Grain Boundary Strengthening Achieves Extraordinary Module Efficiency in GeTe‐Based Thermoelectric Materials

Abstract The most common planar defects in GeTe thermoelectric materials include grain boundaries, phase boundaries, twin boundaries, and stacking faults, all of which have a significant impact on phonon and carrier transport. Here, the simultaneous achievement of high zT and robust mechanical strength in highly alloyed GeTe are reported. By Cd doping and Cu 2 Se‐PbSe co‐alloying, they formed dense nano‐grain boundaries and point defects that significantly strengthen the phonon scattering and approach the amorphous limit of lattice thermal conductivity. Meanwhile, the optimized carrier concentration and valence band convergence lead to a high electronic transport performance. Consequently, a peak zT of 2.1 at 700 K together with an average zT of 1.4 (300–800 K) is achieved in (Ge 0.98 Cd 0.02 Te) 0.88 (Cu 2 Se) 0.02 (PbSe) 0.1 , and the hierarchical structure features results in high Vickers hardness up to ≈210 H V . Benefiting from the synergistic improved zT value and mechanical strength, the fabricated thermoelectric module realizes a high output power density of 0.86 W cm −2 and a conversion efficiency of ≈11% at Δ T = 501 K.