Transport phenomena in the close-spaced sublimation deposition process for manufacture of large-area cadmium telluride photovoltaic panels: Modeling and optimization

With increasing national and global demand for energy and concerns about the effect of fossil fuels on global climate change, there is an increasing emphasis on the development and use of renewable sources of energy. Solar cells or photovoltaics constitute an important renewable energy technology but the major impediment to their widespread adoption has been their high initial cost. Although thin-film photovoltaic semiconductors such as cadmium sulfide-cadmium telluride (CdS/CdTe) can potentially be inexpensively manufactured using large area deposition techniques such as close-spaced sublimation (CSS), their low stability has prevented them from becoming an alternative to traditional polycrystalline silicon solar cells. A key factor affecting the stability of CdS/CdTe cells is the uniformity of deposition of the thin films. Currently no models exist that can relate the processing parameters in a CSS setup with the film deposition uniformity. Central to the development of these models is a fundamental understanding of the complex transport phenomena which constitute the deposition process which include coupled conduction and radiation as well as transition regime rarefied gas flow. This thesis is aimed at filling these knowledge gaps and thereby leading to the development of the relevant models. The specific process under consideration is the CSS setup developed by the Materials Engineering Group at the Colorado State University (CSU).;Initially, a 3-D radiation-conduction model of a single processing station was developed using the commercial finite-element software ABAQUS and validated against data from steady-state experiments carried out at CSU. A simplified model was then optimized for maximizing the steady-state thermal uniformity within the substrate. It was inferred that contrary to traditional top and bottom infrared lamp heating, a lamp configuration that directs heat from the periphery of the sources towards the center results in the minimum temperature variation within the substrate. Next, the models for individual processing stations were coupled to form a combined model for the transient evolution of the substrate temperature and temperature uniformity as it traverses from one station to the next. Procedures for station geometry design and transient control of source temperatures are presented.;High vacuum or line-of-sight deposition gives the 'worst-case' thickness uniformity in a physical vapor deposition setup. A line of sight deposition model for the CSU source was developed by employing an analogy between thermal radiation and free-molecular flow. The modeling procedure was benchmarked against literature data as well as actual high vacuum experiments. The results indicate that large variations in film thickness can exist in the CSU source in the absence of a background gas.