Scientists Find Strange New Effect for Future Solar Cells: Flexo-photovoltaics

Poking something sharp into a plain silicon crystal turns it into a solar cell that might shoot past today’s efficiency limits

2 min read
Artist's impression of squeezing more power out of solar cells by physically deforming each of the crystals in the semiconductors used by photovoltaic cells.
Illustration: Mark Garlick/University of Warwick

Scientists at the University of Warwick report that they’ve discovered a new kind of photovoltaic effect. What they dub “flexo-photovoltaics” is really the realization that ordinary crystals—including silicon—and other materials can be made to exhibit a long-known but underutilized type of energy conversion. The key is to poke the material—hard—and with something sharp.

Today’s solar cells are typically made from silicon, into which a built-in electric field has been engineered. That field comes from a p-n junction, the spot where a region with excess positive charge carriers (holes) meets a region with excess negative charge carriers (electrons). When a photon is absorbed, it becomes an electron and a hole. Because of the p-n junction, the pair is naturally separated to produce a voltage. Such solar cells have an inherent ceiling to the efficiency they can possibly reach. Called the Shockley-Queisser limit, it slams the door on efficiencies higher than 33.7 percent.

University of Warwick professor Marin Alexe. Marin Alexe, a professor at the Univerity of Warwick, recently described a new kind of photovoltaic effect. Photo: University of Warwick

But a different effect has no such limit. Called the bulk photovoltaic effect, it occurs in only materials whose crystal structure lacks what’s called centrosymmetry, explains Marin Alexe, the physics professor who led the research at Warwick in the U.K. Having centrosymmetry means that you can rotate a crystal’s unit structure around the center and wind up with the same structure. Materials that lack centrosymmetry, such as barium titanate, can display the bulk photovoltaic effect—you can get some current out despite the lack of a p-n junction—but they don’t make good solar cells for other reasons.

Alexe, along with his student Ming-Min Yang and postdoctoral researcher Dong Jik Kim, set out to see if they could cause a centrosymmetric material that’s pretty good as a solar cell—silicon, for example—to exhibit bulk photovoltaic effect.

They did this through the microscopic version of brute force: They mashed an atomic-force-microscope tip into the crystal. The result was a strain in the crystal so severe that it was no longer centrosymmetric, and “that automatically kicks in the alternative photovoltaic effect,” says Alexe. The effect should work in many types of crystal; they tested strontium titanate, titanium oxide, and silicon. And even better: “This effect has no thermodynamic limits, because it’s not p-n junction–based,” he says.

But was it more efficient than an ordinary solar cell? “We cannot say anything about efficiency,” says Alexe. Those experiments will have to come once they’ve more fully characterized the effect. “What we can actually say is that nothing prevents us, principally, to use both effects in the same device.”

Alexe imagines an array of microspikes pressed atop a conventional silicon solar cell. “That’s the easiest way; not necessarily cheapest way or smartest way,” says Alexe. Another solution might be to engineer strain-inducing defects into the silicon. (The type of engineered strain used to speed transistors in microprocessors doesn't work for this effect.) “This is a completely new range of research, which can be opened—engineering this type of effect,” he says.

Alexe and his colleagues reported their results last week in Science Advances.

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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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