The key to the getting a full spectrum solar cell, hence increasing efficiency and cutting costs, is to select the right material, insulate the intermediate band, and exploit a common, affordable semiconductor manufacturing process.  (Source: LBNL)

A new cell design by LBNL team members, including staff scientist Kin Man Yu and team leader Wladek Walukiewicz, pictured here, offers full spectrum capture in a single-alloy cell created by common semiconductor manufacturing techniques. The resulting blend -- made from gallium, arsenic, and nitrogen -- also is advantageous in that it doesn't rely on overly scarce elements like indium.  (Source: LBNL)
Vapor deposition process will allow for less expensive mass production of the cells, LEDs

A significant fraction of visible light goes to waste at solar farms, as current generation cells generally can only take advantage of a very specific band of the spectrum.  Researchers have spent the last decade cooking up cells that could capture the entire spectrum and operate at higher efficiency, therefore lowering costs.  But those cells were hard to manufacture.

Now a research collaboration between researchers in the Solar Energy Materials Research Group in the Materials Sciences Division (MSD) at the U.S. Department of Energy’s Lawrence Berkeley National Laboratory (Berkeley Lab), and a corporate partner -- Sumika Electronics Materials, Inc. in Phoenix, Arizona -- may have discovered [press release] the key to mass producing these more efficient cells.

The key was in cooking up the right semiconductor.  

I.  Finding the Right Material

Ideally a semiconductor would absorb the entire visible and non-visible spectrum of light, including infrared, the visible colors (white light), and ultraviolet light.  Unfortunately no single semiconductor can do that.

Wladek Walukiewicz [profile], head of Berkley Lab's MSD team explains, "Since no one material is sensitive to all wavelengths, the underlying principle of a successful full-spectrum solar cell is to combine different semiconductors with different energy gaps."

Many semiconductors used in the industry are actually blends of two or more elements -- one or more of which are semiconducting elements like silicon or gallium.  The blend composition (how much of each element) determines what wavelength of light is absorbed.  What Dr. Walukiewicz's team discovered in 2002 was that wiring layers of semiconductor could create a full spectrum cell with different blends (and thus different absorption wavelengths) in series.  That study used blends of indium and gallium in indium gallium nitride.

Then in 2004, his team discovered a way to make a single alloy have three bands (a valence band, an "intermediate" band, and a conduction band), thus absorbing the full spectrum in a single layer of alloy.  The alloy used was zinc (plus manganese) and tellurium, doped with oxygen.  While the results were good, the resulting material was too expensive and difficult to mass-produce.

Now the team has cooked a semiconductor blend that they feel is ready for prime time.  Like the zinc/manganese/tellurium/oxygen blend, the new mix is a tri-band device.  Taking gallium arsenide and replacing some of the arsenic atoms with nitrogen, to form the third band, form the semiconductor blend.  This replacement can be done using metalorganic chemical vapor deposition (MOCVD), one of the most common methods of fabricating compound semiconductors.

The findings are a beautiful example of how research that is far from production can iteratively evolve and mature into something commercially viable.  It is also a testament to theory pointing the way for manufacturing progress.  

The team used a special model developed in lab, called the band anticrossing model.  That model states, among other things, that the element replaced in the alloy must be from the same group in Mendeleev’s original periodic table.  It also gives clues as to what percent to use.  The original zinc/manganese/tellurium/oxygen blend was a so-called "II-VI" semiconductor alloy, while the new blend (gallium/arsenic/nitrogen) is a "III-V" alloy.

II.  Preparing the Blend for Production

Though the researchers had a good idea what material and what percent of it they wanted to use for this study, the key to building a working device was in doping the semiconductor so as to isolate the intermediate band.  Describes Dr. Walukiewicz, "The intermediate band must absorb light, but it acts only as a stepping stone and must not be allowed to conduct charge, or else it basically shorts out the device."

By adding additional doped layers the researchers were able to essentially isolate the central layer, and assured it was passing on its energy and not shorting out.

Part of the verification that the device worked was taken from when current was applied to it.  When a solar material is subject to current, it reverses from absorbing photons, to emitting them (goes from a photodiode to a light-emitting diode).  The resulting LED admitted emitted three peaks in the visible range and a peak in the infrared range.  This means that not only does the device promise high efficiency absorbance of sunlight, but it may also be promising as a light-emitting diode for applications such as lighting and lasers.

It was a long path to the full-spectrum cell, but the results in "Engineering the Electronic Band Structure for Multiband Solar Cells" certainly offer a convincing argument that a commercially viable solution has finally been reached.  The paper was written by team members Nair Lopez, Lothar Reichertz [profile], Kin Man Yu [profile], and Wladyslaw Walukiewicz, plus Sumika Electronic's Ken Campman, and published [abstract] in the January 10, 2011 edition of the peer-reviewed journal Physical Review Letters.

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