Comparative analysis of chemically and electrochemically formed porous Si antireflection coating for solar cells

From the viewpoint of the porous Si (PS) application as an antireflection coating (ARC) in crystalline Si solar cells, a comparative study has been made on the morphological, kinetic, and optical properties of both chemically and electrochemically etched PS. These results are correlated and interpreted in terms of a simplified empirical model for the simultaneous PS formation and dissolution of Si. The fact that PS mainly carries potential as an ARC stresses the importance of the reflectance study. Chemically etched (stain-etched) PS has a pronounced porosity gradient in depth which is the result of a simultaneous etching at the PS/Si interface and Si dissolution throughout the existing porous layer. This implies that a part of the solar cell emitter is removed during stain etching. The predominant etching at the PS/Si interface in the case of electrochemically etched PS only converts part of the solar cell emitter into a porous layer and results in a nearly constant porosity in the thickness range of interest (up to 200 nm) for solar cell applications. Multicrystalline Si (mc-Si) behaves strongly similar to monocrystalline Si when applying an electrochemical porous etching, while stain etching is intrinsically more sensitive to the Si surface structure. The typical reflectivity behavior of both types of PS is interpreted in terms of the morphological differences. The integrated reflectance of both electrochemically formed and stain-etched porous layers is comparable to the value obtained for a silicon nitride coating on textured mc-Si, which stresses the potential of a PS ARC for crystalline Si solar cells. (C) 2003 The Electrochemical Society.