February 16, 2010
Caltech Scientists Hit New Solar Conversion Efficiency

Caltech scientists used thin silicon wires to achieve a previous unheard of level of efficiency in converting photons into electrical flow.

PASADENA, Calif.—Using arrays of long, thin silicon wires embedded in a polymer substrate, a team of scientists from the California Institute of Technology (Caltech) has 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 solar cells.

"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.

This is an amazing accomplishment. At a competitive price such high efficiency in photovoltaic material would allow a much smaller footprint of land area to supply a very large fraction of all the energy we use. Now, if they can manage to do this cheaply it'll be a game changer. See below for reasons why this approach has big cost reduction potential.

They don't waste precious photons.

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.

Is a low price possible? They use far less silicon than in a conventional silicon PV cell. Plus, the flexibility of the material lends itself to lower cost manufacturing techniques.

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."

Technology Review has a very good write-up on this advance.

Share |      Randall Parker, 2010 February 16 11:18 PM  Energy Solar

PacRim Jim said at February 17, 2010 2:49 AM:

With efficiency improving almost monthly, how could a manufacturer invest in a technology, knowing it would soon be obsolete?

John said at February 17, 2010 5:02 AM:

Well this sounds amazing. Almost perfect efficiency, just with (a little!) silicon and plastic? None of the rare earth elements (Selenium, Indium, etc.) that make thin-films an iffy bet at scale?

The efficiency implies something like an 8X to 9X advantage in output over current commercial PV, and the materials used suggest much lower costs than the current CIGS heroes (who are supposed to be at $.80 per watt now, right?) can do.

If anything like this pans out, it's a revolution. Even cars, I'd think, could get a reasonable chunk of their power from 90% efficient cells on the roof and hood (at least for the drive home from work).

Dan said at February 17, 2010 8:00 AM:

I wonder how efficient these are as solar cells? i.e. They are efficient absorbers of incident light, but how much of that light gets converted into useful electricity vs going to heat the material; with such short nanowires, they must incur significant loss at the interconnect/grain boundary (or just be really small samples). In addition to the efficiency improvements from being a lower-cost cell, an ultra-high efficiency cell would also substantially reduce balance-of-plant (installation, shipping, etc) costs which are now the major expense in residential installation.

BTW: Silicon is quite inexpensive ... damn-near free. The silicon in a conventional solar cell is expensive due to the preprocessing required to produce ingots of large geomety and near-perfect purity. The manufacturing techniques required for a silicon nanowire (that I'm aware of) start from powdered pure silicon which is FAR cheaper and easier to manufacture than silicon wafers.


Nick G said at February 17, 2010 11:18 AM:

This seems way too good to be true, and yet I don't see any reason to doubt it.


How much investor money is showing up at their doorsteps??

Yes, this could be used to provide a very meaningful % of power for all manner of transportation: cars, trains, planes, ships...

James Bowery said at February 17, 2010 1:05 PM:

What are the manufacturing barriers?

PacRim Jim asks With efficiency improving almost monthly, how could a manufacturer invest in a technology, knowing it would soon be obsolete?

By knowing it wouldn't soon be obsolete.

Nick G said at February 17, 2010 4:53 PM:

They don't seem to expect to develop PV cells with 85% efficiency any time soon. They seem to be aiming for the roughly 20% that the best commercial cells get:

""Our goal is to make a thin-film [solar] cell that gets you the efficiency of a regular wafer-based solar cell," Kelzenberg says. "I certainly hope to see that come to fruition within the next few years.""

James Bowery said at February 22, 2010 6:28 AM:

A paper that sheds light on the manufacturing barriers: PREDICTED EFFICIENCY OF SI WIRE ARRAY SOLAR CELLS.

Paul said at February 23, 2010 11:32 AM:

Understand that "quantum efficiency" is not the same thing as "energy efficiency". Much of the energy in each absorbed photon is converted to heat, not electrical energy, in any PV cell.

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