OLED Materials Research Articles

Disilane- and siloxane-bridged biphenyl and bithiophene derivatives as electron-transporting materials in OLEDs

Optical, electrochemical, and electron-transporting properties of disilane- and siloxane-bridged biphenyl and bithiophene derivatives were investigated, in comparison with those of the monosilane-bridged analogues (siloles). The UV spectra and cyclic voltammograms indicated that elongation of the silicon bridge suppresses the π-conjugation, in accordance with the results of DFT calculations. The DFT calculations indicated also that the disilane-bridged biphenyl and siloxane-bridged bithiophene should have the low-lying HOMOs and LUMOs. The electron-transporting properties were evaluated by the performance of triple-layered OLEDs having vapor-deposited films of the Si-bridged compound, Alq 3, and TPD, as the electron-transport, emitter, and hole-transport, respectively. Of these, the device with a disilane-bridged biphenyl exhibited the high performance with the maximum current density of 590 mA/cm 2 at the applied electric field of 12 × 10 7 V/m (applied bias voltage = 13 V) and the maximum luminance of 22 000 cd/m 2 at 13 × 10 7 V/m.

Work function and valence band structure of tin-doped indium oxide thin films for OLEDs

Transparent conducting ITO thin film has been widely used as anode material in OLEDs due to its good optical transparency, low electrical resistivity, ease of patterning, high work function and efficient hole injection properties. The interface between ITO and organic layer in OLED device is thus important and can influence the electrical and luminescent properties. In this report, ITO substrates were treated with 20% H 3PO 4 solution. The corresponding changes in crystalline morphology were studied by X-ray diffraction. X-ray photoelectron spectroscopy (XPS) and ultraviolet photoelectron spectroscopy (UPS) were performed at ∼10 −9 Torr to study the work function and the valence band structure of ITO substrates. It was found that work function became slightly lower after the treatment, probably caused by the formation of metal complex compounds and metal hydroxides. The binding energy of In 3d 5/2 shifted from 444.6 to 445.3 eV. This shifting was referred to the formation of In–OH bonding. It would be easier to provide electron by In–OH bonding than by In–O–In or Sn–O–Sn when photons reached ITO surface. The interface between ITO and CuPc was improved through polar surface and less aggregation. In addition, the OLED devices exhibited improved performance in both external quantum efficiency and luminescence efficiency.

Blue light-emitting and hole-transporting amorphous molecular materials based on diarylaminobiphenyl-functionalized bimesitylenes

The diarylaminobiphenyl-functionalized bimesityls and exhibit amorphous nature, high thermal stability and excellent blue emission in the solid state. They serve as both hole-transporting and emissive materials in OLEDs for blue emission with high external quantum efficiencies.

Photoluminescence, photoabsorption and photoemission studies of hydrazone thin film used as hole transporting material in OLEDs

A fotoluminescencia de filmes finos de 1-(3-metilfenil)-1,2,3,4-tetrahidroquinolina-6carboxialdeido-1,1’-difenilhidrazona foi monitorada em funcao da irradiacao com luz UV. A intensidade da emissao decresce exponencialmente com o tempo de exposicao, sugerindo degradacao das amostras. Com o objetivo de investigar os mecanismos de degradacao e determinar a estrutura eletronica desse material orgânico usado com sucesso como camada transportadora de buracos na fabricacao de diodos orgânicos emissores de luz (OLEDs), foram empregadas as tecnicas de fotoabsorcao e de fotoemissao nas bordas 1s do carbono e do nitrogenio bem como na banda de valencia. A influencia da luz solar foi simulada usando radiacao sincrotron nao-monocromatica. Apos exposicao, todos os espectros apresentam um decrescimo nos sinais de fotoabsorcao e de fotoemissao, que e menos acentuado na borda do carbono, apresentando, entretanto, um decrescimo drastico na borda do nitrogenio e na regiao de valencia. O estudo sugere que a perda de nitrogenio e a principal causa para a quebra do sistema π, levando, dessa forma, a falha do dispositivo fabricado com esse composto. Photoluminescence (PL) emission of 1-(3-methylphenyl)-1,2,3,4-tetrahydroquinoline-6carboxyaldehyde-1,1 ’ -diphenylhydrazone (MTCD) thin films was monitored as a function of UV irradiation, and it was found to decrease exponentially with the exposure time. In order to gain insight into the degradation mechanisms and evaluate the electronic structure of this organic material used with good results as hole transporting layer (HTL) in the fabrication of organic light emitting diodes (OLEDs), synchrotron radiation-based photoabsorption and photoemission techniques at the carbon and nitrogen 1s edges as well as at the valence band were employed. The influence of sunlight was simulated using non-monochromatized synchrotron radiation. After exposure all the spectra show a decrease of the photoabsorption and photoemission signals, however, while it is less accentuated at the carbon edge, at the nitrogen edge and at the valence region it decreases drastically. The loss of nitrogen is suggested to be the main step in the disruption of the π system, leading to the failure of the devices fabricated with this compound as hole transporting layer.

Latest advances in substrates for flexible electronics

Abstract— Recent advances in both organic‐ and inorganic‐based electronics processed on flexible substrates offer substantial rewards in terms of being able to develop displays that are thinner, lighter, robust, and conformable, and can be rolled away when not required. In addition, plastic‐based substrates coupled with the recent developments in solution deposition and ink‐jet printing for laying down OLED materials and active‐matrix thin‐film‐transistor (TFT) arrays open up the possibility of cost‐effective processing in high volumes using roll to roll (R2R) processing. To replace glass, however, a plastic substrate needs to be able to offer some or all of the properties of glass, i.e., clarity, dimensional stability, thermal stability, barrier, solvent resistance, and low coefficient of thermal expansion (CTE) coupled with a smooth surface. In addition, a conductive layer may be required. No plastic film offers all these properties so any plastic‐based substrate will almost certainly be a multilayer composite structure. This paper will discuss the issues associated with selecting plastic materials, contrast the various options, and highlight how to gain optimum performance through process control. This will be illustrated with examples of film in use in flexible electronic applications.

Hole Mobilities of 2,7- and 2,2′-Disubstituted 9,9′-Spirobifluorene-Based Triaryldiamines and Their Application as Hole Transport Materials in OLEDs

Hole mobilities of up to 2 × 10−3 cm2 V−1 s−1 of 2,7- and 2,2′-disubstituted spirobifluorene-based triaryldiamine derivatives have been measured using time-of-flight (TOF) techniques. Among these derivatives, 27DPSF (possessing diphenylamino groups attached onto the same biphenyl branch of spirobifluorene) and 22DPASF (possessing extended π-conjugation of the chromophore) exhibited the highest hole mobilities because of their greater intermolecular interactions. The charge transport parameters were extracted, using a Gaussian disorder model (GDM), from detailed temperature- and field-dependent hole mobility measurements. For 2,2′-disubstituted systems possessing different diarylamino groups, the hole mobilities decreased in the order 22DPSF > 22DTSF > 22DBPSF, indicating that spatial hindrance had a crucial effect on the hole transport behavior. To verify these disubstituted spirobifluorene-based amines can be used as HTLs in OLED devices. A standard device was fabricated with the configuration ITO/m-MTDA...

Steric Inhibition of π-Stacking: 1,3,6,8-Tetraarylpyrenes as Efficient Blue Emitters in Organic Light Emitting Diodes (OLEDs)

The sterically congested tetraarylpyrenes 1-3, which can be readily accessed by Suzuki coupling, exhibit no-aggregation (pi-stacking) behavior in both solution and solid states. The indisposed tendency of 1-3 toward crystallization and their moderate molecular dimensions permit exploitation as blue light emitting materials in OLEDs with respectable device performances.

Synthesis and Electron Transport Layer Properties of Zinc Metallic Complexes Containing Quinoline Moieties in OLED

Metal chelate complexes are often studied as a carrier transport material in OLEDs, because they have a good electron transport property. We have synthesized metal chelate complexes, Bis(8-hydroxyquinolinato)zinc (Znq2) and Bis(10-hydroxybenzo[h]quinolinato)zinc (Zn(bq)2) as well as Bis(10-hydroxybenzo[h]quinolinato)beryllium Be(bq)2 and compared EL performance with commercialized Alq3. We will report the basic EL performance of synthesized complex materials such as EL spectrum, current efficiency and CIE value. When we applied Znq2 and Zn(bq)2 to conventional blue OLED device, the device exhibited better power efficiency of 0.4 lm/w and 0.28 lm/w compared to 0.2 lm/w of Alq3 device. We also introduce overall EL performance of metal chelate device including Be(bq)2.

Molecular Ordering and the Current-Voltage Characteristics of m-MTDATA Thin Films

Molecular ordering and current-voltage characteristics of vacuum-deposited m-MTDATA(4,4’,4’’-tris[N,-(3-methylphenyl)-N-phenylamino]triphenylamine), widely used as a hole injection material in OLEDs, thin films were investigated. Molecular ordering was induced by thermal annealing under electromagnetic field after deposition of m-MTDATA onto the pre-patterned ITO glass. AFM and XRD analysis were employed to characterize the topology and molecular ordering of m-MTDATA thin films. The XRD and AFM results show that m-MTDATA can be molecularly ordered by means of thermal annealing under electromagnetic field. Thermal annealing at 100°C was desirable to get a high degree of molecular ordering with dendritic grains. It was shown that molecular ordering as well as larger dendritic grains in the thin films influenced on improving the current-voltage characteristics and increasing the leakage current of the ITO/m-MTDATA/Al device. Electromagnetic field improved the surface roughness, as well. It is regarded that Rpv seems more significant than the other roughness parameters. Significantly lower Rpv(peak-to-valley roughness) obtained by both thermal annealing and electromagnetic field resulted in enhancing the stability of the current ITO/m-MTDATA/Al device. Ra(average roughness) and Rrms(root-mean-square roughness), however, did not significantly relate with leakage current.

Triplet State Properties of the OLED Emitter Ir(btp)2(acac): Characterization by Site-Selective Spectroscopy and Application of High Magnetic Fields

The well-known red emitting complex Ir(btp)2(acac) (bis(2-(2'-benzothienyl)-pyridinato-N,C3')iridium(acetylacetonate)), frequently used as emitter material in OLEDs, has been investigated in a polycrystalline CH2Cl2 matrix. The studies were carried out under variation of temperature down to 1.2 K and at magnetic fields up to B=10 T. Highly resolved emission and excitation spectra of several specific sites are obtained by site-selective spectroscopy. For the preferentially investigated site (I-->0 at 16268 cm-1), the three substates I, II, and III of the T1 triplet state are separated by DeltaEII-I=2.9 cm-1 and DeltaEIII-I=25.0 cm-1, respectively. DeltaEIII-I represents the total zero-field splitting (ZFS). The individual decay times of these substates are tauI=150 micros, tauII=58 micros, and tauIII=2 micros, respectively. The long decay time of the lowest substate I indicates its almost pure triplet character. The time for relaxation from state II to state I (spin-lattice relaxation, SLR) is as long as 22 micros at T=1.5 K, while the thermalization between the two lower lying substates and substate III is fast. Application of a magnetic field induces Zeeman mixing of the substates of T1, resulting in an increased splitting between the two lower lying substates from 2.9 cm-1 at zero field to, for example, 6.8 cm-1 at B=10 T. Further, the decay time of the B-field perturbed lowest substate IB decreases by a factor of about 7 up to 10 T. The magnetic field properties clearly show that the three investigated states belong to the same triplet parent term of one single site. Other sites show a similar behavior, though the values of ZFS vary between 15 and 27 cm-1. Since the amount of ZFS reflects the extent of MLCT (metal-to-ligand charge transfer) parentage, it can be concluded that the emitting state T1 is a 3LC (ligand centered) state with significant admixtures of 1,3MLCT (metal-to-ligand charge transfer) character. Interestingly, the results show that the MLCT perturbation is different for the various sites. An empirical correlation between the amount of ZFS and the compound's potential for its use as emitter material in an OLED is presented. As a rule of thumb, a triplet emitter is considered promising for application in OLEDs, if it has a ZFS larger than about 10 cm-1.

Photoluminescence and Electroluminescence Characteristics of Benzoxazole Dendrimers

Various studies have been done using dendrimers as emissive materials for OLEDs, although efficiencies are still significantly lower than those offered by evaporated devices. In this study, dendrimers were synthesized. Photoluminescence (PL) and Electroluminescence (EL) characteristics of Benzoxazole dendrimers were investigated by fabricating the device of structure such as ITO/PVK: Benzoxazole dendrimers: PBD/Li:Al. We used PVK as hole transport material and blended with PBD as electron transport material. The Benzoxazole dendrimers studied here used emissive material. The PL spectra of 1st generation and 2nd generation Benzoxazole dendrimers show blue emission at 438 nm. The EL spectra of 1st generation and 2nd generation Benzoxazole dendrimers were observed at the wavelength of 441 nm and 447 nm.

Fully substituted ethylene as a new class of efficient sky‐blue emitting materials for OLEDs

We synthesized new blue and bluish green emitting materials by using fully substituted ethylene moieties. Multi‐layered EL devices were fabricated with synthesized materials and evaluated in terms of emission color and luminescence efficiency. TBBPE[EML 2] device showed bluish‐green CIE value of (0.236, 0.412) and 5.02cd/A at 10mA/cm2. BPBBPE[EML 3] device also showed sky‐blue CIE value of (0.218, 0.355) and 2.31cd/A at 10mA/cm2.

Novel EDOT and fluorene-based electroluminescent “bricks” as materials for OLEDs

Luminescent materials based on 3,4-ethylenedioxythiophene (EDOT) and dihexylfluorene were designed. EDOT–fluorene– EDOT molecule, C3, was synthesized in high yields by a Stille cross-coupling reaction. The formylation of EDOT units yielded C4 molecule and a subsequent coupling reaction of C4 with a Wittig reagent produced C5. Electrochemical and optical experiments allowed us to accurately determine the energy level diagram corresponding to these three new molecules. Finally, in order to compare the electroluminescent behaviour of the bricks C3, C4 and C5, a simple monolayer OLED structure [ITO/PEDOT-PSS(50 nm)/Cx(50 nm)/Ca(100 nm)/Al(100 nm)] was designed. In the case of C5, energetic barriers at both interfaces were significantly reduced. For these reasons, devices based on C5 exhibit very high luminance values of approximately 1000 Cd/m2 at low applied voltage (below 10 V) with a turn-on voltage of 4.5 V. Maximum quantum and luminous efficiencies of 0.3 Cd/A and 0.1 lm/W are reported. According to the basic single layer structure, such values are very promising if C5 is used as emissive material in a multilayer-device.

Optical behaviors of conjugated-chain compounds containing benzene and furan units

Thirteen conjugated-chain compounds which contain benzene and furan units were prepared, their optical behaviors, including UV–vis absorption coefficient ( ɛ), absorption wavelengths ( λ a), fluorescence emission wavelengths ( λ e), and quantum yields ( Φ) were measured. Meanwhile, their LUMO and HOMO energy were determined by cyclic voltammetry and their second-order polarizations ( β xxx ) values were determined by solvatochromic method, respectively. The results showed that this kind of compounds possess a shorter λ a (320–365 nm) and performance a higher Φ values, especially for 2aa, 2ab, 2ac and 2bb, their Φ values are all more than 90%. These compounds, except 2db, showed a higher β xxx values in DMSO, especially for 2dc (75.77 × 10 −30 m 5 C −1) and 2dd (83.32 × 10 −30 m 5 C −1), than that 10-methyl-acridone (6.578 × 10 −30 m 5 C −1) or 10-benzylacridone (6.845 × 10 −30 m 5 C −1) in DMSO did, and second harmonic generation value of 10-methylacridone and 10-benzylacridone in powder are, respectively, 1.381 and 1.861 times of that value of urea. The β xxx values and Φ values determined for these compounds in this work were lower than these values which were desired in the original work, this phenomena was explained from their molecular structures. This work confirmed that as these compounds performance shorter λ a and higher Φ values, they could be good blue-color optical materials for some fields, such as OLED materials, two-photo absorption materials, fluorescent dyes.

P-183: The Barrier Problem in Flexible OLED Display: A Modeling Approach

One of the major challenges in the development of flexible OLED display is the protection of the OLED materials from damage by ambient moisture and oxygen. This requires the use of polymeric substrates with exceedingly low levels of moisture and oxygen permeabilities. This problem is typically addressed through the use of multi-layer barrier coatings comprising alternating organic/inorganic layers, an approach developed and promoted by Vitex Corp. The multi-layer approach is critically examined through a numerical model based on a defect-dominated diffusion process combined with experiments involving face-to-face lamination of two barrier films. The modeling results identify two regimes that correspond to two distinct permeation mechanisms, and provide general scaling relationships and design criteria for producing multi-layered coatings with superior barrier performance. The results suggest that the most significant gain in barrier performance can be realized when the thickness of the organic/adhesive layer(s) is less than the average pinhole (defect) size in the inorganic barrier layer(s).

Perfect polar stacking of parallel beloamphiphile layers. Synthesis, structure and solid-state optical properties of the unsymmetrical acetophenone azine DCA

Extraordinary high degrees of polar order can be achieved by a rational design that involves the polar stacking of parallel beloamphiphile monolayers (PBAM). This strategy is exemplified by the acetophenone azines MCA (4-methoxy-4'-chloroacetophenone azine) and DCA (4-decoxy-4'-chloroacetophenone azine). The beloamphiphile design aims to achieve strong lateral interactions by way of arene-arene, azine-azine, arene-azine and halogen-bonding interactions. Dipole-induced interactions and halogen bonding dominate interlayer interactions and halogen bonding is shown to effect the layer stacking. Crystals of DCA contain PBAMs with perfect polar order and perfect polar layer stacking, while crystals of MCA features perfect polar order only in one of two layers and layer stacking is polar but not entirely perfect. We report the synthesis of the beloamphiphile DCA, its crystal structure, and we present a comparative discussion of the structures and intermolecular interactions of MCA and DCA. Absorbance and photoluminescence measurements have been carried out for solutions of DCA and for DCA crystals. DCA exhibits a broad emission centered at 2.5 eV when excited with UV radiation. The nonlinear optical response was studied by measuring second harmonic generation (SHG). Strong SHG signals have been observed due to the polar alignment and the DCA crystal's NLO response is 34 times larger than that of urea. Optimization of the beloamphiphile and systematic SAR studies of the polar organic crystals, which are now possible for the very first time, will further improve the performance of this new class of functional organic materials. The materials are organic semiconductors and show promise as blue emitters, as nonlinear optical materials and as OLED materials.

P-188: Fully Substituted Ethylene as a New Class of Highly Efficient Blue Emitting Materials for OLEDs

We synthesized new blue and bluish green emitting materials by using fully substituted ethylene moieties. Multi-layered EL devices were fabricated with synthesized materials and evaluated in terms of emission color and luminescence efficiency. BPBAPE[EML 4] having high Tg of 155 °C showed luminance and power efficiency of 10.33cd/A and 4.0 lm/W without any doping agent. BTBPPA[EML 5] exhibited 5cd/A and 1.67lm/W efficiency with blue CIE value of (0.165, 0.195).

Synthesis, thermal, electrochemical, and photophysical characterization of 1,5-bis(diarylamino)naphthalene derivatives as potential hole transport OLED materials

Novel 1,5-bis(diarylamino)naphthalene derivatives (1–9), which have potential as hole-transporting materials for electroluminescent devices, were obtained through palladium-catalyzed coupling of diarylamines and 1,5-dibromonaphthalene. The thermal, electrochemical, and photophysical properties of these compounds were examined and the effects of the N-aryl substituents on these properties were investigated. These materials possess glass transition temperatures (Tg) that range from 70–131 °C and these values are related to the identity of the aryl substituents. Cyclic voltammetric measurements demonstrated that these compounds possess two reversible oxidation processes and were further used to estimate the HOMO energy levels of these materials by comparison with the ferrocene/ferrocenium couple. The intramolecular charge mobility, as gauged by the difference between the two oxidation potentials, indicates that compounds 1–8 have a similar degree of delocalization as meta-diaminobenzene derivatives while that of 9 is similar to TPD. Compounds 1–9 emit in the blue-green region and optical absorption and emission data for these materials can be rationalized in terms of the electronic donating properties of the aryl substituents. This data when combined with the electrochemically determined HOMO energies allowed estimation of the LUMO energy levels.Key words: hole transport materials, diaminonaphthalene, thermal analysis, palladium-catalyzed arylation.

Theoretical investigation of carbazole derivatives as hole-transporting materials in OLEDs

Carbazole and its derivatives were widely used as the hole-transporting material. In this study, ab initio DFT B3LYP/6-31G* was performed for the carbazole and 1-, 2-, 3-, 4- and 9-mono-substituted as well as 3,6-di-substituted carbazoles with electron-donating and -withdrawing substituents. The total energies, HOMO and LUMO orbital energies of these compounds at neutral, cation and anion states were obtained based on the optimized geometrical structures. The ionization potential ( I p) and electron affinity ( E a) were generated by means of the DFT calculated total energy of neutral, cation and anion states. These theoretical I p and E a were consistent with experimental data. Further investigation on 1-, 2-, 3- and 4-mono-substituted carbazole, we conclude that these carbazole derivatives with electron-donating substituents should decrease their I p and E a values while increasing the HOMO and LUMO energies. On the contrary, the mono-substituted carbazole with electron-withdrawing substituent have the opposite results to those carbazole derivatives with electron-donating substituents. Comparing to carbazole, Δ E decreased in the mono-substituted carbazole derivatives with electron-donating and -withdrawing substituents. The substituent effect in 3,6-di-substituted carbazoles is more obvious than that of 3-substituted carbazole. According to the calculated results, the calculated I p has an excellent agreement with the experimental one-electron oxidation potential ( E p/2). Presumably, this calculation can be employed to predict EL character for other OLED materials.

A New AC-Driving Method for Active-Matrix OLED Displays

The lifetime of organic light-emitting devices can be influenced by the OLED material and structure, electrode material, driving mode and so on. Among all these factors, OLED driving method has been regarded as one of the most important factors, and it is well known that ac-driving mode improves the lifetime of OLED, especially pulse-driving mode with a reversed-biased voltage on OLED. In this article, a new ac-driving mode for Active-Matrix OLED is proposed on basic fabrication of two-TFT current source pixel circuit. Simulation has been done on the pixel and 1×4 matrix circuits, and simulation results have demonstrated that OLED is in reversed-biased state during recovery time. At last, a practical peripheral circuit that can provide achieve an ac-driving mode has been designed.