Minuscule tweak can produce big increase in efficiency for organic solar cells

A solar energy material that is remarkably durable and affordable is regrettably also unusable if it barely generates any electricity, which has tended to be the problem with organic solar technologies. But lately, a shift in the underlying chemistry has boosted power output, and a new study has now revealed counterintuitive tweaks that can make the new chemistry even more successful.

The shift is from fullerene acceptors to non-fullerene acceptors (NFAs). 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. In the new study, chemist Jean-Luc Brédas at the Georgia Institute of Technology has now found a way to advance this technology.

"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," said Brédas, a professor in Georgia Tech's School of Chemistry and Biochemistry.

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 all 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 said. "And it goes for the donors, too."

In just the past four years, tuning NFA chemistry has boosted organic photovoltaic technology from initially converting only 1% of sunlight into electricity, to up to 18% 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," said Tonghui Wang, a postdoctoral researcher in Brédas' lab and first author of a paper on this work in Matter.

Morphology, or 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 the corresponding energy conversion in a common NFA electron donor/acceptor pairing.

This revealed that improved performance comes not from tweaks to the metaphorical hand of the catcher nor from the donor's pitching hand, but from something akin to the positions of the catcher's feet. The model showed that some positions better aligned the ‘body’ of the acceptor with that of the electron donor.

The ‘feet’ were a tiny component, known as a methoxy group, on the acceptor. Brédas and Wang found that two out of the four possible positions the ‘feet’ could take boosted the conversion of light into electricity from 6% to 12%.

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," said 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."

This story is adapted from material from the Georgia Institute of Technology, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.