Highly efficient non-doped blue fluorescent OLEDs with low efficiency roll-off based on hybridized local and charge transfer excited state emitters

Designing a donor–acceptor (D–A) molecule with a hybridized local and charge transfer (HLCT) excited state is a very effective strategy for producing an organic light-emitting diode (OLED) with a high exciton utilization efficiency and external quantum efficiency. Herein, a novel twisting D–π–A fluorescent molecule (triphenylamine–anthracene–phenanthroimidazole; TPAAnPI) is designed and synthesized. The excited state properties of the TPAAnPI investigated through photophysical experiments and density functional theory (DFT) analysis reveal that its fluorescence is due to the HLCT excited state. The optimized non-doped blue OLED using TPAAnPI as a light-emitting layer exhibits a novel blue emission with an electroluminescence (EL) peak at 470 nm, corresponding to the Commission International de L''Eclairage (CIE) coordinates of (0.15, 0.22). A fabricated device termed Device II exhibits a maximum current efficiency of 18.09 cd A⁻¹, power efficiency of 12.35 lm W⁻¹, luminescence of ≈29 900 cd cm⁻², and external quantum efficiency (EQE) of 11.47%, corresponding to a high exciton utilization efficiency of 91%. Its EQE remains as high as 9.70% at a luminescence of 1000 cd m⁻² with a low efficiency roll-off of 15%. These results are among the best for HLCT blue-emitting materials involved in non-doped blue fluorescent OLEDs. The performance of Device II highlights a great industrial application potential for the TPAAnPI molecule.

A new pure fluorescent blue HLCT-emitter was designed and synthesized. Highly efficient non-doped blue OLEDs with low efficiency roll-off were achieved.     

Pyrene-Benzimidazole Derivatives as Novel Blue Emitters for OLEDs

Three novel small organic heterocyclic compounds: 2-(1,2-diphenyl)-1H-benzimidazole-7-tert-butylpyrene (compound A), 1,3-di(1,2-diphenyl)-1H-benzimidazole-7-tert-butylpyrene (compound B), and 1,3,6,8-tetra(1,2-diphenyl)-1H-benzimidazolepyrene (compound C) were synthesized and characterized for possible applications as blue OLED emitters. The specific molecular design targeted decreasing intermolecular aggregation and disrupting crystallinity in the solid-state, in order to reduce dye aggregation, and thus obtain efficient pure blue photo- and electroluminescence. Accordingly, the new compounds displayed reasonably high spectral purity in both solution- and solid-states with average CIE coordinates of (0.160 ± 0.005, 0.029 ± 0.009) in solution and (0.152 ± 0.007, 0.126 ± 0.005) in solid-state. These compounds showed a systematic decrease in degree of crystallinity and intermolecular aggregation due to increasing steric hindrance, as revealed using powder X-ray diffraction analysis and spectroscopic studies. An organic light-emitting diode (OLED) prototype fabricated using compound B as the non-doped emissive layer displayed an external quantum efficiency (EQE) of 0.35 (±0.04)% and luminance 100 (±6) cd m⁻² at 5.5 V with an essentially pure blue electroluminescence corresponding to CIE coordinates of (0.1482, 0.1300). The highest EQE observed from this OLED prototype was 4.3 (±0.3)% at 3.5 V, and the highest luminance of 290 (±10) cd m⁻² at 7.5 V. These values were found comparable to characteristics of the best pure blue OLED devices based on simple fluorescent small-molecule organic chromophores.     

Copolyphenylenes with pendant benzimidazolyl and diethanolaminohexyloxy groups: Synthesis and electron‐transporting application in PLEDs

Two new electron‐transporting copolyphenylenes P1NH and P2NH possessing balanced charges crucial to emission efficiency of polymer light‐emitting diodes (PLEDs) have been synthesized and applied as an electron‐transporting layer (ETL). The main chain structure is all para‐linkage for P1NH and both para‐ and meta‐linkage for P2NH , with the same pendant electron‐withdrawing benzimidazolyl and polar diethanolaminohexyloxy groups. Both copolymers possess excellent thermal stability (T _d > 300 °C, T _g > 100 °C) due to their rigid backbones. In addition, the pendant groups effectively lower LUMO (～ ?2.70 eV) and HOMO (～ ?5.70 eV) levels, resulting in improved electron‐transporting and hole‐blocking capabilities. Multilayer yellow‐emitting PLEDs with a configuration of ITO/PEDOT:PSS/SY/ETL/LiF/Al were successfully fabricated by the spin‐coating process. The maximum luminance and maximum current efficiency of the P1NH ‐based device were 12,881 cd/m² and 10.94 cd/A, respectively, superior to the performance of P2NH ‐based device (4938 cd/m², 3.70 cd/A) and the device without ETL (8690 cd/m², 2.78 cd/A). Current results indicate that P1NH is highly effective in enhancing electron transport and device performance. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017 , 55 , 2494–2505     

Hermetic Seal of Organic Light Emitting Diode with Glass Frit

The components of OLED encapsulation with hermetic sealing and a 1026-day lifetime were measured by PXI-1033. The optimal characteristics were obtained when the thickness of the TPBi layer was 20 nm. This OLED obtained a maximum luminance (Lmax) of 25,849 cd/m² at a current density of 1242 mA/cm², an external quantum efficiency (EQE) of 2.28%, a current efficiency (CE) of 7.20 cd/A, and a power efficiency (PE) of 5.28 lm/W. The efficiency was enhanced by Lmax 17.2%/EQE 0.89%/CE 42.1%/PE 41.9%. The CIE coordinates of 0.32, 0.54 were all green OLED elements with wavelengths of 532 nm. The shear strain and leakage test gave results of 16 kgf and 8.92 × 10⁻⁹ mbar/s, respectively. The reliability test showed that the standard of MIL-STD-883 was obtained.     

Multifunctional zig-zag-shaped D-π-A-π-D emitters with high conjugation extent for blue FOLEDs and host for PHOLEDs

We have synthesized zig-zag shaped, meta- and para-linked D-π-A-π-D blue emitters, m-BTPAPy and p-BTPAPy based on a non-symmetrical connection strategy of two identical π-conjugated groups. The phenanthrimidazole moiety coupled to pyridine via naphthyl spacer by para- and meta-linking modes. Both m-BTPAPy (T_d/T_g, °C: 564/281) and p-BTPAPy (T_d/T_g, °C: 502/246) exhibit excellent thermal stability and can form a stable amorphous film. Changing the connection strategy from para to meta mode, m-BTPAPy shows deep blue emission with CIE (0.15, 0.07). The highly twisted m-BTPAPy exhibit higher Photoluminescence quantum yield (PLQY)_s/f of 0.98/0.85 than p-BTPAPy (0.95/0.80) owing to the suppression of intermolecular stacking. The non-doped blue device (BOLEDs) with multifunctional m-BTPAPy/p-BTPAPy show external quantum efficiency (EQE) of 7.12/5.12% with small roll-off efficiency of 1.68/2.14%, power efficiency (PE) of 5.92/5.42 lm/W, the luminance of 58675/76234 cd/m², and current efficiency (CE) of 6.12/5.86 cd/A. The non-doped device using m-BTPAPy/p-BTPAPy as both emitting and electron-transporting material exhibit luminance of 40671/49539 cd/m², CE of 5.01/5.08 cd/A, PE of 4.68/4.76 lm/W, EQE of 6.12/4.81%, roll-off efficiency of 1.63/1.87%, and CIE (0.15, 0.10)/(0.15, 0.11). These bipolar materials with high triplet energy were employed as hosts in green and red PhOLEDs. The green (m-BTPAPy: Ir(ppy)₃)/red device (m-BTPAPy: Ir(MDQ)₂(acac)) exhibit maximum EQE of 29.85/20.09%, luminance of 79523/42412 cd/m², CE of 78.62/27.56 cd/A, and PE of 72.36/23.86 lm/W, and CIE (0.33, 0.60)/(0.65,0.33).     

Electroluminescent properties of a partially‐conjugated hyperbranched poly(p‐phenylene vinylene)

In this paper, the electroluminescent properties of a new partially‐conjugated hyperbranched poly (p‐phenylene vinylene) (HPPV) were studied. The single layer light‐emitting device with HPPV as the emitting layer emits blue‐green light at 496 nm, with a luminance of 160 cd/m² at 9 V, a turn‐on voltage of 4.3 V and an electroluminescent efficiency of 0.028 cd/A. By doping an electron‐transport material [2‐(4‐biphenylyl)‐5‐phenyl‐1,3,4‐oxadiazole, PBD] into the emitting layer and inserting a thin layer of tris(8‐hydroxy‐quinoline)aluminum (Alq₃) as electron transporting/hole blocking layer for the devices, the electroluminescent efficiency of 1.42 cd/A and luminance of 1700 cd/m² were achieved. The results demonstrate that the devices with the hyperbranched polymers as emitting material can achieve high efficiency through optimization of device structures. Copyright © 2006 John Wiley & Sons, Ltd.     

Electroluminescence of a bichromophoric molecule with a benzoxyazolylcoumarin and carbazole moiety

The electroluminescent properties of a bichromophoric molecule in which a benzoxyazolylcoumarin and carbazole moiety is combined with 1,2-ethylene linkage, i.e. 3-(2-benzoxyazolyl)-7-[2-(9-carbazolyl)ethoxy]-coumarin (CmCz), were investigated. CmCz exhibits fluorescence of different colors in a solid state and solution. Two types of device were made. One consisted of a vacuum vapor-deposited film of CmCz as an emission layer to utilize fluorescence in the solid state; the second consisted of a spin-cast film doped with CmCz as an emission material to utilize fluorescence in the solution. The device with a vapor-deposited CmCz film between electrodes shows a green emission with a luminance of less than 10⁻² cd/m². The multiple layer device in which the CmCz film was sandwiched between a hole transport layer and electron transport layer showed a green emission whose spectrum is identical to the photoluminescent spectrum in the vapor-deposited CmCz film. A maximum luminance of the multiple layer device is about 5000 cd/m². On the other hand, the devices consisting of a spin-cast film containing a hole transport material, an electron transport material and CmCz showed a blue emission whose spectra are identical to the photoluminescent spectrum of CmCz in chloroform. Luminance of these devices is over 100 cd/m². © 1997 John Wiley & Sons, Ltd.     

Phosphine Sulfide-Based Bipolar Host Materials for Blue Phosphorescent Organic Light-Emitting Diodes

Three phosphine sulfide-based bipolar host materials, viz CzPhPS, DCzPhPS, and TCzPhPS, were facilely prepared through a one-pot synthesis in excellent yields. The developed hosts exhibit superior thermal stabilities with the decomposition temperatures (T_d) all exceeding 350 °C and the melting temperatures (T_m) over 200 °C. In addition, their triplet energy (E_T) levels are estimated to be higher than 3.0 eV, illustrating that they are applicable to serve as hosts for blue phosphorescent organic light-emitting diodes (PhOLEDs). The maxima luminance, current efficiency (CE), power efficiency (PE), and external quantum efficiency (EQE) of 17,223 cd m⁻², 36.7 cd A⁻¹, 37.5 lm W⁻¹, and 17.5% are achieved for the blue PhOLEDs hosted by CzPhPS. The PhOLEDs based on DCzPhPS and TCzPhPS show inferior device performance than that of CzPhPS, which might be ascribed to the deteriorated charge transporting balance as the increased number of the constructed carbazole units in DCzPhPS and TCzPhPS molecules would enhance the hole-transporting ability of the devices to a large extent. Our study demonstrates that the bipolar hosts derived from phosphine sulfide have enormous potential applications in blue PhOLEDs, and the quantity of donors should be well controlled to exploit highly efficient phosphine sulfide-based hosts.     

The Effect of Controlled Dopant Concentration on the Performance of Blue Polymer Light‐emitting Diodes

The performance of a blue polymer light‐emitting diodes (PLED) was significantly improved by doping a controlled amount (<1%) of a hole transport molecule N,N′‐bis‐(1‐naphthyl)‐N,N′‐diphenyl‐1,1′‐biphenyl‐4,4″‐diamine (NPB) into the emitting layer. Hole carrier mobility of the blue emitting polymer, BP105 (trade name of The Dow Chemicals Co.), increased from 5.27 × 10^‐7 cm^‐2/Vs of the pristine BP105 to 1.80 × 10^‐6 cm^‐2/Vs with the addition of 1% NPB in BP105. The enhanced carrier mobility greatly promoted performance of a blue PLED device with a device structure of ITO/PEDOT:PSS/BP105+x% NPB/LiF/Ca/Al. Luminance increased from 573 cd/m² to 2,720 cd/m² at 6V and efficiency increased from 1.1 lm/W to 1.6 lm/W at 1,000 cd/m² with 1% NPB in BP105. The most important improvement was an increase in the lifetime of the blue device from 80 to 120 hours at an initial luminance of 400 cd/m². We found that by choosing the appropriate dopant with good energy alignment and controlled dopant concentration, the performance of a blue PLED device could be greatly improved.     

Improved stability of molecular organic EL devices

Influence of emission sites on emission efficiency and running durability of molecular organic electroluminescent devices was investigated. When fluorescent dye rubrene was doped into TPD diamine hole transport layer, rubrene molecule emitted bright yellow light with a max. luminance of 61 000 cd/m². The device had the highest efficiency of 9.8 Im/W at 100 cd/m². Half-decay time of the device from initial luminance of 530 cd/m² under constant direct current was 3 600 h. On the other hand, when rubrene was doped into BeBq₂ complex electron transport layer, the efficiency was 4.4 Im/W and the half-decay time was 110 h. Thus doping site is found to exert significant influence both on the emission efficiency and the running durability.     

Synthesis,characterization, and electroluminescent performance of a novel copolyfluorene containing pendant crown ether groups

Copolyfluorene PFC containing pendant crown ether moieties was prepared by the palladium‐catalyzed Suzuki coupling reaction. The photo‐physical and electrochemical properties were investigated by absorption, photoluminescence (PL) spectroscopy, and cyclic voltammetry to elucidate the influence of the crown ether groups. In film state, its PL spectra (peaked at 430 and 452 nm) show noticeable red‐shift relative to 423 and 448 nm of poly(9,9‐dihexylfluorene) ( PF ). Thermal annealing leads to appearance of new emission at about 520 nm which has been attributed to formation of excimer. The highest occupied molecular orbital and lowest unoccupied molecular orbital levels of PFC were estimated to be ?5.68 and ?2.65 eV which contributed to balanced charges injection. Double‐layer electroluminescent device using PFC as emitting layer (ITO/PEDOT:PSS/ PFC /Ca/Al) revealed maximum luminance (7910 cd/m²) and maximum luminance efficiency (2.3 cd/A) superior to those of PF device (860 cd/m², 0.29 cd/A). Moreover, inserting a PFC layer between the PF emitting layer and calcium cathode led to reduced turn‐on voltage (4.1 V), much lower than 7.1 and 6.6 V of the double‐layer PFC and PF devices, respectively, and enhanced device performance (2800 cd/m² and 0.53 cd/A). © 2009 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 47: 2985–2995, 2009     

Tetradentate cyclometalated platinum complex enables high-performance near-infrared electroluminescence with excellent device stability

Due to the high decay rate of the non-radiative transition of long wavelengths, the molecular design of efficient and stable near-infrared (NIR) electroluminescent materials remains a big challenge. Herein, a new tetradentate cyclometalated platinum(II) complex with an N^∧C^∧C^∧N coordinated framework has been developed and used as a dopant for NIR organic light-emitting diodes (OLEDs). The complex exhibited a short-lived (0.5–1.5 µs) metal-to-ligand charge transfer (MLCT) excited state in doped and neat films. The resulting NIR OLEDs (λ_EL = 730 nm) achieved maximum external quantum efficiency (EQE_max) of 5.2% and radiance of 74626 mW sr^?1 m^–2. Of note, the device exhibited excellent stability with operational lifetime of 119 h for LT₉₀. This work demonstrated the great potential of tetradentate platinum(II) complexes in the field of NIR OLEDs.     

A CuO Nanowire‐Based Alternating Current Oxide Powder Electroluminescent Device with High Stability

An AC‐driven powder electroluminescent (EL) device has been achieved by constructing a CuO nanowire–Zn₂GeO₄:Mn phosphor heterogeneous junction. The CuO nanowires enhance the local electric field, resulting in electroluminescence of an oxide‐based phosphor in EL devices owing to field injection at the nanowire tips. The CuO nanowire array was synthesized by an in situ thermal oxidation method at 400 °C in air and employed as an electric field enhancement layer in the EL device. The heterogeneous structures were created through drop coating of a phosphor suspension on the CuO nanowire array. The initial EL device tests show good luminescent performance with very promising brightness maintenance for over 360 h, with a loss of luminescent intensity of under 1 % at over 10 cd m^?2 luminance. The fabrication method offers the prospect of simple, low‐cost, large‐scale EL devices with the potential to solve the limited operational lifetime of sulfide‐based AC powder EL devices.     

Charge transport polymers for organic electroluminescent devices

Triple-layer-type organic electroluminescent devices were fabricated using charge-transporting poly(N-vinylcarbazole) (PVK) as a hole-transporting emitter layer. Electron-transporting layers consisting of a triazole derivative (TAZ) and an aluminum complex (Alq) layer were used to maximize the carrier recombination efficiency. The EL device with a structure of glass substrate/indium-tinoxide/PVK/TAZ/AIq/Mg:Ag showed bright blue emission from the PVK layer with a luminance of over 700 cd/m². The emission color was tuned to a desirable color in the visible region through doping the PVK layer with fluorescent dyes. Bright white emission, in particular, was obtained for the first time at a high luminance level of over 3000 cd/m² by using three kinds of fluorescent dyes each emitting red, green or blue.     

A novel oligothiophene derivative as a hole‐transport with emitting material for OLED

A novel oligothiophene derivative containing the triphenylamine moiety with high glass transition temperature (T_g; 135 °C), 5,5′‐{bis[4‐di(4‐thiophenyl)amino]phenyl}‐2,2′‐bithiophene (TTPA‐dimer) was synthesized by the dimerization of tris[4‐(2‐thienyl)phenyl]amine (TTPA) with a palladium catalysis. Some types of electroluminescent (EL) devices that use the amorphous material for a hole‐ and an electron‐transporting with an emitting layer were fabricated. These devices emitted a bright green‐yellowish light (λ_emi; around 510 nm) with a small full width at half maximum (FWHM) rather than that of Alq₃. The single layer EL device showed a maximum luminance of 221 cd/m² at 8 V (0.06 lm/W at 100 cd/m²). On the other hand, the double layer (TTPA‐dimer/Alq₃) EL device that used Alq₃ as the electron transport material was increased up to 10830 cd/m² at 12 V (0.89 lm/W at 300 cd/m²) and with a lower turn‐on voltage (3.2 V at 0.1 cd/m²) than other types of EL devices. Copyright © 2003 John Wiley & Sons, Ltd.     

Hybrid Passivated Red Organic LEDs with Prolonged Operation and Storage Lifetime

In addition to mobile and TV displays, there is a trend of organic LEDs being applied in niche markets, such as microdisplays, automobile taillights, and photobiomodulation therapy. These applications mostly do not require to be flexible in form but need to have long operation lifetimes and storage lifespans. Using traditional glass encapsulation may not be able to fulfill the rigorous product specification, and a hybrid encapsulation method by combining glass and thin-film encapsulation will be the solution. Conventional thin-film encapsulation technology generally involves organic and inorganic multilayer films that are thick and have considerable stress. As a result, when subjected to extreme heat and stress, the film easily peels off. Herein, the water vapor transmission rate (WVTR) of a 2 µm silicon nitride film prepared at 85 °C is less than 5 × 10⁻⁵ g/m²/day and its stress is optimized to be 23 MPa. Red organic LEDs are passivated with the hybrid encapsulation, and the T₉₅ lifetime reaches nearly 10 years if the LED is continuously driven at an initial luminance of 1000 cd/m². In addition, a storage lifespan of over 17 years is achieved.     

Asymmetric Host Molecule Bearing Pyridine Core for Highly Efficient Blue Thermally Activated Delayed Fluorescence OLEDs

In this study, two host materials, pCzBzbCz and pCzPybCz , are synthesized to achieve a high efficiency and long lifetime of blue thermally activated delayed fluorescence organic light-emitting diodes (TADF-OLEDs). The molecular design strategy involves the introduction of a pyridine group into the core structure of pCzPybCz as an electron-withdrawing unit, and an electron-donating phenyl group into the structure of pCzBzbCz . These host materials demonstrate good thermal stability and high triplet energy (T₁=3.07 eV for pCzBzbCz and 3.06 eV for pCzPybCz ) for the fabrication of blue TADF-OLEDs. In particular, pCzPybCz -based OLED devices demonstrate an external quantum efficiency (EQE) of 22.7 % and an operational lifetime of 24 h (LT₉₀, time to attain 90 % of initial luminance) at an initial luminance of 1000 cd m⁻². This superior lifetime could be explained by the C−N bond dissociation energy (BDE) in the host molecular structure. Furthermore, a mixed-host system using the electron-deficient 2,4-bis(dibenzo[b,d]furan-2-yl)-6-phenyl-1,3,5-triazine (DDBFT) is proposed to inhibit the formation of the anion state of our host materials. In short, the device operational lifetime is further improved by applying DDBFT. The carbazole-based asymmetric host molecule containing a pyridine core realizes a high-efficiency blue TADF-OLED showing a positive effect on the operating lifetime, and can provide useful strategies for designing new host materials.     

Electrophosphorescent Heterobimetallic Oligometallaynes and Their Applications in Solution‐Processed Organic Light‐Emitting Devices

By combining the iridium(III) ppy‐type complex (Hppy=2‐phenylpyridine) with a square‐planar platinum(II) unit, some novel phosphorescent oligometallaynes bearing dual metal centers (viz. Ir^III and Pt^II) were developed by combining trans‐[Pt(PBu₃)₂Cl₂] with metalloligands of iridium possessing bifunctional pendant acetylene groups. Photophysical and computational studies indicated that the phosphorescent excited states arising from these oligometallaynes can be ascribed to the triplet emissive Ir^III ppy‐type chromophore, owing to the obvious trait (such as the longer phosphorescent lifetime at 77 K) also conferred by the Pt^II center. So, the two different metal centers show a synergistic effect in governing the photophysical behavior of these heterometallic oligometallaynes. The inherent nature of these amorphous materials renders the fabrication of simple solution‐processed doped phosphorescent organic light‐emitting diodes (PHOLEDs) feasible by effectively blocking the close‐packing of the host molecules. Saliently, such a synergistic effect is also important in affording decent device performance for the solution‐processed PHOLEDs. A maximum brightness of 3 356 cd m^?2 (or 2 708 cd m^?2), external quantum efficiency of 0.50 % (or 0.67 %), luminance efficiency of 1.59 cd A^?1 (or 1.55 cd A^?1), and power efficiency of 0.60 Lm W^?1 (or 0.55 Lm W^?1) for the yellow (or orange) phosphorescent PHOLEDs can be obtained. These results show the great potential of these bimetallic emitters for organic light‐emitting diodes.     

Synthesis and blue light-emitting properties of 4,4＇-bis（diphenylamino）-quinque（p-phenyl）s

An excellent organic blue light-emitting diode based on 4,4＇-bis（diphenylamino）-quinque（p-phenylene）s （OPP（5）-NPh） with a maximum luminance of up to 5000 cd/m^2 and a luminanous efficiency of 1.3 cd/A was reported. This diode was made by using a wide band-gap hole-blocking layer, F-TBB instead of PBD in the OLED devices. We attribute the good performance to the one trade-off involved in the use of F-TBB to obtain higher luminance is the increased turn-on voltages and slightly decreased device efficiencies.     

Organic electroluminescent devices using europium complex‐doped poly(N‐vinylcarbazole)

Electroluminescence (EL) properties of europium (Eu) complex‐doped poly(N‐vinylcarbazole) (PVK) were investigated. A device structure of glass substrate/indium‐tin oxide/hole‐injection layer/Eu complex‐doped PVK/hole‐blocking layer/electron transport layer/electron‐injection layer/Al was employed. Red emission originating from Eu complex was observed. Relatively high luminance of 50 cd/m² and an efficiency of 0.2% were obtained. Copyright © 2004 John Wiley & Sons, Ltd.