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Researchers increase the efficiency of cheaper quantum dot solar cells

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February 21, 2011

Quantum dot solar cells could offer a cheaper, more efficient alternative to conventional ...

Quantum dot solar cells could offer a cheaper, more efficient alternative to conventional photovoltaic materials

Developing solar cells that are cheaper to produce and can harness the sun's energy more efficiently are both important factors in ensuring the widespread use of solar energy to provide a clean alternative to fossil fuels in the future. Stanford researchers have found that adding a single layer of organic molecules can achieve both these goals by increasing three-fold the efficiency of quantum dot solar cells, which are cheaper to produce than traditional solar cells.

Quantum dot solar cells

Quantum dot solar cells use tiny particles of semiconductors – the "quantum dots" – as the photovoltaic material instead of bulk materials such as silicon, copper indium gallium selenide or CdTe. While they are cheaper to produce as they can be made using simple chemical reactions, they have lagged well behind traditional solar cells in terms of efficiency.

"I wondered if we could use our knowledge of chemistry to improve their efficiency," said Stacey Bent, a professor of chemical engineering at Stanford. She realized that is she was successful, the reduced cost of these solar cells could lead to mass adoption of the technology.

Bent says that, in principle, quantum dot solar cells should be able to reach much higher efficiency due to a fundamental limitation of traditional solar cells.

Solar cells work by using energy from the sun (photons) to excite electrons, which jump from a lower energy level to a higher one, leaving a "hole" where the electron used to be. Solar cells use a semiconductor to pull an electron in one direction, and another material to pull the hole in the other direction. It is this flow of electrons that leads to an electric current.

The amount of minimum energy required to fully separate the electron and the hole is specific to different materials and affects what color, or wavelength of light the material best absorbs. Because the energy required to excite its electrons corresponds closely to the wavelength of visible light, silicon is commonly used to make solar cells. Although higher efficiencies have been achieved with multi-junction solar cells, those made of a single material have a maximum efficiency of about 31 percent – a limitation of the fixed energy level they can absorb.

Size does matter

Because quantum dots don't share this limitation, they can theoretically be far more efficient. Instead, the energy levels of electrons in quantum dot semiconductors depends on their size – the smaller the quantum dot, the larger the amount of energy needed to excite electrons to the next level.

This allows quantum dots to be tuned to absorb a certain of wavelength of light just by changing their size. By building more complex solar cells that have more than one size of quantum dot, the solar cells can absorb multiple wavelengths of light.

Organic molecule layer

In an attempt to take advantage of these properties, the Stanford researchers coated a titanium dioxide semiconductor in their quantum dot solar cell with a very thin single layer of organic molecules. These self-assembling molecules packed together in an ordered way, with the quantum dots present at the interface of this organic layer and the semiconductor.

The researchers tried several different organic molecules in an attempt to learn which ones would most increase the efficiency of the solar cells, but found that the exact molecule didn't matter. Just having a simple organic layer less than a nanometer thick was enough to triple the efficiency of the solar cells.

"We were surprised, we thought it would be very sensitive to what we put down," said Bent.

New theory

But in hindsight, Bent said the result made sense, and the researchers came up with a new model with the length of the molecule, and not its exact nature that mattered. Molecules that are too long don't allow the quantum dots to interact well with the semiconductor.

Bent's theory is that once the sun's energy creates an electron and a hole, the thin organic layer helps keep them apart, preventing them from recombining and being wasted. Although the group hasn't yet optimized the solar cells, currently achieving an efficiency of 0.4 percent at most, they can tune several aspects of the cell and believe, once they do, the three-fold increase caused by the organic layer would be even more significant.

Bent says the cadmium sulfide quantum dots she is currently using are not ideal for solar cells, and the group plans to try different materials. The group will also try other molecules for the organic layer, and could change the design of the solar cell to try and absorb more light and produce more electrical charge. Once the Stanford team has found a way to increase the efficiency of quantum dot solar cells, Bent hopes their lower cost will lead to wider acceptance of solar energy.

About the Author
Darren Quick Darren's love of technology started in primary school with a Nintendo Game & Watch Donkey Kong (still functioning) and a Commodore VIC 20 computer (not still functioning). In high school he upgraded to a 286 PC, and he's been following Moore's law ever since. This love of technology continued through a number of university courses and crappy jobs until 2008, when his interests found a home at Gizmag.   All articles by Darren Quick
3 Comments

The big stumbling block might be the durability of these organic materials. Organics tend to degrade rather quickly during extended exposure to bright sunlight, the exact conditions that solar cells must be exposed to.

Gadgeteer
21st February, 2011 @ 05:29 pm PST

Like everything else in business the more you create a demand which then feeds research and competition the more quickly you will end up with viable products, which is why me must continue to push solar, wind, thermal and wave power as we can get to the stage where these are better than alternative energy sources if we stick at it and do not fold to political pressures to find nasty alternatives. Every home and business should be able to be a power generator in the long term so that we are power self-reliant and can then move onto other problems....

svengali49
22nd February, 2011 @ 05:40 pm PST

The Victorian government in Australia has just gone in the opposite direction. New laws prohibit a wind turbine being constructed within 2km of an existing dwelling, unless there is written consent from the owner of the dwelling. State parks, coastlines and several other extensive regions are also prohibited areas. More than half of future wind farms proposed in Victoria will not be built as a direct result of this policy. The government is also looking at drastically reducing the payment made for households feeding power back into the grid from solar panels, making this alternate energy source less viable for most people. Stupid people and politicians (often in the same body) are the main stumbling blocks to alternate energy sources. Federal Opposition Leader Tony Abbott says nuclear power is the only realistic way for Australia to cut its carbon emissions. What hope do we have with these "leaders"?

Steve Balogh
2nd September, 2011 @ 06:13 pm PDT
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