Ellipsometry, Reflectometry, and XPS Comparative Studies of Oxidation Effects on Graded Porous Silicon Antireflection Coatings

Efficient antireflection coatings (ARC) improve the light collection and thereby increase the current output of solar cells. By simple electrochemical etching of the Si wafer, porous silicon (PS) layers with excellent broadband antireflective properties can be fabricated. The close relation between porosity and refractive index, modeled using the Bruggeman effective medium approximation, allows PS multilayers to be tailored to fabricate ARCs optimized for use in solar cells. In this work, the effect of ageing on the reflection properties of multilayered PS ARCs is studied. The reflection is correlated to the oxidation of the structures through the use of x-ray photoelectron spectroscopy (XPS) and spectroscopic ellipsometry (SE). It is found that even after extensive oxidation, very small changes in reflectivity are measured.