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The photoelectric effect is a well known principle in physics and chemistry.
A photon of some frequency strikes a metal.
If the frequency of the photon is higher than the threshold or cutoff frequency of the metal, an electron will be “knocked loose” and any excess energy of the photon will be converted into kinetic energy of the electron.
Experiments have shown that the typical result is one electron knocked loose for every individual incident photon.
However, under special circumstances, more than one electron may be released from the metal.
Radiation incident on nanocrystals, or quantum dots, seems to be one of those special circumstances that allows for ejection of multiple electrons from a metal.
Nanocrystal technology is helping scientists overcome the one to one photon/electron ratio to create a more efficient solar cell.
Researchers found that a lead-selenium quantum dot approximately eight nanometers across produced as many as seven electrons under ultra-violet radiation.
These multiple electron ejections are believed to be caused by an electron, which when ejected from the metal has sufficient kinetic energy to “knock loose” other electrons.
Ultra thin solar energy films 1000 times smaller than the width of a human hair made out of cadmium-selenide (CdSe) and cadmium-telluride(CdTe) are being synthesized at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory in California.
These inorganic nanocrystal-based solar cells have properties similar to both traditional photovoltaic cells and organic photocells: they can be processed in solution, are resistant to substitutional doping, have broadband adsorption, and excellent transport properties.
In addition, unlike many organic solar cells, they are stable in air and don’t deteriorate with the passage of time.
While these new solar cells have many benefits, there are also drawbacks.
The new cells convert solar energy to electricity with only 3% efficiency.
This is well below the efficiency of the standard silicon based semiconducting photovoltaic cell (~20%), but further research is being conducted to improve efficiency of nanocrystal solar cells.
Once the efficiency is increased to around 10%, the nanocrystal solar cell will viable for marketing as an alternative energy source.
Sources:
Science Daily | Sunny Future for Nanocrystal Solar Cells
Science News | Quantum-Dot Leap
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