CuI-enhanced thermoelectric performance in GeTe by synchronous modulation of hole concentration and thermal conductivity

Germanium telluride (GeTe) is a promising thermoelectric material for low- to medium-temperature applications and has garnered increasing attention owing to its rich structural variants, including ferroelectric domain structures. However, the key step toward thermoelectric performance enhancement is to reduce the excessive hole concentration, which is caused by the intrinsic Ge vacancies. In this work, we demonstrate that the addition of copper iodide (CuI) salt can effectively optimize the hole concentration by either suppressing these vacancies or forming interstitial Cu-Ge vacancy pairs. The weakened carrier scattering from Ge vacancies improves carrier mobility, enabling high power factors in Ge 1- x /2 Te 1- x + x CuI samples with simultaneous band convergence. Furthermore, the Cu-rich planar defects function as phonon-scattering centers, thereby contributing to the low thermal conductivity while maintaining carrier transport due to the continuous bonding. Further Bi-Sb co-doping resulted in a high ZT of 1.9 at 773 K and an average ZT exceeding 1.2 over the 300–773 K range for Ge 0.89 Bi 0.05 Sb 0.04 Te 0.96 + 0.04CuI. Utilizing this optimized composition, a segmented single-leg thermoelectric module was designed and fabricated, delivering an output power of 74 mW and a conversion efficiency of 10.4% under a temperature gradient, Δ T , of 445 K. These findings not only underscore the critical role of CuI in regulating the intrinsic vacancies and subtle microstructures in GeTe to achieve superior thermoelectric performance but also provide a technical route for intrinsic-defect modulation to obtain better functional materials. • Adding CuI in GeTe can regulate Ge vacancies, optimizing carrier concentrations • Cu doping realizes band convergence, leading to a high Seebek coefficient • The Cu-rich planar defects scatter phonons while maintaining carrier transport • A high average ZT over 1.2 and a high conversion efficiency of 10.4% is achieved Adding CuI in GeTe can regulate Ge vacancies, optimizing carrier concentrations Cu doping realizes band convergence, leading to a high Seebek coefficient The Cu-rich planar defects scatter phonons while maintaining carrier transport A high average ZT over 1.2 and a high conversion efficiency of 10.4% is achieved This study demonstrates that copper iodide (CuI) doping synergistically enhances the thermoelectric performance of GeTe by concurrently optimizing its electrical and thermal transport properties. The incorporation of CuI effectively suppresses native Ge vacancies through the formation of V Ge -Cu i complexes, which reduces carrier scattering and enhances carrier mobility, leading to superior electrical performance when combined with intrinsic valence band convergence. Simultaneously, the introduced Cu-rich layer defects and secondary phases provide strong phonon scattering, maintaining low thermal conductivity despite the reduction in vacancy-related scattering. Building on this CuI-mediated defect and microstructure engineering, Bi-Sb co-doping was further employed to achieve a peak ZT of 1.9 and a high average ZT > 1.2. The practical viability was confirmed by a segmented GeTe/(Bi,Sb) 2 Te 3 single-leg device achieving a high output power of 74 mW and a conversion efficiency of 10.4%, validating CuI doping as a highly effective strategy for advancing GeTe-based thermoelectrics, with broader implications for other semiconductor materials.