Doping, Alloying, or Compositing? How Copper Introduction Pathways Dictate Thermoelectric Performance in GeTe

The thermoelectric performance of GeTe is critically governed not only by the choice of dopant but also by the pathway through which it is introduced. Herein, we systematically compare three distinct Cu introduction routes in a Ge_0.95Bi_0.05Te matrix: direct Cu doping, BaCu₂Te₂ alloying, and BaCu₂Te₂ compositing. All three approaches effectively reduce the excessively high hole concentration, thereby decreasing electrical conductivity and increasing the Seebeck coefficient. However, they exhibit markedly different regulation mechanisms. Direct Cu doping achieves the most pronounced reduction in carrier concentration and optimizes carrier mobility but yields only a moderate improvement in the dimensionless thermoelectric figure of merit zT (∼1.86) due to limited phonon scattering. In contrast, the compositing strategy allows limited Cu diffusion into the matrix, while Ba induces additional Ge vacancies that partially compensate for the carrier reduction. The secondary phase introduces strong carrier scattering, suppressing carrier mobility, and also significantly enhances the density-of-states effective mass and reduces lattice thermal conductivity. Consequently, the Ge_0.95Bi_0.05Te + 2.0 wt % BaCu₂Te₂ composite attains a peak zT exceeding 2.0 at 623 K. BaCu₂Te₂ alloying exhibits intermediate behavior, with more Cu incorporation and stronger vacancy compensation, also achieving a zT near 2.0. This work demonstrates that the Cu introduction pathway dictates the balance between carrier concentration modulation, mobility preservation, effective mass enhancement, and phonon scattering, providing a paradigm for synergistically integrating doping and secondary-phase engineering in GeTe-based thermoelectrics.