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

Abstract

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, <b>4CzIPN</b>, was adopted as a probe to examine the feasibility of a co-host composing of a hole transporter SimCP and an electron transporter <i>o</i>CF₃-T2T. As a result, a sky blue device with 1 wt % <b>4CzIPN</b> doped in SimCP:<i>o</i>CF₃-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, <b>4CzIPN-CF</b>_<b>3</b>, <b>3CzIPN-H-CF</b>_<b>3</b>, and <b>3CzIPN-CF</b>_<b>3</b>, modified either by lowering the electron-withdrawing ability of the acceptor group or by reducing the number of carbazole donors of <b>4CzIPN</b>, have been synthesized and characterized. Among them, <b>4CzIPN-CF</b>_<b>3</b> and <b>3CzIPN-H-CF</b>_<b>3</b> display hypsochromic shift emissions compared to that of <b>4CzIPN</b>. These new compounds were then explored for their potential applications as TADF emitters. Blue TADF OLEDs with 1 wt % of <b>4CzIPN-CF</b>_<b>3</b> and <b>3CzIPN-H-CF</b>_<b>3</b> dispersed in SimCP:<i>o</i>CF₃-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.