Moving away from costly III−V compound semiconductor sensors and silicon-based readout circuits found in commercial NIR imaging applications, researchers from Kyushu University have shown a fully organic and easy to process thin-film near-infrared to visible optical upconversion device with a tunable output.

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).

 
NIR upconversion principle (cr) Figure 1: Principle of the NIR-to-visible optical upconversion mechanism.  

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 that 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.

 
NIR upconversion device schematic diagram (cr) Figure 2: Schematic 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.

 
NIR upconversion principle fig3 (cr) Figure 3: Principle of the upconversion devices in the dark (turn-off) and under NIR illumination (turn-on).  

Led by Dr. Takuma Yasuda, professor at the INAMORI Frontier Research Centre, the researchers fabricated and evaluated an upconversion device 400mm² in area, containing a green TADF emitter. Shining a NIR light source at the film, through a patterned mask with the shape of Mount Fuji, they obtained a green contrasted visible image. At a NIR power density of 100mWcm&sup-2;, the luminance of the prototype reached over 150cdm&sup-2; (yielding an external EL quantum efficiency of 12%).

 
NIR upconversion (cr) Figure 4: NIR-to-green upconversion device under 810nm NIR illumination through a shadow mask of mount Fuji at 10V voltage.  

The full organic stack together with the metal electrodes is less than 0.4μm thick, so is the thin film device semi-transparent in daylight or could it be processed with transparent electrodes so as to be practically transparent in the off-state? We asked Prof. Yasuda. The idea being that such films could find their way on helmet visors or car windscreens to augment drivers with natural night vision.

 
NIR white upconversion (cr) Figure 5: NIR-to-white light upconversion device.  

"These upconversion devices are not transparent in this stage because we use a 100nm-thick Al layer as a cathode. We can fabricate semi-transparent devices by using a much thinner metal cathode. Also, flexible devices can be produced using plastic substrates," Yasuda told EE Times Europe, adding that these results show a device at a very fundamental stage.

"We do not contemplate commercialising these devices for actual applications at this point. But in the future, some useful applications may be tested."

First seen on EE Times Europe.