First-principles study on the electron and phonon transport properties of layered Bi2OX2 (X = S, Se)

The electron and phonon transport properties of layered bismuth oxychalcogenides Bi2OX2 (X = S, Se) are studied by combining density functional theory calculation with the Boltzmann transport theory. It is found that Bi2OS2 and Bi2OSe2 are semiconductors with direct bandgaps of 0.86 eV and 0.63 eV, respectively. A large Seebeck coefficient is found in both p- and n-doped Bi2OX2 (X = S, Se) at 300 K together with their low phonon thermal conductivity (kappa (ph)). Through a detailed analysis of the phonon dispersion relation, relaxation time, and joint density of states, we find that the low frequency modes contribute dominantly to kappa (ph) than the high frequency modes. Owing to the high Seebeck coefficient and the low kappa (ph), the largest figure of merit (ZT) value can reach 0.5 for the Bi2OX2. The results are useful for further tuning the thermoelectric properties of Bi2OX2 (X = S, Se).