The carbon substrate in RST Si ribbon technology for solar cells

The carbon substrate used in the RST (silicon Ribbon growth on a Sacrificial carbon Template) process, a silicon ribbon growth technology aimed at the fabrication of silicon solar cells, is a composite made of a compressed expanded natural graphite (CENG) ribbon coated by a protective rough lamellar high-temperature (RLHT) pyrocarbon. It is shown that the reactivity of RLHT pyrocarbon with molten silicon is stimulated by the presence of topological defects of the CENG ribbon. This reactivity generates a contamination of the silicon melt by carbon and its metallic impurities and, in turn, adversely affects the performances of solar cells made from RST silicon films. Reactivity with molten silicon can be drastically reduced by the control of the deposition conditions of pyrocarbon and of the roughness of the CENG ribbon. A new model is proposed to explain the mechanism of the intrinsic reactivity of RLHT pyrocarbon with molten silicon. Based on this model the reactivity problem can virtually be eliminated by the deposition of a SiC coating about 0.3 μm on the RLHT pyrocarbon. Improvements of the carbon ribbon, described in this study, especially its pyrocarbon coating, should lead to flexible silicon solar cells 70–80 μm thick with conversion efficiency above 18%.