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.

Efficient red organic light-emitting diodes based on a dinuclear europium complex

Pure red organic light-emitting diodes based on a dinuclear europium complex with a structure of (TTA)(3)Eu(PYO)(2)Eu(TTA)3 (TTA = thenoyltrifluoroacetonate, PYO = pyridine N-oxide) were presented. The devices showed pure red emission at a peak wavelength of 612 nm with a full width at half maximum of 3 nm, which is a characteristic emission from Eu3+ ion based on D-5(0) -> F-7(2) transition. The maximum brightness and electroluminescent (EL) efficiency reached 340 cd/m(2) at a driving voltage of 19 V and 2.4 cd/A (0.78 lm/W) at a current density of 0.14 mA/cm(2), respectively.

Tunability of resonant wavelength by Fabry-Perot microcavity in organic light-emitting diodes

Distributed Bragg reflectors (DBR) with different reflection wavelengths were designed, and were used to fabricate microcavity organic light-emitting diodes (OLEDs) based on tris(8-hydroxyquinoline)-aluminum (Alq(3)) as the emitter and N, N'-di(naphthalene-1-yl)-N, N'-diphenyl-benzidine (NPB) as the hole-transporting layer. The microcavity was composed of DBR dielectric mirror and metal electrode aluminum (Al) mirror. Some effects of vertical optical Fabry-Perot microcavity on spontaneous emission in OLEDs were investigated. Spectral narrowing, enhancement of emitting intensity and anglular dependence of emission were observed due to the microcavity effect. It was found experimentally that the utilization of DBR is a better method to adjust the emissive mode in the resonant cavity in OLEDs well. Thus the realization of different color light emission becomes possible by the combination of carefully designed microcavity and electroluminescent organic semiconductors in a single LED.

High-efficiency blue light-emitting diodes based on a polyphenylphenyl compound with strong electron-accepting groups

The synthesis and characterization of two new polyphenylphenyl compounds is reported. One compound (CPP) acts as a blue light-emitting material, but contains strong electron-accepting groups that form exciplexes with electron-donating arylamines that are widely used as hole-transporting materials. Inserting a layer of the other compound into the organic light-emitting diodes (see figure) suppresses the formation of exciplexes, and gives high-efficiency blue-light emission from the CPP layer.

Novel heteroleptic Cu-I complexes with tunable emission color for efficient phosphorescent light-emitting diodes

A series of orange-red to red phosphorescent heteroleptic Cu-I complexes (the first ligand: 2,2 '-biquinoline (bq), 4,4 '-diphenyl2,2 '-biquinoline (dpbq) or 3,3 '-methylen-4,4 '-diphenyl-2,2 '-biquinoline (mdpbq); the second ligand: triphenylphosphine or bis[2-(diphenylphosphino)phenyl]ether (DPEphos)) have been synthesized and fully characterized. With highly rigid bulky biquinoline-type ligands, complexes [Cu(mdpbq)(PPh3)(2)](BF4) and [Cu(mdpbq)(DPEphos)](BF4) emit efficiently in 20 wt % PMMA films with photoluminescence quantum yield of 0.56 and 0.43 and emission maximum of 606 nm and 617 nm, respectively. By doping these complexes in poly(vinyl carbazole) (PVK) or N-(4-(carbazol-9-yl)phenyl)-3,6-bis(carbazol-9-yl) carbazole (TCCz), phosphorescent organic light-emitting diodes (OLEDs) were fabricated with various device structures. The complex [Cu(mdpbq)(DPEphos)](BF4) exhibits the best device performance. With the device structure of ITO/PEDOT/ TCCz:[Cu(mdpbq)(DPEphos)](BF4) (15 wt %)/TPBI/LiF/Al (III), a current efficiency up to 6.4 cd A(-1) with the Commission Internationale de L'Eclairage (CIE) coordinates of (0.61, 0.39) has been realized. To our best knowledge, this is the first report of efficient mononuclear Cu complexes with red emission.

Effect of dye concentration on the charge carrier transport in molecularly doped organic light-emitting diodes

The effect of the concentration of 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7, 7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB) as dopant in tris(8-quinolinolato) aluminum (Alq(3)) on the charge carrier transport in Alq(3):DCJTB was investigated by measuring the steady current-voltage characteristics and the transient electroluminescence. The dopant concentration dependence of the current-voltage relationship clearly indicates the carrier trapping by the DCJTB molecule. The DCJTB concentration significantly affects the electron mobility in Alq(3):DCJTB. The mobility has a nontrivial dependence on the doping level. For relatively low doping levels, less than 1%, the electron mobility of Alq(3):DCJTB decreases with the doping level. An increasing mobility is then observed if the dopant concentration is further increased, followed by a decrease for doping levels larger than similar to2%. The change of the electron mobility with the DCJTB concentration in Alq(3) is attributed to the additional energetic disorder due to potential fluctuations caused by the dipole-dipole interaction of random distribution dopant at the relatively low doping concentration, and to the phase separation at the high doping concentration.

Modeling of organic light-emitting diodes with graded concentration in the emissive multilayer

We model the electrical behavior of organic light-emitting diodes whose emissive multilayer is formed by blends of an electron transporting material, tris-(8-hydroxyquinoline) aluminum (Alq(3)) and a hole transporting material, N,N-'-diphenyl-N,N-'-bis(1,1(')-biphenyl)-4,4-diamine. The multilayer is composed of layers of different concentration. The Alq(3) concentration gradually decreases from the cathode to the anode. We demonstrate that these graded devices have higher efficiency and operate at lower applied voltages than devices whose emissive layer is made of nominally homogeneous blends. Our results show an important advantage of graded devices, namely, the low values of the recombination rate distribution near the cathode and the anode, so that electrode quenching is expected to be significantly suppressed in these devices.

Highly efficient green phosphorescent organic light-emitting diodes based on Cu-I complexes

Mononuclear Cu-I complexes with mixed ligands are used to fabricate green phosphorescent organic light-emitting diodes. The electroluminescence (EL) maximum at 524 nm coincides well with its photoluminescent (PL) spectrum in poly(methyl methacrylate) film (see Figure). A maximum current efficiency of 10.5 cd A(-1) at 105 cd m(-2) and a maximum brightness up to 1663 cd m(-2) are

Novel soluble N-phenyl-carbazole-containing PPVs for light-emitting devices: Synthesis, electrochemical, optical, and electroluminescent properties

Novel PPV derivatives (PCA8-PV and PCA8-MEHPV) containing N-phenyl-carbazole units on the back-bone were successfully synthesized by the Wittig polycondensation of 3,6-bisformyl-N-(4-octyloxy-phenyl)carbazole with the corresponding tributyl phosphonium salts in good yields. The newly formed and dominant trans vinylene double bonds were confirmed by FT-IR and NMR spectroscopy. The polymers (with (M) over bar (w) of 6289 for PCA8-PV and 7387 for PCA8-MEHPV) were soluble in common organic solvents and displayed high thermal stability (T(g)s are 110.7 degreesC for PCA8-PV and 92.2 degreesC for PCA8-MEHPV, respectively) because of the incorporation of the N-phenyl-carbazole units. Cyclic voltammetry investigations (onsets: 0.8 V for PCA8-PV and 0.7 V for PCA8-MEHPV) suggested that the polymers possess enhanced hole injection/transport properties, which can be also attributed to the N-phenyl-carbazole units on the backbone. Both the single-layer and the double-layer light-emitting diodes (LEDs) that used the polymers as the active layer emitted a greenish-blue or bluish-green light (the maximum emissions located 494 nm for PCA8-PV and 507 nm for PCA8-MEHPV, respectively).

Highly efficient pure-white-light-emitting diodes from a single polymer: Polyfluorene with naphthalimide moieties

Light-emitting diodes exhibiting efficient pure-white-light electroluminescence have been successfully developed by using a single polymer: polyfluorene derivatives with 1,8-naphthalimide chromophores chemically doped onto the polyfluorene backbones. By adjusting the emission wavelength of the 1,8-naphthalimide components and optimizing the relative content of 1,8-naphthalimide derivatives in the resulting polymers, white-light electroluminescence from a single polymer, as opposed to a polymer blend, has been obtained in a device with a configuration of indium tin oxide/poly(3,4-ethyleiledioxythiophene)(50 nm)/polymer(80 nm)/Ca(10 nm)/Al(100 nm). The device exhibits Commission Internationale de I'Eclairage coordinates of (0.32,0.36), a maximum brightness of 11900 cd m(-2), a current efficiency of 3.8 cd A(-1), a power efficiency of 2.0 lm W-1. an external quantum efficiency of 1.50 %, and quite stable color coordinates at different driving voltages, even at high luminances of over 5000 cd m(-2).

White polymeric light-emitting diodes with high color rendering index

The efficient white polymeric light-emitting diodes based on a white emissive polymer doped with a red phosphorescent dopant were fabricated by spin-coating method. The emission spectrum of the device is broadened to cover the full visible region by doping the red phosphorescent dye and thereby realizes white emission with high color-rendering index (CRI). By controlling the contents of the doped electron-transporting 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole and the red phosphorescent dopant, a luminous efficiency as high as 5.3 cd/A and a power efficiency of 3 lm/W were obtained with a CRI of 92.

Highly efficient iridium(III) complexes with diphenylquinoline ligands for organic light-emitting diodes: Synthesis and effect of fluorinated substitutes on electrochemistry, photophysics and electroluminescence

A series of novel cyclometalated iridium(III) complexes bearing 2,4-diphenylquinoline ligands with fluorinated substituent were prepared and characterized by elemental analysis, NMR and mass spectroscopy. The cyclic voltammetry, absorption, emission and electroluminescent properties of these complexes were systematically investigated. Electrochemical studies showed that the oxidation of the fluorinated complexes occurred at more positive potentials (in the range 0.57-0.69 V) than the unfluorinated complex 1 (0.42 V). In view of the energy level, the lowering of the LUMO by fluorination is significantly less than that of the HOMO. The weak and low energies absorption bands in the range of 300-600 nm are well resolved, likely associated with MLCT and (3)pi-pi* transitions. These complexes show strong orange red emission both in the solution and solid state. The emission maxima of the fluorinated complexes showed blue shift by 9, 24 and 15 nm for 2, 3 and 4, respectively, with respect to the unfluorinated analogous 1. Multilayered organic light-emitting diodes (OLEDs) were fabricated by using the complexes as dopant materials. Significantly higher performance and lower turn-on voltage were achieved using the fluorinated complexes as the emitter than that using the unfluorinated counterpart 1 under the same doping level.

High-efficiency spin-coated organic light-emitting diodes based on a europium complex

An organic light-emitting diode fabricated by doping a europium, complex tris(dibiphenoylmethane)-mono (phenanthroline)-europium (Eu(DBPM)(3) (Phen)) into polymer poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4-phenylene) and poly(N-carbazole) was realized by spin coating. Comparison with other europium complexes, due to the existence of a larger spectral overlap between Eu(DBPM)(3)(Phen) and poly(2-methoxy-5-(2-ethyl-hexyloxy)-1,4phenylene), a high efficiency red emission was achieved. The device showed a turn-on voltage of 5.2 V The maximum efficiency reached 0.47 cd/A at luminance of 50 cd/m(2). The maximum luminance can reach 150 cd/m(2) at 95 mA/cm(2). To the best of our knowledge, this is one of the best results based on europium complexes by spin-casting method.