Why are solar panels blue?

Some solar panels are blue, while others are black. What causes this color difference?

If you’ve noticed a neighbor’s roof with solar panels, or just seen pictures of panels, you might notice that they’re very often distinctly blue in color. Why is that?

The color of solar panels depends on the type of silicon, the type and thickness of anti-reflective coatings, and the surface texture of the silicon cell. In general, polycrystalline (also known as multicrystalline) solar panels are usually blue, while monocrystalline panels are black. While rarely used, a full range of color options is possible with technologies that color the cells directly or apply dye to the glass overlay.

Blue and black polycrystalline and monocrystalline solar cells. (Wikimedia) Blue and black polycrystalline and monocrystalline solar cells. [credit]

I just threw out a few terms that you might not be familiar with, so here’s an indepth explanation.

What are polycrystalline and monocrystalline solar panels?

Most solar panels are made with silicon, which is a non-toxic element. Beach and desert sand is mostly made of silicon, and so is glass.

Silicon for solar cells is refined to eliminate impurities, and then grown into large blocks or ingots. Those large blocks are cut with a saw into smaller bricks. Finally, the bricks are sliced with a wire saw - much like you would slice a block of cheese - into thin wafers that make up solar cells.

Monocrystalline solar cells

The more expensive type of silicon solar cells is called monocrystalline. The manufacturing process involves forming a large ingot that is a single crystal of silicon. Because there are no grain boundaries to interfere with the movement of electrons, monocrystalline cells are more efficient that polycrystalline.

Monocrystalline solar cells are black and have an even texture due to the lack of grain boundaries. They’re also more expensive because of the more complicated manufacturing process.

Polycrystalline solar cells

Polycrystalline solar panels are usually blue.

Manufacturing polycrystalline silicon, which is also called multicrystalline, is a less complicated and expensive process. To make polycrystalline silicon, purified silicon is melted in a large crucible and slowly cooled. If you’ve ever done an experiment with your kids where you grow sugar or salt crystals by evaporation, you’ll be familiar with the type of geometry that results in a collection of small crystals.

While forming polycrystalline silicon doesn’t involve evaporation, it does create a material composed of small crystals. Those crystals give polycrystalline solar cells a flakey, iridescent appearance that comes from the visible grain boundaries. That iridescence tends to emphase the blue color of the cells.

Those grain boundaries also impede the movement of electrons, which is one of the reasons that polycrystalline solar panels tend to have a lower efficiency rating. On the plus side, they’re usually less expensive.

Which is better: monocrystalline or polycrystalline?

Because there is a tradeoff between price and efficiency, neither type of silicon is inherently better. Sometimes it makes sense to go with the lowest price. In another case, maybe you have a small roof and need the highest efficiency panels you can get.

One thing to know is that monocrystalline has steadily grown in popularity, to the point where it now is 90% of the home solar market:

Market share of mono and poly solar cells (courtesy Berkeley Lab) Market share of mono and poly solar cells (courtesy Berkeley Lab).

This change in market share is why you might notice that older solar installations tend to have blue panels, while newer ones are black.

The choice between mono and poly is one of the key decisions for a solar panel buyer, so I wrote an in-depth article about mono vs poly.

Other factors that affect the appearance of a solar panel

While the biggest thing that determines the color and general appearance of a solar panel is whether it uses blue polycrystalline or black monocrystalline, there other factors too.

Anti-reflective coatings

A solar cell works by absorbing specific frequencies of light and using that energy to move electrons. Because of this, you want a solar cell to receive as much light as possible.

It turns out that even a solar cell that appears black reflects some of the light away - as much as 30%. To reduce this reflection, anti-reflective coatings are used.

If you’ve seen eyeglasses or windows with anti-reflective coatings, you might have noticed that these coatings can cause the glass to have a slight tint, because not all wavelengths of light are affected equally by the coating. In the same way, the use of AR coatings on a solar cell can affect the color of the cell.

Different types of coatings used for solar cells include silicon nitride (Si₃N₄) and titanium dioxide (TiO₂). Titanium dioxide is intrinsically blue, even though you may be most familiar with its white color when used in sunscreens. Silicon nitride coatings also have a blueish cast.

In addition to the chemistry, the thickness of the AR coating will affect the color.

Surface texturing

Another way that reflection can be reduced is through the use of surface texturing. By making the surface of a cell rough, less light will be reflected. This can also affect the color of the cell.

Other factors that affect the appearance of a solar panel

The color of the solar cells isn’t the only thing that determines the overall appearance of a solar panel. Here are some other factors that matter:

Busbars and fingers

If you look at the image at the top of this article that compares the appearance of mono and poly, you’ll notice that one of the cells has wires that are obviously visible.

The larger pair of wires are called busbars and the thinner pair of perpendicular wires are called fingers.

In this example, only the polycrystalline panel has wires visible, but don’t be mislead by that. Most mono panels also have visible fingers and busbars. It’s the technical design of the panel that determines how visible the wires are.

The current trend is to increase the number of busbars, because having wires closer together reduces the electrical resistance. But having more wires detracts from the nice homogeneous black color of a monocrystalline cell, so this might not be desirable from an aesthetic point of view.

Manufacturers have taken different approaches to busbars, including using round rather than flat wires, and even eliminating the front wiring in favor of interconnections on the rear, as Sunpower does with their Maxeon cells.

There are tradeoffs to all these approaches, but I think it’s safe to say that the smoother appearance of a busbar-free cell is more appealing.

Colored solar panels

It’s even possible for conventional silicon solar panels to come in any color under the rainbow. This is definitely a niche market, and there are only a few companies doing this. These include Kameleon Solar and Colored Solar.

There are different technologies used to achieve different colors, including different surface coatings and dyeing the glass covering.

For example, these different colors in the cells below are achieved only by modifying the thickness of the anti-reflective coating:

Example of different colored solar cells (courtesy PVEducation Example of different colored solar cells (courtesy PVEducation).

So far, there isn’t a large market for this type of product, but if you’re a homeowner with a colored metal roof, this could be an attractive option for you.

More about attractive solar panels

For people who care about the appearance of their solar panels, there’s a growing selection of products. These include solar shingles that look just like normal roofing products, and all-black conventional panels that blend nicely into a roof with dark shingles. To learn more, you can read my article about solar shingles.

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