Rapid Synthetic Routes and Descriptor-Based Design of Thermoelectric Chalcogenides

Abstract

The development of high performing technologies is crucial in order to address the global demand for energy. Thermoelectric devices are promising candidates, since heat can be directly converted into electricity without secondary, hazardous by-products. Two of the main issues concerning thermoelectric materials are the low performance of the current state-of-the-art materials and the scarcity and high cost of its components. Further, the development of these materials must be accelerated so that they can be competitive in both traditional waste heat recovery as well as upcoming miniaturized applications. One of the main driving forces towards innovation in thermoelectrics is the discovery of new materials with novel physical/chemical phenomena that result in enhanced performance. However, investigating new materials is only practical in combination with rapid material synthesis and characterization, as well as rapid performance evaluation. Amongst the possible material synthesis, chemical methods have the advantage of being fast, scalable and cost effective. Electrodeposition has been widely adopted to obtain traditional thermoelectric materials, such as bismuth telluride (Bi2Te3). However, thermoelectric measurements are challenging, due to the metallic seed layer underneath the semiconducting thin film. This issue is discussed in Chapter 3. A parallel resistor model is used to separate the thermoelectric contributions of seed and film. The approach is experimentally validated electrodepositing Bi2Te3 on indium tin oxide, measuring their temperature dependent thermoelectric properties and conducting a literature benchmark. Results indicate that this method shows potential to reliably evaluate the thermoelectric properties of electrodeposited thin films in a direct fashion. The scarcity and toxicity of tellurium have driven researchers towards using earth abundant, nontoxic thermoelectric materials, such as sulphides. In Chapter 4, a top-down approach for the synthesis of bismuth sulphide (Bi2S3) is introduced. Leveraging upon the electron beam sensitivity of bismuth(III) ethylxanthate, the nanoscale patterning of pristine Bi2S3 with sub-10 nm features is realized. Moreover, micron-sized pristine Bi2S3 thin films are fabricated, their carrier concentration increased using a self-doping methodology and the thermoelectric performance evaluated. At room temperature, an electrical conductivity of 6.00 ± 0.60 S m−1 and a Seebeck coefficient of −21.41 ± 0.21 µV K−1 are measured for a directly written, substoichiometric Bi2S3 thin film. This Chapter demonstrates the first n-type, directly writable thermoelectric material, that is potentially compatible with micro thermoelectric generators for on-chip applications. Considering the bigger picture, however, there is a larger performance gain for new materials rather than for traditional materials fabricated using rapid synthesis. One way to increase the efficacy of successfully choosing a candidate material is through its evaluation using transport descriptors. However, state-of-the-art descriptors for the evaluation of the potential of a material as a good thermoelectric are derived assuming that charge scattering events are dominated by acoustic phonons. In Chapter 5, a data-driven screening is combined with ab initio electronic transport calculations of the best performing candidates, including their scattering rates. From the transport data, two ground-state electronic transport descriptors are derived for the screening of materials with high mobility and powerfactor, accounting for both polar optical phonon and impurity scattering of free carriers in doped materials. The good agreement with theoretical and experimental data proves that the descriptors are robust, hence validating their use as first-level screening parameters in the search of novel materials for thermoelectric applications.

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ORCID for Iris Nandhakumar: orcid.org/0000-0002-9668-9126

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