Synthesis and characterization of benzothiazole derivatives for blue electroluminescent devices

Two benzothiazole derivatives, 4-(benzo[d]thiazol-2-yl)-N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-phenylbenzenamine (BBPA) and 4-(benzo[d]thiazol-2-yl)-N-(4-(benzo[d]thiazol-2-yl)phenyl)-N-naphthylbenzenamine (BBNA), were synthesized and characterized. Electroluminescent devices with compound BBPA or BBNA as the blue-emitting layer were fabricated. The triple-layer device, in which BBPA acted as the blue-emitter, NPB as the hole-transporting layer and TPBI as the electron-transporting layer (Device 1), showed a current efficiency of 5.24 cd/A, a power efficiency of 1.21 lm/W and an external quantum efficiency of 2.88% at a driving current density of 20 mA/cm². The double-layer device with BBNA as the emitting layer and electron-transporting layer and NPB as the hole-transporting layer (Device 4) exhibited a maximum brightness of 1430 cd/m² at 13 V with the CIE coordinates (0.21, 0.23).     

Blue organic light-emitting materials based on diphenylaminofluorene and N-phenylcarbazole derivatives

A series of diphenylaminofluorene- and phenylcarbazole-derived, blue fluorescent molecules have been synthesized via the Hornor–Wadsworth–Emmons and Suzuki-cross coupling reactions. To explore the electroluminescent properties of these molecules, multilayer devices were fabricated with a structure of ITO/NPB/(1–6) doped in MADN/Bphen/Liq/Al, yielding a device that exhibited highly efficient sky-blue emissions with the luminous efficiency of 11.2 cd/A at 20 mA/cm², a power efficiency of 7.35 lm/W at 20 mA/cm², and CIE_x,y coordinates of (x = 0.16, y = 0.26) at 8 V. Also, a deep blue OLED with CIE_x,y coordinates of (x = 0.16, y = 0.13) at 8 V showed a luminous efficiency of 2.13 cd/A and power efficiency of 1.20 lm/W at 20 mA/cm², respectively.     

3,12-Dimethoxy-7,8-dicyano-[5]helicene as a novel emissive material for organic light-emitting diode

3,12-Dimethoxy-7,8-dicyano-[5]helicene (DDH) was introduced as a novel emissive material for organic light-emitting diode. It shown good thermal stability and no glass transition temperature was observed. The LUMO, HOMO and energy band gap (?3.3, ?5.9 and 2.6 eV) of this compound were determined using cyclic voltammetry technique. Fluorescence quantum yield of DDH in chloroform is 0.27. The turn-on voltage of OLEDs with a configuration of ITO/PEDOT:PSS/DDH/Ca/Al was not a function of DDH thickness in a range of 60–100 nm. The best OLED, in which DDH thickness was 100 nm, exhibited a turn-on voltage of 3.7 V with maximum brightness of 1587 cd/m² at 8.0 V and 281 mA/cm². The maximum current efficiency and power efficiency were 0.64 cd/A and 0.29 lm/W, respectively. The CIE coordinates of the OLED electroluminescence, however, appeared to depend on the applied voltage as they were (0.38,0.47) at 5.0 V and (0.51,0.44) at 8.0 V.     

Highly efficient solution-processed red organic light-emitting diodes with long-side-chained triplet emitter

Newly synthesized red Ir complexes tris[2-(4-n-hexyl-phenyl)quinoline]iridium(III) and tris[(4-n-hexylphenyl)isoquinoline)]iridium(III) with long alkyl side chains are utilized to demonstrate the high efficiency multi-layer solution-processed red organic light-emitting diodes. Solubilities of these triplet emitters are high which enable them to be uniformly dispersed in the polymer host. Blade coating method is utilized to prepare organic multi-layers without mutual dissolution between different layers. 17 cd/A current efficiency, 10 lm/W power efficiency, and 8.8% external quantum efficiency can be achieved for the device with CsF/Al cathode. 10,000 cd/m² is reached at 10 V. Similar quantum efficiency is also achieved with an electron-transport layer and LiF/Al cathode.     

Pure red electrophosphorescent organic light-emitting diodes based on a new iridium complex

A new iridium complex was synthesized and demonstrated a saturated red light emission in organic light-emitting diodes (OLEDs). The maximum brightness of 2800 cd/m² and the external quantum efficiency of 5.5% were achieved in multilayer OLEDs. The peak wavelength of the emission was found to be at 677 nm with the Commission Internationale de l’Eclairage (CIE) coordinates of (0.71, 0.27).     

Enhanced color stability and improved performance in white organic light-emitting devices by utilizing a double-graded structure

High efficiency fluorescent white organic light-emitting device (WOLEDs) was investigated by using a double-graded (DG) structure. This strategy can greatly improve the device performances such as better color stability, higher luminance and enhanced efficiency. The optimized WOLED gives the Commission Internationale de L’Eclairage (CIE) color coordinates of (0.324, 0.341) at 20 mA/cm², a negligible color shift of Δ_{x, y} = ±[0.000.001] from 4 to 200 mA/cm², a maximum brightness of 39,740 cd/m² at 17 V and a maximum luminance efficiency of 11.5 cd/A at 16 V. In addition, current-induced fluorescence quenching is mostly controlled.     

Synthesis and electrophosphorescent performances of alkyl-substituted bicycloiridium complexes in polymer light-emitting diodes

In this paper, two complexes (mppy)₂Ir(tmd) and (bppy)₂Ir(tmd) were synthesized and doped into PVK. We find that the alkyl substitution of bicycloiridium complexes on acetylacetonate ligand has a great effect on optimizing the PLED performances. The best device performance is observed for the (bppy)₂Ir(tmd)-doped PVK–PBD (40 wt%) device with the concentration of 1 wt%. A maximal external quantum efficiency of QE_ext = 14.2% ph/el and the luminous efficiency of LE = 33 cd/A with a luminance of 2099 mA/cm² were achieved at a current density of 6.4 mA/cm².     

Optimization of emitting efficiency in organic LED cells using Ir complex

Multilayer organic light-emitting devices (OLED) with phosphorescent guest emitter, tris(2-phenylpyridine) iridium doped in a host 4,4′-N,N′-dicarbazol-biphenyl layer, were prepared. We optimized the cell structure paying special attention to the multiple reflection at the multilayers’ interfaces and succeeded in improving the luminance efficiency. Our method consists of adjusting optical distances between emission sites and dominant reflective surfaces, organic/cathode and ITO/glass interfaces. The device with the 8.7 wt.% guest emitter exhibited external quantum efficiency and power luminous efficiency of 14.9% and 43.4 lm/W, respectively at the luminance of 100 cd/m² driven at the voltage of 4.2 V. In addition, we investigated the emission site in the electrophosphorescent cells and recalculated the external quantum efficiency by the actual emission pattern.     

Pure red electroluminescence from novel heteroleptic cyclometalated platinum(II) emitters embedded in polyvinylcarbazole

We fabricated molecularly doped, polymer-based light-emitting diodes possessing a single emitting layer containing a hole-transporting host polymer poly(N-vinylcarbazole) and an electron-transporting auxiliary, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole, doped with novel phosphorescent cyclometalated Pt(II) complexes bearing arylpyridine and 1,3-diketone ligands. These novel cyclometalated Pt(II) complexes emit pure red color both in steady-state emissions (poly(methyl methacrylate) films)) and electrophosphorescence. They exhibited pure red emissions with the Commission Internationale de l’Eclairage coordinates (X = ～0.67, Y = ～0.33), which is almost identical to the coordinates of standard red (0.66, 0.34) demanded by the National Television System Committee. The color coordinates remained unchanged over a range of operating voltages, even at luminances greater than 1 × 10⁴ cd/m². The maximum external quantum efficiency of these devices exceeded 3.6% and the maximum brightness was greater than 1 × 10⁴ cd/m².     

A new family of solution-processible tris-(pinene-phenylpyridine) iridium(III) derivatives for polymer light-emitting diodes

A new family of iridium(III) complexes featuring pinene groups and various alkoxy side chains were synthesized and characterized. The complexes show emission peaked at around 492 nm with the phosphorescence quantum yield of Φ_P = 0.4–0.6 and the emission lifetime of τ = 2–4 μs. Polymer light emitting devices (PLEDs) with the configuration of ITO/PEDOT:PSS (50 nm)/PVK_0.7:PBD_0.3:(x wt.%)Ir(III) complex(80 nm)/CsF(1.5 nm)/Mg:Ag(200 nm) were fabricated by spin-coating. The device with new iridium(III) complex doping concentration of 3.2 wt.% exhibits a maximum luminance efficiency 19.9 cd/A (7.8 lm/W) at 9.1 V, CIE coordinates (0.21, 0.53), and a maximum luminance of 15,700 cd/m² at 8.4 V was observed.     

N-Aryl carbazole derivatives for non-doped red OLEDs

Two N-aryl carbazole derivatives: 3-2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)vinyl-N-naphthyl-carbazole (NCz-2CN) and 3,6-bis(2-(3,3-dicyanomethylene-5,5-dimethyl-1-cyclohexylidene)vinyl-N-phenyl-carbazole (PCz-4CN), with the molecular structure of donor-π-acceptor, have been synthesized and characterized. They are red emitters in the solid films with a peak wavelength at 630 nm of NCz-2CN and 666 nm of PCz-4CN. Non-doped orange-red electroluminescent devices with the structure of ITO/NPB/NCz-2CN/BCP/Alq₃/LiF/Al were fabricated. The device showed orange-red emission at λ_max = 628 nm and a maximum luminance of 4110 cd/m² obtained at 15 V. The maximum luminous efficiency was 0.49 lm/W and the current efficiency was 2.09 cd/A.     

A highly efficient deep blue fluorescent OLED based on diphenylaminofluorenylstyrene-containing emitting materials

We have designed and synthesized five blue emitters based on diphenylaminofluorenylstyrene emitting core groups. Multilayered OLEDs were fabricated using these materials as dopants in a 2-methyl-9,10-di(naphthen-2-yl)anthracene (MADN) host. One of them in particular a deep blue OLED using dopant 9-[4-(2-diphenylamino-9,9-diethylfluoren-7-yl)phenyl]-9-phenylfluorene (3) at 15% doping concentration exhibited a maximum luminance of 4720 cd m^?2 at 9.0 V, a luminous efficiency of 5.3 cd A^?1 at 20 mA cm^?2, a power efficiency of 2.9 lm W^?1 at 20 mA cm^?2, an external quantum efficiency of 4.8% at 20 mA cm^?2, and CIE coordinates (x = 0.15, y = 0.13) at 8.0 V. Furthermore, this deep blue device had very stable CIE coordinates of (x = 0.15, y = 0.13) that did not vary with doping concentration from 5% to 15%.     

Luminescent properties of a novel naphthalimide-fluorene molecule

A novel naphthalimide-fluorene molecule, 4-(N,N-dimethylamino)-N-(2′-fluorenyl)-1,8-naphthalimide (DFN), has been synthesized and characterized, and its luminescent properties have been studied. DFN has an absorption maximum at 420 nm and possesses solvent polarity dependent changes. The environmental sensitivity exhibited the characteristics of an excited state charge transfer complex. DFN also showed strong luminescence, good electron-affinity, and temperature independence of fluorescence. The application of DFN in organic light-emitting diodes (OLEDs) as an electron-transporting electroluminescent material was investigated. The OLED with a structure of ITO/N,N′-bis(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine/DFN/Al shows a yellow-green emission (chromaticity coordinates: x = 0.424, y = 0.543) with a brightness of 3563 cd/m². The external quantum efficiency and the highest luminous efficiency of the device reach 0.2% and 0.55 lm/W, respectively.     

Synthesis and luminescence of red,fluorinated iridium (III) complexes containing alkenyl benzothiazole ligand

Two novel iridium(III) complexes (2-FSBT)₂Ir(acac) and (4-FSBT)₂Ir(acac) (2-FSBT, (E)-2-(2-fluorostyryl)benzo[d]thiazole; 4-FSBT, (E)-2-(4-fluorostyryl)benzo[d]thiazole; acac, acetylacetone) were synthesized and characterized by ¹H NMR and mass spectrometry. The organic light emitting diodes based on these complexes with the structure of ITO/m-MTDATA(10 nm)/NPB(20 nm)/CBP:Ir-complex(X %, 30 nm)/BCP(10 nm)/Alq₃(30 nm)/LiF(1 nm)/Al(100 nm) were fabricated. The device based on (2-FSBT)₂Ir(acac) exhibited a maximum efficiency of 9.32 cd/A, a luminance of 8800 cd/cm²; and the device based on (4-FSBT)₂Ir(acac) showed a maximum efficiency of 8.5 cd/A, a luminance of 6986 cd/cm². The Commission International de L’Eclairage (CIE) coordinates (1931) of these complexes were (0.619, 0.381) and (0.621, 0.378), respectively.     

Improved performance of Si-based top-emitting organic light-emitting device using MoOx buffer layer

Highly efficient Si-based top-emitting organic light-emitting device (TOLED) using MoO_x buffer layer is demonstrated. With tris(8-hydroquinoline) aluminum as emitting and electron-transport layer, the p-Si/MoO_x based TOLED shows a maximum luminous efficiency of 1.1 cd/A and a power efficiency of 0.68 lm/W, which are almost double those (0.64 cd/A and 0.34 lm/W) of p-Si/SiO₂ based TOLED. Moreover, in comparison with the widely used thermally grown SiO₂ buffer layer, MoO_x can be deposited by conventional evaporation technology and thereby simplifying fabrication process.     

Doped and non-doped organic light-emitting diodes based on a yellow carbazole emitter into a blue-emitting matrix

A new carbazole derivative with a 3,3′-bicarbazyl core 6,6′-substituted by dicyanovinylene groups (6,6′-bis(1-(2,2′-dicyano)vinyl)-N,N′-dioctyl-3,3′-bicarbazyl; named (OcCz2CN)₂, was synthesized by carbonyl-methylene Knovenagel condensation, characterized and used as a component of multilayer organic light-emitting diodes (OLEDs). Due to its π-donor–acceptor type structure, (OcCz2CN)₂ was found to emit a yellow light at λ_max = 590 nm (with the CIE coordinates x = 0.51; y = 0.47) and was used either as a dopant or as an ultrathin layer in a blue-emitting matrix of 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi). DPVBi (OcCz2CN)₂-doped structure exhibited, at doping ratio of 1.4 weight %, a yellowish–green light with the CIE coordinates (x = 0.31; y = 0.51), an electroluminescence efficiency η_EL = 1.3 cd/A, an external quantum efficiency η_ext = 0.4 % and a luminance L = 127 cd/m² (at 10 mA/cm²) whereas for non-doped devices utilizing the carbazolic fluorophore as a thin neat layer, a warm white with CIE coordinates (x = 0.40; y = 0.43), η_EL = 2.0 cd/A, η_ext = 0.7%, L = 197 cd/m² (at 10 mA/cm²) and a color rendering index (CRI) of 74, were obtained. Electroluminescence performances of both the doped and non-doped devices were compared with those obtained with 5,6,11,12-tetraphenylnaphtacene (rubrene) taken as a reference of highly efficient yellow emitter.     

Yellow–green organic light-emitting diode based on tris(2-methyl-8-quinolinolate) scandium

Efficient yellow–green electroluminescence emission at λ_max = 530 nm with CIE coordinates x = 0.3913, y = 0.4947 was obtained with organic light-emitting devices based on tris(2-methyl-8-quinolinolate) scandium (1). The device with the configuration of indium tin oxide/N,N′-bis(3-methylphenyl)-N,N′-diphenylbenzidine/1/Yb exhibits current efficiency of 3.1 cd/A and power efficiency of 1.8 lm/W at a luminance of 100 cd/m². The DFT calculations demonstrate that structural changes of the scandium complex 1 influence the electroluminescence spectrum, the better agreement with experimental data being achieved when monodentate ligands are taken into consideration.     

Photoluminescence and electroluminescence of 3-methyl-8-dimethylaminophenazine

A new phenazine dye—3-methyl-8-dimethylaminophenazine (MDAP) with intramolecular charge transfer (ICT) property was synthesized. The photoluminescence and electroluminescence of were investigated. The device with a configuration of ITO/TPD (30 nm)/TPD:MDAP (30 nm)/Alq₃:MDAP (35 nm)/Alq₃ (30 nm)/Mg:Ag (200 nm) showed a good performance with a brightness of 21650 cd/m² at 250 mA/cm², a maximum luminous efficiency of 9.97 cd/A and a yellow emission peaked at about 564–586 nm.     

High-efficiency red electrophosphorescent devices based on new osmium(II) complexes

In this paper, we report on highly efficient red-phosphorescent light-emitting diodes using a series of osmium complexes 1–3 doped into poly(N-vinyl-carbazole) (PVK) matrix as emitters, thermally stable 1,3,5-tris(4′-fluorobiphenyl-4-yl)benzene (F-TBB) as a hole-blocking layer, and Alq₃ as an electron injection layer. The CIE 1931 chromaticity coordinates of complexes 1–3 are around (0.650, 0.347), (0.681, 0.317), and (0.696, 0.302), respectively, and remain almost unchanged over a wide range of operation voltages. The maximum luminous efficiencies reached 7.0, 3.5, and 1.2 cd/A for devices based on 10 wt.% of osmium complexes 1, 2 and 3, respectively, even with air stable aluminum as the cathode. We systematically studied the dependence of device performance on the osmium doping concentrations. It was found that the best device performance was observed at 10 wt.% doping concentration for all three osmium complexes. Both maximum luminance and luminous efficiency increased with increasing osmium complex concentrations in the beginning and reached maximum at 10 wt.% doping concentration. However, a further increase in the doping level resulted in a reduction in both device brightness and efficiency due to concentration quenching and triplet–triplet annihilation. This is consistent with the absolute photoluminescence quantum yields of PVK thin films doped with different concentration of osmium complexes.     

New host materials for blue emitters

Triarylbenznes and tetraarylbenzenes were synthesized as host materials for the blue emitter Ide 102. Among them, 1,3,5-tri(1-pyrenyl)benzene (TPB3) showed highest performance. A device with a combination of TPB3 as a host and Ide 102 as the guest showed high luminance and high efficiency. The highest luminance was 142,000 cd/m² at 12 V. The efficiency was 6.0 lm/w at 5 V with 820 cd/m² and maintained still 4.0 lm/w even at 48,000 cd/m² with 2.4% external efficiency. The lifetime of the device was remarkably improved by use of TPB3.