Episode 24: Substrate cracking compromises integrity of flexible electronic devices

In this podcast episode, MRS Bulletin’s Laura Leay interviews Nitin Padture, who is the Otis E. Randall University Professor and the founding Director of the Initiative for Sustainable Energy at Brown University, about his group’s work uncovering the cracks in a substrate that was coated with a transparent-conducting oxide thin film. This cracking, they discovered, contributes toward the degradation in the electronic properties of devices. The group’s next step was to mitigate the cracking. This work was published in a recent issue of NPJ Flexible Electronics.

Transcript

LAURA LEAY: Welcome to MRS Bulletin’s Materials News Podcast, providing breakthrough news & interviews with researchers on hot topics in materials research. My name is Laura Leay. Scientific curiosity often leads to surprising discoveries. Flexible solar cells are comprised of ceramic layers deposited on a polymeric substrate. Although the ceramic is expected to crack when the solar cell flexes it has always been assumed that the substrate, with its excellent toughness, remains intact. Over time this cracking causes the efficiency of the solar cell to degrade. New research has shown that the polymer also cracks, and mitigating the cracking of the polymer inhibits degradation of solar cell efficiency.

NITIN PADTURE: People say “you have solved a problem that people didn’t know they had. They didn’t know the substrate cracked but then now you’ve fixed it.”

LAURA LEAY: That was Nitin Padture, who is the Otis E. Randall University Professor and the founding Director of the Initiative for Sustainable Energy at Brown University. It was the curiosity of one graduate student that uncovered the cracks in the substrate which was coated with only a transparent-conducting oxide thin film. The cracks were initially seen after focused ion beam milling was used to create a hole in the thin film, and then confirmed when etching was used to remove the thin film. The degradation in the electronic properties is thought to be caused by debris in the cracks preventing closure after the device has been flexed. Preventing the cracking then limits this degradation mechanism. 

NITIN PADTURE: It was amazing, the resourcefulness and creativity that students demonstrate to try something different that people have not tried. And so that leads to all these things, and that’s inspiring.

LAURA LEAY: Modelling originally published in the 1970s suggested that elastic mismatch causes the polymeric substrate to crack. 

NITIN PADTURE: After you understand the model, it makes sense in hindsight: of course, that’s exactly what’s going to happen. But it is exciting to see something that you didn’t expect. That’s what research is about. You find something unexpected and you figure out why it is happening and then as engineers we go one step beyond that and say, “OK, well now we understand this so how do we fix it?”

LAURA LEAY: To mitigate the cracking, an interlayer was incorporated. In a proof-of-concept, polymethyl methacrylate was deposited on a polyethylene naphthalate substrate using spin coating and an indium-zinc oxide layer then deposited using sputtering. The stiffness of the interlayer relative to the other layers is important for arresting crack propagation. A map of the elastic mismatch between the top and bottom layers was created and areas on the map were identified for crack mitigation.

NITIN PADTURE: If you just use the elastic modulus as the criterion that’s not enough because the mechanics are more complicated. It has to be the elastic mismatch parameters which contain other constants of the different materials.

LAURA LEAY: The interlayer doesn’t affect the properties of the solar cell as its transparent to light and deposited below the conducting layer. The thickness is critical as it needs to be thick enough to mitigate cracking but retain its transparency. This means that the mitigation technique has applications beyond solar cells: it could be used for photochromics, touch screens, and foldable displays. Test over thousands of cycles showed that the inclusion of an interlayer prevented the change in resistance that would usually signal that cracking has caused the electronic properties to degrade. The research can now move on to looking at how to build a full stack to see how a complete device performs when deposited on top of the interlayer. This work was published in a recent issue of NPJ Flexible Electronics. My name is Laura Leay from the Materials Research Society. For more news, log onto the MRS Bulletin website at mrsbulletin.org and follow us on X, @MRSBulletin. Don’t miss the next episode of MRS Bulletin Materials News – subscribe now. Thank you for listening.