Quantum dots with built-in charge boost solar cell efficiency by 50%

May 13, 2011 by Lisa Zyga feature
(Left) A diagram of a quantum dot structure. (Center) A comparison of solar cells with different levels of doping. (Right) The 3D potential profile in quantum dot structures. Image credit: Sablon, et al. ©2011 American Chemical Society.

(PhysOrg.com) -- For the past few years, researchers have been using quantum dots to increase the light absorption and overall efficiency of solar cells. Now, researchers have taken a step further, demonstrating that quantum dots with a built-in electric charge can increase the efficiency of InAs/GaAs quantum dot solar cells by 50% or more.

The researchers, Kimberly Sablon and John W. Little (US Army Research Laboratory in Adelphi, Maryland), Vladimir Mitin, Andrei Sergeev, and Nizami Vagidov (University of Buffalo in Buffalo, New York), and Kitt Reinhardt (AFOSR/NE in Arlington, Virginia) have published their study on the increased in a recent issue of .

In their study, the researchers studied heterostructure with InAs/GaAs quantum dots. As photovoltaic materials, the quantum dots allow for harvesting of the to convert it into electric energy. However, the quantum dots also enhance the recombination of photocarriers and decrease the photocurrent. For this reason, up to now the improvement of photovoltaic efficiency due to quantum dots has been limited by several percent.

Here, the researchers have proposed to charge quantum dots by using selective interdot doping. In their experiments, the researchers compared doping levels of 2, 3, and 6 additional electrons per quantum dot, which resulted in photovoltaic efficiency increases of 4.5%, 30%, and 50%, respectively, compared to an undoped solar cell. For the 6-electron doping level, that 50% increase corresponds to an overall efficiency increase from 9.3% (for undoped solar cells) to 14%.

The researchers attributed this radical improvement of the photovoltaic efficiency to two basic effects. First, the built-in-dot charge induces various transitions of the electrons and enhances harvesting of the infrared radiation. Second, the built-in-dot charge creates potential barriers around dots and these barriers suppress capture processes for electrons and do not allow them to return back into the dots. The effect of potential barriers has been previously used by the researchers to improve the sensitivity of infrared detectors.

In addition, the researchers predict that further increasing the doping level will lead to an even stronger efficiency enhancement, since there was no evidence of saturation. In the future, the researchers plan to further investigate how these effects influence each other at higher doping levels. They predict that further increasing the doping level and radiation intensity will lead to an even stronger efficiency enhancement, since there was no evidence of saturation.

“The methodology and principles developed during this research are applicable to a number of photovoltaic devices with quantum dots and nanocrystals, such as polymer plastic cells and dye-sensitized porous metal oxide Gratzel cells,” Dr. Sergeev told PhysOrg.com. “Effective harvesting and conversion of infrared radiation due to optimized electron-hole kinetics in structures with and nanocrystals will lead to potential breakthroughs in the area of solar energy conversion.”

Explore further: Two-element atomic chain synthesized using microscopic space inside a carbon nanotube

More information: Kimberly A. Sablon, et al. “Strong Enhancement of Solar Cell Efficiency Due to Quantum Dots with Built-In Charge.” Nano Letters, Article ASAP. DOI: 10.1021/nl200543v

4.9 /5 (22 votes)

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Vendicar_Decarian
1.5 / 5 (6) May 13, 2011
A 15% conversion efficiency is quite acceptable.

Eight square meters of panels at that efficiency would provide for all of my electric power needs. Including the production of hot water - which would be an insane wast of electricity.

If they can boost the efficiency to 20%, only 5 panels would be needed.

Government research at it's best.

SteveL
4.7 / 5 (3) May 13, 2011
It will be interesting to see how many additional electrons they can reach per quantum dot before saturation and what efficiency level they can achieve with this technique. With what little data is provided it looks like there may be a reducing rate of return for the number of electrons. So at some point the addition of more electrons will not really provide any benefit. Panels that are 18-20% efficient over a 20+ year life span could be a phenominal boost to this market and to plans for energy independence.

Next will be the questions of whether engineers can find a way to cheaply mass produce this new technique and then how long the panels retain this level of efficiency. Exciting stuff.
gvgoebel
1 / 5 (3) May 13, 2011
WHAT? No denunciations of the solar power "fraud"? The trolls are slipping.

I will concede that renewables have their significant weaknesses. However, as this article suggests, we are now in the early stages of an era of materials and quantum device design that is likely to go stratospheric.
dnatwork
5 / 5 (1) May 13, 2011
As I understand it, quantum dots absorb light at selective wavelengths based on their size. How about stacking a bunch of them (ultraviolet, blue, green, yellow, red, infrared) in columns, add this doping, and you get something that absorbs the full spectrum at high efficiency?
Modernmystic
3 / 5 (4) May 13, 2011
And how much do they cost? Like someone advertising a product that doesn't tell you the price it's usually a bad sign...
gvgoebel
not rated yet May 13, 2011
As I understand it, quantum dots absorb light at selective wavelengths based on their size. How about stacking a bunch of them (ultraviolet, blue, green, yellow, red, infrared) in columns, add this doping, and you get something that absorbs the full spectrum at high efficiency?


A very interesting notion, monocrystalline solar cells are sometimes made in a multispectral format. In that specific case they are highly efficient but VERY expensive and basically only used for space applications -- high launch costs mean a smaller solar array is cheaper overall even if the array itself is much more expensive.
wealthychef
4 / 5 (1) May 13, 2011
And how much do they cost? Like someone advertising a product that doesn't tell you the price it's usually a bad sign...

LOL -- you must be new here. I have yet to see a working product appear out of any of these amazing discoveries. They always suffer from some form of "it's great but in reality it doesn't work because it (fill in the blank)" where the blank can be things like "is too expensive", "requires a huge infrastructure change", "is impractical for users," "relies on scarce materials," etc. There really ought to be a standard way to express this so I know how far we really are from getting something real for my house or car. :-)
ricarguy
2.3 / 5 (3) May 13, 2011
The skeptics towards renewables are only those who decry exorbitant costs or other often excluded details which render the technology uncompetitive for wide spread application. If solar energy can someday soon be implemented without "necessarily causing electricity rates to skyrocket", who would be against it?

Of course even if this proves to be a practical solution for generation, there is still the energy storage issue. Getting the electrical power when you want/need it, not just when the sun is bright.
bzzzz
1 / 5 (1) May 13, 2011
just find the notes that J.P Morgan stole from Tesla and we will all go DAhhhhhh...............
bzzzz
1 / 5 (2) May 13, 2011
We are all correct without the what ifs thrown back at us. WE ALL WANT tHE BEST AND SAFEST. BUT???????? we need to step away from the everything weve been taught. move up over and beyond our masters, teachers, professors, scientists ect ect and the like. Be more like N. Tesla....... find the notes that J.P Morgan stole from Tesla and we will all go DAhhhhhh...............
SlowGenius
not rated yet May 13, 2011
Thats exactly what I was thinking... Along with the over smarted up language they chose to explain it in.
StandingBear
2.3 / 5 (3) May 14, 2011
We have a whole southwest in this country to fill with photo-voltaic collectors such as this. The only power they use would be for the auto solar tracking devices to keep them on target. The only labor would be for an army of dusters to keep them clear, and windshield wipers would handle most of that. We have armies of prisoners for helping install them, and larger armies of unemployed to work in fabrication facilities that we could build. A crash program could get us energy self sufficient in an amazing short time. couple that with a similar strategy to replace the fresh air Otto cycle 7% efficient polluters with cars like the Chevy Volt, and some changes to codes like California's that favor the oil companies by penalizing electric car use with higher per kilowatt rates to ordinary households, and over a few years we won't need imported oil at all....and..o yes, those solar systems should be government owned and run so that we have quality collectors and a loyal laborforce.
Wulfgar
not rated yet May 15, 2011
Arsenic and Indium. So its both toxic and expensive! Well if its a broadly applicable technology, then we can ignore the elements used in this experiment.