A simple fabrication technique for subwavelength structured (SWS) surfaces by means of anodic porous alumina masks directly formed on Si substrates was proposed and demonstrated. By this technique, SWS surfaces were fabricated on polished single-crystalline Si and chemically etched as-cut multicrystalline Si wafers. Smoothly tapered SWS surfaces with a periodicity of 100nm and a height of 300–400nm were obtained. A low reflectivity below 1% was observed from 300to1000nm for both of the wafers, in agreement with numerical simulation. After thermal annealing at 800°C, the reflectivity of the SWS surface increased to 3%.
The angle-dependent reflectivity of several surface structures was analyzed and evaluated with the viewpoint of solar cell applications. Numerical analysis showed that a Si subwavelength structure (SWS) maintains a lower reflectivity at large incident angles than conventional light-trapping techniques such as a random pyramid texture, and that it can contribute to increasing the output power of solar cells under oblique irradiation. This wide-angle antireflection effect was demonstrated by fabricating test crystalline Si cells with several surface structures including a SWS and measuring their angle-dependent short-circuit current densities.
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