Why solar cells are sometimes blue

These are untextured silicon solar cells with a layer of silicon nitride anti-reflection coating on top.  They appear blue because the anti-reflection coating is optimized for a certain wavelength of the solar spectrum.  They are most likely poly-silicon cells because it has been difficult to texture the front surface of polysilicon (the crystal grains aren't all aligned in the same direction).

Sunpower cells with surface texturing appear black

It is easier to texture monocrystalline silicon because it is a single crystal grain.  Just dip it in KOH for some amount of time and the anisotropic etching process will produce random pyramids on the surface.  If a company for some reason decides not to texture a monocrystalline silicon cell or improperly texture it, it will also appear blue.  With recent advances in acid etching, however, texturing polysilicon will become more mainstream, and you might not see blue cells off the line anymore.

A textured cell without the anti-reflection coating will appear gray.  With the coating, it will appear black or close to black.

Textured silicon image from here, courtesy of M. Hines/Cornell University

Note that acid etching does not produce the pyramidal structures like KOH etching on monocrystalline silicon.  Below is an image comparing different texturing methods.  It is reproduced from this article, which seems to have been taken straight from this document.  I highly recommend the latter for anyone interested in more details.

Image of different textures on silicon.  The alkaline KOH etch is on monocrystalline silicon, whereas the acidic etch is on polysilicon.


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