New Polymer Coating Cuts Perovskite Cell Degradation by 14×: MIT

MIT researchers have recently developed a nearly impenetrable, lightweight polymer film for gas molecules, which could be used as a protective coating to prevent solar cells and other infrastructure from corrosion. It can also slow the aging of packaged food and medicines.

The research found that the polymer, which can be applied as a film only a few nanometers thick, completely repels nitrogen and other gases, as far as can be detected by laboratory equipment. This degree of impermeability rivals the impermeability of molecularly thin crystalline materials such as graphene.

This research was funded in part by the Center for Enhanced Nanofluidic Transport–Phase 2, an Energy Frontier Research Center funded by the US Department of Energy Office of Science, and the National Science Foundation. It was carried out, in part, using MIT.nano’s facilities. The polymer film, which the researchers describe today in Nature, is made using a process that can be scaled up to large quantities and applied to surfaces much more easily than graphene.

“Our polymer is quite unusual. It’s obviously produced from a solution-phase polymerization reaction, but the product behaves like graphene, which is gas-impermeable because it’s a perfect crystal. However, when you examine this material, one would never confuse it with a perfect crystal,” says Michael Strano, the Carbon P. Dubbs Professor of Chemical Engineering at MIT.

Bubbles that don’t collapse

Strano’s lab first reported the material — a two-dimensional polymer called a 2D polyaramid that self-assembles into molecular sheets using hydrogen bonds — in 2022. To create such 2D polymer sheets, which had never been done before, the researchers used a building block called melamine, which contains a ring of carbon and nitrogen atoms.

Under the right conditions, these monomers can expand in two dimensions, forming nanometer-sized disks. These disks stack on top of each other, held together by hydrogen bonds between the layers, which make the structure very stable and strong. That polymer, which the researchers call 2DPA-1, is stronger than steel but has only one-sixth the density of steel.

Previous study

In their 2022 study, the researchers focused on testing the material’s strength but also conducted preliminary studies of its gas permeability. For those studies, they created “bubbles” out of the films and filled them with gas. However, the researchers found that bubbles made of 2DPA-1 did not collapse — in fact, bubbles they made in 2021 are still inflated.

Traditional polymers allow gases to pass through because they consist of a tangle of spaghetti-like molecules that are loosely joined together. This leaves tiny gaps between the strands, allowing gas molecules to seep through. However, the new 2D polymer is essentially impermeable because of the way the layers of disks adhere to each other.

“The fact that they can pack flat means there’s no volume between the two-dimensional disks, and that’s unusual. With other polymers, there’s still space between the one-dimensional chains, so most polymer films allow at least a little bit of gas to get through,” Strano says.

“Normally, polymers are reasonably permeable to gases, but the polyaramids reported in this paper are orders of magnitude less permeable to most gases under conditions with industrial relevance,” says Schatz, who was not involved in the study.

A protective coating

Scientists have been working on developing graphene coatings as a barrier to prevent corrosion in solar cells and other devices. However, scaling up graphene films is difficult, in large part because they can’t simply be painted onto surfaces.

In addition to nitrogen, the researchers also exposed the polymer to helium, argon, oxygen, methane, and sulfur hexafluoride. They found that 2DPA-1’s permeability to those gases was at least 1/10,000 that of any other existing polymer. That makes it nearly as impermeable as graphene, which is completely impermeable to gases because of its defect-free crystalline structure.

However, the 2DPA-1 polymer sticks easily because of the strong hydrogen bonds between the layered disks. In this paper, the researchers showed that a layer just 60 nanometers thick could extend the lifetime of a perovskite crystal by weeks. Perovskites hold promise as cheap and lightweight solar cells, but tend to break down much faster than the silicon solar panels now widely used.

A 60-nanometer coating extended the perovskite’s lifetime to about three weeks, but a thicker coating would offer longer protection, the researchers say. The films could also be applied to a variety of other structures.

The other application demonstrated in this paper is a nanoscale resonator — essentially a tiny drum that vibrates at a particular frequency. Larger resonators, around 1 millimeter or less, are found in cell phones, where they allow the device to pick up the frequency bands it uses to transmit and receive signals. Resonators can also be used as sensors to detect extremely tiny molecules, including gas molecules.

“Using an impermeable coating such as this one, you could protect infrastructure such as bridges, buildings, rail lines — basically anything exposed to the elements. Automotive vehicles, aircraft, and ocean vessels could also benefit. Anything that needs to be sheltered from corrosion. The shelf life of food and medications can also be extended using such materials,” says Strano.