Efficient Organic Light-emitting Diodes Research Articles

Enhanced Operational Stability by Cavity Control of Single-Layer Organic Light-Emitting Diodes Based on Thermally Activated Delayed Fluorescence.

Highly efficient organic light-emitting diodes (OLEDs) based on thermally activated delayed fluorescence (TADF) emitters have been realized in recent years, but the device lifetime needs further improvement for practical display or lighting applications. In this work, we present a device design principle by tuning the optical cavity of single-layer undoped devices, to realize efficient and long-lived TADF OLEDs. Extending the cavity length to the second-order interference maximum by increasing the emissive layer thickness broadens the recombination zone, while the optical outcoupling efficiency remains close to that of the thinner first-order devices. Such a device design leads to efficient and stable single-layer undoped OLEDs with a maximum EQE of 16%, a LT90 of 452h and LT50 of 3693h at an initial luminance of 1000cd m-2 , which is doubled compared to the first-order counterparts. We further demonstrate that the widely-used empirical relation between OLED lifetime and light-intensity originates from triplet-polaron annihilation, resulting in an extrapolated LT50 at 100cd m-2 of close to 90,000h, approaching the demands for practical backlight applications. This article is protected by copyright. All rights reserved.

Cooperative Passivation Exciton Quenching Triggers Surpassing 30% External Quantum Efficiency for a Deep‐Blue Organic Light‐Emitting Diode

AbstractOvercoming exciton quenching and improving electroluminescence (EL) performances continue to pose persistent challenges in the development of efficient deep blue thermally activated delayed fluorescence (TADF) emitters. Herein, an asymmetric multi‐donors strategy is employed that provides the potential of establishing multiple radiative transition channels for fluorescence emission and reverse intersystem crossing (RISC), while allowing flexible modulation of packing modes in single‐crystal state and optimization of morphology in films by modifying the substitution site of the donor units. As a result, intermolecular π–π interactions are effectively alleviated and a very smooth surface with a reduced defect density is simultaneously achieved, which can significantly reduce the density of triplet excitons, accelerate the charge transfer rate, and enhance carrier injection efficiency. Consequently, a super‐high photoluminescence quantum yield (PLQY) of 99% and fast kRISC of 1.1 × 105 s−1 are concurrently achieved for the proof‐of‐concept emitter 3,4‐Cz‐SF‐SFAC. The corresponding deep‐blue organic light‐emitting diode (OLED) demonstrates an exceptional external quantum efficiency (EQE) of 31.1%, accompanied by an emission peak at 457 nm, one of the preeminent TADF materials within the deep blue region, ranking among the most efficient OLEDs with an EL spectra below 460 nm on record.

New ionic iridium(III) complexes based on methoxyl-modified 1-phenylisoquinoline for solution-processed red organic light-emitting diodes

Three transition-metal phosphorescent iridium(Ⅲ) complexes (Ir1, Ir2, and Ir3) are designed and synthesized with 1-phenylisoquinolines modified by methoxy as the main ligands and o-phenanthroline as the auxiliary ligands. The introduction of different amounts of methoxy can change the luminescence color of the complexes, and Ir1, Ir2, and Ir3 show orange, dark red, and red phosphorescence under the excitation of 365 nm UV lamp, peaking at 588, 648, and 622 nm, respectively. Ionic complexes are conducive to the efficient solution-processed phosphorescent red organic light-emitting diodes (OLEDs). Among the three devices, the Ir2-based device achieved a relatively high external quantum efficiency, current efficiency, and power efficiency of 1.12 %, 0.75 cd/A, and 0.37 lm/W, respectively. These results indicated that polymethoxy-based ionic Ir(III) complexes have great potential for the fabrication of red OLEDs with wet method.

Efficient and Bright Organic Radical Light-Emitting Diodes with Low Efficiency Roll-Off.

Organic radicals have been of interest due to their potential to replace nonradical-based organic emitters, especially for deep red/near-infrared (NIR) electroluminescence (EL), based on the spin-allowed doublet fluorescence. However, the performance of the radical-based EL devices is limited by low carrier mobility that causes a large efficiency roll-off at high current densities. Here, we report highly efficient and bright doublet EL devices by combining a thermally activated delayed fluorescence (TADF) host that supports both electron and hole transport and a tris(2,4,6-trichlorophenyl)methyl-based radical emitter. Steady-state and transient photophysical studies reveal the optical signatures of doublet luminescence mechanisms arising from both host and guest photoexcitation. The host system presented here allows balanced hole and electron currents, and we report a high maximum external quantum efficiency (EQE) of 17.4% at 707nm peak emission with substantially improved efficiency roll-off: over 70% of the maximum EQE (12.2%) is recorded at 10mA cm-2 , and even at 100mA cm-2 , nearly 50% of the maximum EQE (8.4%) is maintained. Charge recombination is primarily at the radical guest sites, but there is evidence from magneto-EL for host recombination and later energy transfer to the radical guest. We consider this is an important step to the practical application of organic radicals to NIR light-emitting devices. This article is protected by copyright. All rights reserved.

Highly efficient non-doped deep-blue organic light-emitting diodes based on pyreneimidazole derivative with external quantum efficiency > 11% and negligible efficiency roll-off

Highly efficient non-doped deep-blue organic light-emitting diodes based on pyreneimidazole derivative with external quantum efficiency > 11% and negligible efficiency roll-off

Stably doped graphene transparent electrode with improved light-extraction for efficient flexible organic light-emitting diodes

Stably doped graphene transparent electrode with improved light-extraction for efficient flexible organic light-emitting diodes

Efficient and stable green organic light-emitting diode utilizing an asymmetric butterfly-shaped chromophore with enhanced reverse intersystem crossing and suppressed exciton annihilation

Intermolecular electron-exchange interactions involving long-lived triplet state excitons dominate the quenching process of the films. In this work, a butterfly-shaped thermally activated delayed fluorescence (TADF) emitter 10,10′-(5-((3,5-di(9H-carbazol-9-yl)phenyl)sulfonyl)-1,3-phenylene)bis(9,9-dimethyl-9,10-dihydroacridine) (BCZ-DPS-BAD) with diphenylsulfone skeleton is constructed by modulating the rigid steric hindrance on the electron donor fragment to weaken intermolecular electron-exchange interactions. Analysis of the ground and excited state properties, crystal structure, steady-state and transient photophysical characteristics reveals that BCZ-DPS-BAD exhibits loose intermolecular arrangements with no direct facing close packing and weak π–π interactions of 4.18 Å and 4.67 Å. The triplet state excitons can swiftly return to the single exciton state in accordance with a short delayed fluorescence lifetime (2.3 μs) and a quick reverse intersystem crossing rate (1.85 × 106 s−1), obstructing the way for the triplet state to lose energy. A remarkable maximum external quantum efficiency of 21.8 %, maximum current efficiency of 56.8 cd/A and exciton utilization rate of 82 % is achieved for BCZ-DPS-BAD-based green non-doped OLED, concomitant with Commission Internationale d'Eclairage (CIE) coordinates of (0.30, 0.50). Notably, BCZ-DPS-BAD behaves stable non-doped OLED performance with efficiency roll-off of 4 % at 1000 cd m−2.

Expanded multiple-resonance structure for highly efficient narrowband deep-blue organic light-emitting diodes

Excellent color purity and high external quantum efficiency (EQE) are major requirements in the development of deep-blue organic light-emitting diodes (OLEDs). To achieve this, multiple-resonance (MR)–thermally activated delayed fluorescence (TADF) emitters have been considered as promising options. Herein, we suggest a novel expanded MR design strategy to fabricate deep-blue MR–TADF emitters derived from a fused indolo[3,2,1-jk]carbazole framework. The expanded MR structure managed a triplet excited state for the accelerated spin–vibronic coupling-assisted reverse intersystem crossing and increased the emission dipole orientation while maintaining the high efficiency and deep-blue emission color. The rigid and planar structure of the MR core yielded a small full-width at half-maximum (FWHM; less than 16 nm), high photoluminescence quantum yield (over 97%), and high horizontal emitting dipole orientation (over 90%), and facilitated a second-order spin–vibronic coupling-assisted triplet-to-singlet spin crossover. The fabricated MR–TADF OLEDs recorded a high EQE of 24.3% and FWHM of 21 nm at a CIEy of 0.044, thereby satisfying the BT.2020 blue standard. Additionally, further optimized device architecture provided an EQE of 26.8%.

Highly Efficient Phosphorescent Organic Light-Emitting Diodes Using Benzo[4',5']imidazo[2',1':2,3]imidazo[4,5,1-jk]carbazole as a Rigid and Effective Electron Acceptor in Bipolar Host.

A novel bipolar host architecture was investigated to improve the external quantum efficiency (EQE) of green phosphorescent organic light-emitting diodes (PhOLEDs). The host was developed by incorporating carbazole as a hole-transport unit and fused rigid benzo[4',5']imidazo[2',1':2,3]imidazo[4, 5, 1-jk]carbazole (BzICz) as a new electron transport unit. The primary goal of the BzICz-based host design was to achieve a high triplet energy and bipolar charge transport characteristics. The green PhOLEDs fabricated using the new BzICz and carbazole-based host demonstrated a high EQE of 26.6% due to their high triplet energy and good bipolar charge transporting characteristics.

Low-trigger voltage and efficient phosphorescent organic light-emitting diodes with bipolar TADF host

Thermally activated delayed fluorescence (TADF) materials have continued to attract interest due to their rapid improvements in efficiency, non-toxic and potential to be manufactured at energy-saving organic light-emitting diode for full-color display and solid-state lighting. Here, The bipolar and electroluminescence (EL) characteristics of TADF host, 11-(3-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)-12,12-dimethyl-11,12-dihydroindeno[2,1-a]carbazole (DPDDC), were investigated. By incorporating the phosphorescent tris[2-(p-tolyl)pyridine]iridium(III) [Ir(mppy)3] dopant into the matrix, DPDDC showed more excellent EL performance compared with the typical 4,4′-bis(N-carbazolyl)-1,1′-biphenyl (CBP) host. The turn-on voltage for phosphorescent organic light-emitting diodes (PhOLEDs) was decreased to 2.2 V from 2.5 V by exploiting the TADF host instead of CBP. Crucially, we found that the ratio of Ir(mppy)3 in TADF host plays a significant role in EL process. Remarkable low efficiency roll-off was achieved with above 90% of the current efficiency maintained even at a brightness of 3000 cd m−2, along with high power efficiency of 84.8 lm·W−1 and the external quantum efficiency of 20.6% for DPDDC: 5 wt% Ir(mppy)3-based device.

Mild Synthesis of Polychlorinated Arenes for Efficient Organic Light-emitting Diodes with Dual Thermally Activated Delayed Fluorescence.

Polychlorinated (hetero)arenes have shown great promise for organic optoelectronics applications. However, the harsh synthesis of polychlorinated compounds and the possible luminescence quenching from the compact intermolecular π-π stacking induced by chlorine atoms limit their investigations and applications in luminescent materials. Herein, two isomeric polychlorinated polycyclic aromatic hydrocarbon (PAH) compounds JY-1-Cl and JY-2-Cl consisting of rigidified aryl ketones and amine are designed and synthesized under mild conditions through nucleophilic chlorination intermediated by electron donor-acceptor complex. Among them, due to the strong π-π interactions induced by chlorine atoms, JY-2-Cl exhibits bright monomer and dimer emissions with dual thermally activated delayed fluorescence (TADF) characters. Notably, in compared with the non-chlorinated compounds, high photoluminescence quantum yield is well maintained after introducing multiple chlorine atoms in JY-2-Cl. Eventually, the first dual-TADF organic light-emitting diodes are also successfully fabricated with maximum external quantum efficiency as high as 29.1% employing JY-2-Cl as emitter. This work presents a new paradigm and synthesis of polychlorinated amine-carbonyl PAHs and demonstrates the great potential of the chlorinated materials for luminescent applications.

Enhancing Light Extraction Efficiency in OLED Using Scattering Structure-Embedded DMD-Based Transparent Composite Electrodes.

This study investigates the application of scattering structures to the metal layer in a DMD (Dielectric/Metal/Dielectric) configuration through plasma treatment. The purpose is to enhance the light extraction efficiency of organic light-emitting diodes (OLEDs). Different plasma conditions were explored to create scattering structures on the metal layer. The fabricated devices were characterized for their electrical and optical properties. The results demonstrate that the introduction of scattering structures through plasma treatment effectively improves the light extraction efficiency of OLEDs. Specifically, using O2-plasma treatment on the metal layer resulted in significant enhancements in the total transmittance, haze, and figure of merit. These findings suggest that incorporating scattering structures within the DMD configuration can effectively promote light extraction in OLEDs, leading to enhanced overall performance and light efficiency.

Rational design of dibenzo[a,c]phenazine-derived isomeric thermally activated delayed fluorescence luminophores for efficient orange-red organic light-emitting diodes

It is an immense challenge to develop efficient long-wavelength (orange-to-red) thermally activated delayed fluorescence (TADF) materials due to the increasing nonradiative decay rates following the energy-gap law. Herein, two pairs of asymmetric isomers; DPyPzTPA and TPAPzDPy, and PyPzDTPA and DTPAPzPy based on electron-deficient moieties dibenzo[a,c]phenazine (Pz) and pyridine (Py) combined with electron-donor units of triphenylamine (TPA) were designed and synthesized. Their photophysical properties could be finely modulated by changing the position and number of Py groups as well as TPA fragments onto Pz cores. DPyPzTPA and DTPAPzPy possess much more rigidity and thus less geometry relaxation and non-radiative decay between ground states and excited states than those of PyPzDTPA and TPAPzDPy. Intriguingly, DPyPzTPA exhibits the highest relative photoluminescence quantum yield (ΦPL) and the fastest reverse intersystem crossing (rISC) rate among them owing to relatively stronger rigidity and spin-orbit coupling (SOC) interactions between the lowest singlet (S1) and energetically close-lying excited triplet state and therefore, the device showed the highest maximum external quantum efficiency (EQEmax) of 16.6% (60.9 lm/W, 53.3 cd/A) with Commission Internationale de I'Eclairage (CIE) coordinates of (0.43, 0.55), peak wavelength 556 nm. In stark contrast, due to its lower rigidity and extremely weak delayed fluorescence (DF) characteristic and thus the much lower ΦPL, TPAPzDPy-based devices are only half as efficient (30.8 lm/W, 27.5 cd/A, 8.3% EQE) despite the isomers possessing equal singlet-triplet energy gaps (ΔEST) of 0.43 eV. On the other hand, the device based on DTPAPzPy also demonstrated a strongly enhanced performance (59.1 lm/W, 52.7 cd/A, 16.1% EQE) than its isomer PyPzDTPA-based device (39.5 lm/W, 35.2 cd/A, 10.3% EQE). This work explicitly implicates that the asymmetric and isomeric molecular design is a potential strategy for promoting the development of highly efficient long-wavelength TADF materials.

2,3-Diarylquinoline -Based Iridium (III) Complexes: Synthesis, Photophysical Properties and DFT studies

Phosphorescent transition–metal complexes have received considerable interest in the development of efficient organic light-emitting diodes (OLEDs). Iridium complexes derived from heterocyclic ligands have brought a paradigm shift in the optoelectronic materials. Quinoline-based molecular architectures offer excellent materials for such applications due to their exceptional electronic properties, extended π-conjugation and strong ligand–metal binding. Iridium complexes of 2,3-diaryl quinolines with electron rich and electron deficient substituents were synthesized and their electronic properties were studied. Iridium complexes Ir1 and Ir2 bearing the electron withdrawing (-CF3) and electron donating (–OCH3) groups, respectively at the para position of the 2-phenyl ring of 2,3-diarylquinoline exhibited prominent absorption bands in the UV range. Further, the complex Ir3 with unsubstituted 2,3-diarylquinoline ligand and CF3 containing Ir1 showed intensive red phosphorescence near 603 and 611 nm, respectively due to metal-to-ligand and ligand-to-ligand (MLCT/LLCT) charge transfer characteristics. The photoluminescence (PL) spectra of the Ir4 complex derived from 2-phenylquinoline ligand exhibited a bright yellow phosphorescent emission at 589 nm. An additional C3 aryl group on the quinoline ring resulted in a red shift in the emission wavelength of Ir1 and Ir3 complexes compared to Ir4. The PL spectra of Ir2 on the other hand was weak, and blue-shifted to 489 nm, which can be attributed to a ligand-centered (LC) transition. The complexes showed photoluminescence quantum yield (PLQY) in the range of 0.004 to 0.17 and fluorescence lifetime ranged from 1.5 ns to 399 ns, implying the significant effect of the ligand structure on the properties of the complexes. Photophysical investigation of the complexed affirms these complexes to be potent materials for OLEDs fabrication. Cyclic voltammetry was used to investigate the electrochemical characteristics of Ir1-Ir4 complexes in acetonitrile. All of the complexes Ir1-Ir4 exhibited irreversible oxidation waves at 0.44 V, 0.23 V, 0.25 V, and 0.24 V respectively. Ir1-Ir4 complexes showed a reduction potential at −0.98 V, −0.96 V, −1.02 V and −0.97 V respectively. The DFT calculations were used to explore HOMO-LUMO energies, and NBO characteristics by B3LYP level of theory using ECP type double zeta (DZ) quality basis set LanL2DZ for metal and 6-311G(d) basis sets for non-metal atoms. The theoretical calculations were in significant agreement with the experimental studies of these complexes.

Steric groups fusion strategy for green multi-resonance emitters toward efficient OLEDs with narrowband emission

Multi-resonance induced thermally activated delayed fluorescence (MR-TADF) emitters have demonstrated significant advantages in high-efficient organic light-emitting diodes (OLEDs) with wide color gamut due to their high photoluminescence quantum yields (ΦPL) and narrow spectra. Herein, two novel green MR-TADF emitters, MFCzBN and SFCzBN, were designed and synthesized by integrating steric groups into the classic framework of BNCz. These emitters exhibited narrowband green emission both in solution and films, fast radiative decay rates of approximately 108 s−1, as well as high ΦPL values of up to 90%. The fabricated vacuum-deposited OLEDs displayed bright green emission at 508 nm, achieving remarkable maximum external quantum efficiencies (EQEmax) of 24.3% and 24.7% for MFCzBN and SFCzBN, respectively. Notably, the MFCzBN-based device demonstrated low efficiency roll-off values of 1.6% and 2.5% at practical luminance of 100 cd m−2 and 1000 cd m−2. This work presents an effective strategy for attaining high-efficiency MR-OLEDs through the fusion of steric groups.

Metal-Organic Framework-Modified Silver Nanowire Transparent Film with Enhanced Conductivity and Stability for Efficient Organic Light-Emitting Diodes.

Silver nanowire (AgNW) is recognized as a critical material for developing the next generation of transparent conductive films (TCFs); however, poor stability remains a major issue. Herein, we demonstrate a stable AgNW TCF passivated by a metal-organic framework (MOF) via a facile solution process. The MOF is chemically bonded to the surface of the AgNWs as a chemical inhibitor, which contributes to passivating highly active sites and providing chemical protection, leading to enhanced resistance to corrosive molecules and thereby offering exceptional stability under an ambient atmosphere. Simultaneously, the binding interaction with the MOF anchors silver atoms at the surface of the nanowires, suppressing their diffusion at high temperatures and allowing the AgNW film to maintain excellent conductivity up to 300 °C. Additionally, the hydrogen bonding between the MOF and the substrate, along with the tight connection of the MOF with AgNWs, improves the welding between the nanowires, enhancing the conductivity of the AgNW film at mild conditions while offering good flexibility and adhesion properties. Furthermore, the OLED device integrating the MOF-modified AgNW electrode shows comparable performance to an indium tin oxide-based device, verifying its huge potential for applications in optoelectronic devices.

Dimethyl-dihydroindenocarbazole-anthracene-phenanthroimidazole based hybridized local and charge-transfer molecules as efficient nondoped deep-blue emitters for highly efficient fluorescent organic light-emitting diodes

The pursuit of hybridized local and charge transfer (HLCT) excited state-based emitters is a promising strategy to achieve high external quantum efficiency in blue organic light-emitting diodes (OLEDs). Two isomeric dimethyl-dihydroindenocarbazole-anthracene-phenanthroimidazole-based emissive molecules (PhAnsIn and PhAnoIn) were designed and explored based on the HLCT scheme. All molecules possessed the HLCT excited-state characters, according to the photophysical investigation and density functional theory calculations. Consequently, PhAnsIn and PhAnoIn showed good blue emission with peaks at 457 and 464 nm and photoluminescence quantum yield of 80.6% and 56.5% in neat films, respectively. The fabricated blue OLEDs with PhAnsIn and PhAnoIn as emitters also realized the corresponding blue emission with electroluminescence peaks at 458 and 457 nm, respectively. The nondoped devices based on PhAnsIn and PhAnoIn exhibited maximum external quantum efficiencies of 8.2% and 8.5% with CIE coordinates of (0.14, 0.12) and (0.14, 0.11), respectively.

Tailoring Extremely Narrow FWHM in Hypsochromic and Bathochromic Shift of Polycyclo-Heteraborin MR-TADF Materials for High-Performance OLEDs.

Developing double boron-based emitters with extremely narrow band spectrum and high efficiency in organic light-emitting diodes (OLEDs) is crucial and challenging. Herein, we report two materials, NO-DBMR and Cz-DBMR, hinge on polycyclic heteraborin skeletons based on role-play of the highest occupied molecular orbital (HOMO) energy levels. The NO-DBMR contains an oxygen atom, whereas the Cz-DBMR has a carbazole core in the double boron-embedded ν-DABNA structure. The synthesized materials resulted in an unsymmetrical pattern for NO-DBMR and surprisingly a symmetrical pattern for Cz-DBMR. Consequently, both materials showed extremely narrow full width at half maximum (FWHM) of 14 nm in hypsochromic (pure blue) and bathochromic (Bluish green) shifted emission without losing their high color fidelity. Furthermore, both materials show high photoluminescence quantum yield (PLQY) of over 82 %, and an extremely small singlet-triplet energy gap (ΔEST ) of 0.04 eV, resulting in high reverse intersystem crossing process (kRISC ) of 105 s-1 . Due to the efficient thermally activated delayed fluorescence (TADF) characteristics, the fabricated OLEDs based on these heteraborins manifested maximum external quantum efficiency (EQEmax ) of 33.7 and 29.8 % for NO-DBMR and Cz-DBMR, respectively. This is the first work reported with this type of strategy for achieving an extremely narrow emission spectrum in hypsochromic and bathochromic shifted emissions with a similar molecular skeleton.

Carbazole-based thermally activated delayed fluorescent emitters for efficient pure blue organic light-emitting diodes

Carbazole-based thermally activated delayed fluorescent emitters for efficient pure blue organic light-emitting diodes

Achieving Bright Organic Light-Emitting Field-Effect Transistors with Sustained Efficiency through Hybrid Contact Design.

Organic light-emitting field-effect transistors (OLEFETs) with bilayer structures have been widely studied due to their potential to integrate high-mobility organic transistors and efficient organic light-emitting diodes. However, these devices face a major challenge of imbalance charge transport, leading to a severe efficiency roll-off at high brightness. Here, we propose a solution to this challenge by introducing a transparent organic/inorganic hybrid contact with specially designed electronic structures. Our design aims to steadily accumulate the electrons injected into the emissive polymer, allowing the light-emitting interface to effectively capture more holes even when the hole current increases. Our numerical simulations show that the capture efficiency of these steady electrons will dominate charge recombination and lead to a sustained external quantum efficiency of 0.23% over 3 orders of magnitude of brightness (4 to 7700 cd/m2) and current density (1.2 to 2700 mA/cm2) from -4 to -100 V. The same enhancement is retained even after increasing the external quantum efficiency (EQE) to 0.51%. The high and tunable brightness with stable efficiency offered by hybrid-contact OLEFETs makes them ideal light-emitting devices for various applications. These devices have the potential to revolutionize the field of organic electronics by overcoming the fundamental challenge of imbalance charge transport.