Abstract
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 (κph). 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 κph than the high frequency modes. Owing to the high Seebeck coefficient and the low κph, 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).
Highlights
Thermoelectric materials have received much attention in recent years because of their ability to directly convert waste heat into electricity.1,2 The dimensionless figure of merit ZT [ZT = σS2T/(κe + κph)] can be used to characterize the thermoelectric performance, where T, σ, S, κe, and κph are the absolute temperature, electrical conductivity, Seebeck coefficient, electrical thermal conductivity, and phonon thermal conductivity, respectively
Our first-principles calculations were performed using the Vienna abinitio Simulation Package (VASP),23,24 which is based on the density functional theory (DFT)
It can be seen that both Bi2OS2 and Bi2OSe2 are semiconductors with direct bandgaps of 0.86 eV and 0.63 eV, respectively, which are in agreement with other theoretical values
Summary
A good thermoelectric material usually has a high ZT value; increasing the power factor (σS2) or decreasing the thermal conductivity (κe and κph) can improve ZT.. It is difficult to do that simultaneously due to the strong coupling of S, σ, and κe. Searching materials with low phonon thermal conductivity has become one of the most important strategies to improve ZT.. The anharmonicity due to the interlayer coupling of layered materials can lead to strong scattering of phonons resulting in low κph.. Zhao et al. reported that the special layered structure makes SnSe have a low κph of 0.24 ± 0.03 W/m K at 973 K and a high ZT of 2.6 ± 0.3 at 923 K Searching materials with low phonon thermal conductivity has become one of the most important strategies to improve ZT. Generally, the anharmonicity due to the interlayer coupling of layered materials can lead to strong scattering of phonons resulting in low κph. For example, Zhao et al. reported that the special layered structure makes SnSe have a low κph of 0.24 ± 0.03 W/m K at 973 K and a high ZT of 2.6 ± 0.3 at 923 K
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