Balancing Effective Mass and Carrier Mobility via Ga-Induced Multi-Band Valley Engineering in GeTe Thermoelectrics

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_0.94Bi_0.06Te, 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.