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Flexible OPVs may lead to clothing that generates power

Despite the fact that graphene has been known to exist for decades, only recently have researchers begun to study graphene in earnest. The reason for the relatively late start into research using grapheme is that the material was difficult to make in quantities needed for research.

Once that hurdle was overcome, researchers quickly started to use graphene in all manner of study for a variety of uses. Graphene is being studied for use in semiconductors as a multiplier able to make processors many times faster than what we have access to today. Researchers are still studying better and cheaper methods of making grapheme and one important method was discovered in October of 2009 for growing graphene.

A team of researchers at the University of Southern California has discovered a way to use graphene to produce a flexible, printable sheet only four or less atoms thick. These graphene sheets can then be bound to a flexible polymer sheet to create a new type of organic photovoltaic cell or solar panel.

The resulting solar panel is thin and very flexible and the team believes that the material may one day be used for creating clothing that can generate power when worn in the sun. The big advantage of the graphene solar panel is that it is very flexible, much more so than existing solar panel materials.

"Organic photovoltaic (OPV) cells have been proposed as a means to achieve low cost energy due to their ease of manufacture, light weight, and compatibility with flexible substrates," wrote Chongwu Zhou, a professor of electrical engineering in the USC Viterbi School of Engineering, in a paper recently published in the journal ACS Nano.

The team creates the flexible graphene sheets needed for the solar panels by chemical vapor deposition. Carbon atoms are deposited in the form of graphene films onto a nickel plate from a methane gas. After the molecules are deposited, a thin protective layer of thermo plastic covers them. After the protective layer is deposited, the nickel plate is dissolved in an acid bath.

The left over film is very flexible and can be incorporated into an OPV. The downside to an OPV compared to a traditional solar panel made of silicon is that the OPV is able to convert less solar radiation into electricity.

"For every 1000 watts of sunlight that hits a one square meter area of the standard silicon solar cell, 14 watts of electricity will be generated," says Lewis Gomez De Arco, a doctoral student and a member of the team that built the graphene OPVs. "Organic solar cells are less efficient; their conversion rate for that same one thousand watts of sunlight in the graphene-based solar cell would be only 1.3 watts."

The team thinks that in the future these OPV sheets could be made into a fabric that could be hung like curtains or worn as clothing that can generate power. Another interesting use would be to cover the seats in an electric car or hybrid with the material to help capture power for the vehicles battery systems.

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Wrong Numbers
By MrTeal on 7/26/2010 2:09:40 PM , Rating: 2
"For every 1000 watts of sunlight that hits a one square meter area of the standard silicon solar cell, 14 watts of electricity will be generated," says Lewis Gomez De Arco, a doctoral student and a member of the team that built the graphene OPVs. "Organic solar cells are less efficient; their conversion rate for that same one thousand watts of sunlight in the graphene-based solar cell would be only 1.3 watts."

That must be a typo. That would put standard solar cells at 1.4% efficiency, and these new ones at 0.13%. Actual efficiencies are 10x for silicon cells.

RE: Wrong Numbers
By Fraggeren on 7/26/2010 2:25:18 PM , Rating: 2
"June 24, 2010 SunPower Corporation set a new world record by manufacturing a solar cell with sunlight-to-electricity conversion efficiency of 24.2%"

What's going on?

RE: Wrong Numbers
By Kurz on 7/26/2010 3:05:13 PM , Rating: 2
Concentrated Sunlight is more efficient.
~40% i think is the world record

RE: Wrong Numbers
By Kurz on 7/26/2010 3:11:34 PM , Rating: 3
1000 watts probably means everything in the spectrum.
From IR to UV. Solar cells only extract a certain wavelenth if I recall.

RE: Wrong Numbers
By DanNeely on 7/26/2010 4:39:11 PM , Rating: 2
It's not quite as bad as a single wavelength, but they do only capture energy from fairly narrow spectral bands. IIRC some of the current high efficiency designs work by layering several different types of transparent cell over top of each other to capture a larger fraction of the total.

RE: Wrong Numbers
By 3DoubleD on 7/26/2010 4:55:24 PM , Rating: 2
Solar cells, such as those based on silicon, capture most radiation in the solar spectrum above 1.1 eV (~1360 nm wavelength). The problem is that photons captured above this level are not efficiently converted. Instead, the additional energy is lost as heat.

If you use a higher bandgap material, such as GaAs, you have a higher voltage across your solar cell, but you lose current because you are absorbing less of the spectrum (fewer photons). This fundamental trade-off between current and voltage is called the Shockley-Queisser limit. The highest efficiency for a 2 level system (a single bandgap material) is 31%. The best single junction solar cells have based on Si and GaAs have reached ~25 and 29%, respectively.

As you mentioned, if you stack multiple layers of different bandgaps, you can more efficiently absorb the solar spectrum. By doing this, researchers have reached efficiencies of 41% under 1000x concentration. However, these designs are fundamentally limited by the enormous cost of manufacturing. That 41% solar cell was likely ~1 cm x 1cm and I wouldn't even want to know how much it cost. Production solar cells are typically far from these records as they are extremely difficult to mass produce.

Crystalline silicon solar cells were the first generation of solar cells. They are/were costly to produce and had mediocre efficiencies. Organic solar cells fall into the category of second generation. They hope to be dirt cheap, but low efficiency in an effort to get a better $/Watt than the first generation. Organic solar cells will never be used in large scale power production because they will never have a high enough efficiency. Currently, a very successful second generation design based on CdTe is being used by First Solar. The First Solar CdTe cell produces ~15% efficiency and is said to cost ~$1/watt. This is the solar cell technology to beat.

Wrong idea & title
By cparka23 on 7/26/2010 5:26:46 PM , Rating: 2
This lab didn't create a flexible organic photovoltaic panel. What they've created is a replacement for ITO (indium-tin-oxide), which is a clear conducting film that laminates photovoltaic cells and conducts electrons from the semiconducting material. Their graphene sheet could be used in organic photovoltaics and touch screens, but that doesn't mean that it produces electricity on its own. You still need a semiconducting material such as CIGS, silicon, or an organic-based photovoltaic to do that.

Also, this tech has been around for awhile from a company called Unidym, so I'm not really sure how this is newsworthy unless they've made a significant increase in yield. But then again I'm not actively up to date with the industry.

Solar what?
By gregpet on 7/27/2010 2:05:48 PM , Rating: 2
Instead of making solar clothes why don't we work on solar parking lots & roads! Put a use to all that flat surface!

By Iridium130m on 7/26/2010 5:05:56 PM , Rating: 1
In other words, when the heck can I cover my roof with inexpensive solar cells with a break even within 3-5 years of cost, and produce near as much or more power than I consume?

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