New OLED could enable compact, lightweight night vision

A new type of OLED (organic light-emitting diode) could replace bulky night vision goggles with lightweight ones, making them cheaper and more practical for extended use, according to researchers at the University of Michigan.

Their research is published in the journal Photonics of nature.

A memory effect in OLEDs could also lead to computer vision systems capable of detecting and interpreting incoming light signals and images.

Current night vision systems rely on image intensifiers that convert near-infrared light into electrons, which are then accelerated in a vacuum in a thin disk containing hundreds of tiny channels. As they pass through and collide with the walls of the channels, the electrons release thousands more electrons and hit a phosphor screen, which converts them into visible light. The incoming light is amplified 10,000 times during this process, allowing the wearer to see at night.

The newly developed OLED device also converts near-infrared light into visible light and amplifies it more than 100 times, but without the weight, high voltage and bulky vacuum layer required by traditional image intensifiers. The researchers say much higher amplification is possible by optimizing the device design.

“One of the most attractive features of this new approach is that it amplifies light in a thin stack of films that are less than a micron thick. That’s much thinner than a human hair, which is about 50 microns thick,” said Chris Giebink, a UM professor of electrical and computer engineering and of physics and corresponding author of the study.

Since the device operates at a much lower voltage than a traditional image intensifier, it significantly reduces power consumption and therefore extends battery life.

The device works by integrating a photon-absorbing layer, which converts infrared light into electrons, and a stack of five OLED layers, where those electrons are converted into visible light photons. Ideally, five photons are produced for every electron that passes through the OLED stack.

Some of these photons are emitted into the user’s eye, but others are reabsorbed in the photon-absorbing layer, producing even more electrons that move through the OLED in a positive feedback loop. This chain reaction dramatically amplifies the amount of output light that produces a given amount of input light.

Previous OLEDs were able to convert near-infrared light to visible light, but there was no gain, meaning that one input photon produced one output photon.

“This marks the first demonstration of high photon gain in a thin-film device,” said Raju Lampande, a UM postdoctoral researcher in electrical and computer engineering and lead author of the study.

The device also exhibits a kind of memory behavior that could have applications in computer vision. Known as hysteresis, its light output at any given time depends on the intensity and duration of past input illumination.

“Normally, when you turn on an upconverting OLED, it starts emitting light, and when you turn off the light, it stops emitting light. This device can stay on and remember things over time, which is unusual,” Giebink said.

While the memory behavior poses some challenges for night vision applications, it could pave the way for image processing that works more like the human visual system, where biological neurons transmit or not signals based on the timing and strength of incoming signals. The ability to remember past inputs could make these OLEDs good candidates for the kind of neuron-like connections that allow an input image to be interpreted and classified without having to process the data in a separate computational unit.

The researchers fabricated the device using commercially available materials and methods already widely used in OLED manufacturing, which should improve both the cost-effectiveness and scalability of future applications of the technology.