Epitaxial Silicon Solar Cells

Abstract

Commercial solar cells are made on crystalline silicon wafers typically 300 ┤m thick with a cost corresponding to a large fraction of their total cost. The potential to produce good quality layers (of about 50 ┤m thickness), in order to decrease the cost and improve in the same time the efficiency of cells, has entered to the photovoltaic cell manufacturer priorities. The wafers thickness has been significantly decreased from 400 ┤m to 200 ┤m, between 1990 and 2006 while the cell’s surface has increased from 100 cm2 to 240 cm2, and the modules efficiency from 10% to already 13 %, with the highest values above 17% (Photovoltaic Technology Platform; 2007). Advanced technology’s solar cells have been fabricated on wafers of 140 ┤m thicknesses, resulting to efficiencies higher than 20% (Mason.N et al 2006). The cost associated to the substrate of a crystalline silicon solar cell represents about 50-55% on module level and is equally shared between the cost of base material, crystallization and sawing (Peter. K; et al 2008). The cost related to the Si base material can be reduced fabricating thinner cells, while the cost of crystallization and sawing is eliminated by depositing the Si directly on a low cost substrate, like metallurgical grade Si. The epitaxial thinfilm solar cells represent an attractive alternative, among the different silicon thin film systems, with a broad thickness range of 1-100┤m (Duerinckh. F; et al 2005). Conversion efficiencies of 11.5-12 % have been achieved from epitaxial solar cells grown on Upgraded Metallurgical Grade Silicon (UMG Si) substrates with an active layer of 30 ┤m, and an efficient BSF (Hoeymissenet. J.V; al 2008). Epitaxial cells with the same active layers deposited on highly doped multi-crystalline Si substrates by Chemical Vapor Deposition and the front and back surfaces prepared by phosphorous diffusion as well as screen printing technique, have confirmed also efficiencies 12.3% (Nieuwenhuysen. K.V; 2006). Solar cells developed by a specific process for low cost substrates of UMG silicon have led to efficiencies of 12.8% (Sanchez-Friera. P; et al 2006). Better results have been achieved from cells with an emitter epitaxially grown by CVD, onto a base epitaxialy grown (Nieuwenhuysen. K.V; et al 2008). The emitter creates a front surface field which leads to high open-circuit voltages (Voc) resulting to cell efficiencies close to 15% by optimizing the doping profile and thickness of epitaxial layers and by including a light trapping mechanism. This chapter first describes the manufacturing procedures of epitaxial silicon solar cells, starting from the construction of the base layer until the development of solar cells. Then a onedimensional (1D) (Perraki.V; 2010) and a three dimensional (3D) computer program (Kotsovos. K & Perraki.V; 2005), are presented, for the study of the n+pp+ type