Defeng Bi

Very high-efficiency organic light-emitting diodes based on cyclometallated rhenium (I) complex

A phosphorescent (Ph) cyclometallated rhenium (I) (ReI) complex was synthesized by reacting 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline with pentacarbonylbromorhenium in refluxing toluene solutions. The precipitates were easily sublimed to obtain a pure electrically neutral carbonyl diamine ReI complex, (2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline)Re(CO)3Br. The Re complex was used as an orange emitting dopant in 4,4′-N,N′-dicarbazole-biphenyl host to fabricate Ph organic light-emitting diodes (PhOLEDs). The maximum electroluminescence (EL) efficiency and luminance of 21.8cd∕A and 8315cd∕cm2 were harvested, respectively, which were the highest EL results among the PhOLEDs based on ReI complexes. The improvements of the EL performances could be ascribed to the synergistic effects of together incorporation of two reciprocally repulsive phenyl and methyl groups on the backbone of 1,10-phenanthroline molecule of the ligand.

Highly efficient green organic light-emitting diodes from single exciplex emission

Spectral single and stable green exciplex emission was demonstrated from organic light-emitting diodes (OLEDs) with 4,4′,4″-tris[3-methylphenyl(phenyl)amino] triphenylamine and 4,7-diphenyl-1,10-phenanthroline that function as electron donor (D) and acceptor (A), respectively. As 8-hydroxyquinoline aluminum (Alq3) was attached to the acceptor layer, electroluminescent (EL) properties of the two exciplex-type OLEDs with D/A-bilayer and D:A mixture layer configurations were markedly improved, i.e., a peak current efficiency of 7.6cd∕A at 2.38mA∕cm2 in three-layer device and a maximum luminance of 6620cd∕m2 at 8.7V in blend layer device were obtained, respectively, without changing the peak position (535nm) and the shape of EL spectrum. Discussion is given on the harvest of the pure green exciplex emission and enhancement of luminance which is obtained by inserting Alq3 layer.

Effect of acceptor on efficiencies of exciplex-type organic light emitting diodes

The relationship between the electroluminescent (EL) efficiencies of interfacial exciplex emission and the lowest unoccupied molecular orbital (LUMO) of the acceptors systematically investigated. A nearly linear relationship was observed between LUMOs of acceptors and exciplex efficiency for a fixed donor of 4,4′,4″-tris[3-methyl-phenyl(phenyl)amino]-triphenylamine in exciplex-type devices. This result indicates that the lower the LUMO of the acceptor is, the higher the EL efficiency of the exciplex is. The effect of acceptor on the efficiencies of exciplex-type devices is attributable to the interactions between the donor and acceptor molecules, which were closely related to the matched LUMOs and intermolecular conformations overlap between donor and acceptor molecules.

Broad wavelength modulating and design of organic white diode based on lighting by using exciplex emission from mixed acceptors

Modulating electroluminescent (EL) spectra from interfacial exciplex emissions were observed by varying the ratios of two acceptors of exciplex-type devices in which the emissive wavelengths were tuned from 530to656nm. In the devices 4,4′,4″-tris[3-methyl-pheny(phenyl)amino]triphenylamine and (bathocuproine: scdolinium-(dibenzoyl-methane)3bathophenyl-phenathroline) mixtures were used as donor and acceptor materials, respectively. In terms of the exciplex broad band emission a white organic light emitting diode was demonstrated by skillfully designing the structure when blue subband was subjoined in the white spectrum. The white device behaves the Commission Internationale de l’Eclairage coordinates of (0.32, 0.35) with higher color stability at various biases, a color rendering index of 90.4, and a maximum luminance of 425cd∕m2, respectively, although the EL efficiency needs to be further improved. The emission mechanism of the broad exciplex band formed by two mixed acceptors was also discussed.

Highly sensitive organic ultraviolet optical sensor based on phosphorescent Cu (I) complex

Ultraviolet light-sensitive organic optical sensor based on photovoltaic diode was demonstrated by using a phosphorescent Cu complex and a diamine derivative as electroacceptor and electrodonor, respectively. The Cu complex is Cu(DPEphos)((Bphen))BF4, in which DPEphos and Bphen denote 6,7-dicyanodipyrido [2,2-d:2′,3′-f] quinoxaline and bathophenanthroline. And the diamine derivative, m-MTDATA, is 4, 4′,4″-tris-(2-methylphenyl phenylamino) triphenylamine. The sensor is highly sensitive to UV light band from 300to420nm while it has almost no response to the visible light, and under illumination of 365nm light with power of 1.7mW∕cm2, the sensor exhibits an open circuit voltage of 1.86V, a short circuit current of 105.3μA∕cm2, a fill factor of 0.246, and a power conversion efficiency of 2.83%. The dependences of ultraviolet responsive sensitivity on illumination intensity and working temperature were also discussed.

High efficiency electrophosphorescence device using a thin cleaving layer in an Ir-complex doped emitter layer

The authors demonstrate a considerable increase in current efficiency of fac-tris(2-phenylpyridine) iridium doped phosphorescent organic green-light emitting diode in which a thin 4,7 dipheny-1,10-phenanthroline (Bphen) layer acts as a cleaving layer. As 4nm Bphen layer divides the emitting layer (EML) into two sub-EMLs, a maximum current efficiency of 53cd∕A (corresponding to external efficiency quantum of 15%) is obtained, which is higher for 2.3 folds than that of the device without it, especially the current efficiency increases 64% over the reference device at a luminance of 40000cd∕m2. The increases are demonstrated to the high electron mobility and special energy level alignment of Bphen with 4,4′-N,N′-dicarbazole-biphenyl host. The efficiency improvement attributes to a higher exciton formation probability in the recombination zone and better balance of the carrier injection. The detail enhancement mechanism of the efficiency is also discussed.

Low efficiency roll off at high current densities in Ir-complex based electrophosphorescence diode with exciton diffusing and fluorescence compensating layers

We demonstrate a fac-tris(2-phenylpyridine) iridium-based electrophosphorescent organic green-light emitting diode with a considerably reduced current-efficiency roll off at high current density. Such a low roll off of efficiency was achieved by inserting nondoped 4,4′-N,N′-dicarbazole-biphenyl (CBP) layer and a tris-(8-hydroxy-quinoline) aluminum (Alq3) layer between the emitting and electron-transporting layers to diffuse excitons from the emitting layer. The Alq3 layer is found to contribute as a complementary green fluorescent emitter at high current density. Thus, only a small decrease from 20.7to16.7cd∕A was detected in current efficiency when the current density increases from 3.9to100mA∕cm2. A high current efficiency of 12.6cd∕A was achieved even at 350mA∕cm2, indicating that the efficiency roll off was drastically reduced when compared with the device without CBP and Alq3 layers.

Enhanced electrophosphorescence of copper complex based devices by codoping an iridium complex

Evidently enhanced electrophosphorescence of [Cu(bis[2-(diphenylphosphino)phenyl] ether)(6,7-dicyanodipyrido [2,2-d:2′,3′-f] quinoxaline)] BF4 (CuI complex) by codoping bis[(4,6-difluorophenyl)-pyridinato-N, C2](picolinato)Ir(III) (FIrpic) into the same host, 4,4′-N,N′-dicarbazole-biphenyl, was demonstrated. The device codoped with 8wt% FIrpic and 2wt% CuI complex shows a maximum current efficiency and power efficiency of 26.6cd∕A and 17.8lm∕W, respectively, which were increased by factors of 2.6 and 2.1 compared with the 2wt% CuI complex monodoped device. The improvements of the devices were testified to the efficient energy transfer from FIrpic to the CuI complex. The detail of the energy transfer mechanism between the two phosphors was also proposed.

Investigation of dye-doped red emitting organic electroluminescent devices with metal-mirror microcavity structure

Organic electroluminescent (EL) devices with planar microcavity structure, indium-tin-oxide/Ag∕N,N′-diphenyl-N , N ′-bis(3-methylphenyl)-1,1′-biphenyl-4 , 4′-diamine/tris(8-hydroxyquinoline)-aluminum (AlQ):4-(dicyanomethylene)-2-methyl-6-(p-dimethyl aminostyryl)-4H-pyran/AlQ∕LiF∕Al, were fabricated. The Ag and Al layers acted as not only hole-injection layer and cathode, respectively, but reflective mirrors, resulting in strong microcavity effects, such as spectral narrowing and directional emission. The effects of device parameters on the EL performance were studied in detail and were discussed in terms of conventional microcavity theory. On-axis light magnification with a coefficient (EL enhancement ratio between cavity and noncavity devices) of ∼5 was observed, which was consistent with the theoretical calculation. At the same time, optimized microcavity device with bright pure red emission showed maximum luminance of 5140cd∕m2, peak at 624nm, Commission International de I’Eclairage coordinates of x=0....

Sensitized photo- and electroluminescence from Er complexes mixed with Ir complex

We investigated the sensitized effect of fac-tris(2-phenylpyridine) iridium [Ir(ppy)3] on tris-(dibenzoylmethanato)-mono-(bathophenanthroline) erbium (Er-DB) for 1.5μm near-infrared emission. Compared with the neat Er-DB film, the blend film composed of Er-DB and Ir(ppy)3 with a 1:1 of the weight ratio shows 20 times and 4 times enhancement on intensity of photoluminescence (PL) and electroluminescence (EL), respectively. The improvement in both PL and EL intensities was conjectured to be the result of the energy transfer from Ir(ppy)3 to Er-DB complex, and the detailed mechanisms of the enhancement were also discussed.