Polymorphic Effects and Optoelectronic Excellence in Mixed-Pb/Sn Halide Perovskites for Next-Generation Solar Harvesting

Polymorphic effects in mixed-metal halide perovskites (MHP) are critical for predicting optoelectronic performance, as the high-symmetry cubic phase (Pm3̅ m) alone is insufficient to explain observed performance. Here, we performed first-principles calculations with an advanced statistical model to describe the thermodynamic ensembles of CsPb$_{1–x}$Sn$_x$Cl $_3$ and CsPb$_{1–x}$Sn$_x$Br$_3$ , from which the optoelectronic behavior is described through polymorphic contributions by integrating an innovative workflow automation within the SimStack framework. We employed a generalized quasichemical approximation (GQCA) approach to perform a weighted contribution of different polymorphic degrees based on symmetry breaking to confer a more realistic description for the alloys, predominantly driven by distortions and rotations of the octahedra (e.g., based on Glazer’s notations). We found that power conversion efficiencies at operating temperature for both materials exhibit maximum performance for x > 0.70, highlighting the value of ∼29% for the Br-based alloy, which present advantageous low critical temperatures of approximately 28 K (whereas it is 110 K for CsPb$_{1–x}$Sn$_x$Cl$_3$). Therefore, mixing Sn and Pb as MHP alloys preserves high PCE values and mitigates the oxidation tendency of Sn-based MHPs in air, supporting the development of long-term stable photovoltaic devices.