Post-transition metal Sn-based chalcogenide perovskites: a promising lead-free and transition metal alternative for stable, high-performance photovoltaics

Abstract

Chalcogenide perovskites (CPs) have emerged as promising materials for optoelectronic applications due to their stability, non-toxicity, small bandgaps, high absorption coefficients, and defect tolerance. Although transition metal-based CPs, particularly those incorporating Zr and Hf, have been well-studied, they often exhibit higher bandgaps, lower charge carrier mobility, and reduced efficiency compared to lead-based halide perovskites (HPs). Tin (Sn), a post-transition metal with a similar oxidation state (+4) to Zr and Hf in ABX₃ structures but with different valence characteristics, remains underexplored in CPs. Given the influence of valence states on material properties, Sn-based CPs are of great interest. This study employs density functional theory (DFT), density functional perturbation theory (DFPT), and many-body perturbation theory (GW and BSE) to investigate a series of distorted Sn-based CPs (ASnX₃, A = Ca, Sr, Ba; X = S, Se). Our results demonstrate that these perovskites are mechanically stable and exhibit lower direct G₀W₀ bandgaps (0.79–1.50 eV) compared to their Zr- and Hf-based counterparts. Analysis of carrier-phonon interactions reveals that the charge-separated polaronic state is less stable than the bound exciton state in these materials. Additionally, polaron-assisted charge carrier mobilities for electrons (21.33–416.02 cm² V⁻¹ s⁻¹) and holes (7.02–260.69 cm² V⁻¹ s⁻¹) are comparable to or higher than those in lead-based HPs and significantly exceed those of Zr- and Hf-based CPs, owing to reduced carrier-phonon coupling. The estimated spectroscopic limited maximum efficiency (24.2–31.2%)—confirmed through perovskite solar cell (PSC) simulations using SCAPS-1D software—indicates that these materials are promising candidates for photovoltaic applications. Overall, this study highlights the potential of Sn as a superior alternative to transition metals in CPs, particularly for photovoltaic applications, where smaller electronegativity differences lead to reduced bandgaps and reduced polaronic effects improved charge carrier mobility. These findings are expected to stimulate further experimental investigation into Sn-based CPs.