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What is a quantum dot?

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Quantum dots are tiny spheres of semiconductor material measuring only about 2-10 billionths of a meter in diameter.

Quantum dots are a leading candidate for a third generation of solar-cell technologies.

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Achieving significant gains in solar photovoltaic efficiencies depends on new technologies like quantum dots.

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In 2001, NREL predicted that quantum dots would be capable of generating more than one exciton, or electron-hole pair, from a single photon of light. The effect was later proven experimentally.

Game Changer

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Quantum dots have the potential to dramatically increase the efficiency of converting sunlight into energy—perhaps even doubling it in some devices—because of their ability to generate more than one bound electron-hole pair, or exciton, per incoming photon.

Today's solar cells produce only one exciton per incoming photon, but the "multiple exciton generation" effect of quantum dots promises to wring more energy out of each photon.

An illustration of the first all-quantum- dot solar cell, which combines a 70-nanometer-thick layer of lead sulfide quantum dots (QDs) with a 150-nanometer-thick layer of zinc oxide nanocrystals (NCs). Current is collected from the transparent conductive oxide layer, formed from indium tin oxide (ITO), and the gold back contact. The top of the solar cell, shown face down in this illustration, is protected with a layer of glass.
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In 2011, NREL researchers built the first all-quantum-dot solar cell by starting with a piece of glass, then depositing a transparent oxide coating that serves as an electrical contact. This was followed by a layer of zinc oxide nanocrystals, then the quantum dots, and finally a gold back layer that serves as the other electrical contact.

Once assembled, the solar cell is flipped over to expose it to the sun.

What's next?

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Before technologically significant quantum-dot solar cells become a reality, scientists must first learn how to split the excitons created by quantum dots and collect the resulting free electrons and holes with high efficiency. NREL is pursuing a better understanding of the fundamental science of "multiple exciton generation" processes to develop solar cells with predicted high efficiencies.

Before technologically significant quantum-dot solar cells become a reality, scientists must first learn how to split the excitons created by quantum dots and collect the resulting free electrons and holes with high efficiency. NREL is pursuing a better understanding of "multiple exciton generation."

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Learn more about NREL's solar innovation impacts.