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Full-spectrum Solar Cell

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  • William Tamblyn
    http://www.sciencedaily.com/releases/2002/11/021119072756.htm Source: Lawrence Berkeley National Laboratory Date: 11/19/2002 An Unexpected Discovery Could
    Message 1 of 1 , Nov 20, 2002
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      http://www.sciencedaily.com/releases/2002/11/021119072756.htm

      Source: Lawrence Berkeley National Laboratory
      Date: 11/19/2002

      An Unexpected Discovery Could Yield A Full Spectrum Solar Cell


      BERKELEY, CA � Researchers in the Materials Sciences Division (MSD) of Lawrence
      Berkeley National Laboratory, working with crystal-growing teams at Cornell
      University and Japan's Ritsumeikan University, have 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.
      The serendipitous discovery means that a single system of alloys incorporating
      indium, gallium, and nitrogen can convert virtually the full spectrum of
      sunlight -- from the near infrared to the far ultraviolet -- to electrical
      current.

      "It's as if nature designed this material on purpose to match the solar
      spectrum," says MSD's Wladek Walukiewicz, who led the collaborators in making
      the discovery.

      What began as a basic research question points to a potential practical
      application of great value. For if solar cells can be made with this alloy,
      they promise to be rugged, relatively inexpensive -- and the most efficient
      ever created.

      In search of better efficiency

      Many factors limit the efficiency of photovoltaic cells. Silicon is cheap, for
      example, but in converting light to electricity it wastes most of the energy as
      heat. The most efficient semiconductors in solar cells are alloys made from
      elements from group III of the periodic table, like aluminum, gallium, and
      indium, with elements from group V, like nitrogen and arsenic.

      One of the most fundamental limitations on solar cell efficiency is the band
      gap of the semiconductor from which the cell is made. In a photovoltaic 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. For example, red
      light photons are not absorbed by high-band-gap semiconductors. While photons
      with energy higher than the band gap are absorbed -- for example, blue light
      photons in a low-band gap semiconductor -- their excess energy is wasted as
      heat.

      The maximum efficiency a solar cell made from a single material can achieve in
      converting light to electrical power is about 30 percent; the best efficiency
      actually achieved is about 25 percent. To do better, researchers and
      manufacturers stack different band gap materials in multijunction cells.

      Dozens of different layers could be stacked to catch photons at all energies,
      reaching efficiencies better than 70 percent, but too many problems intervene.
      When crystal lattices differ too much, for example, strain damages the
      crystals. The most efficient multijunction solar cell yet made -- 30 percent,
      out of a possible 50 percent efficiency -- has just two layers.

      A tantalizing lead

      The first clue to an easier and better route came when Walukiewicz and his
      colleagues were studying the opposite problem -- not how semiconductors absorb
      light to create electrical power, but how they use electricity to emit light.

      "We were studying the properties of indium nitride as a component of LEDs,"
      says Walukiewicz. In light-emitting diodes and lasers, photons are emitted when
      holes recombine with electrons. Red-light LEDs have been familiar for decades,
      but it was only in the 1990s that a new generation of wide-band gap LEDs
      emerged, capable of radiating light at the blue end of the spectrum.

      The new LEDs were made from indium gallium nitride. With a band gap of 3.4 eV,
      gallium nitride emits invisible ultraviolet light, but when some of the gallium
      is exchanged for indium, colors like violet, blue, and green are produced. The
      Berkeley Lab researchers surmised that the same alloy might emit even longer
      wavelengths if the proportion of indium was increased.

      "But even though indium nitride's band gap was reported to be 2 eV, nobody
      could get light out of it at 2 eV," Walukiewicz says. "All our efforts failed."


      Previously the band gap had been measured on samples created by sputtering, a
      technique in which atoms of the components are knocked off a solid target by a
      beam of hot plasma. If such a sample were to be contaminated with impurities
      like oxygen, the band gap would be displaced.

      To get the best possible samples of indium nitride, the Berkeley Lab
      researchers worked with a group at Cornell University headed by William Schaff,
      renowned for their expertise at molecular beam epitaxy (MBE), and also with a
      group at Ritsumeikan University headed by Yasushi Nanishi. In MBE the
      components are deposited as pure gases in high vacuum at moderate temperatures
      under clean conditions.

      When the Berkeley Lab researchers studied these exquisitely pure crystals,
      there was still no light emission at 2 eV. "But when we looked at a lower band
      gap, all of a sudden there was lots of light," Walukiewicz says.

      The collaborators soon established that the alloy's band-gap width increases
      smoothly and continuously as the proportions shift from indium toward gallium,
      until -- having covered every part of the solar spectrum -- it reaches the
      well-established value of 3.4 eV for simple gallium nitride.

      Promising signs

      At first glance, indium gallium nitride is not an obvious choice for solar
      cells. Its crystals are riddled with defects, hundreds of millions or even tens
      of billions per square centimeter. Ordinarily, defects ruin the optical
      properties of a semiconductor, trapping charge carriers and dissipating their
      energy as heat.

      In studying LEDs, however, the Berkeley Lab researchers found that the way
      indium joins with gallium in the alloy leaves indium-rich concentrations that,
      remarkably, emit light efficiently. Such defect-tolerance in LEDs holds out
      hope for similar performance in solar cells.

      To exploit the alloy's near-perfect correspondence to the spectrum of sunlight
      will require a multijunction cell with layers of different composition.
      Walukiewicz explains that "lattice matching is normally a killer" in
      multijunction cells, "but not here. These materials can accommodate very large
      lattice mismatches without any significant effect on their optoelectronic
      properties."

      Two layers of indium gallium nitride, one tuned to a band gap of 1.7 eV and the
      other to 1.1 eV, could attain the theoretical 50 percent maximum efficiency for
      a two-layer multijunction cell. (Currently, no materials with these band gaps
      can be grown together.) Or a great many layers with only small differences in
      their band gaps could be stacked to approach the maximum theoretical efficiency
      of better than 70 percent.

      It remains to be seen if a p-type version of indium gallium nitride suitable
      for solar cells can be made. Here too success with LEDs made of the same alloy
      gives hope. A number of other parameters also remain to be settled, like how
      far charge carriers can travel in the material before being reabsorbed.

      Indium gallium nitride's advantages are many. It has tremendous heat capacity
      and, like other group III nitrides, is extremely resist to radiation. These
      properties are ideal for the solar arrays that power communications satellites
      and other spacecraft. But what about cost?

      "If it works, the cost should be on the same order of magnitude as traffic
      lights," Walukiewicz says. "Maybe less." Solar cells so efficient and so
      relatively cheap could revolutionize the use of solar power not just in space
      but on Earth.

      The Berkeley Lab is a U.S. Department of Energy national laboratory located in
      Berkeley, California. It conducts unclassified scientific research and is
      managed by the University of California.

      Additional information

      * "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 In1-xGaxN alloys," by
      Wu, Walukiewicz, Yu, Ager, Haller, Lu, and Schaff, Applied Physics Letters, 24
      June 2002.


      * More on the new full-spectrum photovoltaic materials --
      http://www.lbl.gov/msd/PIs/Walukiewicz/02/02_8_Full_Solar_Spectrum.html


      Editor's Note: The original news release can be found here.[LINK IN ORIGINAL
      TO:
      http://www.lbl.gov/Science-Articles/Archive/MSD-full-spectrum-solar-cell.html ]

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      Note: This story has been adapted from a news release issued for journalists
      and other members of the public. If you wish to quote any part of this story,
      please credit Lawrence Berkeley National Laboratory as the original source. You
      may also wish to include the following link in any citation:

      http://www.sciencedaily.com/releases/2002/11/021119072756.htm


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