Alq3 Emission Layer Research Articles

Characterization of vacuum-deposited and solution-processed N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine hole transport layers based organic light emitting devices

Abstract We report the operation characteristics of green tris-(8-hydroxyquinoline)aluminium (Alq3) organic light emitting devices (OLEDs) with a N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB) hole transport layer forming by either vacuum evaporation or solution processing. The maximum luminous yield of device having a vacuum-deposited NPB layer was 2.97 cdA-1 while that of device with solution-processed NPB layer was 2.56 cdA-1. To evaluate the operation characteristics of OLED devices, current densities with respect to bias voltages were investigated by using a modified Shockley equation. Similar behavior was obtained for ideality factors n in two types of devices suggesting the equivalent potential barriers and interface states for injection of holes from indium tin oxide anode to Alq3 emission layer. However, a lower parasitic series resistance (RS) and significant larger saturation current density (J0) were observed from the device with vacuum deposition method, which we attributed to a more dense and homogeneous NPB layer that is more favorable for hole current flow.

Tetraalkyl-substituted zinc phthalocyanines used as anode buffer layers for organic light-emitting diodes**Project supported by the Shenzhen Personal Maker Project, China (Grant No. GRCK2017082316173208), the Shenzhen Overseas High-level Talents Innovation Plan of Technical Innovation, China (Grant No. KQJSCX20180323140712012), and the Special Funds for the Development of Strategic Emerging Industries in Shenzhen, China (Grant No. JCJY20170818154457845).

Two soluble tetraalkyl-substituted zinc phthalocyanines (ZnPcs) for use as anode buffer layer materials in tris(8-hydroxyquinoline)aluminum (Alq3)-based organic light-emitting diodes (OLEDs) are presented in this work. The hole-blocking properties of these ZnPc layers slowed the hole injection process into the Alq3 emissive layer greatly and thus reduced the production of unstable cationic Alq3 () species. This led to the enhanced brightness and efficiency when compared with the corresponding properties of OLEDs based on the popular poly-(3, 4-ethylenedioxythiophene): poly(styrene sulfonate) (PEDOT: PSS) buffer layer. Furthermore, because of the high thermal and chemical stabilities of these ZnPcs, a nonaqueous film fabrication process was realized together with improved charge balance in the OLEDs and enhanced OLED lifetimes.

Surface defects impeded excitons in Alq3 based hetero junction OLEDs

Hetero junction organic light emitting device consists of N,N′-diphenyl-N,N′-bis(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/tris-(8-hydroxyquinoline)aluminum (Alq3)/LiF/Al were fabricated on indium tin oxide coated corning glass substrate under various Alq3 deposition rates. We demonstrate the significance of deposition rate on the morphology of Alq3 layers and subsequently generation of photons in TPD/Alq3 hetero junction. The pin holes formed on the surface of Alq3 at low deposition rates are responsible for exciton quenching. The device fabricated with the deposition rate of 3.0Å/s exhibits the maximum luminescence and maximum current efficiency of 1.02×104cd/m2 and ∼5cd/A, respectively, at 15V owing to the efficient recombination of excitons in Alq3 emissive layer.

A possible mechanism to tune magneto-electroluminescence in organic light-emitting diodes through adjusting the triplet exciton density

At 15 K, the high field decay of magneto-electroluminescence (MEL) in tris (8-hydroxyquinoine) aluminum (Alq3)-based organic light-emitting diodes is greatly suppressed by mixing hole transport material N,N′-bis(naphthalen-1-y)-N,N′-bis(phenyl) benzidine (NPB) into Alq3 emission layer. The positive correlation between the MEL decay and the triplet exciton density indicates the reduced triplet exciton density is the major contributing factor. The change of triplet exciton density and then the MEL upon NPB mixing may result from the strengths change of charge-triplets interaction, in which the injected charges flowing through the exciton formation zone without forming static space charges play the most important role.

Similar Roles of Electrons and Holes in Luminescence Degradation of Organic Light-Emitting Devices

Intrinsic degradation in organic light-emitting devices (OLEDs), the dominating cause of the short device lifetime, continues to be a critical issue for wider commercial application. For many years, intrinsic degradation in tris(8-hydroxyquinoline) aluminum (AlQ3)-based OLEDs has been known to be caused by excessive hole injection into the AlQ3 emissive layer, due largely to earlier observations that electrons and holes act differently in the devices. However, a further investigation here leads to the discovery that excessive electrons can also induce significant degradation of the AlQ3 layer, similar to what has been established for holes. The new understanding of the degradation mechanism of OLEDs is instrumental in directing the efforts of developing stable devices.

Study on Electron Injection Effect of Low Molecular Organic Light-emitting Diodes Utilizing LiF/Al and Li/Al Cathode

ABSTRACTThe electron injection effects of organic light-emitting diodes (OLED) from the cathode interface between LiF or Li and Al on the tris-(8-hydroxyquinolinato) aluminum (Alq3) emissive layer have been investigated. Efficient electron injection was achieved in the case of thin layer of LiF or Li deposited on Alq3 emissive layer at the thickness of 0.5 nm and 2 nm, respectively. The results indicate that the coexistence of LiF and Al layer or the Li and Al layer on the Alq3 emissive layer result in the efficient electron injection. Electron injection effect in the case of LiF/Al and Li/Al electrode configurations and also the effect of air explosion during the electrode formation have been discussed. In the case of the device with Li, the diffused Li in the Al layer acts as the efficient carrier injection process.

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.

Time resolved luminescence properties of Al q3 for spin-injection into organic semiconductor

The electroluminescence and conduction properties of organic light emitting devices (OLEDs) with Al q3 emissive layer and with Fe cathode were investigated. The relative luminescence intensity for the device became lower than that with Al cathode. Time resolved photoluminescence of Al q3 was measured to investigate the electronic state of the interface between Fe and Al q3 layer of OLEDs. The difference of luminescence intensities of OLEDs could not be explained only due to the work functions, but also due to the essential increase of quenching centers.

Synthesis of TPD-Containing Polymer Thin Films by Physical Vapor Deposition

ABSTRACTPolymer thin films having tetraphenyl diaminobiphenyl (TPD) side chain were prepared by evaporating its acrylate monomer by the ionization-assisted deposition (IAD) method. IR absorption and GPC analyses showed that the electron irradiation to the evaporated material works efficiently to initiate the polymerization reaction on the substrate surface, while the simple evaporation produced films mainly consisting of the monomers. The reaction was enhanced by increasing the substrate temperature. However, the surface roughness increased with increasing substrate temperature. Post annealing can be an alternative method to enhance the polymer yield. Light emitting diodes were prepared by stacking the TPD polymer layer, Alq3 emissive layer, and Al electrode on ITO-coated glass. The IAD polymerization can be a useful method for preparing polymer thin films for organic optoelectronic devices.

Effect of location and width of doping region on efficiency in doped organic light-emitting diodes

We systematically investigated an optimum structure (doping width and location) of organic light-emitting diodes (OLED) with Coumarin 540 (C540)-doped aluminum quinoline (Alq3) emission layer. Our OLEDs consisted of ITO/triphenyl diamine derivative (TPD) [50 nm]/Alq3 emission layer [50 nm]/AlLi. We fabricated seven kinds of partially C540-doped OLEDs. These OLEDs are classified into two types of doping device structure: the OLEDs having continuous C540-doping region with a different doping width; the OLEDs having separated C540-doping regions. When the doping width was changed from 0 nm to 50 nm, the OLEDs with a 10-nm thick doping region had the highest EL efficiency of all. The main recombination zone was 10-nm thick near the TPD interface. C540 molecules are found to prevent electrons and holes from recombining on Alq3 molecules in an Alq3 layer by regarding C540 molecules in Alq3 as carrier traps and hopping sites.

Organic electroluminescence cells based on thin films deposited by ultraviolet laser ablation

Organic thin films of copper-phthalocyanine (CuPc) and aluminum tris-8-hydroxyquinline (Alq3) were fabricated via KrF laser ablation. In spite of poor crystallinity exhibited in the reconstructed organic films, the CuPc film still works as a hole-transport layer in electroluminescence cells based on the CuPc and Alq3 layers. Electroluminescence at ∼500 nm was obtained with a low initiation voltage of ∼12 V. There is no significant difference in electroluminescence performance for devices fabricated via laser ablation and thermal evaporation. Emission at ∼600 nm caused by energy transfer from Alq3 to 4-(Dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) is also observed from a DCM-doped Alq3 emission layer.

A study of the electroluminescence process of an organic electroluminescence diode with an Alq3 emission layer using a dye-doping method

The authors prepared an organic electroluminescent diode (organic LED) with hole transport material and aluminium quinoiline and another organic LED with an additional electron blocking layer using the DCM partial doping method. The authors studied the change in the EL spectra accompanied by the applied current in the specimens by means of the peak separation method. The electron-hole recombination did not always occur at the emission layer near the interface between the hole transport layer and the Alq3 emission layer from the initial EL stage after electrons had been transported from cathode to the interface. It was proved that the emitting region is shifted from the cathode side to the anode side in the emission layer with increasing applied current, especially by electron injection from the cathode. In addition, from the measurement of the specimen with an electron blocking layer, the authors found that the dominant EL process is not due to the carrier trap model in an organic DCM-doped Alq3 LED.