Transparent coating cools solar cells while boosting efficiency

September 22, 2015 // By Paul Buckley
Stanford university engineers have invented a transparent material that claims to improve the efficiency of solar cells by radiating thermal energy (heat) into space.

Three Stanford engineers have developed a technology that improves on solar panel performance by shunting 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.

The work by Shanhui Fan, a professor of electrical engineering at Stanford, research associate Aaswath P. Raman and doctoral candidate Linxiao Zhu is described in the current issue of Proceedings of the National Academy of Sciences.

The group's discovery, tested on a Stanford rooftop, addresses a problem that has long bedeviled the solar industry: The hotter solar cells get, the less efficient they become at converting the photons in light into useful electricity.

The Stanford solution is based on 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.

"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."

In 2014, the same trio of inventors developed an ultrathin material that radiated infrared heat directly back toward space without warming the atmosphere. They presented that work in Nature, describing it as 'radiative cooling' because it shunted thermal energy directly into the deep, cold void of space.

In their new paper, the researchers applied that work to improve solar array performance when the sun is beating down.

The Stanford team tested their technology on 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. Their 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.