Dimensionless figure-of-merit (zT) is the most typical descriptor to evaluate thermoelectric materials, which is defined as zT = S 2 σT/κ, where S, σ, T and κ are the Seebeck coefficient, the electrical conductivity, the absolute temperature, and the thermal conductivity, respectively. [4,5] κ includes the lattice thermal conductivity (κ l ) and the electrical thermal conductivity (κ e ). [6,7] To achieve high energy conversion efficiency, high zT values with high power factor (S 2 σ) and low κ are required. According to extensive investigations, S 2 σ can be increased by quantum confinement, [8,9] modulation doping, [10] band convergence, [5,11] and resonant state engineering. [12,13] Whereas, κ (mainly κ l ) is mainly reduced by introducing a variety of lattice defects as phonon scattering centers such as point defects, [14,15] dislocations, [16,17] stacking faults, [18,19] nano precipitates, [20,21] and hierarchical architecture structures. [22,23] GeTe is a promising mid-temperature material with a phase transition from the rhombohedral phase (R-GeTe) to the cubic phase (C-GeTe) at ≈700 K. However, the intrinsic high carrier concentration (n h for p-type GeTe, ≈10 21 cm −3 due to massive Ge vacancies) limits its S 2 σ and thus zT (≈0.75 at ≈740 K). [24,25] To According to theMott's relation, the figure-of-merit of a thermoelectric material depends on the charge carrier concentration and carrier mobility. This explains the observation that low thermoelectric properties of GeTe-based materials suffer from the degraded carrier mobility, on account of the fluctuation of electronegativity and ionicity of various elements. Here, highperformance CuBiSe 2 alloyed GeTe with high carrier mobility due to the small electronegativity difference between Cu and Ge atoms and the weak ionicity of CuTe and BiTe bonds, is developed. Density functional theory calculations indicate that CuBiSe 2 alloying increases the formation energy of Ge vacancies and correspondingly reduces the amount of Ge vacancies, leading to an optimized carrier concentration and a high power factor of ≈37.4 µW cm −1 K −2 at 723 K. Moreover, CuBiSe 2 alloying induces dense point defects and triggers ubiquitous lattice distortions, leading to a reduced lattice thermal conductivity of 0.39 W m −1 K −1 at 723 K. These synergistic effects result in an optimization of the carrier mobility, the carrier concentration, and the lattice thermal conductivity, which favors an enhanced peak figure-of-merit of ≈2.2 at 723 K in (GeTe) 0.94 (CuBiSe 2 ) 0.06 . This study provides guidance for the screening of GeTe-based thermoelectric materials with high carrier mobility.

Compatible p-and n-type materials are necessary for high-performance GeTe thermoelectric modules, where the n-type counterparts are in urgent need. Here, it is reported that the p-type GeTe can be tuned into n-type by decreasing the formation energy of Te vacancies via AgBiTe 2 alloying. AgBiTe 2 alloying induces Ag 2 Te precipitates and tunes the carrier concentration close to the optimal level, leading to a high-power factor of 6.2 µW cm −1 K −2 at 423 K. Particularly, the observed hierarchical architectural structures, including phase boundaries, nano-precipitates, and point defects, contribute an ultralow lattice thermal conductivity of 0.39 W m −1 K −1 at 423 K. Correspondingly, an increased ZT of 0.5 at 423 K is observed in n-type (GeTe) 0.45 (AgBiTe 2 ) 0.55 . Furthermore, a single-leg module demonstrates a maximum η of 6.6% at the temperature range from 300 to 500 K. This study indicates that AgBiTe 2 alloying can successfully turn GeTe into n-type with simultaneously optimized thermoelectric performance.

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Dense point defects can strengthen phonon scattering to reduce the lattice thermal conductivity and induce outstanding thermoelectric performance in GeTe‐based materials. However, extra point defects inevitably enlarge carrier scattering and deteriorate carrier mobility. Herein, it is found that the interstitial Cu in GeTe can result in synergistic effects, which include: 1) strengthened phonon scattering, leading to ultralow lattice thermal conductivity of 0.48 W m−1 K−1 at 623 K; 2) weakened carrier scattering, contributing to high carrier mobility of 80 cm2 V−1 s−1 at 300 K; 3) optimized carrier concentration of 1.22 × 1020 cm−3. Correspondingly, a high figure‐of‐merit of ≈2.3 at 623 K can be obtained in the Ge0.93Ti0.01Bi0.06Te‐0.01Cu, which corresponds to a maximum energy conversion efficiency of ≈10% at a temperature difference of 423 K. This study systematically investigates the doping behavior of the interstitial Cu in GeTe‐based thermoelectric materials for the first time and demonstrates that the localized interstitial Cu is a new strategy to enhance the thermoelectric performance of GeTe‐based thermoelectric materials.

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