Abstract
Multi-band valley engineering offers an effective route to achieving high thermoelectric performance; however, the associated increase in the density-of-states effective mass (m(*)) inevitably compromises carrier mobility (µ), posing a fundamental challenge for further enhancement of the figure of merit (ZT). Here, Ga doping is employed to tailor the electronic band structure in Ge(0.94)Bi(0.06)Te, inducing the simultaneous convergence of three valence band edges and the emergence of a midgap band, which markedly enhances m(*) and the Seebeck coefficient. Reduced electron localization arising from Ga-Te bonding, together with interfaces featuring small lattice mismatch, effectively mitigates carrier scattering and preserves a high µ. As a result, an optimized balance between m(*) and µ yields an outstanding weighted mobility and power factor. Furthermore, Ga-induced lattice vibration disorder, in synergy with engineered multi-scale crystal defects, strongly suppresses the lattice thermal conductivity. Consequently, a high ZT exceeding 2.1 is achieved at 653 K. A single-stage lead-free device based on the optimized material delivers a competitive power conversion efficiency of 7.7% under a temperature difference of 440 K. This study provides new insights into the rational design of high-performance lead-free thermoelectric materials and devices.