Fabrication Of Organic Light-emitting Diodes Research Articles

Space charge and polarization effects upon doping organic light-emitting diodes with pyran-containing donor-acceptor molecules

Doping of organic light-emitting devices using highly luminescent molecules is a well-established strategy to improve brightness, efficiency and lifetime or to adjust emission color and color purity. Laser dyes from the pyran-containing donor-acceptor family are particularly interesting for emission in the yellow and red spectral domain. In solution, these polar dyes show marked solvatochromic shifts in the absorption and emission. The fluorescence quantum efficiency also depends strongly on solvent polarity, which suggests the occurrence of twisted excited states. In thin solid films such as the ones used in the fabrication of organic light-emitting diodes, additional effects have to be considered. When pyran derivatives are used as doping molecules in a Tris(8-hydroxyquinolinato)aluminium (Alq3) host matrix, the emission color is highly dependent on doping concentration. This is attributed to polarization effects induced by the doping molecules themselves. For some pyran derivatives, such as 4-(dicyanomethylene)-2-methyl-6-{2[(4-diphenylamino) phenyl]ethenyl}-4H-pyran (DCM-TPA), the electroluminescence spectrum is not pure and shows a green Alq3 shoulder, even at high concentrations. We have shown that these effects depend on the position of the frontier orbital energy levels with respect to those of the host. Charge carrier trapping by the doping molecules influences the charge density distribution. At high current densities, this can lead to a shift of the recombination zone out of the doped region. To impede charge recombination processes taking place in the undoped host matrix, charge blocking layers efficiently confine the recombination zone and give rise to increased efficiency.

Realization of Polymeric Optical Integrated Devices Utilizing Organic Light-Emitting Diodes and Photodetectors Fabricated on a Polymeric Waveguide

Direct fabrication of organic light-emitting diodes (OLEDs) and organic photodetectors (OPDs) on polymeric substrates, i.e., polymeric waveguide substrates to form flexile optical integrated devices is demonstrated. The OELD and OPD were fabricated by organic molecular beam deposition (OMBD) technique on a polymeric or a glass substrate, for comparison. The device fabricated on a polymeric substrate shows similar device characteristics to that on a glass substrate. Optical signals of faster than 100 MHz have been created by applying pulsed voltage directly to the OLED utilizing diamine derivative, or rubrene or porphine doped in 8-hydoxyquinolinum aluminum derivatives, as an emissive layer. Electrical signals are successively converted to optical signals for optical transmission of moving picture signals with OLED fabricated on a polymeric waveguide. OPDs utilizing phthalocyanines derivatives with superlattice structure provide increased pulse response with input optical signals, and the OPD with the cutoff frequency of more than 5 MHz has been realized.

Quinoxalines Incorporating Triarylamines: Potential Electroluminescent Materials with Tunable Emission Characteristics

Dipolar compounds featuring quinoxaline acceptors and various triarylamine donors were prepared in good yields and successfully employed in the fabrication of organic light-emitting diodes (OLEDs). It is also demonstrated that the emission color of these compounds can be easily tuned from bluish green to orange by suitably modifying the diarylamine and quinoxaline units independently. Increasing the donor and acceptor strengths bathochromically shifts the absorption and emission bands. These molecules possess moderate glass transition temperatures (114−152 °C) and exhibit high decomposition temperatures (441−554 °C). The two-layer OLEDs fabricated using these materials as hole-transporting and emitting layers and 1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene or tris(8-hydroxyquinolinato)aluminum as the electron-transport layer display promising characteristics, i.e., emission color, luminance, and efficiency. Incorporation of the hole-blocking quinoxaline segments in the hole-transporting triarylamine molecules leads to the confinement of the recombination zone in it, and thus emission is realized mainly from these materials for both types of devices. The factors leading to the funneling of light through the hole-transporting layer in these layers are critically analyzed.

Criteria for ITO (indium–tin-oxide) thin film as the bottom electrode of an organic light emitting diode

TCO (transparent conductive oxide) thin films such as ITO (indium tin oxide) have important roles in fabrication of OLED (organic light emitting diode) displays. Although ITO is the most widely used transparent conductor in the field of FPD (flat panel display) industry, there are some unique requirements from the viewpoint of OLED, which is different from other display devices such as LCD (liquid crystal display) and PDP (plasma display panel). Especially surface roughness and work function are very important to enhance the stability and efficiency of OLED. Because all functional organic layers which act as injection, transportation and emission layers are deposited on ITO, surface morphology of ITO is directly transferred to them and uneven interface is not desirable for the efficiency and stability of OLED. On the other hand, there are several different terminologies— R a (average roughness), R r.m.s. (root-mean-square roughness) and R pv (peak to valley roughness)—which are used to interpret the surface roughness, are now all used altogether. In this work, we tried to define which parameter is the most suitable to characterize ITO for OLED and to explain the relationship between surface and I– V characteristics of OLED. To characterize the effect of surface morphology of the ITO electrode, light emitting diodes were fabricated using various types of ITO films which have different surface morphology and their dynamic characteristics were compared.

Organic Light-Emitting Diodes Based on 2-(Stilben-4-yl)benzoxazole Derivatives: An Implication on the Emission Mechanism

A series of 2-(stilben-4-yl)benzoxazole derivatives (BOXSB-X) was prepared and used as a dopant in the fabrication of organic light-emitting diodes. With a device structure of ITO/NPB/CBP/TPBI:3%BOXSB-X/TPBI/Mg:Ag, where NPB, CBP, and TPBI stand for 4,4‘-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl, 4,4‘-dicarbazolyl-1,1‘-biphenyl, and 2,2‘,2‘ ‘-(1,3,5-phenylene)tris-[1-phenyl-1H-benzimidazole], respectively, light emission from the dopant was observed under electric bias, presumably due to energy transfer between the host TPBI exciton and the dopant. Bright blue emission with a luminance ranging between 8 000 and 13 000 cd/m2 was obtained depending on the substituent on the stilbene moiety. However, with a device structure of ITO/NPB/TPBI:3%BOXSB-X/TPBI/Mg:Ag, where no CBP layer was present, the recombination/emission region shifted to the NPB layer except in the device doped with dimethylamino-substituted benzoxazole derivative, in which case the carrier trap mechanism is suggested to be responsible for ...

Organic light-emitting diodes using a gallium complex

A gallium complex (GaMq2Cl) consisting of two 2-methyl-8-hydroxyquinolines (Mq) and a chlorine was synthesized and used for the fabrication of organic light-emitting diodes (OLEDs). The photoluminescent peak of GaMq2Cl at 492 nm was as strong in intensity as that of tris(8-hydroxyquinolinato)aluminum (Alq3). The OLED using GaMq2Cl as an emitting material showed blue-green luminance of 10 490 cd/m2. When it was used as an electron transport material in a rubrene doped cell, an OLED with a high luminance of 27 700 cd/m2 was obtained. We found that GaMq2Cl also was useful as a host material.

Ionization-Assisted Deposition of 8-Hydroxyquinoline Aluminum for Organic Light Emitting Diode

Ionization-assisted deposition (IAD) was applied for the preparation of 8-hydroxyquinoline aluminum (Alq) in the fabrication of organic light-emitting diodes (LEDs) in the structure of indium-tin-oxide glass/spin-coated polyvinylcarbazole (PVCz)/Alq/vacuum-deposited Mg. When Alq was deposited by IAD directly on PVCz, the luminance at a low drive voltage increased with increasing ion acceleration voltage up to 150 V. However, high ion acceleration caused a leakage current that does not contribute to luminescence, due probably to radiation damage in the PVCz/Alq junction. To avoid ion irradiation at the interface, Alq was deposited in two steps, in which the ion acceleration voltage was turned off in the first stage of deposition. The device characteristic was improved with the addition of this buffer layer. When the first two-thirds of the Alq layer was deposited without ion acceleration, luminescence efficiency was two fold higher than that in the case of no-acceleration deposition or vacuum deposition. The Alq layer formed by IAD had higher surface smoothness, which would contribute to the improvement of the Alq/Mg junction.

Organic light-emitting diodes with radio frequency sputter-deposited electron injecting electrodes

We report a successful fabrication of organic light-emitting diodes (LEDs) with electron injecting electrodes prepared by the radio frequency (rf) magnetron sputtering technique. These LEDs consist of a poly(methylphenylsilane) and a tris-(8-hydroxyquinoline) aluminum (Alq3) layer successively formed on an indium–tin–oxide coated glass, and either an aluminum or magnesium electron injecting electrode. Substantial improvements were observed in such device characteristics as intensity and durability of electroluminescence for LEDs with an rf-sputter-deposited electrode as compared to those with a vacuum-heat-deposited electrode.

Oligomeric bis(1,3-indandiylidene)azines: preparation, electrochemical and spectroscopic properties, and implications for the use of polyazines as conducting materials

Four oligomeric azines of the general structure FlN(NC9H4N)nNFl, where Fl represents the 9-fluorenylidene group, C9H4 the 1,3-indandiylidene unit, and n has the values 1 to 4, have been prepared and characterised. Their electrochemical reduction has been studied by cyclic voltammetry and the results compared with the behaviour of fluorenone azine. Two reversible peaks have been observed for each of the compounds and the peak potentials observed show an unusual alternation in their values as n increases. Vibrational spectra have been recorded for all five compounds. The strong Raman lines associated with the collective stretching of the C–C bonds involved in π-delocalisation [[graphic omitted](ya) modes] have been studied. The effect of chain length on the frequency and intensity of this mode is the reverse of that found, for example, with polyacetylene. This suggests that, while polyazines are unlikely to produce materials of very high electrical conductivity, they may be candidates as materials for electroluminescence purposes in the fabrication of organic light-emitting diodes.