New research suggests path toward more durable smartphones and flexible electronics

New research suggests path toward more durable smartphones and flexible electronics

Researchers from Brown University's School of Engineering have discovered new details about how destructive cracks form in flexible electronic devices - and how to prevent them.

PROVIDENCE, R.I. [Brown University] - From health monitors and smartwatches to foldable phones and portable solar panels, demand for flexible electronics is growing rapidly. But the durability of those devices - their ability to stand up to thousands of folds, flexes and rolls - is a significant concern.

New research by engineers from Brown University has revealed surprising details about how cracks form in multilayer flexible electronic devices. The team shows that small cracks in a device's fragile electrode layer can drive deeper, more destructive cracks into the tougher polymer substrate layer on which the electrodes sit. The work overturns a long-held assumption that polymer substrates usually resist cracking.

"The substrate in flexible electronic devices is a bit like the foundation in your house," said Nitin Padture, a professor of engineering at Brown and corresponding author of the study published in npj Flexible Electronics. "If it's cracked, it compromises the mechanical integrity of the entire device. This is the first clear evidence of cracking in a device substrate caused by a brittle film on top of it."

The layers used in flexible electronics have specific jobs. The top layer conducts electricity across the surface to keep the device running. That layer is usually made of special ceramic oxide materials because they are transparent and also good conductors, which is essential for things like display screens, sensors and solar cells. But ceramics are brittle and prone to cracking, so the substrate's job is to add some toughness. Substrates are generally made from polymer materials that are highly flexible and resist cracking.

While using these materials to make flexible solar cells, Anush Ranka, a postdoctoral researcher at Brown who performed the work as a Ph.D. student in materials science, became increasingly curious about the mechanism by which fatigue can degrade performance. He decided to take a closer look at the cracking processes.

For the study, Ranka made small experimental devices using various types of ceramic electrodes and polymer substrates. He then subjected them to bending tests and used a powerful electron microscope to examine the cracks. In places where he found cracks in the ceramic layer, he used a focused ion beam - a kind of nanoscale sandblaster - to etch away the ceramic and reveal the substrate directly beneath a ceramic crack.

The work showed that cracks in the ceramic layer often drive deeper cracks into the substrate. The effect occurred across ceramic and polymer combinations, suggesting this is a common - and surprising - failure mechanism in flexible electronics. Once cracks form deep in the polymer, the researchers say, they become permanent structural defects. With repeated bending, these cracks widen, misalign or fill with debris, which then prevents the ceramic crack faces from reconnecting. That causes electrical resistance to increase and device performance to degrade.

Working with Haneesh Kesari, a Brown engineering professor who specializes in theoretical and applied mechanics, and solid mechanics Ph.D. student Sayaka Kochiyama, the researchers analyzed this cracking problem. They showed that a mismatch in the elastic properties of the two layers was driving the deep cracking phenomenon in the substrate. Understanding the cracking mechanism led the team toward a potential fix: Adding a third layer of material between the ceramic and the substrate that mitigates the elastic mismatch.

"We created a design map that identified hundreds of polymers that, with the correct thickness, could potentially mitigate this elastic mismatch and prevent cracking in a wide range of electrode-substrate combinations," said Padture who leads Brown's Initiative for Sustainable Energy. "Using this design map, we were able to choose a specific polymer for the third layer and experimentally demonstrate the feasibility of our approach."

The researchers are hopeful that the design diagram will make for more durable devices. Just as important, however, is the discovery that cracks do indeed affect polymer substrates - a fact that was not apparent before this research.

"We're essentially solving a problem people didn't know they had," Padture said. "We think this could significantly improve the cyclic life of flexible devices."

Other researchers from Brown, Yale University and University of Rome Tor Vergata also contributed to the work.

The research was supported by the U.S. Department of Energy (DE-EE0009511, DE-SC0025180), the U.S. National Science Foundation (DMR-2102210, CBET-2315077, DGE-2139841) and the Office of Naval Research (N00014-21-1-2851, N00014-24-1-2200, N00014-21-1-2054).