High-Performance Fluorescent Organic Light-Emitting Diodes Utilizing an Asymmetric Anthracene Derivative as an Electron-Transporting Material

Enhancing Light Outcoupling Efficiency via Anisotropic Low Refractive Index Electron Transporting Materials for Efficient Perovskite Light-Emitting Diodes

Thanks to the extensive efforts toward optimizing perovskite crystallization properties, high-quality perovskite films with near-unity photoluminescence quantum yield are successfully achieved. However, the light outcoupling efficiency of perovskite light-emitting diodes (PeLEDs) is impeded by insufficient light extraction, which poses a challenge to the further advancement of PeLEDs. Here, an anisotropic multifunctional electron transporting material, 9,10-bis(4-(2-phenyl-1H-benzo[d]imidazole-1-yl)phenyl) anthracene (BPBiPA), with a low extraordinary refractive index (ne) and high electron mobility is developed for fabricating high-efficiency PeLEDs. The anisotropic molecular orientations of BPBiPA can result in a low ne of 1.59 along the z-axis direction. Optical simulations show that the low ne of BPBiPA can effectively mitigate the surface plasmon polariton loss and enhance the photon extraction efficiency in waveguide mode, thereby improving the light outcoupling efficiency of PeLEDs. In addition, the high electron mobility of BPBiPA can facilitate balanced carrier injection in PeLEDs. As a result, high-efficiency green PeLEDs with a record external quantum efficiency of 32.1% and a current efficiency of 111.7 cd A-1 are obtained, which provides new inspirations for the design of electron transporting materials for high-performance PeLEDs.

Improving electron transportation and operational lifetime of full color organic light emitting diodes through a "weak hydrogen bonding cage" structure

Efficient electron-transporting materials (ETMs) are critical to achieving excellent performance of organic light-emitting diodes (OLEDs), yet developing such materials remains a major long-term challenge, particularly ETMs with high electron mobilities (μeles). Herein, we report a short conjugated ETM molecule (PICN) with a dipolar phenanthroimidazole group, which exhibits an electron mobility of up to 1.52 × 10-4 cm2 (V-1 s-1). The origin of this high μele is long-ranged, regulated special cage-like interactions with C-H⋯N radii, which are also favorable for the excellent efficiency stability and operational stability in OLEDs. It is worth noting that the green phosphorescent OLED operation half-lifetimes can reach up to 630 h under unencapsulation, which is 20 times longer than that based on the commonly used commercial ETM TPBi.

Controllable π-π coupling of intramolecular dimer models in aggregated states

π-π coupling as a common interaction plays a key role in emissions, transport and mechanical properties of organic materials. However, the precise control of π-π coupling is still challenging owing to the possible interference from other intermolecular interactions in the aggregated state, usually resulting in uncontrollable emission properties. Herein, with the rational construction of intramolecular dimer models and crystal engineering, π-π coupling can be subtly modulated by conformation variation with balanced π-π and π-solvent interactions and visualized by green-to-blue emission switching. Moreover, it can rapidly respond to temperature, pressure and mechanical force, affording a facile way to modulate π-π coupling in situ. This work contributes to a deeper understanding of the internal mechanism of molecular motions in aggregated states.

Unlocking Structurally Nontraditional Naphthyridine-Based Electron-Transporting Materials with C-H Activation-Annulation

The inherent benefits of C-H activation have given rise to innovative approaches in designing organic optoelectronic molecules that depart from conventional methods. While theoretical calculations have suggested the suitability of the 2,6-naphthyridine scaffold for electron transport materials (ETMs) in organic light-emitting diodes (OLEDs), the existing synthetic methodologies have proven to be insufficient for the construction of multiple arylated and fully aryl-substituted molecules. Herein, we present a solution for the synthesis of 2,6-naphthyridine derivatives, with the rhodium-catalyzed consecutive C-H activation-annulation process of fumaric acid with alkynes standing as the pivotal step within this strategy. The ETMs, purposefully designed and synthesized based on the 2,6-naphthyridine framework, exhibit an impressively high glass-transition temperature (Tg) of 282 °C and high electron mobility (μe), setting a new benchmark for ETMs in OLEDs with a μe exceeding 10-2 cm2 V-1 s-1. These materials prove to be versatile ETM candidates suitable for red, green, and blue phosphorescent OLED devices.

An unsymmetrical thermally activated delayed fluorescence emitter enables orange-red electroluminescence with 31.7% external quantum efficiency

The thermally activated delayed fluorescence (TADF) emitters based on donor-acceptor (D-A) configuration were continuously developed in the past few years, whereas an unsymmetrical TADF emitter with A-D-A' configuration has never been reported. Herein, an A-D-A' type TADF emitter of TRZ-SBA-NAI was firstly developed by simultaneously integrating 2-phenyl-1H-benzo[de]isoquinoline-1,3(2H)-dione and 2,4,6-triphenyl-1,3,5-triazine acceptors into a spirobiacridine donor core. Due to the coexistence of double charge-transfer excited states, TRZ-SBA-NAI displayed dual emission containing a dominant orange-red emission and an anti-Kasha's rule sky-blue emission shoulder in solution. As doped into the host matrix, TRZ-SBA-NAI only exhibited an orange-red emission, together with a high photoluminescence quantum yield of 87%. The linear molecular shape imparted TRZ-SBA-NAI with a high horizontal dipole ratio of 88%. As a result, the TRZ-SBA-NAI based devices achieved a record-high external quantum efficiency of 31.7% with an electroluminescence peak at 593 nm. This finding not only enriches the diversity in TADF molecular design, but also unlocks the huge potential of A-D-A' type TADF emitters for excellent device performance.

Highly sensitive detecting system to precisely evaluate the emission spectra and quantum efficiency of organic crystal light-emitting transistors

The lack of a reliable and sensitive measure system has strongly hindered the progress of organic single-crystal light-emitting transistors, since their emission spectra and absolute intensity were rather difficult to be measured as a consequence of small area and weak emission, as well as their edge emission feature. Hence, a tailor-made detecting system was set up by combining a semiconductor analyzer and a rotatable microscope coupling with a highly sensitive grating spectrometer. The detecting system solved the important issue of quantitatively characterizing the optoelectronic properties of organic transistors, especially those based on single crystals. With organic lamellar single crystals, we confirmed that the measurement system was reliable and had enough sensitivity to precisely evaluate the performance of light-emitting transistors corresponding to the emission spectra.

Triazolotriazine-based thermally activated delayed fluorescence materials for highly efficient fluorescent organic light-emitting diodes (TSF-OLEDs)

Thermally activated delayed fluorescence (TADF) sensitized fluorescent organic light-emitting diodes (TSF-OLEDs) have shown great potential for the realization of high efficiency with low efficiency roll-off and good color purity. However, the superior examples of TSF-OLEDs are still limited up to now. Herein, a trade-off strategy is presented for designing efficient TADF materials and achieving high-performance TSF-OLEDs via the construction of a new type of triazolotriazine (TAZTRZ) acceptor. The enhanced electron-withdrawing ability of TAZTRZ acceptor, fused by triazine (TRZ) and triazole (TAZ) together, enables TADF luminogens with small singlet-triplet energy gap (ΔE_ST) values. Meanwhile, the increased planarity from the TRZ-phenyl linkage (6:6 system) to the TAZ-phenyl linkage (5:6 system) can compensate the decrease of oscillator strength (f) while lowing ΔE_ST, thus achieving a trade-off between small ΔE_ST and high f. As a result, the related TSF-OLED achieved an extremely low turn-on voltage of 2.1 V, an outstanding maximum external quantum efficiency (EQE_max) of 23.7% with small efficiency roll-off (EQE₁₀₀₀ of 23.2%; EQE₅₀₀₀ of 20.6%) and an impressively high maximum power efficiency of 82.1 lm W^-1, which represents the state-of-the-art performance for yellow TSF-OLEDs.

When Anthracene and Quinone Avoid Cycloaddition: Acid-Catalyzed Redox Neutral Functionalization of Anthracene to Aryl Ethers

Benzoquinone and 9-phenylanthracene barely undergo anticipated cycloaddition under acid catalysis. Instead, 9-anthracenyl aryl ethers are obtained as unexpected products. Mechanistic studies indicate that the reaction likely undergoes an ionic mechanism between protonated anthracene species and nucleophilic oxygen of 1,4-benzoquinone or 1,4-hydroquinone. A variety of 9-anthracenyl aryl ethers are constructed with this method. Produced anthracenyl aryl ethers are potential scaffolds for new fluorescent molecules.

Emerging Self-Emissive Technologies for Flexible Displays

Featuring a combination of ultrathin and lightweight properties, excellent mechanical flexibility, low power-consumption, and widely tunable saturated emission, flexible displays have opened up a new possibility for optoelectronics. The demands for flexible displays are growing on a continual basis due not only to their successful commercialization but, more importantly, their endless possibilities for wearable integrated systems. Up to now, self-emissive technologies for displays, flexible active-matrix organic light-emitting diodes (flex-AMOLED), flexible quantum dot light-emitting diodes (flex-QLEDs), and flexible perovskite light-emitting diodes (flex-PeLEDs) have been widely reported, but despite the significant progress made in these technologies, enormous obstacles and challenges remain for the vision of truly wearable applications, in particular with flex-QLEDs and flex-PeLEDs. Here, a review of the recent progress of all three self-emissive technologies for flexible displays is conducted, including the emissive active materials, device structures and approaches to manufacturing, the flexible substrates, and conductive electrodes, as well as the encapsulation techniques. The fast-paced improvement made to the efficiency of flexible devices in recent years is also summarized. The review concludes by making suggestions on the future development in this area, and is expected to help researchers in gaining a comprehensive understanding about the newly emerging technologies for flexible displays.

Hydrogen bond modulation in 1,10-phenanthroline derivatives for versatile electron transport materials with high thermal stability, large electron mobility and excellent n-doping ability

4,7-Bisphenyl-1,10-phenanthroline (BPhen) is a promising electron transport material (ETM) and has been widely used in organic light-emitting diodes (OLEDs) because of the large electron mobility and easy fabrication process. However, its low glass transition temperature would lead to poor device stability. In the past decades, various attempts have been carried out to improve its thermal stability though always be accomplished by the reduced electron mobility. Here, we present a molecular engineering to modulate the properties of BPhen, and through which, a versatile BPhen derivative (4,7-bis(naphthalene-β-yl)-1,10-phenanthroline, β-BNPhen) with high thermal stability (glass transition temperature = 111.9 °C), large electron mobility (7.8 × 10^-4 cm²/(V s) under an electrical field of 4.5 × 10⁵ V/cm) and excellent n-doping ability with an air-stable metal of Ag is developed and used as multifunctional layers to improve the efficiency and enhance the stability of OLEDs. This work elucidates the great importance of our molecular engineering methodology and device structure optimization strategy, unlocking the potential of 1,10-phenanthroline derivatives towards practical applications.

Unsymmetric 2-phenylpyridine (ppy)-type cyclometalated Ir(iii) complexes bearing both 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene and phenylsulfonyl groups for tuning optoelectronic properties and electroluminescence abilities

Unsymmetric Ir(iii) complexes with 5,9-dioxa-13b-boranaphtho[3,2,1-de]anthracene and phenylsulfonyl groups showing high EL efficiencies.

Understanding and Manipulating the Interplay of Wide-Energy-Gap Host and TADF Sensitizer in High-Performance Fluorescence OLEDs

Comprising an emitting layer (EML) constituting a wide-energy-gap host, a thermally activated delayed fluorescence (TADF) sensitizer and a conventional fluorescent dopant, TADF-sensitizing-fluorescence organic light-emitting diodes (TSF-OLEDs) highly depend on component interplay to maximize their performance, which, however, is still under-researched. Taking the host type (TADF or non-TADF) and the recombination position (on the host or on the TADF sensitizer) into consideration, the interplay of host and TADF sensitizer is comprehensively studied and manipulated. A wide-energy-gap host with TADF and recombination of charges on it are both required to maximize device performances by triggering multiple sensitizing processes to eliminate exciton losses. Based on those findings, a maximum external quantum efficiency (EQE)/power efficiency (PE) of 23.2%/76.9 lm W^-1 is realized with a newly developed TADF host, significantly outperforming the reference devices. Further device optimization leads to unprecedently high EQE/PE of 24.2%/89.5 lm W^-1 and a half-lifetime of over 400 h at an initial luminance of 2000 cd m^-2 , with the peak PE being the highest value among the reported TSF-OLEDs. This work reveals the importance of manipulating the component interplay in EMLs, opening a new avenue toward highly efficient TSF-OLEDs.

Quinoxaline and Pyrido[x,y-b]pyrazine-Based Emitters: Tuning Normal Fluorescence to Thermally Activated Delayed Fluorescence and Emitting Color over the Entire Visible-Light Range

Quinoxaline (Q), pyrido[2,3-b]pyrazine (PP) and pyrido[3,4-b]pyrazine (iPP) are used as electron acceptors (A) to design a series of D-π-A-type light-emitting materials with different donor (D) groups. By adjusting the molecular torsion angles through changing D from carbazole (Cz) to 10-dimethylacridine (DMAC) or 10H-phenoxazine (PXZ) for a fixed A, the luminescence is tuned from normal fluorescence to thermally activated delayed fluorescence (TADF). By gradually enhancing the intramolecular charge-transfer extent through combining different D and A, the emission color is continuously and regularly tuned from pure blue to orange-red. Organic light-emitting diodes (OLEDs) containing these compounds as doped emitters exhibit bright electroluminescence with emission colors covering the entire visible-light range. An external quantum efficiency (η_ext ) of 1.2 % with excellent color coordinates of (0.16, 0.07) is obtained for the pure-blue OLED of Q-Cz. High η_ext values of 12.9 (35.9) to 16.7 % (51.9 cd A^-1 ) are realized in the green, yellow, and orange-red TADF OLEDs. All PP- and iPP-based TADF emitters exhibit superior efficiency stabilities to that of analogues of Q. This provides a practical strategy to tune the emission color of Q, PP, and iPP derivatives with the same molecular skeletons over the entire visible-light range.

Exciplex System with Increased Donor-Acceptor Distance as the Sensitizing Host for Conventional Fluorescent OLEDs with High Efficiency and Extremely Low Roll-Off

Exciplex systems with efficient thermally activated delayed fluorescence as the sensitizing hosts for fluorescent organic light-emitting diodes (OLEDs) have been flourished recently, while the device performances are still lagging behind. Here, a donor molecule sterically encapsulated with tert-butyl units is designed and synthesized to increase the donor-acceptor separation in an exciplex system, leading to reduced singlet-triplet energy gap (Δ E_STs) and improved reverse intersystem crossing (RISC) efficiency. OLEDs utilizing exciplexes with increased donor-acceptor distance ( r_DA) as the hosts for conventional fluorescent dopants exhibit a maximum external quantum efficiency (EQE_max) as high as 16.5%, benefiting from the enhanced RISC process and suppressed exciton loss by the Dexter interaction. Furthermore, extremely low efficiency roll-off is obtained with EQEs of 16.2% at 5000 cd/m² and 15.2% at 10 000 cd/m². The results here represent the state-of-the-art performances for devices based on exciplexes as the hosts for conventional fluorescent dopants, manifesting the superiority of exciplexes with increased r_DA as the sensitizing hosts for fluorescent dopants.

Polycyclic Aromatic Hydrocarbon Derivatives toward Ideal Electron-Transporting Materials for Organic Light-Emitting Diodes

One seemingly fundamental issue for electron-transporting materials (ETMs) in organic light-emitting diodes (OLEDs) is the mutual exclusion of a high electron-transporting mobility ( μ_e) and a large molecular triplet energy for good exciton confinement. Very recently, a trade-off was realized by adopting polycyclic aromatic hydrocarbon (PAH) derivatives as bipolar ETMs. Though the intrinsic low triplet energy of PAH moieties, good exciton confinement abilities are realized by manipulating the peripheral groups, integrating large μ_e values and long-term stabilities simultaneously. The resulting state-of-the-art OLED performances manifest the bright future of such ETMs. This Perspective summarizes the theoretical and experimental work relating to the relationship between the molecular structure of PAH-type ETMs and their electronic properties, focusing on charge transfer and exciton confinement abilities; the Perspective concludes by providing a vision of future developments in this field.

Unveiling the Role of Langevin and Trap-Assisted Recombination in Long Lifespan OLEDs Employing Thermally Activated Delayed Fluorophores

Recent research studies on noble-metal-free thermally activated delayed fluorescent (TADF) materials have boosted the efficiencies of organic light-emitting diodes (OLEDs) to unity. However, the short lifespan still hinders their further practical application. Carrier recombination pathways have been reported to have a significant influence on the efficiencies of TADF devices, though their effects on device lifetimes remain rarely studied. Here, we have designed and synthesized five pyrimidine or pyrazine/carbazole isomers as hosts for TADF OLEDs to explore the inherent role of Langevin recombination (LR) and trap-assisted recombination (TAR) in device lifetimes. It is revealed that for LR dominant devices, lifetimes would increase by reducing the host triplet energy levels, whereas for TAR dominant devices, lifetimes are insensitive to the host triplet excitons as recombination mainly takes place on dopants. Still, LR dominant devices are favored as they offer more room for optimization. We further apply this concept in designing a stable LR dominant blue TADF device, achieving a long LT50 (lifespan up to 50% of the initial luminance) of 269 h and high external quantum efficiency of 17.9% at 1000 cd m^-2 simultaneously.

Breaking the Efficiency Limit of Fluorescent OLEDs by Hybridized Local and Charge-Transfer Host Materials

Hybridized local and charge-transfer (HLCT) states with "hot exciton" properties are effective in harvesting high-lying triplet excitons for electroluminescence in organic light-emitting diodes (OLEDs). Here, we propose a technique based on the HLCT mechanism at the high-lying excited states to develop HLCT-sensitized fluorescent (HLCT-SF) OLEDs using HLCT host molecules and metal-free fluorescent dopants for highly efficient OLEDs. A maximum external quantum efficiency (EQE) up to 6.3% and an exciton utilizing efficiency (EUE) of 64% were achieved, apparently exceeding the upper limits of the EQE (5%) and EUE (25%) in conventional fluorescent OLEDs. The HLCT-SF process via long-range Förster resonance energy transfer from the singlet excited states of the HLCT host to that of the fluorescent guest is efficient in harvesting "hot triplet excitons" by efficient high-lying reverse intersystem crossing, and the newly proposed HLCT-SF OLEDs represent an important advance in realizing high-performance OLEDs.