Short-Circuit Current Densities Exceeding 30 $ \hbox{mA/cm}^{2}$ by Use of Chirped Porous-Silicon Reflectors and Shallow Emitters in Thin-Film (20- $\mu\hbox{m}$) Epitaxial Silicon Solar Cells

We demonstrate the use of chirped porous-silicon broadband optical reflectors for thin-film epitaxial silicon solar cells. The benefits of chirped multilayer structures over conventional Bragg reflectors on the cell level are presented. By combining these chirped reflectors with shallow emitters, we show that both low- and high-energy photons are more effectively absorbed in the thin (20-mum ) epitaxial active layer of the cell. This resulted in a high conversion efficiency of 14.2% on screen-printed epitaxial solar cells made on highly doped multicrystalline silicon substrate. The corresponding photogenerated current densities are well above 30 mA/cm
²
.