Dongyu Zhang

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.

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.

Reduced efficiency roll-off in electrophosphorescent devices by a short-living rhenium emitter with well-matched energy levels

We demonstrated reduced efficiency roll-off in electrophosphorescent devices based on a rhenium [Re(I)] complex, {9, 9-Di-[9′-(4″-phenoxybutyl)-9′H-carbazyl]-9-H-4,5-Diazafluorene Re(CO)3Br} (Re-PCDF). The devices based on Re-PCDF exhibited the peak luminance of 7888 cd/m2 and the maximum efficiency of 7.41 cd/A, respectively. Remarkably, the devices exhibited very small efficiency roll-off with only ca. 28%, which is much better than the reported Re(I) complexes based devices. Such excellent performances could be ascribed to the short luminescent lifetime of Re-PCDF and well-matched energy levels between the singlet host and the triplet emitter. The detailed mechanisms of such small efficiency roll-off were also investigated.

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.

A very high efficiency electrophosphorescent device doped with short triplet lifetime phosphor using multi-recombination zones

We have demonstrated a high-efficiency electrophosphorescent device doped with bis(2-phenyl-benzoimidazole) iridium (III) acetylacetonate with a triplet lifetime in the range of nanoseconds. Then the doped mixed host was sandwiched by two doped single-host layers. Multi-recombination zones were formed in the emission layer. Such a structure could increase the proportion of recombined carriers to injected carriers and extend the recombination zone. As a result, a maximum external quantum efficiency of 18.6% was attained due to the presence of multi-recombination zones. The mechanism of improving the electroluminescence performance was also discussed.

Highly Air-Stable Electron-Transport Material for Ink-Jet-Printed OLEDs

A novel cross-linkable electron-transport material has been designed and synthesized for use in the fabrication of solution-processed OLEDs. The material exhibits a low LUMO level of -3.51 eV, a high electron mobility of 1.5×10-5  cm2  V-1  s-1 , and excellent stability. An average 9.3 % shrinkage in film thickness was observed for the film after thermal curing. A maximum external quantum efficiency (EQE) of 15.6 % (35.0 cd A-1 ) was achieved for blue-phosphorescent OLEDs by spin-coating and 13.8 % (31.0 cd A-1 ) for an ink-jet-printed device, both of which are better than the EQE of a control device prepared by vacuum-deposition (see figure).

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.

Novel bluish green benzimidazole-based iridium(iii) complexes for highly efficient phosphorescent organic light-emitting diodes

Three multifluorinated benzimidazole-based iridium [Ir(III)] complexes abbreviated as 1F-Ir, 2F-Ir, and 3F-Ir were designed and synthesized. Their photoluminescent, thermal, and electrochemical properties were investigated in detail. These Ir(III) complexes exhibited strong bluish green emission with a high quantum efficiency of 0.68–0.81. Organic light-emitting diodes (OLEDs) with the simple structure of ITO/1,1-bis(4-(N,N-di(p-tolyl)amino)phenyl)cyclohexane (TAPC) (20 nm)/4,4′-N,N′-dicarbazole-biphenyl (CBP):Ir(III) complex (30 nm)/1,3,5-tris(N-phenylbenzimidazol-2-yl)benzene (TPBi) (50 nm)/8-hydroxyquinolinato lithium (Liq) (2 nm)/Al were fabricated to evaluate the potential applications of these fluorinated Ir(III) complexes. Good device performance (current efficiency: 48.0–70.1 cd A−1 and external quantum efficiency: 15.6–21.7%) was achieved. In particular, OLEDs with 2F-Ir as a dopant emitter showed the best performance with a maximum current efficiency of 70.1 cd A−1 and a maximum external quantum efficiency of 21.7% along with low efficiency roll-off. In this study, a very simple strategy has been reported to regulate the emitting color and improve the luminous efficiency of the Ir(III) phosphorescent emitters with great potential for practical applications in the field of OLEDs.

Pyridine-Based Electron-Transport Materials with High Solubility, Excellent Film-Forming Ability, and Wettability for Inkjet-Printed OLEDs

Film morphology has predominant influence on the performance of multilayered organic light-emitting diodes (OLEDs), whereas there is little reported literature from the angle of the molecular level to investigate the impact on film-forming ability and device performance. In this work, four isomeric cross-linkable electron-transport materials constructed with pyridine, 1,2,4-triazole, and vinylbenzyl ether groups were developed for inkjet-printed OLEDs. Their lowest unoccupied molecular orbital (∼3.20 eV) and highest occupied molecular orbital (∼6.50 eV) levels are similar, which are mainly determined by the 1,2,4-triazole groups. The triplet energies of these compounds can be tuned from 2.51 to 2.82 eV by different coupling modes with the core of pyridine, where the 2,6-pyridine-based compound has the highest value of 2.82 eV. Film formation and solubility of the compounds were investigated. It was found that the 2,6-pyridine-based compound outperformed the 2,4-pyridine, 2,5-pyridine, and 3,5-pyridine-based compounds. The spin-coated blue OLEDs based on the four compounds have achieved over 14.0% external quantum efficiencies (EQEs) at the luminance of 100 cd m-2, and a maximum EQE of 12.1% was obtained for the inkjet-printed device with 2,6-pyridine-based compound.