Low-cost epitaxial techniques for solar-cell fabrication. Final report, 25 September 1979-24 September 1980

Abstract

This research and development effort was designed to investigate epitaxial growth processes which will allow the use of low-cost forms of silicon for fabricating high-efficiency, cost-effective solar cells. This report covers the results of a 1-year effort which involved characterization of potentially low-cost silicon substrates, epitaxial growth studies, and the fabrication and evaluation of solar cells made in the epitaxial layers. Three grades of generic metallurgical grade (MG) silicon material designated upgraded metallurgical grade (UMG) silicon were investigated in some detail. These three grades have a wide range of purity, defect structure, and crystallinity, all of which are representative of the characteristics expected in future low-cost silicon. The electrical, physical, and structural properties of this material were measured. An initial epitaxial growth technique was standardized and qualified for application to the growth-of'diagnostic and solar-cell structures. Such structures were grown on the three UMG grades of substrates, and the properties of these layers were compared to those of the substrates. It was generally found that the defect density in the epiiaxial films wa~ lower than in the substrates, but greater aerial variation was observed for the less pure, multicrystalline grades of substrates. Sol•a•r cells were fabricated in epitaxial structures grown on all three UMG grades' of substrates. With a 15-~m-thick epitaxial structure grown on the purest form of UMG substrate, an average cell AM-1 efficiency of 12.5% was obtained. •similar structures grown on the less pure multicrystalline substrate grades result •in efficiencies of 9 to 10.3%. A novel way of slicing these substrates which•exposes larger grain areas was implemented and resulted in higher average artd peak efficiencies for cells made on such substrates. Two additional substrate materials which-were obtained •from other research efforts supported by SERI were briefly explored. On one of these, a silicon ingot solidified by the HEM method applied to MG sili'c•on, epitaxial cells of efficiency as high as 12% were obtained. The second material was representative of the early• stages of a low-cost substrate development program, and peak effiCiencies of•about 10% were obtained, but material defects resulted in a large variation •in •cell performance. The limitations experienced with these preliminary niateriais are pointed out. The practical aspects of making epitaxial •solar celis were explored by growing and fabricating large-area cells. In this work, a si~nificant achievement ~as the attainment of efficiencies close to 10% with epitaxial cells of 10.8-cm area grown on zhe lowes~ ~rade silicon S!lbstrat: under study. For larger areas (23.1 em) an eff1c1ency of 7.0% was obta1ued. The use of low-cost surface preparation techniques was successfully for both epitaxial growth arid the resultant solar-cell properties. ments on the surface finish obtainable with chemical etchants which the subsequent application of AR coatings are poizit...