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Symmetry Breaking Assisted Fast Reverse Intersystem Crossing for Efficient TADF Materials.

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New asymmetric molecules accelerate the reverse intersystem crossing (RISC) process in thermally activated delayed fluorescence (TADF) materials for organic light-emitting diodes (OLEDs). This breakthrough enhances electroluminescence efficiency and operational stability, paving the way for advanced OLED technology.

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HyperfluorescenceOrganic light‐emitting diodesReverse intersystem crossingSymmetry breakingThermally activated delayed fluorescence

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Area of Science:

  • Materials Science
  • Organic Electronics
  • Photochemistry

Background:

  • The reverse intersystem crossing (RISC) process is crucial for thermally activated delayed fluorescence (TADF) materials in organic light-emitting diodes (OLEDs).
  • Insufficient RISC rates in current TADF materials limit electroluminescence (EL) efficiency, operational stability, and device lifetime.
  • Developing strategies to accelerate RISC is essential for high-performance OLEDs.

Purpose of the Study:

  • To design and synthesize novel asymmetric TADF molecules with accelerated RISC rates.
  • To investigate the impact of molecular asymmetry and vibrations on RISC efficiency.
  • To evaluate the performance of these new TADF materials in OLED devices.

Main Methods:

  • A symmetry breaking strategy was employed using asymmetric electron-withdrawing backbones (benzonitrile and xanthone/thioxanthone).
  • Two new asymmetric TADF molecules, 4tCzCN-pXT and 4tCzCN-pTXT, were synthesized with 3,6-di-tert-butylcarbazole donors.
  • The RISC rates, photophysical properties, and device performance in OLEDs were characterized.

Main Results:

  • The asymmetric TADF molecules exhibited significantly enhanced molecular vibrations, promoting faster intrinsic RISC rates (up to 1.24 × 107 s-1), an order of magnitude higher than symmetric controls.
  • OLEDs fabricated with these materials achieved high external quantum efficiencies (EQEs) up to 31.2% (non-doped) and 35.8% (doped) with minimal roll-off.
  • When used as sensitizers for multi-resonance emitters, EQEs exceeded 40% with exceptional operational stability (LT90 of 24974 h at 1000 cd m-2).

Conclusions:

  • The proposed symmetry breaking strategy effectively accelerates RISC processes in TADF materials.
  • The developed asymmetric molecules demonstrate excellent potential for high-efficiency and stable OLED applications.
  • These findings offer a promising pathway for the advancement of organic light-emitting diode technology.