Controlling Horizontal Dipole Orientation and Emission Spectrum of Ir Complexes by Chemical Design of Ancillary Ligands for Efficient Deep-Blue Organic Light-Emitting Diodes

Exceptionally high brightness and long lifetime of efficient blue OLEDs for programmable active-matrix display

Blue phosphorescent OLEDs (Ph-OLEDs) have long faced critical challenges in efficiency, stability and brightness, which are crucial for advanced display. Herein, we introduce two novel Ir(III) emitters featuring a 3,6-di(tert-butyl)-9H-carbazolyl (tBuCz) substituted tridentate carbene pincer ligand, significantly improving efficiency and stability. The tBuCz-m-CF3 and tBuCz-p-CF3 complexes are designed to enhance steric encumbrance and minimize exciton accumulation. These innovations lead to exceptional photoluminescence quantum yields (PLQY) of 98% and an impressive decay rate constant of 7.97 × 105 s-1 in doped thin films. The Ph-OLEDs emit blue light with a peak wavelength of 485 nm and CIE coordinates of (0.175, 0.446), exhibiting a peak external quantum efficiencies (EQE) of 31.62% and brightness up to 214,255 cd m-2. Notably, they shown minimal efficiency roll-off, retaining an EQE of 27.76% at 10,000 cd m-2, and 20.58% at 100,000 cd m-2. These consistent performances across various brightness levels represent a significant milestone for blue Ph-OLED technology. The devices also exhibit impressive stability, with an operational lifetime (LT50, the time taken for luminance to decrease by 50%) reaching 1237 h at 1000 cd m-2, setting new benchmarks for blue Ph-OLEDs. To enhance the color purity, hyper-OLEDs were developed with a full width at half maximum (FWHM) of 20 nm and the CIEy of 0.233, achieving an EQEm of 29.78% and LT50 of 318 h at 1000 cd m-2. We also fabricated the active-matrix (AM) blue Hyper-OLEDs with 400 pixels per inch to demonstrate their application in AM displays.

Solution-Processed Blue Narrowband OLED Devices with External Quantum Efficiency Beyond 35 % through Horizontal Dipole Orientation Induced by Electrostatic Interaction

The multiple resonance thermally activated delayed fluorescence (MR-TADF) device has drawn great attention due to their outstanding efficiency and color purity. However, the efficiency of solution-processed MR-TADF devices is still far behind their vacuum-deposited counterparts, due to the uncontrollable horizontal emitting dipole orientation for emitters during solution process, resulting in low light out-coupling efficiency. Here, we proposed a new strategy namely electrostatic interaction between a dendritic host with high positive electrostatic potential (ESP) and dendritic emitter with multiple negative ESP sites, which could induce high horizontal dipole ratio (Θ||) up to 83.0 % in solution-processed films. For this couple, the largest plane of dendritic host tends to anchor on the substrate, and thus the strong positive electrostatic site mainly lies at the exposed tetraphenylsilicon, which could electrostatically attract the multiple negative electrostatic sites of the dendritic emitter, realizing horizontal dipole orientation. Moreover, the highly twisted structure of dendritic host and dendron encapsulation of emitter could effectively suppress aggregation, leading a high photoluminescence quantum yield of 98.6 %. As a result, the solution-processed blue MR-TADF devices exhibit a record-break external quantum efficiency of 35.3 %, as well as narrow bandwidth of 17 nm and pure blue color with CIE coordinates of (0.137, 0.176).

Carbazolylpyridine (cp)-based tetradentate platinum(II) complexes containing fused 6/5/6 metallocycles

A series of carbazolylpyridine (cp)-based 6/5/6 Pt(II) complexes featuring tetradentate ligands with nitrogen or oxygen atoms as bridging groups was designed and synthesized, and the bridging nitrogen atoms were derived from acridinyl (Ac), azaaceridine (AAc) and carbazole (Cz). Systematic experimental and theoretical studies reveal that the ligand structures have a significant effect on the electrochemical, photophysical and excited state properties of these complexes. Their oxidation processes mainly occur on the carbazole-Pt moieties, whereas the reduction processes typically occur on the electron-deficient pyridine (Py) moieties. Time-dependent density functional theory (TD-DFT) and natural transition orbital (NTO) calculations reveal that the cp-based Pt(II) complexes have a metal-to-ligand charge transfer (3MLCT) state mixed with ligand-centered (3LC) and intra-ligand charge-transfer (3ILCT) characteristics. Pt(cp-1) shows strong red luminescence with a dominant peak at 611 nm and an excited-state lifetime of 10.7 μs in dichloromethane at room temperature, 602 nm and 10.9 μs in toluene, and 602 nm and 8.2 μs in PMMA films. It also exhibits high photoluminescence quantum efficiencies of 85%, 84% and 60% in dichloromethane, toluene and PMMA, respectively. These studies indicate the potential application of the cp-based Pt(II) complexes as phosphorescent emitters in the field of organic light-emitting diodes (OLEDs).

Excited-State Engineering of Chalcogen-Bridged Chiral Molecules for Efficient OLEDs with Diverse Luminescence Mechanisms

The exploration of circularly polarized luminescence is important for advancing display and lighting technologies. Herein, by utilizing isomeric molecular engineering, a novel series of chiral molecules are designed to exploit both thermally activated delayed fluorescence (TADF) and room-temperature phosphorescence (RTP) mechanisms for efficient luminescence. The cooperation of a small singlet-triplet energy gap, moderate spin-orbital coupling (SOC), and large oscillator strength enables efficient TADF emission, with photoluminescence quantum yields exceeding 90 %. By altering the symmetry of molecular structures, it is demonstrated that the intrinsic electronic SOC and vibrational SOC effects can be greatly enhanced to facilitate RTP emission. Notably, through modulating simultaneous TADF and RTP emissions, single-molecule white emission is successfully achieved. Accordingly, the TADF-based organic light-emitting diode (OLED) achieves a maximum external quantum efficiency up to 30 %, representing exceptional performance of non-aromatic amine-based emitters. Furthermore, the first single-molecule white OLED based on TADF and RTP dual-emissive chiral material is developed, establishing a benchmark for the development of advanced display and lighting technologies.

Enhanced Emitting Dipole Orientation Based on Asymmetric Iridium(III) Complexes for Efficient Saturated-Blue Phosphorescent OLEDs

Three novel asymmetric Ir(III) complexes have been rationally designed to optimize their emitting dipole orientations (EDO) and enhance light outcoupling in blue phosphorescent organic light-emitting diodes (OLEDs), thereby boosting their external quantum efficiency (EQE). Bulky electron-donating groups (EDGs), namely: carbazole (Cz), di-tert-butyl carbazole (tBuCz), and phenoxazine (Pxz) are incorporated into the tridentate dicarbene pincer chelate to induce high degree of packing anisotropy, simultaneously enhancing their photophysical properties. Angle-dependent photoluminescence (ADPL) measurements indicate increased horizontal transition dipole ratios of 0.89 and 0.90 for the Ir(III) complexes Cz-dfppy-CN and tBuCz-dfppy-CN, respectively. Analysis of the single crystal structure and density functional theory (DFT) calculation results revealed an inherent correlation between molecular aspect ratio and EDO. Utilizing the newly obtained emitters, the blue OLED devices demonstrated exceptional performance, achieving a maximum EQE of 30.7% at a Commission International de l'Eclairage (CIE) coordinate of (0.140, 0.148). Optical transfer matrix-based simulations confirmed a maximum outcoupling efficiency of 35% due to improved EDO. Finally, the tandem OLED and hyper-OLED devices exhibited a maximum EQE of 44.2% and 31.6%, respectively, together with good device stability. This rational molecular design provides straightforward guidelines to reach highly efficient and stable saturated blue emission.

Phosphorescent PdII-PdII Emitter-Based Red OLEDs with an EQEmax of 20.52

Three dinuclear Pd(II) complexes (1, 2, and 3) with intense red phosphorescence at room temperature are here synthesized using strong ligand field strength compounds. All three complexes are characterized by nuclear magnetic resonance, high-resolution mass spectrometry, and elemental analyses. Complexes 2 and 3 are characterized by single-crystal X-ray diffraction. The crystalline data of 2 and 3 reveal complex double-layer structures, with Pd-Pd distances of 2.8690(9) Å and 2.8584(17) Å, respectively. Furthermore, complexes 1, 2, and 3 show phosphorescence at room temperature in their solid states at the wavelengths of 678, 601, and 672 nm, respectively. In addition, they show phosphorescence at 634, 635, and 582 nm, respectively, in the 2 wt.% (PMMA) films, and phosphorescence at 670, 675, and 589 nm, respectively, in the deoxygenated CH2Cl2 solutions. Among three complexes, complex 1 shows red emission at 634 nm with phosphorescent quantum yield Ф = 67% in the 2 wt.% PMMA film. Furthermore, complex 1-based organic light-emitting diode is fabricated using a vapor-phase deposition process, and their maximum external quantum efficiency reaches 20.52%, which is the highest percentage obtained by using the dinuclear Pd(II) complex triplet emitters with the CIE coordinates of (0.62, 0.38).

Cationic Ir(III) Complexes with 4-Fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile as the Cyclometalating Ligand: Synthesis, Characterizations, and Application to Ultrahigh-Efficiency Light-Emitting Electrochemical Cells

Light-emitting electrochemical cells (LECs) promise low-cost, large-area luminescence applications with air-stabilized electrodes and a versatile fabrication that enables the use of solution processes. Nevertheless, the commercialization of LECs is still encountering many obstacles, such as low electroluminescence (EL) efficiencies of the ionic materials. In this paper, we propose five blue to yellow ionic Ir complexes possessing 4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile (ppfn) as a novel cyclometalating ligand and use them in LECs. In particular, the device within di[4-fluoro-4'-pyrazolyl-(1,1'-biphenyl)-2-carbonitrile]-4,4'-di-tert-butyl-2,2'-bipyridyl iridium(III) hexafluorophosphate (DTBP) shows a remarkable photoluminescence quantum yield (PLQY) of 70%, and by adjusting the emissive-layer thickness, the maximal external quantum efficiency (EQE) reaches 22.15% at 532 nm under the thickness of 0.51 μm, showing the state-of-the-art value for the reported blue-green LECs.

Double boron-oxygen-fused polycyclic aromatic hydrocarbons: skeletal editing and applications as organic optoelectronic materials

An efficient one-pot strategy for the facile synthesis of double boron-oxygen-fused polycyclic aromatic hydrocarbons (dBO-PAHs) with high regioselectivity and efficient skeletal editing is developed. The boron-oxygen-fused rings exhibit low aromaticity, endowing the polycyclic aromatic hydrocarbons with high chemical and thermal stabilities. The incorporation of the boron-oxygen units enables the polycyclic aromatic hydrocarbons to show single-component, low-temperature ultralong afterglow of up to 20 s. Moreover, the boron-oxygen-fused polycyclic aromatic hydrocarbons can also serve as ideal n-type host materials for high-brightness and high-efficiency deep-blue OLEDs; compared to single host, devices using boron-oxygen-fused polycyclic aromatic hydrocarbons-based co-hosts exhibit dramatically brightness and efficiency enhancements with significantly reduced efficiency roll-offs; device 9 demonstrates a high color-purity (Commission International de l'Eclairage CIEy = 0.104), and also achieves a record-high external quantum efficiency (28.0%) among Pt(II)-based deep-blue OLEDs with Commission International de l'Eclairage CIEy < 0.20; device 10 achieves a maximum brightnessof 27219 cd/m2 with a peak external quantum efficiency of 27.8%, which representes the record-high maximum brightness among Pt(II)-based deep-blue OLEDs. This work demonstrates the great potential of the double boron-oxygen-fused polycyclic aromatic hydrocarbons as ultralong afterglow and n-type host materials in optoelectronic applications.

Neoteric Advances in Oxygen Bridged Triaryl Boron-based Delayed Fluorescent Materials for Organic Light Emitting Diodes

Since their first demonstration, thermally activated delayed fluorescence (TADF) materials have been emerged as the most promising emitters because of their promising applications in optoelectronics, typified by organic light-emitting diodes (OLEDs). In which, the rigid oxygen bridged boron acceptor-featured (DOBNA) emitters have gained tremendous impetus for OLEDs, which is ascribed to their excellent external quantum efficiency (EQE). However, these materials often displayed severe efficiency roll-off and poor operational stability. Therefore, there needs to be a comprehensive understanding of the aspect of the molecular design and structure-property relationship. To the best of our knowledge, there is no detailed review on the structure-function outlook of DOBNA-based emitters emphasizing the effect of the nature of donor units, their number density, and substitution pattern on the physicochemical properties, excited state dynamics and OLED performance were reported. To fill this gap, herein we presented the recent advancements in DOBNA-based acceptor featured TADF materials by classifying them into several subgroups based on the molecular design i. e. donor-acceptor (D-A), D-A-D, A-D-A, and multi-resonant TADF (MR-TADF) emitters. The detailed design concepts, along with their respective physicochemical and OLED performances were summarized. Finally, the prospective of this class of materials in forthcoming OLED displays is also discussed.

New [3+2+1] Iridium Complexes as Effective Phosphorescent Sensitizers for Efficient Narrowband Saturated-Blue Hyper-OLEDs

Two newly designed and synthesized [3+2+1] iridium complexes through introducing bulky trimethylsiliyl (TMS) groups are doped with a terminal emitter of v-DABNA to form an coincident overlapping spectra between the emission of these two phosphors and the absorption of v-DABNA, creating cascade resonant energy transfer for efficient triplet harvesting. To boost the color quality and efficiency, the fabricated hyper-OLEDs have been optimized to achieve a high external quantum efficiency of 31.06%, which has been among the highest efficiency results reported for phosphor sensitized saturated-blue hyper-OLEDs, and pure blue emission peak at 467 nm with the full width at half maxima (FWHM) as narrow as 18 nm and the CIEy values down to 0.097, satisfying the National Institute of Standards and Technology (NIST) requirement for saturated blue OLEDs display. Surprisingly, such hyper-OLEDs have obtained the converted lifetime (LT50 ) up to 4552 h at the brightness of 100 cd m-2 , demonstrating effective Förster resonance energy transfer (FRET) process. Therefore, employing these new bulky TMS substituent [3+2+1] iridium(III) complexes for effective sensitizers can greatly pave the way for further development of high efficiency and stable blue OLEDs in display and lighting applications.

Color Stable Deep Blue Multi-Resonance Organic Emitters with Narrow Emission and High Efficiency

The development of highly efficient and deep blue emitters satisfying the color specification of the commercial products has been a challenging hurdle in the organic light-emitting diodes (OLEDs). Here, deep blue OLEDs with a narrow emission spectrum with good color stability and spin-vibronic coupling assisted thermally activated delayed fluorescence are reported using a novel multi-resonance (MR) emitter built on a pure organic-based molecular platform of fused indolo[3,2,1-jk]carbazole structure. Two emitters derived from 2,5,11,14-tetrakis(1,1-dimethylethyl)indolo[3,2,1-jk]indolo[1',2',3':1,7]indolo[3,2-b]carbazole (tBisICz) core are synthesized as the MR type thermally activated delayed fluorescence emitters realizing a very narrow emission spectrum with a full-width-at-half-maximum (FWHM) of 16 nm with suppressed broadening at high doping concentration. The tBisICz core is substituted with a diphenylamine or 9-phenylcarbazole blocking group to manage the intermolecular interaction for high efficiency and narrow emission. The deep blue OLEDs achieve high external quantum efficiency (EQE) of 24.9%, small FWHM of 19 nm, and deep blue color coordinate of (0.16, 0.04) with good color stability with increase in doping concentration. To the authors' knowledge, the EQE in this work is one of the highest values reported for the deep blue OLEDs that achieve the BT.2020 standard.

Forthcoming hyperfluorescence display technology: relevant factors to achieve high-performance stable organic light emitting diodes

Over the decade, there have been developments in purely organic thermally activated delayed fluorescent (TADF) materials for organic light-emitting diodes (OLEDs). However, achieving narrow full width at half maximum (FWHM) and high external quantum efficiency (EQE) is crucial for real display industries. To overcome these hurdles, hyperfluorescence (HF) technology was proposed for next-generation OLEDs. In this technology, the TADF material was considered a sensitizing host, the so-called TADF sensitized host (TSH), for use of triplet excitons via the reverse intersystem crossing (RISC) pathway. Since most of the TADF materials show bipolar characteristics, electrically generated singlet and triplet exciton energies can be transported to the final fluorescent emitter (FE) through Förster resonance energy transfer (FRET) rather than Dexter energy transfer (DET). This mechanism is possible from the S₁ state of the TSH to the S₁ state of the final fluorescent dopant (FD) as a long-range energy transfer. Considering this, some reports are available based on hyperfluorescence OLEDs, but the detailed analysis for highly efficient and stable devices for commercialization was unclear. So herein, we reviewed the relevant factors based on recent advancements to build a highly efficient and stable hyperfluorescence system. The factors include an energy transfer mechanism based on spectral overlapping, TSH requirements, electroluminescence study based on exciplex and polarity system, shielding effect, DET suppression, and FD orientation. Furthermore, the outlook and future positives with new directions were discussed to build high-performance OLEDs.

Enhanced stability and complex phase behaviour of organic-inorganic green-emitting ionic manganese halides

Light-emitting materials based on earth-abundant metals, such as manganese hold great promise as emitters for organic lighting devices. In order to apply such emitter materials and, in particular, to overcome the problem of self-quenching due to cross-relaxation, we investigated a series of tetrabromidomanganate ([MnBr₄]^2-) salts with bulky tetraalkylphosphonium counter cations [P_nnn]⁺, namely [P_nnnn]₂[MnBr₄] (n = 4 (1), 6 (2) and 8 (3)), which can be obtained by a straightforward reaction of the respective phosphonium bromide and MnBr₂. Variation of the cation size allows control of the properties of the resulting ionic materials. 1 and 3 qualify as ionic liquids (ILs), where 1 features a melting point of 68 °C, and 3 is liquid at room temperature and even at very low temperatures. Furthermore, 1 and 2 show the formation of higher-ordered thermotropic mesophases. For 1 a transition to a thermodynamically metastable smectic liquid crystalline phase can be observed at room temperature upon reheating from the metastable glassy state; 2 appears to form a plastic crystalline phase at ∼63 °C, which persists up to the melting point of 235 °C. The photoemission is greatly affected by phase behaviour and ion dynamics. A photoluminescence quantum yield of 61% could be achieved, by balancing the increase in Mn²⁺-Mn²⁺ separation and reducing self-quenching through increasingly large organic cations which leads to adverse increased vibrational quenching. Compared to analogous ammonium compounds, which have been promoted as ̈inorganic hybrid perovskites̈, the phosphonium salts show superior performance, with respect to photoluminescent quantum yield and thermal and air/humidity stability. As the presented compounds are not sensitive to the atmosphere, in particular moisture, and show strong visible electroluminescence in the green region of light, they are important emitter materials for use in organic light-emitting devices.

Multiresonant TADF materials: triggering the reverse intersystem crossing to alleviate the efficiency roll-off in OLEDs

The hunt for narrow-band emissive pure organic molecules capable of harvesting both singlet and triplet excitons for light emission has garnered enormous attention to promote the advancement of organic light-emitting diodes (OLEDs). Over the past decade, organic thermally activated delayed fluorescence (TADF) materials based on donor (D)/acceptor (A) combinations have been researched for OLEDs in wide color gamut (RGB) regions. However, due to the strong intramolecular charge-transfer (CT) state, they exhibit broad emission with full-width-at-half maximum (FWHM) > 70 nm, which deviates from being detrimental to achieving high color purity for future high-end display electronics such as high-definition TVs and ultra-high-definition TVs (UHDTVs). Recently, the new development in the sub-class of TADF emitters called multi-resonant TADF (MR-TADF) emitters based on boron/nitrogen atoms has attracted much interest in ultra-high definition OLEDs. Consequently, MR-TADF emitters are appeal to their potentiality as promising candidates in fabricating the high-efficient OLEDs due to their numerous advantages such as high photoluminescence quantum yield (PLQY), unprecedented color purity, and narrow bandwidth (FWHM ≤ 40 nm). Until now many MR-TADF materials have been developed for ultra-gamut regions with different design concepts. However, most MR-TADF-OLEDs showed ruthless external quantum efficiency (EQE) roll-off characteristics at high brightness. Such EQE roll-off characteristics were derived mainly from the low reverse intersystem crossing (k_RISC) rate values. This feature article primarily focuses on the design strategies to improve k_RISC for MR-TADF materials with some supportive strategies including extending charge delocalization, heavy atom introduction, multi-donor/acceptor utilization, and a hyperfluorescence system approach. Furthermore, the outlook and prospects for future developments in MR-TADF skeletons are described.

Improving Electroluminescence Efficiency by Linear Polar Host Capable of Promoting Horizontal Dipole Orientation for Dopant

In doped organic light-emitting diodes (OLEDs), the host materials play an important role in emitting layers. Most studies about host materials mainly focus on their energy levels and carrier transport behaviors, while less attention is paid to their influence on the dipole orientation of dopants, which closely associate with the light out-coupling efficiency (ηout ) of the device. Herein, a linear polar host material (l-CzTRZ) consisting of carbazole donor, triazine acceptor, and the conjugated para-terphenyl skeleton is developed and its crystal and electronic structures, thermal and electrochemical stabilities, optical property, and carrier transport ability are investigated. l-CzTRZ prefers ordered horizontal orientation and favors electron transport in neat film. More importantly, it can promote horizontal dipole orientation for the dopants via dipole-dipole interaction, furnishing an excellent horizontal dipole ratio of 91.5% and thus a high ηout of 43% for the phosphorescent dopant (PO-01-TB). Consequently, the OLED with l-CzTRZ host and PO-01-TB dopant attains state-of-the-art electroluminescence efficiencies of 135.5 cd A-1 , 135.7 lm W-1 and 41.3%, with a small roll-off of 9.7% at 5000 cd m-2 luminance. The presented significant impact of the host on the dipole orientation of the dopant shall enlighten the design of host materials to improve OLED performance.

Realization of Long Operational Lifetimes in Vacuum-Deposited Organic Light-Emitting Devices Based on para-Substituted Pyridine Carbazolylgold(III) C^C^N Complexes

A new series of robust C^C^N carbazolylgold(III) complexes is designed and synthesized through the introduction of inert and sterically bulky oligophenyl substituents on the pyridyl moiety of the cyclometalating ligand. High photoluminescence quantum yields of up to 96% are recorded with these complexes doped in solid-state thin films, and short excited-state lifetimes of 0.3 μs or less in the solid state at room temperature are found. Promising electroluminescence (EL) performances are shown by the vacuum-deposited organic light-emitting devices (OLEDs) based on this series of gold(III) complexes. High external quantum efficiencies of up to 19.5% with efficiency roll-offs of down to 10% at a practical luminance brightness level of 1000 cd m^-2 are achieved. More importantly, record-long operational lifetimes (LT₅₀) of up to 470,700 h at 100 cd m^-2 are realized, which is currently the highest value among all classes of gold(III) complexes with tridentate pincer ligands. Particularly, by introducing a sterically bulky terphenyl moiety on the reactive site of the pyridine ring, the LT₅₀ value is shown to attain ∼7 times longer half-lifetime than that based on the unsubstituted complex. These unprecedented EL performances and the simple synthetic route in a mercury-free fashion make them promising emitting materials for practical OLEDs toward commercialization.

Double boron-embedded multiresonant thermally activated delayed fluorescent materials for organic light-emitting diodes

The subclass of multi resonant thermally activated delayed fluorescent emitters (MR-TADF) containing boron atoms has garnered significant attention in the field of organic light emitting diode (OLED) research. Among boron-based MR-TADF emitters, double boron-embedded MR-TADF (DB-MR-TADF) emitters show excellent electroluminescence performances with high photoluminescence quantum yields, narrow band emission, and beneficially small singlet-triplet energy levels in all the full-color gamut regions. This article reviews recent progress in DB-MR-TADF emitters, with particular attention to molecular design concepts, synthetic routes, optoelectronic properties, and OLED performance, giving future prospects for real-world applications.

Organic radical emitters: nature of doublet excitons in emissive layers

Organic radical emitters have received significant attention as a new route to efficient organic light-emitting diodes (OLEDs). The electronic structure of radical emitters allows bypassing the triplet harvesting issue in current OLED devices. However, the nature of doublet excited states remains elusive due to the complex nature of emissive layers. To date, the computational efforts have treated radical carrying materials as isolated entities in the gas phase. However, OLED materials are applied as thin solid films where intermolecular interactions significantly impact optoelectronic properties of the devices. Here, we combine molecular dynamics simulations and quantum chemical calculations to evaluate the effect of emitter-host interactions on the performance of radical-based emissive layers. Results demonstrate that intermolecular interactions remarkably modulate the electronic properties of the radicals in the thin solid films. The doublet excitons of isolated emitters demonstrate a hybrid character of charge-transfer (CT) and local-excitation (LE), while the emitter-host clusters present a significant CT character. Further, the impact of static and dynamic disorders on the hole-electron recombination is studied. Although the host-emitter interactions simultaneously decrease radiative rates and increase non-radiative rates, the latter rates are 100 times smaller than the former rates, allowing internal quantum efficiency to reach 100% for the doublet-based emission process. The results of this study highlight the significant impact of host-emitter interactions on radiative and non-radiative recombination processes and offer guidelines to tune these interactions for advancing radical-based OLEDs.

Fine Emission Tuning from Near-Ultraviolet to Saturated Blue with Rationally Designed Carbene-Based [3 + 2 + 1] Iridium(III) Complexes

We designed and synthesized a new class of six phosphorescent [3 + 2 + 1] iridium(III) complexes [(pbib)Ir(C^C)CN] bearing a tridentate 1,3-bis(1-butylimidazolin-2-ylidene) phenyl N-heterocyclic carbene (NHC)-based pincer ligand (pbib), bidentate imidazole-based NHC ligands (C^C), and a monodentate cyano group and investigated their photophysical, electrochemical, and thermal stabilities and electroluminescent properties. The extended π-conjugation of the imidazole-based C^C ligand is found to be the key to fine-tune the emission energies from ultraviolet blue (λ = 378 nm) to saturated blue (λ = 482 nm), as shown by electrochemical and photophysical studies, which is also revealed by the density functional theory (DFT) and time-dependent DFT calculations. Vacuum-deposited organic light-emitting diode devices have been fabricated with these newly synthesized emitters and exhibited the best external quantum efficiency of 6.4% and Commission International de L'Éclairage (CIE) coordinates of (0.163, 0.096), where the CIE y is very similar to the National Television System Committee standard blue CIE (x, y) coordinates of (0.149, 0.085). These results indicate that the novel [3 + 2 + 1] coordinated iridium(III) complexes [(pbib)Ir(C^C)CN], having a saturated blue emission, not only could alleviate the photodegradation of the emitters when compared to [(pbib)Ir(pmi)CN] but also provide new design strategies of saturated-blue-emitting iridium(III) complexes.

Theoretical studies on boron dimesityl-based thermally activated delayed fluorescence organic emitters: excited-state properties and mechanisms

At 300 K, S1 excitons could emit fluorescence or undergo ISC to T1, where rISC exceeds the phosphorescence emission enabling TADF.

AIE-active Ir(iii) complexes functionalised with a cationic Schiff base ligand: synthesis, photophysical properties and applications in photodynamic therapy

Two AIE-active Ir(iii) cationic complexes containing Schiff base ligands were synthesised. Ir-2-N+ NPs are shown to be good PSs in vitro for PDT.

Dinuclear PtII Complexes with Strong Blue Phosphorescence for Operational Stable Organic Light-Emitting Diodes with EQE up to 23% at 1000 cd m-2

Here we describe the synthesis and characterization of a new class of dinuclear PtII complexes with blue phosphorescence. Bulky N-heterocyclic carbene and tethered bridging ligands were employed to suppress photo-induced structural changes and to improve thermal stability of the complexes. These complexes show mixed 3IL/3MLCT blue emission (~460 nm) with emission quantum yields of up to 0.95, emission lifetimes of as low as 1.3 µs and radiative decay rate constants of up to 7.3 × 105 s-1 in 4 wt% doped PMMA films; the latter is attributed to a 1MLCT excited state having high metal character (resulting in large SOC) and a large transition dipole moment, based on DFT calculations. Phosphor-sensitized blue hyperOLEDs with Commission Internationale de L'Eclairage (CIE) coordinates of (0.13, 0.12) showed maximum EQE of 23.4% with full-width-at-half-maximum of 18 nm and LT50 > 250 h at an L0 of 1000 cd m-2.

Phosphorescent [3 + 2 + 1] coordinated Ir(iii) cyano complexes for achieving efficient phosphors and their application in OLED devices

A series of neutral [3 + 2 + 1] coordinated iridium complexes bearing tridentate bis-NHC carbene chelates (2,6-bisimidazolylidene benzene), bidentate chelates (C^N ligands, e.g. derivatives of 2-phenylpridine), and monodentate ions (halides and pseudo-halides, such as Br, I, OCN and CN ions) have been systematically designed and synthesized. X-ray single crystal structure characterization revealed that the nitrogen atom in C^N ligands is located trans to the carbon atom in the benzene ring in tridentate chelates, while the coordinating carbon atom in C^N ligands is located trans to the monodentate ligands. Photophysical studies reveal that the C^N ligands play a vital role in tuning the UV absorption and emission properties, while the tridentate bis-NHC carbene chelates influence the lowest absorption band and emission energy when compared to heteroleptic Ir(ppy)2(acac) [i.e. molar absorptivities at ∼450 nm for ppy-OCN and Ir(ppy)2(acac) are 350 M-1 cm-1 and 1520 M-1 cm-1 and emission maximum peaks are at 465 nm and 515 nm respectively]. Among monodentate ligands that the complexes bear, the group containing the cyanide ligand displays higher emission energy, higher photophysical quantum yields, longer triplet lifetimes and better electrochemical and thermal stabilities than those of cyanate and bromide. Particularly, a blue organic light-emitting diode (OLED) based on dfppy-CN exhibited a maximum external quantum efficiency of 22.94% with CIE coordinates of (0.14, 0.24). Furthermore, a small efficiency roll-off of 5.7% was observed for this device at 1000 cd m-2.

Electroluminescent materials toward near ultraviolet region

Near ultraviolet (NUV) light-emitting materials and devices are significant due to unique applications in anti-counterfeit, manufacturing industries, and hygienic treatments. However, the development of high-efficiency NUV electroluminescent devices encounters great challenges and is far behind their RGB emitter counterparts. Besides the photoluminescence quantum yields (PLQYs) of NUV materials being higher than 40%, charge injection and lopsided carrier transport also determine the device performance, leading to great efforts in optimizing the frontier molecular orbitals to fit the adjacent function layer. In the exploration of NUV materials, organic molecules are one of the primary candidates, given their preparative facility and structural variability. Recently, all-inorganic quantum-dot light-emitting diodes (QLEDs) of Cd-based, ZnSe, graphene and inorganic perovskite emitters and organic-inorganic hybrid lead halide perovskite nanocrystals (NCs) were demonstrated for achieving NUV electroluminescence. Owing to the great efforts devoted to NUV material engineering and device configuration, NUV materials and devices have achieved great advances over the last two decades. In this review, we retrospect the development of NUV materials and devices covering all promising systems, which may inspire the enthusiasm of researchers to explore the huge potential in the NUV region.

Improved Efficiency Roll-Off and Operational Lifetime of Organic Light-Emitting Diodes with a Tetradentate Platinum(II) Complex by Using an n-Doped Electron-Transporting Layer

The efficiency roll-off and operational lifetime of organic light-emitting diodes (OLEDs) with a tetradentate Pt(II) emitter is improved by engaging an n-doped electron-transporting layer (ETL). Compared to those devices with non-doped ETL, the driving voltage is lowered, the charged carrier is balanced, and the exciton density in the emissive layer (EML) is decreased in the device with n-doped ETL with 8-hydroxyquinolinolatolithium (Liq). High luminance of almost 70,000 cd m^-2 and high current efficiency of 40.5 cd A^-1 at high luminance of 10,000 cd m^-2 is achieved in the device with 50 wt%-Liq-doped ETL. More importantly, the extended operational lifetime of 1945 h is recorded at the initial luminance of 1000 cd m^-2 in the 50 wt%-Liq-doped device, which is longer than that of the device with non-doped ETL by almost 10 times. This result manifests the potential application of tetradentate Pt(II) complexes in the OLED industry.

High-Efficiency Red Electroluminescence Based on a Carbene-Cu(I)-Acridine Complex

How to develop efficient red-emitting organometallics of earth-abundant copper(I) is a formidable challenge in the field of organic light-emitting diodes (OLEDs) because Cu(I) complexes have weak spin-orbit coupling and a serious excited-state reorganization effect. Here, a red Cu(I) complex, MAC*-Cu-DPAC, was developed using a rigid 9,9-diphenyl-9,10-dihydroacridine donor ligand in a carbene-metal-amide motif. The Cu(I) complex achieved satisfactory red emission, a high photoluminescence quantum yield of up to 70%, and a sub-microsecond lifetime. Thanks to a linear geometry and the acceptor and donor ligands in a coplanar conformation, the complex exhibited a high horizontal dipole ratio of 77% in the host matrix, first demonstrated for coinage metal(I) complexes. The resulting OLEDs delivered high external quantum efficiencies of 21.1% at a maximum and 20.1% at 1000 nits, together with a red emission peak at ∼630 nm. These values represent the state-of-the-art performance for red-emitting OLEDs based on coinage metal complexes.

A Brief History of OLEDs-Emitter Development and Industry Milestones

Organic light-emitting diodes (OLEDs) have come a long way ever since their first introduction in 1987 at Eastman Kodak. Today, OLEDs are especially valued in the display and lighting industry for their promising features. As one of the research fields that equally inspires and drives development in academia and industry, OLED device technology has continuously evolved over more than 30 years. OLED devices have come forward based on three generations of emitter materials relying on fluorescence (first generation), phosphorescence (second generation), and thermally activated delayed fluorescence (third generation). Furthermore, research in academia and industry toward the fourth generation of OLEDs is in progress. Excerpts from the history of green, orange-red, and blue OLED emitter development on the side of academia and milestones achieved by key players in the industry are included in this report.

A New Benchmark of Charges Storage in Single-Layer Organic Light-Emitting Diodes Based on Electrical and Optical Characteristics

The storage of charges in organic light-emitting diodes (OLEDs) has drawn much attention for its damage to device performance as well as the loss to carriers. Thus, it is essential to address the issue and do further investigation. The traditional approach to storage analysis is mainly based on transient measurement since it is sensitive to transient instead of steady signal. In this paper, we proposed a new benchmark to investigate the single-layer OLEDs capable of stored charges with poly (methyl methacrylate) (PMMA), which is just based on electrical and optical characteristics. Since the stored charges contribute both to luminance and current of the devices with PMMA, the area between them can be taken as a benchmark and evaluated the storage of charges. In our experiment, the areas of 4 nm, 6 nm, 8 nm, and 10 nm PMMA devices are 0.348, 0.554, 0.808, and 0.894, respectively, indicating a higher capability of storage in thicker PMMA. It is exactly in line with the results taken from transient electroluminescence (EL) measurement. Thus, this new benchmark is practical and provides a more accessible approach to investigate the storage of charges in OLEDs.

Peripheral Ligand Effect on the Photophysical Property of Octahedral Iridium Complex: o-Aryl Substitution on the Phenyl Units of Homoleptic IrIII(C∧C)3 Complexes (C∧C = 1-Phenyl-3-methylimidazolin-2-ylidene-C,C2') for Deep Blue Phosphorescence

To evaluate the efficacy of ortho-arylation in the second coordination sphere of octahedral iridium complex, a series of homoleptic N-heterocyclic carbene (NHC)-based Ir(C^∧C^R)₃-type complexes were designed and prepared by introducing various substituents (R = H, Me, Ph, MePh, and diMePh) at the ortho-position of the aryl unit of the orthometalated phenyl group. In solution, an unnoticeable increase of emission quantum yields was observed within the variation of the ortho-substituent of the sterically demanding side-branch, a diMePh- group, showing the radiative quantum yield of mer-Ir(C^∧C^diMePh)₃ (Φ_PL = 1.9%), being higher than that of the unsubstituted carbene-based mer-Ir(C^∧C^H)₃ (Φ_PL = 1.2%), due to a considerable difference in the nonradiative decay rate (k_nr = 65.40 × 10⁵ s^-1 for mer-Ir(C^∧C^diMePh)₃ vs k_nr = 141.1 × 10⁵ s^-1 for mer-Ir(C^∧C^H)₃). Such a difference is attributed to the reduction of nonradiative pathway via the ³MLCT → ³MC transition by the widening gap between triplet emissive states and ³MC state, and a rigidity increase in structure by steric hindrance of bulky aryl substituent. In contrast, significant increase of emission quantum yield was observed in the films cast by spin coating, and fac-/mer-Ir(C^∧C^diMePh)₃ (Φ_PL = 60.1/49.1%) were the most efficient ones among NHC-Ir(III) complexes, compatible with the assumption that the secondary coordination effect, i.e. a peripheral constraint, was put into action. As the substituent R increases in size on going from H, Me, Ph, MePh, to diMePh, notable structural changes in the periphery are evident, while an increase of emission quantum yields is also seen. Such a peripheral difference was under scrutiny first with X-ray structural studies, and its manifestation in photophysics was investigated along with quantum calculations that finally addressed the peripheral effect being maximized at R = diMePh. In the application of PhOLED, the mer-Ir(C^∧C^diMePh)₃-doped multilayer device showed highly enhanced efficiency with an external quantum efficiency (EQE) of up to 8.1%, compared to that of the mer-Ir(C^∧C^H)₃-based device (1.2%), indicating the multiple positive effects of bulky aryl substitution of Ir(III) dopant. A deep-blue CIE chromaticity diagram (0.16, 0.09) was achieved from the device using mer-Ir(C^∧C^diMePh)₃ as a dopant.

Acceptor plane expansion enhances horizontal orientation of thermally activated delayed fluorescence emitters

Manipulating orientation of organic emitters remains a formidable challenge in organic light-emitting diodes (OLEDs). Here, expansion of the acceptor plane of thermally activated delayed fluorescence (TADF) emitters was demonstrated to selectively modulate emitting dipole orientation. Two proof-of-the-concept molecules, PXZPyPM and PXZTAZPM, were prepared by introducing a planar 2-phenylpyridine or 2,4,6-triphenyl-1,3,5-triazine substituent into a prototypical molecule (PXZPM) bearing a pyrimidine core and two phenoxazine donors. This design approach suppressed the influence of substituents on electronic structures and associated optoelectronic properties. Accordingly, PXZPyPM and PXZTAZPM preserved almost the same excited states and similar emission characteristics as PXZPM. The expanded acceptor plane of PXZPyPM and PXZTAZPM resulted in a 15 to 18% increase in horizontal ratios of emitting dipole orientation. PXZPyPM supported its green device exhibiting an external quantum efficiency of 33.9% and a power efficiency of 118.9 lumen per watt, competitive with the most efficient green TADF OLEDs reported so far.

Organic-Inorganic Charge Transfer Complex with Charge Modulation after Electrical Pre-biasing

Several breakthroughs in organic optoelectronic devices with new applications and performance improvement have been made recently by exploiting novel properties of charge transfer complexes (CTCs). In this work, a CTC film formed by coevaporating molybdenum(VI) oxide and pentacene (MoO₃:pentacene) shows a strong dipole of 2.4 eV with direction controllability via pre-biasing with an external voltage. While CTCs are most widely known for their much red-shifted energy gaps, there is so far no report on their controllable dipoles. By controlling this dipole with an electrical pre-bias in a MoO₃:pentacene CTC based device, current changes over 2 orders of magnitude can be achieved. Kelvin probe force microscopy further confirms that surface potential of the MoO₃:pentacene film can be modulated by an external electric field. It is shown for the first time that a dipole of controllable direction can be set up inside a CTC layer by pre-biasing. This concept is further tested by incorporating the CTC layer in organic photovoltaic (OPV) devices. It was demonstrated that by pre-biasing the OPV devices in different directions, their open circuit voltages (V_oc) can be significantly tuned via the built-in potentials.

Color-Tunable Organic Light Emitting Diodes for Deep Blue Emission by Regulating the Optical Micro-Cavity

Nowadays, most blue organic light emitting diodes (OLEDs) are fabricated by using sky-blue emitters which are more easily synthesized when compared with other deep blue emitters. Herein, we put forward a new idea of using an optical micro-cavity based on metal electrodes to regulate electroluminance (EL) spectra of sky-blue organic light emitting diodes to obtain a saturated deep blue emission with a narrowed full-width at half-maximum (FWHM). First, we simulate micro-cavity OLEDs and find that the transmission of the anode plays an important role in the forward emission. Meanwhile, the optical path of micro-cavity OLEDs as well as the phase shifting from electrodes influence the EL spectra and induce the extra intensity enhancement. The results show that when the resonant cavity optical path is regulated by changing the thickness of emitting layer (EML) from 25 nm to 75 nm in the micro-cavity, the EL peak of blue OLEDs has a redshift from 479 nm to 493 nm with FWHM shifting from 69.8 nm to 83.2 nm, when compared to the device without the micro-cavity, whose approximate EL peak and FWHM are 487 nm and 87 nm, respectively. However, the efficiency of electroluminescence decreases in micro-cavity OLEDs. We speculate that this is on account of the ohmic contact between ITO and Ag, the surface plasma effect and the rough morphology induced by Ag electrodes.

Molecular Orientations of Delayed Fluorescent Emitters in a Series of Carbazole-Based Host Materials

Molecular orientation is one of the most crucial factors to boost the efficiency of organic light-emitting devices. However, active control of molecular orientation of the emitter molecule by the host molecule is rarely realized so far, and the underlying mechanism is under discussion. Here, we systematically investigated the molecular orientations of thermally activated delayed fluorescence (TADF) emitters in a series of carbazole-based host materials. Enhanced horizontal orientation of the TADF emitters was achieved. The degree of enhancement observed was dependent on the host material used. Consequently, our results indicate that π-π stacking, CH/n (n = O, N) weak hydrogen bonds, and multiple CH/π contacts greatly induce horizontal orientation of the TADF emitters in addition to the molecular shape anisotropy. Finally, we fabricated TADF-based organic light-emitting devices with an external quantum efficiency (ηext) of 26% using an emission layer with horizontal orientation ratio (Θ) of 79%, which is higher than that of an almost randomly oriented emission layer with Θ of 62% (ηext = 22%).

Ancillary ligand increases the efficiency of heteroleptic Ir-based triplet emitters in OLED devices

The excellent contrast ratio, visibility, and advantages in producing thin and light displays let organic light emitting diodes change the paradigm of the display industry. To improve future display technologies, higher electroluminescence efficiency is needed. Herein, the detailed study of the non-radiative decay mechanism employing density functional theory calculations is carried out and a simple, general strategy for the design of the ancillary ligand is formulated. It is shown that steric bulk properly directed towards the phenylisoquinoline ligands can significantly reduce the non-radiative decay rate.

Though Ir-based dopants with ancillary ligands are attractive for realizing efficient organic light-emitting diodes, a strategy for designing these materials remains elusive. Here, the authors report a design strategy for heteroleptic triplet OLED emitters featuring functionalized ancillary ligands.

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.

Rational Design of Efficient Organometallic Ir(III) Complexes for High-Performance, Flexible, Monochromatic, and White Light-Emitting Electrochemical Cells

Highly efficient light-emitting electrochemical cells (LECs) have attracted tremendous interest because of their simple structures and low-cost fabrication processing, showing great potential for full-color displays and solid-state lighting. In this work, we rationally designed and synthesized two red-emitting cationic Ir(III) complexes, [Ir(tBuPBI)₂(biq)]PF₆ (R1) and [Ir(tBuPBI)₂(qibi)]PF₆ (R2), in which a tert-butyl-functionalized 1,2-diphenyl-1H-benzo[d]imidazole (PBI) unit and conjugated 2,2'-biquinoline (biq) and 2-(1-phenyl-1H-benzo[d]imidazol-2-yl)quinolone (qibi) were employed as cyclometalated and ancillary ligands, respectively. The introduced tert-butyl group led to homogeneous and highly emissive thin films by increasing the solubility and suppressing the strong intermolecular interactions due to steric hindrance. Based on the abovementioned high-quality emissive layer, high-efficiency LECs were achieved. An efficient red-emitting LEC fabricated on a glass substrate achieved a current efficiency (η_C) of 7.18 cd/A and an external quantum efficiency (η_ext) of 9.32%. By doping both complexes into a blue-green-emitting cationic Ir(III) complex, high-performance white LECs were also successfully fabricated with Commission International de L'Eclairage (CIE) coordinates of (0.39,0.39), a η_C of 17.43 cd/A, and a η_ext of 8.92%. In addition, we also fabricated flexible red and white LECs with outstanding efficiencies and mechanical flexibilities. The η_C and η_ext values of a flexible white LEC could be as high as 13.50 cd/A and 6.86%, respectively. The efficiency of the flexible device remained at approximately 95% of the initial value after 500 bends with a radius of curvature of 5 mm, demonstrating the great potential of these complexes for full-color displays and flexible optoelectronics.

Unraveling the marked differences of the phosphorescence efficiencies of blue-emitting iridium complexes with isomerized phenyltriazole ligands

Quantum chemical insights into the marked quantum efficiencies of blue-emitting iridium complexes with isomerized ptz ligands.