Synergistic Cu Doping and Yb Alloying Enhance Thermoelectric Performance of p-Type Mg _1.8 Zn _1.2 Sb ₂ -Based Material toward High-Efficiency All-Mg ₃ Sb ₂ Devices

<i>AB</i>₂Sb₂-type Zintl phases, particularly Mg₃Sb₂-based materials, have recently garnered significant attention owing to their earth-abundant, nontoxic constituents and excellent <i>n</i>-type thermoelectric (TE) performance in the medium temperature range. However, achieving high-performance all Mg₃Sb₂-based TE devices remains difficult due to the lack of a compatible and efficient <i>p</i>-type counterpart. Herein, a combined approach of Cu doping and Yb alloying is employed to synergistically optimize the carrier concentration, carrier mobility, and reduce lattice thermal conductivity, thereby achieving a high-performance <i>p</i>-type Mg₃Sb₂ TE material. Consequently, a high <i>zT</i> value of 0.95 is obtained for the optimized composition Mg_1.18Cu_0.02Zn_1.2Yb_0.6Sb₂ at 673 K. To demonstrate practical applicability, a fully compatible Mg₃Sb₂-based TE device is fabricated using cupronickel as a common diffusion barrier for both <i>p-</i> and <i>n</i>-type legs, ensuring excellent interfacial stability and device reliability. A crack-free interface with a specific contact resistivity of ρ_c ∼ 2.76 μΩcm² is achieved. The single-leg p-type device exhibited a maximum efficiency, η_max ∼ 5.5%, while for a two-pair TE device, η_max ∼ 9.2% at a temperature difference (Δ<i>T</i>) of 374 K is realized. These findings demonstrate the potential of compositional engineering for developing efficient <i>p</i>-type Mg₃Sb₂ materials and fully integrated TE devices for waste heat recovery.