Interface Engineering Boosting High Power Density and Conversion Efficiency in Mg₂Sn_0.75Ge_0.25‐Based Thermoelectric Devices

Abstract Electrode contact interfaces for practical thermoelectric (TE) devices require high bonding strength, low specific contact resistivity, and superb stability. Herein, the state‐of‐the‐art Cu₂MgFe/Mg₂Sn_0.75Ge_0.25 interface is designed for Mg₂Sn_0.75Ge_0.25‐based TE devices, adhering to the general strategy of high bonding propensity, thermal expansion matching, diffusion passivation, and dopant inactivation. The interfacial stability is verified by the in situ transmission electron microscopy analysis, thereby confirming the contributions from decreasing the chemical potential gradient and increasing the diffusion activation energy barrier. The single‐leg device exhibits a high power density (ω_max) of 2.6 W cm⁻² and conversion efficiency (η_max) of 8% under a temperature difference (ΔT) of 370 °C, which is the record‐breaking value in comparison to other Mg₂(Si, Ge, Sn)‐based TE devices. Additionally, a two‐couple device with p‐type Bi₂Te₃ shows an excellent ω_max of 1.3 W cm⁻² and η_max of 5.4% under a ΔT of 270 °C, comparable to commercial Bi₂Te₃ devices. The proposed interface design strategy provides a general technique for constructing high‐performance devices using cutting‐edge TE materials.