Bismuth telluride‐based materials are already being commercially developed for thermoelectric (TE) cooling devices and power generators. However, the relatively low efficiency, which is characterized by a TE figure of merit, zT, is the main obstacle to more widespread application. Significant advances in the TE performance have been made through boundary engineering via embedding nanoinclusions or nanoscale grains. Herein, an effective approach to greatly enhance the TE performance of p‐type BiSbTe material by incorporating carbon microfibers is reported. A high zT of 1.4 at 375 K and high average zT of 1.25 for temperatures in the range of 300 to 500 K is achieved in the BiSbTe/carbon microfiber (BST/CF) composite materials. Their superior TE performance originates from the low thermal conductivity and the relatively high power factor. A TE unicouple device based on the p‐type BST/CF composite material and the commercially available n‐type bismuth telluride‐based material shows a huge cooling temperature drop in the operating temperature range of 299–375 K, and is greatly superior to the unicouple device made of both commercial p‐type and n‐type bismuth telluride‐based material. The materials demonstrate a high average zT and excellent mechanical properties and are strong candidates for practical applications.

trical conductivity, S is Seebeck coefficient, T is the absolute temperature, and κ is the thermal conductivity (κ = electronic (κ e) + lattice thermal conductivity (κ L)). [2,3] The state-of-the-art bismuth telluride-based thermoelectric (TE) materials have been used for refrigeration applications. [4] However, their ZT is limited to about 1 at room temperature, making such cooling devices less powerful and cost-competitive than other conventional technologies such as mechanical vapor-compression cooling systems. Further improving the ZT would facilitate their widespread application in industrial waste heat harvesting and electronic device cooling. [5] Maximizing ZT requires the enhancement of the power factor (PF = S 2 σ) and the reduction of thermal conductivity. [6,7] Several approaches have recently been implemented to enhance ZT, including improvement of PF by optimizing carrier concentration, [8-10] band convergence, [11,12] resonant levels, [13] energy barrier filtering, [14] and reducing κ L by alloying, [15] all-scale hierarchical architectures [16-18] and nanostructuring. [19-21] In particular, reduction of κ L by nanostructuring or through formation of nanocomposites has been demonstrated to be an effective Based on the Seebeck and Peltier effects, state-of-the-art bismuth telluridebased thermoelectric materials, which are capable of direct and reversible conversion of thermal to electrical energy, have great potential in energy harvesting and solid-state refrigerators. However, their widespread use is limited by their low conversion efficiency, which is determined by the dimensionless figure-of-merit (ZT). Significant enhancement of ZT is a great challenge owing to the common interdependence of electrical and thermal conductivity. Here, it is demonstrated that by incorporating nanoamorphous boron into the p-type Bi 0.5 Sb 1.5 Te 3 , a record high ZT of 1.6 at 375 K is achieved. It is shown that a high density of nanostructures and dislocations due to the incorporation of the boron inclusions, leads to a significant reduction of thermal conductivity and improved charge transport. The findings represent an important step to further promote the development of thermoelectric technology and its widespread application in solid-state refrigeration and power generation from waste heat.