LT) can be achieved for weakly absorbed photons with energies close to the absorption edge of silicon. [ 15 ] These properties of b-Si are particularly useful for photovoltaic applications.The limiting effi ciency of a solar cell is given by the detailed balance of absorption and radiative recombination [ 16 ] and by nonradiative processes like Auger-and impurity recombination. [17][18][19] b-Si can help to approach those limits in two ways. On the one hand b-Si improves the coupling of light into the solar cell and the absorption of near band edge photons. This in turn increases the short circuit current and on a logarithmic scale also the open circuit voltage. On the other hand, due to excellent light-trapping properties b-Si might also allow reducing the solar cell thickness substantially below 100 µm while sustaining a high light absorption. This reduces nonradiative bulk recombination losses that scale linearly with the solar cell thickness [ 17,18 ] and hence, increases the open-circuit voltage. Of course, reducing the solar cell thickness also increases the cost effi ciency. Decreasing the amount of required silicon feedstock is a major industry concern as can be seen by the growing interest in kerf-free crystalline silicon solar cell technologies. [20][21][22] Unfortunately, besides bulk effects, surface recombination imposes a very critical limit to the solar This article presents an overview of the fabrication methods of black silicon, their resulting morphologies, and a quantitative comparison of their optoelectronic properties. To perform this quantitative comparison, different groups working on black silicon solar cells have cooperated for this study. The optical absorption and the minority carrier lifetime are used as benchmark parameters. The differences in the fabrication processes plasma etching, chemical etching, or laser processing are discussed and compared with numerical models. Guidelines to optimize the relevant physical parameters, such as the correlation length, optimal height of the nanostructures, and the surface defect densities for optoelectronic applications are given.
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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