Films For Organic Light-emitting Diodes Research Articles

Effect of the conformer distribution on the properties of amorphous organic semiconductor films for organic light-emitting diodes.

With the remarkable improvement in the electrical and optical properties of organic light-emitting diodes (OLEDs) in recent years, the details of the higher-order structure of vacuum-deposited amorphous organic films and its formation mechanism need to be understood. In particular, to clarify the effect of the higher-order structure on the film properties, it is necessary to analyze the molecular aggregation states in the vacuum-deposited amorphous films. Toward their deep understanding, the higher-order structure and film properties have often been discussed with relation to the surface diffusion and structural relaxation of the molecules immediately after deposition on the film surface. However, the effect of the variety of conformers, which is specific to amorphous organic materials, on the thermal and electrical properties of the films has not been deeply discussed. In this study, we focused on three structural isomers of OLED materials and discuss the effect of the conformer distribution on the molecular aggregation states and thermal and electrical properties of the vacuum-deposited films. From their comparison, we found that the properties of the film composed of a relatively small number of stable conformers are superior to those of the other two films composed of relatively large numbers of stable conformers. This superiority originates from formation of aggregates of the same conformer, which become the starting points for crystallization when the film is heated. Our detailed comparison and discussion focusing on the variety of conformers will lead to a deeper understanding of the molecular aggregation states and physical properties of amorphous organic films.

Improved Intrapixel Thickness Uniformity of Slot-Coated PEDOT:PSS Films for OLEDs via Dilution and Predrying Treatments

Due to capillary force, solution coating of pristine poly(3,4-ethylenedioxythiophene):poly(4-styrenesulfonate) (PEDOT:PSS) solution in the presence of banks exhibits poor intrapixel thickness uniformity, shrinking the emission area of organic light-emitting diodes (OLEDs) within pixels. To enhance it, we have applied the predrying treatments (predrying under atmospheric pressure, by heat conduction, and under vacuum) to wet films. However, its impact is minimal because the evaporation rate of wet films is not well-matched to the capillary flow rate of solute during the predrying process. It is tackled by diluting the pristine PEDOT:PSS solution with deionized water such that the evaporation of wet films is retarded and in turn, the convective flow of solute occurs from the pixel edge to the center. With increasing dilution, a noticeable enhancement in its intrapixel thickness uniformity is observed without any sacrifice on the surface morphology. It is found that the predrying effect by heat conduction is more pronounced than that under vacuum. Furthermore, we have demonstrated that slot-die coating rather than spin coating is preferable in the coating of diluted PEDOT:PSS solutions. With the diluted solution and predrying by heat conduction, the thickness uniformity can be enhanced by a factor of 2 and the emission area of OLEDs within pixels is broadened to a great extent.

An Approach for Measuring the Dielectric Strength of OLED Materials.

Surface roughness of electrodes plays a key role in the dielectric breakdown of thin-film organic devices. The rate of breakdown will increase when there are stochastic sharp spikes on the surface of electrodes. Additionally, surface having spiking morphology makes the determination of dielectric strength very challenging, specifically when the layer is relatively thin. We demonstrate here a new approach to investigate the dielectric strength of organic thin films for organic light-emitting diodes (OLEDs). The thin films were deposited on a substrate using physical vapor deposition (PVD) under high vacuum. The device architectures used were glass substrate/indium tin oxide (ITO)/organic material/aluminum (Al) and glass substrate/Al/organic material/Al. The dielectric strength of the OLED materials was evaluated from the measured breakdown voltage and layer thickness.

A thermally cross-linked hole-transporting film with the remarkable solvent resistance for solution-processed OLEDs

Abstract In this work, we report a systematic study of the properties of the thermal cross-linkable triarylamine based hole-transporting material (HTM) N,N′-di-p-tolyl-N,N′-bis(4-vinylphenyl)-[1,1′-biphenyl]-4,4′-diamine (V-p-TPD) and the distinctive nature of its cross-linked thin films fabricated by solution process under mild thermal polymerization without any initiator. The cross-linked V-p-TPD film exhibited remarkable solution resistance to common solute such as chlorobenzene, toluene, chloroform and tetrahydrofuran. The cross-linked V-p-TPD film showed better morphological stability than vacuum-deposited film of its monomer TPD. The space-charge-limited current (SCLC) measurements revealed that the cross-linked V-p-TPD film afforded comparable hole mobility with vacuum-deposited TPD film. Finally, we applied the cross-linked V-p-TPD film in organic light-emitting diodes (OLEDs) to evaluate its performance in all-solution-processed OLEDs.

High-refractive-index polymers of poly (carbazole phenoxy-based polyurethane) for a refractive index matching film in organic light-emitting diodes

ABSTRACTA high-refractive index polymer of carbazole phenoxy-based polyurethane (CPP) was prepared by step-growth polymerization. Initially, bicarbazole phenoxy (BCP) derivatives were synthesized by the reaction between 9-(2-oxiranylmethyl)-9H-carbazole and 4,4′-(1-phenylethylidene)bisphenol. The step-growth polymerization of BCP and isophorone diisocyanate took place at 80°C in the presence of dibutyltin dilaurate as a catalyst affording the prepolymer with isocyanate end-groups. The CPP was obtained by the chain extension reaction of the prepolymer and bis-(2-hydroxyethyl) disulfide as an extender agent. The polymer was characterized by 1H NMR, FT-IR, and GPC. The CPP solution was spin-coated on a silicon wafer and dried under nitrogen. The transmittance of the CPP film was almost 100% as analyzed by a UV-visible spectrometer and the refractive index of the polymer was 1.63 as measured by spectroscopic ellipsometry. The result indicated that the high-refractive index CPP could be a promising material for optoelectronic device applications.

Extremely High Barrier Performance of Organic-Inorganic Nanolaminated Thin Films for Organic Light-Emitting Diodes.

This work presents a novel barrier thin film based on an organic-inorganic nanolaminate, which consists of alternating nanolayers of self-assembled organic layers (SAOLs) and Al2O3. The SAOLs-Al2O3 nanolaminated films were deposited using a combination of molecular layer deposition and atomic layer deposition techniques at 80 °C. Modulation of the relative thickness ratio of the SAOLs and Al2O3 enabled control over the elastic modulus and stress in the films. Furthermore, the SAOLs-Al2O3 thin film achieved a high degree of mechanical flexibility, excellent transmittance (>95%), and an ultralow water-vapor transmission rate (2.99 × 10-7 g m-2 day-1), which represents one of the lowest permeability levels ever achieved by thin film encapsulation. On the basis of its outstanding barrier properties with high flexibility and transparency, the nanolaminated film was applied to a commercial OLEDs panel as a gas-diffusion barrier film. The results showed defect propagation could be significantly inhibited by incorporating the SAOLs layers, which enhanced the durability of the panel.

Characteristics of vanadium-doped indium oxide thin films for organic light-emitting diodes fabricated by spray chemical vapor deposition

An indium oxide transparent electrode for organic light-emitting diodes was fabricated by the inexpensive spray chemical vapor deposition method. The high work function (5.1 eV) necessary for a transparent anode and a hole-injection layer was successfully achieved with a vanadium doping concentration of 1.5 at. % V without any significant increase in resistivity and surface roughness or loss of transparency. The effect of vanadium doping on indium oxide was systematically investigated. The resistivity, average transmittance in the visible range, and surface roughness (Ra) were 1.08 × 10−3 Ω·cm, 84%, and 4.0 nm, respectively, for the vanadium-doped indium oxide.

Advantages and disadvantages of vacuum-deposited and spin-coated amorphous organic semiconductor films for organic light-emitting diodes

Film densities, transition temperatures, and degrees of horizontal molecular orientation of vacuum-deposited and spin-coated amorphous organic semiconductor films used for OLEDs are systematically and quantitatively compared, and their general differences are discussed.

Effects of flow rate of oxygen and hydrogen gases on characteristics of ITO thin films for OLEDs

We have investigated the effect of ambient gases on the structural, electrical and optical characteristics of indium tin oxide (ITO) thin films intended for use as anode contacts in organic light emitting diode (OLED) devices. These ITO thin films are deposited by radio frequency magnetron sputtering under different ambient gases (Ar, Ar+O2 and Ar+H2) at 300°C. In order to investigate the influences of the oxygen and hydrogen, the flow rate of oxygen and hydrogen in argon mixing gas has been changed from 0·5 to 5 sccm and from 0·1 to 1 sccm, respectively. The intensity of the (400) peak in the ITO thin films increases with increasing H2 flow rate, while the (400) peak was nearly invisible in an atmosphere of Ar+O2. The electrical resistivity of the ITO thin films increases with increasing O2 flow rate, whereas the electrical resistivity decreases sharply under an Ar+H2 atmosphere and is nearly similar regardless of the H2 flow rate. The change of electrical resistivity with changes in the ambient gas composition is mainly interpreted in terms of the charge carrier concentration rather than the charge carrier mobility. All the films show an average transmittance of >80% in the visible range. The optical band gap of the ITO films increases with increasing H2 flow rates, whereas the optical band gap of the ITO films deposited in an O2 atmosphere decreases with increasing O2 flow rates. The current density and the luminance of the OLED devices with ITO thin films deposited in 1 sccm of H2 ambient gas are the highest among all the films. The optical band gap energy of ITO thin films plays a major role in OLED device performance, especially the current density and luminance.

Study on the Optical and Electrical Properties of Organic Semiconductor Thin Films for OLEDs

Organic semiconductor thin films of aluminum (III) bis(2-methyl-8-quninolinato)-4- phenylphenolate (BAlq), -naphthylphenylbiphenyl amine (NPB), and tris(8-hydroxy-quinoline) aluminum (AlQ) for organic light-emitting diodes (OLEDs) were deposited by the vacuum sublimation technique. The optical properties in the UV-visible region of the thin films were investigated by optical transmittance and absorption spectra. The band gaps were obtained from direct allowed transitions at room temperature by means of the Tauc plots. The Urbach energy and the slope of Urbach edge were evaluated, respectively according to the Urbach-edges method. The thin film devices of sandwich structure were fabricated using these organic semiconductor materials, in addition, the effective carrier mobility, free carrier density, and electrical conductivity of the thin films were calculated in terms of the measured current-voltage characteristics of the devices.

Electrospray deposition of polymer thin films for organic light-emitting diodes

Electrospray process was developed for organic layer deposition onto polymer organic light-emitting diode [PLED] devices in this work. An electrospray can be used to produce nanometer-scale thin films by electric repulsion of microscale fine droplets. PLED devices made by an electrospray process were compared with spin-coated ones. The PLED device fabricated by the electrospray process showed maximum current efficiency of 24 cd/A, which was comparable with that of the spin-coating process. The electrospray process required a higher concentration of hole and electron transport materials in the inks than spin-coating processes to achieve PLED maximum performance. Photoluminescence [PL] at 407 nm was observed using electrosprayed poly(N-vinyl carbazole) films, whereas a peak at 410 nm was observed with the spin-coated ones. Similar difference in peak position was observed between aromatic and nonaromatic solvents in the spin-coating process. PLED devices made by the electrospray process showed lower current density than that of spin-coated ones. The PL peak shift and reduced current of electrosprayed films can therefore be attributed to the conformation of the polymer.

Reliability Improvement of Barrier Thin Films for Organic Light-Emitting Diodes

Reliability Improvement of Barrier Thin Films for Organic Light-Emitting Diodes

Passivation Effect of Diamond-Like Carbon Films for Organic Light-Emitting Diodes

Passivation Effect of Diamond-Like Carbon Films for Organic Light-Emitting Diodes

Passivation Effect of Diamond-Like Carbon Films for Organic Light-Emitting Diodes

We have studied organic light-emitting devices (OLEDs) with double-layered inorganic molybdenum trioxide (MoO3)/diamond-like carbon (DLC) as passivation films. The initial device characteristics were identical to those of devices without a passivation film, namely, the maximum luminance and electro luminescence (EL) efficiency were 15,000 cd/m2 and 1.9 lm/W, respectively. However, in the device without a 20 nm MoO3 inorganic film, the device durability was markedly lower than that of the device with a double-layered structure. By adding MoO3 as an additional passivation layer, plasma damage in the peripheral region of the organic layer was markedly reduced, and the resultant device durability was improved significantly.

Investigating Morphology and Stability of Fac‐tris (2‐phenylpyridyl)iridium(III) Films for OLEDs

AbstractStable film morphology is critical for long‐term high performance organic light‐emitting diodes (OLEDs). Neutron reflectometry (NR) is used to study the out‐of‐plane structure of blended thin films and multilayer structures comprising evaporated small molecules. It is found that as‐prepared blended films of fac‐tris(2‐phenylpyridyl)iridium(III) [Ir(ppy)3] in 4,4′‐bis(N‐carbazolyl)biphenyl (CBP) are uniformly mixed, but the occurrence of phase separation upon thermal annealing is dependent on the blend ratio. Films comprised of the ratio of 6 wt% of Ir(ppy)3 in CBP typically used in OLEDs are found to phase separate with moderate heating while a higher weight percent mixture (12 wt%) is found to be stable. Furthermore, it is found that thermal annealing of a multilayer film comprised of typical layers found in efficient devices ([tris(4‐carbazoyl‐9‐ylphenyl)amine (TCTA)/Ir(ppy)3:CBP/bathocuproine (BCP)]) causes the BCP layer to become mixed with the emissive blend layer, whereas the TCTA interface remains unchanged. This significant structural change causes no appreciable difference in the photo­luminescence of the stack although such a change would have a dramatic effect on the charge transport through the device, leading to changes in performance. These results demonstrate the effect of thermal stress on the delicate interplay between the chemical composition and morphology of OLED films.

Mechanism of droplet generation in silver thin films for organic light-emitting diode displays

Thin silver film is widely used as the cathode in organic light-emitting diode displays and it is generally fabricated using the thermal evaporation method. But during the evaporation process, there is an inevitable outgassing problem and this creates high viscosity bubbles in melted silver. When the bubbles break, the high energy released scatters silver droplets which damage the silver surface. In this study, we were able to decrease the number of droplets from 6,171 to 278 with a degassing process of 400 °C for 6 h before proceeding with a thermal evaporation process.

Spin-coated Ga-doped ZnO transparent conducting thin films for organic light-emitting diodes

Gallium doped zinc oxide (GZO) thin films have been prepared by a simple sol–gel spin coating technique. XRD results showed the preferential c-axis orientation of the crystallites and the presence of the wurtzite phase of ZnO. A lowest resistivity of 3.3 × 10−3 Ω cm was obtained for the ZnO film doped with 2 at% of Ga after post-annealing at 500 °C for 45 min in a H2 atmosphere. All the films showed more than 80% of transparency in the entire visible region. Blue shifting of the optical band gap was observed with an increase in Ga doping, which can be explained on the basis of the Burstein–Moss effect. OLED devices were fabricated using 2 at% Ga-doped ZnO thin films as anodes. Preliminary results obtained demonstrated that spin-coated GZO films can be used as a promising TCO for optoelectronic device applications.

Dependence of surface morphology on molecular structure and its influence on the properties of OLEDs

Most organic light-emitting diodes (OLEDs) have a multilayer structure composed of organic layers such as a hole injection layer (HIL), a hole transport layer (HTL), an emission layer (EML), an electron transport layer (ETL) and an electron injection layer (EIL) sandwiched between two electrodes. The organic layers are thin solid films with a thickness from a few nano meters to a few tenths nano meter, respectively. Surface morphology of an organic thin solid film in OLEDs depends on the molecular structure of the organic material and has an affect on device performance. To analyze the effect of surface morphology of an organic thin solid film on fluorescence and electroluminescence (EL) properties, thin solid films of 4-(dicyanomethylene)-2-methyl-6-(julolidin-4-yl-vinyl)-4 H-pyran (DCM2) and new red fluorophores, (2 E,2′ E)-3,3′-[4,4″-bis(dimethylamino)-1,1′:4′,1″-terphenyl-2′,5′-diyl]bis[2-(2-thienyl)acrylonitrile] (ABCV-Th) and (2 Z,2′ Z)-3,3′-[4,4″-bis(dimethylamino)-1,1′:4′,1″-terphenyl-2′,5′-diyl]bis(2-phenylacrylonitrile) (ABCV-P) were investigated by atomic force microscopy (AFM). The samples for EL and AFM measurement were fabricated by the high-vacuum thermal deposition (8×10 −7 Torr) of organic materials onto the surface of indium tin oxide (ITO)-coated glass substrate, in which the layer structures of samples for AFM measurement and those for EL measurement were ITO/NPB (40 nm)/red emitters (80 nm) and ITO/NPB (40 nm)/red emitters (80 nm)/BCP (30 nm)/Liq (2 nm)/Al (100 nm), respectively. The analysis based on AFM measurements well supported that the photoluminescence properties and the device performance were very much dependent upon surface morphology of an organic thin layer.

Fullerene-doped hole transport molecular films for organic light-emitting diodes

C 60 -doped N,N′-bis(l-naphthyl)-N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine(NPB) film is studied as hole injection layer between indium tin oxide (ITO) and NPB. The doped films on ITO substrates were found to be thermally stable after being annealed at temperatures up to 120°C. This was attributed to a strong interaction between NPB and C60 providing a dipole force crosslinking NPB molecules, similar to a crosslinked thermoset polymer networks. Furthermore, the C60-doped NPB p-type hole injection layers yield devices having better efficiency and low driving voltage as compared with standard devices with CuPc as the hole injection layers.

Work-function changes of treated indium-tin-oxide films for organic light-emitting diodes investigated using scanning surface-potential microscopy

Local surface work-function (WF) measurement on indium-tin-oxide (ITO) films prepared by different cleaning methods for use as anode materials in organic light-emitting diodes were studied using scanning surface-potential microscopy. The ITO WF changes with standard wet-cleaning treatment correspond directly to asperities on the film surface. The maximum difference value (MDV) in the local WF reached 0.41eV. However, after wet-cleaning ITO with the ultraviolet ozone or O-plasma treatment, the WF distribution is more uniform than the original distribution. Owing to the sufficient ITO surface oxidization, the mean local WF value increases effectively to more than 5.00eV and the MDV is less than 0.05eV. Furthermore, the changes in roughness and conductivity on the ITO surface at different treatment times are also discussed. The proper exposure time for the O-plasma treatment can thereby be determined.