Environmentally Friendly Thermoelectrics for Sustainable Energy Generation

Thermoelectric devices provide a simple and practical route for converting low-grade waste heat into electrical energy. Given that more than 60\% of industrial energy is lost as heat, thermoelectric generators offer a promising solution for energy recovery. They operate without moving parts, are noiseless, and require minimal maintenance. However, their widespread application is limited by the reliance on rare and expensive materials. In this context, environmentally friendly chalcogenide-based compounds with zinc-blende-derived structures, such as Cu2+yZn1-ySnSxSe4-x (CZTSSe) and CuFeS2 (CFS), are gaining attention due to their earth-abundant composition, low toxicity, and structural versatility, enabling defect engineering and control of chemical disorder. This doctoral work investigates the synthesis of bulk and thin-film chalcogenide thermoelectric materials, their structural and transport properties, and device fabrication based on CZTSSe and CFS systems. In CZTSSe, high-energy ball milling stabilizes a metastable cubic phase at room temperature. Its origin is clarified through stacking-fault X-ray diffraction modelling and ab initio simulations. Upon sintering, the material transforms into an ordered tetragonal kesterite phase, with reversible disorder associated with Cu/Zn randomization. Partial Cu substitution for Zn improves electronic transport, while chalcogen substitution enhances chemical disorder, leading to an unconventional thermal conductivity trend where Se-rich compositions exhibit higher thermal conductivity due to microstructure-driven grain-boundary scattering. For thin-film devices, CFS-based systems achieve record power density among sulfur-based thermoelectrics through a three-step process combining ball milling, thermal evaporation, and sulfurization. The coexistence of Cu2S and Fe2S phases enhance electronic transport by bridging CuFeS2 grains. Overall, this work elucidates how chemical and microstructural disorder can be tuned in zinc-blende-derived chalcogenides, develops X-ray diffraction methodologies for their characterization, and enhances the efficiency of sustainable thermoelectric energy conversion in both bulk and thin-film systems.