Organic electronics lead to new ways to sense light

Osaka, Japan – The past few decades have seen astonishing advances in imaging technology, from high-speed optical sensors that process over two million frames per second to tiny lensless cameras that record images using a single pixel. 

In a study recently published in Advanced Materials, researchers from SANKEN (The Institute of Scientific and Industrial Research), at Osaka University have developed an optical sensor on an ultrathin, flexible sheet that can be bent without breaking. In fact, this sensor is so flexible, it will work even after it has been crumpled into a ball.

In a camera, the optical sensor is the device that senses the light that has passed through a lens, similar to the retina inside a human eye.

“Conventional optical sensors are built using inorganic semiconductors and ferroelectric materials,” says Rei Kawabata, lead author of the study. “This makes the sensors stiff and unable to bend. To avoid this problem, we looked at a different way to detect light.”

Instead of traditional light sensors, the researchers use an array of tiny carbon nanotube photodetectors printed on an ultra-thin polymer substrate (less than 5 μm). When exposed to light, the carbon nanotubes heat up, creating a thermal gradient that then generates a voltage signal. Doping the nanotubes with chemical carriers during printing further increases their sensitivity. Using these nanotubes, visible light as well as infrared light such as those related to heat or molecules can be measured.

Along with the carbon nanotube sensors, organic transistors are also printed on the polymer substrate to organize the voltage signals into an image signal. To read this signal, a computer does not need to be physically connected by wires to the sensor. Instead, a wireless Bluetooth module is used.

“Together with this wireless system, our imager can attach soft and curved objects to analyse their surfaces or insides without damaging them,” says Teppei Araki, senior author of the study.

The researchers built a prototype of the sheet-type optical sensor and tested its ability to sense heat from objects like human fingers or wires as well as glucose flowing through tubes. They found the optical sensor has high sensitivity over a wide range of wavelengths. Moreover, at room temperature and atmospheric conditions, tests showed it has high bending durability and worked even after it was crumpled.

The unique advantages of this wireless measurement system and sheet-type optical sensor will lead to new and simpler ways to perform many tasks such as evaluating the quality of liquid without needing to sample it. The researchers believe it holds high promise in many applications such as non-destructive imaging, wearable devices, and soft robotics.   

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The article, “Ultraflexible Wireless Imager Integrated with Organic Circuits for Broadband Infrared Thermal Analysis,” was published in Advanced Materials at DOI: https://doi.org/10.1002/adma.202309864.

The related review article, “Broadband Photodetectors and Imagers in Stretchable Electronics Packaging” is available in Advanced Materials at DOI: https://doi.org/10.1002/adma.202304048

About Osaka University
Osaka University was founded in 1931 as one of the seven imperial universities of Japan and is now one of Japan's leading comprehensive universities with a broad disciplinary spectrum. This strength is coupled with a singular drive for innovation that extends throughout the scientific process, from fundamental research to the creation of applied technology with positive economic impacts. Its commitment to innovation has been recognized in Japan and around the world, being named Japan's most innovative university in 2015 (Reuters 2015 Top 100) and one of the most innovative institutions in the world in 2017 (Innovative Universities and the Nature Index Innovation 2017). Now, Osaka University is leveraging its role as a Designated National University Corporation selected by the Ministry of Education, Culture, Sports, Science and Technology to contribute to innovation for human welfare, sustainable development of society, and social transformation. Website: https://resou.osaka-u.ac.jp/en