Improving the Quality of Epitaxial Foils Produced Using a Porous Silicon-based Layer Transfer Process for High-Efficiency Thin-Film Crystalline Silicon Solar Cells

A porous silicon-based layer transfer process to produce thin (30-50 mu m) kerfless epitaxial foils (epifoils) is a promising approach toward high-efficiency solar cells. For high efficiencies, the epifoil must have high minority carrier lifetimes. The epifoil quality depends on the properties of the porous layer since it is the template for epitaxy. It is shown that by reducing the thickness of this layer and/or its porosity in the near-surface region, the near-surface void size is reduced to <65 nm and in certain cases achieve a 100 nm-thick void-free zone below the surface. Together with better void alignment, this allows for a smoother growth surface with a roughness of <35 angstrom and reduced stress in the porous silicon. These improvements translate into significantly diminished epifoil crystal defect densities as low as similar to 420 defects/cm(2). Although epifoils on very thin porous silicon were not detachable, a significant improvement in the lifetime (diffusion length) of safely detachable n-type epifoils from similar to 85 (similar to 300 mu m) to similar to 195 mu s (similar to 470 mu m) at the injection level of 10(15)/cm(3) is achieved by tuning the porous silicon template. Lifetimes exceeding similar to 350 mu s have been achieved in the reference lithography-based epifoils, showing the potential for improvement in porous silicon-based epifoils.