Efficient multilayer white polymer light-emitting diodes with aluminum cathodes

Efficient multilayer white polymer light-emitting diodes (WPLEDs) with aluminum cathodes are fabricated. The multilayer structure is composed of a water soluble hole-injection layer, a toluene-soluble emissive layer, and an alcohol-soluble emissive layer. The polarity difference of the solvents used for spin coating these polymers allows for realization of the multilayer polymer structure. The recombination zone confined at the interface of the two emissive polymers avoids exciton quenching by electrodes, and white emission is realized by harvesting photons emitted from the two emissive polymers. A maximum luminous efficiency of 16.9 cd/A and a power efficiency of 11.1 lm/W are achieved for this WPLED.

Solution-processible multi-component cyclometalated iridium phosphors for high-efficiency orange-emitting OLEDs and their potential use as white light sources

The synthesis and photophysical studies of several multifunctional phosphorescent iridium(III) cyclometalated complexes consisting of the hole-transporting carbazole and fluorene-based 2-phenylpyridine moieties are reported. All of them are isolated as thermally and morphological stable amorphous solids. Extension of the pi-conjugation through incorporation of electron- pushing carbazole units to the fluorene fragment leads to bathochromic shifts in the emission profile, increases the highest oc- cupied molecular orbital levels and improves the charge balance in the resulting complexes because of the propensity of the carbazole unit to facilitate hole transport. These iridium-based triplet emitters give a strong orange phosphorescence light at room temperature with relatively short lifetimes in the solution phase. The photo- and electroluminescence properties of these phosphorescent carbazolylfluorene-functionalized metalated complexes have been studied in terms of the coordinating position of carbazole to the fluorene unit. Organic light-emitting diodes (OLEDs) using these complexes as the solution-processed emissive layers have been fabricated which show very high efficiencies even without the need for the typical hole-transporting layer.I These orange-emitting devices can produce a maximum current efficiency of similar to 30 cd A(-1) corresponding to an external quantum efficiency of similar to 10 % ph/el (photons per electron) and a power efficiency of similar to 14 Im W-1.

Change of the dominant luminescent mechanism with increasing current density in molecularly doped organic light-emitting devices

We have fabricated and measured a series of electroluminescent devices with the structure of ITO/TPD/Eu(TTA)(3)phen (x):CBP/BCP/ ALQ/LiF/Al, where x is the weight percentage of Eu(TTA)3phen (from 0% to 6%). At very low current density, carrier trapping is the dominant luminescent mechanism and the 4% doped device shows the highest electroluminescence (EL) efficiency among all these devices. With increasing current density, Forster energy transfer participates in EL process. At the current density of 10.0 and 80.0mA/ cm(2), 2% and 3% doped devices show the highest EL efficiency, respectively. From analysis of the EL spectra and the EL efficiency-current density characteristics, we found that the EL efficiency is manipulated by Forster energy transfer efficiency at high current density. So we suggest that the dominant luminescent mechanism changes gradually from carrier trapping to Forster energy transfer with increasing current density. Moreover, the conversion of dominant EL mechanism was suspected to be partly responsible for the EL efficiency roll-off because of the lower EL quantum efficiency of Forster energy transfer compared with carrier trapping.

Efficient organic electroluminescent devices based on an organosamarium complex

Several organic electroluminescent devices with different device structures were fabricated based on an organosamarium complex Sm(HFNH)(3)phen[HFNH=4, 4, 5, 5, 6, 6, 6-heptafluoro-l-(2-naphthvl)hexane-1, 3-dione; phen=1, 10-phenanthroline] as emitter. Their electroluminescent properties were investigated in detail. Although the devices with the optimal structure ITO/TPD (50nm)/ Sm(HFNH)(3)phen (xwt%):CBP (50nm)/BCP (20nm)/AIQ (30nm)/LiF (1 nm),/Al (200nm) show high brightness (more than 400cd/m(2)) and high current efficiency (about 1 cd/A), there are emissions from CBP, BCP and even from AIQ existing in the electroluminescence (EL) spectra besides emission from Sm(HFNH)(3)Phen. The reason to this was discussed. The device with the structure ITO/TPD (50 nm)/ Sm(HFNH)(3)phen (50 nm)/AIQ (30 nm)/LiF (1 nm)/Al (200 nm) exhibits the maximum brightness of 118 cd/m(2) and current efficiency of 0.029 cd/A, and shows emissions from AIQ and Sm(HFNH)(3)phen at high voltages. However, with the BCP hole-block layer added, the device [ITO/TPD (50 nm)/Sm(HFNH)(3)phen (50 nm)/BCP (20 nm)/AIQ (30 nm)/LiF (1 nm)/Al (200 nm)] exhibits pure Sm3+ emission in 2 the EL spectra even at high voltages, with the maximum current efficiency of 0.29cd/A and brightness of 82cd/m(2)

Improved performances of organic light-emitting diodes with metal oxide as anode buffer

We demonstrate extremely stable and highly efficient organic light-emitting diodes (OLEDs) based on molybdenum oxide (MoO3) as a buffer layer on indium tin oxide (ITO). The significant features of MoO3 as a buffer layer are that the OLEDs show low operational voltage, high electroluminescence (EL) efficiency and good stability in a wide range of MoO3 thickness. A green OLED with structure of ITO/MoO3/N,N-'-di(naphthalene-1-yl)-N,N-'-diphenyl-benzidene (NPB)/NPB: tris(8-hydroxyquinoline) aluminum (Alq(3)):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)/Alq(3)/LiF/Al shows a long lifetime of over 50 000 h at 100 cd/m(2) initial luminance, and the power efficiency reaches 15 lm/W. The turn-on voltage is 2.4 V, and the operational voltage at 1000 cd/m(2) luminance is only 6.9 V. The significant enhancement of the EL performance is attributed to the improvement of hole injection and interface stability at anode.

Improved efficiency by doping blue iridium (III) complex in europium complex-based light-emitting diodes

We demonstrated high-efficiency red organic light-emitting diodes (OLEDs) employing a europium complex, Eu (III) tris( thenoyltrifluoroacetone) 3,4,7,8-tetramethyl-1,10-phenanthroline (Eu(TTA)(3)(Tmphen)), as an emitter and a blue electrophosphorescent complex, Iridium ( III) bis[4,6-di-fluorophenyl-pyridinato-N,C-2] picolinate (FIrpic), as an assistant dopant codoped into 4,4-N, N-dicarbazole-biphenyl (CBP) host as an emissive layer. A pure red electroluminescence (EL) only from Eu3+ ions at 612 nm with a full width at half maximum of 3 nm was observed and the EL efficiency was significantly enhanced. The maximum EL efficiency reached 7.9 cd A(-1) at 0.01 mA cm(-2) current density, which is enhanced by 2.8 times compared with electrophosphorescence-undoped devices. The large improvements are attributed to energy transfer assistance effects of FIrpic, indicating a promising method for obtaining efficient red OLEDs based on rare-earth complexes.

Ligand effect on the performance of organic light-emitting diodes based on europium complexes

A series of europium complexes were synthesized and their electroluminescent (EL) characteristics were studied. It was found by comparison that the different substituted groups, such as methyl, chlorine, and nitryl, on ligand 1,10-phenanthroline affect significantly the EL performance of devices based on these complexes. The more methyl-substituted groups on ligand 1,10-phenanthroline led to higher device efficiency. A chlorine-substituted group showed the approximate EL performance as two methyl-substituted groups, whereas a nitryl substituent reduced significantly the EL luminous efficiency. However, beta-diketonate ligand TTA and DBM exhibited similar EL performance. The improved EL luminous efficiency by proper substituted groups on the 1, 10-phenanthroline was attributed to the reduction of the energy loss caused by light hydrogen atom vibration, as well as concentration quenching caused by intermolecular interaction, and the match of energy level between the ligand and Eu3+.

Improved color purity and efficiency by a coguest emitter system in doped red light-emitting devices

We demonstrate red organic light-emitting diodes (OLEDs) with improved color purity and electroluminescence (EL) efficiency by codoping a green fluorescent sensitizer 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1, 1, 7,7-tetramethyl-1H, 5H, 11H-(1)-benzopyropyrano(6,7-8-ij)quinolizin-11-one (C545T) as the second dopant and a red fluorescent dye 4-(dicyanomethylene)-2-t-butyl-6(1,1,7,7tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as the lumophore into tris(8-hydroquinoline) aluminum (Alq(3)) host. It was found that the C545 T dopant did not by itself emit but assisted the carrier trapping from the host Alq(3) to the red emitting dopant. The red OLEDs realized by this approach not only kept the purity of the emission color, but also significantly improved the EL efficiency. The current efficiency and power efficiency, respectively, reached 12 cd/A at a current density of 0.3 mA/cm(2) and 10lm/W at a current density of 0.02 mA/cm(2), which are enhanced by 1.4 and 2.6 times compared with devices where the emissive layer is composed of the DCJTB doped Alq(3), and a stable red emission (chromaticity coordinates: x = 0.64, y = 0.36) was obtained in a wide range of voltage. Our results indicate that the coguest system is a promising method for obtaining high-efficiency red OLEDs.

Effect of doped dye on the charge-carrier transport and electroluminescent performance in molecularly doped polymer light-emitting diodes

The effects of the concentration of 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H, 5H, 11H-(1)-benzopyropyrano(6, 78-i,j)quinolizin-11-one (C545T) as dopant in polyfluorene (PFO) on the charge-carrier transport and electroluminescence (EL) performance were investigated by steady-state and transient EL measurements. A fully green emission from C545T was observed and the EL performance depends strongly on the C545T concentration. The mobility in the C545T-doped PFO film was determined by transient EL. The dopant concentration dependence of the current-voltage relationship indicated clearly the carrier trapping by the C545T molecules. The mobility in C545T:PFO changed significantly with the C545T concentration, and showed a nontrivial dependence on the doping level. The behavior may be understood in terms of the formation of an additional energy disorder due to potential fluctuation caused by the Coulomb interaction of the randomly distributed doping molecules.

Improved color purity and efficiency in polyfluorene-based light-emitting diodes

Polyfluorene (PF) is a class of typical blue electroluminescent (EL) material, but it exhibits undesired feature in the green spectral region under operation condition. We investigated the spectral properties of different device structures of poly(9,9-dioctylfluorene) (PFO)-based light-emitting diodes, and found that the interaction between cathode and PFO is the main origination of green emission in EL devices. The general method of inserting a buffer layer between the PFO and cathode can decrease the low energy band emission to purify the color and improve the EL performance of devices.

Near infrared light-emitting diodes based on composites of near infrared dye, CdSe/CdS quantum dots and polymer

Near infrared (NIR) light emitting diodes employing composites of an IR fluorescent dye, CdSe/CdScore/shell semiconductor quantum dots and poly( N-vinylcarbazole) (PVK) have been demonstrated. The device, with a configuration of indium-tin-oxide (ITO)//PEDOT:PSS//PVK:NIR Dye:CdSe/CdS//Al, had a turn-on voltage of 7 V, emitted the NIR light with a maximum at 890 nm and the irradiance intensity of 96 mu W. The electroluminescence efficiency of 0.02% was achieved at a current density of 13 mA cm(-2).

Effects of doped dye on the charge carrier injection, transport, and electroluminescent performance in polymeric light-emitting diodes

The effects of doped fluorescent dye 4-(dicyanomethylene)-2-i-propyl-6-(1, 1, 7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTI) on the charge carrier injection, transport and electroluminescence (EL) performance in polyfluorene (PFO)-based polymer light-emitting diodes (PLEDs) were investigated by steady-state current-voltage (I-V) characteristics and transient EL measurements. A red EL from DCJTI was observed and the EL performance depended strongly on the DCJTI concentration. The analysis of the steady-state I-V characteristics at different DCJTI concentrations found that three regions was shown in the I-V characteristics, and each region was controlled by different processes depending on the applied electric field. The effect of the dopant concentration on the potential-barrier height of the interface is estimated using the Fowler-Nordheim model. The dopant concentration dependence of the current-voltage relationship indicated clearly the carrier trapping by the DCJTI molecules. The mobility in DCJTI: PFO changed significantly with the DCJTI concentration, and showed a nontrivial dependence on the doping level. The behavior may be understood in terms of the formation of an additional energy disorder due to potential fluctuation caused by the Coulomb interaction of the randomly distributed doping molecules.

Charge carrier transporting, photoluminescent, and electroluminescent properties of zinc(II)-2-(2-hydroxyphenyl)benzothiazolate complex

Zinc(II)-2-(2-hydroxyphenyl)benzothiazolate complex is an excellent white-light-emitting material. Despite some studies devoted to this complex, no information on the real origin of the unusually broad electroluminescent (EL) emission is available. Therefore, we investigate photoluminescent and EL properties of the zinc complex. Orange phosphorescent emission at 580 nm was observed for the complex in thin film at 77 K, whereas only fluorescent emission was obtained at room temperature. Molecular orbitals, excitation energy, and emission energy of the complex were investigated using quantum chemical calculations. We fabricated the device with a structure of ITO/F16CuPc(5.5 nm)/Zn-complex/Al, where F16CuPc is hexadecafluoro copper phthalocyanine. The EL spectra varied strongly with the thickness of the emissive layer. We observed a significant change in the emission spectra with the viewing angles. Optical interference effects and light emission originating both from fluorescence and from phosphorescence can explain all of the observed phenomena, resulting in the broad light emission for the devices based on the Zn complex. We calculated the charge transfer integral and the reorganization energy to explain why the Zn complex is a better electron transporter than a hole transporter.

Tuning the energy level and photophysical and electroluminescent properties of heavy metal complexes by controlling the ligation of the metal with the carbon of the carbazole unit

Four novel Ir-III and Pt-II complexes with cyclometalated ligands bearing a carbazole framework are prepared and characterized by elemental analysis, NMR spectroscopy, and mass spectrometry. Single-crystal X-ray diffraction studies of complexes 1, 3, and 4 reveal that the 3- or 2-position C atom of the carbazole unit coordinates to the metal center. The difference in the ligation position results in significant shifts in the emission spectra with the changes in wavelength being 84 nm for the Ir complexes and 63 nm for the Pt complexes. The electrochemical behavior and photophysical properties of the complexes are investigated, and correlate well with the results of density functional theory (DFT) calculations. Electroluminescent devices with a configuration of ITO/NPB/CBP:dopant/BCP/AlQ(3)/LiF/Al can attain very high efficiencies.

Green light-emitting polyfluorenes with improved color purity incorporated with 4,7-diphenyl-2,1,3-benzothiadiazole moieties

By incorporating 4,7-diphenyl- 2,1,3 benzothiadiazole instead of 2,1,3-benzothiadiazole into the backbone of polyfluorene, we developed a novel series of green light- emitting polymers with much improved color purity. Compared with the state-of-the-art green light-emitting polymer, poly(fluorene-co-benzothiadiazole) (lambda max = 537 nm), the resulting polymers (lambda(max) = 521 nm) showed 10-20 nm blueshifted electroluminescence (EL) spectra and greatly improved color purity because the insertion of two phenylene units between the 2,1,3-benzothiadiazole unit and the fluorene unit reduced the effective conjugation length in the vicinity of the 2,1,3-benzothiadiazole unit. As a result, the resulting polymers emitted pure green light with CIE coordinates of (0.29, 0.63), which are very close to (0.26, 0.65) of standard green emission demanded by the National Television System Committee (NTSC). Moreover, the insertion of the phenylene unit did not affect the photoluminescence (PL) and EL efficiencies of the resulting polymers. PL quantum efficiency in solid films up to 0.82 was demonstrated. Single-layer devices (ITO/PEDOT/ polymer/Ca/Al) of these polymers exhibited a turn-on voltage of 4.2 V, luminous efficiency of 5.96 cd A(-1) and power efficiency of 2.21 lm W-1. High EL efficiencies and good color purities made these polymers very promising for display applications.