Published on October 28th, 2013 | by Tina Casey7
Record Breaking Solar Cell Efficiency From A “Perfect Crystal”
October 28th, 2013 by Tina Casey
Gallium is already on its way to becoming the workhorse of the solar tech field, and now it looks like the soft metal is is on track to become a thoroughbred. A team of US scientists has hit upon an improved method for growing indium gallium nitride (InGaN) crystals that could lead to record-breaking solar cell efficiency. So far the method has resulted in a film of InGaN that has “almost ideal characteristics.”
To ice the cake, an analysis of the film revealed the precise reason why the results of the new InGaN growing method were so good, which could lead to further improvements in LED technology as well as solar cells.
A Perfect InGaN Crystal
Nitride refers to a compound of nitrogen, in this case in conjunction with indium, a soft silvery-white, zinc-like metal, as well as gallium.
If InGaN already rings a bell, you might be thinking of the world record-setting concentrating solar cell module developed by the company Amonix. That module is based on a record setting solar cell developed by Solar Junction, that incorporates a layer of antimony-doped InGaN.
Gallium in particular is an effective material for LEDs as well as solar cells due to its band gap characteristics, most familiarly in CIGS thin film solar cells (CIGS is the semiconductor copper-indium-gallium-(di)selenide). The potential has barely been scratched, though.
Arizona State University and the Georgia Institute of Technology collaborated on the new method, which addressed the problem at its core. The obstacle has been irregularities in the atomic structure of the crystal, as explained by ASU team leader Fernando Ponce:
Being able to ease the strain and increase the uniformity in the composition of InGaN is very desirable, but difficult to achieve. Growth of these layers is similar to trying to smoothly fit together two honeycombs with different cell sizes, where size difference disrupts a periodic arrangement of the cells.
The new method is called metal modulated epitaxy. It is a variation of the epitaxial deposition method first developed at Bell Labs in the 1960’s, which involves applying a thin layer of material to a substrate that takes on the crystal structure of the lower layer.
The result was a more film that resembles a perfect crystal, both in its uniformity of structure and in the desirable trait of luminosity.
As for why the improvement occurred, the analysis credited “strain relaxation at the first atomic layer of crystal growth.”
We Built This Next-Generation Solar Cell
Solar cell efficiency is not the only factor leading to a drop in the cost of solar power, since the “soft costs” of installing a solar system still account for a considerable chunk of change.
However, solar cell efficiency is still a key factor, and if the new findings translate from the lab to commercial development, let’s throw ourselves a taxpayer appreciation party.
The latest development has roots in a 2008 paper published by Georgia Tech team leader Alan Doolittle with other collaborators, titled “Metal modulation epitaxy growth for extremely high hole concentrations above 1019cm−3 in GaN.” It described how the metal modulated epitaxy method yielded an enhanced doping efficiency of up to 10 percent, which compares favorably to the 1 percent efficiency under the conventional method.
That research was funded by grants from the Office of Naval Research, the Air Force Office of Scientific Research, and the Defense Advanced Research Projects Agency as well as the National Science Foundation.
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