Photovoltaic Properties of ABSe₃ (A = Ca, Sr, Ba; B = Zr, Hf) Chalcogenide Perovskites

Chalcogenide perovskites have sparked interest as promising optoelectronic materials due to their stability, nontoxicity, small bandgaps, large absorption coefficients, and high defect tolerance. Nevertheless, a thorough theoretical investigation of excitonic and polaronic properties is not explored rigorously due to the high computational expense. In this study, we have therefore performed a systematic and comprehensive investigation of electronic, optical, transport, excitonic, and polaronic properties of chalcogenide perovskites ABSe₃ (A = Ca, Sr, Ba; B = Zr, Hf), including their needle-like (α-phase) and distorted (β-phase) phases within the framework of density functional theory (DFT), density functional perturbation theory (DFPT), and many-body perturbation theory (MBPT) based GW and BSE. We observe that the phonon band structures and elastic properties confirm the stability of these perovskites. These materials exhibit direct electronic bandgap in the range of 1.02 - 1.97 eV, which are calculated using the G₀W₀@PBE method. Also, the computed effective masses of both carriers are found to be small (< 1), suggesting good charge carrier mobility of these compounds. Following that, the optical properties are computed by solving the Bethe-Salpeter equation (BSE), which indicates a high absorption coefficient (~ 10⁵ cm^-1) for all compounds, and β-phases also have absorption onset in the visible region. Interestingly, they exhibit smaller exciton binding energy (0.02 - 0.10 eV) than conventional halide and sulfur-based chalcogenide perovskites, and β-phases have a longer exciton lifetime than the α-phases. In addition, from Fröhlich’s mesoscopic model, an intermediate electron (hole)-phonon coupling has been observed, which results in higher polaronic mobility in the range of 8.26 - 77.59 cm²V⁻¹s⁻¹ for electrons and 19.05 - 100.49 cm²V⁻¹s⁻¹ for holes, and these are found to be much higher than that of sulfur-based chalcogenide perovskites. Further, the polaron energy calculations confirm that charge-separated polaronic states of α-phases (β-phases) are found to be less (more) stable than the bound exciton. All the examined properties suggest β-ABSe₃ to be promising environmentally friendly stable materials for photovoltaic applications. This has been further confirmed by estimating spectroscopic limited maximum efficiency, which is calculated as ∼ 17.5% - 23% for these materials.