Diffusion barrier of sol-gel derived silica for sprayed tin oxide film on soda-lime glass
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K. H. Pannell Department of Chemistry, University of Texas at El Paso, El Paso, Texas 79968
S. P. Albright Photon Energy, Inc., El Paso, Texas 79924 (Received 2 December 1992; accepted 9 September 1993)
Diffusion barriers of silica were deposited on soda-lime glass using a sol-gel technique. The migration of sodium from the glass through these layers and into a spray-pyrolyzed layer of fluorine-doped tin oxide was studied by SIMS depth profiling. The silica layer reduced sodium in the tin oxide overlayer by two orders of magnitude.
I. INTRODUCTION Tin oxide thin films are utilized in a variety of applications where a transparent conductor is needed, as in large area solar cells. The goal of maximizing the transparency of the film while minimizing the use of tin motivates the use of doped films with higher conductivity than pure tin oxide. As the thickness is decreased, contamination from the substrate becomes more important. Because the cost of materials is a significant portion of total cost for large area solar cells, soda-lime glass is frequently used. Economical and effective deposition processes such as spray pyrolysis are preferred. This work describes the effect of a silica diffusion barrier on the migration of sodium from soda-lime glass into a fluorine-doped tin oxide film. These form the transparent substrate and contact used for CdS/CdTe photovoltaic panels developed by Photon Energy, Inc.1'2 Silica layers are formed by a sol-gel process3 which is compatible with spray deposition of tin oxide, and thus adds only one step to the entire production process: the dip-coating of the glass substrate prior to placement in the spray pyrolysis ovens. This integrated manufacturing process is much less expensive than the use of commercially available silica-coated glass, which is used in other devices, such as display panels, where the cost is less important. The concentration of Na in the tin oxide layers was studied for several reasons. Na is a major constituent of the low-cost soda-lime glass used in these panels; it diffuses readily; it is easier to detect in SIMS depth profiling than other contaminants (such as Ca or K); ^Current address: Centro de Investigation y de Estudios Avanzados del IPN-Unidad Merida, A.P. 73 "Cordemex", 97310 Merida, Yucatan, Mexico. 226
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J. Mater. Res., Vol. 9, No. 1, Jan 1994
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it is known to affect the conductivity of tin oxide4; and it is thought to be deleterious to CdS/CdTe device performance.5 We find that the silica layer reduces sodium concentrations in the tin oxide by two orders of magnitude, relative to that in films without the diffusion barrier. II. EXPERIMENTAL DETAILS Samples for this study were produced in production equipment at Photon Energy, Inc.1'2 One set of samples was cut from a square sheet (25.4 cm on a side) of commercial 3 mm float glass that had been dip-coated on only one-half of its surface with a commercial solgel precursor for silica (from Emulsitone Co., Whippany, NJ). This glass pane was put in
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