Polysilicon vs. upgraded metallurgical-grade silicon (UMG-Si): Technology, quality and costs

摘要

Throughout the severe plummet of PV prices that took place during 2008-2012 as a result of overcapacity, the polysilicon sector suffered a major adjustment of costs and capacity to face the reduction in prices and the mismatch between demand and supply. In 2012 that significant drop in prices provoked the bankruptcy of many polysilicon producers, with only the large and efficient players still surviving. However, there was also an impact on the (at that time) promising and immature industry of metallurgical purification of metal silicon, also known as upgraded metallurgical-grade silicon (UMG-Si). The strong selling point of UMG-Si producers - the production costs - was no longer an asset, leaving UMG-Si with nothing but its weakness - the quality. The generation costs for solar energy are currently comparable to those for conventional fuels. The solar industry is self-sustaining and is not dependent on government subsidies. In this current situation, the industry requires an updated comparison between the two main routes of silicon purification and their products, which is the aim of this paper. The first route is the indirect route (or chemical route), in which the metallurgical-grade silicon (MG-Si) is transformed into a silane (typically trichlorosilane TCS, or silane MS), which is refined and finally introduced into a chemical vapour deposition (CVD) reactor where the silicon is deposited. The second route is the direct route (or metallurgical route), whereby the silicon is purified by means of metallurgical processes. The principal technologies within each route, along with their main advantages and challenges, are briefly described in this paper. Subsequently, the inherent quality of the product and the costs associated with each purification route are assessed by compiling the producers' published information and the authors' analyses.