Stable Organic Light-emitting Diodes Research Articles

Triptycene-Fused Sterically Shielded Multi-Resonance TADF Emitter Enables High-Efficiency Deep Blue OLEDs with Reduced Dexter Energy Transfer.

Designing multi-resonance (MR) emitters that can simultaneously achieve narrowband emission and suppressed intermolecular interactions is challenging for realizing high color purity and stable blue organic light-emitting diodes (OLEDs). Herein, a sterically shielded yet extremely rigid emitter based on a triptycene-fused B,N core (Tp-DABNA) is proposed to address the issue. Tp-DABNA exhibits intense deep blue emissions with a narrow full width at half maximum (FWHM) and a high horizontal transition dipole ratio, superior to the well-known bulky emitter, t-DABNA. The rigid MR skeleton of Tp-DABNA suppresses structural relaxation in the excited state, with reduced contributions from the medium- and high-frequency vibrational modes to spectral broadening. The hyperfluorescence (HF) film composed of a sensitizer and Tp-DABNA shows reduced Dexter energy transfer compared to those of t-DABNA and DABNA-1. Notably, deep blue TADF-OLEDs with the Tp-DABNA emitter display higher external quantum efficiencies (EQEmax =24.8 %) and narrower FWHMs (≤26 nm) than t-DABNA-based OLEDs (EQEmax =19.8 %). The HF-OLEDs based on the Tp-DABNA emitter further demonstrate improved performance with an EQEmax of 28.7 % and mitigated efficiency roll-offs.

Modeling the impacts of energetic disorder and charge transport on the exciton formation zone in organic light-emitting diodes

The width of exciton formation zone plays a vital role in determining the long-term stability of organic light-emitting diodes (OLEDs). Here, based on the general model of carrier device lifetimes, the dependences of the width and location of exciton formation zone on energetic disorder and charge transport have been simulated in OLEDs with emissive layer featuring hole and electron transport on host and guest molecules, respectively. The width of exciton formation zone increases with emissive layer’s energetic disorder increasing. The emissive layer with properly imbalanced carrier mobilities enables larger width of exciton formation zone than that with balanced carrier mobilities, due to the different hole and electron trap densities. Moreover, increasing hole transport layer’s hole mobility or decreasing electron transport layer’s electron mobility helps increase the width of exciton formation zone. There is no width of exciton formation zone obtained, when hole (electron) transport layer’s hole (electron) mobility is smaller than a certain value. The location of exciton formation relies on not only emissive layer’s carrier mobilities but also hole (electron) transport layer’s hole (electron) mobility. The general model provides the comprehensive picture to elucidate the roles of energetic disorder and charge transport in enhancing the stability of OLEDs, beneficial to improve the OLEDs designs towards the high stability at high luminance.

Biological Interfacial Materials for Organic Light-Emitting Diodes.

Organic optoelectronic devices have received appreciable attention due to their low cost, mechanical flexibility, band-gap engineering, lightness, and solution processability over a broad area. Specifically, realizing sustainability in organic optoelectronics, especially in solar cells and light-emitting devices, is a crucial milestone in the evolution of green electronics. Recently, the utilization of biological materials has appeared as an efficient means to alter the interfacial properties, and hence improve the performance, lifetime and stability of organic light-emitting diodes (OLEDs). Biological materials can be known as essential renewable bio-resources obtained from plants, animals and microorganisms. The application of biological interfacial materials (BIMs) in OLEDs is still in its early phase compared to the conventional synthetic interfacial materials; however, their fascinating features (such as their eco-friendly nature, biodegradability, easy modification, sustainability, biocompatibility, versatile structures, proton conductivity and rich functional groups) are compelling researchers around the world to construct innovative devices with enhanced efficiency. In this regard, we provide an extensive review of BIMs and their significance in the evolution of next-generation OLED devices. We highlight the electrical and physical properties of different BIMs, and address how such characteristics have been recently exploited to make efficient OLED devices. Biological materials such as ampicillin, deoxyribonucleic acid (DNA), nucleobases (NBs) and lignin derivatives have demonstrated significant potential as hole/electron transport layers as well as hole/electron blocking layers for OLED devices. Biological materials capable of generating a strong interfacial dipole can be considered as a promising prospect for alternative interlayer materials for OLED applications.

Efficient and Stable OLEDs with Inverted Device Structure Utilizing Solution‐Processed ZnO‐Based Electron Injection Layer

AbstractTo realize low‐cost, efficient, and stable organic light‐emitting diodes (OLEDs) in future large area displays and lighting, the development of suitable solution‐processed functional materials is highly desirable. Herein, a series of efficient and stable OLEDs with an inverted device architecture is reported, employing both a vapor‐deposited phosphorescent aggregate emitter, i.e., Pd(II) 7‐(3‐(pyridine‐2‐yl‐κN)phenoxy‐κC)(benzo‐κC)([c]benzo[4,5]imidazo‐κN)[1,2‐a][1,5]naphthyridine, and a solution‐processed ZnO layer as potential electron injection layer and electron‐transporting layer. One of the optimized OLED devices exhibits its peak external quantum efficiency (EQE) of 23.9% and retains EQEs of 23.5% and 18.7% at 1000 and 10 000 cd m−2, with a low efficiency roll‐off. Such an efficient device also demonstrates a measured lifetime (LT95) of 98.6 h with an initial brightness of 10 435 cd m−2 corresponding to an estimated LT95 of 5313 h at 1000 cd m−2. By depositing a 2 nm Al on the ZnO surface, an estimated LT95 at 1000 cd m−2 of such a device can be further extended to 73 244 h, making it the longest‐lived OLED reported in the literature domain. This study lays the strong foundation for the future deployment of efficient and stable inverted OLEDs with solution‐processed ZnO layers for a wide range of displays and lighting applications.

Efficient deep-blue luminescence based on dual-channel intra/intermolecular exciplexes

Highly efficient and stable blue organic light-emitting diodes (OLEDs) cannot be easily obtained simultaneously. In particular, the efficiency roll-off as a reference index to evaluate the lifetime of deep-blue OLED at high luminescence is still severe. A novel molecule (CzSiTrz) connected with carbazole and triazine fragments by a nonconjugated silicon atom is designed. An intramolecular charge transfer emission and intermolecular exciplex luminescence in the aggregated state are obtained, resulting in a dual-channel intra/intermolecular exciplex (DCIE) emission with fast and efficient reverse intersystem crossing (RISC). Deep-blue OLED with Commission Internationale de l'Eclairage (CIE) coordinates of (0.157, 0.076) and a record-high external quantum efficiency (EQE) of 20.35% at high luminance (5000 cd m-2) is accomplished. Simple molecular synthesis and device fabrication of this strategy give a unique approach to realizing high-performance deep-blue electroluminescence.

Multiple donor–acceptor design for highly luminescent and stable thermally activated delayed fluorescence emitters

A considerable variety of donor–acceptor (D–A) combinations offers the potential for realizing highly efficient thermally activated delayed fluorescence (TADF) materials. Multiple D–A type compounds are one of the promising families of TADF materials in terms of stability as well as efficiencies. However, those emitters are always composed of carbazole-based donors despite a wide choice of moieties used in linearly linked single D–A molecules. Herein, we developed a multiple D–A type TADF compound with two distinct donor units of 9,10-dihydro-9,9-dimethylacridine (DMAC) and carbazole as the hetero-donor design. The new emitter exhibits high photoluminescence quantum yield (PLQY) in various conditions including polar media blend and high concentrations. Organic light-emitting diodes (OLEDs) showed a reasonably high external quantum efficiency (EQE). In addition, we revealed that the multiple-D–A type molecules showed better photostability than the single D–A type molecules, while the operational stability in OLEDs involves dominant other factors.

Asymmetric Decoration of a meta-Linked Bitriazine-Based Host for Highly Efficient and Stable Blue Phosphorescent Organic Light-Emitting Diodes

High triplet energy hosts for blue phosphorescent organic light-emitting diodes were developed by decorating a meta-linked bitriazine core with carbazole and tetraphenylsilyl functional groups. A symmetric host with two carbazole units as the two triazine units of the core and an asymmetric host with one carbazole unit and one tetraphenylsilyl unit as the two triazine units were prepared. The triplet energy of these two hosts was 2.97 eV, suitable for triplet exciton harvesting of blue phosphors. Comparing the two host designs, the asymmetric decoration of the two triazine units with carbazole and tetraphenylsilyl units was superior to the symmetric decoration of the two triazine units with two carbazoles in terms of high external quantum efficiency (EQE) and long-term device stability. A high EQE of 19.7% and a long device lifetime of 2093.6 h at 100 cd m-2 were achieved using the asymmetrical host.

Highly Air-Stable, Flexible, and Water-Resistive 2D Titanium Carbide MXene-Based RGB Organic Light-Emitting Diode Displays for Transparent Free-Form Electronics.

Flexible see-through displays are considered to be the next generation smart display, providing improved information flow, safety, situational awareness, and overall user experience in smart windows, automotive displays, glass-form biomedical displays, and augmented reality systems. 2D titanium carbides (MXenes) are promising material as electrodes of the transparent and flexible displays due to their high transparency, metallic conductivity, and flexibility. However, current MXene-based devices have insufficient air stability and lack engineering schemes to develop matrix-addressable display forms with sufficient pixels to display information. Here, we develop an ultraflexible and environmentally stable MXene-based organic light-emitting diode (OLED) display by combining high performance MXene electrodes, flexible OLEDs, and ultrathin and functional encapsulation systems. The MXene material was synthesized and used to fabricate a highly reliable MXene-based OLED that can stably operate in air condition for over 2000 h, endure repetitive bending deformation of 1.5 mm radius, and maintain environmental stability for 6 h when exposed to wet surroundings. The RGB MXene-based OLEDs were fabricated, (1691 cd m-2 at 40.4 mA cm-2 for red, 1377 cd m-2 at 4.26 mA cm-2 for green, and 1475 cd m-2 at 18.6 mA cm-2 for blue) and a matrix-addressable transparent OLED display was demonstrated that could display letters and shapes.

Blue Transparent OLEDs with High Stability and Transmittance for Modulating Sleep Disorders (Adv. Mater. Interfaces 11/2023)

Blue Transparent Organic Light-Emitting Diodes In article number 2202443, Kyung-Cheol Choi and co-workers develop high-transmittance and stable blue transparent organic light-emitting diodes (TrOLEDs) using optimized dielectric-metal-dielectric (DMD) electrodes with silver, seed, and capping layers. They also demonstrated the TrOLEDs on glasses for visual stimulus devices and proposed an animal experimental protocol for modulating NREM sleep duration with blue light stimulation. The proposed blue TrOLEDs could be a practical and convenient solution for modulating sleep disorders in a wearable form factor.

Efficient, Color-Stable, and Long-Lived White Organic Light-Emitting Diodes Utilizing Phosphorescent Molecular Aggregates.

Highly efficient and stable single-stack hybrid white organic light-emitting diode (WOLED) devices are developed using two emissive layers: one with amber-colored phosphorescent molecular-aggregate emission from the Pd(II) complex, Pd(II) 7-(3-(pyridine-2-yl-κN)phenoxy-κC)(benzo-κC)([c]benzo[4,5]imidazo-κN)[1,2-a][1,5]naphthyridine, Pd3O8-Py5, and the other with blue fluorescence emission. An optimized device structure achieves high color stability under various current densities, an external quantum efficiency (EQE) of 45.5%, a power efficiency of 97.4 Lm W-1 , and an estimated LT95 (operational time to 95% of the initial luminance) of 50 744 h at an initial luminance of 1000cd m-2 .

Efficient Spin-Flip between Charge-Transfer States for High-Performance Electroluminescence, without an Intermediate Locally Excited State.

Thermally activated delayed fluorescence (TADF) materials with both high photoluminescence quantum yield (PLQY) and fast reverse intersystem crossing (RISC) are strongly desired to realize efficient and stable organic light-emitting diodes (OLEDs). Control of excited-state dynamics via molecular design plays a central role in optimizing the PLQY and RISC rate of TADF materials but remains challenging. Here, 3 TADF emitters possessing similar molecular structures, similar high PLQYs (89.5% to 96.3%), and approximate energy levels of the lowest excited singlet states (S1), but significantly different spin-flipping RISC rates (0.03 × 106 s-1 vs. 2.26 × 106 s-1) and exciton lifetime (297.1 to 332.8 μs vs. 6.0 μs) were systematically synthesized to deeply investigate the feasibility of spin-flip between charge-transfer excited states (3CT-1CT) transition. Experimental and theoretical studies reveal that the small singlet-triplet energy gap together with low RISC reorganization energy between the 3CT and 1CT states could provide an efficient RISC through fast spin-flip 3CT-1CT transition, without the participation of an intermediate locally excited state, which has previously been recognized as being necessary for realizing fast RISC. Finally, the OLED based on the champion TADF emitter achieves a maximum external quantum efficiency of 27.1%, a tiny efficiency roll-off of 4.1% at 1,000 cd/m2, and a high luminance of 28,150 cd/m2, which are markedly superior to those of the OLEDs employing the other 2 TADF emitters.

Achieving high-performance non-doped sky-blue OLEDs with negligible efficiency roll-off by combining AIE, HLCT and mechanoluminescence features

The hot exciton materials with high-lying reverse intersystem crossing provide great potential for the realization of efficient, low-cost and stable organic light-emitting diodes (OLEDs). However, it is highly desired but still challenging to achieve high-performance non-doped blue OLEDs with hot exciton luminogens. In this work, a new hybridized local and charge transfer (HLCT) luminogen 2-(4-(2-(4-(1,2,2-triphenylvinyl)phenyl)-1H-phenanthro[9,10-d]imidazole-1-yl)phenyl)phenanthro[9,10-d]oxazole (abbreviated as TPE-PI-PO) with the incorporation of tetraphenylethylene is obtained for constructing the high-performance non-doped blue OLEDs. TPE-PI-PO possesses the multifunctional features of aggregation-induced emission (AIE), HLCT and mechanoluminescence with high photoluminescence quantum yield (58%) in the neat film. The non-doped OLEDs based on TPE-PI-PO exhibits sky-blue emission, high luminance of 13,020 cd m−2 and the maximum efficiencies of 11.62 cd A−1, 5.08% with the negligible efficiency roll-off as low as 1.6% at 1000 cd m−2. The electroluminescent performance is highly competitive with the best reported non-doped blue fluorescent OLEDs based on AIE luminogens. These results can shed light on a feasible design principle to develop high-quality HLCT-type luminogens for high-performance non-doped blue OLEDs.

Highly efficient stable blue organic light-emitting diodes based on a novel anthracene[2,3-b]benzofuran framework

Organic blue fluorescent emitters’ emissions are highly desirable for high-resolution organic light-emitting diode (OLED) display technology. In this work, a design strategy to realize narrowband emission of organic blue fluorescent emitters by connecting carbazole and amine, named ABFCz and ABFAn, respectively, with anthracene[2,3-b]benzofuran as rigid groups is reported for the first time. The developed materials can achieve high efficiency and long life. Studies of the doped devices show that ABFCz-based blue-emitting OLEDs achieve a maximum current efficiency of 6.96 cd A−1 with a narrow full width at half maximum (FWHM) of 44 nm, as well as a Commission Internationale de l’Eclairage (CIE) coordinate of (0.15, 0.14). A maximum external quantum efficiency (EQEmax) of 7.7% is also achieved. Meanwhile, the ABFAn emitter-based devices exhibit blue emission with a maximum current efficiency of up to 9.20 cd A−1 (EQEmax of 11.4%) with a FWHM of 44 nm, as well as a CIE coordinate of (0.14, 0.13). The devices are based on an ABFAn operating lifetime test LT90 (luminance decays to 90% of the initial brightness) of 202 h at an initial luminance of 1000 cd m−2. The results show that anthracene[2,3-b]benzofuran can be modified with a higher glass transition temperature (T g) to achieve high-efficiency and stable-operating-lifetime blue light devices. Our work may provide a new approach for designing high-efficiency blue materials.

Highly efficient and stable deep-blue organic light-emitting diode using phosphor-sensitized thermally activated delayed fluorescence.

Phosphorescent and thermally activated delayed fluorescence (TADF) blue organic light-emitting diodes (OLEDs) have been developed to overcome the low efficiency of fluorescent OLEDs. However, device instability, originating from triplet excitons and polarons, limits blue OLED applications. Here, we develop a phosphor-sensitized TADF emission system with TADF emitters to achieve high efficiency and long operational lifetime. Peripheral carbazole moieties are introduced in conventional multi-resonance–type emitters containing one boron atom. The triplet exciton density of the TADF emitter is reduced by facilitating reverse intersystem crossing, and the Förster resonant energy transfer rate from phosphor sensitizer is enhanced by high absorption coefficient of the emitters. The emitter exhibited an operational lifetime of 72.9 hours with Commission Internationale de L’Eclairage chromaticity coordinate y = 0.165, which was 6.6 times longer than those of devices using conventional TADF emitters.

3-Aminobenzeneboronic acid self-assembled monolayer-modified ITO for efficient and stable OLED devices

Interface modification of indium tin oxide (ITO) is a hot research topic to obtain highly efficient and stable organic light-emitting diodes (OLEDs), and the key point is to improve the hole injection efficiency between the electrode and organic layer. In this study, we demonstrate an optimization strategy based on the use of 3-aminobenzeneboronic acid (3ABBA) for modifying ITO at the anode interface. The protonation reaction of boronic acid groups with oxides and hydroxyl groups forms a self-assembled monolayer (SAM). Since the ionization potential (IP) of 3ABBA-SAM is 5.44 eV, the hole injection barrier at the interface is minimized. Compared with a control device using PEDOT:PSS, the synergistic effects of the optimized 3ABBA-SAM significantly improves the maximum power efficiency (PEmax of 8.52 lm W−1) and maximum current efficiency (CEmax of 10.45 cd A−1) of the device, and the turn-on voltage is reduced by 0.2 V. In addition, effective interface modification reduces the hydrophilicity of the device and provides support for the stability of the OLED in the ambient environment. Therefore, we believe that a simple 3ABBA-SAM interface modification project can promote the development of high-performance and low-cost OLEDs.

Zero-Zero Energy-Dominated Degradation in Blue Organic Light-Emitting Diodes Employing Thermally Activated Delayed Fluorescence.

Blue-emitting organic light-emitting diodes (OLEDs) fall significantly behind other OLEDs in operational stability. To better understand the key factors governing the stability of blue OLEDs employing thermally activated delayed fluorescence (TADF), nine efficient sky-blue to green TADF emitters with different frontier orbital energy levels and different TADF lifetimes have been designed and synthesized on the basis of charge-transfer (CT) acridine/phenyltriazine derivatives. Among them, ToDMAC-TRZ, a molecule composed of a 9,9-dimethyl-2,7-di-o-tolyl-9,10-dihydroacridine donor and a 2,4,6-triphenyl-1,3,5-triazine acceptor, shows a quantum yield of nearly 1 and a TADF lifetime as short as 0.59 μs in thin film. However, the stability of OLEDs is independent of the frontier orbital energy levels and TADF lifetime of the emitter. In contrast, the device half-life is found to decrease by five-sixths as the 0-0 energy of the singlet excitons increases by about 0.06 eV, which can be well-explained by the Arrhenius equation employing a photoreaction model. Whether in photoluminescence or electroluminescence, the contribution of long-lifetime triplet excitons to degradation is much lower than expected, which can be accounted for by how the solid-state solvation effect reduces the energy of the 3CT state and how most molecules have a low-lying locally excited triplet state.

Stable organic light-emitting diodes based on thioxanthone derivative with shortened photoluminescence delayed lifetime

The device stability of the devices based on thermally active delayed fluorescence is still unsatisfactory, although their efficiencies of the reported devices have reached those of phosphorescent emitters for application. Reducing the triplet related nonradiative process is the most efficient method to enhance the device stability. In this work, we designed and synthesized a TADF molecule 4,5-TXO-PhCz with a reduced delayed fluorescence lifetime of only 11.3 μs. Based on this molecule, the OLED device can reach a maximum EQE of 18.6% with a low efficiency roll-off, the device can even retain 9.38% at luminescence of 5000 cd/m2. The short PL delayed lifetime suppressed the triplet-related nonradiative annihilation and reduced the efficiency roll-off, affording an excellent device stability with a half lifetime of 380 h. These results indicate that reducing the delayed fluorescence lifetime can effectively reduce the device efficiency roll-off and improve the device stability.

High Efficiency of over 25% and Long Device Lifetime of over 500 h at 1000 nit in Blue Fluorescent Organic Light-Emitting Diodes.

In this study, a multiple resonance (MR) type blue emitter is synthesized, characterized, and evaluated for highly efficient and stable blue fluorescent organic light-emitting diodes (OLEDs). The MR blue fluorescent emitter has a di-tert-butyl benzene substituent in the MR core structure to minimize quenching mechanisms by intermolecular interaction. The emitter shows a high photoluminescence quantum yield and small full width at half maximum of 22 nm, which realize high external quantum efficiency (EQE) of 11.4% in the single unit OLED and device lifetime up to 95% of the initial luminance (LT95 ) of 208 h at 1000 cd m-2 and over 10000 h at 100 cd m-2 . The optimized tandem device of the new blue emitter achieves high EQE over 25% and extremely long LT95 of over 500h at 1000 cd m-2 and 30000 h at 100 cd m-2 . The lifetime of this work is one of the best data of blue OLED lifetime reported in the literature.

Understanding the Structure and Energy Transfer Process of Undoped Ultrathin Emitting Nanolayers Within Interface Exciplexes

Organic light-emitting diodes (OLEDs) have great potential for display, lighting, and near-infrared (NIR) applications due to their outstanding advantages such as high efficiency, low power consumption, and flexibility. Recently, it has been found that the ultrathin emitting nanolayer technology plays a key role in OLEDs with simplified structures through the undoped fabricated process, and exciplex-forming hosts can enhance the efficiency and stability of OLEDs. However, the elementary structure and mechanism of the energy transfer process of ultrathin emitting nanolayers within interface exciplexes are still unclear. Therefore, it is imminently needed to explore the origin of ultrathin emitting nanolayers and their energy process within exciplexes. Herein, the mechanism of films growing to set ultrathin emitting nanolayers (<1 nm) and their energy transfer process within interface exciplexes are reviewed and researched. The UEML phosphorescence dye plays a key role in determining the lifetime of excitons between exciplex and non-exciplex interfaces. The exciplex between TCTA and Bphen has longer lifetime decay than the non-exciplex between TCTA and TAPC, facilitating exciton harvesting. The findings will be beneficial not only to the further development of OLEDs but also to other related organic optoelectronic technologies.

Through-Space Conjugated Electron Transport Materials for Improving Efficiency and Lifetime of Organic Light-Emitting Diodes.

Thermally stable electron transport (ET) materials with high electron mobility and high triplet state energy level are highly desired for the fabrication of efficient and stable organic light‐emitting diodes (OLEDs). Herein, a new design strategy of constructing through‐space conjugated folded configuration is proposed to explore robust ET materials, opposite to the widely used planar configuration. By bonding two quinolines to the 9,10‐positions of phenanthrene, two novel folded molecules with high thermal and morphological stabilities and high triplet state energy levels (>2.7 eV) are created. These folded molecules possess excellent ET ability with electron mobilities of three orders of magnitude higher than those of linear and planar counterparts. Theoretical calculation and crystallography analysis demonstrate the through‐space conjugated folded configuration has not only reduced reorganization energy but also enlarged charge transfer integral at various dimensions, bringing about efficient multi‐dimensional ET, independent of molecular orientation. By adopting the folded molecule as ET layers, OLEDs with no matter delayed fluorescence or phosphorescence emitters can achieve high external quantum efficiencies and long operational lifetimes simultaneously. This work paves a new avenue towards robust ET materials to improve efficiency and stability of OLEDs.