Very Low Temperature Epitaxial Growth of Silicon Films for Solar Cells

Abstract

Low-temperature n-type silicon epitaxial growth has been studied for the application to n+/p homojunction solar cells. Epitaxial Si films can be grown at the substrate temperatures from room temperature (RT) to 200 °C by plasma-enhanced chemical vapor deposition. Two-step growth is effective for obtaining high-quality layers above 150 °C. In this growth, a very thin Si layer has been deposited on Si substrates at a low hydrogen-to-silane gas-flow ratio, and subsequently an epitaxial Si layer has been grown at a higher ratio. Smooth nucleation with reduced local stress has been enhanced in the initial stage of the epitaxial growth. An open-circuit voltage, Voc, of 0.610 V and a conversion efficiency, η, of 13.54% have been obtained for an n+/p cell, where the n+ layer was deposited at 200 °C. Film growth mode changes below 100 °C. The stress in the film during the epitaxial growth is relieved by introducing a small fluctuation of crystal orientation and the voids in the film at these temperatures. We have succeeded in fabricating a solar cell with a Voc of 0.608 V and an η of 13.52% having an n+ layer deposited at RT.