Solution-processed white organic light-emitting diode based on a single-emitting small molecule

A pure-white organic light-emitting diode (WOLED) with a simple device architecture of indium tin oxide (ITO)/poly-(3,4-ethylenedioxythiophene):poly-(styrenesuphonic acid) (PEDOT:PSS)/L-Z-390/1,3,5-tri(N-phenylbenzimidazol-2-yl)benzene (TPBi)/LiF/Al was realized by using a single small molecule 3,3′-(2,7-Dis((9-(2,3,5,6-tetrafluorophenyl)-9H-carbazol-3-yl)ethynyl)-9H-fluorene-9,9-diyl)bis(9-heptyl-9H-carbazole) (L-Z-390) as the light-emitting material. The WOLED exhibited efficient white emission with a turn-on voltage of 6 V, a maximum luminous efficiency of 3.6 cd/A at 6 V, a maximum luminous of 204 cd/m² at 13 V and Commission Internationale de l’Eclairage (CIE) coordinates of (0.33, 0.34). The device performance characteristics are among the best ever reported for solution-processed WOLEDs with a single-emitting small molecule. The white light comes from the combination of the blue emission originating from the excimer of L-Z-390 and the long-wavelength orange emission originating from the electromer of L-Z-390 under a high electric field. The double-layer device based on 3,3′-(2,7-Dis((9-(2,3,5,6-tetrafluorophenyl)-9H-carbazol-3-yl)ethynyl)-9H-fluorene-9,9-diyl)bis(9-(2,3,5,6-tetrafluorophenyl)-9H-carbazole) (L-Z-398), the other molecule with two more electron-withdrawing 2,3,5,6-tetrafluorophenyl units than L-Z-390, shows green EL under the high voltages. That L-Z-398 cannot form electromer under a high voltage may be because the L-Z-398 cannot stably form L-Z-398⁺ component and L-Z-398^? component simultaneously due to the too strong electron-withdrawing ability of the four 2,3,5,6-tetrafluorophenyl units connected to each carbazole.     

Organic light-emitting diodes based on multilayer structures

We study various organic structures and explain the obtained improvements by the use of protective layer or hole transport layer in multilayer OLEDs based on Alq₃ as emitter.     

Doped and non-doped organic light-emitting diodes based on a yellow carbazole emitter into a blue-emitting matrix

A new carbazole derivative with a 3,3′-bicarbazyl core 6,6′-substituted by dicyanovinylene groups (6,6′-bis(1-(2,2′-dicyano)vinyl)-N,N′-dioctyl-3,3′-bicarbazyl; named (OcCz2CN)₂, was synthesized by carbonyl-methylene Knovenagel condensation, characterized and used as a component of multilayer organic light-emitting diodes (OLEDs). Due to its π-donor–acceptor type structure, (OcCz2CN)₂ was found to emit a yellow light at λ_max = 590 nm (with the CIE coordinates x = 0.51; y = 0.47) and was used either as a dopant or as an ultrathin layer in a blue-emitting matrix of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi). DPVBi (OcCz2CN)₂-doped structure exhibited, at doping ratio of 1.4 weight %, a yellowish–green light with the CIE coordinates (x = 0.31; y = 0.51), an electroluminescence efficiency η_EL = 1.3 cd/A, an external quantum efficiency η_ext = 0.4 % and a luminance L = 127 cd/m² (at 10 mA/cm²) whereas for non-doped devices utilizing the carbazolic fluorophore as a thin neat layer, a warm white with CIE coordinates (x = 0.40; y = 0.43), η_EL = 2.0 cd/A, η_ext = 0.7%, L = 197 cd/m² (at 10 mA/cm²) and a color rendering index (CRI) of 74, were obtained. Electroluminescence performances of both the doped and non-doped devices were compared with those obtained with 5,6,11,12-tetraphenylnaphtacene (rubrene) taken as a reference of highly efficient yellow emitter.     

Organic light-emitting diodes and organic light-emitting electrochemical cells based on silole–fluorene derivatives

Silole groups are known to present a high electron affinity. Initially, copolymerization of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of organic light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission colour to the green part of the spectrum and to stop the well-known spectral shift degradation occurring in fluorene-based materials. In this paper we report the synthesis and the characterisation of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two soluble conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either methyl 11a or n-hexyl 11b. Silole 4 crystallizes in the triclinic P-1 space group with a = 9.8771(8), b = 10.6240(10), c = 16.585(2) Å, α = 95.775(8), β = 97.025(7), and γ = 111.738(8)°. The results obtained with this molecule, operating in a single-layer OLED (luminance ≈450 Cd/m² at 12 V; η_max = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from organic light-emitting electrochemical cells (LEC's) made from silole–fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances electrical characterisation data and atomic force microscope (AFM) imaging were combined.     

The role of carbazole in organic light-emitting devices

New organic oligomers and polymers based on the carbazole molecule are explored for possible applications in light-emitting devices. In one case, (butyl- or octyl-) carbazole dimers and poly(N-butyl-3,6-carbazolylene) polymer were used as the hole-transporting and light-emitting layer in multilayer light-emitting diodes (LEDs). These devices yielded bright blue light (as much as about 6000 cd m⁻²) with high external quantum (about 10%) and luminance efficiencies (about 2 lm W⁻¹). The other case involved (3-octylthiophene]-[bis-(N-ethyl or octyl carbazolylene)]) multiblock copolymers as the active emitting layer in single-layer LEDs. Color tuning was achieved in these devices by changing the number of monomer units contained in the thiophene chain. We also observed an increase of the external quantum efficiency in diodes based on the copolymers with short thiophene segments that we attributed to a more balanced charge injection.     

Organic light-emitting devices (OLED) based on new triphenylamine derivatives

In this paper, we reported the synthesis of two new triphenylamine derivatives: 1,4-bis[-E-4-(N,N-diphenylamino)styryl]naphthalene (Np-G1) and 2,8-bis[-E-4-(N,N-diphenylamino) styryl]dibenzothiophene (ST-G1) and investigated the electroluminescence characteristics of the three-layer devices with Np-G1 or ST-G1 as emitting layer. The results have shown that introduction electron-acceptor group, naphthalene, into triphenylamine units, Np-G1 with linear geometric conformation can lower its LUMO level obviously and resulting facilitate electron injection and transport for the device. Thus, the three-layer device (ITO/TCTA/Np-G1/BCP /Mg:Ag) improved the electroluminescence properties the best, presenting the brightness of ～10,000 cd/m² and current efficiency of ～3.0 cd/A. On the other hand, ST-G1 with V-shaped conformation containing dibenzothiophene linked triphenylamine groups cannot effectively decreased its LUMO level. As a result, ST-G1 has little contribution to the carrier recombination within itself.     

Polymer light-emitting diode using a new trialkoxyalkyl substituted PPV derivative

A new light-emitting poly(p-phenylenevinylene) (PPV) derivative containing trialkoxyalkyl substituents (TAA) as color tunable groups was synthesized through a Gilch polymerization, and the light-emitting properties of the polymer (TAA-PPV) were investigated. The emission of TAA-PPV was found to be highly blue-shifted to the green region, with peak absorption and band edge near 399 and 490 nm, respectively. The PL emission maximum of TAA-PPV thin films was found to be at approximately 493 nm. Light-emitting devices were fabricated with the configuration ITO (indium–tin oxide)/PEDOT/polymer/Ca/Al and produced very bright green emission. This device with TAA-PPV as the emissive layer had a maximum brightness of 3600 cd/m² and a maximum power efficiency of 0.77 lm/W.     

Molecular nanocrystals in polyaniline-based light-emitting diode structures

It is found that one of the types of molecular organic nanocrystals, the so-called J-aggregates, is able to modify optoelectronic properties of electroactive polymers. It is intrinsic that the polyaniline interpolymeric complex, known as p-type semiconductor, turns into electroluminescent material in the presence of J-aggregates of 3,3-di(gamma-sulfopropyl)-5,5-dichlorotiamonomethincyanine triethylammonium salt. During the studying of new organic light-emitting diode structures, in which the polyaniline/J-aggregates polymeric composite forms both recombination and emitting layers, a polyaniline intrinsic electroluminescence spectrum is obtained. The electroluminescence band completely coincides with the polyaniline photoluminescence spectrum; it has maximum at 400 nm. The electroluminescence mechanism in the studied nanocomposites is discussed. Original Russian Text ? E.I. Mal’tsev, D.A. Lypenko, O.M. Perelygina, V.F. Ivanov, O.L. Gribkova, M.A. Brusenrseva, A.V. Vannikov, 2008, published in Fizikokhimiya Poverkhnosti i Zashchita Materialov, 2008, Vol. 44, No. 5, pp. 478–481.     

Near-IR electromer emission from new ambipolar carbazole containing phosphorescent dendrimer based organic light emitting diode

The solution process is more suitable for large-sized OLED in terms of small material usage and no need for fine mask patterning technique. Among soluble materials, dendrimers are acknowledged as a new class of emissive OLED materials that can carry both the emissive chromophore and the charge transporting moiety simultaneously. Here, we report on a new Ir(III)-based dendrimer and an efficient single emission layer OLED device by spin-coating the dendrimer itself on PEDOT:PSS polymer. This dendrimer carries a pair of carbazole functional groups on the pyridine side and a fluorinated phenyl on the other side of the Ir(ppy)₃ ligand. The latter functional group acts as an electron transporting material and the former as a host and hole-transporting material. The dendrimer device has a green emission at 540 nm with current efficiency as high as 25 Cd/A. Interestingly, we have found that the two neighboring carbazole form the electromer under the applied electrical current and, thus, that the EL device shows new near-IR emission over 720 nm.     

Performance of a polymer light-emitting diode with enhanced charge carrier mobility

The device characteristics of a polymer light-emitting diode (PLED) based on a poly(p-phenylene vinylene) (PPV) derivative with an enhanced charge carrier mobility have been investigated. Improvement of the mobility, which has been obtained by a decrease of the energetic disorder in the polymer, is expected to increase the power efficiency of a PLED. However, it is demonstrated that an increased mobility leads to a decrease as well as to a slower rise of the quantum efficiency with voltage. This performance reduction is explained in terms of an increased quenching of the electroluminescence (EL) at the cathode.     

Static and dynamic electrical investigations on AVB polymer light-emitting diode

We report investigations of current–voltage (I–V), capacitance–voltage (C–V) and impedance–frequency (Z–ω) measurements performed on ITO/1,4-bis-(9-anthrylvinyl)-benzene (AVB)/Al light-emitting diodes under different bias conditions. These measurements give information about the nature of the conduction mechanism in AVB. It was found that the current is space-charge limited with traps (SCLC) and conductivity exhibits a power low frequency dependence. Moreover, we give an equivalent circuit of the device designed as a parallel resistor R_p and capacitor C_p network in series with resistor R_s. Using experimental data, we deduce numerical values of R_p, C_p and R_s.     

Enhancement of a top emission organic light-emitting diode with a double buffer layer

We report on luminance characteristics of top emission organic light-emitting diode (TEOLED) containing transparent cathode with various structures of buffer layers. Especially, we have focused on the buffer layers preventing OLED degradation from plasma damages which occur during deposition of inorganic passivation layer. The TEOLED with a double buffer layer showed much higher luminance and lower leakage current than that with single buffer layer of organic layer or inorganic layer.     

Electrical characteristics of light-emitting diode based on poly(p-phenylenevinylene) derivatives: CzEH-PPV and OxdEH-PPV

In the light-emitting devices (LEDs) based on the π-conjugated polymers, the relationship between the quantum efficiency and the balance of hole (μ_h) and electron (μ_e) mobility has been investigated. In order to measure the μ_h and μ_e of the LEDs based on π-conjugated polymers, we fabricated the hole transport device (HTD) and the electron transport device (ETD) by using various metal electrodes with different work functions. For the materials of light emitting layer, we synthesized poly[2-(N-carbazolyl)-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (CzEH-PPV) and poly[2-{4-[5-(4-tert-butylphenyl)-1,3,4-oxadiazolyl]-phenyl}-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (OxdEH-PPV) with electron-rich groups. The poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV), which is well known material for the polymer-based LED, was synthesized for the reference. We measured the current density vs. applied field (J–E) characteristics of the HTD and ETD with various thickness at different temperatures. The results of the J–E curves were analyzed by using the space charge limited conduction (SCLC) model. Based upon the SCLC model, μ_h and μ_e of MEH-PPV sample was measured to be 10⁻⁶ cm²/V s and 10⁻⁸ cm²/V s, respectively. For CzEH-PPV and OxdEH-PPV samples with electron-rich groups, μ_h was similar to μ_e with 10⁻¹⁰–10⁻¹¹ cm²/V s. The μ_h and μ_e of CzEH-PPV and OxdEH-PPV samples was lower than that of MEH-PPV sample, but more balanced. The quantum efficiency of the LED by using CzEH-PPV or OxdEH-PPV materials was 10 times higher than that prepared from MEH-PPV. The balance of the μ_h and μ_e plays an important role for the quantum efficiency. We analyze the balance of the μ_h and μ_e and the relatively low mobilities of CzEH-PPV and OxdEH-PPV samples in terms of the heavier effective mass due to the asymmetric dipole distribution in the side chains. The results of photocurrent of the systems qualitatively agreed with the result of the electrical measurements. From AC impedance measurement of the LEDs, we observed that the relaxation time of MEH-PPV was shorter than that of OxdEH-PPV sample because of the higher mobility of MEH-PPV sample.     

Optimizing efficiency of polycrystalline p-Si anode organic light-emitting diode

Optimizing efficiencies of organic light-emitting diodes (OLEDs) with a structure of Al/glass/nanometer-thick polycrystalline p-Si (NPPS) anode/SiO₂/N′-bis-(1-naphthl)-diphenyl-1,1′-biphenyl-4,4′-diamine (NPB)/tris (8-hydroxyquinoline) aluminum (Alq₃)/4,7-diphenyl-1,10-phenanthroline (BPhen):Cs₂CO₃/Sm/Au were studied. The NPPS anodes were fabricated by magnetron sputtering (MS) Si and Ni layers followed by Ni-induced crystallization of the amorphous Si layers. By adjusting the resistivity of the p-Si target adopted in MS, the electroluminescent efficiency of the OLED was optimized. When the resistivity of the p-Si target is 0.01 Ω·cm, the current and power efficiencies of the NPPS anode OLED reach maximum values of 6.7 cd·A^?1 and 4.64 lm·W^?1, respectively, which are 2.7 and 3.1 times those of the resistivity-optimized bulk p-Si anode counterpart and 2.9 and 3.7 times those of the indium tin oxide (ITO) anode counterpart, and then, the physical reasons were discussed.     

Methylene blue is a good background stain for tuberculosis light-emitting diode fluorescence microscopy

背景:达米恩基金会孟加拉国结核病(TB)控制项目.目的:比较蓝墨水、高锰酸钾和美蓝用于透射发光二极管(LED)TB荧光显微镜(FM)的背景染色.设计:将萋-尼染色(Z-N)抗酸杆菌阳性或阴性的痰标本制成一式三份的金胺染色涂片,使用上述一种背景染液染色,而后选用LED FM进行盲法镜检.对与萋-尼染色结果不一致的涂片,由参比实验室在金胺复染前后进行复检,金胺复染结果作为金标准.结果:在评价的1977份涂片中,991(50.1％)来自Z-N阳性标本.其中用蓝墨水、高锰酸钾和美蓝进行背景染色的真阳性分别为919、942和958张涂片,而假阳性数分别为12、12和16张涂片.美蓝复染的敏感性(95.6％,95％CI:94.2～96.8)要高于蓝墨水(92.7％,95％CI: 90.9～94.3,P＜0.01)或高锰酸钾(93.6％,95％CI:91.9～95.0,P＜0.05).在180例不一致涂片中,经过或未经复染,85％的涂片未找到抗酸杆菌,15％的涂片未找到抗酸杆菌.结论:在使用透射蓝光激发的LED荧光显微镜观察时,美蓝复染至少与高锰酸钾效果相当,而比蓝墨水更便宜.对金胺染色涂片进行室间质量评估盲法复检时,需要在第一次复检前对所有涂片进行复染.     

Synthesis and characterization of light-emitting materials composed of carbazole,pyrene and fluorene

A series of ethynyl-linked π-conjugated light-emitting molecules with good photoluminescence properties were designed and synthesized through Pd/Cu-catalyzed Sonogashira coupling reaction. The main structure comprised of different fluorophors (pyrene, carbazole, and fluorene), which were linked with triple bonds in order to be screened for high emission efficiency. Optical properties of these synthesized compounds (I_a–I_f) were examined in solutions, thin films and solid states, respectively. All of them showed relative high quantum yields both in solutions (Φ_s: 0.70–0.83) and in films (Φ_f: 0.17–0.34). Light-emitting diode using I_e as an emissive layer was fabricated in the ITO/poly(3,4-ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS)/I_e/2, 2′, 2″-(1, 3, 5)-benzene-triyl)tris[1-phenyl-1H-benzimidazole] (TPBI)/Al configuration. The maximum emission wavelength of the device was 496 nm. The maximum luminance and electroluminescence efficiency was 1000 cd/m² and 0.41%, respectively.     

Organic light-emitting devices with triphenylphosphine oxide layer

We have developed organic light-emitting devices using triphenylphosphine oxide (Ph₃PO) layers. The operating voltage of device is substantially reduced by using a Ph₃PO layer. For example, the required voltages for a current density of 20 mA/cm² are 3.5 and 9.7 V for the devices with Ph₃PO and Alq₃ layers, respectively. Good electron transporting property of Ph₃PO results in a high luminance of 1000 cd/m² at a low driving voltage of 4.1 V in a device with a structure of ITO/2-TNATA (15 nm)/α-NPD:rubrene (1%, 10 nm)/α-NPD (30 nm)/Ph₃PO (60 nm)/LiF (0.5 nm)/Al.     

Organic light-emitting devices of thiophene vynilic derivatives

In this work, we present the results obtained for the polymerisation in non-aqueous media of thiophenes derivatives; [2-Tenal-acetotienone] and [1,5-bis(2-Tenal)-1,4-pentadien-3-one], via electrochemical deposition on indium tin oxide (ITO) glass electrodes. The I–V curves and electroluminescent response was evaluated. It was studied that the film thickness affects the metal/polymer interface properties during the devices operation. The results indicate that diodes and electroluminescent response occurs at 2–3 V though anomalous device behaviour can be found when the reverse bias was applied.     

The interface surfaces of a CN-substituted poly(phenylenevinylene) light-emitting diode,a morphological study

The morphology of the polymer/electrode interfaces within polymer light-emitting diodes with a poly(cyanoterephthalylidene) polymer as the active layer, poly (2,5,2′,5′-hexyloxy-8,7′-dicyano-di-para-phenylenevinylene) (CN-PPV), was studied by scanning force microscopy (SFM). The structure and roughness of (i) the substrates (indium-tin oxide (ITO)), (ii) the CN-PPV films spin-coated from different solvents and concentrations, and (iii) the top electrodes of vapour-deposited aluminium were studied. Film thicknesses were determined by ellipsometry. CN-PPV films were also studied on smooth silicon substrates, to reduce the influence of substrate roughness on film morphology, and for comparison of SFM and ellipsometry results on ITO.