Geometry Optimization Strategies for Dual Enhancement of Power and Efficiency in All‐Mg ₃ Sb 1.5 Bi 0.5 Thermoelectric Devices

Mg 3 Sb 2 –Mg 3 Bi 2 solid‐solutions exhibit excellent thermoelectric (TE) performance in power generation and cooling. However, reports on fully Mg 3 (Sb,Bi) 2 ‐based TE devices remain limited, due to performance imbalance between p‐type and n‐type counterparts. In such cases, geometric design optimization is essential for achieving either a maximum power density (ω max ) or maximum conversion efficiency (η max ). Simultaneous improvement is often difficult due to compromises in the device fill factor. In this study, simultaneous enhancement of ω max and η max is demonstrated in a fully Mg 3 Sb 1.5 Bi 0.5 device through optimized design. Two‐pair devices are fabricated using p‐ and n‐type doped Mg 3 Sb 1.5 Bi 0.5 materials with Cu/Fe multilayer electrical contacts.The optimum cross‐sectional area ratio ( A n / A p ) is calculated using the constant properties model. Accordingly, a revised device architecture is designed to accommodate the optimum A n / A p = 1:2 without compromising the fill factor. Finite element simulations predict a combined improvement in ω max and η max compared to a non‐optimized 1:1 design. Simulations are experimentally validated with A n / A p = 1:1 and 1:2 devices, showing a“double‐high” improvement more than 10 % in ω max and η max at Δ T ≈465 K. The proposed optimization strategy provides a general approach to enhance both power output and conversion efficiency in various p‐n thermoelectric material combinations.