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

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.