An efficient material via meta-position connection as thermally activated delayed fluorescence emitter for organic light-emitting diodes

A novel compound was designed and synthesized by connecting a dicyanobenzene acceptor and two 9,9-dimethyl-9,10-dihydroacridine donors to the 1,3,5-position of a phenyl ring by meta-position connection. This compound, which is a novel emitter for OLED devices, exhibits preferable heat stability. Moreover, the energy gap between its singlet and triplet states is as small as 0.04 eV, resulting in this molecule possesses thermally activated delayed fluorescence. Therefore, the corresponding device showed efficient electroluminescent performances. The maximum external quantum efficiency, maximum current efficiency, maximum power efficiency and maximum luminance were 16.5%, 40.8 cd A^?1, 45.8 lm W^?1 and 5120 cd m^?2, respectively. In addition, the CIE_x,y only changed from (0.22, 0.38) to (0.22, 0.39) over the entire operating voltage range, which confirms that the device possesses highly stable chromaticity with respect to the current density. Based on these experimental results, meta-connected type structures may provide a new approach for developing high-performance TADF emitters for OLED applications.     

A solution-processable phosphonate functionalized deep-blue fluorescent emitter for efficient single-layer small molecule organic light-emitting diodes

Based on a p-type scaffold, a novel solution-processable phosphonate functionalized deep-blue fluorescent emitter has been designed and synthesized. The corresponding non-doped single-layer SMOLED shows a peak current efficiency of 0.76 cd A(-1) with CIE coordinates of (0.15, 0.09), which is about three orders of magnitude higher than that of the prototype with tert-butyl substituents.     

Organometallic complexes as hole-transporting materials in organic light-emitting diodes

The use of metal complexes fac-tris(1-phenylpyrazolato-N,C(2)('))cobalt(III) [fac-Co(ppz)(3)], fac-tris(2-phenylpyridinato-N,C(2)(') cobalt(III) [fac-Co(ppy)(3)], and [tris[2-((pyrrole-2-ylmethylidene)amino)ethyl]amine]gallium(III) [Ga(pma)] as materials for hole-transporting layers (HTL) in organic light-emitting diodes (OLEDs) is reported. Co(ppz)(3) and Co(ppy)(3) were prepared by following literature procedures and isolated as mixtures of facial (fac) and meridional (mer) isomers. The more stable fac isomers were separated from the unstable mer forms via column chromatography and thermal gradient sublimation. Crystals of fac-Co(ppz)(3) are monoclinic, space group P2(1)/c, with a = 13.6121(12) A, b = 15.5600(12) A, c = 22.9603(17) A, beta = 100.5 degrees, V = 4781.3(7) A(3), and Z = 8. [Tris[2-((pyrrol-2-ylmethylidene)amino)ethyl]amine]gallium [Ga(pma)] was prepared by the reaction of gallium(III) nitrate with the pmaH(3) ligand precursor in methanol. Ga(pma) crystallizes in the cubic space group I3d with cell parameters a = 20.2377(4) A, b = 20.2377(4) A, c = 20.2377(4) A, beta = 90.0 degrees, V = 8288.6(3) A(3), and Z = 16. These cobalt and gallium complexes are pale colored to colorless solids, with optical energy gaps ranging 2.6-3.36 eV. A two-layer HTL/ETL (ETL = electron-transporting layer) device structure using fac-Co(ppz)(3) and fac-Co(ppy)(3) as the HTL does not give efficient electroluminescence. However, the introduction of a thin layer of a hole-transporting material (N,N'-bis(1-naphthyl)-N,N'-diphenylbenzidine, NPD) as an energy "stair-step" and electron/exciton-blocker dramatically improves the device performance. Both fac-Co(ppz)(3) and fac-Co(ppy)(3) devices give external quantum efficiencies higher than 1.0%, with brightness 5000 and 7000 Cd/m(2) at 10 V, respectively. Ga(pma) also functions as an efficient interface layer, giving device performances very similar to those of analogous devices using NPD as the interface layer. Stability tests have been carried out for Co(ppz)(3)/NPD/Alq(3) and Co(ppy)(3)/NPD/Alq(3) devices. While fac-Co(ppy)(3) gave stable OLEDs, the fac-Co(ppz)(3)-based devices had very short lifetimes. On the basis of the experimental results of chemical oxidation of fac-Co(ppz)(3), the major cause for the fast decay of the fac-Co(ppz)(3) device is proposed to be the decomposition of fac-Co(ppz)(3)(+) in the HTL layer during the device operation.     

Designing interfaces that function to facilitate charge injection in organic light-emitting diodes

Layer-by-layer (LbL) assembly of triarylamine (TAA)-containing polymers has been applied for anode functionalizations in organic light-emitting diodes (OLEDs). Surface work function of the ITO electrodes was significantly altered with the functionalizations, and the values changed depending on electron affinity of the substituents (X) on the TAA units. When the functionalized ITO electrodes were utilized for the conventional TPD/Alq OLED, the multilayers of P1 (X = 4-OMe) and P2 (X = none) were found to promote better energy matching at the ITO/TPD interface to reduce the hole injection barrier. Furthermore, the multilayers having heterodeposited structure of several TAA polymers provided stepped and graded electronic profiles to facilitate hole mobility from ITO to TPD, so that the resulting OLED devices can exhibit appreciably reduced turn-on voltage and higher luminous intensities.     

Quinoline-based aggregation-induced delayed fluorescence materials for highly efficient non-doped organic light-emitting diodes

Three new emitters,namely 10,10'-(quinoline-2,8-diyl)bis(10 H-phenoxazine)(Fene),10,10'-(quinoline-2,8-diyl)bis(10 H-phenothiazine)(Fens) and 10,10'-(quinoline-2,8-diyl)bis(9,9-dimethyl-9,10-dihydroacridine)(Yad),featuring quinoline as a new electron acceptor have been designed and conveniently synthesized.These emitters possessed small singlet-triplet splitting energy(ΔEst) and twisted structures,which not only endowed them show thermally activated delayed fluorescence(TADF)properties but also afforded a remarkable aggregation-induced emission(AIE) feature.Moreover,they also showed aggregation-induced delayed fluorescence(AIDF) property and good photoluminescence(PL) property,which are the ideal emitters for non-doped organic light-emitting diodes(OLEDs).Furthermore,high-performance non-doped OLEDs based on Fene,Fens and Yad were achieved,and excelle nt maximum external quantum efficiencies(EQE_(max)) of 14,9%,13.1% and 17,4%,respectively,were obtained.It was also found that all devices exhibited relatively low turn-on voltages ranging from 3.0 V to3.2 V probably due to their twisted conformation and the AIDF properties.These results demonstrated the quinoline-based emitters could have a promising application in non-doped OLEDs.     

Design rules for charge-transport efficient host materials for phosphorescent organic light-emitting diodes

The use of blue phosphorescent emitters in organic light-emitting diodes (OLEDs) imposes demanding requirements on a host material. Among these are large triplet energies, the alignment of levels with respect to the emitter, the ability to form and sustain amorphous order, material processability, and an adequate charge carrier mobility. A possible design strategy is to choose a π-conjugated core with a high triplet level and to fulfill the other requirements by using suitable substituents. Bulky substituents, however, induce large spatial separations between conjugated cores, can substantially reduce intermolecular electronic couplings, and decrease the charge mobility of the host. In this work we analyze charge transport in amorphous 2,8-bis(triphenylsilyl)dibenzofuran, an electron-transporting material synthesized to serve as a host in deep-blue OLEDs. We show that mesomeric effects delocalize the frontier orbitals over the substituents recovering strong electronic couplings and lowering reorganization energies, especially for electrons, while keeping energetic disorder small. Admittance spectroscopy measurements reveal that the material has indeed a high electron mobility and a small Poole-Frenkel slope, supporting our conclusions. By linking electronic structure, molecular packing, and mobility, we provide a pathway to the rational design of hosts with high charge mobilities.     

Synthesis and properties of a thiophene-substituted diaza[7]helicene for application as a blue emitter in organic light-emitting diodes

Carbazole-based diaza[7]helicene substituted by thiophene groups, 2,12-dithiophene-5,15-dihexyl-5,15-diaza[7]helicene (6), was synthesised successfully and confirmed by ¹H NMR, ¹³C NMR, High Resolution Mass Spectrometry, Time of Flight Mass Spectrometry. Compound 6 exhibited good solubility and excellent thermal stability with no melting point and a high decomposition temperature of 453.64 °C. A doped device with a structure of ITO/NPB (50 nm)/CBP: 10% 6 (30 nm)/Bphen (20 nm)/Mg:Ag (150 nm)/Ag (50 nm) emitted the blue light at 460 nm with Commission Internationale de LEclairage (CIE) coordinate of (0.176, 0.26). The maximum brightness and external quantum efficiency (EQE) were 2306 cd m^?2 and 0.41%, respectively.     

Carbazole compounds as host materials for triplet emitters in organic light-emitting diodes: polymer hosts for high-efficiency light-emitting diodes

A carbazole homopolymer and carbazole copolymers based on 9,9'-dialkyl-[3,3']-bicarbazolyl, 2,5-diphenyl-[1,3,4]-oxadiazole and 9,9-bis(4-[3,7-dimethyloctyloxy]phenyl)fluorene were synthesized and their electrical and photophysical properties were characterized with respect to their application as host in phosphorescent polymer light-emitting diodes. It is shown that the triplet energy of a polymer depends on the specific connections between its building blocks. Without changing the composition of the polymer, its triplet energy can be increased from 2.3 to 2.6 eV by changing the way in which the different building blocks are coupled together. For poly(9-vinylcarbazole) (PVK), a carbazole polymer often used as host for high-energy triplet emitters in polymer light-emitting diodes, a large hole-injection barrier of about 1 eV exists due to the low-lying HOMO level of PVK. For all carbazole polymers presented here, the HOMO levels are much closer to the Fermi level of a commonly used anode such as ITO and/or a commonly used hole-injection layer such as PEDOT:PSS. This makes high current densities and consequently high luminance levels possible at moderate applied voltages in polymer light-emitting diodes. A double-layer polymer light-emitting diode is constructed comprising a PEDOT:PSS layer as hole-injection layer and a carbazole-oxadiazole copolymer doped with a green triplet emitter as emissive layer that shows an efficacy of 23 cd/A independent of current density and light output.     

Quinolinate zinc complexes as electron transporting layers in organic light-emitting diodes

We describe the synthesis and properties of a bis(8-quinolinolato-N¹,O⁸)-zinc(II) complex (Znq₂) which can be used as an efficient electron transfer layer in organic light-emitting diodes. In this material, the electrons are much more mobile at room temperature than in the well-known tris(8-quinolinolato-N¹,O⁸)-aluminium(III) complex (Alq₃). Preliminary results concerning a series of related compounds are presented.     

白光有机发光二极管的最新进展

有机发光二极管(Organic Light-Emitting Diodes,OLEDs)以其制备工艺简单、成本低、发光颜色可在可见光区内任意调节以及易于大面积制作和柔韧弯曲等优点,被认为是未来重要的显示技术之一,在未来照明光源领域也显示了诱人的应用前景.一般认为,如果OLED的发光效率超过100 lm/W,就有可能取代一般照明.本文综述了实现白光OLED的方法及其最新进展,并对白光OLED存在的问题及其发展趋势进行了讨论.     

Platinum-functionalized random copolymers for use in solution-processible, efficient, near-white organic light-emitting diodes

A near-white light-emitting polymer, consisting of hole transport (HT), electron transport (ET), and phosphorescent Pt functionalities, was prepared and utilized as the emissive layer in solution-processed OLEDs. The air-stable devices had a maximum external quantum efficiency of 4.6%.     

Fused-seven-ring anthracene derivative with two sulfur bridges for high performance red organic light-emitting diodes

A fused-seven-ring anthracene derivative with two sulfur bridges, benzobisthioxanthene (BTA), was synthesized, facilely. OLEDs employing BTA as the emitter exhibited bright (maximum 40,752 cd m(-2)) and efficient red emission (CIE, x = 0.64, y = 0.36) with a luminous efficiency of 4.4 cd A(-1).     

A robust yellow-emitting metallophosphor with electron-injection/-transporting traits for highly efficient white organic light-emitting diodes

With the aim of endowing triplet emitters in the development of organic light-emitting devices (OLEDs) with electron-injection/-transporting (EI/ET) features, the phenylsulfonyl moiety was introduced into the phenyl ring of a 2-phenylpyridine (Hppy) ligand and the yellow phosphorescent heteroleptic iridium(III) complex 1 was developed. It was shown that the SO(2)Ph unit could provide EI/ET character to 1, as indicated from both electrochemical and computational data. Complex 1 is a promising yellow-emitting material for both monochromatic OLEDs and white OLEDs (WOLEDs). The outstanding electronic traits associated with 1, coupled with careful device design, afforded very attractive electroluminescent performances for two-element WOLEDs, including a low turn-on voltage of less than 3.7 V, a maximum brightness of 48,000 cd m(-2), an external quantum efficiency of 13.0%, a luminance efficiency of 34.7 cd A(-1), and a power efficiency of 24.3 Lm W(-1). In addition, a good color rendering index (CRI) of about 74, a stable white color with a Commission Internationale de L'Eclairage (CIE(x,y)) variation of Δ(x, y) < ±(0.02, 0.02), and a correlated color temperature higher than 5130 K were obtained. These encouraging results indicate the potential of these WOLEDs as good candidates for warm indoor lighting sources, as well as the critical contribution of such key EI/ET properties to triplet emitters to advance new OLED research.     

Rare-earth metal 8-hydroxyquinolinate complexes as materials for organic light-emitting diodes

Rare-earth metal 8-hydroxyquinolinates (q) were studied as emission layers for organic light-emitting diodes. Compounds Dyq₃, Tbq₃, and Smq₃ possess weak electroluminescence due to the emission of the ligand, and the electroluminescence of Hoq₃, Tmq₃, and Ybq₃ is due to electronic transitions of the lanthanide ion. The best electroluminescence characteristics were demonstrated by Scq₃ and Yq₃. The power efficiency of the organic light emitter based on ITO/TPD/Scq₃/Yb (ITO is tin-alloyed indium oxide, TPD is N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine) is 2.6 lm W⁻¹ at a luminance of the device of 300 cd m⁻².     

Highly efficient green organic light-emitting diodes containing luminescent three-coordinate copper(I) complexes

A series of highly emissive three-coordinate copper(I) complexes, (dtpb)Cu(I)X [X = Cl (1), Br (2), I (3); dtpb =1,2-bis(o-ditolylphosphino)benzene], were synthesized and investigated in prototype organic light-emitting diodes (OLEDs). 1-3 showed excellent photoluminescent performance in both degassed dichloromethane solutions [quantum yield (Φ) = 0.43-0.60; lifetime (τ) = 4.9-6.5 μs] and amorphous films (Φ = 0.57-0.71; τ = 3.2-6.1 μs). Conventional OLEDs containing 2 in the emitting layer exhibited bright green luminescence with a current efficiency of 65.3 cd/A and a maximum external quantum efficiency of 21.3%.     

Performance and defects in phosphorescent organic light-emitting diodes

Phosphorescent heavy metal complexes can utilize both singlet and triplet excitons and thus are interesting for doping polymer to obtain highly efficient organic light-emitting diodes. In this study, we have investigated devices using a new phosphorescent–metal complex containing fluorene and platinum added to a luminescent polymer blend, composed of 2-(4-biphenylyl)-5-(4-tert-butyl-phenyl)-(1,3,4-oxadiazole) (PBD) and poly(9-vinylcarbazole) (PVK). The performance of devices (luminance and yield) is measured in indium tin oxide (ITO)/poly(3-4 ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS)/(PVK–PBD-complex)/Al diodes. The devices emit an orange light with a brightness of 607 cd/m² and an external quantum efficiency of 0.28 cd/A at 25 V. In order to investigate the structural modifications of the polymer by the incorporation of phosphorescent–metal complex, we have studied the defect states in diodes by charge-based Deep Level Transient Spectroscopy (Q-DLTS). Analysis of Q-DLTS spectra obtained in undoped and doped devices, revealed at least three trap levels distributed in the range 0.2–0.5 eV within the band gap of the hybrid composite with trap density in the range around 10¹⁶ cm^?3. Incorporation of Pt complex into the polymer blend modified the trap states by reducing the density of traps in the blend and by creating new trap levels in the band gap.     

Progress in small-molecule luminescent materials for organic light-emitting diodes

Organic light-emitting diodes(OLEDs) have been extensively studied since the first efficient device based on small molecular luminescent materials was reported by Tang. Organic electroluminescent material, one of the centerpieces of OLEDs, has been the focus of studies by many material scientists. To obtain high luminosity and to keep material costs low, a few remarkable design concepts have been developed. Aggregation-induced emission(AIE) materials were invented to overcome the common fluorescence-quenching problem, and cross-dipole stacking of fluorescent molecules was shown to be an effective method to get high solid-state luminescence. To exceed the limit of internal quantum efficiency of conventional fluorescent materials, phosphorescent materials were successfully applied in highly efficient electroluminescent devices. Most recently, delayed fluorescent materials via reverse-intersystem crossing(RISC) from triplet to singlet and the "hot exciton" materials based on hybridized local and charge-transfer(HLCT) states were developed to be a new generation of low-cost luminescent materials as efficient as phosphorescent materials. In terms of the device-fabrication process, solution-processible small molecular luminescent materials possess the advantages of high purity(vs. polymers) and low procession cost(vs. vacuum deposition), which are garnering them increasing attention. Herein, we review the progress of the development of small-molecule luminescent materials with different design concepts and features, and also briefly examine future development tendencies of luminescent materials.     

Spiro-configured bifluorenes: highly efficient emitter for UV organic light-emitting device and host material for red electrophosphorescence

[structure: see text] Remarkable UV OLED devices using novel spiro-configured bifluorenes as UV emitters have been successfully achieved with external quantum efficiencies up to 2.6-3.1% and emission wavelengths as short as 370 nm. In addition, these large-gap materials have been shown to be effective hosts for red phosphorescent emitters in phosphorescent OLEDs. This result represents the first example that pure hydrocarbon molecules can serve as an efficient host for red electrophosphorescence devices.