Experimental testing of a random medium optical model of porous silicon for photovoltaic applications

We have developed a model for Eight propagation in porous silicon (PS) based on the theory of wave propagation in random media. The low porosity case is considered, with silicon being the host material assuming randomly distributed spherical voids as scattering particles. The specular and the diffuse part of the light could be determined and treated separately. The model is applied to the case in which porous silicon would be used as a diffuse back reflector in a thin-film crystalline silicon solar cell realized in an ultrathin (1-3 mum) epitaxially grown Si layer on PS. Three-layer structures (epi/PS/Si) have been fabricated by atmospheric pressure chemical vapor deposition (APCVD) of 150-1000 nm epitaxial silicon layers on silicon wafers of which 150-450 nm of the surface has been electrochemically etched. An excellent agreement is found between the experimentally measured reflection data in the 400-1000 nm wavelength range and those calculated using the proposed model. The values of the layer thickness agree, within a reasonable experimental error, with those obtained independently by cross-sectional transmission electron microscopy (XTEM) analysis. This provides an experimental verification of the random-medium approach to porous silicon in the low porosity case. The analysis shows that the epitaxial growth process has led to appreciable porosity decrease of an initially high-porosity layer from about 60% to 20-30%. Copyright (C) 2001 John Wiley & Sons, Ltd.