Abstract
We investigated the anisotropic thermoelectric properties of the Bi2Te2.85Se0.15Ix (x = 0.0, 0.1, 0.3, 0.5 mol.%) compounds, synthesized by ball-milling and hot-press sintering. The electrical conductivities of the Bi2Te2.85Se0.15Ix were significantly improved by the increase of carrier concentration. The dominant electronic scattering mechanism was changed from the mixed (T ≤ 400 K) and ionization scattering (T ≥ 420 K) for pristine compound (x = 0.0) to the acoustic phonon scattering by the iodine doping. The Hall mobility was also enhanced with the increasing carrier concentration. The enhancement of Hall mobility was caused by the increase of the mean free path of the carrier from 10.8 to 17.7 nm by iodine doping, which was attributed to the reduction of point defects without the meaningful change of bandgap energy. From the electron diffraction patterns, a lattice distortion was observed in the iodine doped compounds. The modulation vector due to lattice distortion increased with increasing iodine concentration, indicating the shorter range lattice distortion in real space for the higher iodine concentration. The bipolar thermal conductivity was suppressed, and the effective masses were increased by iodine doping. It suggests that the iodine doping minimizes the ionization scattering giving rise to the suppression of the bipolar diffusion effect, due to the prohibition of the BiTe1 antisite defect, and induces the lattice distortion which decreases lattice thermal conductivity, resulting in the enhancement of thermoelectric performance.
Highlights
A thermoelectric device can directly convert heat into electric energy and transfer heat by electric bias with no moving parts, no noise, and no greenhouse gas emissions. [1]Recently, because the application fields are expanding to the flexible or wearable thermoelectric devices [2], the demand for high-performance thermoelectric materials near room temperature is increasing
Thermoelectric performance mainly depends on the thermoelectric figure of merit which is defined by zT = S2 σT/κ, where S, σ, T and κ are the Seebeck coefficient, electrical conductivity, absolute temperature, and total thermal conductivity, respectively [1,2]
We propose that the lattice distortion by charge density wave (CDW) suppresses the bipolar diffusion effect and results in the enhancement of thermoelectric performance at high temperature regions
Summary
A thermoelectric device can directly convert heat into electric energy and transfer heat by electric bias with no moving parts, no noise, and no greenhouse gas emissions. [1]Recently, because the application fields are expanding to the flexible or wearable thermoelectric devices [2], the demand for high-performance thermoelectric materials near room temperature is increasing. The n-type bismuth tellurides show relatively high zT values such as hot-deformed Bi2 Te2.3 Se0.7 [14] To reach high thermoelectric performance in n-type bismuth tellurides, there have been many studies such as the control of Se concentration Bi2Te3−xSex [4,11,15,16], Cu-doping [8,17], I-doping [9,17] Ga-doping [18], CuI-doping [10,19,20], hot-press and hot-deformation processes [8,13,21,22,23,24], etc. From the electron diffraction experiment, the iodine doping in Bi2 Te2.85 Se0.15 enhances lattice distortion by the formation of the charge density wave
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