Scientists at Lawrence Berkeley National Lab in California are super excited about a new way to squeeze more electricity out of a solar cell, but right now it only works if the air is made of nitrogen. Bummer! Well, that’s early stage research for you. On the bright side, they solved a problem that has been puzzling other researchers for a good five years, so it’s an important step toward the ultra-efficient, low cost solar cell of the future.
Why Gild The Solar Cell Lily?
At this point you may be asking yourself: why bother making solar cells more efficient? The global solar industry has already gone full mainstream with the photovoltaic technology at hand today.
Solar employment is growing in the US, too, allaying fears that President* Trump’s new tariff on solar modules would kill the industry and send 80,000 jobs down the tubes.
That’s nice, but solar power still accounts for a small fraction of global electricity production. Here in the US, for example, the 2017 figure comes in at just 1.3% and that includes concentrating solar power as well as photovoltaic modules.
Consider what will happen to global resources — including metals and other materials used for racks and other hardware — if and when solar power generation gains a larger market share.
Now add in emissions and other impacts related to manufacturing, transportation and installation, and you’ve just made the case for ensuring that the solar cell of the future is as compact, lightweight and efficient as possible.
The day of reckoning is fast approaching, too. Last year the International Energy Agency painted this rosy picture:
Solar PV is entering a new era. For the next five years, solar PV represents the largest annual capacity additions for renewables, well above wind and hydro. This marks a turning point and underpins our more optimistic solar PV forecast which is revised up by over one-third compared to last year’s report…
Yikes!
Aside from all that, if solar cells can be made smaller and cheaper without a loss of efficiency, that would help accelerate solar adoption.
Molecular Foundry To The Rescue
Where were we? Oh right, that new PV breakthrough from Berkeley Lab. One main pathway to increasing solar cell efficiency is to tweak the solar materials so they can absorb the greatest possible range of the light spectrum.
That includes invisible light, and that’s where the latest breakthrough happened.
Three members of the Berkeley Lab team — Bruce Cohen, P. James Schuck, and Emory Chan — had been working for a good ten years to unlock the mysteries of nanoparticles that can convert near-infrared light to visible light.
Part of the mystery was solved in 2012, when one study indicated that dyes on the surface of the nanoparticles were doing most of the heavy lifting.
So, for the past five years various researchers tried to duplicate the results, because that’s what researchers do. However, most found that the dyes simply degraded when exposed to light, which made it kind of difficult to figure out what was going on.
The new study approached the problem from an interdisciplinary angle, which was made possible by something called the Molecular Foundry, which is this:
The Molecular Foundry is a national scientific user facility and independent division at Berkeley Lab, and one of five Nanoscale Science Research Centers sponsored by the Basic Energy Sciences Office of the DOE Office of Science. [It] is both a multidisciplinary research center at the forefront of nanoscale science and a knowledge-based user facility that provides its state-of-the-art expertise, methods, and instrumentation to over 800 users per year.
Group hug for US taxpayers! By the way, they’re looking for a new Director right now so if you know of a “dynamic leader with an international scientific reputation and record of accomplishment” send them the link.
Lanthanides To The Rescue
The research team credits the “unique mix” of resources and know-how at the Molecular Foundry with the breakthrough.
The idea is that the nanoparticles — called upconverting nanoparticles or UCPNs — owe their conversion ability to lanthanide metal ions.
Lanthanides are the silvery metals at the bottom of the periodic table. They are called rare-earth metals, although as it turns out, some of them are rather abundant.
The team showed demonstrated that the dye was interacting with the lanthanide metals in a specific way:
The proximity of the dyes to the lanthanides in the particles enhances the presence of a dye state known as a “triplet,” which then transfers its energy to the lanthanides more efficiently. The triplet state allowed a more efficient conversion of multiple infrared units of light, known as photons, into single photons of visible light.
Got all that? Here’s the plain language version:
“The dyes act as molecular-scale solar concentrators, funneling energy from near-infrared photons into the nanoparticles,” Schuck said. Meanwhile, the particles themselves are largely transparent to visible light, so they would allow other usable light to pass through, he noted.
Thanks, Schuck.
The team also found that they could tweak the nanoparticles to make them more efficient:
They then found that by increasing the concentration of lanthanide metals in the nanoparticles, from 22 percent to 52 percent, they could increase this triplet effect to improve the nanoparticles’ light-converting properties.
Got A Protective Coating?
On the downside, the new study was carried out in a nitrogen atmosphere. The team is currently looking around for a protective coating that would enable the UCPNs to exist in the real world.
Now that they know how the UCPN mechanism works, researchers can organize the search more efficiently.
It that’s starting to sound familiar, you may be thinking of perovskites and graphene.
Perovskites form another class of promising new PV cell materials that can’t cope with actual life in the real world, but the integration of perovskites with PV technology is coming on quickly.
A similar transformation is at hand for the “wonder material” graphene. The finicky stuff is tricky to work with but graphene PV technology is also on the verge of commercialization.
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Image (cropped): via Berkeley Lab. “An erbium atom (red) in a nanocrystal emits visible, green light via a process known as upconversion that could lead to the development of improved solar cells that capture some previously missed solar energy. Scientists discovered that coating the particles with dyes (blue and purple molecules at right) can greatly enhance this light-converting property.
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