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Simple manufacturing process boosts stretchable semiconductor performance

By Prachi Patel June 21, 2019
(a) Roll of meter-scale, roll-to-roll-coated conjugated-polymer/elastomer phase-separation-induced elasticity (termed CONPHINE) films on a polyethylene terephthalate substrate. (b) Three-dimensional schematic representing alignment of polymer semiconductor nanofibers through the solution-shearing method using a microtrench-patterned blade. Credit: Nature Materials

Electronics that stretch and flex will enable wearable sensors, implantable medical devices, and electronic-skins for prosthetics and robots. But the stretchable semiconducting materials that could be used to make such electronics have had lackluster performance so far. With a new solution-based manufacturing process, researchers have now boosted the electrical performance of stretchable semiconductors sixfold.

Transistors made with the elastic semiconducting films maintain their performance when stretched to twice their original size. And the manufacturing technique is compatible with roll-to-roll fabrication methods, so it could allow the mass manufacture of low-cost stretchable semiconductors for skin-like electronics, says Jie Xu, an assistant scientist at Argonne National Laboratory and co-author of the article on this work published in a recent issue of Nature Materials.

There are a few different ways to make stretchable circuits. One is to create microscopic waves or buckles on thin pieces of rigid semiconductors like silicon and fix them on a flexible substrate; the microstructures allow the rigid materials to stretch. But the rigid materials can still break or separate from the substrate. Plus such devices are expensive to make and hard to manufacture on a large scale.

Another approach is to use intrinsically stretchable semiconducting materials. Stanford University chemical engineering professor Zhenan Bao and others have made such materials by embedding semiconducting polymer nanofibers in a silicone matrix. The spin-coating techniques typically used to deposit such films, though, result in kinky polymer chains that are loosely packed and disordered. “Charge carrier transport in semiconductors favors ordered structures,” Xu says.

So Xu, Bao, and their colleagues came up with a novel method to make the films. They made a blade patterned with tiny teeth that are 20 µm high and 5 µm wide, and used it to spread solutions of conductive polymers that are placed on a substrate. As the solution passes beneath the blade, the microtrenches create an intense one-directional flow that stretches out polymer chains and lines them up. Multiple polymer chains then bunch together into nanofibrils that also line up through the entire film. The multiscale ordering increases charge-carrier mobility while maintaining excellent stretchability, Xu says.

Using this method, the research team fabricated stretchable semiconducting films from five different polymers, all of which showed improved charge-carrier mobilities by up to sixfold when compared with their spin-coated counterparts. As a representative system, they chose films of a high-mobility semiconducting polymer (DPPDTSE) and a soft elastomer, polystyrene-block-poly(ethylene-ran-butylene)-block-polystyrene (SEBS) to make stretchable transistors. The devices had an average mobility of 1.5 cm2 V s−1. Furthermore, the team demonstrated using a roll-to-roll technique to make a meters-long film in just a few minutes. They first coat a polyethylene terephthalate (PET) roll with water-soluble Dextran and then print the stretchable semiconductor film on top. Washing away the Dextran releases the meters-long semiconductor film.

“This work is a breakthrough in materials design and development,” says Cunjiang Yu, a mechanical engineering professor at the University of Houston. The simple fabrication process is clever and exciting, he says, and it could push stretchable semiconductor films toward practical applications.

Read the abstract in Nature Materials.