Researchers improve efficiency of low-cost solar cells

Dec 07, 2010 By Lisa Zyga feature
This image shows a cross-section of a CZTSSe thin film solar cell. The film exhibits densely packed grains, which leads to a high efficiency. Image credit: Qijie Guo, et al. ©2010 American Chemical Society.

( -- As part of the recent progress in improving solar cells for widespread use, researchers from Purdue University have designed solar cells made of low-cost, abundant materials that are easily scalable and very stable. The researchers have increased the solar cells' total area efficiency to 7.2% and plan to make further improvements in the future.

The researchers, Qijie Guo, Grayson M. Ford, Wei-Chang Yang, Bryce C. Walker, Eric A. Stach, Hugh W. Hillhouse, and Rakesh Agrawal, have published their study on the improved in a recent issue of the . They fabricated the solar cells from copper-zinc-tin-chalcogenide (CZTSSe), which is an Earth-abundant material, using a solution-based thin-film deposition method. Previous research has shown that these methods can provide high yields at lower manufacturing costs compared to other methods.

The solar cell design is based on the researchers' earlier study in which they demonstrated that solar cells fabricated using CZTS nanocrystals are potentially viable, although they had efficiencies of less than 1%. Here, the researchers made significant improvements to the design by tuning the composition of the nanocrystals as well as developing a more robust thin-film coating method.

After synthesizing the nanocrystals and applying them on a for a total film thickness of 1 micrometer, the researchers observed that the nanocrystal film featured large, densely packed , which leads to improved . In testing, the solar cells could achieve a total area efficiency of 7.2%. As coauthor Hugh Hillhouse explained, the total area efficiency refers to the entire cell, rather than just the “active area.”

"It is the total area efficiency that matters most," he told “Some people report an ‘active area’ efficiency, which only includes areas that the light reaches. However, all thin film solar cells are made with metal contacts that block the light from reaching some areas. When you include this loss, we use the term ‘total area’ efficiency. It is the most fair and important efficiency.”

The 7.2% efficiency was reached after “light soaking” for 15 minutes under one-sun illumination; when the light was turned off, the efficiency dropped to 6.89%.

“Light soaking simply means that we shine normal intensity simulated sunlight on the cell for a period of time before we make the measurement,” Hillhouse said. “Most likely, the light soaking allows photogenerated carriers to fill traps, shift the quasi-Fermi levels, and/or screen barriers created by band offsets. It doesn’t present a problem since real solar cells are naturally light soaked – they sit in the sun.”

Although currently there are no CZTS or CZTSSe solar cells on the market for comparison, the solar cells in this study are very competitive with other fabrication methods.

“The best cells formed by vacuum processes have only reached 6.7%,” Hillhouse said. “Typically, solar cells produced by vacuum-based processes have been more efficient, but also more expensive. For the case of CZTS, the solution phase approach (our nanocrystal route and IBM’s hydrazine route) is more efficient.”

One potential area for improvement for these solar cells lies in improving their low quantum efficiency for light of longer wavelengths (i.e., the near-infrared range). The researchers attempted to improve this efficiency by increasing the thickness of the absorber, although their initial experiments showed that thicker absorber layers also had increased resistance. In the future, they plan to optimize the fabrication for thicker films, which could further increase the overall efficiency.

“There is a lot of compositional freedom in the CZTSSe system, and it is likely that the optimum compositions, device structure, and processing conditions have not yet been found – but we are working on it,” Hillhouse said.

Explore further: Thinnest feasible nano-membrane produced

More information: Qijie Guo, et al. “Fabrication of 7.2% Efficient CZTSSe Solar Cells Using CZTS Nanocrystals.” J. Am. Chem. Soc. ASAP. DOI: 10.1021/ja108427b

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5 / 5 (3) Dec 07, 2010
Well, it's already 7.2 times more efficient than photosynthesis, doesn't require re-planting and a 2 month growing period every year to achieve full ground cover, doesn't waste energy continuously constructing roots and stems, needs no irrigation or increasingly rare mineral phosphorous.

I like it.
not rated yet Dec 07, 2010
I wonder what would be the estimated price per watt of this technology? From the description materials and manufacturing are cheap. This also should scale up without foreseeable problems.
not rated yet Dec 07, 2010
besides cost, I would question real world performance, for instance the performance above or below room temperature. I would also wonder about durability in terms of how fast the material will weather, oxidize or decompose when exposed to sunlight and moisture outdoors. Even if it's dirt cheap, it's not affordable if you have to replace it so often that it costs 100 times what you normally pay for electricity. Also, at 7% efficiency you would need a huge area of panels to provide a modest return of power. The more compact types of cells which cost much more but offer much higher efficiency are actually a better deal since the places where power is needed most are always crowded with other things.
5 / 5 (5) Dec 07, 2010
Also, at 7% efficiency you would need a huge area of panels to provide a modest return of power.
Well yes, this is more a materials paper than an energy paper, but to go from less than 1% efficiency to over 7% without impacting cost per process is a huge leap. This is certainly a noteworthy accomplishment.
not rated yet Dec 07, 2010
Yes, now they are certainly invited to the table with the big boys, since the 7% club is hard to get into with thin film. My other technical questions remain though. 7% doesn't mean squat unless it's actually something that can be used in the real world.
not rated yet Dec 07, 2010
According to the Food and Agriculture Organization of the United Nations, photosynthesis has an overall efficiency of between 3 and 6% of total solar radiation.
1 / 5 (1) Dec 07, 2010
..This is certainly a noteworthy accomplishment...
IBM claimed 9.6% efficiency for CZTSS cells already...


Unfortunatelly, zinc-selenium cells are known to be unstable, as the ultraviolet light causes photoreduction of selenium (zinc selenide turns grey, when illuminated).
5 / 5 (1) Dec 07, 2010
Thales: FAO is posing absolute theoretically optimal figures. This paper from U Illinois states more useful figures.

Due to losses at all steps in biochemistry, one has been able to get only about 1 to 2% energy efficiency in most crop plants. Sugarcane is an exception as it can have almost 8% efficiency. However, many plants in Nature often have only 0.1 % energy efficiency.

NB: Their proposal of "up to" 8% efficiency for sugar cane is directly contradicted in other documents, which state "about 3%".
not rated yet Dec 07, 2010
And this link ptovides a credible calculation:

which collects more usable energy, an acre of sugar cane or an acre of solar panels?

So I ran the numbers. I chose sugar cane because it produces the largest number of calories per acre of any agricultural product. From we learn that you can harvest 10 tons of sugar per hectare = 10,000 kg/10,000m^2 = 1 kg /m^2. This means a square meter of sugar can yields a kilogram of sugar a year. As there are 4 calories (or kcal) per gram this gives you 4,000 kcal/m^2.

At this site we learn that a square meter of 11% efficient solar panels collects around 550Wh a day in Austin Texas. So for a whole year that gives you 550 * 365 = 200 kWh/yr/m^2....1 kcal = 1.16222222 watt hour. So our 4,000 kcals of sugar = 4.65 kWh. That means the 11% efficient solar panel generates 200/4.65 = 50 times more usable energy a year than the sugar cane does.

not rated yet Dec 07, 2010
SO if the 11% efficient solar array yields 50x more NET ENERGY than the sugar cane (which BTW I would downrate to about 33x due to weak math, cloudy days etc.), its fair to assume the sugar case is operating OVERALL at about 0.33% efficiency. Difference from "instantaneous theoretical efficiency" to year-round real-world efficiency, including root growth, months after cutting when the new shoots have very little leaf cover, dry periods, over-hot afternoons, less than perfect nutrition, etc. etc.
not rated yet Dec 07, 2010
Note that NONE of the above penalizes the biomass system for planting / harvesting energy or resources, nor credits the solar --> electric system for the much more useful form of the output energy (high quality electricity vs. low-quality sugar, which either needs to be converted to ethanol at further great loss, or burned in a boiler at only 33% efficiency).

NO plant can get near the energy output of an equivalent area of solar-electric generation. Solar thermal (trough) easily and cheaply achieves 15% Net Electricity out NOW, and the costs aren't terrible compared to fossil peaker turbines.

Biomass --> energy is a complete waste of time and resources.
1 / 5 (2) Dec 07, 2010
Biomass --> energy is a complete waste of time and resources.
Try to imagine, how incredible amount of biomass gets rotten by the autumn every year, while releasing methane gas into air. Why not to use this energy and oxidize this biomass for our own purposes instead?
not rated yet Dec 07, 2010
What is going on here? Is this work of 10 years ago?? Boeing is already committed to mass-producing solar panels with over 30% efficiency!! and others have achieved over 40% !!!

not rated yet Dec 07, 2010
tkjtkj- You conveniently left out what the cost would be per kilowatt/hour.
not rated yet Dec 07, 2010
apex01. You're SURE not going to get net energy out of biomass at a lower cost than the $0.12 per kwh now the standard for parabolic trough solar thermal (15% efficiency). See


Assessment of Parabolic Trough and Power Tower Solar Technology Cost and Performance Forecasts - Sargent and Lundy LLC Consulting Group Chicago, Illinois - NREL data

initial electricity costs in the range of 10 to 12.6 cents per kWh and eventually achieving costs in the range of 3.5 to 6.2 cents per kWh. (assuming 2 to 8 GW constructed by 2020)
not rated yet Dec 07, 2010
KwasniczJ - Try to imagine - what the world is going to use to feed its population once the reserves of phosphorous fertilizer run out in 85 years.
not rated yet Dec 08, 2010
Solar panels in space then beam electricity to earth. Is this the next step?
5 / 5 (1) Dec 08, 2010

You are on the wrong website. This is a science website not a teletubbies website
not rated yet Dec 08, 2010
KwasniczJ - Try to imagine - what the world is going to use to feed its population once the reserves of phosphorous fertilizer run out in 85 years.

the phosphorous is still around. Don't have a cow man. It's in the rivers and oceans (particularly deltas,) where it can be harvested, given the near-infinite energy supply of the Sun.

With the exception of whatever quantities that many have been in spacecraft we've launched, the amount of phosphorous on earth is about the same it has been since earth existed. Heck, if you count meteors, it's actually gone up.

It's all there. Just need enough solar farms and you can mine, filter, recycle anything as often as you want.
5 / 5 (1) Dec 08, 2010
@neiorah You are on the wrong website. This is a science website not a teletubbies website

Japan and China have seriously claimed they want to do just that. Machio Kaku, a leading American physicist has proposed the same thing.
5 / 5 (1) Dec 09, 2010
Energy beamed from space stations cannot possibly compete economically with surface-installed solar energy, at least until the cost of boosting a ton of material into geosynch orbit drops by at least a factor of 100, which likely cannot even happen with a space elevator where the installation of the ribbon is free. Just too much energy required for lifting and accelerating.

not rated yet Dec 09, 2010
I believe DARPA is funding research to determine the viability of a system to beam small amounts of power down from space in order to power remote millitary bases where it's hard to transport fuel for standard generators. I think I read something about a 10 Mw system possibly moving forward to proof of concept phase if they can get funding. I don't think it's a priority though.

It's certainly not cost effective compared to ground based coal plants, but could be usefull in emergency situations or in war.
not rated yet Dec 11, 2010
So the idiot who wrote this got them to define total area efficiency and state the efficiency of their cell using that stat but didn't have sense enough to ask what the total area efficiency is of current other cells who use the active area efficiency stat?
not rated yet Dec 12, 2010
So what type of solar cell (in production now) currently has the lowest $/watt?
not rated yet Dec 15, 2010
@neiorah You are on the wrong website. This is a science website not a teletubbies website

Japan and China have seriously claimed they want to do just that. Machio Kaku, a leading American physicist has proposed the same thing.

Fraudic Psuedo-scientists, just want their pockets filled.

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