* Corresponding author. Tel.: #32-16-281 270; fax: #32-16-281 501. E-mail address: bilyalov@imec.be (R. Bilyalov).  Also Professor at Katholieke Universiteit Leuven, Electrotechnical Department, Belgium. Solar Energy Materials & Solar Cells 65 (2001) 477} 485 Porous silicon as an intermediate layer for thin-"lm solar cell R. Bilyalov*, L. Stalmans, G. Beaucarne, R. Loo, M. Caymax, J. Poortmans, J. Nijs IMEC vzw, Kapeldreef 75, B-3001 Leuven, Belgium Abstract The potential of porous silicon (PS) with dual porosity structure as an intermediate layer for ultra-thin "lm solar cells is described. It is shown that a double-layered PS with a porosity of   % allows to grow epitaxial Si "lm at medium temperature (7253}8003C) and at the same time serves as a gettering/di!usion barrier for impurities from potentially contaminated low-cost substrate. A 3.5 m thin-"lm cell with reasonable e$ciency is realized using such a PS intermediate layer.  2001 Elsevier Science B.V. All rights reserved. Keywords: Porous silicon; Thin "lms; Si solar cells 1. Introduction Thin-crystalline silicon solar cells have a great potential for photovoltaic applica- tions. The use of low-cost Si-based substrates such as metallurgical grade Si (MG-Si) [1] or Si sheets from powder [2] looks very attractive from the viewpoint of cost. On the other hand, these materials are contaminated by di!erent impurities (particularly transition metals), which might di!use into the active Si layer during the CVD process and result in a degradation of the solar cell characteristics. Secondly, these substrates cannot act as a backside re#ector, which is however crucial for ultra-thin-"lm solar cells. One possible way to overcome these problems is to use PS as an intermediate layer between substrate and thin Si "lm. It was shown already that a single PS layer with 0927-0248/01/$ - see front matter  2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 7 - 0 2 4 8 ( 0 0 ) 0 0 1 3 0 - 6