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What Are Quantum Dot Solar Cells?

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  • Written By: Ray Hawk
  • Edited By: E. E. Hubbard
  • Last Modified Date: 08 September 2016
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Quantum dot solar cells are solar cells built upon a network of crystals manufactured at the nanometer scale that have the potential to outperform conventional solar cell technologies due to a fundamental limitation of how solar cells capture sunlight. A standard solar cell is built upon a layer of material that is most efficient at capturing one particular band or wavelength of light. The quantum dots in quantum dot solar cells, however, can be created to capture multiple bands of light by varying their size and chemical makeup in the manufacturing process. This makes an array of different kinds of quantum dots on one layer of substrate potentially able to capture a wide range of light wavelengths, making them much more efficient and economical to produce than standard solar cells.

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The technical limit for converting sunlight to electrical energy with a solar cell material made up of one type of chemical structure is theoretically a maximum of 31%. Commercial solar cells as of 2011 only have a practical efficiency level of 15% to 17% at their maximum level, however. Research has been underway for decades to find improvements to solar cell technology from several vantage points, such as reducing the expense of photovoltaic material based on highly-pure silicon by substituting flexible polymer and metallic substrates. Solar cell research has also focused on capturing a wider band gap range of light, both by stacking different layers of solar cell materials or engineering unique crystals, known as quantum dots, on one solar cell layer. All of the approaches have their drawbacks, and quantum dot solar cells also attempt to make use of their advantages where possible.

The emerging technology of quantum dot solar cells is built upon the physics and chemistry of the quantum dots themselves, but also includes the principle of a multi-layered solar cell, and the ability to incorporate these components into a more easily-manufactured, potentially-flexible substrate. Ideally, the technology is targeting at producing what is known as a full-spectrum solar cell, capable of capturing up to 85% of radiant, visible light and converting it to electricity, as well as capturing some light in the infrared and ultraviolet bands. Energy outputs for such solar cells have reached 42% efficiency in the laboratory as of 2011, and current efforts involve finding practical, cost-effective chemical structures for such technology so that it can be mass produced.

Approaches to next generation solar cells have focused on the three band gap or multi-junction model, where different layers of semiconducting alloys of gallium-arsenide-nitrate are interconnected. Another multi-junction chemical composition has used a zinc-manganese-tellurium alloy and quantum dot solar cells are also being made from cadmium-sulfide on a titanium dioxide substrate that is coated with organic molecules to interconnect the metal substrate and the quantum dots. Other variations on the three band gap layers include research using indium-gallium-phosphide, indium-gallium-arsenide, and germanium. Many chemical combinations seem to work, and the size of the molecules used in the process, such as the organic interconnect layer, appear to have more of a direct impact on the efficiency of quantum dot solar cells to capture a broad spectrum of light than the actual chemistry of the materials themselves. The layers in a multi-junction solar cell, however, including the quantum dots themselves, often have to be less than two nanometers thick, which requires an extremely fine level of precision to produce that only microchip fab facilities that make computer processors and memory are capable of at a mass scale.

The goal of quantum dot solar cells research is to make solar cells both more efficient and less expensive to manufacture. Ideally, they will be built upon flexible polymer materials so that they can be painted onto buildings or used as a coating for portable electronics. They would then also be capable of being weaved into synthetic fabrics for clothing and upholstery in cars. This would give solar cell technology widespread applications in electrical generation that could supplement or supplant the need for fossil fuel use for many common consumer needs including in climate control, telecommunications, transportation, and lighting. Such solar cells have been created in the laboratory in the US, Canada, Japan, and other nations, and the first company to find a method of inexpensive mass production of the technology is likely to capture a world market for it of unprecedented scale.

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