Graduate Thesis Or Dissertation
 

Growth, characterizations and applications of copper sulfide thin films by solution-based processes

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  • Copper sulfides (Cu[subscript x]S) are compound semiconductor materials that exhibit considerable optical and electrical properties varying significantly as a function of the composition. Copper sulfide thin films can be used in many applications, such as solar control coatings, solar cells, photothermal conversion of solar energy, electroconductive coatings, and microwave shielding coatings. A variety of solution-based and vapor-based techniques are suitable for their deposition. Solution-based processes have the advantages of simplicity, low capital cost, and low processing temperature. In this work, copper sulfide thin film deposition by a number of solution-based processes was investigated. These processes include chemical bath deposition (CBD), Microreactor Assisted Solution Deposition (MASD), and PhotoChemical Deposition (PCD). The growth kinetics of copper sulfide thin films by CBD was monitored using an in-situ quartz crystal microbalance for the first time. CBD growth was studied as a function of time, temperature, concentrations of reactants, and pH. The reaction activation energy was determined based on initial growth rates. The result indicates the rate limiting step of the deposition is the chemical reaction rather than mass transport. The structure, morphology, composition and optical absorption of the films were found to depend strongly on the deposition conditions. Results from the study of CBD reactions indicated the need to de-reduce the undesirable homogeneous particle formation. The MASD process was developed to achieve this objective. The continuous flow process together with the microreactor design not only improve the mixing of reactants and provide a better temporal control over the reaction which result in higher quality films and a higher deposition rate. A particle-free flux was obtained after adjusting the key process parameters (concentration of mixed reactants, solution temperature, substrate temperature, and residence time). Significantly improved copper sulfide thin film deposition with a good selectivity of heterogeneous surface reactions was achieved. PCD basically employs the UV illumination to excite the irradiated region of the substrate in a deposition solution. It has the potential to reduce the homogeneous particle formation. We investigated the growth kinetics of copper sulfide thin films by PCD under various deposition conditions (e.g. pH, substrate position, reactant concentration, deposition time, and temperature) that influence on the film properties and characteristics. Moreover a detailed mathematical model that describes the multiple chemical reactions in the deposition mechanism was also developed in this work to have a better understanding of the reaction mechanism. Reaction rate constants were successfully estimated from the experimental data based on this model. The calculated results agree well with the experimental data. This model could serve as a useful tool for the control and optimization of photochemical deposition of copper sulfide thin films. Both CBD and PCD processes suffer from severe homogeneous particle formation which has resulted in lower deposition rate. In contrast, MASD provides good selectivity towards heterogeneous surface deposition using molecular precursors at a much higher deposition rate. Thus MASD process was used to deposit copper sulfide layers on textured substrates with nice conformal coverage. Dense, crack-free CuInSe₂ thin films were fabricated successfully after adding an indium precursor layer, and followed by a selenization process. This approach offers a potential low-cost route to fabricate thin absorber solar cells.
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