Investigating the potential of triclinic ABSe3 (A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) perovskites as a new class of lead-free photovoltaic materials

In recent years, chalcogenide perovskites have emerged as promising candidates with favorable structural, electrical, and optical properties for photovoltaic applications. This paper explores the structural, electronic, and optical characteristics of ABSe₃ perovskites (where A = Li, Na, K, Rb, Cs; B = Si, Ge, Sn) in their triclinic crystallographic phases using density functional theory. The stability of these materials is ensured by calculating formation energies, tolerance factors (T_f), and phonon dispersion. The E_form values of all ABSe₃ are negative, suggesting favorable thermodynamic stability. The T_f values range between 0.82 and 1.1, which is consistent with stable perovskites. The phonon dispersion analysis of the chalcogenide perovskites revealed no imaginary frequencies in any of the vibrational modes, confirming their stability. The electronic band structures and corresponding density of states are computed to unveil the semiconducting nature of the studied compounds. These perovskites are promising for high-performance solar cells due to their indirect bandgaps (E_g, 1.10-2.33 eV) and a small difference between these indirect and direct gaps (0.149-0.493 eV). The E_g values increase as the ionic radii of A-site elements increase (Li < Na < K < Rb < Cs). At the B-site, Si-based chalcogenides have the largest E_g values, followed by Sn-based and then Ge-based materials. Furthermore, optical properties such as the real part and imaginary part of the dielectric function, refractive index extinction coefficient, optical conductivity, absorption coefficient, reflectivity, and energy loss are predicted within the energy range of 0-50 eV. Several ABSe₃ materials, particularly LiGeSe₃ and NaGeSe₃, demonstrated optical properties comparable to both traditional and emerging materials, suggesting their potential for effective use in solar cells.