Excited-state engineering of universal ambipolar hosts for highly efficient blue phosphorescence and thermally activated delayed fluorescence organic light-emitting diodes

The development of high-performance universal host materials for both blue electrophosphorescence and thermally activated delayed fluorescence (TADF) is one of the main challenges for organic light-emitting diode (OLED) technology. In this work, based on four quaternary ambipolar hosts with insulated donor-acceptor (D-A) structures, the influence of the excited-state characteristics for these hosts on their device performance was systematically investigated. It was showed that on the premise of the similar electrical properties and excited-state energy levels for positive host-dopant energy transfer (ET), the weak exciton capture ability, embodied as low exciton binding energy (E-b), for the hosts could facilitate the Dexter ET to the dopant through charge migration. Since the charge-transfer -featured excited states and small Eb of blue TADF emitters, in contrast to the blue phosphorescence (PH) counterparts, the influence of the host Eb on the device efficiencies of blue TADF dyes was dramatically enlarged, reflecting the higher sensitivity of the latter to host characteristics. In consequence, 9CzFDBFDPO with the smallest Eb endowed its blue TADF and PH devices with the state-of-the-art performance, e.g. EQE beyond 20% and EQE roll-off within 20% at 500 cd m(-2). This work showed that based on the systematically optimized excited-state characteristics, the ambipolar hosts can be competent in both high-efficiency blue PH and TADF OLEDs, which is crucial for their commercial applications.