One way to make solar cells more efficient is to find
a material that will capture energy from a large portion of the spectrum of
sunlight -- from infrared to visible light to ultraviolet.
Energy transfers from photons to a photovoltaic
material when the material absorbs lightwaves that contain the same amount of
energy as its bandgap. A bandgap is the energy required to push an electron from
a material's valence band to the conduction band where electrons are free to
flow.
Most photovoltaic materials absorb a relatively
narrow range of light energy, however. The most efficient silicon solar cells
capture only about 25 percent. Multijunction solar cells made from several
different materials boost efficiency as high as 36 percent, but are relatively
difficult to make and therefore expensive.
Researchers from Lawrence Berkeley National
Laboratory, the University of California, and the Massachusetts Institute of
Technology have engineered a single material that contains three bandgaps and is
capable of capturing more than 50 percent of the sun's energy. The researchers
made the material by forcing oxygen into a zinc-manganese-tellurium crystal. The
oxygen split the crystal's band gap and formed a third one of its
own.
The material could lead to relatively inexpensive,
highly-efficient solar cells that would be much simpler to make than today's
high-end multijunction solar cells.
It will take to three years to assess the technical
feasibility of the multiband solar cell, according to the researchers. The work
appeared in the December 12, 2003 issue of Physical Review Letters.
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