Efficient triplet utilization in conventional solution-processed phosphorescent organic light emitting diodes using a thermal activated delayed fluorescence polymer as an assistant host

Abstract

Solution processed Ir(mPPy)₃ based phosphorescent organic light emitting diodes (PhOLEDs) were fabricated, in which mCP and a TADF copolymer, COPO, were selected as host and assistant host components, respectively. The COPO has a lower triplet energy and a higher singlet energy than Ir(mPPy)₃. Compared with traditional Ir(mPPy)₃ doped mCP type PhOLEDs, the introduction of COPO as an assistant host can effectively improve the electroluminescent performance. Utilizing 20% COPO and 70% mCP (w/w) as a mixed-host system for Ir(mPPy)₃ in solution-processed devices, a considerably higher current efficiency of 56.8 cd A⁻¹ has been achieved with great improvement, compared to traditional Ir(mPPy)₃ based devices with mCP as the single host (24.5 cd A⁻¹). Moreover, the EQE displays only a slight roll-off from an EQE_max of 17.3% to an EQE of 16.0% at 500 cd m⁻². The improved PhOLED performance is mainly attributed to the full utilization of triplet excitons as COPO may facilitate the triplet transfer due to a closer physical contact with mCP and Ir(mppy)₃ molecules. Moreover, the presence of COPO also improves the ambipolar transport of charge carriers.