Organic Electroluminescent Diodes Research Articles

Highly efficient fluorescent organic electroluminescent diodes based on doping-free phosphorescent sensitization system

Highly efficient fluorescent organic electroluminescent diodes based on doping-free phosphorescent sensitization system

Synthesis of Organic Phosphor Materials for Display Devices

An organic light emitting diode (OLED) also known as organic electroluminescent diode is a display device like light emitting diode in which an emissive electroluminescent layer is a film of organic compound material that emits light in response to an electric current. This organic layer is situated in between two electrodes; typically at least one of these electrodes is transparent. The organic molecules have conductivity levels ranging from insulators to conductors, and are therefore considered as organic semiconductors. Organic semiconductors were synthesised byone of the method known as the method of Friedlander condensation reaction. Originally, the basic polymer synthesised organic material is group of Diphenylquinoline (DPQ) family which consisted of a single organic layer for OLED. The family members of DPQ were synthesised by bonding chlorine- methyl, bromine, methyl, methoxy, P- hydroxyl, P-Acetyl-biphenyl and P- Acetyl bi-chlorine to the original structure of DPQ at various positions. All the synthesised polymers show crystalline in nature and emits blue colour under UV in various acidic as well as basic solvents like acidic acid, formic acid, chloroform, dichloromethane, tetrahydrofuran etc. The synthesised phosphors were characterised by different techniques to study physical, optical and chemical properties such as Fourier Transform infra-red (FTIR), UV- Visible absorption and photoluminescence spectra, X-Ray diffraction spectra (X-RD), Thermo gravimetric analysis (TGA) and Differential thermal analysis (DTA). All The blue emitting organic phosphors has generated considerable interest owing to their good photoluminescence efficiencies.

Heteroleptic Zinc(II) complex based stable solution-processed organic electroluminescent diodes

Synthesis, characterization, and photophysical properties of heteroleptic Zn(II) complex. The complexes (Zn-1 and Zn-2) were well-characterized by UV–Vis, fluorescence spectroscopy, phosphorescence spectroscopy, cyclic voltammetry, photoluminescence quantum yield, thermogravimetric analysis, and atomic force microscopy. Moreover, density functional theory (DFT) and time-dependent density functional theory (TD-DFT) also performed to examine the geometries and electronic structures of the ground states, transition energies and excited-state structures. The Zn-2 exhibits favorable triplet energy, thermal stability, and quantum yield compared to Zn-1. Stable solution-processed OLEDs were fabricated and Zn-2 based device exhibits an efficient bluish-green emission with a maximum EQE of 1.6% and CE of 3.2 cd/A. • Novel heteroleptic Zn(II) complexes is designed and synthesized. • Excellent photophysical and high thermal stabilities, and electroluminescent features of the complexes. • Stable solution-processed OLEDs were fabricated of synthesized Zn(II) complexes. • Stable device of Zn(II) complexes with bluish-green emission and low turn-on voltage.

Improved stability of blue TADF organic electroluminescent diodes via OXD-7 based mixed host.

Thermally activated delayed fluorescence (TADF) organic light-emitting diodes (OLEDs) have been demonstrated in applications such as displays and solid-state lightings. However, weak stability and inefficient emission of blue TADF OLEDs are two key bottlenecks limiting the development of solution processable displays and white light sources. This work presents a solution-processed OLED using a blue-emitting TADF small molecule bis[4-(9,9-dimethyl-9,10-dihydroacridine) phenyl]sulfone (DMAC-DPS) as an emitter. We comparatively investigated the effects of single host poly(N-vinylcarbazole) (PVK) and a co-host of 60% PVK and 30% 2,2'-(1,3-phenylene)-bis[5-(4-tert-butylphenyl)-1,3,4-oxadiazole] (OXD-7) on the device performance (the last 10% is emitter DMAC-DPS). The co-host device shows lower turn-on voltage, similar maximum luminance, and much slower external quantum efficiency (EQE) rolloff. In other words, device stability improved by doping OXD-7 into PVK, and the device impedance simultaneously and significantly reduced from 8.6 × 103 to 4.2 × 103 Ω at 1000 Hz. Finally, the electroluminescent stability of the co-host device was significantly enhanced by adjusting the annealing temperature.

Double layer printed high performance OLED based on PEDOT:PSS/Ir(bt)2acac:CDBP

The preparation of organic electroluminescent diodes (OLEDs) by printing with the potential advantages of high material utilization, low equipment cost, simple process flow, and favorable for large-area device preparation, is expected to be the next generation of OLED fabrication method. In this paper, PEDOT: PSS and CDBP: Ir(bt)2acac functional layers were prepared by spin coating/printing and characterized, respectively. The results show that the double layer printing device achieved the best performance, the maximum brightness and external quantum efficiency reach 4357cd/m2 and 2.45%, even better than that of the spin coated reference. It indicates that the ink-jet printing process has a potential to overwhelm the spin-coating one for constructing high performance OLED.

Recent Developments in AIEgens for Non‐doped and TADF OLEDs

AbstractThe design of innovative and more efficient Aggregation‐Induced Emission (AIE) chromophores has kept a very high scientific interest since the first report on 2001. Among the possible applications, the field of the organic electroluminescent diodes (OLEDs) is highly attractive. This review will focus on very recent development in the design of AIE molecules for OLEDs, with particular attention on the performance of different emitting devices. Another key point is the description of the new class of AIE luminogens showing Thermally Activated Delayed Fluorescence (TADF), which appears as possible solution to overcome the limitation of fluorescent dyes employed in electroluminescent devices.

Effect of positively charged particles on sputtering damage of organic electro-luminescent diodes with Mg:Ag alloy electrodes fabricated by facing target sputtering

We investigated the influence of the positively charged particles generated during sputtering on the performances of organic light-emitting diodes (OLEDs) with Mg:Ag alloy electrodes fabricated by sputtering. The number of positively charged particles increased by several orders of magnitude when the target current was increased from 0.1 A to 2.5 A. When a high target current was used, many positively charged particles with energies higher than the bond energy of single C–C bonds, which are typically found in organic molecules, were generated. In this situation, we observed serious OLED performance degradation. On the other hand, when a low target current was used, OLED performance degradation was not observed when the number of positively charged particles colliding with the organic underlayer increased. We concluded that sputtering damage caused by positively charged particles can be avoided by using a low target current.

Electronic Structures of Platinum(II) Complexes with 2-Arylpyridine and 1,3-Diketonate Ligands: A Relativistic Density Functional Study on Photoexcitation and Phosphorescent Properties

The electronic structures of heteroleptic cyclometalated platinum(II) complexes a–f with 2-arylpyridine derivatives and 1,3-bis(3,4-dibutoxyphenyl)propane-1,3-dione which are candidate materials for phosphorescent organic electroluminescent diodes (OLEDs) were theoretically investigated using relativistic density functional theory (DFT) calculations including spin–orbit effects. The electronic excitation bands and phosphorescent emission wavelengths of the complexes were theoretically calculated and compared with the corresponding experimental results. It was theoretically confirmed that the color of the phosphorescent emission is controlled by the π–π* electronic excited state localized on each 2-arylpyridine derivative ligand. Our calculations also gave good predictions for spectral red shifts originating from excimer formations for a–f.

The New Nanosized Systems on the Basis Eu(III) Complexes as Precursors for Organic Electroluminescent Diodes

Novel Eu(III) complex with 2-methyl-5-phenylpentene-1-dione-3,5, metallopolymer based on and copolymer with N-vinylcarbazole were obtained. The kinetics of polymerization was studed by dilatometric method. The complex and metallopolymer were investigated and characterized by a number of physicochemical methods. The obtained results allowed to determine the composition, structure and symmetry of synthesized compounds. On the basis of metallopolymers was constructed the electroluminescent diode.

ChemInform Abstract: Dicyanomethylene‐4H‐pyran Chromophores for OLED Emitters, Logic Gates and Optical Chemosensors

AbstractReview: 111 refs.

Enhanced performance of organic electroluminescent diodes by UV–ozone treatment of molybdenum trioxide

The operating voltage is reduced and luminous efficiency is improved in the organic light-emitting diodes (OLEDs) with UV–ozone treatment of molybdenum trioxide (MoO3) anode buffer layer. The interface of indium tin oxide (ITO)/MoO3 was investigated by X-ray photoelectron spectroscopy. It was found that the binding energies of Mo 3d, O 1s, In 3d and C 1s were shifted to higher after UV–ozone treated MoO3. The enhanced properties are impervious to the UV–ozone treatment of ITO. Therefore, the improved performance can be attributed that the binding energies of Mo 3d, O 1s, In 3d and C 1s are changed to higher and also that the sheet resistance of ITO/MoO3 films is decreased after UV–ozone treated MoO3.

Tuning the bandgaps of polyazomethines containing triphenylamine by different linkage sites of dialdhyde monomers

Two series of conjugated aromatic polyazomethines (PAMs) have been designed and obtained by solution polycondensation of N,N′-bis(4-aminophenyl)-N,N′-di-2-naphthalenyl-4-benzene diamine, N,N′-bis(4-aminophenyl)-N,N′-di-1-naphthalenyl-(1,1′-biphenhyl)-4,4′-diamine with different aromatic dialdehydes, namely terephthalaldehyde, 1,3-isophthalaldehyde, phthaldialdehyde, 4,4′-diformyltriphenylamine, 2,5-thiophenedialdehyde, respectively. The structures of polymers were characterized by means of FTIR, 1H NMR spectroscopy, which showed an agreement with the proposed structure. All the polymers were amorphous determined by XRD technique with good solubility in many organic solvents, such as CHCl3, THF, N-methyl-2-pyrrolidinone (NMP) and N,N′-dimethylacetamide (DMAc), and could be cast into tough and flexible polymer films. The morphologies of PAMs have been detected by atom force microscopy (AFM) which showed more planar surface.All polymers displayed outstanding thermal stability, i.e. 20wt% loss in excess of 515°C under nitrogen atmosphere. Hybrid density functional theory (DFT) method was used to calculate the optimized geometry and electronic structure of PAMs. The calculated HOMO and LUMO energy levels of these polymers by theory method are in the range of −4.879 to −5.494 and −2.344 to −2.927eV vs. the vacuum level, respectively, which are similar to the experiment results that indicated the PAMs would be used in organic layer electroluminescent diode (OLED) and photovoltagic cell as hole transporting materials or functional dyes. All obtained PAMs revealed excellent stability of electrochromic characteristics, changing color from original yellowish to red or blue. And multiple reversible colors states were observed upon I2, HCl, FeCl3, UV irradiation doping and dedoping. The properties prove that the PAMs are multifunctional materials which will be subject of hole-transporting, electrochromic, chemical sensor application in near future.

Dicyanomethylene-4H-pyran chromophores for OLED emitters, logic gates and optical chemosensors

Dicyanomethylene-4H-pyran (DCM) chromophores are typical donor-π-acceptor (D-π-A) type chromophores with a broad absorption band resulting from an ultra-fast internal charge-transfer (ICT) process. In 1989, Tang et al. firstly introduced a DCM derivative as a highly fluorescent dopant in organic electroluminescent diodes (OLEDs). Integration of ICT chromophore-receptor systems based on DCM chromophores with ion-induced shifts in absorption or emission is a convenient method to perform the logic expression for molecular logic gates. In recent years, various DCM-type derivatives have been explored due to their excellent optical-electronic properties and diverse structural modification. This feature article provides an insight into how the structural modification of DCM chromophores can be utilized for OLED emitters, logic gates and optical chemosensors. In addition, the aggregation-induced-emission (AIE) of DCM derivatives for further optical applications was also introduced.

Probing electric field in double-layer electroluminescence diode by optical second harmonic generation

Optical second harmonic generation (SHG) measurement has been employed for studying electric field distribution in organic double-layer electroluminescence diode. Taking advantage of the material dependence on the SHG, we measured the electric field induced SHG (EFISHG) from IZO/α-NPD/Alq3/Al diode. The generated SH signal wavelength was governed by the active organic layers, and two peaks were generated at 940nm from the α-NPD and at 1050nm from the Alq3 layers. The SHG intensity was dependent on applied d.c. voltages, but the voltage dependence was totally opposite between the two peaks. Interface carrier charging caused by the Maxwell–Wagner effect accounted for the results. We concluded that the EFISHG provides an effective way to probe the electric field distribution in double-layer electroluminescence diodes.

Improvement of voltage and charge balance in inverted top-emitting organic electroluminescent diodes comprising doped transport layers by thermal annealing

We present investigations of top emitting organic light emitting devices (OLED) comprising n- and p-doped organic charge transport layers. It has been found previously that in comparison to noninverted p-i-n OLEDs, inverted n-i-p OLEDs show reduced device performances after fabrication. These differences can be eliminated by subsequent thermal annealing of the whole n-i-p OLED. After this process, the n-i-p OLED exhibits a superior low driving voltage of 2.9 V at 1000 cd/m2 and shows an increase in external quantum efficiency from 11% to almost 15% which we ascribe to a modified charge balance within the intrinsic organic emission layer.

A novel red organic light-emitting diode with ultrathin DCJTB and Rubrene layers

A novel red organic electroluminescent diode based on an ultrathin 4-(dicyanomethylene)-2- t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) layer has been fabricated. The device with the ultrathin DCJTB layer inserted between N, N′-bis-(1-naphthyl)- N, N′-diphenyl-1,1′-biph-enyl-4,4′- diamine (NPB) and tri-(8-hydroxyquinoline) aluminum (Alq 3) layers has an emission from both DCJTB and Alq 3. When utilizing the 5,6,11,12-tetraphenylnaphthacene (Rubrene) bilayer as the emitting-assistant layer in the fabrication of the device, the emission from Alq 3 disappears. We attribute the improvement of color purity to the exciton confinement and energy transfer from the Rubrene molecular. The device with 0.1 nm DCJTB layer has a luminance of 1130 cd/m 2 at 20 mA/cm 2 with an EL efficiency of 5.6 cd/A, and the CIE coordinates was [0.617, 0.379]. The maximum brightness of this device reaches 21,525 cd/m 2.

Structural, Magnetic and Magnetoresistive Properties of PTCTE Based Organic Spin Valves

Organic semiconductors are very promising materials in the field of spintronic devices like spin valves (SV) because of large spin relaxation time in ¿-conjugated molecules and the advantage of rather simple and low energy consuming device fabrication techniques. Most of the studies of spin injection in organic semiconductors focused on hole transport materials and one electron transport material: the tris(8-hydroxyquinoline) aluminum (Alq <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ). In this article, we present our first results on SV based on a perylene derivative, PTCTE (PTCTE: tetraethyl perylene 3, 4, 9, 10-tetracarboxylate). Perylene molecules were traditionally used as electron transport materials in optoelectronic devices like OLED (organic electroluminescent diodes) and solar cells. NiFe/PTCTE/Co devices were prepared by dc sputtering for the electrodes (with direct or off axis sample configuration for the upper electrode), and vacuum sublimation for the organic layer. CPP (current perpendicular to plane) devices geometry was obtained with millimetric contact masks determining a CPP spin valve area of 1 mm × 1 mm. Up to now, our best samples have a spin valve magnetic behavior and exhibit up to 3 % MR response at low temperature for a 300 nm organic spacer layer.

Enhanced fluorescence by surface plasmon coupling of Au nanoparticles in an organic electroluminescence diode

A significant increase in electroluminescence was achieved through coupling with localized surface plasmons in a single layer of Au nanoparticles. We fabricated a thin-film organic electroluminescence diode, which consists of an indium tin oxide (ITO) anode, a Au nanoparticle array, a Cu phthalocyanine hole transport layer, a tris(8-hydroxylquinolianato) aluminum (III) electron transport layer, a LiF electron injection layer, and an Al cathode. The device structure, with size-controlled Au particles embedded on ITO, can be used to realize the optimum distance for exciton-plasmon interactions by simply adjusting the thickness of the hole transport layer. We observed a 20-fold increase in the molecular fluorescence compared with that of a conventional diode structure.

Probing of carrier behavior in organic electroluminescent diode using electric field induced optical second-harmonic generation measurement

By using the electric field induced optical second-harmonic generation (EFISHG) measurements, we probed the transient electric field in a double-layer indium zinc oxide (IZO)/N, N′-di-[(1-naphthyl)-N,N′-diphenyl]-(1,1′-biphenyl)-4, 4′-diamine(α-NPD)/tris(8-hydroxy-quinolinato)aluminum(III) (Alq3)/Al electroluminescent (EL) diode. Results evidently showed that EL was initiated by the injected hole transport across α-NPD layer, and holes accumulated at the α-NPD/Alq3 interface while EL was enhanced. Analysis based on the Maxwell–Wagner effect model well accounted for the hole accumulation. EFISHG measurement is useful as a tool for probing carrier behavior in organic EL devices.

Synthesis, Electrochemical and Photophysical Properties, and Electroluminescent Performance of the Octa‐ and Deca(aryl)[60]fullerene Derivatives

AbstractThree multifunctionalized organo[60]fullerene derivatives, C60Ph5(C6H4‐tBu‐4)5Me2 (cyclophenacene, 1), C60Ph5(C6H4‐tBu‐4)5Me2 (fused corannulene, 2), and C60Ph5(C6H4‐tBu‐4)3Me2 (phenylene‐bridged fused corannulene, 3) are synthesized by the reaction of C60Ph5Me with 4‐tert‐butylphenylcopper reagent in the presence of pyridine, followed by treatment with MeI. Compounds 1–3 undergo reduction in the range from −1.8 to −2.5 V versus Fc/Fc+ and exhibit photoluminescence behavior with fluorescent quantum yields of 18.5%, 2.5%, and 3.2% with fluorescent lifetimes of 67, 1.1, and 27 ns (1–3, respectively). Organic electroluminescent diode devices using 1–3 are fabricated with π‐conjugated polymers and show external electroluminescent efficiencies of 0.04%, 0.07%, and 0.03% emitting yellow, green, and red light, respectively. The device containing all three compounds emits white light. This result indicates that the bulky addends in 1–3 can effectively isolate the π‐conjugated systems of the molecules in the solid state and retard the intermolecular excited‐state quenching process.