JUNE 2016 IN

Infrared Solar Cells

When you think of solar power, you probably think of the conventional solar cells that use photons of visible light to move electrons across a gap, generating an electric current. But only half of the energy in sunlight is visible light. Most of the rest is infrared, which we typically think of as the heat of the sun rather than as light.

Could solar cells pick up energy from infrared photons too? Engineers have been working on dozens of possible approaches to make this work. Perhaps the most intriguing is the transparent solar panel that could be built into the sun-facing windows of a building. These windows let between 50 and 90 percent of visible light through, similar to an ordinary tinted office window, while capturing solar energy in proportions that, in the best cases, compare to those of traditional solar cells. Solar car windows could provide a small fraction of the power needed to operate a car.

But it’s more than just windows. A clear infrared solar panel could be placed directly over an existing solar panel to provide a combined power generation that in optimistic estimates might be 30 percent higher. The higher conversion rates are especially important in cities, where electric use is high and the areas where solar panels can be placed are limited.

Another possibility is that a clear coating over a solar panel could convert many of the infrared photons into visible light photons that the solar panel could capture. In a process called upconverting, two infrared photons are combined to form a single green or blue photon. Upconversion coatings have been demonstrated in the laboratory, and prototypes can’t be far behind. There are concerns over the durability of the coatings, but since it is only a coating and not part of the electrical functioning of a solar panel, it could be reapplied halfway through the solar panel’s life or even added to the glass surface of an old solar panel. There is reason to hope that such a coating might be inexpensive to manufacture.

Hybrid solar cell designs are designed with multiple photovoltaic layers, each layer concentrating on a narrow bandwidth of light. Typically the top layer captures near infrared photons. In this approach the layers have to be so thin that they miss some photons, but by combining several layers the total electric production can be higher than with a single layer. As you might guess, manufacturing is a challenge, but one that will surely be solved in a matter of years. A completely different approach uses black carbon to capture infrared photons only. This design might be attractive if it can be made very cheaply — a possibility, since carbon is such an easy material to work with.

It is easy to imagine more distant applications of infrared solar cells. They have efficiency advantages in hot, cloudy conditions — the planet Venus comes to mind, for example. Similarly, they would be needed to power stations in orbit around brown dwarf stars, which don’t produce much visible light. Closer to the present, perhaps they can be used as part of cooling systems for machinery, in furnaces or ovens or in nuclear power plants, recapturing some of the waste heat as electricity. Infrared is just one of 20 or 30 ways solar cells might evolve in the coming decade.

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