The InTe has intrinsically low lattice thermal conductivity κL originating from the anharmonic bonding of In1+ ion in the lattice, which scatters the phonons. Here we report the enhancement of thermoelectric properties in Te-deficient InTe1−δ (δ = 0, 0.01, 0.1, and 0.2) polycrystalline compounds by lattice softening and energy band gap opening. Te-deficiency gives rise to more weak chemical bonding between In1+ atoms and In3+Te2− clusters than those of pristine InTe, resulting in the reduction of κL near the room temperature. The weak ionic bonding is confirmed by the increase of lattice volume from the X-ray diffraction and lattice softening by the decrease of Debye temperature with increasing Te-deficiency. We observed the low lattice thermal conductivity κL of 0.53 W m−1 K−1 at 300 K for InTe0.99, which is about 25 % decreased value than those of InTe. The Te-deficiency also induces energy band gap so that the electrical resistivity and Seebeck coefficient are increased due to the decrease of carrier concentration. Temperature-dependent thermoelectric properties shows the high Seebeck coefficient at high temperature and high electrical conductivity near room temperature, resulting in the temperature-insensitive high power factor S2σ over a wide temperature range. Owing to the temperature-insensitive high power factor and intrinsic low lattice thermal conductivity by Te-deficiency, the thermoelectric performances of figure-of-merit ZT and engineering ZTeng are enhanced at mild temperature range (≤550 K).

We investigate the thermoelectric properties of (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 /Mo (Mo: 0.0, 0.9, 1.3, 1.8, 3.1, and 4.3 vol %) composites, which were synthesized by extrinsic phase mixing with hot press sintering. From X-ray diffraction (XRD) and energy-dispersive X-ray spectroscopy (EDX) measurements, we confirm that micro-sized Mo particles are dispersed homogeneously in the (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 matrix without doping. While the electrical resistivity of Mo-dispersed (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 composites is not changed significantly, the Seebeck coefficient is significantly increased. Because the work function (5.3 eV) of the (CuI) 0.003 Bi 2 Te 2.7 Se 0.3 compounds, measured by ultraviolet photoelectron spectroscopy (UPS), is larger than that of Mo particles (4.95 eV), we expect the potential barrier near the interfaces between the BTS matrix and Mo particles. The band bending effect and potential barrier can give rise to the low-energy carrier filtering. For a low concentration dispersion of Mo particles (<2 vol %), a decrease of Hall carrier concentration, an increase of Hall mobility, a decrease of effective mass, and an increase of Seebeck coefficient also support the formation of low-energy carrier filtering. The Mo dispersion does not affect the decrease in the lattice thermal conductivity but enhances the power factor significantly, leading to the high ZT value above 1.0 at room temperature, which is a high level in n-type thermoelectric room-temperature applications.

Bi 2 Te 3 -based compounds have long been studied as thermoelectric materials in cooling applications near room temperature. Here, we investigated the thermoelectric properties of CuI-doped Bi 2 Te 2.1 Se 0.9 compounds. The Cu/I codoping induces the lattice distortion partially in the matrix. We report that the charge density wave caused by the local lattice distortion affects the electrical and thermal transport properties. From the high-temperature specific heat, we found a first-order phase transitions near 490 and 575 K for CuI-doped compounds (CuI) x Bi 2 Te 2.1 Se 0.9 (x = 0.3 and 0.6%), respectively. It is not a structural phase transition, confirming from the high-temperature X-ray diffraction. The temperature-dependent electrical resistivity shows a typical behavior of charge density wave transition, which is consistent with the temperature-dependent Seebeck coefficient and thermal conductivity. The transmission electron microscopy and electron diffraction show a local lattice distortion, driven by the charge density wave transition. The charge density wave formation in the Bi 2 Te 3 -based compounds are exceptional because of the possibility of coexistence of charge density wave and topological surface states. From the Kubo formula and Boltzmann transport calculations, the formation of charge density wave enhances the power factor. The lattice modulation and charge density wave decrease lattice thermal conductivity, resulting in the enhancement of thermoelectric performance simultaneously in CuI-doped samples. Consequently, an enhancement of thermoelectric performance ZT over 1.0 is achieved at 448 K in the (CuI) 0.003 Bi 2 Te 2.1 Se 0.9 sample. The enhancement of ZT at high temperature gives rise to a superior average ZT avg (1.0) value than those of previously reported ones.

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Electronic ZT value with chemical potential for rhombohedral α- (black line) and cubic β-phase (red line) (a) and the temperature-dependent ZT value of GeTe1−xIx compounds with reference data (b).