Application In Organic Light-emitting Diodes Research Articles

New donor–π–acceptor AIEgens: Influence of π bridge on luminescence properties and electroluminescence application

Aggregation-induced emission (AIE) offers a solution to harvest strong solid-state fluorescence, which is crucial to achieving efficient organic light-emitting diodes (OLEDs). In this study, two novel AIE luminogens (AIEgens) were designed by integrating the advantages of triphenylamine (TPA) and phenanthroimidazole (PI), which exhibited excellent electron-donating ability of TPA and bipolar characteristics and suitable carrier transporting ability of PI. The two structural units were connected by a π bridge of tris/tetrastyrene group with a different number of benzene rings. Intra-/inter-molecular charge transfer can be obtained by adjusting the conjugation of the π bridge, and the AIE activity is also manipulated accordingly. The dilute solutions of TPE-TPAPPI present concentration- and polarity-dependent emission, demonstrating emission Adjustability by controlling the ratio of intramolecular and intermolecular charge transfer. It is found that intermolecular charge transfer dominates in film state. A non-doped OLED based on TPE-TPAPPI shows a maximum external quantum efficiency (EQE) of 4.01% with a low turn-on voltage of 3.2 V. The efficiency roll-off is only 4.75% even at a high brightness of 1000 cd m−2, demonstrating the superiority of our AIEgens in OLED application.

Creation of a thermally cross-linkable encapsulated TADF molecule for highly efficient solution-processed hybrid white OLEDs

Robust luminogens that can be used in application for high efficiency solution-processed hybrid white organic light-emitting diodes (WOLEDs) are rarely reported. Herein, a thermally cross-linkable encapsulated thermally activated delayed fluorescence (TADF) molecule, DVCz-2CzCN, consisting of blue emissive core, encapsulation unit and thermal cross-linkable group is designed and synthesized. Systematic studies demonstrate that DVCz-2CzCN as the emitter can form a stable and uniform thin film by the solution-process, and simultaneously suppress the triplet excitons concentration quenching. Consequently, the nondoped blue solution-processed device using DVCz-2CzCN as the emissive layer exhibits blue emission with maximum external quantum efficiency (EQE) of 5.6% is achieved. Especially to solution-processed hybrid WOLED with the construction of employing the cross-linked DVCz-2CzCN as the blue-emitting component combined with an orange-emitting phosphor PO-01, exhibits a superior performance with maximum forward-viewing EQE of 14.9% and a stable Commission International de L'Eclairage (CIE) coordinate of (0.33, 0.45). This work indicates that molecular topology construction is crucial for the development of high-performance solution-processed hybrid WOLEDs. Herein, a highly efficient solution-processed hybrid white OLED was reported based on the strategy of constructing a thermally cross-linkable encapsulated TADF molecule. • Solution-processed hybrid white OLED was fabricated by utilizing a thermally cross-linkable encapsulated TADF molecule. • DVCz-2CzCN possesses good morphologic stability, sufficient solvent resistance and efficient triplet excitons confinement. • The DVCz-2CzCN-based device can afford a high blue TADF OLED by the solution-process with EQE max of 5.6%. • Solution-processed hybrid white OLED exhibits a superior performance with EQE max of 14.9%.

Synthesis and optical properties of the isomeric phenanthroimidazole Imidazo[1,2-a]pyridine conjugates: Effects of donor and their linking topology

In the present work, four imidazopyridine based isomeric emitters were synthesized as deep-blue emitters for application in organic light-emitting diodes. The position of triphenylamine/carbazole and phenanthroimidazole is changed to obtain different isomers. Their structure-property relationships were investigated through photophysical, electrochemical, thermal and electroluminescence properties. All the isomers exhibited excellent thermal stability and high fluorescence quantum efficiency. The position of the triphenylamine, carbazole and phenanthroimidazole units on the 2-phenylimidazopyridine core considerably influenced the optical and electroluminescence properties. Among all, triphenylamine substituted on the para-position of the phenyl ring attached to the C2 of the imidazopyridine exhibited the longest wavelength emission maximum. The positive solvatochromism and larger Stokes shifts of this compound revealed the participation of ICT from triphenylamine to imidazopyridine unit. Electrochemical measurement presented the high-lying HOMO for the triphenylamine containing dyes and low lying LUMO for the carbazole containing dyes. Thus, variation in the orbital energies attributable to the structural isomers is realized. Further, these isomeric dyes were used as emitting dopants in multilayer solution-processed organic light-emitting diodes. The doped devices fabricated with a dye containing carbazole as a donor exhibited deep-blue emission with CIE coordinates (0.16, 0.08) and decent device characteristics.

Design of Organic Electronic Materials With a Goal-Directed Generative Model Powered by Deep Neural Networks and High-Throughput Molecular Simulations.

In recent years, generative machine learning approaches have attracted significant attention as an enabling approach for designing novel molecular materials with minimal design bias and thereby realizing more directed design for a specific materials property space. Further, data-driven approaches have emerged as a new tool to accelerate the development of novel organic electronic materials for organic light-emitting diode (OLED) applications. We demonstrate and validate a goal-directed generative machine learning framework based on a recurrent neural network (RNN) deep reinforcement learning approach for the design of hole transporting OLED materials. These large-scale molecular simulations also demonstrate a rapid, cost-effective method to identify new materials in OLEDs while also enabling expansion into many other verticals such as catalyst design, aerospace, life science, and petrochemicals.

Fabrication of cylindrical microlens array using rotatable multi-arrayed needle coating module for light out-coupling of OLEDs

We have performed coatings of poly(methyl methacrylate) (PMMA) using a rotatable multi-arrayed needle coating module to achieve high-density and high-aspect-ratio cylindrical microlens arrays (C-MLAs) for potential applications in organic light-emitting diodes (OLEDs). It is shown that high-density C-MLAs can be fabricated by rotating the coating module in such a way that the spacing between microlenses can be reduced (tuned). To enhance the aspect ratio of C-MLAs, we have analyzed the coating behaviors by varying the material parameters such as the molecular weight of PMMA, solute concentration, and wettability of the PMMA solution. It is demonstrated that high-aspect-ratio C-MLAs can be achieved using a coating solution with a small amount of a hydrophobic solvent, in which a high concentration of low-molecular-weight PMMA is dissolved. To further enhance the aspect ratio, we have come up with a new process method; repetitive coating and pre-drying treatment in ambient air. Such a repetitive coating method is shown to boost the microlens thickness without a much compromise on the microlens width. With this scheme, we have fabricated the C-MLA with the average thickness of 22.6 μm and width of 125 μm (aspect ratio as high as 0.18) at a coating velocity of 3 mm/s. The light out-coupling efficiency of OLED with the C-MLA increases by 20%, which is in good agreement with the result by ray tracing simulations.

Synergetic Insulation and Induction Effects Selectively Optimize Multiresonance Thermally Activated Delayed Fluorescence.

Multiresonance (MR) emitters featuring narrowband emissions and theoretically 100% exciton harvesting are great potential for organic light-emitting diode (OLED) applications. However, how to functionalize MR molecules without scarifying emission color purity is still a key challenge. Herein, we report a feasible strategy for selective optimization of MR molecules, which is demonstrated by a blue MR emitter tCBNDASPO substituted with a diphenylphosphine oxide (DPPO) group. Compared to its DPPO-free parent molecule, tCBNDASPO preserves narrowband feature with full widths at half maximum (FWHM) values of 28 nm in film and 32 nm in OLEDs and achieves 40% increased photoluminescence (92%) and electroluminescence quantum efficiencies (28%). It is showed that insulation effect of P=O effectively confines the singlet excited state on MR core to keep emission color purity, and its induction effect enhances singlet radiation and triplet-to-singlet conversion. This synergism for selective optimization is based on rational linkage between MR core and functional groups.

The impact of ZnO nanotube on the performance of hybrid inorganic/organic light-emitting diode as a single-mode ring-core UV waveguide

After a systematic survey in hybrid inorganic/organic light-emitting heterostructure devices based on ZnO in the last decay, in this novel work, the impact of the single-mode ring-core waveguide based on ZnO nanotube (NT)/MEH-PPV for ultraviolet organic light-emitting diode (UV-OLED) application has been carefully scrutinized for the first time. The proposed structure has been fabricated, simulated and compared with conventional ZnO nanorod (NR)/MEH-PPV structure. To synthesize ZnO NTs, the as-grown chemical bath deposited ZnO NRs have been etched in KCL solution in various molar (M) concentration, etching time and etching temperature. The optimized etching condition is obtained in 1 M concentration of KCL solution, 4 h etching time and 90 °C temperature. The structural properties (such as strain, stress and texture coefficient), electrical properties (such as band gap energy) and optical properties (such as Urbach energy, absorbance and photoluminescence spectra) of ZnO NTs have been investigated, systematically. In continue, hybrid UV-OLEDs have been fabricated based on ZnO NRs and ZnO NTs. According to the results, ZnO NT-based OLED depicts superior electrical and optical results including lower turn-on voltage (11.2 V < 15 V) and higher UV peak in electroluminescence spectra with respect to ZnO NR-based device. To acquire more enlightenment about UV emission mechanism, the proposed devices have been simulated through Silvaco TCAD and Lumerical FDTD software. The results from simulations illustrate great agreement with experimental results. Higher radiative recombination rate, higher Purcell factor and single-mode waveguiding effect of ring-core ZnO NT lead to major superiority of the ZnO NT-based UV-OLED fabricated and simulated in this work.

Highly Efficient Heteroleptic Cerium(III) Complexes with a Substituted Pyrazole Ancillary Ligand and Their Application in Blue Organic Light-Emitting Diodes.

Compared with red and green organic light-emitting diodes (OLEDs), blue is the bottleneck that restricts the wide development of OLEDs from being the next-generation technology for displays and lighting. As a new type of emitter, a Ce(III) complex shows many satisfactory advantages, such as a short excited-state lifetime, 100% theoretical exciton utilization efficiency, and tunable emission color. Herein we synthesized three heteroleptic Ce(III) complexes Ce(TpMe2)2(dtfpz), Ce(TpMe2)2(dmpz), and Ce(TpMe2)2(dppz) with the hydrotris(3,5-dimethylpyrazolyl)borate (TpMe2) main ligand and different substituted pyrazole ancillary ligands, namely, 3,5-di(trifluomethyl)pyrazolyl (dtfpz), 3,5-dimethylpyrazolyl (dmpz), and 3,5-diphenylpyrazolyl (dppz), and studied their structures and luminescence properties. All the Ce(III) complexes exhibited a near-unity photoluminescence quantum yield both in solution and as a powder with maximum emission wavelengths in the range of 450-486 nm. The OLED employing Ce(TpMe2)2(dppz) as the emitter showed the best performance, including a turn-on voltage, maximum luminance, and external quantum efficiency of 3.2 V, 29 200 cd m-2, and 12.5%, respectively.

Boosting external quantum efficiency to 38.6% of sky-blue delayed fluorescence molecules by optimizing horizontal dipole orientation.

To achieve high electroluminescence efficiency, great efforts are devoted to enhancing photoluminescence quantum yield (ΦPL) and exciton utilization of luminescent molecule, while another important factor, light out-coupling efficiency (ηout), receives less attention in molecule design. Here, we focus on horizontal dipole orientation engineering of the molecule to increase ηout and external quantum efficiency (ηext). A series of tailor-made luminescent molecules consisting of an electron-accepting carbonyl core plus double electron-donating groups of spiro[acridine-9,9′-fluorene] and carbazole derivatives [e.g., 1,3,5-tri(carbazol-9-yl)benzene] are developed and systematically investigated. These molecules hold distinguished merits of strong sky-blue delayed fluorescence with excellent ΦPL values, large horizontal dipole ratios, and balanced bipolar carrier transport, which furnish record-high ηext values of up to 26.1 and 38.6% in nondoped and doped sky-blue organic light-emitting diodes (OLEDs), respectively. Moreover, the state-of-the-art nondoped hybrid white OLED and all-fluorescence single-emitting layer white OLED are also realized, demonstrating great potentials in OLED industry of these molecules.

Platinum Poly-yne Featuring N-Heterocyclic Carbene Ligands: Synthesis, Properties, and Organic Light-Emitting Diode Application

We report the synthesis, characterization, and organic light-emitting diode (OLED) application of the first example of a platinum poly-yne (NPtAP) featuring N-heterocyclic carbene (NHC) ligands. NP...

Self‐Host Thermally Activated Delayed Fluorescence Material with Aggregation‐Induced Emission Character: Multi‐Functional Applications in OLEDs

AbstractThermally activated delayed fluorescence (TADF) materials that can exhibit high efficiencies in non‐doped organic light‐emitting diodes (OLEDs) are highly desired. On the base of a reference TADF molecule (4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)(phenyl)methanone (BP‐DMAC) containing benzoyl/acridine as acceptor/donor, 9‐phenyl‐9H‐3,9“‐bicarbazole (PBCz) is incorporated to the benzoyl side to design and develop a novel emitter, (4‐(9,9‐dimethylacridin‐10(9H)‐yl)phenyl)(9‐phenyl‐9H‐[3,9″‐bicarbazol]‐6‐yl)methanone (PBCz‐BP‐DMAC). The introduction of PBCz unit is proved to endow PBCz‐BP‐DMAC the additional self‐host feature, in addition to its original aggregation‐induced emission and TADF characters. As a result, the non‐doped OLED of PBCz‐BP‐DMAC exhibits sky‐blue emission with remarkable external quantum efficiency (EQE) of 23.0% and rather low efficiency roll‐off (6%) at a brightness of 1000 cd m−2, while its doped device reveals a maximum EQE of 27.5%; the spectra and color coordinates of the doped and non‐doped devices are almost identical. It also acts as a host to sensitize green iridium phosphor (fac‐tris(2‐phenylpyridine) iridium(III), Ir(ppy)3) with high EQE of 23.6%. More fascinatingly, the simplified single‐emitting‐layer all‐TADF white OLED utilizing PBCz‐BP‐DMAC as host emitter in combination with an orange TADF emitter (4CzTPN‐Ph) realizes a state‐of‐the‐art EQE of 20.6%. Evidently PBCz‐BP‐DMAC is one of the best sky‐blue TADF materials and exhibits multi‐functional applications in OLEDs with excellent performance.

Differences in ITO Surfaces According to the Formation of Aromatic Rings and Aliphatic Self-Assembled Monolayers for Organic Light-Emitting Diode Applications.

In this study, we investigated the effects on the characteristic changes in OLED devices of using self-assembled monolayers with different functional groups as the hole injection layer, resulting in changes in their performance. Thus, we confirmed that it is possible to control the wetting properties, surface roughness, and work function of the indium tin oxide (ITO) surface by introducing self-assembled monolayers (SAMs). The contact angle measurements confirmed that the substrate surface contact angle tended to increase with SAM deposition. In addition, AFM measurements confirmed that the substrate surface roughness tended to decrease when SAM was deposited on the surface. Finally, it was confirmed through the work function measurement results that the work function increased when the ITO surface was modified by SAM. Furthermore, compared to OLEDs using only the ITO anode, the SAM-modified device showed a higher current density (359.68 A/cm2), improved brightness (76.8 cd/cm2), and a smaller turn-on voltage (7 V). This approach provides a simple route for fabricating organic light-emitting diode applications.

Achieving host-free near-ultraviolet electroluminescence via electronic state engineering with phosphine oxide

Developing near-ultraviolet (NUV) emitters with board band gap and high photoluminescence quantum efficiency (PLQY) is a formidable challenge for organic light-emitting diode (OLED) applications. Herein, two NUV emitters PhImPOCz and PhImPOtBuCz were designed on the basis of phosphine oxide (PO) modification. It is showed that based on crescent phenanthrene (Phen) fused with acceptor imidazole (Im) as NUV chromophore, carbazole (Cz) or 3,6-di-tertbutyl carbazole (tBuCz) donors make the molecule bipolar character to balance carrier recombination in emissive layer (EML). More significantly, besides steric hindrance for aggregation suppression, the introduction of triphenylphosphine enhances localized excited feature of the electronic states, and prevents the red shift induced by intra- and inter- molecular charge transfer, which establishes the basis for fabricating host-free NUV OLEDs. The non-doped devices based on the PhImPOCz and PhImPOtBuCz emitters achieved the low turn-on voltages of 4.2 V and 4.4 V, and the state-of-the-art external quantum efficiency (EQE) up to 2.81% and 3.42%, respectively. This work demonstrates the significance of electronic state engineering for high-energy OLED applications.

Analysis of Transient Degradation Behaviors of Organic Light-Emitting Diodes under Electrical Stress

Despite the wide application of organic light-emitting diodes (OLEDs), the performance of OLED devices is sometimes limited by their reliabilities. In this paper, we report the transient degradation behaviors of fluorescent blue OLEDs, where both the current and luminance initially increase under electrical stress within a short stress time. We analyze the degradation mechanism in terms of the carrier recombination and transport. From the comprehensive analyses of electrical and optical characteristics, it is suggested that the electron transport is responsible for the initial transient behavior of the device.

Molecular design strategy for orange-red thermally activated delayed fluorescence emitters via intramolecular energy transfer and their application in solution processable organic light-emitting diodes

Realizing effective orange-red/red emission is still far behind blue and green emission in thermally activated delayed fluorescence (TADF) materials. In this context, a novel strategy for achieving efficient orange-red/red TADF emitters is proposed via intramolecular energy transfer by encapsulating the red-emitting core with a blue emitting chromophore. Two novel compounds called TS-1 and TS-2 were prepared and characterized, in which two blue TADF emitting units are used as the end groups and a red TADF emitter as the molecular core (TA). Both compounds show clear TADF character with red emission at 655 nm in a neat film, suggesting an effective intramolecular energy transfer from the blue chromophores to the red one. Both TS-1 and TS-2 present higher photoluminescence quantum yields (70–90%) than the TA core (42%) due to the encapsulation and energy transfer. Solution-processable organic light-emitting diodes (OLEDs) exhibit a maximum external quantum efficiency of ∼13% for compound TS-1, much higher than that of TA-based OLEDs (2.3%). This research is the first example for red TADF emitter via intramolecular energy transfer, which clearly provides an effective method for designing highly efficient red TADF molecules.

Fabrication of heteromorphic microlens arrays built in the TiO2/ormosils composite films for organic light-emitting diode applications

Fabrication of heteromorphic microlens arrays built in the TiO2/ormosils composite films for organic light-emitting diode applications

Speeding‐up Thermally Activated Delayed Fluorescence in Cu(I) Complexes Using Aminophosphine Ligands

AbstractLuminescent copper(I) complexes presenting thermally activated delayed fluorescence (TADF) have drawn attention as emitters for organic light emitting diodes (OLEDs). While the majority of ligands used have nitrogen as donor atoms, in this work, we report the synthesis and characterization of three copper(I) complexes with the diimine ligand 1,10‐phenanthroline and aminophosphine‐derived ligands containing the piperazine and N,N’‐dimethylethylenediamine to evaluate their effect into the emission properties. The photophysical studies as a function of temperature suggested TADF and phosphorescence emission, supported by detailed density functional theory (DFT) calculations. The use of aminophosphine ligands enhance the TADF decay pathway in comparison with copper complex containing the usual POP ligand. These properties, combined with the appropriate HOMO‐LUMO energy levels and thermal stability, make these compounds a promising alternative for application in OLEDs.

A red-orange carbazole-based iridium(III) complex: Synthesis, thermal, optical and electrochemical properties and OLED application

A novel heteroleptic iridium(III) acetylacetonate (acac) complex, (L-5-CHO)2Ir(acac) (3b), was synthesised from 2-(9’-hexylcarbazole-3’-yl)-5-formylpyridine (L-5-CHO) (1b). The complex 3b was determined to be thermally and electrochemically stable. The photoluminescence properties of the compound were studied, with a dichloromethane solution of 3b giving structureless emission at 662 nm, showing that the formyl group red-shifted the emission by 151 nm compared to the parent complex. Complex 3b was also shown to possess a moderate photoluminescence quantum yield (67%) and a short emission lifetime (τ = 280 ns). Organic light-emitting diodes (OLEDs) were fabricated with a solution-processed emissive layer (EML) consisting of poly(N-vinylcarbazole) (PVK), 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) and iridium complex (3b). The OLEDs containing complex 3b showed red-orange electroluminescence (EL) at 624 nm. The influence of the host materials was studied and the best performance was achieved with both PVK and PBD in the emissive layer, with the resulting OLEDs exhibiting a current efficiency of 0.84 cd/A, a power efficiency of 0.20 lm/W, and an external quantum efficiency (EQE) of 0.66% at a brightness of 2548 cd/m2.

High-performance red and white organic light-emitting diodes based on a novel red thermally activated delayed fluorescence emitter in an exciplex matrix

Blue and green organic emitters based on thermally activated delayed fluorescence (TADF) mechanism have made great progress in organic light-emitting diode (OLED) applications, whereas red and white OLEDs with this kind of emitters are lagging behind. In this text, we develop high-performance red and white OLEDs by doping a novel red TADF emitter 10,10'-(11,12-diphenyldibenzo[a,c]phenazine-3,6-diyl)bis(10H-phenoxazine) (DBBPZ-DPXZ) into a blue emissive exciplex matrix. The highly rigid components and twisted donor-acceptor (D-A) structure of DBBPZ-DPXZ can suppress non-radiation decays and guarantee a small ΔEST, which results in effective exciton utilization. Meanwhile, the exciplex matrix can also provide additional reverse intersystem crossing (RISC) channel and enhance the utilization of triplet excitons. And thus, with doping 6 wt% DBBPZ-DPXZ into the exciplex host, red OLED with an electroluminescence peak at 628 nm and a high EQE of 20.8% is achieved. Moreover, by dropping doping concentration of DBBPZ-DPXZ into 0.2 wt%, a white OLED with Commission Internationale de l'Eclairage of (0.40, 0.38) at the luminance of 1000 cd m−2 and EQE of 20.7% is realized.

Deep Blue Fluorescent Material with an Extremely High Ratio of Horizontal Orientation to Enhance Light Outcoupling Efficiency (44%) and External Quantum Efficiency in Doped and Non-Doped Organic Light-Emitting Diodes

A novel bis-4Ph-substituted 9,10-dipehnylanthracene deep blue [1931 CIE (0.15, 0.08)] fluorescent compound, AnB4Ph, has been synthesized and characterized for organic light-emitting diode (OLED) applications. Our experimental study of AnB4Ph excludes the possibility of triplet-triplet annihilation, hybridized local and charge transfer, or thermally activated delayed fluorescent characteristics of the material. Since the solid-state photoluminescence quantum yield of AnB4Ph was determined to be 48%, assuming a 100% for the charge recombination efficiency, the light outcoupling efficiency (ηout) of an AnB4Ph non-doped OLED achieving an external quantum efficiency (EQE) of 5.3% is at least 44%, which is more than twofold higher than 20% for conventional OLEDs. Both grazing incidence wide-angle X-ray scattering (GIWAXS) and angle-dependent photoluminescence (ADPL) measurements reveal AnB4Ph having a high value of order parameter (SGIWAXS) of 0.61 for a ππ stacking along the normal direction and an orientation order parameter (SADPL) for a horizontal emitting dipole moment of -0.50 or Θ (horizontal-dipole ratios) of 100%, respectively. Otherwise, a refractive index (n) measurement provides a n = 1.80 for AnB4Ph thin films. Based on ηout = 1.2 × n-2, the calculated ηout is 37%, which is also in accordance with the results of GIWAXS and ADPL. We have also fabricated the classical fluorescent DPAVBi-doped AnB4Ph OLEDs, which display a true blue [1931 CIE (0.15 and 0.16)] electroluminescence with a high efficiency (EQE = 6.9%), surpassing the conventional ∼5% EQE. Based on an ηout of 42% for DPAVBi-doped AnB4Ph OLEDs, our studies suggest that the extremely horizontally aligned AnB4Ph host material exerts the same horizontal alignment on the DPAVBi dopant molecules.