Lithium Quinolate Research Articles

Enhancement of Luminance Efficiency in Organic Light-Emitting Devices Utilizing Mixed Layers Embedded in Electron Transport Layers

While the hole transport decreased in the mixed layer in the organic light-emitting devices (OLEDs) with an Al:lithium quinolate (Liq) mixed layers in the electron transport layer (ETL), the electron injection increased the Al:Liq mixed layer. The enhancement of luminance efficiency in the OLEDs with an Al:Liq mixed layer in the ETL originated from the more balance between the electrons and the holes in the emitting layer due to the decrease in the hole injection and the increase in the electron injection resulting from the existence of the Al:Liq mixed layer.

Efficient electron injection in organic light-emitting diodes using lithium quinolate/Mg:Ag/Al cathodes

A cathode structure of lithium quinolate (Liq)/Mg:Ag/Al was developed and device performances were investigated according to the thickness of Mg:Ag interlayer between Liq and Al. Device performances of Liq/Mg:Ag/Al were optimized at an interlayer thickness of 1.0 nm and power efficiency was enhanced by 50% by introducing Mg:Ag interlayer because of low driving voltage and high current efficiency.

Effect of the Thickness of the Lithium Quinolate and Aluminum Mixed Layer Acting as an Electron Injection Layer on the Efficiency of Organic Light-Emitting Devices

The thickness of the lithium quinolate and aluminum mixed layer acting as an electron injection layer in organic light-emitting diodes (OLEDs) was optimized to enhance the efficiencies of OLEDs. The electrical properties of OLEDs with a mixed layer were compared with those of hole-rich or electron-only devices. The current densities of OLEDs with a mixed layer were lower than those of OLEDs without a mixed layer. The luminance efficiencies of OLEDs with a mixed layer were higher than those of OLEDs without a mixed layer due to an increase in electron current and a decrease in hole current, resulting in achieving more balance of electrons and holes in an emitting layer.

Effect of Ca interlayer thickness on electron injection of lithium quinolate/Ca/Al cathodes

Electron injection behavior of lithium quinolate (Liq)/Ca/Al cathode was investigated by using X-ray photoelectron spectroscopy and ultraviolet photoelectron spectroscopy (UPS). Interfacial energy barrier lowering of Liq/Ca/Al cathode was dependent on Ca thickness and maximum energy level shift was observed at a Ca thickness of 1 nm. Maximum current density could be obtained in Liq/Ca/Al device at a Ca thickness of 1 nm and it was well correlated with energy level shift from UPS measurement. Power efficiency of Liq/Al device could be improved by more than 70% by inserting Ca layer between Liq and Al.

Efficient electron injection in organic light-emitting diodes using lithium quinolate/Ca∕Al cathodes

Device performances of green devices with cathode structure of lithium quinolate (Liq)∕Ca∕Al were investigated and electron injection mechanism was studied using ultraviolet photoelectron spectroscopy. Power efficiency could be improved by 70% by using Liq∕Ca∕Al cathode structure due to efficient electron injection, and interfacial energy barrier lowering by Liq∕Ca∕Al metal cathode was observed.

Efficient Red Electrophosphorescent Devices Based on Iridium Complexes of Fluorinated 1-Phenylisoquinoline

The iridium complexes containing fluorinated 1-phenylisoquinoline ligands were prepared and their luminescent properties were investigated. Density functional theory (DFT) was applied to calculate the energies of highest occupied molecular orbitals (HOMOs) and lowest unoccupied molecular orbitals (LUMOs) in the iridium complexes. Electroluminescent devices were fabricated with a configuration of indium tin oxide (ITO)/4,4',4''-tris[2-naphthylphenylamino]triphenylamine (2-TNATA)/4,4'-bis[N-(naphthyl)-N-phenyl-amino]biphenyl (NPB)/4,4,N,N'-dicarbazolebiphenyl (CBP):dopant/bathocuproine (BCP)/tris-(8-hydroxy-quinoline) aluminum (Alq3)/lithium quinolate (Liq)/Al. The device using bis[1-(5-fluorophenyl)isoquinolinato-C2,N] iridium acetylacetonate [Ir(5-Fpiq)2(acac)] as a dopant showed red emission with 1931 Commission Internationale de I'Eclairage (CIE) chromaticity coordinates (x=0.68, y=0.30). Organic light-emitting device (OLED) based on bis[1-(4,5-difluorophenyl)isoquinolinato-C2,N] iridium(III) acetylacetonate [Ir(4,5-F2piq)2(acac)] yielded luminous efficiency of 9.9 cd/A at a luminance of 11840 cd/m2.

Synthesis and photophysical characterization of the free‐radical copolymerization of metaloquinolate‐pendant monomers with methyl methacrylate

AbstractA series of novel metaloquinolate [aluminum quinolate (Alq3), zinc quinolate (Znq2), and lithium quinolate (Liq)]‐containing copolymers were prepared. This is the first report of the synthesis of metaloquinolate‐containing polymers by free‐radical copolymerization. The structures of the metaloquinolate monomers and copolymers were characterized by 1H NMR and Fourier transform infrared techniques. The differential scanning calorimetry and thermogravimetric analysis results showed that the copolymers were more thermally stable than the methyl methacrylate homopolymer. The copolymers (<25 wt % Alq3, <20 wt % Znq2, or <15 wt % Liq) could be dissolved in common solvents without crosslinking. The ultraviolet–visible absorption and photoluminescence (PL) emission properties of the copolymers were consistent with the literature data of metaloquinolate complexes. The PL efficiencies of the metaloquinolate‐containing copolymers with 25 wt % Alq3, 20 wt % Znq2, or 15 wt % Liq were 19.89, 13.24, and 11.82%, respectively. The experimental results indicated that these kinds of materials could be used for practical applications in organic light‐emitting diodes. © 2004 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 43: 397–406, 2005

Improved electron injection in organic LED with lithium quinolate/aluminium cathode

Lithium quinolate (Liq), covered with aluminium, was used as an electron injection layer in a double layer organic light emitting diode consisting of NPD as the hole transport layer and Alq as the emitting layer resulting in lower turn on voltage and increased power efficiency. The driving voltage required to achieve a luminance of 100 cd/m 2 decreased from 5.8 V for the Ca/Al to 4.2 V when Liq/Al was used, improving device power efficiency from 2.3 to 4.1 lm/W. The performance tolerance to layer thickness of Liq devices is also better than that of the devices with lithium fluoride (LiF). Due to the highly insulating nature of LiF, it can only be used when deposited as an ultra-thin layer, while the Liq can be deposited into layers as thick as 5 nm without significantly affecting the EL properties. An Liq electron injecting layer has also been tried in combination with Ca, Mg and Ag cathodes. Our experiments support the assumption that free lithium is released from lithium quinolate, as in the case of lithium fluoride, when Liq is over coated with active metals such as Al.

Lithium−Quinolate Complexes as Emitter and Interface Materials in Organic Light-Emitting Diodes

We synthesized lithium−quinolate complexes, 8-hydroxyquinolinolatolithium (Liq) and 2-methyl-8-hydroxyquinolinolatolithium (LiMeq), as emitter and electron injection/transport materials to be used in conventional two-layer organic light-emitting diodes in combination with N,N‘-bis(p-methoxyphenyl)-N,N‘-diphenylbenzidine (DMeOTPD) as hole transport material (HTL). The lithium complexes were also examined as interface materials in combination with 8-hydroxyquinolinolato-Al(III) (Alq3) as emitter material. The device efficiency with these complexes was optimized by combinatorial methods. We also compared the electron injection, transport, and emission properties of Li complexes with the well-known emitter Alq3 in the same experiment by taking advantage of the combinatorial approach. The Li quinolates are found to be efficient emitter molecules. But the efficiencies of lithium quinolate devices are lower than that of Alq3 devices. Contrary to the Alq3 emission, the Li quinolates exhibit a bathochromic shift of emission compared to the respective photoluminescence spectra. No clear evidence of exciplex formation was seen by comparing the photoluminescence spectrum of an equimolar mixture of Li quinolate and DMeOTPD with the observed electroluminescence spectrum. However, the lithium complexes increase the efficiency of an optimized indium−tin oxide (ITO)/DMeOTPD/Alq3/Al device considerably when used as a thin interface layer between Alq3 and aluminum. The improvement of device characteristics with lithium quinolates is similar to that obtained with LiF salt.