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Welcome back. Part 1 of this primer explains how a solar cell turns sunlight into electricity and why silicon is the semiconductor that usually does it. But silicon cells have a maximum theoretical efficiency of about 32%, so researchers are exploring new materials and cell designs that can improve conversion and performance. Here are the most promising ones:
Layering Up with Multijunction Solar Cells
Some researchers are working to improve cell efficiency by layering multiple different semiconductors to make multijunction solar cells. These cells are essentially semiconductor stacks, as opposed to single-junction cells, which have only one semiconductor. Each layer absorbs a different part of the solar spectrum, making greater use of sunlight than single-junction cells do.
The amount and type of light a semiconductor absorbs is determined by its bandgap, a property that signifies the minimum amount of energy needed to free electrons so the material can conduct electricity. Without this energy, silicon acts like an insulator. Multijunction solar cells can reach record efficiency levels because the light that doesn’t get absorbed by the first semiconductor layer is captured by a layer beneath it. Different layers absorb different parts of the solar spectrum. Once light is absorbed, the energy is converted to electrical current, and less energy is lost since the bandgap is closer to the energy of the absorbed light.
While all solar cells with more than one bandgap are multijunction solar cells, a solar cell with exactly two bandgaps is called a tandem solar cell. Multijunction solar cells that combine semiconductors from columns III and V in the periodic table are called multijunction III-V solar cells.
Multijunction solar cells have demonstrated efficiencies higher than 45%, but they’re costly and difficult to manufacture, so they’re reserved for space exploration. The military is using III-V solar cells in drones, and researchers are exploring other uses for them where power conversion efficiency is key.
The Skinny on Thin-Film Solar Cells
Silicon may be the most common type of solar cell, but thin-film solar cells generally cost less and can be easier to fabricate. Thin films make up 3% to 5% of the global market but are usually less efficient than silicon.
Thin-film solar cells are made by coating a thin layer of a highly absorptive semiconductor material on a sheet of glass, plastic, or metal foil called a substrate rather than creating a crystal wafer. This material can be deposited on flexible surfaces, which keeps costs down and the solar cells versatile. Thin films are typically dark or partially transparent, so the modules look more uniform than the speckled, blue or black crystalline-silicon modules. The record high thin-film cell efficiency is 22.1%, while monocrystalline-silicon cells have reached 25%, and polycrystalline, over 20%.
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