After Gutenberg

“In a photo voltaic cell, negatively doped (n-type) material, with extra electrons in its otherwise empty conduction band, makes a junction with positively doped (p-type) material, with extra holes in the band otherwise filled with valence electrons. Incoming photons of the right energy — that is, the right color of light — knock electrons loose and leave holes; both migrate in the junction’s electric field to form a current… Photons with less energy than the band gap slip right through.” Multi-junction photo voltaic cells made with a single system of Indium Gallium Nitride alloys promises to solve this problem.

In November 2002 researchers in the Materials Sciences Division (MSD) of Lawrence Berkeley National Laboratory learned that the band gap of the semiconductor indium nitride is not 2 electron volts (2 eV), as previously thought, but instead is a much lower 0.7 eV. Because the researchers looked elsewhere than expected, they made what one industry insider has described as “a breakthrough scientific discovery that accelerates the concept of full spectrum solutions in the global photo voltaic markets.”

Working with crystal-growing teams at Cornell University and Japan’s Ritsumeikan University, the Berkeley Lab discovered that photons over virtually the full spectrum of sunlight — from the near infrared to the far ultraviolet could be harvested using “a single system of alloys incorporating indium, gallium, and nitrogen”.

Wladek Walukiewicz, who led the collaborators in the search for greater efficiency, has overcome one of the most fundamental limitations: the band gap of the semiconductor from which the cell is made. Four years later developers at Boeing Spectrolab are working with advanced multi-junction PV cells with efficiencies approaching 38%.

Unfortunately, while the solar cells that Spectrolab produces are of very high efficiency, they also are “NASA budget” expensive. As yet, solar panels using Indium Gallium Nitride solar cells cannot compete with typical flat panels intended for household use, e.g., BP 120W solar panel using wafers made of polycrystalline, amorphous silicon.

Nevertheless, there is interest in multi-junction solar cells manufactured by Boeing-Spectrolab or other high-tech firms. For instance, on 21 November Lion Energy signed a letter of intent to purchase solar farm quantities from Pyron Solar. Boeing – Spectrolab multi-junction photo voltaic cells, along with Pyron’s proprietary power electronics and a proprietary system of lenses to concentrate the sun’s energy by 400 times, are the key components of an innovative solar package for high-efficiency conversion of sunlight into electricity that will be utilized at power generation locations in the United States, Germany, India and China.

References

• “Effects of the narrow band gap on the properties on InN,” by J. Wu, W. Walukiewicz, W. Shan, K. M. Yu, J. W. Ager III, E. E. Haller, Hai Lu, and William J. Schaff, appears in the journal Physical Review B, 15 November 2002.
• Investigations of indium gallium nitride have also been reported in “Unusual properties of the fundamental band gap of InN,” by Wu, Walukiewicz, Yu, Ager, Haller, Lu, Schaff, Yoshiki Saito, and Yasushi Nanishi, Applied Physics Letters, 27 May 2002, and in “Small band gap bowing in In_1-xGa_xN alloys,” by Wu, Walukiewicz, Yu, Ager, Haller, Lu, and Schaff,
  Applied Physics Letters, 24 June 2002.

• More on the new full-spectrum photo voltaic materials