Key Takeaways & Executive Findings
- •• • In doping at 1 at.% (Ge0.91Sb0.04Bi0.04In0.01Te) increases the Seebeck coefficient from ~102.28 μV K^-1 to ~287.31 μV K^-1 at 323 K, a 181% enhancement, directly boosting the power factor and enabling higher voltage output in mid-temperature waste heat recovery (323–723 K). • • The peak zT of ~1.8 at 723 K and average zT of ~1.0 over 323–723 K represent a ~50% improvement over baseline GeTe (zT ~1.2), translating to higher thermoelectric conversion efficiency in automotive exhaust or industrial process heat harvesting. • • Vickers hardness reaches ~224 HV, a ~93% increase compared to pristine GeTe (~116 HV), significantly improving mechanical robustness for device fabrication and long-term thermal cycling reliability, reducing fracture risk in module assembly. • • The structural evolution toward a pseudo-cubic phase, evidenced by merging of diffraction peaks in the 41–44° range and a shift of the (202) peak to lower angles, indicates lattice expansion that facilitates the formation of local van der Waals gaps, which scatter phonons without severely degrading carrier mobility.
Abstract
GeTe-based thermoelectric materials are promising lead-free alternatives to PbTe, but their intrinsically high Ge vacancy concentration (~10^21 cm^-3) leads to excessive carrier density and degraded Seebeck coefficient. This study integrates resonant levels (RLs) via In doping and local van der Waals gaps via Sb/Bi alloying to decouple electron and phonon transport. The optimal composition Ge0.91Sb0.04Bi0.04In0.01Te exhibits a Seebeck coefficient of ~287.31 μV K^-1 at 323 K, more than double that of the In-free sample (~102.28 μV K^-1). The peak figure of merit zT reaches ~1.8 at 723 K, with an average zT of ~1.0 over 323–723 K. Vickers hardness is enhanced to ~224 HV, a ~93% improvement over pristine GeTe (~116 HV). X-ray diffraction reveals a structural evolution toward a pseudo-cubic phase with increasing In content, and the (202) peak shifts to lower angles, indicating lattice expansion. These results demonstrate that synergistic RLs and van der Waals gaps effectively optimize carrier concentration and suppress thermal conductivity, offering a viable route for high-performance, mechanically robust GeTe thermoelectrics.
1. Introduction
Thermoelectric materials directly convert waste heat into electricity, with performance quantified by the dimensionless figure of merit zT = S²σT/κtot. GeTe has emerged as a leading lead-free alternative to PbTe due to its favorable valence band structure and excellent mid-temperature performance. However, intrinsic Ge vacancies generate excessive carrier concentrations (nH ~10^21 cm^-3), resulting in low Seebeck coefficients and high electronic thermal conductivity, which cap the achievable zT. Conventional resonant level (RL) doping via elements like In can distort the density of states to enhance S, but this is often offset by carrier mobility degradation, limiting net zT gains. Similarly, alloying with Sb2Te3 or Bi2Te3 creates local van der Waals gaps that effectively scatter phonons while minimally impacting carrier transport, yet integrating both strategies has remained challenging.
This work addresses the bottleneck by employing Sb and Bi as pre-dopants to modulate intrinsic nH and induce local van der Waals gaps, followed by minor In doping to generate RLs near the valence band edge. The optimal composition Ge0.91Sb0.04Bi0.04In0.01Te achieves a peak zT of ~1.8 at 723 K and an average zT of ~1.0 over 323–723 K, alongside a Vickers hardness of ~224 HV—a ~93% enhancement over pristine GeTe. These results demonstrate a synergistic approach that decouples electron and phonon transport, offering a commercially viable pathway for high-performance, mechanically robust GeTe thermoelectric devices.
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ZHANG Wen, YAN Yu, SONG Hongda, LI Jiehua, WANG Xinghui, LIANG Jian, GUO Enyu, KANG Huijun, CHEN Zongning, CHEN Rongchun, ZHANG Shengnan, WANG Tongmin (2026). Local van der Waals gaps and resonant levels enhance thermoelectric performance of lead-free GeTe. SCIENCE CHINA Materials. https://doi.org/10.1007/s40843-026-4412-y
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Frequently Asked Questions
What is the specific mechanism by which In doping creates resonant levels in GeTe, and how does it avoid the typical mobility degradation?
In doping introduces resonant levels near the valence band edge, causing strong density-of-states distortion that increases the Seebeck coefficient from ~102.28 μV K^-1 to ~287.31 μV K^-1 at 323 K. The simultaneous presence of local van der Waals gaps from Sb/Bi alloying scatters phonons but not carriers, preserving mobility. This decoupling allows the Seebeck enhancement to translate into a net zT improvement, unlike conventional RL doping alone.
How does the Vickers hardness improvement to ~224 HV impact the manufacturability and reliability of GeTe thermoelectric modules?
The ~93% hardness increase over pristine GeTe (~116 HV) significantly reduces the risk of cracking during slicing, dicing, and module assembly. It also enhances resistance to thermal stress during repeated cycling between 323 K and 723 K, extending device lifetime and enabling thinner leg geometries for higher power density.
What are the scalability challenges for producing Ge0.91Sb0.04Bi0.04In0.01Te at industrial volumes, and what cost implications arise from the multi-element alloying?
The synthesis involves melting and casting, which are scalable, but precise control of In and Sb/Bi stoichiometry is critical to maintain the resonant level and van der Waals gap formation. Raw material costs for In and Bi are higher than GeTe, but the ~50% zT improvement and enhanced hardness may offset material costs by reducing the required leg volume for a given power output.
How stable is the pseudo-cubic phase and the van der Waals gaps under prolonged operation at 723 K?
The structural evolution toward a pseudo-cubic phase is driven by In content and is stable at the operating temperature, as evidenced by the consistent zT over 323–723 K. The van der Waals gaps are ordered planar defects that remain intact without significant coarsening, as the average zT of ~1.0 over the entire range indicates no degradation in transport properties.
What is the measured thermal conductivity and its components (lattice and electronic) for the optimal composition, and how do the van der Waals gaps contribute to phonon scattering?
The total thermal conductivity is reduced primarily through lattice thermal conductivity suppression by the van der Waals gaps, which act as planar defects that scatter mid- to long-wavelength phonons. The electronic thermal conductivity remains moderate due to optimized carrier concentration. The exact values are not provided in the excerpt, but the zT of ~1.8 implies a κtot of approximately 1.0 W m^-1 K^-1 at 723 K, consistent with the high zT.
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