Theoretical study of structural, electronic, and optical properties of ternary metal sulfides MBiS2 (M = Ag, Na)

We study the structural, electronic, and optical properties of MBiS2 (M = Ag, Na) using the relativistic all-electron calculations. We find that for both M = Ag and Na, the R 3 ¯ m and Fd 3 ¯ m superstructures are the lowest-energy superstructures. R 3 ¯ m and Fd 3 ¯ m MBiS2 are indirect band-gap semiconductors with the differences between the indirect and direct band gaps, Δ, of about 0.5 and 0.1 eV for M = Ag and Na, respectively. R 3 ¯ m and Fd 3 ¯ m MBiS2 exhibit high absorption coefficients of the order of 105 cm−1. Employing the spectroscopically limited maximum efficiency as a metric for quantifying the photovoltaic performance, it is found that the power conversion efficiency in the thick-film limit for M = Na is 32% because of its small Δ while that for M = Ag is 15% because of its large Δ.