Local van der Waals gaps and resonant levels enhance thermoelectric performance of lead-free GeTe

• • In doping at 1 at.% (Ge0.91Sb0.04Bi0.04In0.01Te) increases the Seebeck coefficient from ~102.28 μV K^-1 to ~287.31 μV K^-1 at 323 K, a 181% enhancement, directly boosting the power factor and enabling higher voltage output in mid-temperature waste heat recovery (323–723 K). • • The peak zT of ~1.8 at 723 K and average zT of ~1.0 over 323–723 K represent a ~50% improvement over baseline GeTe (zT ~1.2), translating to higher thermoelectric conversion efficiency in automotive exhaust or industrial process heat harvesting. • • Vickers hardness reaches ~224 HV, a ~93% increase compared to pristine GeTe (~116 HV), significantly improving mechanical robustness for device fabrication and long-term thermal cycling reliability, reducing fracture risk in module assembly. • • The structural evolution toward a pseudo-cubic phase, evidenced by merging of diffraction peaks in the 41–44° range and a shift of the (202) peak to lower angles, indicates lattice expansion that facilitates the formation of local van der Waals gaps, which scatter phonons without severely degrading carrier mobility.