Electrochemical Deposition and Characterization of Thermoelectric Thin Films of Bismuth Telluride and Its Derivatives

Abstract: Thermoelectric materials have attracted enormous academic and industrial interest recently due to the fact that they can be used in solid state devices for power generation and cooling applications. It is expected that such thermoelectric applications will eventually contribute to meet the energy challenge in the future if the performance of the current devices can be further promoted through an increase in thermoelectric efficiency of the materials. Structural nano-engineering of conventional thermoelectric materials has lately shown its potential in improving the efficiency. However, nano-engineering especially for top-down techniques for thin film materials often involves complicated preparation processes and expensive equipment. In contrast, electrochemical deposition provides an easy-to-handle and inexpensive bottom-up method for nano-engineering. It requires a complete understanding of the fundamental processes regarding electrochemical interfaces between liquid and solid phases, electron transfer involving reduction and oxidation reactions, and electro-crystallization and deposition mechanism, etc. This thesis concerns thin film fabrication of the thermoelectric materials bismuth telluride (Bi2Te3) and its derivatives using electrochemical deposition. Systematic investigations of the deposition mechanisms were first carried out using a variety of analytical electrochemical techniques including cyclic voltammetry, chronoamperometry and combined electrochemical quartz crystal microbalance. Different mechanisms were found for Bi2Te3 deposition on stainless steel substrates and gold-coated silicon substrates. A follow-up study revealed a complex underpotential deposition and nucleation process of the deposition on the gold surface. Based on the mechanistic studies, thin films (< 1μm) of binary Bi2Te3 and Sb2Te3 and ternary Bi0.5Sb1.5Te3 and Bi2Te2.7Se0.3 were prepared using continuous and pulse electrodeposition. Comparison of film composition, surface morphology, and crystal structure for the two deposition approaches and for different deposition parameters was made and correlated to the thermoelectric and transport properties which were measured after the films were transferred to non-conducting substrates. It was found that the thermoelectric properties of the as-deposited thin film materials studied are generally lower than their bulk counterparts due to factors such as unusual high carrier concentration, presence of grain boundaries and low crystallinity. Adjustment of the deposition conditions did not provide an effective improvement of the thermoelectric properties of the as-deposited films. However, for Bi2Te3-based films, annealing of the as-deposited films at elevated temperate resulted in significant reduction of carrier concentration with an accompanying increase in Seebeck coefficient.