Effect of O 2 on thickness uniformity of CdTe thin film deposited using CSS
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Effect of O2 on thickness uniformity of CdTe thin film deposited using CSS Nirav Vora, Ramesh Dhere National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401 ABSTRACT Incorporating O2 in the closed space sublimation (CSS) of CdTe thin film has resulted in improved cell efficiencies. Many studies have been undertaken to understand this effect on cell efficiency. In this work we study the effect of oxygen on lateral uniformity of the deposited CdTe film. A finite element model has been developed to represent the mass and heat transfers involved in the CSS process. The model takes into consideration the effect of O2 by modeling its reaction with Cd vapors in the space between the source and the substrates and with the CdTe source. So a gradient of O2 from the edges to the center of the substrate can result in non-uniform oxidation of the source and subsequently a laterally non-uniform film. A steady state model solved at various temperatures, pressures, and separation distances. O2 concentration gradient was found to depend on the oxidation rate of Cd vapors and thus on temperature, total pressure, and oxygen partial pressure in the system. INTRODUCTION Closed space sublimation is a commonly used method for depositing CdTe thin films on glass substrates coated with CdS. The method consists of sublimating Cd and Te2 vapors from a CdTe source at high temperatures and subsequent condensation of these vapors as CdTe on a substrate maintained at lower temperature than the source. The deposition rate will thus depend on the rate of sublimation from the source, the rate of condensation on the substrate, and on the diffusion of Cd and Te2 vapors in the space between them. Depending on the process conditions, the deposition will be either reaction (sublimation/condensation) controlled or diffusion controlled or both. Another important factor that affects the deposition rate is the amount of oxygen in the system. Rose [1] found that presence of O2 in the system shifts the control towards reaction control. He attributed this to the oxidation of the CdTe source to form CdTe2O3, which subsequently decreases the amount of Cd and Te2 vapors being sublimated. An increase in the source temperature shifts the control back towards diffusion control as more vapors are sublimated. Various studies have been undertaken to understand the effect of O2 on the deposition rates of CdTe thin film and on efficiencies of CdTe solar cells, but little effort has been made to study the effect of oxygen on the uniformity of the film deposited. In most CSS systems, O2 is introduced from the sides. This can result in an O2 concentration gradient in the system with less O2 reaching the center depending on the pressure in the system and its reaction with Cd vapor. Subsequently, there is a preferential oxidation of the source near the edges. This can be a crucial factor when scaling up the process to incorporate it in a manufacturing line. To understand this phenomenon better a finite element model has been developed which takes
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