Interplay between chemical bonding, band structure and charge transport in thermoelectric chalcogenides
Thermoelectric power generators could aid reducing the carbon footprint of mankind by converting waste heat into electricity. Extensive research is being conducted to identify materials with suitable thermoelectric figures of merit. Yet, the values required have been elusive so far. The best thermoelectric performances to-date have been found in crystalline group IV chalcogenides, whose outstanding properties have been attributed to complex band structures and intrinsically low lattice thermal conductivities. The present dissertation aims at establishing a novel approach to identify chalcogenides suitable for thermoelectric applications. The work starts by inferring a correlation between metavalent bonding, octahedral coordination and high thermoelectric performance in crystalline p3 chalcogenides. A link between the three properties is proven by investigating charge transport in crystalline GeSexTe1-x and GexSn1-xTe alloys. The strong anisotropy of the effective mass tensor of the relevant charge carriers is found to be the underlying mechanism. Other aspects of charge transport are also studied. Afterwards, a tight-binding model is defined for the band structure of crystalline group IV chalcogenides with metavalent bonding and octahedral coordination, which relates band gap and effective mass tensor to intuitive chemical coordinates. The model can explain the experimental findings and enables the definition of simple design rules for thermoelectric chalcogenides. Additionally, it is combined with the k⋅p method to determine tight-binding parameters (bond energies) and estimate the band structures of the samples under study from the experimental data. The work provides a better understanding of thermoelectric chalcogenides and is expected to accelerate the discovery of new materials for thermoelectric applications.
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