Four-Junction Solar Cell Claims Efficiency Record

Moves closer to theoretical limit

2 min read
Four-Junction Solar Cell Claims Efficiency Record
Photo: Rob Largent/UNSW

Researchers in Australia have built a solar cell with light conversion efficiency far in excess of commercial solar cells.

Mark Keevers and Martin Green of the University of New South Wales say their four-junction mini-module breaks the world record for efficiency for that type of solar cell. The cell converted 34.5 percent of the sunlight striking it into electricity, the two said at a press conference on Tuesday. That’s well on the way to the 53-percent theoretical maximum efficiency of such a device. Most commercially available silicon photovoltaics—the types found on rooftops and in solar farms—top out at around 16 to 18 percent efficiency.

The researchers have not published a peer-reviewed paper on their results, but the efficiency was certified by the National Renewable Energy Laboratory.

The device consists of a triple-junction cell comprising three semiconductor materials: indium-gallium-phosphide, indium-gallium-arsenide, and germanium. Each of those absorbs different wavelengths of sunlight. Additional wavelengths are reflected toward a fourth layer made of silicon. The overall effect is that, because the cell can handle a much wider range of wavelengths than is possible with a silicon cell alone, more of the sunlight striking the cell is converted to electricity.

Because the cell is shaped like a prism, says Green, who is director of the Australian Centre for Advanced Photovoltaics at UNSW, it passively gathers light coming from a wide range of angles. The prism also guides the light from the triple-junction cell on one side to the silicon on the other, Martin said in an email.

The device will likely cost more to make than conventional solar cells, both because of the complexity of the design and the cost of the materials. But Green says the manufacturing cost may be offset by increased efficiency. He adds that he would like to use this design with lower cost materials such as copper-zinc-tin-sulfide and perovskites—a class of materials increasingly seen as well suited for photovoltaics.

Green and Keevers, a senior researcher at the center, are aiming for a cell with a conversion efficiency of 40 percent. They’d already achieved that with a cell that used a lens to concentrate sunlight, but this design would not require a lens.

Many people like to claim world records for solar cell efficiency. In March, First Solar, of Tempe, Ariz., claimed a record for cadmium-telluride solar cells when it achieved an efficiency of 22.1 percent. Alta Devices, of Sunnyvale, Calif., which makes solar cells for unmanned aerial vehicles, announced in April that a dual-junction device made of gallium-arsenide and indium-gallium-phosphide had reached an efficiency of 31.6 percent. And a collaboration between Soitec, a French semiconductor material company; CEA, a French government research agency; and the Fraunhofer Institute for Solar Energy Systems in Germany, announced a cell with 46- percent efficiency in December 2014. The cell they produced—a four-junction cell that uses gallium, indium, and germanium and relies a concentrating lens—is very expensive and is mainly sold to NASA for applications in space.


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Smokey the AI

Smart image analysis algorithms, fed by cameras carried by drones and ground vehicles, can help power companies prevent forest fires

7 min read
Smokey the AI

The 2021 Dixie Fire in northern California is suspected of being caused by Pacific Gas & Electric's equipment. The fire is the second-largest in California history.

Robyn Beck/AFP/Getty Images

The 2020 fire season in the United States was the worst in at least 70 years, with some 4 million hectares burned on the west coast alone. These West Coast fires killed at least 37 people, destroyed hundreds of structures, caused nearly US $20 billion in damage, and filled the air with smoke that threatened the health of millions of people. And this was on top of a 2018 fire season that burned more than 700,000 hectares of land in California, and a 2019-to-2020 wildfire season in Australia that torched nearly 18 million hectares.

While some of these fires started from human carelessness—or arson—far too many were sparked and spread by the electrical power infrastructure and power lines. The California Department of Forestry and Fire Protection (Cal Fire) calculates that nearly 100,000 burned hectares of those 2018 California fires were the fault of the electric power infrastructure, including the devastating Camp Fire, which wiped out most of the town of Paradise. And in July of this year, Pacific Gas & Electric indicated that blown fuses on one of its utility poles may have sparked the Dixie Fire, which burned nearly 400,000 hectares.

Until these recent disasters, most people, even those living in vulnerable areas, didn't give much thought to the fire risk from the electrical infrastructure. Power companies trim trees and inspect lines on a regular—if not particularly frequent—basis.

However, the frequency of these inspections has changed little over the years, even though climate change is causing drier and hotter weather conditions that lead up to more intense wildfires. In addition, many key electrical components are beyond their shelf lives, including insulators, transformers, arrestors, and splices that are more than 40 years old. Many transmission towers, most built for a 40-year lifespan, are entering their final decade.

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