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Engineers reveal record-setting flexible phototransistor

31 October 2015

Inspired by mammals' eyes, engineers have created what they believe is the fastest, most responsive flexible silicon phototransistor ever made.

Developed by UW-Madison electrical engineers, this novel phototransistor is flexible, and claimed to be faster and more responsive than any similar phototransistor in the world (photo: Jung-Hun Seo)

The device could improve the performance of products ranging from digital cameras, night-vision goggles and smoke detectors to surveillance systems and satellites, all of which rely on electronic light sensors. Integrated into a digital camera lens, for example, it could reduce bulkiness and boost both the acquisition speed and quality of video or still photos.

Developed by University of Wisconsin-Madison (UW-Madison) collaborators Zhenqiang Ma, professor of electrical and computer engineering, and research scientist Jung-Hun Seo, the phototransistors are flexible, meaning they more easily mimic the behaviour of mammalian eyes.

"We actually can make the curve any shape we like to fit the optical system," Ma says. "Currently, there's no easy way to do that."

One important aspect of the success of the new phototransistors is the researchers' innovative 'flip-transfer' fabrication method, in which their final step is to invert the finished phototransistor onto a plastic substrate. At that point, a reflective metal layer is on the bottom.

"In this structure - unlike other photodetectors - light absorption in an ultra-thin silicon layer can be much more efficient because light is not blocked by any metal layers or other materials," Ma says.

The researchers also placed electrodes under the phototransistor's ultra-thin silicon nanomembrane layer, and the metal layer and electrodes each act as reflectors to improve light absorption without the need for an external amplifier. "There's a built-in capability to sense weak light," Ma says.

The demonstration shows the capabilities of high-sensitivity photodetection and stable performance under bending conditions, which Ma claims have never been achieved at the same time.

An article describing the work is published in the journal, Advanced Optical Materials.

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