Ab initio study of alkali-metal-based bismuth selenides (A BiSe2; A = K, Na) for photovoltaic and thermoelectric applications

The structural, electronic, mechanical, and thermoelectric properties of alkali-metal-based bismuth selenides (ABiSe2; A = Na, K) are investigated using a combination of ab initio density functional theory and semiclassical Boltzmann transport theory. The computed lattice constants are in close agreement with experimental results obtained from the Crystallography Open Database. The calculation of the phonon dispersion, elastic tensor, and formation energy confirm that both compounds are dynamically, mechanically, and thermodynamically stable. The band gaps of KBiSe2 and NaBiSe2 are indirect, and their room-temperature values computed at the HSE06 level including spin-orbit coupling (SOC) are 1.20 and 1.13 eV, respectively. SOC interactions impact the band gaps of these compounds which in turn influence the optical and thermoelectric properties, respectively. The absorption coefficients and figure-of-merit values obtained from optical and thermoelectric calculations suggest these materials as potential candidates for photovoltaic and thermoelectric applications.