Highly efficient hybrid warm white organic light-emitting diodes using a blue thermally activated delayed fluorescence emitter: exploiting the external heavy-atom effect

Abstract

To attain high efficiencies in hybrid white organic light-emitting diodes (WOLEDs), mutual quenching of the fluorophors and phosphors should be minimized. Efforts have been devoted to reducing the triplet quenching of phosphors; however, the quenching of fluorophors by the external heavy-atom effect (EHA) introduced by the phosphors is often ignored. Here, we observed that conventional fluorophors and fluorophors with thermally activated delayed fluorescence (TADF) behave differently in the presence of EHA perturbers. The efficiencies of the conventional fluorophors suffer greatly from the EHA, whereas the TADF fluorophors exhibit negligible changes, which makes TADF materials ideal fluorophors for hybrid devices. WOLEDs using a blue TADF fluorophor and an orange phosphor achieve a maximum forward viewing external quantum efficiency of 19.6% and a maximum forward viewing power efficiency of 50.2 lm W 21 , among the best values for hybrid WOLEDs. This report is the first time that the EHA effect has been considered in hybrid WOLEDs and that a general strategy toward highly efficient hybrid WOLEDs with simple structures is proposed. Keywords: external heavy-atom effect; hybrid white OLEDs; thermally activated delayed fluorescence INTRODUCTION Recent conceptual advancements have led to many exciting approaches to white organic light-emitting diodes (WOLEDs). 1-4 For practical use, high efficiency and long lifetime are the most critical parameters. Blue phosphors can obtain high luminous efficiency but suffer from short lifetime, while blue fluorophors are exactly the opposite. 5,6 Efforts have been devoted to resolve the tradeoff between the high efficiency and long lifetime by combining the advantages of phosphors and fluorophors, that is, by fabricating hybrid WOLEDs with blue fluorophors and green/red or orange phosphors. [6][7][8][9] To attain high efficiencies in hybrid WOLEDs, mutual quenching of the fluorophors and phosphors should be minimized. It has been long realized that the quenching of the green and red phosphorescence by the low triplet energy fluorophors is one of the main losses of excitation energy in these devices. 6 Therefore, a 'triplet harvesting' strategy by doping a blue fluorophor and red/green phosphors in separated regions of high-triplet-energy host materials has been proposed 1,10 and is further improved by incorporating blue fluorophors with high triplet energy (T 1 ) to prevent the undesirable 'back energy transfer' from the phosphors to the blue fluorophors. 2,7,8 However, the heavy metal in the phosphors introduce an external heavy-atom effect (EHA) in the emissive layer (EML) and may lead to

Stable green phosphorescence organic light-emitting diodes with low efficiency roll-off using a novel bipolar thermally activated delayed fluorescence material as host

“…To our knowledge, devices based on a TADF host at high-brightness levels rarely achieve an operation lifetime above 600 min. 44,45 Furthermore, we also measured the lifetime of device C-3 in a humid air atmosphere with 20% relative humidity (Fig. S11, in the ESI†).…”

Shorter Exciton Lifetimes via an External Heavy‐Atom Effect: Alleviating the Effects of Bimolecular Processes in Organic Light‐Emitting Diodes

“…It was already successfully utilized in white OLEDs by combining blue TADF fluorophores and yellow phosphors. [32] However this approach is not applicable to monochrome pixels, as needed in active matrix display applications. An increase in OLED external quantum efficiency (EQE) after the insertion of a heavy atom perturber layer has also been reported.…”

High‐Performance Hybrid White Organic Light‐Emitting Diodes with Superior Efficiency/Color Rendering Index/Color Stability and Low Efficiency Roll‐Off Based on a Blue Thermally Activated Delayed Fluorescent Emitter

“…[ 7 ] Because the generated triplet excitons can be harvested by a reverse intersystem crossing (RISC) process induced by suffi ciently small singlet-triplet energy gap (Δ E ST ), thus the TADF materials can achieve 100% exciton utilization effi ciency as the alternative of phosphorescent emitters. [ 8,9 ] Nowadays, the highly effi cient blue TADF materials with high T 1 and PLQY have been successfully synthesized, [ 10,11 ] and used for WOLEDs, such as the combination of TADF-assisted materials and conventional fl uorescent emitters, [ 12 ] the introduction of the external heavy-atom effect in the TADF-based hybrid WOLED, [ 13 ] and the utilization of the TADF blue exciplex system in the single-EML hybrid WOLED. [ 14 ] As shown, although some TADF-based hybrid WOLEDs have achieved an impressive electroluminescence (EL) effi ciency, the problems are still obvious in terms of severe effi ciency rolloff, poor color stability, and low CRI.…”

“…[ 14 ] As shown, although some TADF-based hybrid WOLEDs have achieved an impressive electroluminescence (EL) effi ciency, the problems are still obvious in terms of severe effi ciency rolloff, poor color stability, and low CRI. [13][14][15][16] The realization of high effi ciency, superior color stability, high CRI, and low efficiency roll-off simultaneously in TADF-based hybrid WOLEDs remains a big challenge.…”