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) base d on the theory of wave propagation in random media. The low porosity case is considered, with silicon being the host material assuming randomly distr ibuted spherical voids as scattering particles. The specular and the diffus e part of the light could be determined and treated separately. The model i s applied to the case in which porous silicon would be used as a diffuse ba ck reflector in a thin-film crystalline silicon solar cell realized in an u ltrathin (1-3 mum) epitaxially grown Si layer on PS. Three-layer structures (epi/PS/Si) have been fabricated by atmospheric pressure chemical vapor de position (APCVD) of 150-1000 nm epitaxial silicon layers on silicon wafers of which 150-450 nm of the surface has been electrochemically etched. An ex cellent agreement is found between the experimentally measured reflection d ata in the 400-1000 nm wavelength range and those calculated using the prop osed model. The values of the layer thickness agree, within a reasonable ex perimental error, with those obtained independently by cross-sectional tran smission electron microscopy (XTEM) analysis. This provides an experimental verification of the random-medium approach to porous silicon in the low po rosity case. The analysis shows that the epitaxial growth process has led t o appreciable porosity decrease of an initially high-porosity layer from ab out 60% to 20-30%. Copyright (C) 2001 John Wiley & Sons, Ltd.