Local van der Waals gaps and resonant levels enhance thermoelectric performance of lead-free GeTe
GeTe-based thermoelectric materials are promising lead-free alternatives to PbTe, but their intrinsically high Ge vacancy concentration (~10^21 cm^-3) leads to excessive carrier density and degraded Seebeck coefficient. This study integrates resonant levels (RLs) via In doping and local van der Waals gaps via Sb/Bi alloying to decouple electron and phonon transport. The optimal composition Ge0.91Sb0.04Bi0.04In0.01Te exhibits a Seebeck coefficient of ~287.31 μV K^-1 at 323 K, more than double that of the In-free sample (~102.28 μV K^-1). The peak figure of merit zT reaches ~1.8 at 723 K, with an average zT of ~1.0 over 323–723 K. Vickers hardness is enhanced to ~224 HV, a ~93% improvement over pristine GeTe (~116 HV). X-ray diffraction reveals a structural evolution toward a pseudo-cubic phase with increasing In content, and the (202) peak shifts to lower angles, indicating lattice expansion. These results demonstrate that synergistic RLs and van der Waals gaps effectively optimize carrier concentration and suppress thermal conductivity, offering a viable route for high-performance, mechanically robust GeTe thermoelectrics.