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Question : Why are solar cells usually dark blue?

Solar cells are usually dark blue. The most solar cells are poly-crystalline silicon, but this silicon is grey. The blue color is caused by a special anti-reflection layer coated on the silicon.

Every light particle that is not absorbed by the solar cell, but is reflected on the surface of the solar cell, is lost. Reflected light particles can not be converted into electricity. By adding a special anti-reflection layer, the reflection is decreased. And the color of this special layer is dark blue. That's why most photovoltaic solar cells colour blue.

It is possible to make solar cells in all colours, and it is done! Architects use red, green, yellow,... solar cells to integrate in a building. But their is a price for wanting another colour. The efficiency of the photovoltaic solar panels is lower, because the reflection is the lowest with dark blue cells.

Answer: because of the dark blue anti-reflection layer.