Assessing CH₃NH₃CaBr₃ as a lead-free hybrid perovskite: DFT insights into stability and potential applications

Since the development of metal halide hybrid perovskite (MHP)-based solar cells, many investigations have aimed to overcome the shortcomings of the most widely used MHP, methylammonium (CH₃NH₃, MA) lead iodide (CH₃NH₃PbI₃, MAPI), such as its toxicity and low stability. In this respect, many MHPs have been proposed to reduce or replace the Pb content in MAPI. This work studies the vibrational, thermodynamic, electronic, and optical properties of a proposed CH₃NH₃CaBr₃ (MACB) cubic perovskite by using first-principles density functional theory and density functional perturbation theory to evaluate its potential as a solar cell material. The results show that MA lies stably only at certain positions within the perovskite lattice, and the barrier energy for an internal rotation to the next stable position is relatively low. Also, the mechanical properties show fragility in directions perpendicular to the C-N bond of MA. The electronic properties show a large energy gap; that is, the optical properties of MACB appear outside the ideal range for solar cell applications. These results could help understand the stabilization mechanisms of MACB and reveal its potential applications, as a computationally promising material whose optical properties are consistent with UV-protective applications in solar cells, pending experimental validation, as well as for applications as ultraviolet detectors and photonic devices such as perovskite-based light-emitting diodes.