Why The Organic Solar Cell Of The Future Will Twist When Excited

Low cost solar power is already breathing new life into the beleaguered US manufacturing sector, and this is just the beginning. After all, silicon is king in today’s solar industry, and silicon is rather pricey compared to other solar cell materials. Just imagine what will happen when some other, less expensive material comes along and knocks silicon off the hill.

An atomic-level twist is the key to the ultra-efficient singlet fission organic solar cell of the future (image via NREL).

The Road To An Even Cheaper Solar Cell

Solar cell researchers already imagined that possibility years ago, when they came up with an organic alternative to silicon.

That’s “organic” as in carbon-based, in contrast with silicon which is, well, silicon.

For those of you keeping score at home, silicon is a brittle, crystalline material residing in Group 14 of the Periodic Table, sandwiched in between carbon and the rest of the group, including geranium, tin, lead, and a radioactive synthetic newcomer called flerovium.

If you caught that thing about crystalline, that’s the key to the silicon solar conversion magic.

A carbon-based solar cell, on the other hand, does not have a crystalline structure. It is more like a form of plastic. That’s why an organic solar cell is thin, lightweight, and flexible.

Those characteristics mean that organic solar cells can be applied to buildings, windows, cars, clothing, and anywhere else that silicon solar cells are impractical because they are more rigid, thick and heavy.

“This [organic photovoltaic] technology also has the theoretical potential to provide electricity at a lower cost than first- and second-generation solar technologies,” enthuses the US Department of Energy. “Because various absorbers can be used to create colored or transparent OPV devices, this technology is particularly appealing to the building-integrated PV market.”

The Low Cost Organic Solar Cell: So Close & Yet So Far

Like plastic, organic solar cells are also cheap. However, the solar conversion efficiency of an organic solar cell is worlds away from the superior capabilities of silicon.

Last time we checked, organic solar cell conversion efficiency barely scratched past 17%, while the latest iteration of commercial silicon technology is approaching the 25% level.

On top of that shortcoming, organic solar cells also have durability and longevity matters with which to deal.

On the up side, researchers are hot on the trail of a solution to the solar conversion efficiency issue.

Last year, CleanTechnica took note of new photovoltaic research from Columbia University, involving a two-for-one phenomenon that can occur in some types of organic solar cells.

Typically, one photon of light creates one exciton in a solar cell (exciton refers to the state of energy created by sunlight in a solar cell). In a phenomenon called singlet fission, some organic solar cells can generate two excitons from one photon. If all goes according to plan, that translates into a significantly more efficient solar cell.

The Singlet Fission Solar Cell: What A Twist!

All of this sounds fabulous, but there being no such thing as a free lunch, there is a catch. The lifespan of the two excitons is ephemeral. As soon as they split apart, they merge back together again. The Columbia researchers tackled that problem by designing a molecule that can hold the two excitons apart long enough to make a difference.

Now along comes new research from the National Renewable Energy Laboratory that describes exactly how that mechanism works.

You can get all the details from the newly published study in the journal Nature Chemistry under the title, “Spatial separation of triplet excitons drives endothermic singlet fission,” authored by Nadia Korovina, Chris Chang, and Justin Johnson.

“We show that the minimal number of coupled chromophores needed to undergo endothermic singlet fission is three, which provides sufficient statistical space for triplet excitons to separate and avoid annihilation—and a subsequent fast return to the singlet state,” the team explains.

“Our data additionally suggest that torsional motion of chromophores about the molecular axis following triplet-pair separation contributes to the increase in entropy, thus lengthening the triplet lifetime in longer oligomers,” they add.

Shorter version: the twisting motion of the coupled chromophores enables the excitons to remain in their excited state for a longer period of time.

Here’s the explainer from NREL:

“By creating and then refining a model of how the molecules move and interact, the team discovered that a twisting motion gives the molecules the characteristics needed to isolate the triplets. The molecular chain is usually floppy and flexible when not under illumination; but when it absorbs a photon, the chain twists around its central axis and initially stiffens, resulting in a shape that facilitates the formation of two triplets. The subsequent twisting that occurs after the initial process finishes helps to spatially separate the two triplets, lengthening their lifespans.”

Solar Power To The Rescue For US Industry

Got all that? Good! The next steps involve synthesizing  the special molecules and fabricating a solar cell out of them and solving that thing about durability and longevity, to boot.

So, don’t hold your breath for the ultra-efficient singlet fission organic thin film solar cell of the future. The body of research is accelerating but commercial applications are probably somewhere off in the sparking green horizon.

Meanwhile, though, run-of-the-mill thin film solar cells are already popping up in applications where a remote power source comes in handy, like Antarctica, for example.

Lower manufacturing costs are also coming into play for organic solar cells, which helps to offset the lower conversion efficiency.

And of course, the falling cost of silicon solar cells is already eating away at the death grip of fossil fuels upon the nation’s industrial sector.

In the latest development on that score, a massive steel mill in Pueblo, Colorado, is getting a 240-megawatt solar power makeover that will enable it to ditch two of three existing coal units at the Comanche power plant.

With roots going back to 1881, the mill currently comes under the umbrella of Russia-based EVRAZ, which is interesting, but the really interesting part is that the new solar farm will be built on the mill’s property.

That is interesting because the Pueblo mill was originally constructed in Pueblo to take advantage of coal assets. With low cost solar power in hand, heavy industries no longer have to be tied down to coal mines or coal transportation routes. That could make a huge difference moving forward, as the US seeks to restore its manufacturing sector on the heels of the COVID-19 crisis.

The new solar farm, dubbed The Bighorn Solar Project, will be constructed by a UK firm called Lightsource BP, which is also interesting. Lightsource was acquired by the legacy fossil company BP barely three years ago in 2017, and it has been on a renewable energy tear ever since.

BP is taking full advantage of the Bighorn Project to tout its renewable energy cred. While larger solar farms exist, Lightsource BP has stated that Bighorn will be the largest solar farm dedicated to a single customer in the US.

If you’re wondering how the Pueblo mill plans to make steel with clean electricity, that’s a good question. Steelmaking normally involves coal, but the mill is currently focused on recycling scrap steel, and electric arc furnaces are sufficient for that operation.

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Image: via National Renewable Energy Laboratory, “By twisting when excited, some long chains of organic molecules can isolate triplet excitons at opposite ends of the molecule.”