Efficient Spin-Flip between Charge-Transfer States for High-Performance Electroluminescence, without an Intermediate...

Area of Science:

• Materials Science
• Organic Electronics
• Photophysics

Background:

• Thermally activated delayed fluorescence (TADF) materials are crucial for efficient and stable organic light-emitting diodes (OLEDs).
• Achieving high photoluminescence quantum yield (PLQY) and fast reverse intersystem crossing (RISC) simultaneously in TADF materials is a key challenge.
• Molecular design is essential for controlling excited-state dynamics, but optimizing PLQY and RISC rates remains difficult.

Purpose of the Study:

• To systematically investigate the feasibility of a direct spin-flip transition between charge-transfer excited states (³CT-¹CT) for efficient RISC.
• To explore how molecular structure influences RISC rates and exciton lifetimes in TADF emitters.
• To correlate TADF material properties with the performance of fabricated OLED devices.

Main Methods:

• Synthesis of three TADF emitters with similar structures but varying RISC rates and exciton lifetimes.
• Experimental characterization of photophysical properties, including PLQY, RISC rates, and energy levels.
• Theoretical calculations to analyze the energy gaps and reorganization energies of excited states.
• Fabrication and testing of OLED devices using the synthesized TADF emitters.

Main Results:

• Three TADF emitters were synthesized, exhibiting high PLQYs (89.5%–96.3%) and distinct RISC rates (0.03 × 10⁶ s^-1 vs. 2.26 × 10⁶ s^-1).
• Experimental and theoretical data suggest that a small singlet-triplet energy gap and low RISC reorganization energy facilitate efficient RISC via a direct ³CT-¹CT spin-flip.
• The optimal TADF emitter enabled an OLED with a 27.1% external quantum efficiency, minimal roll-off (4.1% at 1,000 cd/m²), and high luminance (28,150 cd/m²).

Conclusions:

• A direct spin-flip ³CT-¹CT transition, without intermediate states, can efficiently achieve fast RISC in TADF materials.
• Molecular design focusing on small energy gaps and low reorganization energies between CT states is a viable strategy for high-performance TADF emitters.
• The developed TADF emitter offers a promising pathway for realizing highly efficient and stable OLEDs.