by Emma Foehringer Merchant
April 20, 2020

The U.S. solar industry is no stranger to records.

Last year, the resource accounted for 40 percent of new electric generating capacity, beating out natural gas and wind. The industry regularly hits new low prices for power-purchase agreements, the most recent being a project contracted last year for under $20 per megawatt-hour by 8minute Solar Energy and the Los Angeles Department of Water and Power. And last quarter, the pipeline for new large-scale solar projects reached a new high at 48.1 gigawatts.

Last week, the industry logged another first. Researchers at the National Renewable Energy Laboratory (NREL), one of the U.S.’ national labs working on renewable energy technologies, designed a solar cell that reached the highest efficiency ever recorded — an upper bound the lab has hit several times in the past few decades.

NREL’s latest record centers on six-junction III-V solar cells. Translated, that means a cell assembled from six different layers made of materials from groups 13 and 15 of the periodic table (elements such as gallium and thallium).

Because the commercial solar industry relies on silicon technology, that puts a ceiling on efficiency.

“Silicon is limited fundamentally, just by being a single material. It’s got a single bandgap,” Ryan France, a scientist at NREL who works on the team that developed the cell, said in an interview. “The multijunction concept can get much higher efficiencies than that by combining different materials with different bandgaps.”

Bandgaps refer to the part of the solar spectrum a material can absorb and use to produce energy. Single bandgap cells can only reach 33.5 percent efficiency in ideal circumstances with regular old sunlight, according to the Department of Energy. But layering more and varied materials means cells can absorb a wider swath of the solar spectrum.

NREL’s new cell hit 39.2 percent efficiency under unconcentrated solar conditions and 47.1 percent using concentrated light.

It’s an impressive topline figure, but that doesn’t mean it will necessarily change the game for every solar project across the globe.

NREL's France calls the near 50 percent efficiency a “psychological milestone.” It may open the door for more applications of the extremely niche — and expensive — III-V technology, but that will depend on commercializing applications and significantly drawing down costs.

Improving solar materials science

Multijunction solar cells have been around for decades. Even the specific construction of NREL’s most recent cell, called an inverted metamorphic device structure, was first invented at the lab about 15 years ago.

Much of the innovation tied to this most recent efficiency number, France said, was in the materials science — understanding how the layers NREL used could fit together to maximize efficiency.

“One of the main challenges that we have overcome in our recent device design is how we can integrate six different materials into one device while really maintaining a high material quality [and] not having a bunch of internal defects that pop up within the material that would lower the solar cell efficiency,” he said. “It’s not easy to integrate them into one device.”

The “inverted” structure means that NREL deposits higher bandgap layers of III-V alloys first before adding lower bandgap layers.

“This material science, I should point out, has been what’s enabled us to make these high-efficiency PV devices, but it is not specific to PV,” said France. “It could be useful for a lot of other optoelectronic devices as well.” That could include LEDs or lasers.

The new cell’s most impressive efficiency numbers arise when using “concentrated light,” or sunlight that’s amplified using mirrors or lenses, which is no longer very common in the U.S.

Right now, III-V cells are most often used in space photovoltaics, attached to weight-conscious satellites headed beyond the Kármán line. But France said solar concentrator systems, or any other terrestrial use with serious space constraints, could be a potential application.

To go mainstream, researchers will have to get the costs down. Making high-tech solar cells in a lab is not cheap.

Lab solar cells: High efficiency, high price tag

For now, commercial adoption of III-V solar cells is limited by their price. NREL has not quantified a dollars-per-watt price for its record cell, but in 2018 the national lab estimated manufacturing costs for III-V cells ranged from $40 to $100 per watt.

Commercial monocrystalline PERC cells cost just $0.13 per watt, according to the latest data from Wood Mackenzie Power & Renewables.

That’s a wide delta, and the lab is still at work on how to bridge it. But France said, because of their higher efficiency, III-V cells may not need to contest silicon cells on price to gain a steadier foothold in the market. A 2018 paper from NREL estimated that, at high-volume production, it may be possible to produce the cells for $0.40 per watt or less if they’re commercialized.  

“Silicon is getting pretty darn cheap, but III-Vs have an efficiency advantage. We don’t necessarily have to get down to the cost of silicon,” said France. “The more costs we can reduce, the larger market we’ll take over. We already have a home in space PV and we’re hoping to grow that.”

The capabilities of multijunction solar cells may also encourage innovation in other PV technologies. High-bandgap perovskites are being layered onto silicon, for instance, to create multijunction cells.

Reaching beyond 50 percent efficiency for III-V cells is also quite possible, France said. But the applicability of the technology does depend on the lab’s future innovations on price. NREL is working to examine simpler solar cells, such as two-junction or three-junction, and innovating the processes for producing its multijunction III-V cells.

“In terms of where III-V technology is going to go, it just depends on what we’re able to do [on] cost,” said France. “[Under] the dollars-per-watt metric, we have a lot of watts, but we have a lot of dollars as well.”

“If we can get that metric down, then we’re good.”