Thermoelectric Studies of Modified Cu2Se: Bulk and Thin Film for Waste Heat Recovery

The rising global emphasis on sustainable energy solutions has driven interest in thermoelectric (TE) materials, which enable the direct conversion of waste heat into electrical power. Among various thermoelectric materials, copper selenide (Cu₂Se) has emerged as a promising p-type material owing to its unique superionic behavior, inherently low thermal conductivity, and decent electrical transport properties. However, its thermoelectric performance still requires further optimization for practical applications, especially under the mid-temperature range and for flexible energy harvesting systems. This thesis presents a comprehensive study on the development, modification, and optimization of Cu₂Se-based thermoelectric materials in both bulk and thin film forms, aiming to enhance their performance for waste heat recovery applications. Bulk Cu₂Se was synthesized using a cost-effective reduction method, ensuring phase purity and high yield. To further enhance its thermoelectric properties, nanocomposites were prepared by incorporating carbon-based additives, graphene, reduced graphene oxide (RGO), and multi-walled carbon nanotubes (MWCNT) in varying concentrations (0.5, 1, 1.5, 2 %). These additives were found to significantly improve electrical conductivity and carrier mobility, leading to a marked enhancement in the power factor and the figure of merit (ZT). In parallel, Cu₂Se thin films were fabricated using radio frequency (RF) magnetron sputtering, where a systematic optimization was carried out by varying sputtering power, substrate temperature, and post-deposition annealing conditions. These parameters played a crucial role in tuning the film’s microstructure, crystallinity, and carrier transport properties, which directly influenced thermoelectric performance. The optimized thin films were further employed in the fabrication of a flexible thermoelectric generator (FTEG) prototype deposited on a polyimide substrate with 9 legs, which demonstrated stable output and mechanical robustness under bending cycles, highlighting their suitability for flexible and wearable energy harvesting devices. Additionally, a custom-built thermal conductivity setup was developed for accurate measurement of thermal transport properties of bulk nanocomposites, enabling precise ZT evaluation. The combined approach of nanostructuring and thin film optimization presented in this research contributes valuable insights toward the design of high-performance, low-cost Cu2Se thermoelectric material for real-world applications.<p></p>