Thermally activated delayed fluorescence (TADF) so called third-generation organic light emitting diodes (OLEDs) materials are now being actively studied to replace current phosphorescent and fluorescence materials. Such TADF can achieve 100% internal quantum efficiency by harvesting electrical excited triplet excitons through spin up-conversion from triplet (T1) to singlet (S1) state . To incresase the efficiency of TADF OLEDs, high T1 host materials are desired to suppress triplet exciton quenching from dopant to host material. In addition, bipolar type host materials are desired for good charge balance and suppress the exciton polaron annihilation in the dopant. Furthermore, bipolar hosts could result in good device stability and better device performances such as low driving voltage, high efficiency, and low efficiency roll-off characteristic. To date, phosphine oxide series have been widely used due to their high T1 characteristics, but their bipolar characteristics and chemical stability are known to be poor .
In this work, we report newly synthesized two bipolar host materials, KHU-TBH 1 and KHU-TBH 2. For the bipolar characteristic, our hosts have carbazole as a hole-transport type moiety and carboline and pyridine as electron-transport type moieties. These host molecules were designed to have high T1 values by reducing the conjugation length with introducing steric hindrance between each moiety. The measured T1 values of KHU-TBH 1 and KHU-TBH 2 were 2.98 eV and 2.97 eV, respectively. It indicates that our bipolar hosts have suitable high T1 values for blue TADF OLEDs. We evaluated our two host materials with well-known hole-transport type host, 1,3-bis(N-carbazolyl)benzene (mCP). For a blue TADF dopant, DMAC-DPS was employed for the device fabrication . As expected, the current density-voltage-luminance characteristics of KHU-TBH 1 and KHU-TBH 2 were better than mCP due to thier bipolar characteristics. In addition, the device efficiencies of two hosts were higher than that of mCP. The maximum EQEs of KHU-TBH 1, KHU-TBH 2, and mCP were 22.9%, 18.8%, and 17.5% and the reduced efficiencies from the maximum to 1,000 cd/m2 were 0.16, 0.22, and 0.38, respectively.
This work was supported by Grant No. NRF-2016R1A6A3A11930666 and the Human Resources Development program (no. 20154010200830) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Trade, Industry and Energy.
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