Revealing the origin of promising photovoltaic performance in ternary chalcogenide NaBiS2 from first principles

Abstract

Ternary chalcogenide NaBiS2 has emerged as a promising candidate for light absorbers of solar cells, but its optoelectronic properties are not yet fully understood. Here, we report a study of quasiparticle (QP) electronic structure and optical excitons in NaBiS2 using the GW plus Bethe–Salpeter equation (BSE) approach within the ab initio many-body perturbation theory. Through calculations of phonon dispersion with self-consistent phonon theory and elastic constants, we reveal that cubic NaBiS2 is stable dynamically and mechanically at elevated temperatures. Our calculations of QP band structure using the G0W0 + BSE approach demonstrate its direct bandgap of 1.41 eV in agreement with the experiment, allowing dipole transition and inhibiting formation of trapped excitons with their small binding energy of 13 meV. Furthermore, we find strong optical absorption with a steep absorption onset and large absorption coefficients exceeding 1 μm−1, which enable high theoretical spectroscopic limited maximum efficiency over 26%. Finally, mobilities of charge carriers are found to become very high, 19.18 and 55.87 cm2 V−1 s−1 for electrons and holes at a carrier concentration of 1016 cm−3, ensuring high photovoltaic performance. Our work highlights atomistic insights into its superior optoelectronic properties and thus contributes to the design of advanced light absorbers.