A trio of Stanford University engineers has developed a transparent coating designed to improve the efficiency of solar cells by radiating heat into space.

Their invention shunts away the heat generated by a solar cell under sunlight and cools it in a way that allows it to convert more photons into electricity, according to a research announcement.

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The Stanford University team addressed this problem: The hotter solar cells become, the less efficient they are at converting the photons in light into useful electricity.

The study is described in the journal Proceedings of the National Academy of Sciences.

Problem and Solution

The discovery, announced Monday (Sept. 21), addresses a problem that has long plagued the solar industry, the university reported.

The hotter solar cells become, the less efficient they are at converting the photons in light into useful electricity.

So, Shanhui Fan, professor of electrical engineering, research associate Aaswath P Raman and doctoral candidate Linxiao Zhu, tackled the problem with a thin, patterned silica material laid on top of a traditional solar cell.

The material is transparent to the visible sunlight that powers solar cells, but captures and emits thermal radiation, or heat, from infrared rays, the team said.

Shanhui Fan / Stanford University

The transparent material improves efficiency of solar cells by radiating thermal energy into space.

"Solar arrays must face the sun to function, even though that heat is detrimental to efficiency," Fan said.

"Our thermal overlay allows sunlight to pass through, preserving or even enhancing sunlight absorption, but it also cools the cell by radiating the heat out and improving the cell efficiency."

Applications

The engineers said the new transparent thermal overlays work best in dry, clear environments, which are also preferred sites for large solar arrays.

The team said it is also possible to scale things up for commercial and industrial applications. They suggested using nanoprint lithography, which is a common technique for producing nanometer-scale patterns.

Further, Zhu said the technology has significant potential for any outdoor device or system that demands cooling but requires the preservation of the visible spectrum of sunlight for either practical or aesthetic reasons.

Testing Details

To test the technology, the team said it used a custom-made solar absorber—a device that mimics the properties of a solar cell without producing electricity—covered with a micron-scale pattern designed to maximize the capability to dump heat, in the form of infrared light, into space.

Norbert von der Groeben / Stanford University

(From left) Doctoral candidate Linxiao Zhu, Professor Shanhui Fan and research associate Aaswath Raman are members of the team. The engineers are shown here with a previous technology.

The experiments showed that the overlay allowed visible light to pass through to the solar cells, but that it also cooled the underlying absorber by as much as 23 degrees Fahrenheit.

For a typical crystalline silicon solar cell with an efficiency of 20 percent, 23 F of cooling would improve absolute cell efficiency by over 1 percent, a figure that represents a significant gain in energy production, the team reported.

Previous Research

The same trio of engineers was responsible for developing an ultrathin material that radiated infrared heat directly back toward space without warming the atmosphere in 2014.

The team presented that work in the journal Nature, describing it as "radiative cooling" because it shunted thermal energy directly into the deep, cold void of space.