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May 30, 2006

Quantum dots could create 65% efficient solar cells

Advancing control and improving processes for the creation of quantum dots could enable vastly superior solar cells.

In a paper published in a May, 2005 issue of the American Chemical Society's Nano Letters journal, an NREL team found that tiny "nanocrystals," also known as "quantum dots," produce as many as three electrons from one high-energy photon of sunlight. When today's photovoltaic solar cells absorb a photon of sunlight, the energy gets converted to at most one electron and the rest is lost as heat. "We have shown that solar cells based on quantum dots theoretically could convert more than 65% of the sun's energy into electricity, approximately doubling the efficiency of solar cells," Nozik said. The best cells today convert about 33% of the sun's energy into electricity. In 2004, Richard Schaller and Victor Klimov of Los Alamos National Laboratory in New Mexico were the first to demonstrate the electron multiplication phenomenon predicted by Nozik, using quantum dots made from lead selenide.

Here is a somewhat technical summary of quantum dots and photovoltaics in general.

Quantum dots, that are irradiated with energy that is to 2 - 4 times their band gap energy, produce excitons that correspond to the second, third or forth excited states depending on the energy of the incident photon. The absorbed energy from this single photon produces two or more excitons in the quantum dot, which means that quantum yields, e.g., the percent of excitons produced/photons absorbed, up to 300% have been achieved.3 This phenomenon is referred to as multiple exciton generation (MEG). In 2004, researchers at Los Alamos were the first to demonstrate that if the photon energy is more than three times the band gap of PbSe quantum dots, two or more excitons can be produced with up to 100% efficiency. Since that time, researchers at the National Renewable Energy Laboratory (NREL), University of Colorado and Naval Research Laboratory (NRL) collaboratively demonstrated ultra-efficient MEG in colloidal PbSe and PbS quantum dots, producing three excitons per photon at photon energies at four times the quantum dots band gap.

One high efficiency solar energy device has not been created. The charges need to be taken from the quantum dots to the electrode efficiently. Then the process needs to be scaled up and the costs lowered to be competitive with other methods

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