m ), [52] enlarging band degeneracy (N V ), [43] and introducing resonant energy levels, [26] can also tune n p effectively. Taking Ge 1−x−y Sb x Zn y Te as an example, Zn doping can reduce the energy offset (ΔE) between L and Σ point. [52] The minimization of thermal transport properties is mainly achieved through suppressing the electrical property-independent κ l . [53][54][55][56][57] Thermoelectric materials with High-performance GeTe-based thermoelectrics have been recently attracting growing research interest. Here, an overview is presented of the structural and electronic band characteristics of GeTe. Intrinsically, compared to lowtemperature rhombohedral GeTe, the high-symmetry and high-temperature cubic GeTe has a low energy offset between L and Σ points of the valence band, the reduced direct bandgap and phonon group velocity, and as a result, high thermoelectric performance. Moreover, their thermoelectric performance can be effectively enhanced through either carrier concentration optimization, band structure engineering (bandgap reduction, band degeneracy, and resonant state engineering), or restrained lattice thermal conductivity (phonon velocity reduction or phonon scattering). Consequently, the dimensionless figure of merit, ZT values, of GeTe-based thermoelectric materials can be higher than 2. The mechanical and thermal stabilities of GeTe-based thermoelectrics are highlighted, and it is found that they are suitable for practical thermoelectric applications except for their high cost. Finally, it is recognized that the performance of GeTe-based materials can be further enhanced through synergistic effects. Additionally, proper material selection and module design can further boost the energy conversion efficiency of GeTe-based thermoelectrics.
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 CuTe and BiTe 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.
Flexible thermoelectric materials and devices show great potential to solve the energy crisis but still face great challenges of high cost, complex fabrication, and tedious postprocessing. Searching for abnormal thermoelectric materials with rapid and scale-up production can significantly accelerate their applications. Here, we develop superlarge 25 × 20 cm2 commercial graphite-produced composite films in batches, achieved by a standard 10 min industrial process. The high cost effectiveness (S 2σ/cost) of 7250 μW g m–1 K–2 $–1 is absolutely ahead of that of the existing thermoelectric materials. The optimized composite film shows a high power factor of 94 μW m–1 K–2 at 150 °C, representing the optimal value of normal carbon materials so far. Furthermore, we design two types of flexible thermoelectric devices fabricated based on such a novel composite, which achieve an output open-circuit voltage of 3.70 mV using the human wrist as the heat source and 1.33 mV soaking in river water as the cold source. Our study provides distinguished inspiration to enrich flexible and cost-effective thermoelectric materials with industrial production.
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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