (18.4%), followed by the cubic phase (17%) and the orthorhombic phase (12.5%). In this work, we use the newly formulated first-principles--based effective Hamiltonian for ${\mathrm{CsPbI}}₃$ to investigate the phase transformation sequence in ${\mathrm{CsPbI}}₃$ thin films. We investigate the effect of film thickness and substrate strain on the phase transformations and the room-temperature phase. The effect of screening is also investigated. The three perovskite phases previously found in bulk by experiments and by computations were also found to exist in the thin film. It is shown that thickness and substrate strain can stabilize photoactive perovskite tetragonal and cubic phases at room temperature, both of which can be used for solar cell applications.