Thermoelectric Performance Of GeTe Research Articles

Versatile Vanadium Doping Induces High Thermoelectric Performance in GeTe via Band Alignment and Structural Modulation

AbstractOwing to the moderate energy offset between light and heavy band edges of the rock‐salt structured GeTe, its figure‐of‐merit (ZT) can be enhanced by the rational manipulation of electronic band structures. In this study, density functional theory calculations are implemented to predict that V is an effective dopant for GeTe to enlarge the bandgap and converge the energy offset, which suppresses the bipolar conduction and increases the effective mass. Experimentally, V‐doped Ge1−xVxTe samples are demonstrated to have an enhanced Seebeck coefficient from ≈163 to ≈191 µV K−1. Extra alloying with Bi in Ge1−x−yVxBiyTe can optimize the carrier concentration to further enhance the Seebeck coefficient up to ≈252 µV K−1, plus an outstanding power factor of ≈43 µW cm−1 K−2. Comprehensive structural characterization results also verify the refinement of grain size by V‐doping, associated with highly dense grain boundaries, stacking faults, nanoprecipitates, and point defects, reinforcing the wide‐frequency phonon scattering and in turn, securing an ultralow thermal conductivity of ≈0.59 W m−1 K−1. As a result, the Ge0.9V0.02Bi0.08Te sample shows a peak ZT of >2.1 at 773 K, with an average plateaued average ZT of >2.0 from 623 and 773 K, which extends better thermoelectric behavior for GeTe over a wider temperature range. This study clarifies the multiple benefits of V‐doping in GeTe‐based derivatives and provides a framework for a new‐type of high‐performance middle‐temperature thermoelectric material.

Effect of excess Ge and Te on thermoelectric performance of GeTe

AbstractGeTe is a medium‐temperature thermoelectric material with excellent performance. The thermoelectric performance of GeTe is affected by the carrier concentration generated by Ge vacancy. Therefore, it is of important to study the effect of excess Ge or Te on the thermoelectric performance of GeTe. In this paper, GexTey materials (x:y = 1:1.08, 1:1.06, 1:1.04, 1:1, 1.05:1, 1.075:1, and 1.1:1) were fabricated by high‐pressure sintering (HPS) and spark plasma sintering (SPS), respectively, to study the effects of different Ge/Te atomic ratios and preparation process on the thermoelectric properties of polycrystalline GeTe. The composition and microstructure were investigated by an X‐ray diffraction method (XRD) and field‐emission scanning electron microscope (FESEM). The thermoelectric performance was tested from 303 to 703 K. The measurement results show that the Seebeck coefficient of GexTey increases and the conductivity decreases with the decreasing in Te content or the increasing in Ge content. Ge1Te1 exhibits the highest power factor because its Seebeck coefficient and conductivity are at an average level. Owing to the presence of pure Ge and the decrease of Ge vacancy, the lattice thermal conductivities of samples with excess Ge are higher than that of Ge1Te1. Ge1Te1 sintered by HPS has the highest ZTmax value, reaching 1.37 at 723 K.

Generation of multi-dimensional defect structures for synergetic engineering of hole and phonon transport: enhanced thermoelectric performance in Sb and Cu co-doped GeTe

The thermoelectric performance of GeTe can be enhanced by Sb/Cu codoping due to the generation of complex defect structures.

Engineering the electronic band structure and thermoelectric performance of GeTe via lattice structure manipulation from first-principles.

GeTe has become a high-performance thermoelectric material with a figure of merit (ZT) over two through alloying and band engineering strategies. Yet, the question on how to effectively engineer the electronic band structure of GeTe toward achieving a better thermoelectric performance still cannot be clearly answered, and its underlying physics has not been well understood. Here, we manipulate the lattice structure of GeTe via modifying the lattice parameters, interaxial angles and reciprocal displacements, and investigate their influence on the electronic band structure and thermoelectric properties using first-principles calculations. The calculation results show that the reciprocal displacement directly manipulates the energy level of the L-band and the Z-band, resulting in an indirect-direct transition of the band gap and a strong Rashba effect. Modifications of lattice parameters and interaxial angles can affect band gaps, band convergence and density of states, which are crucial to determining thermoelectric performance. This work performs a systematic study on how the lattice structure manipulation influences the electronic band structure and thermoelectric properties of GeTe, and can provide a clear route to further enhance its ZT.

Thermoelectric Materials: Compositional Fluctuations Locked by Athermal Transformation Yielding High Thermoelectric Performance in GeTe (Adv. Mater. 1/2021)

In article number 2005612, Pai-Chun Wei, Hsin-Jay Wu, and co-workers leverage the interplay between the spinodal decomposition and athermal phase transition in GeTe-based thermoelectric materials. The exquisite microstructure, featured by strong composition fluctuations and the coexistence of rhombohedral- and cubic-GeTe, yield high zT values of >2.6. This work provides a new thermodynamic route to developing higher-performance thermoelectric materials in general.

The role of electronegativity in the thermoelectric performance of GeTe–I–V–VI2 solid solutions

The influence of the average electronegativity difference between the alloying atom and host atom on alloy scattering potential and thus carrier mobility is studied.

Compositional Fluctuations Locked by Athermal Transformation Yielding High Thermoelectric Performance in GeTe.

Phase transition in thermoelectric (TE) material is a double-edged sword-it is undesired for device operation in applications, but the fluctuations near an electronic instability are favorable. Here, Sb doping is used to elicit a spontaneous composition fluctuation showing uphill diffusion in GeTe that is otherwise suspended by diffusionless athermal cubic-to-rhombohedral phase transition at around 700 K. The interplay between these two phase transitions yields exquisite composition fluctuations and a coexistence of cubic and rhombohedral phases in favor of exceptional figures-of-merit zT. Specifically, alloying GeTe by Sb2 Te3 significantly suppresses the thermal conductivity while retaining eligible carrier concentration over a wide composition range, resulting in high zT values of >2.6. These results not only attest to the efficacy of using phase transition in manipulating the microstructures of GeTe-based materials but also open up a new thermodynamic route to develop higher performance TE materials in general.

Screening of transition (Y, Zr, Hf, V, Nb, Mo, and Ru) and rare-earth (La and Pr) elements as potential effective dopants for thermoelectric GeTe – an experimental and theoretical appraisal

A report on an improvement in the thermoelectric performance of GeTe via intentional creation of more electrically dormant Ge vacancies (in contrast to classical approaches) and modulating/balancing it with optimized Zr-doping.

Realizing High Thermoelectric Performance in GeTe through Optimizing Ge Vacancies and Manipulating Ge Precipitates

Recent vast reinvestigations on GeTe, a promising thermoelectric material at medium temperature, have triggered enormous enthusiasm in the thermoelectric community again. Intrinsically, GeTe, as a p-type semiconductor, possesses plenty of Ge vacancies, resulting in extremely high carrier concentration. Meanwhile, because of the vast off-stoichiometric Ge, both concomitant Ge precipitates and Ge vacancies will influence the electrical and thermal transport properties caused by phonons and carriers scattering. Herein, we report one effective way to optimize the thermoelectric performance of GeTe through manipulating Ge vacancies and Ge precipitates via successive continuous steps. First, the optimization of carrier concentration through Bi counter-doping led to an optimum carrier concentration of ∼2.79 × 1020 cm–3. Second, the Ge phase aggregation was successfully suppressed by heat treatment, which enhanced the electrical properties due to improved carrier mobility. Additionally, the Ge precipitates were uniformly distributed by heat treatment, which provided all-scale phonons scattering and effectively reduced the lattice thermal conductivity. Finally, combined with the significantly increased power factor (PF) and the decreased thermal conductivity, the maximum ZT of ∼1.9 at 723 K was eventually obtained in Ge0.94Bi0.06Te just through optimizing Ge vacancies and manipulating Ge precipitates by means of heat treatment.

Realizing high thermoelectric performance in GeTe through decreasing the phase transition temperature via entropy engineering

Entropy engineering is one of the powerful approaches to suppress phase transitions.

Medium-temperature thermoelectric GeTe: vacancy suppression and band structure engineering leading to high performance

Achieving high electronic mobility and excellent thermoelectric performance in GeTe through Ge vacancy suppression and band structure engineering.