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)

Electrical properties in vanadyl-phthalocyanine-based metal-insulator-semiconductor devices

We investigated electrical properties of vanadyl phthalocyanine (VOPc) metal-insulator-semiconductor (MIS) devices by the measurement of capacitance and conductance, which were fabricated on ordered para-sexiphenyl (p-6P) layer by weak epitaxy growth method. The VOPc/p-6P MIS diodes showed a negligible hysteresis effect at a gate voltage of +/- 20 V and small hysteresis effect at a gate voltage of +/- 40 V due to the low interface trap state density of about 1x10(10) eV(-1) cm(-2). Furthermore, a high transition frequency of about 10 kHz was also observed under their accumulation mode. The results indicated that VOPc was a promising material and was suitable to be applied in active matrix liquid crystal displays and organic logic circuits.

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.

Nonvolatile memory devices based on gold nanoparticle and poly (N-Vinylcarbazole) composite

A rewritable polymer memory device based on gold nanoparticle doped poly (N-vinylcarbazole) (PVK), which can be easily fabricated by simple spin coating, has been described. An electrical bistable phenomenon is observed in the current-voltage characteristics of this device, and it is found that the electrical bistability is repeatable by proper writing voltage and erasing voltage. The unique behavior of the devices provides an interesting approach such that doping nanoparticles in polymer can be used to realize high performance nanovolatile polymer memory devices.

High efficiency tandem organic light-emitting devices with Al/WO3/Au interconnecting layer

An interconnecting layer of Al (2 nm)/WO3 (3 nm)/Au (16 nm) was studied for application in tandem organic light-emitting devices. It can be seen that the Al/WO3/Au structure plays the role of an excellent interconnecting layer. The introduction of WO3 in the connection unit significantly improves the device efficiency as compared to the case of Al/Au. Thus, the current efficiency of the two-unit tandem devices is enhanced by two factors with respect to the one-unit devices. The green two-unit tandem device of indium tin oxide/MoO3/4,4(')-N,N-'-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl(NPB)/tris(8-hydroxylquinoline) aluminum (Alq(3)):10-(2-benzothiazolyl)-1,1,7,7-tetramethyl-2,3,6,7-tetrahydro-1H,5H,11H-[1]benzopyrano[6,7,8-ij]quinolizin-11-one (C545T)/Alq(3)/LiF/Al/WO3/Au/MoO3/NPB/Alq(3):C545T/Alq(3)/LiF/Al showed a maximum current efficiency of 33.9 cd/A and a power efficiency of 12.0 lm/W.

Improved electron injection in organic light-emitting devices with a lithium acetylacetonate [Li(acac)]/aluminium bilayer cathode

Lithium acetylacetonate [Li(acac)] covered with aluminium was used as an efficient electron injection layer in organic light-emitting devices (OLEDs) consisting of NPB as the hole transport layer and Alq(3) as the electron transport and light emitting layer, resulting in lower turn- on voltage and increased current efficiency. The turn- on voltage (the voltage at a luminance of 1 cd m(-2)) was decreased from 5.5 V for the LiF/Al and 4.4 V for Ca/Al to 4.0 V for Li(acac)/Al, and the device current efficiency was enhanced from 4.71 and 5.2 to 7.0 cd A(-1). The performance tolerance to the layer thickness of Li(acac) is also better than that of the device with LiF. LiF can only be used when deposited as an ultra- thin layer because of its highly insulating nature, while the Li(acac) can be as thick as 5 nm without significantly affecting the EL performance. We suppose that the free lithium released from Li(acac) improves the electron injection when Li(acac) is covered with an Al cathode.

Improved efficiency by a fluorescent dye in red organic light-emitting devices based on a europium complex

By doping a fluorescent dye in the emissive layer, we realized high efficient red organic light-emitting diodes (OLEDs) based on a europium complex. The OLEDs realized by this method showed pure red emission at 612 nm with a full width at half maximum Of 3 nm. The Commission International de L'Eclairage Coordination keeps approximately the same as the emission of pure Eu3+. The maximum brightness and EL efficiency reached 2450 cd/m(2) at 20 V and 9.0 cd/A (6.0 lm/w) at a current density of 0.012 mA/cm(2), respectively. At the brightness of 100 cd/m(2), the current efficiency reached 4.4 cd/A.

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).

Novel hole-transporting materials based on 1,4-bis(carbazolyl)benzene for organic light-emitting devices

Novel hole-transporting molecules containing 1,4-bis(carbazolyl)benzene as a central unit and different numbers of diphenylamine moieties as the peripheral groups have been synthesized and characterized. These compounds are thermally stable with high glass transition temperatures of 141-157 degreesC and exhibit chemically reversible redox processes. Their amorphous state stability and hole transport properties can be significantly improved by increasing the number of diphenylamine moieties in the outer part and by controlling the symmetry of the carbazole-based molecules. These compounds can be used as good hole-tran sporting materials for organic electroluminescent (EL) devices. The device performance based on tri- and tetra-substituted carbazole derivatives is comparable to that of a typical 4,4'-bis[N-(1-naphthyl)-N-phenylamino] biphenyl (NPB)-based device.

Negative differential resistance effect in organic devices based on an anthracene derivative

The authors observed a negative differential resistance (NDR) in organic devices consisting of 9,10-bis-(9,9-diphenyl-9H-fluoren-2-yl)-anthracene (DPFA) sandwiched between Ag and indium tin oxide electrodes. The large NDR shown in current-voltage characteristics is reproducible, resulting in that the organic devices can be electrically switched between a high conductance state (on state) and a low conductance state (off state). It can be found that the currents at both on to off states are space-charge limited and attributed to the electron traps at the Ag/DPFA interface. The large and reproducible NDR makes the devices of tremendous potential in low power memory and logic circuits.

White organic light-emitting devices with a phosphorescent multiple emissive layer

A phosphorescent multiple emissive layer, in which a blue emissive layer is sandwiched between red and green ones, is employed in a white organic light-emitting device (OLED). This OLED has a maximum luminance of 48 000 cd/m(2) at 17 V, a maximum power efficiency of 9.9 lm/W at 4 V, and a color rendering index of 82. In addition, the emission color of this device is fairly stable at high luminances: its Commission Internationale de l(')Eclairage coordinate slightly changes from (0.431, 0.436) to (0.400, 0.430) when the luminance ranges from 2000 to 40 000 cd/m(2).

Memory devices based on lanthanide (SM3+, EU3+, Gd3+) complexes

Memory effects in single-layer organic light-emitting devices based on Sm3+, Gd3+, and Eu3+ rare earth complexes were realized. The device structure was indium-tin-oxide (ITO)/3,4-poly(ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT)/Poly(N-vinyl carbazole) (PVK): rare earth complex/LiF/Ca/Ag. It was found experimentally that all the devices exhibited two distinctive bistable conductivity states in current-voltage characteristics by applying negative starting voltage, and more than 10(6) write-read-erase-reread cycles were achieved without degradation. Our results indicate that the rare earth organic complexes are promising materials for high-density, low-cost memory application besides the potential application as organic light-emitting materials in display devices.

Synthesis and characterization of new carbazole/fluorene-based derivatives for blue-light-emitting devices

2,7-Bis(9-ethylcarbazol-3-yl)-9,9-di(2-ethylhexyl)fluorene and a segmented copolymer composed of the same chromophores alternated with hexamethylene fragments were synthesized. The obtained materials possess good solubility in common organic solvents, high thermal stability with 1% weight loss temperature of 350-370 degrees C, and suitable glass transition temperatures. Both derivatives show blue fluorescence in dilute solutions as well as in solid state, demonstrating that excimers are not formed in the thin films. The fluorescence spectra of the materials do not show any peaks in the long-wavelength region even after annealing at 200 degrees C in air. An organic LED with the configuration of ITO/copolymer/Al generates blue electroluminescence with the maximum peak at 416 nm, rather low turn-on voltage (4.0 V), and brightness of about 400 cd/m(2). The heterostructure device based on model derivative emitted stable blue light with low operation voltage (100 cd/m(2) at similar to 11 V) and demonstrated luminescence efficiency of 0.8 cd/A.