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.     

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.     

Cyclometalated iridium(III) complexes with 5-acetyl-2-phenypyridine derived ligands for red phosphorescent OLEDs

A series of new iridium complexes with 5-acetyl-2-phenylpyridine derivatives as ligands was developed. The complexes exhibited high EL performance when applied to OLEDs. These materials showed red emission with a peak at 575–636 nm. In particular, one of the devices in this study showed a maximum luminous efficiency, maximum power efficiency, external quantum efficiency and CIE coordinates of 29.0 cd/A, 6.13 lm/W, 8.86% at 20 mA/cm² and (0.57, 0.43) at 10 V, respectively. In addition, a deep red OLED with CIE coordinates of (0.67, 0.32) at 10 V exhibited a maximum luminous efficiency, maximum power efficiency and external quantum efficiency of 5.61 cd/A, 1.02 lm/W and 5.35% at 20 mA/cm², respectively.     

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.     

A blue organic emitting diode derived from new styrylamine type dopant materials

We have designed and synthesized new dopant materials based on the styrylamine moiety, 4-[(1,2-diphenyl)-4′-(N,N-diphenyl-4-vinylbenzenamine)]biphenyl (4) and 4-[(1,2-diphenyl)-4′-(N,N-diphenyl-4-vinylbenzenamine)]terphenyl (8). Blue OLEDs were obtained from new styrylamine dopant materials and compared with those of blue dopant bis[4-(di-p-N,N-diphenylamino)styryl]stilbene (DSA-Ph) and diphenyl[4-(2-terphenyl vinyl)phenyl]amine (R-BD). The ITO/DNTPD/NPB/MADN:dopant/Alq₃/Al-LiF device obtained from 4 shows blue EL spectrum at 469 nm and high efficiency 3.02 cd/A at 7 V. 8 also shows blue EL spectrum around λ_max = 468 nm, efficiency of 3.51 cd/A and a current density of 25.94 mA/cm² (855.7 cd/m²) at 7 V.     

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.     

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.     

Polypyridyl Ru(II)-sensitizers with extended π-system enhances the performance of dye sensitized solar cells

New polypyridyl ruthenium(II) complexes “cis-Ru(4,4′-dimesityl-2,2′-bipyridine) (Ln) (NCS)₂ H102” and “cis-Ru(4,4′-bis(2,3,6-tri-isopropylphenyl)-2,2′-bipyridine) (Ln) (NCS)₂ H105”, where Ln = 4,4′-dicarboxylic acid-2,2′-bipyridine; were synthesized and successfully applied to sensitization of nano-crystalline TiO₂ based solar cells (DSSCs). The DSSCs of H102 and H105 fabricated from 0.16 cm² TiO₂ electrodes exhibited broader comparable photocurrent action spectra with almost identical solar-to-electrical energy conversion efficiency (η) as compared to N719 sensitizer. The incident photon-to-current conversion efficiency (IPCE) values of 98% and 95% were obtained for H102 and H105 sensitizers respectively. Under 1 sun condition, η-values of 8.39% (short-circuit photocurrent (J_SC) = 16.4 mA/cm², open-circuit photo voltage (V_OC) = 692 mV, fill factor = 0.734), 8.76% (J_SC = 16.3 mA/cm², V_OC = 735 mV, fill factor = 0.734) and 9.12% (J_SC = 16.1 mA/cm², V_OC = 745 mV, fill factor = 0.753) were obtained for H102, H105 and N719 sensitizers respectively.     

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.     

Organic light-emitting diode based on a carbazole compound

A carbazole compound was synthesized by Knovenagel condensation and characterized by the measurements of ¹H NMR, IR and melting point. A multilayer organic light-emitting diode (OLED) using this compound as an active layer was fabricated by vacuum-deposition. This OLED showed a turn-on voltage of approximately 4.5 V and a maximum luminance of 910 cd/m². Additionally, the maximum luminous efficiency was found as 0.95 cd/A, at this point the device luminance was measured as 146 cd/m² at an operating voltage of 7 V. The coordinate value of CIE 1931 was calculated as (x, y) = (0.3843, 0.5345) from the electroluminescence (EL) spectrum, which suggested that the device can emit a yellow-green light.     

Phenothiazine conjugated bipyridine as ancillary ligand in Ru(II)-complexes for application in dye sensitized solar cell

A new high molar extinction coefficient ruthenium(II)-bipyridine complex “cis-Ru(4,4′-bis((E)-2-(10-decyl-10H-phenothiazin-3-yl)vinyl)-2,2′-bipyridine)(4,4′-dicarboxylic acid-2,2′-bipyridine)(NCS)₂ PTZ1″ was synthesized through conjugation of phenothiazine unit with bipyridine and characterized by FT-IR, ¹H-NMR and ESI-MASS spectroscopes. Absorption measurements and time dependent-density functional theory (TD-DFT) calculations show increased spectral response for the ancillary ligand and the corresponding complex. The dye upon anchoring onto mesoporous nanocrystalline TiO₂ solar cells exhibited solar-to-electric energy conversion efficiency (η) of 3.77% short-circuit photocurrent density (J_SC) = 7.79 mA/cm², open-circuit voltage (V_OC) = 640 mV, fill factor = 0.750) under air mass 1.5 sunlight, the reference Z907 and HRS1sensitized solar cells, fabricated and evaluated under identical conditions exhibited η-value of 7.02% (J_SC = 15.25 mA/cm², V_OC = 650 mV, fill factor = 0.705) and 3.05% (J_SC = 8.20 mA/cm², V_OC = 610 mV, fill factor = 0.620) respectively. The lower film absorption of PTZ1on TiO₂ surface could be probably due to larger molecular diameter and planarity of phenothiazine prone to aggregate in solution as well as on TiO₂ surface. The DFT calculations show that the first three HOMOs of PTZ1 have t2g character as observed in case of Z907, while HOMO-4 and HOMO-5 have π-orbitals with major component on phenothiazine moieties of L1.     

Highly efficient deep-blue organic light-emitting diodes with doped transport layers

We demonstrate highly efficient, vapor-deposited blue organic light-emitting diodes (OLEDs) operating at low voltage. For reaching deep-blue color, we used two new fluorophores, 9,10-bis(9,9′-spirobi[9H-fluorene]-2-yl)anthracene (Spiro-Anthracene) from Covion, and 4,4′-bis-(N,N-diphenylamino)-tetraphenyl (4P-TPD) from Syntec-Sensient, sandwiched in between p- and n-type doped wide band-gap transport layers and appropriate blocking layers. These p-i-n OLED devices show high luminance and efficiency at low operating voltages. Both dyes emit deep-blue light at color coordinates of x = 0.15 and y = 0.09 (4P-TPD) and x = 0.15 and y = 0.18 (Spiro-Anthracene). Optimized devices containing Spiro-Anthracene reach a luminance of 100 and 1000 cd/m² already at a voltage of 2.9 and 3.4 V, respectively. At the same time, a deep-blue color with CIE color coordinates of x = 0.14 and y = 0.14 as well as good current efficiencies (3.9 cd/A at 100 cd/m²) and quantum efficiencies (3.7% at 100 cd/m²) are reached, which shows that the concept of doped transport layers and appropriate fluorescent emitters can be applied successfully to the preparation of blue OLEDs.     

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.     

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.     

Highly efficient red electroluminescence induced by efficient electron injection and exciton confinement

Highly efficient DCJTB-doped device was realized by enhanced electron injection and exciton confinement. A fluorine end-capped linear phenylene/oxadiazole oligomer 2,5-bis(4-fluorobiphenyl-4′-yl)-1,3,4-oxadiazole (1) and a trifluoromethyl end-capped oligomer 2,5-bis(4-trifluoromethylbiphenyl-4′-yl)-1,3,4-oxadiazole (2) were designed and incorporated as an electron transporting/hole blocking material in the device structure ITO/NPB (60 nm)/DCJTB:Alq₃ (0.5%, 10 nm)/1 or 2 (20 nm)/Alq₃ (30 nm)/LiF (1 nm)/Al (100 nm). The devices showed highly efficient red luminescence. In particular, the device based on 1 achieved pure red luminescence at 620 nm originating from DCJTB, with a narrow FWHI of 65 nm, maximal brightness of 13,300 cd/m² at voltage of 20.8 V and current density of ca. 355 mA/cm². High current and power efficiencies (>3.6 cd/A, 1.0 lm/W) were retained within a wide range of current densities. Our results show efficient and stable DCJTB-doped red electroluminescence could be anticipated for practical applications by taking advantage of the present approaches. The control experiments using BCP were also studied.     

Electroluminesent divinylene- and trivinylene-molecules with terminal naphthalimide or phthalimide segments

Two new linear divinylenes FN and PN that contained fluorene and phenylene, respectively, as central unit and naphthalimide terminal groups were synthesized by Heck coupling. In addition, two new star-shaped trivinylenes TPA-P and TPP-P that contained triphenylamine and 2,4,6-triphenylpyridine, respectively, as central core, and terminal phthalimide groups were similarly synthesized. All molecules were very soluble in common organic solvents due to their high fraction of aliphatic moieties which were attached to the central unit and/or the terminal imides. Trivinylenes showed higher thermal stability and higher glass transition temperature (118–126 °C) than divinylenes. FN, PN and TPA-P emitted green-orange light with maximum at 518–586 nm, while TPP-P emitted blue light with maximum at 444–462 nm due to the kinked central core of 2,4,6-triphenylpyridine. The maximum luminance among the four molecules is 583 cd/m² at current density of 186 mA/cm² and applied voltage of 19.5 V based on TPA-P, with a luminance efficiency maximum (η_max) of 1.7 cd/A.     

New amorphous electron-transporting materials based on Tris-benzimidazoles for all wet-process OLED devices

New amorphous electron-transporting materials 1,3,5-tris[1-(phenoxy phenyl)-1H-benzimidazol-2-yl]benzene (6a and 6b) were synthesized by dehydration reaction of corresponding triamides and fully characterized. Compounds 6a and 6b showed high thermal stability (T_g = 108 to 110 °C) and good solubility in common organic solvents, such as acetone, 2-methoxyetanol, tetrahydrofuran, chloroform, and 2-butanone. Pinhole-free transparent films were obtained by spin-casting the 2-butanone solution. 6a and 6b were fabricated as electron-transporting layer by spin-casting on a hole-transporting layer that was deposited by spin-cast beforehand and was insoluble in 2-butanone. Both devices showed no emission, but carrier transporting property was observed (400 mA/cm²). The device showed a blue emission (35 cd/m²) from 9,10-diphenylanthracene (DPA) when 10 wt.% DPA was mixed into the 6b layer.     

Synthesis of 2-phenylquinoline-based ambipolar molecules containing multiple 1,3,4-oxadiazole spacer groups

A series of donor–acceptor type ambipolar electroluminescence dyes with the general structure PQ(OXD)_nT (where n = 1, 2 and 3) were prepared, in which PQ is 2-phenylquinoline, T is diphenylamine which constituted the hole transporting triphenylamine moiety with an adjacent phenyl ring, and OXD is an electron transporting 2-phenyl-1,3,4-oxadiazole repeating unit. The compounds fluoresced bluish green to green hue in solid-state, exhibited a positive solvatochromism in solution and their quantum efficiency decreased rapidly with increase in n. The materials are thermally stable with glass transition temperature (T_g) ranging from 83 °C (n = 1) to 130 °C (n = 3). Cyclic voltammetry studies indicated the HOMO remained relatively unchanged with n while the LUMO decreased (away from the vacuum level) with an increase in the number of OXD. For single layer homojunction OLEDs, highest efficiency was obtained when n = 1 (max luminous 3300 cd/m² and current efficiency 0.9 cd/A), whereas for multilayer heterojunction OLEDs, best results was achieved for compounds with n = 1 or 2 assuming the role of the HT layer (over 8200 cd/m² max and 2.0 cd/A). Formation of exciplexes led to significant red-shift and lower emission efficiency for the compound with n = 3.     

Synthesis and electroluminescence of thiophene-based bipolar small molecules with different arylamine moieties

A series of fluorescent dyes consisted of a thiophene unit, an 1,3,4-oxadiazole unit and four different arylamine moieties were prepared using a facile multi-steps synthetic route with high yield. The four arylamine structures studied were triphenylamine, N,N′-diphenyl-(1,1′-biphenyl)-4,4′-diamine, diphenyl(1-naphthyl)amine and 9-phenylcarbazole. Experimental results shown that their HOMOs varied from 5.21 to 5.73 eV strongly affected by the arylamine chemistry while their LUMOs remained relatively unchanged. Their corresponding emission colors ranged from UV (393 nm) to bluish green (483 nm). In general, the thiophene unit enhanced the overall thermal stability of the compounds. According to cyclic voltammetry, the compounds are predominantly hole-transporting while OLED results indicated cpd 10 possess both hole and electron transport properties. Single layer OLED fabricated from 10 resulted in ca. 2000 cd/m² (luminous intensity) and 1.10 cd/A (current efficiency) max, whereas, a multilayer OLED using 10 as the hole transporting layer achieved over 7400 cd/m² and 2.3 cd/A max.