Aggregation-enhanced TADF in deep-red emitters for high-performance OLEDs.

Area of Science:

• Materials Science
• Organic Electronics
• Photophysics

Background:

• Molecular aggregation influences electronic structure and photophysical properties, including fluorescence lifetime.
• The impact of aggregation on thermally activated delayed fluorescence (TADF) mechanisms and device performance is underexplored.
• Molecular structure dictates the extent of changes in photophysical properties upon aggregation.

Purpose of the Study:

• To investigate the aggregation-enhanced TADF (AE-TADF) effect in deep-red emitters.
• To explore the relationship between molecular structure and AE-TADF.
• To understand how AE-TADF influences optoelectronic device performance.

Main Methods:

• Design and synthesis of D-A and D2-A type deep-red TADF emitters using quinoxaline-6,7-dicarbonitrile (QCN) as acceptor and naphthalene-substituted N,N-diphenylamine (ND) as donor.
• Characterization of photophysical properties, including singlet-triplet energy gap (ΔEST) changes upon aggregation.
• Fabrication and testing of organic light-emitting diodes (OLEDs) using the designed emitters.

Main Results:

• The D2-A type emitter, (αND)2-QCN, demonstrated a significant AE-TADF effect compared to D-A types.
• Aggregation led to a substantial reduction in ΔEST, attributed to intermolecular hydrogen bonds in J-aggregates.
• An (αND)2-QCN-based deep-red OLED achieved λEL,max = 622 nm and a maximum external quantum efficiency (EQEmax) of 14.3%.

Conclusions:

• Intermolecular hydrogen bonding in J-aggregates is crucial for the AE-TADF effect in these deep-red emitters.
• The AE-TADF effect enhances reverse intersystem crossing (RISC), leading to improved OLED performance.
• Designed D2-A structures are promising for high-performance deep-red OLED applications.