Oxidation effects on graded porous silicon anti-reflection coatings
Efficient anti-reflection 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 anti-reflection properties can be fabricated. In this work, ageing of graded PS has been studied using Spectroscopic Ellipsometry, Transmission Electron Microscopy and X-ray Photoelectron Spectroscopy. During oxidation of PS elements such as pure Si (Si$^0$), Si$_2$O (Si$^+$), SiO (Si$^{2+}$), Si$_2$O$_3$ (Si$^{3+}$), and SiO$_2$ (Si$^{4+}$) are present. In addition both hydrogen and carbon is introduced to the PS in the form of Si$_3$SiH and CO. The oxide grows almost linearly with time when exposed to oxygen, from an average thickness of 0 - 3.8 nm for the surface PS. The oxidation is then correlated to the optical stability of multi-layered PS ARCs. It is found that even after extensive oxidation, the changes in the optical properties of the PS structures are small.