Unlocking the optoelectronic potential of AGeX3 (A = Ca, Sr, Ba; X = S, Se): A sustainable alternative in chalcogenide perovskites

The quest for environmentally benign and stable optoelectronic materials has intensified, and chalcogenide perovskites (CPs) have emerged as promising candidates owing to their non-toxic composition, stability, small bandgaps, and large absorption coefficients. However, a detailed theoretical study of excitonic and polaronic properties of these materials remains underexplored due to the high computational demands. Herein, we present a comprehensive theoretical investigation of germanium-based CPs, AGeX3 (A = Ca, Sr, Ba; X = S, Se), which adopt distorted perovskite structures (β-phase) with an orthorhombic crystal structure (space group: pnma) by utilizing state-of-the-art density functional theory, density functional perturbation theory (DFPT), and many-body perturbation theory [GW, Bethe-Salpeter Equation (BSE)]. Our calculations reveal that these materials are mechanically stable, having potential thermodynamic accessibility under suitable conditions. The G0W0@PBE bandgaps range from 0.65 to 2.00 eV, suitable for optoelectronics. We analyze the ionic and electronic contributions to dielectric screening using DFPT and BSE methods, finding that the electronic component dominates. The exciton binding energies range from 6.38 to 73.63 meV, indicating efficient exciton dissociation under ambient conditions. In addition, these perovskites exhibit low to high polaronic mobilities (1.67-167.65 cm2 V-1 s-1), exceeding many lead-free CPs and halide perovskites due to reduced carrier-phonon interactions. Among the studied systems, BaGeSe3 exhibits the most robust combination of thermodynamic stability and high carrier mobility, while SrGeSe3 shows a balanced interplay between electronic and optical performance. On the other hand, BaGeS3 and other sulfide members demonstrate noteworthy variations in excitonic and polaronic behavior, offering additional directions for property tuning. The combination of tunable bandgaps, low exciton binding energies, and high carrier mobility underscores the scientific promise of these materials in the context of future optoelectronic applications.