Near-Infrared Electrophosphorescence up to 1.1 µm using a Thermally Activated Delayed Fluorescence Molecule as Triplet Sensitizer

The efficient harvesting of triplet energy by near-infrared (NIR) phosphors via reverse intersystem crossing (RISC) in a thermally activated delayed fluorescence (TADF) host layer also functioning as carrier recombination center, resulting in the nearly complete utilization of electrically generated exciton energy for NIR phosphorescence, was investigated. For each optical measurement, samples with thicknesses of 100 nm were deposited onto clean quartz substrates under high vacuum condition. PL quantum efficiencies were measured with separate absolute PL quantum yield measurement. OLEDs with active areas of 1 mm² were fabricated by thermal deposition onto clean indiumtin-oxide (ITO)-coated glass substrates under high vacuum conditions. After fabrication, the OLEDs were immediately encapsulated with a glass lid using epoxy glue in a N₂-filled glove box. The current-density-voltage characteristics and external-quantum-efficiency-current-density characteristics were measured using a semiconductor parameter analyzer and a calibrated InGaAs photodetector and a calibrated InGaAs photodetector connected to an optical power meter. Electroluminescence spectra of the OLEDs were also measured using a spectrometer. The results indicate that a TADF-based host matrix can provide not only triplet harvesting ability but also balanced carrier-transport properties according to the spatially separated HOMO and LUMO distributions. Although the relatively low phosphorescence yield of the Pc derivatives is still a barrier to further boosting the EQE of NIR-OLEDs with Pc derivatives as NIR phosphors, this can be overcome by the introduction of the heavy metal effect in a TADF-sensitized electrophosphorescence system or the utilization of highly emissive NIR-phosphors.