Looking for natural night vision

February 03, 2017 // By Julien Happich
Researchers from Kyushu University (Japan) have demonstrated a fully organic and easy to process thin-film near-infrared to visible optical upconversion device, whose output can be tuned across the full visible range.

Published in the ACS Photonics journal, their paper "Tunable Full-Color Electroluminescence from All-Organic Optical Upconversion Devices by Near-Infrared Sensing" relates their decision to move away from costly III−V compound semiconductor sensors and silicon based readout circuits found in commercial NIR imaging applications to look for direct upconversion mechanisms which would allow the naked eyes to "see" in the near infrared by simply holding a thin foil in front of them.

In order to devise large-area pixel-less NIR imaging applications, the researchers looked for all-organic optical upconversion systems, stacking an OLED for the secondary emitter and an organic photodetector, all into a single device.

They leveraged prior research around high-efficiency precious-metal-free OLEDs (relying on thermally activated delayed fluorescence or TADF with an internal EL quantum efficiency as high as 100%), combining the TADF-OLED with a NIR-sensitive bulk hetero-junction charge-generation layer (CGL).

The operational principle of the NIR-to-visible optical upconversion.

In the absence of NIR illumination, the OLED is kept in the off-state (through a thin hole-blocking layer (HBL) deposited between the ITO anode and the charge-generation layer (CGL). When illuminated by a near infrared light source, the CGL can absorb the incoming NIR light and generate holes and electrons, which are successively transported to the respective electrodes along the external applied bias. The photo-generated holes are then injected into the hole transport layer (HTL), and they recombine with electrons injected from the Al cathode within the emission layer (EML) to output the upconverted visible light.

Schematic cross-sectional view of organic NIR-to-visible upconversion devices based on TADF-OLEDs and chemical structures of the representative organic semiconductor materials used in the devices.

Hence, upon NIR illumination, the stack directly emits light visible to the naked eye, without any readout electronics or any other form of display. By combining different molecules in the TADF-OLED, the researchers demonstrated that their device could be tuned to output light across the whole visible range from blue to red and white.

Operational principle of the upconversion devices in the dark (turn-off) and under NIR illumination (turn-on).