Epitaxial solar cells over upgraded metallurgical silicon substrates:: The EPIMETSI project

In order to overcome the shortage of silicon feedstock that is currently affecting the photovoltaic industry, many efforts are being dedicated to the search of alternative substrates for solar cells. One of the possible solutions is to use low-cost substrates of upgraded metallurgical silicon, for which there are no supply issues. The solar cell is built on a thin layer of high quality silicon which is epitaxially grown over this substrate, with the use of a high throughput epitaxial reactor of novel design. This reactor can be fed with electronic grade source gases or with gases obtained in situ from metallurgical silicon. A reactor prototype has been constructed with a capacity of 50 wafers per run and a deposition velocity of approximately 2 microns per minute. The production of chlorosilanes by Si etching and the recirculation of gases for maximum efficiency are being studied. A solar cell process specific for these substrates has been developed, achieving an efficiency of 12,8%. Further improvements on the solar cell process are under investigation, with special emphasis on light trapping by plasma texturing and the application of a porous Si layer as internal reflector. A preliminary market study has been performed to understand the role of silicon feedstock in the current photovoltaic market and its projection in the next decades, looking at the expected role of epitaxial solar cells.