Flexible solar cells soak up the sun


Above, a photomicrograph of a silicon wire array embedded within a transparent, flexible polymer film. “Light comes into each wire, and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires makes the array very absorbing,” says Harry Atwater. (Credit: Caltech/Michael Kelzenberg)

CALTECH (US)—Scientists have created a new type of flexible solar cell that enhances the absorption of sunlight and efficiently converts its photons into electrons. The solar cell does all this using only a fraction of the expensive semiconductor materials required by conventional models.

The new solar cells, designed by a team at the California Institute of Technology (Caltech), uses arrays of long, thin silicon wires embedded in a polymer substrate.

“These solar cells have, for the first time, surpassed the conventional light-trapping limit for absorbing materials,” says Harry Atwater, Howard Hughes Professor, professor of applied physics and materials science, and director of Caltech’s Resnick Institute, which focuses on sustainability research.

The light-trapping limit of a material refers to how much sunlight it is able to absorb. The silicon-wire arrays absorb up to 96 percent of incident sunlight at a single wavelength and 85 percent of total collectible sunlight. “We’ve surpassed previous optical microstructures developed to trap light,” he says.

Atwater and his colleagues—including Nathan Lewis, the George L. Argyros Professor and professor of chemistry and graduate student Michael Kelzenberg—assessed the performance of these arrays in a paper appearing in the online edition of the journal Nature Materials.

Atwater notes that the solar cells’ enhanced absorption is “useful absorption.”

“Many materials can absorb light quite well but not generate electricity—like, for instance, black paint,” he explains. “What’s most important in a solar cell is whether that absorption leads to the creation of charge carriers.”

The silicon wire arrays created by Atwater and his colleagues are able to convert between 90 and 100 percent of the photons they absorb into electrons—in technical terms, the wires have a near-perfect internal quantum efficiency. “High absorption plus good conversion makes for a high-quality solar cell,” says Atwater. “It’s an important advance.”

The key to the success of these solar cells is their silicon wires, each of which, says Atwater, “is independently a high-efficiency, high-quality solar cell.” When brought together in an array, however, they’re even more effective, because they interact to increase the cell’s ability to absorb light.

“Light comes into each wire, and a portion is absorbed and another portion scatters. The collective scattering interactions between the wires makes the array very absorbing,” he says.

This effect occurs despite the sparseness of the wires in the array—they cover only between 2 and 10 percent of the cell’s surface area.

“When we first considered silicon wire-array solar cells, we assumed that sunlight would be wasted on the space between wires,” explains Kelzenberg. “So our initial plan was to grow the wires as close together as possible. But when we started quantifying their absorption, we realized that more light could be absorbed than predicted by the wire-packing fraction alone. By developing light-trapping techniques for relatively sparse wire arrays, not only did we achieve suitable absorption, we also demonstrated effective optical concentration—an exciting prospect for further enhancing the efficiency of silicon-wire-array solar cells.”

Each wire measures between 30 and 100 microns in length and only 1 micron in diameter. “The entire thickness of the array is the length of the wire,” notes Atwater. “But in terms of area or volume, just 2 percent of it is silicon, and 98 percent is polymer.”

In other words, while these arrays have the thickness of a conventional crystalline solar cell, their volume is equivalent to that of a two-micron-thick film.

Since the silicon material is an expensive component of a conventional solar cell, a cell that requires just one-fiftieth of the amount of this semiconductor will be much cheaper to produce.

The composite nature of these solar cells, Atwater adds, means that they are also flexible. “Having these be complete flexible sheets of material ends up being important,” he says, “because flexible thin films can be manufactured in a roll-to-roll process, an inherently lower-cost process than one that involves brittle wafers, like those used to make conventional solar cells.”

Atwater, Lewis, and their colleagues had earlier demonstrated that it was possible to create these innovative solar cells. “They were visually striking,” says Atwater. “But it wasn’t until now that we could show that they are both highly efficient at carrier collection and highly absorbing.”

The next steps, Atwater says, are to increase the operating voltage and the overall size of the solar cell. “The structures we’ve made are square centimeters in size,” he explains. “We’re now scaling up to make cells that will be hundreds of square centimeters—the size of a normal cell.”

Atwater says that the team is already “on its way” to showing that large-area cells work just as well as these smaller versions.

The research was supported by BP and the Energy Frontier Research Center program of the Department of Energy, and made use of facilities supported by the Center for Science and Engineering of Materials, a National Science Foundation Materials Research Science and Engineering Center at Caltech. In addition, Shannon Boettcher, a postdoctoral scholar involved with the work, received fellowship support from the Kavli Nanoscience Institute at Caltech.

Caltech news: http://media.caltech.edu/

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  1. UofRgrad

    This is very exciting research! It’s such a shame the general population doesn’t understand the importance of funding science and engineering. The concern I have is how much BP gets to benefit from (or squelch) commercial use of this technology.

  2. robert reed

    I am a builder in Seattle trying to find funding to build a Net Zero home….in Seattle, that will require a 9.6kw array of PV panels, almost 1000sq ft. The price for the solar panels themselves at right now prices is $27,360.00, $2.85 a watt. the total system costs around $60k. It’s a non starter for clients right now.
    It kills the deal dead.
    This is technology that can actually spur growth in the renewable energy sector.
    I personally don’t care if its funded by BP. The oil guys have the money. Good for them.
    Does anybody think that if the whole world goes solar, the oil guys will be poor then?
    How much more carbon do we have to blow off before we get technology like this into the marketplace?

  3. Leif Hartvig-Hansen

    Good with your development.
    I am working a little with a “green” transport system and would like how much power in watt / m^2 you expect will come out your develoipment.
    What will the price be aprowimately / m^2

    I am Danish Engineer and Master shipbuilder and come in New Zealand for do the metal boat/ship building to a more proffesional level and bette quality, as there was some problems with the standard done here.
    But I have not tuch a vessel here for now 14 years. In stead for the yards here had a “door open” was it closed, as they was afraid for it come around the world what real is done here ?????
    In Europe and USA is the thinking much different. Many members in the Marine and Boat org. is not educated in the marine branch. Here is f.ex. also serve contractors and plumbers member, but other proffesional is kept out ?????
    Most of my jobs here in NZ have so been in the trucks transport section, as I not know about ??? But few times have I been overseas and done jobs in my real branch.
    I have meet 3 here i high position, where two had HR functions, as not know what a Ship was, but only about a large Boat. ?????
    Sincerely Yours
    Leif Hartvig-Hansen
    Naval Prod. Tech. / Master Shipbuilder
    Diploma Mech. Eng.
    Diploma Works Eng.
    Certificate Ships Hydraulic Eng.
    1 Class Yacht Master

    6 A Cussen St.
    Hamilton 3210
    New Zealand
    E-mail: siamoana@hotmail.com

  4. Leif Hartwig-Hansen

    I have a little more to add to above story here from New Zealand.
    At a super yacht yard in Auckland was I refused a job, when they hear I I had been in a deaf school as kid. They was afraid for I maybe not could hear a firer alarm ?????
    But “shit” for them, as I later on time I was for visiting, as this yacht Yard through RINA found serious fault so this super yacht was dangerous to sail with and it was reported to US Coastguard, as did a note.
    At another yard was they afraid for I with my dialect not could say “be carefull” to somebody so they could understand it ?????
    I have worked at 8 shipyard in 4 countries without any problems.

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