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The overall goal of the project was direct conversion of sunlight into electrical energy. While many materials in the bulk (or normal) form have been studied for this purpose, materials used for commercialized solar cells can be made into well-controlled nanostructures (materials with diameters of a few nanometers) which hold even more promise. Nanocrystals are solution processable and decrease material expenditures and avoid energy costs as the deposition of bulk materials is very expensive and energy intensive. This project is focused specifically on the synthesis and use of Cadmium Telluride (CdTe). Solar cells made using CdTe nanoparticles have demonstrated an efficiency of up to 5%2. I spent the first part of the summer focusing on the basic synthesis of CdTe. Monodisperse CdTe nanocrystal spheres with diameters between 4 and 6 nanometers were produced using a conventional method. Cadmium Oxide (CdO), Octadecyl Phosphonic Acid (ODPA), and Tri-octylphosphine Oxide were used to create the precursor solution, and tellurium was injected rapidly in a solution of Tri-octylphosphine and Tellurium.
Once I had worked with the conventional synthesis of CdTe, I worked on scaling up the reaction to produce more at once, thus increasing the efficiency of the process and producing more material from which devices were configured. Along the same lines, I tested different injection amounts and precursor methods to expand the size range of the particles. It took some time to optimize the system because every time the conditions are changed, the size and quality of the particles is affected.
The CdTe produced had a maximum diameter of between 6 and 7 nm. For many assembly purposes, such as the creation of binary superlattices, larger particles could prove very interesting. Therefore, experiments were conducted to overcoat the monodisperese CdTe spheres with monolayers of Cadmium Selenide (CdSe) and Cadmium Sulfide (CdS) materials. Overcoating is an interesting process as it increases the size and range of the particles, making them useful for more purposes. Once the CdTe was synthesized, the particles were held in hexane. Using an automated machine called a Dip-coater, the particles were placed onto a glass coated with Indium Tin Oxide (ITO). The glass (substrate) was dipped in six different treatments, starting with the CdTe in hexane followed by various ligand exchange solutions and rinses. After the substrate was dipped enough times to create a thick layer of the particles, the glass was annealed (heated) in air. Once the substrate was prepared, Aluminum was evaporated on to create the solar cells. I tested the cells using a solar simulator. So far, 0.3% efficiency has been obtained. While this project produced many promising results, there are still many opportunities to improve these materials and processes. I have started exploring different synthesis methods, including microwave synthesis. Microwave synthesis has the potential to facilitate more reproducible, consistent materials. Also, chalcopyrite materials such as Copper Indium Diselenide have the potential to produce even more efficient solar cells. I will be working on all of these topics during the upcoming academic year.