Multijunction Solar Cell Sets World Record for Efficiency
Scientists at the Department of Energy’s (DOE) National Renewable Energy Laboratory (NREL) have won an R&D 100 award from R&D Magazine for their world-record multijunction solar cell. The three-layered cell, dubbed the SJ3, converted 43.5% of the energy in sunlight into electrical energy. That rate has perked up demand for the cell for use in concentrator photovoltaic arrays for utility-scale energy production.
In November 2012, the 43.5% efficiency record at 415 suns was exceeded, with 44% efficiency at 947 suns. Concentrator photovoltaic technology (CPV) increases efficiency by using low-cost lenses to multiply the sun’s intensity, which scientists refer to as numbers of suns.
The SJ3 has roots going back 15 years, to when National Renewable Energy Laboratory researchers began looking for materials they could easily grow that also had ideal combinations of band gaps for converting light from the sun into electricity.
Band gaps are energies that characterize how a semiconductor material absorbs photons, as well as how efficiently a solar cell made from that material can extract the useful energy from those photons.
“The ideal band gaps for a solar cell are determined by the solar spectrum,” said Daniel Friedman, manager of the NREL III-V Multijunction Photovoltaics Group. “There’s no way around that.”
Aimed at the market for utility-scale CPV projects, the SJ3 cells are designed for application under sunlight concentrated to 1,000 times its normal intensity via inexpensive lenses gathering the light and directing it at each cell. In areas of clear atmosphere and intense sunlight, like the U.S. desert Southwest, CPV has outstanding potential for lowest-cost solar electricity. Enough sunlight is available in these areas to supply many times the electrical energy needs of the entire United States.
Fundamental Limits of Photovoltaic Cells
Sunlight is comprised of photons of a large range of energies from roughly zero to four electron volts (eV). This widerange of energies makes a fundamental difficulty in conventional solar cells that have a single photovoltaic junction with a single characteristic band gap energy.
The most efficient conversion for conventional cells is for the photons that very nearly match the band gap of the semiconductors in the cell. Higher-energy photons give up their excess energy to the solar cell as waste heat, even as lower-energy photons are not collected by the solar cell, and their energy is entirely lost.
This imposes a fundamental limit on the efficiency of a conventional solar cell. We can overcome that limitation with multijunction solar cells. Using multiple layers of materials in the cells, they create multiple junctions, each with different band gap energies. Each converts a different energy range of the solar spectrum.
An invention in the mid-1980s by NREL’s Jerry Olson and Sarah Kurtz led to the first practical, commercial multijunction solar cell, a GaInP/GaAs two-junction cell with 1.85-eV and 1.4-eV bandgaps that was recognized with an R&D 100 award in 1990, and later to the three-junction commercial cell based on GaInP/GaAs/Ge that won an R&D 100 award in 2001.
Gallium Arsenide Lattice
NREL researchers realised if they could replace the 0.67-eV third junction with one of superior tuning to the solar spectrum, the result would capture more of the sun’s light throughout the day. But they needed a material that had an atomic structure that matched the lattice of the layer above it, and the ideal band gap.
“We knew from the shape of the solar spectrum and modeling solar cells that what we wanted was a third junction that has a band gap of about 1.0 electron volt, lattice-matched to gallium arsenide,” Friedman said. “The lattice match makes materials easier to grow.”
Because the III-V semiconductors have similar crystal structures and ideal diffusion, absorption, and mobility properties for solar cells, they focused on materials from the third and fifth columns of the periodic table.
But there seemed to be no way to capture the benefits of the gallium arsenide material while matching the lattice of the layer below, because no known III-V material compatible with gallium arsenide growth had both the desired 1-eV band gap and the lattice-constant match to gallium arsenide.
In the early 1990s, that all changed when a Tokyo research group at NTT Laboratories made an unexpected discovery while working on an unrelated problem. Although gallium nitride has a higher band gap than gallium arsenide, when you add a bit of nitrogen to gallium arsenide, the band gap shrinks, which is 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 said. “It helped push us to start making solar cells with this new dilute nitride material.”
Molecular Beam Epitaxy
The new nitrided gallium arsenide solar cells NREL developed had good news and bad news that went with them.
“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 said. “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.”
“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 said. “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.”
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 said. But to do that, practically all impurities would need to be removed, and the only way to remove them would be to use a different growth technique.
“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 said.
Another growth technique, known as molecular beam epitaxy (MBE), is done in such an ultra-high vacuum (10 to the minus 13 atmospheres) that it 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.
Dilute-Nitride Junction and Heavy Germanium
Using the new dilute-nitride junction, eliminates the need for the germanium layer, which constitutes about 90% of the weight of the cell. That is a big deal when solar cells are used to power satellites. The reduction in weight means a smaller rocket is needed to launch into space, possibly significantly reducing costs. The lighter weight is also essential for the military, which is increasingly asking soldiers to carry backpacks that include solar devices to power electronics.
Owing to its design and size, SJ3 is a plug-in replacement for the standard cell now used by the space and CPV industries. If a 40%-efficient cell were replaced with a 44%-efficient cell, this would instantly increase the entire system power output by close to 10%.
Utilities are now ordering the SJ3 cells so fast that the manufacturer, Solar Junction, has run out of the pilot-scale stock and gone into partnership with manufacturer IQE to ramp up to full manufacturing scale.