Ultra-thin Single-crystalline Silicon Membrane Solar Cells as a Light-trapping Test Platform

Abstract

<p>The photovoltaics (PV) research community is currently pursuing many approaches to reduce the cost of PV and increase the energy conversion efficiency. Single-crystalline silicon (sc-Si) solar cells are able to achieve high efficiency but have a higher cost relative to other technologies. It may be possible to drastically reduce the cost of sc-Si PV by fabricating solar cells which are an order of magnitude thinner than conventional solar cells, i.e. thinner than 30 microns. Aside from new fabrication paradigms, ultra-thin sc-Si solar cells require advanced light-trapping techniques to enhance the absorption of long-wave radiation which is otherwise transmitted through the cell. In this thesis, a novel process flow for the fabrication of ultra-thin sc-Si solar cells in the laboratory was designed and implemented with the aim of testing light-trapping structures in the context of actual ultra-thin sc-Si devices. The process flow uses 10 micron thick sc-Si membranes, 0.95 cm in diameter, fabricated on silicon-on-insulator wafers using double-sided processing. The best fabricated device incorporated a back surface field, a white paint diffuse rear reflector and a silicon nitride antireflection coating. It achieved a fill factor, efficiency, short circuit current and open circuit voltage of 0.67, 9.9%, 27.9 mA cm^-2 and 0.53 V respectively. Simulations suggest the device efficiency can approach 15.4% without light-trapping and 16.5% with a diffuse rear reflector as a light trapping structure. This process flow is intended to be used as a platform on which to test further light-trapping structures with the continuation of this project.</p>