Progress with epitaxy wrap-through crystalline silicon thin-film solar cells

Wafer-Equivalents are thin-film solar cells that use a low-cost silicon substrate to epitaxially grow a high-quality crystalline silicon active layer. The epitaxy wrap-through (EpiWT) cell is a back-contact version of the Wafer-Equivalent that aims to increase currents and gain other benefits of back contacts. The EpiWT cell can be made in a symmetrically interdigitated configuration with 50% back emitter coverage, or using an isolation layer to lower the back emitter coverage to similar to 10%, which will theoretically increase voltages. The epitaxial deposition through via holes in the substrate depends on many factors, including the sealing of the deposition chamber, and produces various thicknesses and geometrical forms of the layers in the holes. An extended process has been developed to incorporate a passivated selective emitter and the first batch has been fabricated. The best result was an efficiency of 13.2% with similar to 22 mu m base layer thickness. The results are limited most by the fill factors at this stage, e.g. 75% for this cell, which is due to a processing difficulty encountered with screen-printing in via holes. A new isolation layer was tested and successfully implemented for the low back-emitter configuration. Comparable voltages and currents were achieved but the fill factors were lower than for the 50% back emitter cells, resulting in a best efficiency of 11.2%. Copyright (C) 2011 John Wiley & Sons, Ltd.