Realizing wide color gamut OLEDs with superior operational stability is crucial for advanced display applications. The Rec.2020 standard demands specific wavelength peaks and narrow full-width-at-half-maximum (FWHM) for primary RGB colors. While multiple-resonance (MR)-TADF molecules, with their robust n-conjugated frameworks and efficient triplet exciton harvesting, show promise in achieving high color purity, their device durability remains a challenge. Current strategies, such as using stable phosphors as triplet assistants, often rely on rare metals like iridium and platinum. Pure organic TADF molecules offer an alternative, but their application in enhancing the stability of MR-OLEDs has lagged. This is partly due to inappropriate molecule selection and undesired exciton behavior. Traditional p-/n-type host materials also contribute to higher driving voltages and an imbalance in charge carrier injection and transport, making the electron/hole mobilities highly dependent on TADF assistant doping concentration. This research focuses on manipulating the recombination zone and exciton behavior to improve both color purity and stability in OLEDs, addressing the limitations of previous approaches.

The literature extensively discusses the challenges of achieving high color purity and long operational lifetimes in OLEDs, particularly for achieving the Rec.2020 color gamut. MR-TADF molecules have emerged as promising candidates due to their narrowband emission and efficient triplet harvesting. However, the operational stability of devices utilizing these molecules remains inadequate. Several strategies have been explored, including the use of stable phosphors as triplet assistants, which however involves rare-earth metals and fails to address the fundamental stability issues. The use of pure organic TADF molecules as triplet manipulators has shown some progress but has not yet yielded significant improvements in operational stability. The use of traditional p-/n-type host materials has also been identified as a factor contributing to poor stability and high driving voltages due to their wide energy gaps and polarity, further highlighting the need for improved material selection and control over exciton dynamics.

This study employed a comprehensive approach involving material selection, device fabrication, and characterization techniques. The researchers selected an organoboron-nitrogen-carbonyl compound (h-BNCO) as the terminal emitter for its narrowband emission (38 nm FWHM). A bipolar donor-acceptor (D-A) type molecule, PIC-TRZ2, served as the host matrix, and 4CzIPN, a stable green TADF molecule, was used as the TADF assistant dopant to stabilize triplets. The photophysical properties of the materials, including absorption, photoluminescence (PL), and transient PL decay, were characterized using spectrophotometry and time-resolved spectroscopy. OLED devices were fabricated with different emissive layers (EMLs): 1 wt% h-BNCO: mCBP (D1), 1 wt% h-BNCO: PIC-TRZ2 (D2), and 1 wt% h-BNCO: 8 wt% 4CzIPN: PIC-TRZ2 (D3). The electroluminescent (EL) characteristics, including EL spectra, external quantum efficiency (EQE), and operational lifetime (LT95), were measured. A deep-red phosphorescent emitter, Ir(fliq)2(acac), was used as an exciton indicator to experimentally determine the recombination zone in the different devices. Hole-only and electron-only devices (HOD and EOD) were fabricated to study charge carrier dynamics. Transient EL decay measurements were conducted using a pulsed transient EL system. The HOMO energy levels were measured using atmospheric ultraviolet photoelectron spectroscopy. Detailed device fabrication procedures are described, covering substrate cleaning, organic layer deposition, encapsulation, and characterization techniques.

The study revealed that the incorporation of 4CzIPN, with its deep LUMO, into the PIC-TRZ2 host matrix effectively trapped electrons, shifting the recombination zone towards the EML/ETL interface in device D3. This led to improved operational stability (LT95 = 322 h at 1000 cd m⁻²) compared to the mCBP-based device (D1, LT95 = 46 h). However, the recombination zone shift also resulted in some efficiency roll-off. Device D2 (1 wt% h-BNCO: PIC-TRZ2) showed better performance compared to D1 due to the balanced charge carrier injection and transport in the PIC-TRZ2 host. To further optimize the device, a 5 nm layer of 1 wt% h-BNCO: PIC-TRZ2 was used as a functional spacer between the 4CzIPN-doped EML and the ETL in device D4. This strategy effectively shifted the recombination zone away from the EML/ETL interface, resulting in significantly enhanced stability (LT95 = 437 h at 1000 cd m⁻²) with high CIE y (0.69) and minimal efficiency roll-off. The transient EL analysis confirmed the effect of 4CzIPN on electron trapping and the impact of the spacer layer on recombination zone distribution. The study highlights that the rational energy level alignment plays a crucial role in OLED performance. The findings show that the pure organic OLED (D4) outperforms the mCBP-based devices (D1) in terms of stability and driving voltage. Analysis of the EL spectra during degradation revealed that the functional spacer layer effectively reduced exciton diffusion towards the ETL, thus preventing degradation.

The findings demonstrate that manipulating the recombination zone in OLEDs through careful material selection and device architecture significantly impacts device performance and stability. The deep LUMO of 4CzIPN facilitates electron trapping, altering carrier dynamics and shifting the recombination zone. While this improves stability, it can lead to efficiency roll-off. The use of the bipolar host PIC-TRZ2 and the introduction of a functional spacer layer effectively address this issue by enabling a more balanced charge transport and relocating the recombination zone away from the interface, thus reducing exciton quenching and improving operational lifetime. The results highlight the importance of understanding and controlling exciton dynamics for the development of high-performance, stable OLEDs. This study provides valuable insights into the design and optimization of pure-organic OLEDs for achieving the Rec.2020 standard and surpassing the performance limitations of existing approaches.

This research successfully demonstrated highly stable pure-green OLEDs approaching the Rec.2020 standard through recombination zone manipulation. The use of a bipolar host matrix (PIC-TRZ2) and a TADF assistant (4CzIPN) with a deep LUMO, combined with a functional spacer layer, resulted in a device (D4) with an LT95 exceeding 430 h at 1000 cd m⁻², a high CIE y of 0.69, and low driving voltage. This approach avoids the use of rare-metal-containing phosphors and offers a pathway for developing commercially viable OLEDs with superior performance. Future work could explore alternative materials with optimized energy levels and explore further refinement of the spacer layer design to achieve even longer lifetimes and higher efficiencies.

While the study demonstrates a significant improvement in OLED stability, some limitations exist. The study primarily focused on green OLEDs, and the findings might not be directly transferable to other colors. Further investigation is needed to explore the long-term stability of the devices beyond the reported LT95 values. The impact of manufacturing variations and potential scalability challenges requires further research. The precise mechanism of exciton quenching at interfaces still requires more detailed investigation.