Posted on 06/10/2003 11:30:03 AM PDT by Ernest_at_the_Beach
PORTLAND, Ore. — A new shape for semiconductor nanocrystals—tetrapods, rather than simple spheres, rods and disks—could double the efficiency of "plastic" solar cells, according to the inventor of tetrapods.
Paul Alivisatos, the inventor of semiconductor tetrapods and a professor at the University of California at Berkeley, said they promise to convert twice as much incident light into electricity. Tetrapods also promise to improve chemical sensors, biomedicine and optoelectronic devices, as well as serving as strengthening additives to plastic composites.
"We have been studying these materials [II-VI semiconductors like cadmium tellurium, CdTe] because we already take rodsshapes of it and put them inside plastic solar cells—the rods are the light-absorbing materials inside these solar cells," said Alivisatos, Chancellor's Professor of Chemistry and Materials Science at Berkeley and director of the Materials Sciences Division at the Lawrence Berkeley National Laboratory.
"It turns out that tetrapods could be better for [plastic solar cell] application, because the rods need to always point in the same direction, and tetrapods will always do that. You just scatter them on a surface and they all point up."
The researchers said tetrapods could double the efficiency of plastic solar cells.
Tetrapods are shaped like schoolyard "jacks" since they come to rest on their three downward pointing legs. On closer inspection, the nucleus of a tetrapod is shaped like an upside-down pyramid with legs protruding from its four faces.
Nanocrystals are manufactured in only three simple shapes—spheres, rods and disks. Many different types of semiconductor materials can be fabricated in these shapes. Alivisatos said he hopes his technique for growing crystalline tetrapods will advance the industry's understanding, leading to broader use of the tetrapod shape for chemical sensor applications, biomedical diagnostics and optoelectronics.
Alivisatos said careful selection of the type of semiconductor used is critical since his tetrapod growing technique depends on "polytypism," in which the crystal must have two different ways of packing atoms that are close to each other in energy. He used III-V, IV and II-VI semiconductors since all have the advantage of nearby energy levels enabling any of them to switch between atom-packing methods using the technique.
When energy levels are very close together, as in the CdTe semiconductor material, Ailivisatos found a chemically induced manner of switching between which structure is more stable part-way through the nanocrystal's formation. For CdTe, the first packing method forms what chemists call a zinc-blender core (akind of nanoscale pyramid), later switching to the wurtzite-rod packing method, which grows the legs of the tetrapod.
"We form an initial packing that makes a pyramid, and then when the crystal gets bigger it switches to the other kind of packing, making the arms that come out from the faces of the pyramid to form the tetrapod," the researcher said.
The "trick" Alivisatos found to induce the nanocrystal to switch packing methods, was by binding a specific organic molecule (phosphonic acid) to the surface of the pyramid. In the presence of the phosphonic molecule, CdTe will switch from zinc-blender (pyramid building during "nucleation") to wurtzite-rod (leg formation) during its growth phase.
Alivisatos grew the legs to lengths of 50 nm or more and found that they formed according to the controllable kinetic mechanisms previously observed while making nanorods, namely that higher Cd/Te ratios resulted in longer arms with more light-gathering capability.
He also discovered that higher concentrations of phosphonic acid yielded larger arm diameter, which determines the bandgap of the material.
Alivisatos and his staff are attempting to embed tetrapods into hybrid nanocrystal--polymer plastic solar cells instead of rods so that the tetrapod's improved efficiency can be measured.
Alivisatos is meanwhile looking for new shapes to make such as "branching" tetrapods. "The next step for us to try to find a way to get the legs to branch again, forming a branching tree-like structure," he said.
For example, to get a solar output of 7500watts would require 100 solar panels costing $30,000, array mounting structures, another $12-15,000, storage batteries, inverter and wiring. Altogether, you're looking at $45,000 and more to run a small house.
On the other hand, a 7500 watt generator can be bought at Home Depot for $1,000.
Y'know, I was just saying that to my wife....
Dan
I'm a technology fan. Solar power has its place. It would be great if my digital camera, laptop, phone, gps unit etc. etc. never needed batteries.
Anytime you can double the efficiency of any technolnogy it's good news.
Besides, the guy's from my alma matter. Go Bears!
And will just sit there not providing any power...
You forgot to include the fuel bill. And, just how bulky is this generator? And, does it stink up its location, from its exhaust? And, when it breaks down, as all machinery is wont to do, how much will it cost to fix it?
Delivering fuel to a solar cell is easy, there are no emissions, and no moving parts. Maintenance of solar cells is easy - Windex, at most.
Uncontrolled nannites could turn the genetic material of the royal families of Europe into gray goo.
Oops. My mistake. That should be "uncontrolled inbreeding".
To be fair, though, you are not really comparing apples to oranges, for the following reasons:
1st, if you tried to run a 7500 watt generator that you bought from Home Depot 12 hours per day (lets say that is the "average" amount of hours that solar produces) that generator is not going to last you very long. Those cheapo generators that cost you $1000 are designed for emergency use only. And they are not convenient - you have to run out and add gasoline regularly and change the oil frequently. A fair comparison would be those generators that are about the size of a central air conditioner and are hooked up to a transfer switch. Cost of one of those puppies installed is going to run you around $5000. They will work off natural gas so you you don't have to keep adding fuel. Even so, they are designed as a backup system. They are only good for about 2000 hours or so. Maybe one year for the way you want to use it.
2nd, the solar output you get varies tremendously based on location. Solar panels on a house in New England are far less efficient than on a house in the SouthWest. Now that I have said that, consider this: you may have underestimated the costs!
From this source which is apparently prosolar, they estimate that "residential solar system costs about $8,000-$10,000 per kWp installed", and the kWp measurement translates to roughly "1800 kWhrs/year in Southern California". Further, they estimate that it would take "$16-$20,000 to satisfy around 25% " of a homes energy needs in Sacramento, CA. The reality is, where solar is cost effective is for remote areas with no electric grid connection and the alternative is some kind of large diesal generator (and those suckers are expensive).
I realize I didn't lay out the various options thorougly. I've tried quite a few of them including wind, propane and diesel. For sheer reliability and durability, nothing beats diesel. A 60amp generator cost me $17,000 with wiring but I expect it'll last a lifetime and really cranks it out.
Yes, they smell, which is why mine is 200 feet from my residence and downwind. But I have a 200 gal tank which gets refilled regularly and, although less convenient than propane, costs less for the output.
The only point I intended to make in my original post was that, although Tetrapod nanocrystals are a great innovation, in terms of making solar economically competative with fossil fuels, they've got a long way to go.
What's the wind-power parallel for "insolation"? Just like solar, wind isn't a constant source. Energy storage is an issue for both.
A 7500kw generator, running a home @ 3600 rpm will last a year or so. Once it breaks, at $1000 each, you're better off just buying a new one. 5 gallons a day is enough to run a couple loads in the washer/dryer, filter the pool for 2-3 hours, and run all lights, computers and wide-screen tvs.
Total cost...$8.75 a day, $262 a month. This is a bit more than the cost for power in town but if you're in a rural or remote location, it works efficiently and reliably.
The cost for a reliable solar power set up to do the same would run the above for 15 years. In the long run, then, the really long, long run, a solar system is less expensive.
However, you'd need huge storage capability and a powerful inverter to give you anywhere near the surge capability a little generator can output.
Delivering fuel to a solar cell is easy, there are no emissions, and no moving parts. Maintenance of solar cells is easy - Windex, at most.
Right you are. Solar is a miracle. Making it more efficient with the tetrapod nanocrystals is a great leap forward. But ultimately, it's a matter of money. For solar to be viable it has to compete with fossil fuels and it's not there yet..
Yes, delilvering fuel to a solar cell is easy, as long as it's not cloudy. Also, from personal experience, you get an average 6-7 hours actual solar exposure per day over the course of a year.
One advantage of producing all your own energy, you tend to be extremely conservative with it.
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