High‐performance thermoelectric (TE) devices require not only a high figure of merit (ZT) but also mechanical strength and thermal stability. Here, a simultaneous enhancement of ZT as well as mechanical properties is obtained in GeTe‐based alloys by adding boron. This material is then assembled with n‐type CoSb3 skutterudite into TE modules. The improved ZT values result from the increase in charge carrier mobility due to the reduced interfacial barrier height. A peak ZT of 2.2 at 773 K can be achieved in Ge0.84Pb0.1Sb0.06TeB0.07, which shows a negligible change in the coefficient of thermal expansion upon the phase transition from the rhombohedral to the cubic phase, ensuring good thermal stability of the device. Resulting from the boron‐induced grain refinement and dispersion strengthening, the average compressive strength and Vickers hardness of Ge0.9Sb0.1TeB0.07 can be enhanced to ≈227 MPa and ≈202 Hv, respectively. The improved mechanical properties facilitate the assembly of devices and lower the interfacial contact resistance. As a synergy of increased ZT and mechanical strength, a high output power density of ≈1.76 W cm−2 at ΔT = 425 K and an energy conversion efficiency of 7.4% at ΔT = 477 K can be achieved in the TE modules.

To achieve high-performance thermoelectric (TE) devices, constructing a good interfacial connection between TE materials and electrodes is as important as having high figure-of-merit TE materials. Although CoSb 3 -based TE devices have received great attention for power generation recently, the limited long-term service stability is the main obstruct for their applications. In this work, we have prepared two kinds of Ti-based alloys (Ti 83.7 Al 10.7 Si 5.6 and Ti 74 Ni 26 ) as the diffusion barrier layer of CoSb 3 -based TE joints by the spark plasma sintering method and have systematically investigated their interfacial behaviors during the aging process. The performances of contact resistivity and mechanical strength for Ti 74 Ni 26 /Yb 0.4 Co 3.8 Fe 0.2 Sb 12 TE joints are good before aging treatment but gradually deteriorate during the aging process, which should be ascribed to the phase-transition-induced negative thermal expansion in Ti−Ni alloys. On the other hand, Ti 83.7 Al 10.7 Si 5.6 /Yb 0.4 Co 3.8 Fe 0.2 Sb 12 TE joints show both low contact resistivity (<10 μΩ•cm 2 ) and high mechanical strength (>20 MPa) before and after 16-day aging at 500 °C, which is originated from the matching of the coefficient of thermal expansion (CTE) and the formation of network structures in Ti−Al−Si alloys. We have also prepared an eight-couple TE module of p-Ge 0.9 Sb 0.1 TeB 0.01 /n-Yb 0.4 Co 3.8 Fe 0.2 Sb 12 and have measured its corresponding device performance. Our work has demonstrated that the matched CTE and network structures in the Ti−Al−Si alloy are key to obtain high-performance CoSb 3 -based TE joints for long-term service.