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Improved Black Silicon for Photovoltaic Applications

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Abstract

The morphology and the electronic properties of monocrystalline Si (c-Si) with a nano-textured “black” surface, obtained by a metal-catalyzed wet etching process, and the improvement by an additional chemical treatment are examined with regard to solar cell applications. Photoluminescence and optical reflectivity measurements show the presence of a nano-porous Si (np-Si) phase in the as-prepared nano-texture. It is found that an additional wet chemical treatment with the standard clean 1 of the common RCA cleaning process removes the np-Si fraction and significantly alters the surface of the nano-structure. Cross-sectional scanning electron microscopy images reveal a pronounced reduction of the surface area, to values of only 3–6 times that of a planar surface. Electron spin resonance measurements were performed to investigate the type and quantity of defects induced by the nano-texturing process. The optimized nano-texture exhibits a Si dangling bond density comparable to planar c-Si wafers. Electrically detected magnetic resonance spectra reveal an additional paramagnetic defect present in the nano-textured Si, linked to a hydrogen- or oxygen-related double donor. In addition, initial results on the passivation of surface defects via atomic layer deposition of Al2O3 are presented. Photoconductance decay measurements of passivated samples show a tenfold increase of the effective lifetime for nano-textures which have received the additional etching treatment. The improved electronic quality of the nano-textured surface makes it an interesting candidate for application as an anti-reflection surface in solar cells.

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