Whispering-galleries-for-light
Illustration shows the nanoresonator coating, consisting of thousands of tiny glass beads, deposited on solar cells. The coating enhances both the absorption of sunlight and the amount of current produced by the solar cells /K. Dill, D. Ha, G. Holland/NIST

Trapping light with an optical version of a whispering gallery, researchers at the National Institute of Standards and Technology (NIST) have developed a nanoscale coating for solar cells that enables them to absorb about 20 percent more sunlight than uncoated devices.

The coating, applied with a technique that could be incorporated into manufacturing, opens a new path for developing low-cost, high-efficiency solar cells with abundant, renewable and environmentally friendly materials.

Whispering galleries for light were developed about a decade ago, but researchers have only recently explored their use in solar-cell coatings. In the experimental set up devised by a team including Dongheon Ha of NIST and the University of Maryland’s NanoCenter, the light captured by the nanoresonator coating eventually leaks out and is absorbed by an underlying solar cell made of gallium arsenide.

Using a laser as a light source to excite individual nanoresonators in the coating, the team found that the coated solar cells absorbed, on average, 20 percent more visible light than bare cells. The measurements also revealed that the coated cells produced about 20 percent more current.

The team also devised a rapid, less-costly method of applying the nanoresonator coating. Researchers had previously coated semiconductor material by dipping it in a tub of the nanoresonator solution. The dipping method takes time and coats both sides of the semiconductor even though only one side requires the treatment.

In the team’s method, droplets of the nanoresonator solution are placed on just one side of the solar cell. A wire-wound metal rod is then pulled across the cell, spreading out the solution and forming a coating made of closely packed nanoresonators.

This is the first time that researchers have applied the rod method, used for more than a century to coat material in a factory setting, to a gallium arsenide solar cell.

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