Semiconductor-metal transition powers high-efficiency MgAgSb thermoelectrics

Because of the inferior thermoelectric performance of metals, the semiconductor-to-metal transition in thermoelectric materials is always avoided. Here, we demonstrate that α-to-β semiconductor-metal transition in MgAgSb is actually not detrimental but can be strategically exploited to create α/β-MgAgSb junction, enabling 150% enhancement in output power while maintaining high conversion efficiency. This power enhancement lies in the notably reduced internal resistance induced by semiconductor-to-metal transition, which is independent of dimensional changes. Consequently, α/β-MgAgSb can simultaneously achieve high maximum conversion efficiency exceeding 10% (9%) and maximum power density above 1 (0.9) W cm^-2 by simulation (experiment), outperforming most p-type materials under identical conditions. In addition, a two-pair thermoelectric module combining α/β-MgAgSb with n-type Mg₃Sb_0.6Bi_1.4 achieves an unprecedented power density, representing notable advancements over existing Mg₃(Sb,Bi)₂/MgAgSb two-pair system. These findings highlight the immense potential of α/β-MgAgSb for thermoelectric applications and provide insights into the design of high-power thermoelectrics.