Selection of solar cells in the laboratory. The cells include 3D, CZTS, organic photovoltaic, and silicon. (Credit: Branden Camp/Georgia Tech)

How a few tweaks revived near-dead solar tech

Researchers had all but given up on emerging organic solar cell technology, but chemical tweaks brought them back from the brink. Here's how it worked.

Ben Brumfield-Georgia Tech • futurity
Dec. 30, 2019 6 minSource

The solar cells look like small gold and black squares sitting in several clear plastic petri dishes

New research reveals the counterintuitive tweaks to the chemistry of a solar cell material that have boosted its power output.

A solar energy material that’s remarkably durable and affordable is also regrettably unusable if it barely generates electricity, so many researchers had abandoned emerging organic solar technologies. But lately, a shift in the underlying chemistry has boosted power output.

The shift is from “fullerene” to “non-fullerene acceptors” (NFAs), and in photovoltaic electricity generation, the acceptor is a molecule with the potential to be to electrons what a catcher is to a baseball. Corresponding donor molecules “pitch” electrons to acceptor “catchers” to create electric current.

“NFAs are complex beasts and do things that current silicon solar technology does not. You can shape them, make them semi-transparent or colored. But their big potential is in the possibility of fine-tuning how they free up and move electrons to generate electricity,” says Jean-Luc Brédas, a professor in the School of Chemistry and Biochemistry at the Georgia Institute of Technology.

Move over silicon?

In just the last four years, tuning NFA chemistry has boosted organic photovoltaic technology from initially converting only 1% of sunlight into electricity to 18% conversion in recent experiments. By comparison, high-quality silicon solar modules already on the market convert about 20%.

“Theory says we should be able to reach over 25% conversion with organic NFA-based solar if we can control energy loss by way of the morphology,” says first author Tonghui Wang, a postdoctoral researcher in Brédas’ lab.

Morphology, the shapes molecules take in a material, is key to NFA solar technology’s heightened efficiency, but how that works on the molecular level has been a mystery. The new study carefully modeled tiny tweaks to molecular shapes and calculated corresponding energy conversion in a common NFA electron donor/acceptor pairing.

Improved performance came not from tweaks to the metaphorical hand of the catcher, nor from the donor’s pitching hand, but from something akin to positions of the catcher’s feet. Some positions better aligned the “body” of the acceptor with that of the electron donor.

The “feet” were a tiny component, a methoxy group, on the acceptor, and two positions out of four possible positions it took boosted the conversion of light into electricity from 6% to 12%.

The donor/acceptor chemical pair was PBDB-T / IT-OM-1, -2, -3, or -4, with -2 and -3 showing superior electricity generation. PBDB-T is an abbreviation for: poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophen)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c′]dithiophene)-4,8-dione)]

Why organic solar cells are better

Marketable NFA-based solar cells could have many advantages over silicon, which requires mining quartz gravel, smelting it like iron, purifying it like steel, then cutting and machining it. By contrast, organic solar cells start as inexpensive solvents that can be printed onto surfaces.

Silicon cells are usually stiff and heavy and weaken with heat and light stress, whereas NFA-based solar cells are light, flexible, and stress-resistant. They also have more complex photoelectric properties. In NFA-based photoactive layers, when photons excite electrons out of the outer orbits of donor molecules, the electrons dance around the electron holes they have created, setting them up for a customized handoff to acceptors.

“Silicon pops an electron out of orbit when photons excite it past a threshold. It’s on or off; you either get a conduction electron or no conduction electron,” says Brédas. “NFAs are subtler. An electron donor reaches out an electron, and the electron acceptor tugs it away. The ability to adjust morphology makes the electron handoff tunable.”

Fullerene vs. non-fullerene

Like the name says, non-fullerene acceptors are not fullerenes, which are pure carbon molecules with rather uniform and geometric structures of repeating pentagonal or hexagonal elements. Nanotubes, graphene, and soot are examples of fullerenes, which are named after architect Buckminster Fuller, who was famous for designing geodesic domes.

Fullerenes are more ridged in molecular structure and tunability than non-fullerenes, which are more freely designed to be floppy and bendable. NFA-based donors and acceptors can wrap around each other like precise swirls of chocolate and vanilla batter in a Bundt cake, giving them advantages beyond electron donating and accepting—such as better molecular packing in a material.

“Another point is how the acceptor molecules are connected to each other so that the accepted electron has a conductive path to an electrode,” Brédas says. “And it goes for the donors, too.”

As in any solar cell, conduction electrons need a way out of the photovoltaic material into an electrode, and there has to be a return path to the opposite electrode for arriving electrons to fill holes that departing electrons left behind.

The study appears in the journal Matter. Funding for the research came from the Office of Naval Research. Any findings, conclusions, or recommendations are those of the authors and not necessarily of the Naval Office of Research.

Source: Georgia Tech

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