Harnessing a New Co-Host System and Low Concentration of New TADF Emitters Equipped with Trifluoromethyl- and Cyano-Substituted Benzene as Core for High-Efficiency Blue OLEDs
2019
Yi, Chih-Lun | Ko, Chang-Lun | Yeh, Tzu-Chin | Chen, Chih-Yang | Chen, Yi-Sheng | Chen, Deng-Gao | Chou, Pi-Tai | Hung, Wen-Yi | Wong, Ken-Tsung
A strategic approach combining a new co-host system and low concentration of new thermally activated delayed fluorescence (TADF) emitters to make efficient blue TADF organic light-emitting diode (OLED) was developed. The benchmark TADF molecule, 4CzIPN, was adopted as a probe to examine the feasibility of a co-host composing of a hole transporter SimCP and an electron transporter oCF₃-T2T. As a result, a sky blue device with 1 wt % 4CzIPN doped in SimCP:oCF₃-T2T co-host exhibited 100% energy transfer and achieved a high external quantum efficiency (EQE) up to 26.1%. Importantly, this device showed a limited efficiency rolloff with an EQE of 24% at 1000 cd m–². To further shift the emission toward blue, three new TADF molecules, 4CzIPN-CF₃, 3CzIPN-H-CF₃, and 3CzIPN-CF₃, modified either by lowering the electron-withdrawing ability of the acceptor group or by reducing the number of carbazole donors of 4CzIPN, have been synthesized and characterized. Among them, 4CzIPN-CF₃ and 3CzIPN-H-CF₃ display hypsochromic shift emissions compared to that of 4CzIPN. These new compounds were then explored for their potential applications as TADF emitters. Blue TADF OLEDs with 1 wt % of 4CzIPN-CF₃ and 3CzIPN-H-CF₃ dispersed in SimCP:oCF₃-T2T co-host achieved EQEs of 23.1 and 16.5% and retained high EQEs of 20.9 and 14.7% at 1000 cd m–², respectively.
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