Sulfur-locked multiple resonance emitters for high performance orange-red/deep-red OLEDs

Area of Science:

• Materials Science
• Organic Electronics
• Photophysics

Background:

• Multiple resonance thermally activated delayed fluorescence (MR-TADF) materials offer high efficiency and color purity in organic light-emitting diodes (OLEDs).
• Designing effective MR-TADF emitters, especially for the orange-red and deep-red spectral regions, remains a significant challenge in the field.

Purpose of the Study:

• To introduce a novel design strategy for orange-red and deep-red MR-TADF emitters.
• To investigate the impact of intramolecular sulfur (S) locks on the photophysical properties and device performance of MR-TADF materials.

Main Methods:

• A new molecular design incorporating intramolecular sulfur (S) locks into a classical MR framework was proposed.
• The photophysical properties, including emission maxima, photoluminescence quantum yield (PLQY), radiative decay rate, and reverse intersystem crossing (RISC) rate, were characterized.
• Fabrication and testing of organic light-emitting diodes (OLEDs) using the synthesized S-embedded emitters (S-BN and 2S-BN) were performed to evaluate device performance.

Main Results:

• Two proof-of-concept emitters, S-BN (594 nm) and 2S-BN (671 nm), were synthesized and demonstrated significant planarity due to the S-locks.
• Both emitters exhibited high PLQY (~100%), rapid radiative decay rates (~10^7 s^-1), and high RISC rates (~10^5 s^-1), crucial for efficient TADF operation.
• The fabricated OLEDs achieved high maximum external quantum efficiencies of 39.9% for S-BN (orange-red) and 29.3% for 2S-BN (deep-red), with improved efficiency roll-off characteristics.

Conclusions:

• The proposed S-locking strategy effectively enables the design of highly efficient orange-red and deep-red MR-TADF emitters.
• The planar structure induced by S-locks is critical for suppressing non-radiative decay pathways and enhancing device performance.
• This work presents a promising pathway for advancing red-emitting MR-TADF materials for next-generation OLED displays and lighting.