LAUSR

A strategic approach of combining a new cohost system and low concentration of new TADF emitters to make efficient blue TADF OLED was developed. The benchmark TADF molecule, 4CzIPN, was adopted as a probe to examine the feasibility of a cohost composing of a hole-transporter SimCP and an electron-transporter oCF3-T2T. A sky blue device with 1wt% 4CzIPN doped in SimCP:oCF3-T2T cohost exhibited completed energy transfer and achieved high EQE up to 26.1%. More importantly, this device showed limited efficiency roll-off with EQE of 24% at 1000 cd m-2. To further blue-shift the emission, three new TADF molecules, 4CzIPN-CF3, 3CzIPN-H-CF3 and 3CzIPN-CF3 modified either by lower the electron-withdrawing ability of acceptor group or reduce the number of carbazole donors of 4CzIPN have been synthesized and characterized. Among them, 4CzIPN-CF3 and 3CzIPN-H-CF3 display hypsochromic shift emissions as compared to that of 4CzIPN. These new molecules were then explored their applications as TADF emitters. Blue TADF OLEDs with 1wt% of 4CzIPN-CF3 and 3CzIPN-H-CF3 dispersed in SimCP:oCF3-T2T cohost achieved EQE of 23.1% and 16.5%, and retained high EQE of 20.9% and 14.7% at 1000 cd m-2, respectively.