November 09, 2008
Advance In Flexible Thin Plastic Solar Cells
Swiss and Chinese researchers lifted the conversion efficiency of a bendable and thin type of solar cell.
Researchers in China and Switzerland are reporting the highest efficiency ever for a promising new genre of solar cells, which many scientists think offer the best hope for making the sun a mainstay source of energy in the future. The photovoltaic cells, called dye-sensitized solar cells or Grätzel cells, could expand the use of solar energy for homes, businesses, and other practical applications, the scientists say. Their study is scheduled for the November 13 issue of ACS’ The Journal of Physical Chemistry C, a weekly publication.
The research, conducted by Peng Wang and colleagues — who include Michael Grätzel, inventor of the first dye-sensitized solar cell — involves photovoltaic cells composed of titanium dioxide and powerful light-harvesting dyes. Grätzel cells are less expensive than standard silicon-based solar cells and can be made into flexible sheets or coatings. Although promising, Grätzel cells until now have had serious drawbacks. They have not been efficient enough at converting light into electricity. And their performance dropped after relatively short exposures to sunlight.
In the new study, researchers describe lab tests of solar cells made with a new type of ruthenium-based dye that helps boost the light-harvesting ability. The new cells showed efficiencies as high as 10 percent, a record for this type of solar cell. The new cells also showed greater stability at high temperatures than previous formulas, retaining more than 90 percent of their initial output after 1,000 hours in full sunlight.
These cells offer two potential advantages: First off, they should be lower cost due to expected ease in manufacturing. Second, both their flexibility and their light weight will allow installation in places which otherwise would not support the presence of solar cells. For example, cars could have them on their roofs without a big weight penalty. Also, side walls and other vertical surfaces could be covered with lightweight solar cells whereas much heavier weight silicon photovoltaics would weigh too much for easy installation. Skyscrapers might some day get covered with Grätzel solar cell sheeting.
While they achieved 10% efficiency they only achieved 9.1% efficiency using materials that can be incorporated into a plastic.
The new dye absorbs light far better than the conventional dye. Because the dye absorbs light so well, it's possible to cut the thickness of the active material in the solar cell in half, which makes it easier for electrons to escape the solar cell and reach an external circuit. That, in turn, increases efficiency, in this case to 9.1 percent.
The researchers also paired the new dye with a nonvolatile solvent-based electrolyte. It's not quite as stable as an ionic liquid, and it can't be used with plastic. But it allowed slightly higher efficiencies--up to 10 percent.
With advances happening that cut costs for such a large variety of solar photovoltaic materials I am optimistic about the future of solar energy. PV electric power will come down in cost so far that it will compete with grid power during the day first in high sunlight areas such as the US southwest and gradually in lower and lower light areas. This will slow and eventually stop and reverse the growth of coal burning for electric power. It will also cut natural gas consumption and save it for more valuable space heating and fertilizer production.
Our biggest future energy problem continues to be for transportation. While the world recession has cut oil demand and oil prices the eventual resumption of economic growth will push oil prices back up again and the peak in world oil production is not many years away. I'm still not clear on whether advances in battery technology will happen soon enough to make the transition away from liquid fuels.
I think much of solar energy is mental masturbation.
Starting in January of this year, I have contacted several companies that are developing next generation solar technology. I contacted these companies on behalf of Japanese cohort of mine who was working as a contract researcher for a major Japanese chemical company. This company currently buys $100 million worth of silicon solar cells per year for their construction materials division. These solar cells eventually go into residential structures.
The companies I contacted are developing solar cells based on conductive polymers (plastics) or dye-sensitized TiO2 materials (like the one that Randal profiles in his blog entry here). We rejected Te-Cd solar cells for reasons of toxicity and CIGS for reasons of Indium supply issues. Of the companies I contacted, I found three fundamental issues that they are not dealing with at all:
1) Limited lifetime - All of the companies I contacted had material lifetimes of about 1-3 years. A lifetime of 20 years is needed for outdoor commercial deployment. The problem here is that conductive polymers are, well, conductive. This means that they react with the Oxygen in the atmosphere, thus reducing their lifetime. This is an intrinsic problem that only low-cost hermetic encapsulation is necessary to prevent this. There is no other way around this issue. Of course, low cost hermetic encapsulation is surprisingly hard to develop. None of the companies I know of are working on this.
2) Transparent material for electrode - Because these are solar cells and therefor must have transparency on one side so that the light can shine in, one of the electrodes must be transparent at both visible and IR spectrum. All solar cell manufacturers use ITO for this electrode, ITO having been developed by the flat panel manufacturers. The problem with ITO is that Indium is in short supply. You see, Indium is not mined directly. It is produced as a by-product of Zinc refining. Also, Zinc is in somewhat short supply as well. The other problem with ITO is that it can be deposited only by vacuum process. This is because the deposition can only be done at around 350degC. I know this because my aforementioned Japanese cohort spent 4 years trying to develop an ITO replacement that can be deposited by spray or other non-vacuum process method. So far, he has been unsuccessful. Worse of all, NONE of the next-technology solar cell companies are working on this problem. Indeed, everyone of them that I have contacted has asked me if we can develop this for them!
3) Conversion efficiency - This has to be in the 15% range for economic viability. So far, their best is actually around 7-8%, not the 9% that Randal has reported here. Again, I know this because I have signed NDAs with these people and this is the conversion efficiencies that they have achieved. Out of these three problems, this is the only one that they are actually working on.
In short, I have not found a silicon solar cell replacement technology that is ready for the prime time. Indeed, all of the companies I contacted are a good 3-4 years away from such a technology, if they solve all of the above mentioned problems, which they are not.
So, its going to be a while before solar becomes a real solution.
A few thoughts:
1) Silicon cell producers, like Sharp, are expanding production almost as quickly as thin film, and are still cutting costs. They claim that they won't be far behind thin film cost-wise, and will achieve grid-parity fairly soon.
2) "We rejected Te-Cd solar cells for reasons of toxicity"
Could you expand on this? First Solar is growing quickly, and selling all they can make at fat profit margins. The rest of the market has accepted it - do you think it will be a problem that will impede sales eventually? If so, why?
3) "and CIGS for reasons of Indium supply issues."
I have basically the same questions as for Te-CD. IOW, what do you know that others don't?
You've noted that "Indium is not mined directly. It is produced as a by-product of Zinc refining. Also, Zinc is in somewhat short supply as well. ", but this doesn't seem definitive. Could you give more info on Indium supply issues?
As pointed out recently in an ANALOG blurb, the typical power plant puts out about 5 gigawatts. If you allow for nighttime lack of sun and inevitable clouds it would take a solar panel square 11 miles on a side to do the same. And then you would break the panel into a bunch of smaller squares. You would have to have access roads, power lines, maintenance buildings. You would have some panels down for maintenance; some out for hail or lighting storms. In the end your square would be much bigger than 11 miles on a side. The same arguments apply to wind power. Solar will be fine for dingbats, retail, maybe even some light manufacturing. A heavy hitter it is not. How many solar panels do you suppose it would take to power a blast furnace?
Nuclear power is our only answer.
"the typical power plant puts out about 5 gigawatts."
I've never seen a power plant rated higher than about 1.5 GW. The "typical" would be the mean, which is probably about .2GW.
"In the end your square would be much bigger than 11 miles on a side. "
Not really. You should run through the calculations, rather than guessing. In the SW desert you could plan on a 20% capacity factor, including clouds and night time, for about 200 watts per sq meter on average. That gives 25M sq meters, 250M sq ft, or 10 sq miles, or a square 3 miles on edge.
Ultimately, sq miles used is unimportant: what matters is cost per KWH.
"A heavy hitter it is not"
It's the heaviest hitter, in the long run.
oops. I didn't include efficiency of conversion, which might be 20% and get you up to 50 sq miles (before access space is factored in), or 7 miles on edge (much less than the 121 sq miles originally suggested).
Again, desert is cheap; we're not likely to use the kind of PV discussed in this article in the desert; PV is so easy to put on roofs, of which there's plenty; and the important thing is cost, not space requirements.