High thermoelectric performance of n-type PbTe is urgently needed to match its p-type counterpart. Here, we show a peak ZT ∼ 1.5 at 723 K and a record high average ZT > 1.0 at 300-873 K realized in n-type PbTe by synergistically suppressing lattice thermal conductivity and enhancing carrier mobility by introducing CuTe inclusions. Cu performs several outstanding roles: Cu atoms fill the Pb vacancies and improve carrier mobility, contributing to an unexpectedly high power factor of ∼37 μW cm K at 423 K; Cu atoms filling Pb vacancies and Cu interstitials both induce local disorder and, together with nano- and microscale Cu-rich precipitates and their related strain fields, lead to a very low lattice thermal conductivity of ∼0.38 Wm K in PbTe-5.5%CuTe, approaching the theoretical minimum value of ∼0.36 Wm K. This work provides an effective strategy to enhance thermoelectric performance by simultaneously improving electrical and thermal transport properties.

HIGHLIGHTS • A comprehensive review on the recent development of two-dimensional (2D) nanomaterials for bulk or thin-film thermoelectric materials, as well as composite filler, has been extensively presented. • Development of micro-device platform and its application to study the inherent thermoelectric properties of individual single-and few-layer 2D nanomaterials.

We report that KBiSe exhibits multiple conduction bands that lie close in energy and can be activated through doping, leading to a highly enhanced Seebeck coefficient and a high power factor with elevated temperature. Meanwhile, the large unit cell, complex low symmetry crystal structure, and nondirectional bonding lead to the very low lattice thermal conductivity of KBiSe, ranging between 0.42 and 0.20 W m K in the temperature interval 300-873 K. Experimentally, we further support the low thermal conductivity of KBiSe using phonon velocity measurements; the results show a low average phonon velocity (1605 ms), small Young's modulus (37.1 GPa), large Grüneisen parameter (1.71), and low Debye temperature (154 K). A detailed investigation of the microstructure and defects was carried out using electron diffraction and transmission microscopy which reveal the presence of a KBiSe minor phase intergrown along the side of the KBiSe phase. The combination of enhanced power factor and low thermal conductivity results in a high ZT value of ∼1.3 at 873 K in electron doped KBiSe material.

We revisited and investigated a layer-structured thermoelectric material AgCrSe2, which has an extremely low thermal conductivity. After using both differential scanning calorimetry and a comparative laser flash method, we realized that the specific heat of this material, the main contributor to the reported low thermal conductivity, is unlikely to be way below the Dulong-Petit limit as revealed in the literature. Besides, our in situ X-ray diffraction pattern up to 873 K indicated the instability of AgCrSe2 over 723 K, where it begins to decompose into Cr2Se3 and Ag2Se. This unexpected decomposition phenomenon resulted in the gradual increment of specific heat and thermal diffusivity, hence the deterioration of the overall thermoelectric performance. We deliberately introduced Ag and Cr vacancies into the lattice for carrier concentration optimization and could achieve an optimal figure of merit of ZT ∼ 0.5 at 723 K in the nominal composition Ag0.96CrSe2 in the direction perpendicular to the sintering press. Our findings suggest that more thorough investigations are necessary to ensure that AgCrSe2 is a promising thermoelectric material.