New solar cell is world's most efficient

Solar cells efficiencies typically range between 15 and 20 percent, but the Energy Department has just developed a new cell that converts 31.1 percent of the sunlight that strikes it into useful electricity. That is a world record, representing the most efficient solar cell ever produced for a two-layer cell design under normal lighting conditions.

Created at the National Renewable Energy Lab run by the Energy Department, the new solar cell technology was designed under their Foundation Program to Advance Cell Efficiency (F-PACE).

This solar cell is a two-junction design, manufactured with a gallium indium phosphide cell layered on top of one of gallium arsenide. The front of the device is covered in two layers of anti-reflection coating, and it has a back made of gold. It was grown upside-down and turned over during processing.

One of the troubles with current designs is that many of the photons used to generate electricity are lost to the material surrounding the target molecules. The second layer both allows a energy-producing backdrop, as well as captures some photons of light at different wavelengths. This increases both the efficiency and voltage in a form of photon recycling call luminescent coupling. The gold back reflects photons back into the cell, further increasing usage of the light.

"Historically, scientists have bumped up the performance of multijunction cells by gradually improving the material quality and the internal electrical properties of the junctions - and by optimizing variables such as the bandgaps and the layer thicknesses. The scientific goal of this project is to understand and harness the internal optics," Myles Steiner, NREL researcher, said.

The whole device is only 1/32 of a square inch in area, and was tested measured under a light intensity of 317 watts per square foot, about three times that of a typical sunny day.

Currently, lenses are being used to increase the effect of sunlight on the cells, bringing older solar cells to efficiencies around 30 percent. Increasing the realtive output of the cell through the use of a lens would break its own world record. 

"We expect to observe similar enhancements of the solar cell characteristics when measured under concentrated illumination," Steiner said.

The ultimate goal of the F-PACE project is the development of a solar cell achieving 48 percent efficiency. In 2010, the record for a gallium-arsenide cell was 26.4 percent, which grew ro 28.8 percent two years later. The previous record, set by Alta Devices, was 30.8 percent, achieved in April, 2013.

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