Organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters offer a sustainable alternative to those using precious metals. However, current TADF OLEDs struggle to simultaneously achieve high efficiency, color purity, brightness, and stability. Early work by Adachi and colleagues demonstrated metal-free donor-acceptor (DA)-type TADF emitters achieving 100% internal quantum efficiency. Subsequent advancements led to high-EQE blue, green, and red TADF OLEDs. Nevertheless, the charge-transfer (CT) emission nature of DA-type emitters typically results in broad emission, hindering the attainment of green color gamut standards (y-coordinate > 0.7) such as those defined by the National Television Standards Committee (NTSC) (0.21, 0.71) and the more recent BT.2020 standard (0.17, 0.80). In 2016, Hatakeyama et al. introduced multi-resonance emitters (MREs), exhibiting narrowband emission and high photoluminescence quantum yields (PLQYs), addressing the color purity issue. While pure-blue and pure-red MRE-based TADF OLEDs are well-established, pure-green MREs with CIEy ≥ 0.7 and satisfactory stability and efficiency roll-off are less common. This research focuses on addressing this gap by designing and synthesizing novel pure-green MREs to create bright, efficient, and stable OLEDs meeting NTSC or BT.2020 color standards.

The literature extensively covers the development of TADF OLEDs, highlighting the advancements in achieving high EQEs in blue, green, and red regions. However, many studies emphasize the challenges associated with achieving pure green emission with CIEy ≥ 0.7, where the broad emission nature of conventional donor-acceptor based TADF emitters hinders achieving the stringent color purity requirements of modern display standards. The introduction of multi-resonance emitters (MREs) has offered a promising solution, by enabling narrowband emission and high PLQYs. Several reports have explored pure-green MREs, but the efficiency roll-off and device stability remain problematic, particularly for devices achieving CIEy ≥ 0.7. Moreover, while maintaining CIEy is a primary focus, the importance of achieving the desired CIEx value to meet NTSC and BT.2020 standards has often been overlooked or compromised. Therefore, the literature indicates a need for a strategic approach that targets both x and y coordinates to fully achieve the color standards while maintaining high efficiency and stability.

This research involved the design, synthesis, and characterization of two novel pure-green multi-resonance emitters (MREs), namely ω-DABNA-M and ω-DABNA-PH. The synthesis followed a two-step Friedel-Craft borylation process, similar to the previously reported ω-DABNA. The resulting compounds were thoroughly characterized using NMR and mass spectrometry. Density functional theory (DFT) calculations were performed to predict and compare the HOMO and LUMO energy levels of the three MREs (ω-DABNA, ω-DABNA-M, and ω-DABNA-PH). The photophysical properties of the MREs were investigated by doping them (1 wt%) into poly(methyl methacrylate) (PMMA) films. Absorption and photoluminescence (PL) spectra, PL quantum yields (PLQYs), and delayed lifetimes (τd) were measured. Electrochemical properties were studied using cyclic voltammetry to determine HOMO and LUMO energy levels. Two device architectures were employed: a standard TADF OLED structure and a hyperfluorescence (HF) OLED structure. The standard TADF OLEDs used a configuration of ITO/NPD/TCTA/mCP/DOBNA-Ph:MRE/3,4-2CzBN/BPy-TP2/LiF/Al. The HF OLEDs utilized a structure of ITO/HAT-CN/Tris-PCz/mCBP/mCBP:3Cz2DPhCzBN:MRE/SF3-TRZ/SF3-TRZ:Liq/Liq/Al, where 3Cz2DPhCzBN served as a sky-blue TADF assistant dopant. OLED performance was assessed by measuring external quantum efficiency (EQE), luminance, current density-voltage-luminance (JVL) characteristics, and device lifetime (LT95). Transient electroluminescence (TrEL) measurements were conducted on selected devices to investigate exciton dynamics. Different dopant concentrations of the MREs were investigated to optimize device performance and stability. In a final set of experiments, the TADF assistant dopant 3Cz2DPhCzBN was replaced with 4CzIPN to investigate the effect of the assistant dopant on device stability.

The study successfully synthesized and characterized two new pure-green MREs, ω-DABNA-M and ω-DABNA-PH, achieving redshifted emissions compared to the parent ω-DABNA. Both alkyl substitution (ω-DABNA-M) and π-conjugation extension (ω-DABNA-PH) effectively redshifted the emission. DFT calculations supported the experimental findings, showing that alkyl substitution primarily affected the HOMO level, while phenyl substitution significantly lowered the LUMO level, leading to narrower energy gaps and redshifted emissions. The standard TADF OLEDs based on the three MREs showed CIEy ≥ 0.7, with maximum EQEs reaching 32.7%, but suffered from unsatisfactory stability (LT95 ≈ 24-34 h). The hyperfluorescence (HF) OLED architecture significantly improved device stability and brightness, achieving maximum EQEs exceeding 25% and maximum luminance exceeding 10⁵ cd m⁻². HF OLEDs demonstrated suppressed efficiency roll-offs, maintaining high EQEs (≈20%) even at high luminance (10⁵ cd m⁻²). The devices exhibited significantly improved device lifetime with LT95 of ~600 h. The optimal dopant concentration of the MREs in HF architecture was found to be crucial for balancing color purity and stability. Increasing the MRE concentration improved the CIEx,y coordinates but reduced the device stability. The choice of TADF assistant dopant was also important; using 4CzIPN instead of 3Cz2DPhCzBN further improved the device stability (LT95 up to 580h).

The findings demonstrate a successful strategy for designing and synthesizing pure-green MREs for high-performance OLEDs. The judicious molecular design, incorporating either alkyl substitution or π-conjugation extension, enabled precise tuning of the emission wavelength while maintaining high PLQYs. The hyperfluorescence architecture effectively mitigated efficiency roll-off and significantly enhanced device stability. The results highlight the importance of both CIEx and CIEy coordinates in achieving display standards, which have often been overlooked in previous studies. The optimized dopant concentrations and the choice of TADF assistant dopant were crucial for achieving the desired balance between color purity, efficiency, and stability. The improved stability in HF OLEDs might be attributed to several factors, including efficient triplet exciton harvesting by the assistant dopant and reduced direct triplet formation on the MRE. The observed relationship between efficiency roll-off and the reverse intersystem crossing (RISC) rate constant of the MREs indicates that faster RISC rates lead to better performance.

This study presents a significant advancement in the development of pure-green OLEDs by introducing two novel multi-resonance emitters. The hyperfluorescence architecture, coupled with careful optimization of dopant concentrations and the choice of assistant dopant, resulted in ultra-bright, efficient, and highly stable devices meeting the stringent color purity requirements of modern displays. Future work could focus on exploring other MRE structures and assistant dopants to further enhance device performance and potentially extend device lifetimes even further. The demonstrated high-performance pure-green OLEDs pave the way for their application in AR/VR technologies and other high-brightness display applications.

While the study achieved significant improvements in stability, further investigation is needed to fully understand the long-term degradation mechanisms. The optimization of the device structure and material selection could potentially lead to further performance enhancements. The current study focused on a specific set of materials; exploring a broader range of MREs and TADF assistant dopants could lead to even better performance characteristics. The impact of different device fabrication techniques should also be investigated for optimal device performance.