Epitaxial silicon growth for solar cells. Quarterly report No. 2

Abstract

The objective of this program is to develop and apply epitaxial growth techniques to the fabrication of efficient solar cells on low-cost forms of silicon. Progress is reported. The efficiency of baseline epitaxial solar-cell structures of about 1 mil thickness grown on single-crystal substrates and processed with diffused junctions has been reproducibly established at approximately 13% (AM-1). Experiments were conducted in the growth of the entire cell structure including the junction layer. For baseline cells fabricated on these layers, efficiencies of approximately 10% have been obtained. These structures were grown with junction depths of 0.8 to 1.0 ..mu..m which resulted in sort-circuit current densities of approximately 24 mA/cm. Techniques for the reproducible growth of thinner surface layers are being pursued, in order to increase the short-wavelength response of these structures. Epitaxial techniques were applied in the growth of diagnostic layers and solar-cell structures on two potentially, low-cost, polycrystalline silicon substrates, namely Wacker (SILSO) and a Refined Metallurgical Grade (RMS) silicon. X-ray topographic studies have shown improved crystallographic quality in the epitaxial layers grown on both substrate forms. The best solar cells were fabricated on the RMS substrates where an AM-1 efficiency of 10.6% was obtained with a 16-..mu..m thick epitaxial layer. Epitaxial structures on the Wacker substrates yielded efficiencies in the 5.6 to 9.5% range.