An innovative design for a flexible phototransistor could improve the performance of a wide variety of products that rely on electronic light sensors.
Integrated into devices such as digital cameras, night vision goggles, surveillance systems and satellites, the phototransistor could boost both acquisition speed and image or video quality.
According to the team of engineers at the University of Wisconsin who created the phototransistor, the design exceeds previous flexible phototransistor parameters, including sensitivity and response time.
Phototransistors sense and collect light in a way similar to human eyes, converting the light into an electrical charge proportional to its intensity and wavelength.
In human eyes, these electrical impulses transmit the image to the brain.
In a digital camera or other digital imaging device, that electrical charge becomes the string of binary 1s and 0s that create the digital image.
Because the University of Wisconsin team’s phototransistor is so flexible compared to other phototransistors fabricated on rigid surfaces, it is able to mimic the behavior of mammalian eyes.
"We actually can make the curve any shape we like to fit the optical system," said Zhenqiang "Jack" Ma, professor of electrical and computer engineering. "Currently, there's no easy way to do that."
The team’s design uses an innovative “flip-transfer” method of fabrication. This involves a final step of inverting the finished phototransistor onto a plastic substrate. At this point, a reflective metal layer is on the bottom.
"In this structure - unlike other photodetectors - light absorption in an ultrathin silicon layer can be much more efficient because light is not blocked by any metal layers or other materials," Ma said.
The researchers also placed electrodes under the phototransistor's ultrathin silicon nanomembrane layer. The metal layer and electrodes each act as reflectors and improve light absorption without the need for an external amplifier.
"There's a built-in capability to sense weak light," Ma said.
Ultimately, the new phototransistors open the door of possibility, he continued.
"This demonstration shows great potential in high-performance and flexible photodetection systems," said Ma, whose work was supported by the U.S. Air Force. "It shows the capabilities of high-sensitivity photodetection and stable performance under bending conditions, which have never been achieved at the same time."
A paper describing the phototransistor is published in the journal Advanced Optical Materials, and is available here.