Spiral-Locking Strategy for Efficient Narrowband Multiple Resonance Thermally Activated Delayed Fluorescence Emitters

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).
• Planar structures in MR emitters lead to concentration-induced emission quenching, spectral redshift, and broadening, limiting device performance.

Purpose of the Study:

• To develop novel MR-TADF materials that overcome the limitations of planar structures, specifically addressing emission quenching and spectral instability.
• To investigate the impact of steric hindrance via a spiro-carbon bridge on the photophysical properties and device performance of MR-TADF emitters.

Main Methods:

• Synthesis and characterization of two orthorhombic asymmetric conformational materials, SBNO and SBNOS, featuring a sterically hindered spiro-carbon bridge.
• Fabrication and testing of OLED devices using the developed MR-TADF emitters at various doping concentrations.
• Analysis of spectral properties (redshift, FWHM broadening) and device performance metrics (EQE, CIE coordinates).

Main Results:

• SBNOS-based devices exhibited minimal spectral redshift (4 nm) and broadening (7 nm FWHM) across a wide doping range (1-100 wt%).
• The steric effect of the spiro-carbon bridge effectively suppressed intermolecular interactions, leading to pure green emission (CIE y = 0.69).
• Maximum external quantum efficiency (EQEmax) reached up to 32.7%, significantly outperforming a reference material (BNO) lacking the spiro skeleton.

Conclusions:

• The incorporation of a sterically hindered spiro-carbon bridge is a highly effective strategy for developing stable and efficient MR-TADF emitters.
• This approach mitigates concentration-induced spectral shifts and broadening, enabling high color purity and performance in OLEDs.
• The developed SBNO and SBNOS materials represent a promising advancement for next-generation OLED applications.