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

• Published in: Small (Weinheim an der Bergstrasse, Germany) Vol. 18; no. 21; pp. e2201548 - n/a
• Main Authors: Chen, Jia‐Xiong, Wang, Hui, Xiao, Ya‐Fang, Wang, Kai, Zheng, Ming‐Hui, Chen, Wen‐Cheng, Zhou, Lu, Hu, Dehua, Huo, Yanping, Lee, Chun‐Sing, Zhang, Xiao‐Hong
• Format: Journal Article
• Language: English
• Published: Germany Wiley Subscription Services, Inc 01-05-2022
• Subjects: Doping; Emitters; Energy; energy level alignment; Energy levels; Excitation; Excitons; intermolecular interaction; Nanotechnology; Organic light emitting diodes; organic light‐emitting diodes (OLEDs); red emitters; Steric hindrance; thermally activated delayed fluorescence; organic light-emitting diodes (OLEDs); thermally activated delayed fluorescence; red emitters; energy level alignment; intermolecular interaction
• ISSN: 1613-6810; 1613-6829
• DOI: 10.1002/smll.202201548

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.
By introducing peripheral groups into the donor–acceptor backbone, the intermolecular distance can be enhanced, thus weakening the intermolecular interaction. Therefore, with increasing doping concentrations, triplet‐related exciton annihilations can be effectively suppressed, while the alignment of various excited state energy levels can be tuned to achieve high‐efficiency reverse intersystem crossing, resulting in remarkably high‐efficiencies for BPSPXZ over a wide doping concentration.