Georgia Tech develops soft semiconductors that can detect ultra-low light levels

01 Jan 2022

Three engineering laboratories collaborate to achieve new level of photodetector stretchability.

Semiconductors are moving away from rigid substrates, which are cut or formed into thin discs or wafers, to more flexible plastic material and even paper thanks to new material and fabrication discoveries. The trend toward more flexible substrates has led to fabrication of numerous active devices, from light-emitting diodes to solar cells and transistors.

Researchers at Georgia Tech have created a material that acts like a second skin layer. They say it is up to 200% more stretchable than its original dimension without significantly losing its electric current. The researchers say the soft flexible photodetectors could enhance the utility of medical wearable sensors and implantable devices, among other applications. The development is described in Science Advances.

Georgia Tech researchers from both mechanical and computing engineering labs collaborated over three years to demonstrate a new level of stretchability for a photodetector, a device made from a synthetic polymer and an elastomer that absorbs light to produce an electrical current. Given that conventional flexible semiconductors break under a few percentages of strain, the Georgia Tech findings are “an order-of-magnitude improvement,” said Olivier Pierron, professor in the George W. Woodruff School of Mechanical Engineering.

Flexible with solid performance

“Think of a rubber band or something that is soft and stretchable like human skin yet has similar electrical semiconducting properties of solid or rigid semiconductors,” said Canek Fuentes-Hernandez, a co-PI formerly in the School of Electrical and Computer Engineering and now an associate professor in ECE at Northeastern University in Boston.

“We have shown that you can build stretchability into semiconductors that retains the electrical performance needed to detect light levels that are around hundred million times fainter than produced by a light bulb used for indoor illumination,” he said.

Bernard Kippelen, vice provost for International Initiatives and an ECE professor, oversaw the work of Youngrak Park, the study’s first author and a Ph.D. candidate in ECE. Following two-and-a-half years of research, Park uncovered the right combination of chemical compounds that produced a super-soft material with the ability to generate and conduct electricity when exposed to light.

Optimum layer thickness

Park found the perfect ratio for all parts of the semiconductor layer to maintain high performance in the photodetector. But it was painstaking work to prove the materials’ stretchability. Park relied on Kyungjin Kim, then a Georgia Tech Ph.D. mechanical engineering student, to test the material’s reliability. He continued to provide Kim with larger, thicker samples until one with a thickness of 500 nanometers worked.

“It was still super thin. Under dry conditions, it would just crumble. We had to use a water reservoir to keep its shape,” commented Kim, now an assistant professor in the University of Connecticut’s Department of Mechanical Engineering.

Elaborating on how difficult it was to measure pure mechanical properties of a photoactive layer, Pierron noted, “Electronic devices are very brittle typically, which is okay with conventional devices fabricated on rigid substrates. But as soon as you use soft substrates that becomes an issue.”

The water acted like plastic wrap keeping the thin films in place without crumbling or losing shape, enabling the researchers to stretch the material and measure its mechanical properties. To test for electrical signals coming out of the device under illumination, electronic terminals had to be embedded on it. Yet, those terminals had to be deformable, too, or the entire device would become rigid.

“Fabricating stretchable electronic terminals was a major challenge in and of itself,” said ECE PhD graduate Felipe Andres Larrain, who worked closely with Park and focused on the embedded components. Larrain is now an assistant professor at Adolfo Ibáñez University in Chile.

While this breakthrough material has been initially integrated into a photodetector and tested for electrical functionality, more testing and optimization is needed to show the materials’ stretchability under multimodal loads and its shelf stability.

Possible applications

The researchers are most excited about the potential of the material to enhance medical wearables. Typically, wristwatches that use rigid biosensors have limitations since flexing the wrist can completely change the sensor’s measurements. They are subject to what is called “motion artifact,” or degraded image quality, caused when a person moves.

The team foresees rich applications for the soft and stretchable polymer blend beyond wearables for health monitoring. “The soft device also could be attractive for implantable electronics for bio-electronic applications since the interfaces comply with the dynamic motion of the soft biological tissues, reducing the foreign body reaction,” said Kim.

Fuentes sees the material working in smart agriculture applications, where farmers could attach light sensors into fruits or other produce to monitor growth, disease and to better time harvesting. Kippelen believes the rubber-like photodiodes that detect ultralow light levels could find applications in detecting, identifying, and characterizing ionizing radiation for nuclear fuel cycle monitoring.