First-principles study of A-site cation effects in halide double perovskites A2NaMoI6 (A = Cs, Rb) for photovoltaic applications

In this study, the first-principles method was used to analyze the geometric, electronic, elastic, phonon, magnetic, optical, and thermoelectric (TE) characteristics of halide double perovskites (HDPs) A2NaMoI6 (A = Cs, Rb). The stability of cubic A2NaMoI6 (A = Cs, Rb) is evidenced by the calculations of formation enthalpy (Delta H) and tolerance factor (tau). The tau values falling within the stability range and the negative Delta H values confirmed the structural stability of both perovskites. Using the mBJ potential, an analysis of the spin-polarized band structure (BS) and density of states (DOS) was conducted, which indicates that both Rb2NaMoI6 and Cs2NaMoI6 are direct bandgap (Eg) semiconductors. The Eg values for Rb2NaMoI6 are 1.88 eV in the spin-up and 2.71 eV in the spin-down state. For Cs2NaMoI6, the Eg values are 1.83 eV in the spin-up and 2.65 eV in the spin-down state. These compounds exhibit high optical conductivity sigma(omega) in the visible spectrum and demonstrate peak absorption in the UV-visible range. Additionally, the positive Seebeck coefficient values across all temperatures confirm the existence of hole-type charge carriers. From the TE properties, the figure of merit (ZT) shows high values across the studied temperature range, suggesting efficiency. This study highlights the potential of A2NaMoI6 for use in photovoltaic (PV) and TE devices.