A porous silicon intermediate reflector in thin film epitaxial silicon solar cells as a gettering site of impurities

In this work, we study the gettering properties of a porous silicon (Psi) intermediate reflector. This type of reflector is used currently in the thin film epitaxial silicon solar cell approach. The porous layers are electrochemically etched into low-cost, highly contaminated UMG (Upgraded Metallurgical Grade) Si in order to investigate the influence of impurity diffusion to the active layer due to the high thermal budget during epitaxial growth. When implemented into a solar cell, a porous silicon (Psi) multilayer stack consisting of layers with alternating porosities can serve as a gettering site for impurities that originate from the substrate. This statement is confirmed in our present work by several techniques such as SIMS (Secondary Ion Mass Spectrometry) and TXRF (Total Reflection X-Ray Fluorescence). Additionally to these experiments, solar cell results are presented, for which the active layer is grown with two different growth rates on top of porous silicon (Psi) multilayer stack and on two different types of multicrystalline silicon substrates: UMG (Upgraded Metallurgical Grade) and OFFSPEC (Out-of spec silicon material treated as a waste from Integrated Circuits industry). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)