Optimizing Intermolecular Interactions and Energy Level Alignments of Red TADF Emitters for High-Performance Organic Light-Emitting Diodes

Adequately harvesting all excitons in a single molecule and inhibiting exciton losses caused by intermolecular interactions are two important factors for achieving high efficiencies thermally activated delayed fluorescence (TADF). One potential approach for optimizing these is to tune alignment of various excited state energy levels by using different doping concentrations. Unfortunately, emission efficiencies of most TADF emitters decrease rapidly with concentrations which limits the window for energy level tunning. In this work, by introducing a spiro group to increase steric hindrance of a TADF emitter (BPPXZ) with a phenoxazine and a dibenzo[a,c]phenazine, emission efficiency of the resulting molecule (BPSPXZ) is much less affected by concentration increase. This enables exploitation of the concentration effects to tune energy levels of its excited states for obtaining simultaneously small singlet-triplet energy offset and large spin-orbital coupling, leading to high-efficiency reverse intersystem crossing. With these merits, organic light-emitting diodes (OLEDs) using the BPSPXZ emitter from 5 to 60 wt% doping can all deliver EQE of over 20%. More importantly, record-high EQEs of 33.4% and 15.8% are respectively achieved in the optimized and nondoped conditions. This work proposes a strategy for developing red TADF emitters by optimizing the intermolecular interaction and energy level alignments to facilitate exciton utilization over wide doping concentrations.

Automated summary

This study introduces a spiro group to reduce the decrease in emission efficiency of a TADF emitter at higher concentrations. By tuning energy level alignments, the molecule achieves a small singlet-triplet energy offset and a large spin-orbital coupling, resulting in high-efficiency reverse intersystem crossing. OLEDs using this emitter at various doping concentrations exhibit high EQEs, with record-high values of 33.4% and 15.8% achieved.