Game Over For Coal: Scientists Nail Down Pesky Perovskite Solar Cell Problem

February 1st, 2018 by Tina Casey

Remember back when plastic was a new and exciting thing? That’s more or less where we are with that other p-word, perovskites. Legions of scientists around the world have been trying to tease a durable solar cell out of this optically-promising but fussy material, and it looks like a team from the National Renewable Energy Laboratory has put its finger on the solution.

Did we mention that perovskites are cheap and could be manufactured at high volume, too? Until recently natural gas was the main driver pushing coal out of the power generation business, but renewable energy is also becoming a force to be reckoned with, and its influence will grow stronger as the cost of photovoltaic modules continues to drop.

Another Perovskite Solar Cell Breakthrough

Perovskite is a natural occurring mineral with good optical properties, and its crystalline structure can be replicated with relative ease. NREL, for one, is a huge fan of synthetic perovskites for the low cost solar cells of the future, but the problem is that they deteriorate quickly when exposed to ambient air.

That’s quite an Achilles heel, right?

In the latest perovskite development, the NREL team seems to have solved that little thing about air. Here’s the teaser from the lab:

Researchers at the U.S. Department of Energy’s National Renewable Energy Laboratory (NREL) created an environmentally stable, high-efficiency perovskite solar cell, bringing the emerging technology a step closer to commercial deployment.

Do tell! The research team successfully tested a perovskite solar cell in ambient conditions without protection for 1,000 hours, and it retained 94% of its conversion efficiency.

You can get all the details from the study, titled “Tailored Interfaces of Unencapsulated Perovskite Solar Cells for >1000 Hours of Ambient Operational Stability” in the journal Nature.

Basically, instead of relying on a protective capsule around the exterior of the solar cell, the team tinkered with its innards. Here’s a snippet from the study:

Each interface and contact layer throughout the device stack plays an important role in the overall stability which, when appropriately modified, yields devices in which both the initial rapid decay (often termed burn-in) and the gradual slower decay are suppressed.

Got all that? The team focused first on the “weakest link” in a perovskite solar cell, which typically consists of a thin layer of a tricked out organic molecule called spiro-OMeTAD. Earlier research has established this layer as one epicenter of perovskite degradation, so the research team replaced it with another molecule borrowed from the Colorado School of Mines.

That’s interesting because the Colorado School of Mines is all about, well, mining. So, what are they doing with solar research? A lot! The school is partners with NREL and it is the headquarters of the Renewable Energy Materials Research Science and Engineering Center.

What About That New Molecule?

The new molecule, nicknamed EH44, fit the bill because it repels water and because it doesn’t include lithium.

That didn’t solve the whole problem, because there are actually two aspects to perovskite solar cell degradation. One happens quickly at the beginning of exposure, and the other happens gradually some time later.

EH44 solved the gradual aspect. To tackle the other aspect, the research team ditched a bottom layer of titanium oxide (TiO 2 ) and used tin oxide (SnO 2 ) instead. They also made some additional adjustments:

…With both EH44 and SnO 2 in place, as well as stable replacements to the perovskite material and metal electrodes, the solar cell efficiency remained steady. The experiment found that the new SnO 2 layer resolved the chemical makeup issues seen in the perovskite layer when deposited onto the original TiO 2 film.

As For Coal…

Despite President* Trump’s numerous promises to help coal miners, their families and communities, coal power generation in the US slid all the way through his first year in office, and things are already looking bad for his second year.

A new op-ed in The Hill lays out this scenario:

This year, 15,000 MW of coal-fired electricity generation will be retired — double the 2017 total.

Yikes! The piece discusses findings in a new report produced for the Institute for Energy Economics and Financial Analysis, titled “U.S. Coal: More Market Erosion is on the Way.”

The report confirms the industry consensus on natural gas as coal’s main competition, but it also indicates where and how renewable energy will become a greater force:

Utilities in the middle of the country, where many coal-fired generating plants are located, are rapidly increasing their investments in wind, which — like solar — now provides lower-cost electricity during times of peak demand, when prices are highest, thus depriving coal-burning plants of badly needed revenue.

Among the plants set to close is the largest coal power plant left in Pennsylvania, the Bruce Mansfield facility in Shippingport. According to a report in the Pittsburgh Post-Gazette, demand for electricity from the plant has dipped “from constant to intermittent.” Not helping much, a recent fire at the plant did some expensive damage and knocked two of its three generating units offline.

Do read the full piece for details on the financial situation of the plant’s owner, FirstEnergy, and how that impacts its plans for several other coal and nuclear plants. Meanwhile, the Post-Gazette cites FirstEnergy CFO James Pearson:

Pennsylvania and Ohio embraced deregulation in 1996, a decade before a global fracking boom resulted in record-low natural gas prices. That, along with growing investments in wind and solar power, made nuclear and coal noncompetitive.

“Those units cannot generate enough cash to cover costs,” Mr. Pearson said of FirstEnergy’s nuclear and coal-fired plants.

[snip]

The inevitability of the decision has been building since the fracking boom began a little more than seven years ago, he said.

No worries, evidently the President has a plan to protect coal power plants. Well, actually he had a plan, but that got scotched by federal regulators — including his own appointees — on January 9.

After that not so promising start, apparently there is another plan, which consists of cutting the Energy Department’s budget for clean tech and energy efficiency research by almost 75%. Not sure how that will stop more coal power plants from closing, but stay tuned.

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*As of this writing.

Photo (cropped): Perovskite solar cells via NREL.









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