Macrocyclic Covalent Encapsulation of a Multi-Resonant Emitter: Understanding and Controlling Interactions in Highly Efficient Deep-Blue OLEDs

TL;DR

This paper introduces a new method to improve deep-blue OLEDs by encapsulating emitters in a macrocycle, enhancing efficiency and color purity.

Contribution

The study demonstrates a novel use of macrocyclic encapsulation to enhance MR-TADF emitters for OLEDs.

Findings

Macrocyclic encapsulation suppresses nonradiative losses and preserves narrowband deep-blue emission.

The encapsulated emitter achieves a 33% external quantum efficiency and meets BT.2020 blue color standards.

Abstract

Multi-resonant thermally
activated delayed fluorescence
(MR-TADF)
emitters have emerged as popular candidates for the development of
blue organic light-emitting diodes (OLEDs), offering narrowband emission,
high photoluminescence quantum yields (PLQYs), and the ability to
upconvert dark triplet states to bright singlet states. However, their
planar polycyclic structures promote detrimental intermolecular interactions
in the solid-state which diminish the color purity and introduce nonradiative
loss pathways. Furthermore, the intrinsic luminescence of many MR-TADF
emitters fails to satisfy the stringent color purity standards required
for next-generation display technologies. Here, we synthetically address
these issues by covalently encapsulating a blue-shifted MR-TADF emitter
within a protective macrocyclic ring. We identify a previously undiscovered
utility of macrocyclic encapsulation, whereby it can shield the MR
core from the surrounding environment to enhance its radiative rate,
PLQY, and reverse intersystem crossing (RISC) efficiency. Only with
spectrally resolved transient photoluminescence measurements were
we able to identify the weakly emissive aggregate and excimer species,
and definitively confirm that the macrocycle suppresses their formation
in the solid-state, thereby preserving narrowband deep-blue emission
and reducing nonradiative losses. Notably, these performance enhancements
were achieved without compromising thermal stability or vacuum-processability.
When integrated into an OLED device based on the “hyperfluorescent”
strategy, this emitter delivers an exceptional combined maximum external
quantum efficiency (EQE) of 33% and (0.146, 0.046) CIE
x,y
coordinates with peak emission
at 451 nm, satisfying BT.2020 blue color requirement, and significantly
outperforming its nonencapsulated analogue. This material represents
one of the highest efficiency deep-blue OLEDs to date and therefore
establishes macrocyclic encapsulation as a powerful synthetic strategy
for unlocking the full potential of MR-TADF materials for next-generation
OLEDs.

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