Scientists may have overcome one of the major hurdles in developing high-efficiency, long-lasting solar cells -- keeping them cool, even in the blistering heat of the noonday sun.
By adding a specially patterned layer of silica glass to the surface of ordinary solar cells, a team of researchers led by Shanhui Fan, an electrical engineering professor at Stanford University in California has found a way to let solar cells cool themselves by shepherding away unwanted thermal radiation. The researchers describe their innovative design in the premiere issue of The Optical Society's (OSA) new open-access journal Optica.
Solar cells are among the most promising and widely used renewable energy technologies on the market today. Though readily available and easily manufactured, even the best designs convert only a fraction of the energy they receive from the sun into usable electricity.
Part of this loss is the unavoidable consequence of converting sunlight into electricity. A surprisingly vexing amount, however, is due to solar cells overheating.
Under normal operating conditions, solar cells can easily reach temperatures of 130 degrees Fahrenheit (55 degrees Celsius) or more. These harsh conditions quickly sap efficiency and can markedly shorten the lifespan of a solar cell. Actively cooling solar cells, however -- either by ventilation or coolants -- would be prohibitively expensive and at odds with the need to optimize exposure to the sun.
The newly proposed design avoids these problems by taking a more elegant, passive approach to cooling. By embedding tiny pyramid- and cone-shaped structures on an incredibly thin layer of silica glass, the researchers found a way of redirecting unwanted heat -- in the form of infrared radiation -- from the surface of solar cells, through the atmosphere, and back into space.
"Our new approach can lower the operating temperature of solar cells passively, improving energy conversion efficiency significantly and increasing the life expectancy of solar cells," said Linxiao Zhu, a physicist at Stanford and lead author on the Optica paper. "These two benefits should enable the continued success and adoption of solar cell technology."
Solar cells work by directly converting the sun's rays into electrical energy. As photons of light pass into the semiconductor regions of the solar cells, they knock off electrons from the atoms, allowing electricity to flow freely, creating a current. The most successful and widely used designs, silicon semiconductors, however, convert less than 30 percent of the energy they receive from the sun into electricity -- even at peak efficiency.
The solar energy that is not converted generates waste heat, which inexorably lessens a solar cell's performance. For every one-degree Celsius (1.8 degree F) increase in temperature, the efficiency of a solar cell declines by about half a percent.
"That decline is very significant," said Aaswath Raman, a postdoctoral scholar at Stanford and co-author on the paper. "The solar cell industry invests significant amounts of capital to generate improvements in efficiency. Our method of carefully altering the layers that cover and enclose the solar cell can improve the efficiency of any underlying solar cell. This makes the design particularly relevant and important."
In addition, solar cells "age" more rapidly when their temperatures increase, with the rate of aging doubling for every increase of 18 degrees Fahrenheit.
To passively cool the solar cells, allowing them to give off excess heat without spending energy doing so, requires exploiting the basic properties of light as well as a special infrared "window" through Earth's atmosphere.
Different wavelengths of light interact with solar cells in very different ways -- with visible light being the most efficient at generating electricity while infrared is more efficient at carrying heat. Different wavelengths also bend and refract differently, depending on the type and shape of the material they pass through.
The researchers harnessed these basic principles to allow visible light to pass through the added silica layer unimpeded while enhancing the amount of energy that is able to be carried away from the solar cells at thermal wavelengths.
"Silica is transparent to visible light, but it is also possible to fine-tune how it bends and refracts light of very specific wavelengths," said Fan, who is the corresponding author on the Optica paper. "A carefully designed layer of silica would not degrade the performance of the solar cell but it would enhance radiation at the predetermined thermal wavelengths to send the solar cell's heat away more effectively."
To test their idea, the researchers compared two different silica covering designs: one a flat surface approximately 5 millimeters thick and the other a thinner layer covered with pyramids and micro-cones just a few microns (one-thousandth of a millimeter) thick in any dimension. The size of these features was essential. By precisely controlling the width and height of the pyramids and micro-cones, they could be tuned to refract and redirect only the unwanted infrared wavelengths away from the solar cell and back out into space.
"The goal was to lower the operating temperature of the solar cell while maintaining its solar absorption," said Fan. "We were quite pleased to see that while the flat layer of silica provided some passive cooling, the patterned layer of silica considerably outperforms the 5 mm-thick uniform silica design, and has nearly identical performance as the ideal scheme."
Zhu and his colleagues are currently fabricating these devices and performing experimental tests on their design. Their next step is to demonstrate radiative cooling of solar cells in an outdoor environment. "We think that this work addresses an important technological problem in the operation and optimization of solar cells," he concluded, "and thus has substantial commercialization potential."
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