Inorganic-organic hybrid thermoelectric materials for energy harvesting applications

Abstract

Organic-inorganic hybrid thermoelectric composites have emerged as promising candidates for flexible thermoelectric devices due to their low cost, solution processability, and ease of large-scale fabrication. Nevertheless, hybrid composite materials have been scarcely used primarily due to the unavailability of robust, high-performance materials. Therefore, it is essential to develop new materials with attributes such as elevated carrier mobility, electrical conductivity, Seebeck coefficients, and low thermal conductivity.
This dissertation primarily describes to develop organic-inorganic hybrid thermoelectric composites, predominantly composed of tellurium nanowires (TeNWs) and the conducting polymer poly(3-hexylthiophene-2,5-diyl) (P3HT), thereby improving thermoelectric (TE) performance by manipulating charge transport at the interfaces between nanowires and P3HT. Various strategies such as oxidation control, doping, surface modification, extending the dimensions of nanowires, and utilizing high molecular weight polymers are presented in this dissertation to enhancing TE performance of these hybrid composites. Aqueous solution chemical (ASS) method was developed to synthesize TeNWs. TeNWs and tellurium oxide (TeO2) nanowires were combined with P3HT to fabricate two hybrid composite systems, and dispersions were drop-casted on quartz substrates. Hybrid films were doped in FeCl3-acetonitrile solution to investigate the doping level. A significant improvement was observed in the power factor (65 μW/m·K²) of the oxidation-controlled P3HT-TeNWs hybrid composites compared to TeO2NW-P3HT (PF ~ 15 μW/mK²) at room temperature with an optimum doping of 0.06M and 0.03M, respectively (Chapter 3). We then modified the surface of TeNWs with sulfide linkers and encapsulated the synthesized S2--TeNWs in P3HT to fabricate hybrid composite materials. Intriguingly, the S2--TeNWs-P3HT hybrid composites demonstrated improved TE performance, with an electrical conductivity (σ) of 35 S/cm, a Seebeck coefficient (S) of 150 μV/K, resulting a PF of about 78 μW/m-K² at room temperature (Chapter 4). Finally, we scaled up ASS process to synthesize long TeNWs (~ 13 μm) and embedded them into P3HT with varying molecular weights (50-70 kDa and 80-143 kDa) to create hybrid composite systems (Chapter 5). The hybrid composite Te-P3HT (80-143 kDa) exhibited a significant improvement in PF of 303 ± 38 μW/mK², with σ of 91 S/cm, S of 183 μV/K, and a thermal conductivity (к) of 0.25 W/m-K, leading to a ZT value of 0.36 ± 0.06 with an optimum doping of 0.02M FeCl3. Theoretical modelling has confirmed the strong templating of P3HT on the TeNWs surface. This templating enhances the charge carrier concentration, leading to increased σ. Whilst the charge transport induces de-doping at interface, resulting in high S. Both σ and S collectively contributes to improve power factors of composite hybrid materials.

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Identifiers

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

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