Background:

The performance of Organic Light-Emitting Diodes (OLEDs), the next generation of display and lighting technology, is highly dependent on luminescent materials. Thermally activated delayed fluorescence (TADF) materials gain attention for their high emitting efficiency and narrowband. Despite substantial progress with blue and green MR-TADF materials, the development of pure-red MR-TADF emitters has lagged behind. This paper proposes an effective approach for designing pure-red MR-TADF molecules based on the integration of secondary electron-donating units and π-skeleton extension into MR cores, which enables not only a redshift of narrowband emission but also an acceleration of reverse intersystem crossing (RISC) rate. The proof-of-the-concept emitter BNTPA showcases a bright and saturated red emission centered at 613 nm with a full-width at half-maximum of 0.14 eV with an exceptional EQEmax of 43.3% approaching the coordinates of the National Television Standards Committee (NTSC) red standard. It opens up a new paradigm for development of MR-TADF materials.

About the paper:

Figure 1. Molecular design concept and chemical structures of the emitters.

Figure 2. First to third columns: natural transition orbitals (NTOs) for S1, T1 and T2 states of BNCz, TBN3 and BNTPA, respectively. Fourth to fifth columns: the differences in the density of the S1-T1 NTOs and S1-T2 NTOs for BNCz, TBN3 and BNTPA, respectively. Blue for highest occupied natural transition orbitals (HONTOs) and red for lowest unoccupied natural transition orbitals (LUNTOs). All hydrogen atoms are omitted for clarity. ΔEST values as well as associated SOC matrix element < S1|ĤSOC|Tn> of BNCz, TBN3 and BNTPA are also shown.

Figure 3. (a) UV–vis absorption spectra and (b) fluorescence spectra of BNCz, TBN3 and BNTPA in toluene at 298 K. (c) Fluorescence spectra of BNTPA in various solvents with different polarity at 298 K. (d) Transient PL spectra of BNTPA in oxygen-free toluene at 298 K.

Figure 4. (a) Transient absorption contour map (298 K) of BNTPA on picosecond time scales in oxygen-free toluene. (b) Transient absorption contour map (298 K) of BNTPA on nanosecond to microsecond time scales in oxygen-free toluene. (c) Single-wavelength kinetics of BNTPA at 500 and 575 nm. Symbols are raw data and lines are multiexponential fits from single-wavelength kinetics analysis. (d) Transient absorption spectra (298 K) of BNTPA on nanosecond to microsecond time scales in oxygen-free toluene.

Figure 5. (a) Schematic diagram of the device architectures. (b) Electroluminescence (EL) spectra of devices A and B. Inset, color coordinates in the Commission Internationale de l’Eclairage (CIE) chromaticity diagram. (c) External quantum efficiency-luminance (EQE-L), current efficiency-luminance (CE-L) and power efficiency-luminance (PE-L) plots for device A. (d) External quantum efficiency-luminance (EQE-L), current efficiency-luminance (CE-L) and power efficiency-luminance (PE-L) plots for device B.

About publication:

Journal: Journal of the American Chemical Society

Title: Efficient and Stable Narrowband Pure-Red Light-Emitting Diodes with Electroluminescence Efficiencies Exceeding 43%

Authors: Lishuang Ge,‡ Wei Zhang,‡ Yi-Hong Hao, Ming Li, Yuan Liu,* Meng Zhou,* and Lin-Song Cui*

First published: 13 November 2024