news digest ♦ Solar
Friedman comments. “The lattice match makes materials easier to grow.”
They concentrated on materials from the third and fifth columns of the periodic table because these III-V semiconductors have similar crystal structures and ideal diffusion, absorption, and mobility properties for solar cells.
But there was seemingly no way to capture the benefits of the GaAs material while matching the lattice of the layer below, because no known III-V material compatible with GaAs growth had both the desired 1-eV band gap and the lattice-constant match to GaAs.
That changed in the early 1990s, when a research group at NTT Laboratories in Tokyo working on an unrelated problem made an unexpected discovery. Even though GaN has a higher band gap than GaAs, when you add a bit of nitrogen to GaAs, the band gap shrinks - exactly the opposite of what was expected to happen.
“That was very surprising, and it stimulated a great deal of work all over the world, including here at NREL,” Friedman says. “It helped push us to start making solar cells with this new dilute nitride material.”
Good Band Gaps, but Not So Good Solar Material
The new solar cells NREL developed had two things going for them - and one big issue.
“The good things were that we could make the material very easily, and we did get the band gap and the lattice match that we wanted,” Friedman says. “The bad thing was that it wasn’t a good solar cell material. It wasn’t very good at converting absorbed photons into electrical energy. Materials quality is critical for high-performance solar cells, so this was a big problem.”
Still, NREL continued to search for a solution.
“We worked on it for quite a while, and we got to a point where we realized we had to choose between two ways of collecting current from a solar cell,” Friedman says. “One way is to let the electrical carriers just diffuse along without the aid of an electric field. That’s what you do if you have good material.”
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If the material isn’t good, though, “you have to introduce an electric field to sweep the carriers out before they recombine and are lost,” Friedman adds.
But to do that, virtually all impurities would have to be removed. And the only way to remove the impurities would be to use a different growth technique.
Using Molecular Beam Epitaxy to Virtually Eliminate Impurities
Solar cells are typically grown using MOVPE.
“It works great, except you always get a certain level of impurities in the material. That’s usually not a problem, but it would be an issue for this novel material, with the gallium arsenide diluted with nitrogen,” Friedman points out.
However, a different growth technique, MBE, used in an ultra-high vacuum, ( 10-13 atmospheres), can lower the impurities to the point where an electric field can be created in the resulting photovoltaic junction. And that would make the otherwise promising gallium-arsenide-dilute-nitride material work as a solar cell.
“The only problem was that there was no one in the entire world manufacturing solar cells by MBE,” Friedman says.
But that was soon to change.
Partnering with Stanford University Startup, Solar Junction
A Stanford University research group with expertise in the use of MBE for other electronic devices saw an opportunity. In around 2007, they spun out a startup company they named Solar Junction.
Because Solar Junction was a mix of enthusiastic recent Ph.D.s and experienced hands from outside the established solar cell field, “they weren’t tied to the constraints of thinking this couldn’t be done, that the only economically viable way to make solar cells was with MOVPE,” Friedman says.
The federal lab and the startup got together. Solar Junction won a $3 million DOE/NREL Photovoltaic
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