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The Basics of CdTe Solar Cells

First mentioned in a scientific journal in 1963 by a researcher at General Electric named D. A. Cusano, thin-film Cadmium Telluride (CdTe) solar cells are some of the most successful and commercially relevant solar devices in the US. Their success can mainly be attributed to their manufacturing cost, as the low cost per watt of electricity generated by a CdTe cell enables the technology to make profits. In 2020, 40% of the US utility scale solar market was composed of CdTe solar devices. Because of the indirect bandgap of CdTe, these cells are also believed to have a higher max efficiency than Silicon solar cells, though the funding deficit between the two technologies has allowed Si cells to remain more efficient for the time being. The main difference between CdTe cells and other thin film solar devices is that they have CdTe as the primary photoconversion layer. This means that CdTe is the all-important material in the cell that absorbs light in the form of photons and converts it into electricity.

Vocab

Thin Film Photovoltaic Cells

  • Just as they sound, thin-films are a type of solar cell with exceptionally thin layers.

Substrate

  • The material on which something, in this case a solar cell, is grown or developed.

Transparent Conducting Oxide (TCO)

  • A material that allows light to pass through it but conducts electricity.

Electrical Contacts

  • Conductive metal on the side of solar cells that accept excited electrons and send them to an external load for use.

Manufacturing CdTe Solar Devices

CdTe solar cells can be manufactured in a variety of different ways, but the most common practice is known as Vapor Transport Deposition. Before performing Vapor Transport Deposition, a glass substrate must be made. When making the glass, sand is superheated to a molten state and a chemical coating is sprayed onto the material. The chemical spray becomes integrated throughout the glass, transforming the cooled substance into a Transparent Conducting Oxide, or TCO. This TCO serves as a protective container for the solar device and allows light to pass through it to the photoactive CdTe layer.

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A sketched cross-section of a standard CdTe cell.

Image: International Journal of Photoenergy

Once made, the glass substrate is placed in a reactor where Vapor Transport Deposition is performed. Much like the manufacturing process of III-V solar cells, materials in the form of gases (hence the name “vapor phase”) are released into the reactor and settle onto the substrate, forming crystalline wafers of CdTe. These wafers are doped to improve electrical efficiency, then removed from the reactor where they are outfitted with conducting materials like electrical contacts in a process known as metallization and turned into a fully fledged CdTe cell.

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A bent piece of willow glass.

Image: Corning

The Flexibility of CdTe Cells

The flexibility of a solar cell is entirely dependent on the thickness of its layers. For a CdTe cell to be flexible it must be composed of both thin semiconductor layers of CdTe and a thin glass package. Since CdTe cells are commercially manufactured on a relatively thick glass substrate, they are not traditionally flexible like III-Vs.

To make a CdTe cell flexible, the thick glass substrate can be removed and replaced with a thinner glass, though this process is expensive and therefore commercially unfavorable. If a manufacturer wanted to skip that step they could also use a thinner material as the initial substrate, such as willow glass. At the moment, however, the flexibility of solar cells is not a major selling point, so manufacturers of CdTe cells have not focused on it.

References

      Fang, Zhou, et al. “Achievements and Challenges of Cds/Cdte Solar Cells.” International Journal of Photoenergy, Hindawi, 12 Oct. 2011, www.hindawi.com/journals/ijp/2011/297350/.

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      Lefteri, Chris. “Glass: A Flexible Material.” Corning, Blueprint Magazine, www.corning.com/worldwide/en/innovation/the-glass-age/design-and-application/glass-a-flexible-material.html.

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      “Solar Photovoltaic Cell Basics.” Energy.gov, US Department of Energy, www.energy.gov/eere/solar/solar-photovoltaic-cell-basics.

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      Special thanks to the Cadmium Telluride Solar Cells Lab at NREL

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