First principles investigations for structural, electronic, optical and elastic properties of Ag2BeSn(S/Se)4 chalcogenide in kesterite and stannite phases with theoretical SLME calculations

In the present work, density functional theory based computational analysis is carried out for structural, electronic, optical, and mechanical properties of novel chalcogenide Ag2BeSn(S/Se)4 materials. We have employed highly accurate Heyd-Scuseria-Ernzerhof (HSE-06) hybrid functional with generalised gradient approximation (GGA) functional within density functional theory (DFT). The Ag2BeSn(S/Se)4 in their thermodynamically stable phases kesterite (Kes) and stannite (Sta) are optimized and using the optimized lattice constants the bandgap values are calculated. Out of all four structures studied here, the estimated bandgap values are found to be 1.5 eV and 1.26 eV, respectively, for Ag2BeSnS4 (Sta) and Ag2BeSnSe4 (Kes) materials. Partial density of states (PDOS) for all four structures have also been analysed to get more insights into their electronic behaviour. Various optical properties such as the dielectric function, refractive function, absorption spectrum, extinction coefficient, reflectivity and optical conductivity have been calculated and studied in detail for all structures. Calculated elastic constants and moduli to validate the mechanical stability of the materials are studied here. Also, we have calculated spectroscopic limited maximum efficiency (SLME) for all four materials studied here and found that Ag2BeSnS4 (Sta), and Ag2BeSnSe4 (Kes) have single layer efficiencies of 23.7%. 31.3%, respectively. We expect our computational results to encourage photovoltaic researchers globally to experimentally realise these materials and their solar cells.