All-organic infrared-to-visible up-conversion OLEDs for low-cost SWIR imaging

As a low-cost alternative technology to conventional expensive short-wavelength infrared (SWIR) imaging technology based on InGaAs semiconductors, an infrared (IR)-to-visible up-conversion organic light-emitting diode (OLED) has recently been developed with the potential to convert an IR image to a visible image without pixilation in the device, thus enabling a high-quality image at a significantly low-cost. Recent IR-to-visible up-conversion OLEDs clearly demonstrated the pixel-less SWIR imaging capability directly up-converting SWIR images to visible images by using epitaxial-free solution-based PbS/PbSe colloidal quantum dots (CQDs) as the SWIR sensitizer. However, the large volume synthesis of PbS/PbSe CQDs and the solution-processed device fabrication using PbS/PbSe CQDs are still under development and not ready for manufacturing yet. In this study, fabrication of all-organic IR-to-visible up-conversion OLEDs is demonstrated by replacing colloidal quantum dot SWIR sensitizers to organic SWIR sensitizer. In order to achieve the aim, following researches were conducted: (1) discover low bandgap small molecules and polymers with strong IR sensitivity in SWIR wavelength, (2) investigate electron acceptors with an appropriate energy alignment to SWIR sensitive low-bandgap organic electron donors, and (3) study the effect of hole blockers in IR-to-visible up-conversion OLEDs. First, low-bandgap organic materials with SWIR sensitivity up to 1200 nm are demonstrated using a low-bandgap polymer, poly 4-(4,8-bis(5-(2-butyloctyl) thiophen-2-yl) benzo[1,2-b:4,5-b'] dithiophen-2-yl)-6,7-diethyl-[1,2,5]thiadiazolo[3,4-g]quinoxaline (PBDTT-BTQ), newly synthesized by alternating strong electron-withdrawing/donating building blocks, and a low-bandgap small molecule, SnNcCl2, selected by adding additional conjugated aromatic rings on the phthalocyanine framework. Next, photodetectors using the SWIR sensitizing materials are fabricated for demonstrating the photoelectric performance of the SWIR sensitizers in actual devices. The photodetector using PBDTT-BTQ shows the maximum detectivity of 3 x 1011 Jones at the wavelength of 1000 nm, while the SnNcCl2 photodetectors showing the maximum detectivity of 2 x 1011 Jones at the wavelength of 1000 nm. Finally, SWIR-to-visible up-conversion OLEDs using PBDTT-BTQ and SnNcCl2 are demonstrated successfully, with sensitivity up to 1200nm. The operation voltage windows are 7 V for the PBDTT-BTQ up-conversion OLED, and 5 V for the SnNcCl2 up-conversion OLED. The importance of the energy band alignment in the sensitizing layer and the hole blocking layer effect are also studied