Highly anisotropic thermoelectric transport properties responsible for enhanced thermoelectric performance in the hot-deformed tetradymite Bi2Te2S

In general, Bi2Te3 and related alloys show the best thermoelectric performance at near-room temperature region below about 423 K, which is in some cases not high enough for waste heat recovery. In order to raise the operation temperature on the basis of Bi2Te3 technology, S element was introduced into Bi2Te3 to synthesize a ternary tetradymite compound, Bi2Te2S, and the thermoelectric properties were analyzed. In this study, Bi2S3 was considered, instead of using elemental S, to avoid the issues arising from the volatility of S. Compared with Bi2Te3, undoped Bi2Te2S shows higher Seebeck coefficients (Smax = −210.9 μV K−1 at 523 K) but also higher resistivity (ρmin = 26.5 μΩ m at 323 K), resulting in decreased power factor (PFmax = 1.49 mW K−2m−1 at 373 K). However, low thermal conductivity (κmin = 0.987 WK−1m−1 at 473 K) produced higher ZT at T ≥ 423 K than Bi2Te3, reaching about 0.66 at 473 K. At the same time, the bipolar conduction was reduced due to the enlarged band gap of about 0.22 eV, which was estimated from the S-T curve. After additional hot deformation at 793 K, undoped Bi2Te2S shows remarkably improved values of power factor (PFmax = 2.62 mW K−2m−1 at 323 K) and ZT (ZTmax = 0.86 at 473 K), which was ascribed to increased carrier concentration. In addition, the highly anisotropic transport properties of Bi2Te2S were suggested to be another origin, which was revealed by the first-principles calculations. In summary, Bi2Te2S is a promising candidate for thermoelectric generation at the intermediate temperature region 473–573 K where Bi2Te3 shows degraded performances.