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Directory:Black Silicon

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Flat silicon surfaces are normally highly reflective. This is undesirable when it comes to solar power cells made of silicon. The more light they reflect, the less they convert to electricity.

This page considers methods for turning the surface of silicon black so that it absorbs more solar energy, rather than reflecting it.

Table of contents

1 Solutions

1.1 Etching

1.1.1 Laser Etching
1.1.2 Gold Etching
1.1.3 Plasma Etching

1.2 Anti-reflective Coatings
1.3 Misc.

2 See also

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Solutions

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Etching

Techniques to create nanometer pits and bumps on the silicon which end up helping to absorb light.

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Laser Etching

Mazur Group, Harvard University (http://mazur-www.harvard.edu/research/detailspage.php?rowid=1) - Irradiating a silicon surface with femtosecond laser pulses in the presence of a sulfur containing gas transforms the flat, mirror-like surface of a silicon wafer into a forest of microscopic spikes.

In addition to near-unity absorption in the visible, the irradiated surface absorbs over 80 percent of infrared light for wavelengths as long as 2500 nm.


silicon microstructures as viewed under a scanning electron microscope	Black silicon surface viewed at high magnification

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Gold Etching

Technical University of Munich

Nano World: Black silicon for solar power (http://www.physorg.com/news70282948.html) - Silicon surfaces rendered black by pits and bumps only nanometers or billionths of a meter large could in the future help make solar power cells more efficient. (PhysOrg; June 23, 2006)

Excerpt:

Svetoslav Koynov and his colleagues have developed a simple and fast technique that creates these textures using wet chemical processes. Their method works regardless of the crystalline structure of the silicon, its thickness, or what extra chemical additives it possesses. The fact the technique is so flexible could mean it could work on silicon surfaces that cannot handle other kinds of anti-reflection treatments, such as the silicon thin films used in advanced hybrid solar cells, Koynov suggested.

First the researchers deposit grains of gold only nanometers large onto a flat silicon surface. Next the silicon between the areas covered by the clusters is etched away with a solution of hydrogen peroxide and hydrofluoric acid. The gold nanoparticles exhibit catalytic action, "behaving like drills into the surface," Koynov said. The areas covered by the gold form 50-to-100-nanometer-high pits on the surface while the silicon between the gold makes up the bumps. The nanoparticles are then removed with a solution of iodine and potassium iodide.

Koynov cautioned the gold nanoparticles could react unfavorably with the semiconductors in solar cells. He added his team has experimented with using other metal nanoparticles as well, with some success.

The researchers are currently attempting to establish an industrial partner to advance their method forward.

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Plasma Etching

Typical techniques are complex, difficult to work over large surfaces and can incur damage, making large-scale production of solar power cells with them a problem.

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Anti-reflective Coatings

A traditional approach has been to apply an anti-reflective coating. These reduce the amount of average reflection in the wavelengths of light solar power cells use by 85 percent to 92 percent.

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