Achieving 21% External Quantum Efficiency for Nondoped Solution-Processed Sky-Blue Thermally Activated Delayed Fluorescence OLEDs by Means of Multi-(Donor/Acceptor) Emitter with Through-Space/-Bond Charge Transfer.

Xujun Zheng,Fernando B Dias,Manli Huang,Fan Ni,Weimin Ning,Chuluo Yang,Guohua Xie,Rongjuan Huang,Cheng Zhong

doi:10.1002/advs.201902087

Abstract

Although numerous thermally activated delayed fluorescence (TADF) organic light‐emitting diodes (OLEDs) have been demonstrated, efficient blue or even sky‐blue TADF‐based nondoped solution‐processed devices are still very rare. Herein, through‐space charge transfer (TSCT) and through‐bond charge transfer (TBCT) effects are skillfully incorporated, as well as the multi‐(donor/acceptor) characteristic, into one molecule. The former allows this material to show small singlet–triplet energy splitting (ΔE ST) and a high transition dipole moment. The latter, on the one hand, further lights up multichannel reverse intersystem crossing (RISC) to increase triplet exciton utilization via degenerating molecular orbitals. On the other hand, the nature of the molecular twisted structure effectively suppresses intermolecular packing to obtain high photoluminescence quantum yield (PLQY) in neat flims. Consequently, using this design strategy, T‐CNDF‐T‐tCz containing three donor and three acceptor units, successfully realizes a small ΔE ST (≈0.03 eV) and a high PLQY (≈0.76) at the same time; hence the nondoped solution‐processed sky‐blue TADF‐OLED displays record‐breaking efficiency among the solution process‐based nondoped sky‐blue OLEDs, with high brightness over 5200 cd m−2 and external quantum efficiency up to 21.0%.

Highlights

Owing to their superior display quality and flexibility, organic light-emitting diodes (OLEDs) have been leading the innovation in flat panel displays and lighting applications
On one hand, the alternating arrangement of D-A units endows T-CNDF-T-tCz the coexistence of through-space charge transfer (TSCT) and through-bond charge transfer (TBCT) effects, resulting in a small ΔEST and high PLQY.[26]
Our results indicated that such multi-(donor/acceptor) thermally activated delayed fluorescence (TADF) molecules combined with TSCT and TBCT effects should be promising TADF emitters, and may shed light on the molecular design to achieve high-performance OLEDs

Summary

Introduction

Owing to their superior display quality and flexibility, organic light-emitting diodes (OLEDs) have been leading the innovation in flat panel displays and lighting applications. On one hand, the alternating arrangement of D-A units endows T-CNDF-T-tCz the coexistence of through-space charge transfer (TSCT) and through-bond charge transfer (TBCT) effects, resulting in a small ΔEST and high PLQY.[26] On the other hand, the multi-(donor/acceptor) structure in T-CNDF-T-tCz promotes spin-vibronic mixing among the multiple excited states, which is crucial to the efficient multi-channel RISC process.[10] Remarkably, its highly twisted structure could suppress the intermolecular π-π stacking, leading to reduced fluorescence quenching in condensed state.[27, 28]

Results

Conclusion