Casting Technologies for Solar Silicon Wafers: Block Casting and Ribbon-Growth-on-Substrate

Abstract

Multi-crystalline silicon solar wafer are the working horses of the rapidly developing photovoltaic market. The availability and cost efficiency but, even more important, the improved wafer quality and the understanding how to process multi-crystalline silicon wafers into highly efficient solar cells are important factors in this development. In this paper multi-crystalline silicon wafer manufacturing technologies both in industrial production as well as under development are outlined. Important wafer characteristics such as oxygen-, carbon and metallic impurities are described and experimental links to solar cell efficiency are exemplary demonstrated.

Chemical natures and distributions of metal impurities in multicrystalline silicon materials

“…The mc-Si materials analyzed in this study are directionally-solidified ingot-grown, 24 sheet, 25 and ribbon 26,27 materials originating from a variety of vendors, and multicrystalline float zone 28 grown at the National Renewable Energy Laboratory. While comprehensive information about these crystal growth techniques, throughput, and crystal growth parameters can be found in the references hitherto and in a review by T. F. Ciszek, 29 a brief summary is presented here to highlight the most relevant points for this study.…”

“…29,35 In the text, we refer to this mc-Si material as sheet mc-Si. Examples of materials defined as sheet mc-Si herein include ribbon growth on substrate (RGS) 24,27 and SiliconFilm. 36,37 Two types of sheet materials were analyzed, one using high-purity feedstock (Sheet A) and another using low-purity feedstock (Sheet B).…”

“…This includes Bridgman, casting, and heat exchange method growth techniques, 24,29 which currently account for about 50% of the worldwide PV market. 30 A silicon ingot of a few hundred kg in mass is grown relatively slowly ($10 mm/s) from the bottom up within a crucible.…”

Modeling electromagnetically driven free-surface flows motivated by the Ribbon Growth on Substrate (RGS) process

“…For such a discontinuous procedure, unavoidable sawing losses are typically in the range of 40 to 50% of the feedstock. On the contrary, the Ribbon Growth on Substrate (RGS) technology, which was suggested and developed during the last decades [1,2,3], circumvents such a deficit in terms of energy and material almost completely.…”

“…Figure 1. Scheme of the RGS process [4,2]: one half with central cut in the process direction. A narrow slit region is situated between the casting frame and the substrate.…”