Alq3 Host Research Articles

Numerical simulation of charge carriers distribution and the relevant electric fields in host–guest systems of small molecules

The Monte Carlo method has been used to simulate the average number of injected electrons and the related electric potential and field distributions in a single film of tris-(8-hydroxyquinoline) aluminum (Alq3) and 8-hydroxyquinolinato lithium (Liq) blend which is sandwiched between two electrodes of Al. In our simulations, two correlated and uncorrelated states have been taken into account for the energy of the charge carriers which are located on a 3D simple cubic lattice of sites. The layer averaged number of charges is maximized near the injecting and collecting electrodes, and it decreases as the distance increases from the electrodes for the two states. Moreover, results show that the layer averaged number of charges is higher in pristine Alq3, as compared to other materials in the study, but no change is observed in the width of space charge regions in the materials. The electric field distribution related to the electrons injected into the materials is non-uniform, and it has higher values near the electrodes than other places. The electric field distribution shows a nonlinear behavior when the concentration of Liq increases in the Alq3 host, although it moves towards a linear state, and the magnitude of electric field increases as the blending ratio decreases.

MoO3 as p-type dopant for Alq3-based p–i–n homojunction organic light-emitting diodes

Using high-work-function material MoO3 as a p-type dopant, efficient single-layer hybrid organic light-emitting diodes (OLEDs) with the p–i–n homojunction structure are investigated. When MoO3 and Cs2CO3 are doped into the conventional emitting/electron-transport material tris-(8-hydroxyquinoline) aluminum (Alq3), respectively, a significant increase in p- and n-type conductivities is observed compared to that of intrinsic Alq3 thin films. With optimal doping, the hole and electron mobilities in Alq3:MoO3 and Alq3:Cs2CO3 films was estimated to be 9.76×10−6 and 1.26×10−4cm2/Vs, respectively, which is about one order of magnitude higher than that of the undoped device. The p–i–n OLEDs outperform undoped (i–i–i) and single-dopant (p–i–i and i–i–n) OLEDs; they have the lowest turn-on voltage (4.3V at 1cd/m2), highest maximum luminance (5860cd/m2 at 11.4V), and highest luminous efficiency (2.53cd/A at 100mA/cm2). These values are better than those for bilayer heterojunction OLEDs using the same emitting layer. The increase in conductivity can be attributed to the charge transfer process between the Alq3 host and the dopant. Due to the change of carrier concentration in the Alq3 films, the Fermi level of Alq3 is close to the highest occupied molecular orbital (HOMO) or lowest unoccupied molecular orbital (LUMO) energy levels upon p- and n-type doping, respectively, and the carrier injection efficiency can thus be enhanced because of the lower carrier injection barrier. The carriers move closer to the center energy levels of the HOMO or LUMO distributions, which increases the hopping rate for charge transport and results in an increase of charge carrier mobility. The electrons are the majority charge carriers, and both the holes and electrons can be dramatically injected in high numbers and then efficiently recombined in the p–i–n OLEDs. As a result, the improved conductivity characteristics as well as the appropriate energy levels of the doped layers result in improved electroluminescent performance of the p–i–n homojunction OLEDs.

Heavily Doped, Charge-Balanced Fluorescent Organic Light-Emitting Diodes from Direct Charge Trapping of Dopants in Emission Layer.

We studied the effect of direct charge trapping at different doping concentrations on the device performance in tris(8-hydroxyquinoline) aluminum (Alq3):10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)-benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T) as a host-dopant system of a fluorescent organic light-emitting diode. With increasing C545T doping concentration, trap sites could lead to the promotion of hole injection and the suppression of electron injection due to the electron-transport character of Alq3 host for each carriers, as confirmed by hole- and electron-only devices. Direct charge injection of hole carriers from the hole transport layer into C545T dopants and the charge trapping of electron carriers are the dominant processes to improve the charge balance and the corresponding efficiency. The shift of the electroluminescence (EL) spectra from 519 nm to 530 nm was confirmed the exciton formation route from Förster energy transfer of host-dopant system to direct charge trapping of dopant-only emitting systems. Variation in the doping concentration dictates the role of the dopant in the fluorescent host-dopant system. Even though concentration quenching in fluorescent dopants is unavoidable, relatively heavy doping is necessary to improve the charge balance and efficiency and to investigate the relationship between direct charge trapping and device performance. Heavy doping at a doping ratio of 6% also generates heavy exciton quenching and excimer exciton, because of the excitons being close enough and dipole-dipole interactions. The optimum device performance was achieved with a 4%-doped device, retaining the high efficiency of 12.5 cd/A from 100 cd/m(2) up to 15,000 cd/m(2).

Red fluorescent materials based on julolidine/chromene with the bulky tert-butyl and trimethylsilyl substituents for organic light-emitting diodes.

In this work, we designed and synthesized two red emitters 2-(6,8-di-tert-butyl-2-(2-(1,1-dimethyl-7-(trimethylsilyl)-7-((trimethylsilyl)methyl)-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)vinyl)-4H-chromen-4-ylidene)malononitrile (Red 1) and 2-(6,8-di-tert-butyl-2-(2-(1,1,7-trimethyl-7-t-butyl)-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)vinyl)-4H-chromen-4-ylidene)malononitrile (Red 2). To explore the electro-luminescence properties of these materials, multilayered OLEDs using these materials as dopants in a Alq3 host were fabricated. Particularly, by using Red 1 as a dopant in emitting layer, device 1 showed the luminous and power efficiencies of 0.81 cd/A and 0.43 lm/W at 20 mA/cm2, respectively. The CIE coordinates of this device was (0.65, 0.34) at 7.0 V, which is close to the NTSC standard CIE coordinates of (0.67,0.32) for red emission.

Red Fluorescent Donor-π-Acceptor Type Materials based on Chromene Moiety for Organic Light-Emitting Diodes

Two red emitters, 2-(7-(4-(diphenylamino)styryl)-2-methyl-4H-chromen-4-ylidene)malonitrile (Red 1) and 2(7-(julolidylvinyl)-2-methyl-4H-chromen-4-ylidene)malonitrile (Red 2) have been designed and synthesized for application as red-light emitters in organic light emitting diodes (OLEDs). In these red emitters, the julolidine and triphenyl moieties were introduced to the emitting core as electron donors, and the chromederived electron accepting groups such as 2-methyl-(4H-chromen-4-ylidene)malononitrile were connected to electron donating moieties by vinyl groups. To explore the electroluminescence properties of these materials, multilayered OLEDs using red materials (Red 1 and Red 2) as dopants in Alq3 host were fabricated. In particular, a device using Red 1 as the dopant material showed maximum luminous efficiencies and power efficiencies of 0.82 cd/A and 0.33 lm/W at 20 mA/cm 2 . Also, a device using Red 2 as a dopant material presented the CIEx,y coordinates of (0.67, 0.32) at 7.0 V.

Red Fluorescent Emitting Materials Based on Di-<I>tert</I>-Butyl Chromene Derivatives for Organic Light-Emitting Diodes

In this paper are described two di-tert-butyl chromene-containing red fluorescent materials (Red 1 and Red 2). To explore the electroluminescence properties of these materials, multilayered OLEDs using these materials as dopants in a Alq3 host were fabricated. In particular, a device using Red 2 as the dopant material showed maximum luminous efficiencies and power efficiencies of 1.14 cd/A and 0.58 Im/W, respectively. The CIEx,y coordinates of this device were (0.67, 0.32) at 7.0 V.

Rational Design of Charge‐Neutral, Near‐Infrared‐Emitting Osmium(II) Complexes and OLED Fabrication

AbstractA new series of charge neutral Os(II) isoquinolyl triazolate complexes (1–4) with both trans and cis arrangement of phosphine donors are synthesized, and their structural, electrochemical and photophysical properties are established. In sharp contrast to the cis‐arranged complexes 2–4, the trans derivative 1, which shows a planar arrangement of chromophoric N‐substituted chelates, offers the most effective extended π‐delocalization and hence the lowest excited state energy gap. These complexes exhibit phosphorescence with peak wavelengths ranging from 692–805 nm in degassed CH2Cl2 at room temperature. Near‐infrared (NIR)‐emitting electroluminescent devices employing 6 wt % of 1 (or 4) doped in Alq3 host material are successfully fabricated. The devices incorporating 1 as NIR phosphor exhibit fairly intense emission with a peak wavelength at 814 nm. Forward radiant emittance reaches as high as 65.02 µW cm−2, and a peak EQE of ∼1.5% with devices employing Alq3, TPBi and/or TAZ as electron‐transporting/exciton‐blocking layers. Upon switching to phosphor 4, the electroluminescence blue shifts to 718 nm, while the maximum EQE and radiance increase to 2.7% and 93.26 (μW cm−2) respectively. Their performances are optimized upon using TAZ as the electron transporting and exciton‐blocking material. The OLEDs characterized represent the only NIR‐emitting devices fabricated using charge‐neutral and volatile Os(II) phosphors via thermal vacuum deposition.

Asymmetric Pentacene Derivatives for Organic Light-Emitting Diodes

Two aryl-substituted red emitting pentacenes, 5,6,13,14-tetraphenylpentacene (asym-TPP) and 5,14-bis(2,6-dimethylphenyl)-6,13-diphenylpentacene (DMPDPP), have been prepared and spectroscopically characterized. Guest−host films of pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq3) exhibit narrow red emission (λmax = 663−680 nm), indicative of efficient Forster energy transfer from the Alq3 host to the guest molecules. Solid-state absolute photoluminescence quantum yields of the films were measured as a function of guest molecule concentration (ΦPL ∼20% at 0.20−0.50 mol %). Red light-emitting diodes based on the fluorescent DMPDPP yield external electroluminescence quantum efficiency (ηEL ∼1%) close to the theoretical limit.

Functionalized Pentacene Derivatives for Use as Red Emitters in Organic Light-Emitting Diodes

A series of three pentacene derivatives featuring substituted aryl groups at the 6,13-position, namely, 6,13-diphenylpentacene, 6,13-bis(2,6-dimethylphenyl)pentacene, and 6,13-bis(4-tert-butylphenyl)pentacene, have been synthesized and characterized. The optical properties of each compound have been determined in solution and in the solid state. Films containing the pentacene derivatives dispersed in tris(quinolin-8-olato)aluminum(III) (Alq3) emit in the red with small contribution from the Alq3 host, signaling efficient energy transfer from host to guest molecules. The absolute photoluminescence quantum yield (φPL) of composite films is ∼30−32% at optimum dopant concentration. Organic light-emitting diodes based on these composite films as the active emitting layers have been fabricated and characterized. External electroluminescence quantum efficiencies near the theoretical limit have been achieved.

Preparation and Characterization of Organic Light Emitting Devices Using QD Dopant

Two types of the organic light-emitting devices (OLEDs) with different emission structures were prepared using Alq3 (aluminum tris 8-hydroxyquinoline) host material and quinacridone (QD) dopant at the emission layer. One is the OLED device with emission layer consisting of Alq3 host material doped with QD dopant ("codoped OLED"). The another one has a seperated QD dopant film in the Alq3 emission layer ("undoped OLED"). The maximum brightness of the codoped and undoped OLEDs were 3207 cd/m2 and 1570 cd/m2, respectively. The wavelength of the maximum emission peak in the undoped sample was 527 nm and shifted slightly toward longer wavelength with the value of 540 nm for the codoped OLED sample. The maximum luminous efficiency of the undoped OLED was about 1.4 lm/W and increased to 7.0 lm/W for the codoped sample.

42.4: Passive Matrix Displays Based on the New Red Emitting Dopant RedATDB

AbstractWe present investigations on a new red emitting dye RedATDB. Doping small concentrations of RedATDB in an Alq3 host leads to red OLEDs (x=0.650, y=0.346) with good efficiency values (4.3 cd/A; 2.1 lm/W @ 100 cd/m2). To test our material for display applications, we produced monochrome red passive matrix displays (32 × 100 pixels). With appropriate display measurement equipment we investigate the pixel resolved electrical (I‐V) and optical (luminance) characteristics. Homogenous pixel performance data in terms of luminance and driving voltage has been observed.

Bright red organic light-emitting diodes doped with a fluorescent dye

We have evaluated a synthetic red fluorescent dye, 6-methyl-3-[3-(1,1,6,6-tetramethyl-10-oxo2,3,5,6-tetrahydro-1H,4H,10H-11 -oxa-3a-aza-benzo[de]anthracen-9-yl)-acryloyl]-pyran-2,4-dione (AAAP), as a dopant for an organic light-emitting diode (LED). Bright emission of a good red (maximum luminance: 5600 cd/m2, chromaticity coordinates: x=0.63, y=0.36) was obtained. The device consisted of ITO/TPD(50 nm)/Alq3 doped with AAAP(1.5 mol %,15 nm)/bOXDF(20 nm)/Alq3(25 nm)/Mg:Ag (ITO: indium tin oxide, TPD: N, N′-diphenyl- N,N′-di(3-methylphenyl)-1, 1′biphenyl-4,4′-diamine, Alq3: tris (8-hydroxyquinolinato)-aluminum (III), bOXDF:2,2-bis[5-(4-biphenyl)-1,3,4-oxadiazole-2-yl-4,1 -phenylene]-hexafluoropropane). The bands of fluorescence of the Alq3 host and of absorption of the AAAP dopant overlap in an advantageous way, and insertion of the bOXDF layer between the emission layer, doped with AAAP, and Alq3 layer, made for a device with good properties.

Determination of the energy levels of a phosphorescent guest in organic light emitting devices

We have used photoelectron spectroscopy to study the relative binding energies of the highest occupied molecular orbitals (HOMOs) of the phosphorescent guest, 2, 3, 7, 8, 12, 13, 17, 18octaethyl-21H,23H-porphine platinum (PtOEP), and the organic electron transport host, tris(8-hydroxyquinolinato) aluminum (III) (Alq3) in PtOEP:Alq3 composite films. The PtOEP HOMO position was found to be 0.50±0.13 eV above that of the Alq3 host, independent of the guest molecule concentration (0.8%–22% by mass). These results are consistent with the assumption that the vacuum levels of the guest and host align.