This is an excerpt from EERE Network News, a weekly electronic newsletter.

November 20, 2002

Unexpected Discovery Shows Promise for Better Solar Cells

A serendipitous discovery about the electronic properties of indium nitride may eventually yield high-efficiency solar cells that are able to make use of the sun's entire spectrum of radiation. While researching the properties of light-emitted diodes (LEDs), researchers at DOE's Lawrence Berkeley National Laboratory (LBNL) recently discovered that the band-gap energy of indium nitride is 0.7 electron volts, much lower than the 2.0 electron volts previously expected. The band-gap energy is the amount of energy needed to free an electron from its atom; a solar cell material can only capture sunlight at energies equal to or greater than its band-gap energy. The low band-gap energy of indium nitride means that it can capture sunlight at much lower energies than expected, so the material is able to capture low-frequency, red or near-infrared light.

To make practical use of their discovery, the LBNL researchers propose making a solar cell from several layers of varying alloys of indium, gallium, and nitrogen. Such indium gallium nitride solar cells could potentially convert the full spectrum of sunlight-from the near-infrared to the far ultraviolet-into electricity. The LBNL researchers' familiarity with the material, thanks to its use in LEDs, makes them confident that it could be used to create two-layer solar cells that they believe could achieve a sunlight-to-electricity conversion efficiency of 50 percent. They also claim that multi-layered cells could reach the maximum theoretical conversion efficiency of greater than 70 percent. In contrast, the best existing solar cell achieves a conversion efficiency of only about 34 percent.

Teams at Cornell University and Japan's Ritsumeikan University contributed to the LBNL research by preparing the high-purity crystals of indium nitride that were required for the band-gap experiments. See the LBNL press release. The LBNL Web site also features a detailed technical description of the work.