OLED Applications Research Articles

Nano Sized Defect Free Al2O 3 Thin Film for High Performance Gas Barrier

Recently, our interests in flexible display for next generation have been increased. Plastic substrate is used for flexible display. However it has less chemical resistance, heat resistance, gas transmittance resistance characteristics than glass substrate. By these reasons, plastic substrate requires functional coatings. Because the OLED is degraded by moisture and oxygen, it requires intensely ultralow Water Vapor Transition Rate (WVTR) as 1ⅹ10-6gm-2day-1. To achieve the ultralow WVTR, defects must be prevented. Defects generated by particles can be prevented carefully controlling the process, but defects generated in grain boundary have to be annealed to get rid of them. Organic materials have weakness for heat so annealing process has to be avoid. In this study, we developed Al2O3 nano-structure single gas barrier layer using Neutral Beam Assisted Sputtering (NBAS) process. The NBAS process can be continuously changing crystalline structure from amorphous phase to nano-crystal phase of various grain sizes within single inorganic thin film. These NBAS process effect can lead to formation of nano-structure barrier layer which effectively passivate and healing gas diffusion pathways between grain boundaries. As a result, we confirmed nano-structure of NBAS processed Al2O3 single gas barrier layer through the dielectric constant measurement, Ellipsometry, AFM, and TEM analysis. Finally, WVTR property of Al2O3 nano-structure single gas barrier layer is measured under 1ⅹ10-6gm-2day-1. The Al2O3 nano- structure single gas barrier layer is very suitable in the OLED application.

Investigation on the electronic structures and photophysical properties of a series of cyclometalated iridium(III) complexes based on DFT/TDDFT calculations

The electronic structures and photophysical properties of a series of cyclometalated iridium (III) complexes Ir(C∧N)2(H2NNHCOO), including 1 [C∧N=2-phenyl-pyridine], 2 [C∧N=5-fluoro-2-phenylpyridine], 3 [C∧N=2-phenyl-5-trifluoromethylpyridine], 4 [C∧N=6-phenyl-[2, 3] bipyridinyl], 5 [C∧N=7-phenyl-cyclopenta [4] dipyridine] and 6 [C∧N=8-phenyl-[1, 9] phenanthroline], have been theoretically investigated based on density functional theory (DFT) and time-dependent DFT. The characteristics in phosphorescent performances have been outlined for each of the complexes in the applications in OLED. On the basis of the two simplifications presented in this paper and the available experimental data, the magnitudes of phosphorescent radiative rates for complexes 1–6 were approximately calculated to be: 5.56×105s−1, 2.68×105s−1, 1.17×106s−1, 9.78×104s−1, 5.30×106s−1 and 6.71×106s−1, respectively. Meanwhile, the sequence of phosphorescent quantum efficiencies was obtained to be: ФPL(4)<ФPL(2)<ФPL(1)<ФPL(3)<ФPL(5)<ФPL(6), in which ФPL(4) is by far the lowest, ФPL(5) is much larger and ФPL(6) is by far the largest. In contrast to complex 1, the emission wavelengths are slightly red-shifted for 2 and 3 and significantly red-shifted for 4 and 5, while the emission wavelength of 6 is slightly blue-shifted. In comparison, complexes 6 and 5 may be singled out to be the most efficient phosphorescence emitters for the applications in OLED.

Enhanced Permeability Property of Silicon Oxide Thin Films Deposited by Facing Target Sputtering System for OLED Application

ABSTRACTThe thin silicon oxide (SiOx) films were deposited by using the facing target sputtering(FTS) apparatus for the thin film passivation application of OLEDs. The deposition system was investigated under various process conditions such as the array of cathode magnets, oxygen concentration[O2/(Ar + O2)] during deposition, distance between two targets, and working pressure. The optimum conditions for FTS apparatus for the deposition of thin silicon oxide films are as follows: distances between target and substrate (T-S) as well as two targets (T-T) are 90 mm and 120 mm, respectively. Dense structured SiOx film with maximum deposition rate is obtained under a gas pressure of 2 mTorr with an oxygen concentration of 3.3%. Under the optimum conditions, SiOx thin films were grown with deposition rate of 25 nm/min by rf-power of 4.5 W/cm2, which was remarkably enhanced as compared with a deposition rate(3–4 nm/min) of conventional sputtering system. The SiOx thin film showed a very low water vapor transmission rate(WVTR) less than 3 × 10−2 g/m2/day compared to the conventional sputtering system.

Theoretical study of the optical and charge transport properties of π-conjugated three-coordinate organoboron compounds as organic light-emitting diodes materials

A series of π-conjugated three-coordinate organoboron compounds have been designed as luminescent and charge transport materials for OLEDs application.

Quantum chemistry investigation on the photophysical properties of phosphorescent iridium(III) complexes with modified cyclometalating ligands

Based on the quantum chemistry methodologies, the electronic structures and photophysical properties have been investigated for a series of cyclometalated iridium(III) complexes Ir(C^N)2(acac), including 1 [C^N=2-biphenyl-4-yl-quinoline], 2 [C^N=2-(fluoren-2-yl)-quinoline], 3 [C^N=2-dibenzofuran-3-yl-quinoline], 4 [C^N=2-dibenzothiophen-3-yl-quinoline] and 5 [C^N=2-phenanthren-2-yl-quinoline]. The electronic configurations, absorption properties, emission wavelengths and the potential phosphorescent performances have been outlined for each of the complexes. On the basis of the two simplifications presented in this paper and the available experimental data, the magnitudes of phosphorescent radiative rates for each of the complexes were approximately calculated to be: 1.20×105s−1, 8.75×104s−1, 3.58×105s−1, 5.69×104s−1 and 4.73×104s−1, respectively. Meanwhile, the sequence of phosphorescent quantum efficiencies was obtained to be: ФPL(3)>ФPL(1)>ФPL(2)>ФPL(5)>ФPL(4). Our research work is of great importance, consulting the quantum chemistry conclusions and applying adequate chemical engineering, it may be possible to obtain cyclometalated iridium(III) complexes with distinctly improved phosphorescent performances in the application in OLED.

Aging of ITO anodes treated by supercritical CO2/H2O2 fluids for OLEDs

The aging behaviors of indium tin oxide (ITO) anodes treated by supercritical CO2/H2O2 (SCCO2/H2O2) fluids were investigated. As the SCCO2/H2O2-treated ITO anodes were aged, the contact angle and surface energy were analyzed and compared with those of the ITO anodes treated by oxygen plasma. The SCCO2/H2O2 pretreatment yielded a stable polar component of the ITO surfaces after 48 h of aging. The energy reduction in the polar component of the oxygen-plasma-treated ITO due to aging was 20 %, as compared with the 1.1 % decrease in ITO treated with the SCCO2/H2O2 fluids at 4000 psi for 15 min. The X-ray photoelectron spectroscopy analysis revealed that the oxygen content of the ITO surfaces with the SCCO2/H2O2 pretreatment was significantly higher than that of the ITO treated by oxygen plasma. This could result from the formation of hydroxyl products that functioned as a stable buffer layer against further contamination during aging. In addition, the correlated dependence of the OLED performance on the aged ITO anodes was also studied. The OLEDs with the SCCO2/H2O2 pretreatment showed an improved degradation of I–V characteristics and brightness in comparison with those of the devices treated with oxygen plasma after aging the treated ITO anodes for 6 and 12 h. The obtained results suggested that the ITO anodes treated by the SCCO2/H2O2 fluids exhibited a stable surface chemistry and could be useful for OLED applications.

High efficient OLEDs based on novel Re(I) complexes with phenanthroimidazole derivatives

Novel rhenium(I) [Re(I)] complexes with phenanthroimidazole ligands were successfully designed, synthesized and characterized. The Re(I) complexes displayed intense phosphorescence with yellow or orange color around 540–580nm at room temperature with relatively short lifetimes. The phosphorescent simple-structure OLEDs using these Re(I) complexes as dopants exhibited low turn-on voltage of 3.5–3.6V, maximum current efficiencies of 18.7–21.1cdA−1 and maximum power efficiencies of 13.3–18.9lmW−1, which were amongst the highest reported for OLEDs based on Re(I) complexes with phenanthroline ligands as emitters. The excellent performances are due to the bulky steric Re(I) complexes with short lifetime and good electron-transporting ability, which may improve the electron injection and result in greater balance between electron and hole fluxes. The results suggest that these complexes have potential application in OLEDs.

Transparent and Conductive Properties of Nanocomposite Thin Film of MWCNTs and PEDOT-PSS for Applications in OLED

Transparent and Conductive Properties of Nanocomposite Thin Film of MWCNTs and PEDOT-PSS for Applications in OLED

Highly phosphorescent green emitting iridium(iii) complexes for application in OLEDs

Phenanthrimidazole based iridium(iii) complexes show good performance in terms of brightness and power and current efficiencies.

Correction: AIE-active mechanochromic materials based N-phenylcarbazol-substituted tetraarylethene for OLED applications

Correction for ‘AIE-active mechanochromic materials based N-phenylcarbazol-substituted tetraarylethene for OLED applications’ by He Zhao et al., RSC Adv., 2015, 5, 19176–19181.

Rational design of organoboron heteroarene derivatives as luminescent and charge transport materials for organic light-emitting diodes

A series of organoboron heteroarene derivatives have been designed as luminescent and charge transport materials for OLEDs application.

Thickness-Dependent Structural, Optical, and Electrical Characteristics of ZnO:Al Thin Films and Application in OLEDs

Optically transparent and electrically conducting aluminum-doped zinc oxide thin films of thickness 110, 350, and 500 nm have been deposited on unintentionally heated glass substrates by the radio frequency magnetron sputtering method to investigate their structural and optoelectrical properties. All thin films are shown to exhibit: 1) wurtzite-type hexagonal structure similar to ZnO with c-axis perpendicular to the substrate and 2) optical transmittance of >85% in the visible range. The increase in film thickness leads to growth of crystallites with an average size in the range of 16-26 nm, decrease in the sheet resistance, and increase in the figure of merit. Owing to high optical transmittance of 92%, low sheet resistivity of 26 Ω/□, and high figure of merit (9.23 × 10 3 Ω -1 cm -1 ), 500-nm-thick Al-doped ZnO films can be potential transparent conductors for photovoltaic applications. The organic light-emitting diodes fabricated with them and commercial indium-tin oxide thin films are shown to display comparable luminous flux power efficiency.

Synthesis, characterization and photophysical properties of iridium complexes with amidinate ligands

Iridium(III) complexes containing 2-(3-fluorophenyl)-4-methylpyridine and amidinate ligands with alkyl chains were synthesized and characterized by UV/Visible, fluorescence, FTIR, NMR spectroscopies and cyclic voltammetry. Thermal properties of the iridium complexes have been investigated using thermogravimetric analysis (TGA). The influence of alkyl side groups on the photoluminescence spectra of iridium complexes has been investigated. The spectra of all complexes show similar emission which results in the blue region. However, the increase in the solution concentrations of the complexes leads to excimer emission in the amidinate complexes. The effects of different polarities of solvents on photophysical properties have been investigated. The HOMO and LUMO energy levels of the complexes are in the range of 5.32–5.35 eV and 2.48–2.51 eV, respectively. The energy states, thermal and electrochemical stability of the complexes are appropriate for their utilization in OLED applications.

Synthesis and Photophysical Properties of Charged Iridium(III) Cyclometallates Based on Hole-Transporting Carbazole Unit

Four novel charged Ir (III) cyclometallates based on 9-phenylcarbazole unit have been developed. By attaching isoquinoline or 5-trifluoromethylpyridine group, the light-emitting properties of the resulting Ir (III) cyclometallates have been improved. Meanwhile, 1,10-phenanthroline or 4,4′-dimethyl-2,2′-bipyridine has been utilized to modify the thermal stability. The four Ir (III) cyclometallates emit yellow light with the highest photoluminescence quantum yields (ΦP) of 0.32 (C4), which is only less than that of the state-of-art complexfac-tris (2-phenylpyridine) iridium (III) [fac-Ir (ppy)3], which showsΦPof 0.40 under the same condition. This inspiring result reveals that the bulky character of CF3group enhances the performance of reported charged Ir (III) cyclometallates and makes them to be potential phosphors in OLED applications.

Arbitrary Shape Designable Microscale Organic Light-Emitting Devices by Using Femtosecond Laser Reduced Graphene Oxide as a Patterned Electrode

Arbitrarily shape customized microscale organic light-emitting devices (micro-OLEDs) have been fabricated by using reduced graphene oxide (RGO) film as a patterned electrode. A femtosecond (Fs) laser was employed to fabricate the RGO electrode by direct reduction and patterning of the GO films according to preprogrammed patterns. The patterned RGO exhibits good conductivity for electrode applications in OLEDs. Various complex patterned micro-OLEDs were successfully created through this simple Fs laser fabrication process, which exhibit well-defined sizes, shapes, and edges and uniform electroluminescence characteristics. The arbitrarily shape customized micro-OLEDs open the door for new applications in microdisplays and three-dimensional or flexible displays.

Bodipy dyes bearing oligo(ethylene glycol) groups on the meso-phenyl ring: tuneable solid-state photoluminescence and highly efficient OLEDs

Borondipyrromethene (Bodipy) dyes with strong solid-state photoluminescence are highly desirable for their applications in OLEDs. In this work, a series of meso-(4-R–C6H4)-substituted Bodipy dyes (R = H for B1; R = (OCH2CH2)nOCH2CH3, n = 0–3 for B2–B5, respectively) were prepared through an acid-catalyzed reaction. The flexible ether groups attached to the periphery of B2–B5 are expected to influence the molecular arrangement and intermolecular interactions in solid state. Crystallographic analysis of B1–B4 reveals that the meso-phenyl ring is almost orthogonal to the indacene plane. Intermolecular π–π interactions are observed in B1 but absent in B2–B4. As a consequence, the dye B1 is weakly emissive in solid state, while B2–B4 emit strongly in the red region with emission quantum yields of up to 0.33. The PMMA films doped with B2 show two separated emission peaks, and their relative intensity is concentration dependent, leading to the fluorescence color varying from greenish yellow to red as the concentrations successively increased from 5 to 80 wt%. The formation of dimers in the ground state is found to be responsible for the red emission in the condensed state. Efficient OLEDs were fabricated by doping 2 wt% B2–B4 as emitters and gave high luminance, current efficiency, and external quantum efficiency of up to 920 cd m−2, 8.00 cd A−1, and 2.15%, respectively.

ChemInform Abstract: Four‐Coordinate Organoboron Compounds for Organic Light‐Emitting Diodes (OLEDs)

AbstractReview: progress in the molecular construction of four‐coordinate organoboron compounds with an emphasis on their applications in OLEDs; 118 refs.

Quantitative structure-property relationships of electroluminescent materials: Artificial neural networks and support vector machines to predict electroluminescence of organic molecules

Electroluminescent compounds are extensively used as materials for application in OLED. In order to understand the chemical features related to electroluminescence of such compounds, QSPR study based on neural network model and support vector machine was developed on a series of organic compounds commonly used in OLED development. Radial-basis function-SVM model was able to predict the electroluminescence with good accuracy (R = 0·90). Moreover, RMSE of support vector machine model is approximately half of RMSE observed for artificial neural networks model, which is significant from the point of view of model precision, as the dataset is very small. Thus, support vector machine is a good method to build QSPR models to predict the electroluminescence of materials when applied to small datasets. It was observed that descriptors related to chemical bonding and electronic structure are highly correlated with electroluminescence properties. The obtained results can help in understating the structural features related to the electroluminescence, and supporting the development of new electroluminescent materials.

Brightly Blue and Green Emitting Cu(I) Dimers for Singlet Harvesting in OLEDs

With the chelating aminophosphane ligands Ph2P-(o-C6H4)-N(CH3)2 (PNMe2) and Ph2P-(o-C6H4)-NC4H8 (PNpy), the four halide (Cl, Br, I)-bridged copper coordination compounds [Cu(μ-Cl)(PNMe2)]2 (1), [Cu(μ-Br)(PNMe2)]2 (2), [Cu(μ-I)(PNMe2)]2 (3), and [Cu(μ-I)(PNpy)]2 (4) were synthesized and structurally characterized. Their photophysical properties were studied in detail. The complexes exhibit strong blue (λmax = 464 (3) and 465 nm (4)) and green (λmax = 506 (1) and 490 nm (2)) luminescence as powders with quantum yields of up to 65% at decay times as short as 4.1 μs. An investigation of the emission decay behavior between 1.3 and 300 K gives insight into the nature of the emitting states. At temperatures below T ≈ 60 K, the decay times of the studied compounds are several hundred microseconds long, which indicates that the emission originates from a triplet state (T1 state). DFT calculations show that this state is of (metal+halide)-to-ligand charge transfer (3)(M+X)LCT character. Investigations at 1.3 K allow us to gain insight into the three triplet substates, in particular, to determine the individual substate decay times being as long as a few milliseconds. The zero-field splittings are smaller than 1 or 2 cm(-1). With an analysis of these data, conclusions about the effectiveness of spin-orbit coupling (SOC) can be drawn. Interestingly, the large differences of SOC constants of the halides are not obviously displayed in the triplet state properties. With a temperature increase from T ≈ 60 to 300 K, a significant decrease of the emission decay time by almost 2 orders of magnitude is observed, and at ambient temperature, the decay times amount only to ∼4-7 μs without a significant reduction of the emission quantum yields. This drastic decrease of the (radiative) decay time is a result of the thermal population of a short-lived singlet state (S1 state) that lies energetically only a few hundred wavenumbers (460-630 cm(-1)) higher than the T1 state. Such an emission mechanism corresponds to a thermally activated delayed fluorescence (TADF). At ambient temperature, almost only a delayed fluorescence (∼98%) is observed. Compounds showing this mechanism are highly attractive for applications in OLEDs or LEECs as, in principle, it is possible to harvest all singlet and triplet excitons for the generation of light in the lowest excited singlet state. This effect represents the singlet harvesting mechanism.

SnO2 thin films doped indium prepared by the sol–gel method: structure, electrical and photoluminescence properties

This paper reports on the preparation, characterization, electrical and optical properties of tin oxide (SnO2) thin films doped indium prepared by the sol–gel method and deposited on glass substrates with dip coating technique. X-ray diffraction patterns showed an increase in the crystallinity of the films with increase in annealing temperatures. Atomic force microscopy analyses revealed an increase of grain growth with raise in annealing temperature. The film surface revealed positive skewness and kurtosis values less than 3 which make them favorable for OLEDs applications. The lowest resistivity (about 10−7) was obtained for the ITO films annealed at 500 °C. These films acquire n-type conductivity due to the non-stoichiometric in the films like (interstitial tin atoms) and also due to low indium doping concentration. The optical properties of the films have been studied from transmission spectra. An average transmittance of >80 % in ultraviolet–visible region was observed for all the films. Optical band gap energy (Egap) of ITO films was found to vary in the range of 3.69–3.81 eV with the increase in annealing temperature. This slight shift of Egap to higher photon energies could be related to the crystalline nature of the films associated with the decrease in the defect concentration caused by annealing. Photoluminescence spectra of the films exhibited an increase in the emission intensity with increase in annealing temperature. The high temperature annealing would be expected to decrease the density of defects, improve the crystal orientation and reduce the traps for non-radiative transition and also increase the oxidation processes.