Pyridine-Carbonitrile-Carbazole-Based Delayed Fluorescence Materials with Highly Congested Structures and Excellent...

Area of Science:

• Organic Chemistry
• Materials Science
• Photophysics

Background:

• Thermally activated delayed fluorescence (TADF) materials are crucial for efficient organic light-emitting diodes (OLEDs).
• Developing novel TADF emitters with high performance and specific structural features is an ongoing research area.
• Steric hindrance in molecular design can influence photophysical properties and device performance.

Purpose of the Study:

• To synthesize and characterize novel pyridine-carbonitrile-carbazole-based TADF materials.
• To investigate the structure-property relationships of these sterically congested emitters.
• To evaluate the performance of these materials in OLED devices.

Main Methods:

• Synthesis of three donor-acceptor type TADF emitters: 26tCzPPC, 246tCzPPC, and 35tCzPPC.
• Structural analysis revealing significant twisted angles between donor and acceptor units.
• Photophysical characterization including photoluminescence quantum yield (PLQY) and energy gap (ΔEST) measurements.
• Fabrication and testing of OLED devices using these materials as dopants.

Main Results:

• The synthesized molecules possess highly sterically congested structures with large twisted angles.
• These structures lead to spatial separation of frontier molecular orbitals and small ΔEST values, enabling efficient TADF.
• 26tCzPPC and 246tCzPPC dopants promote horizontal molecular alignment, enhancing light outcoupling.
• 35tCzPPC dopant results in perpendicular alignment, reducing light outcoupling.
• Devices incorporating 246tCzPPC achieved a record external quantum efficiency (EQE) of 29.6% among pyridine-carbonitrile-based TADF emitters, attributed to excellent TADF properties, high PLQY, and high triplet quantum yield (Θ).

Conclusions:

• Highly sterically congested pyridine-carbonitrile-carbazole structures are effective for developing high-performance TADF emitters.
• Molecular alignment plays a critical role in device efficiency, with horizontal alignment being favorable for light outcoupling.
• The 246tCzPPC material represents a significant advancement in TADF emitter design, offering superior device performance.