A new type of OLED could replace bulky night vision goggles with lightweight glasses, making them cheaper and more practical for prolonged use, according to University of Michigan researchers. A memory effect in the OLEDs could also lead to computer vision systems that both sense and interpret incoming light signals and images.

Current night vision systems rely on image intensifiers that convert incoming near-infrared light into electrons, which then acce­lerate through a vacuum into a thin disc containing hundreds of tiny channels. As they pass through and collide with the channel walls, the electrons release thousands of additional electrons and go on to strike a phosphor screen, which converts them into visible light. The incoming light is amplified by 10,000 times in 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 cumber­some vacuum layer required for traditional image inten­sifiers. The researchers say much higher amplifi­cation is possible by optimizing the design of the device.

“One of the most attractive features of this new approach is that it amplifies light within a thin film stack that is less than a micron thick.” said Chris Giebink. Because the device operates at much lower voltage than a traditional image inten­sifier, it opens the door to significantly reducing power consumption and thereby extending battery life. The device works by integrating a photon-absorbing layer, which converts infrared light into electrons, and a five-layer stack of OLEDs, where those electrons are converted into visible light photons. Ideally, five photons are produced for each electron that passes through the OLED stack.

Some of these photons are emitted out to the user's eye, but others are reabsorbed back in the photon-absorbing layer, producing still more electrons that move through the OLED in a positive feedback cycle. This chain reaction greatly amplifies the amount of output light that results for a given amount of input light. Previous OLEDs were able to convert near infrared light to visible light, but there was no gain, meaning one input photon yielded one output photon. “This marks the first demonstra­tion of high photon gain in a thin film device,” said postdoc Raju Lampande. The device also exhibits a sort of memory behavior that could have applications in computer vision. Its light output at a given moment depends on the intensity and duration of past input illumination.  

“Normally when you illu­minate an upconversion OLED, it starts outputting light and when you turn off the illumination, it stops outputting light. This device can get stuck on and remember things over time, which is unusual,” Giebink said. Although the memory behavior introduces some challenges for night vision appli­cations, it may create an oppor­tunity for image processing that works more like the human visual system – where bio­logical neurons pass signals on, or not, based on the timing and strength of incoming signals.

The ability to remember past inputs could make these OLEDs a good candidate for the type of neuron-like connections that enable an input image to be inter­preted and classified without having to process the data in a separate computing unit. The researchers fabri­cated the device using “off the shelf” materials and methods that are already widely used in OLED manu­facturing, which should improve both cost effective­ness and scalability for future appli­cations of the techno­logy. (Source: U. Michigan)

Reference: R. Lampande et al.: Positive-feedback organic light-emitting diodes and upconverters, Nat. Photon., online 13 September 2024; DOI: 10.1038/s41566-024-01520-0

Link: Optics and Photonics, Dept. of Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, USA