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Defects, scattering, and mobility in complex thermoelectric materials
Porter, Claire E.
Porter, Claire E.
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2024
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Abstract
Growing numbers of newly discovered materials hold the possibility of high thermoelectric performance.
However, the requirement of the material to conduct electricity while maintaining a high Seebeck coefficient can be limited by native defects or poor electronic mobility which are difficult to predict \textit{a priori}.
The research goal of this dissertation is to investigate how the thermoelectric community can understand defects and scattering in complex thermoelectric materials.
Using recently discovered compound Hg$_2$GeTe$_4$ as a case study, we demonstrate manipulation of native and extrinsic defects to optimize thermoelectric performance. Finally, we explore the impact of carrier scattering on mobility in classic material SnTe using a custom-designed Nernst effect instrument we built.
To understand the impact of defects in complex thermoelectric materials, we adopt a joint computational-experimental approach focusing on Hg$_2$GeTe$_4$ as a case study material. We perform phase boundary mapping, defect calculations, and synthesis of native \& extrinsic doped samples to identify the highest performance in Hg$_2$GeTe$_4$. We succeed in manipulating the carrier concentration of Hg$_2$GeTe$_4$ by half an order of magnitude \textit{via} manipulating concentrations of native defects. We follow our native doping study with an extrinsic doping study which examines the impact of 15 different dopants on the thermoelectric performance in Hg$_2$GeTe$_4$.
Ultimately, we achieve the highest figure of merit to date in Hg$_2$GeTe$_4$ by doping with silver.
Our curiosity about the dominant source of scattering in Hg$_2$GeTe$_4$ leads us to construct an instrument to measure the Nernst effect, which can shed light on the dominant scattering mechanism in a material.
The Nernst coefficient, when combined with three other measured experimental parameters (Hall coefficient, electrical resistivity, and Seebeck coefficient) can be used to solve for material parameters that elucidate scattering mechanisms.
Nernst measurements are scant across the thermoelectric literature in comparison with Hall and Seebeck instruments. We describe our design and build a room temperature apparatus that automatically measures and calculates the Nernst coefficient in a material.
Finally, we apply our Nernst instrument to
explore carrier scattering in SnTe under various doping regimes. Undoped SnTe possesses high mobility, and by synthesizing samples doped with indium, iodine, or natively doped, we explore how mobility evolves in response to two orders of magnitude change in carrier concentration.
We demonstrate that the traditional single parabolic band model is insufficient to describe the scattering in SnTe, but applying the more appropriate two band Kane/parabolic model is a complex optimization problem. We provide an interpretation of the raw Nernst data, along with standard thermoelectric measurements to offer insight into the chemistry dependence of scattering in SnTe.
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