Highly efficient blue electrophosphorescent devices with a novel host material

Highly efficient blue electrophosphorscent light emitting diodes with a new host material N,N′-dicarbazolyl-1,4-dimethene-benzene (DCB) were demonstrated. The energy transfer mechanism of the host–guest material system consisting of DCB and bis[(4,6-difluorophenyl)-pyridinato-N,C²′] (picolinato) Ir(III) (FIrpic) is an exothermic process. The device with a configuration of indium tin oxide/ N,N′-diphenyl-N,N′-bis(1,1′-biphenyl)-4,4′-diamine (NPB)/DCB:FIrpic/4,7-diphenyl-1,10-phenanthroline(BPhen)/Mg:Ag was optimized by adjusting the thickness of emitting layer and the dopant concentration. The device with the 8% (weight ratio) FIrpic and 30 nm emitting layer exhibits the maximum external quantum efficiency and current efficiency of 5.8% and 9.8 cd/A, respectively, at the luminance of 22 cd/m² driven at the voltage of 6.0 V.     

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.     

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.     

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.     

9,9-Bis{4-[di-(p-biphenyl)aminophenyl]}fluorene: a high Tg and efficient hole-transporting material for electroluminescent devices

A promising fluorene derivative 9,9-bis{4-[di-(p-biphenyl)aminophenyl]}fluorene (BPAPF) of high glass-transition temperature (T_g=167 °C) was synthesized and assessed as the hole-transporting material (HTM) in electroluminescent devices. Devices of various configurations, such as single-heterojunction, with or without dopant, and double-heterojunction devices were made. Superior performance was observed for devices based on BPAPF as the HTM, relative to that based on NPB (4,4′-bis[N-(1-naphthyl)-N-phenyl-amino]-biphenyl). In particular, with a device structure of ITO/BPAPF/Alq:0.5% quinacridone/Alq/Mg:Ag, a maximum luminance of ∼140,000 cd/m² was obtained at 15 V and maximum luminance and external quantum efficiencies of 13.7 cd/A and 4.1%, respectively, were obtained at 5.5 V.     

Highly efficient blue phosphorescent organic light-emitting devices using simple structures with thin 1,1-bis[(di-4-tolyamino)phenyl]cyclohexane layers

We have developed highly efficient blue phosphorescent organic light-emitting devices with thin 1,1-bis[(di-4-tolyamino)phenyl]cyclohexane (TAPC) layers. We used simple three organic layers: TAPC, iridium(III) bis[[4,6-di-fluorophenyl]-pyridinato-N,C^2′] picolinate (FIrpic) doped N,N′-dicarbazolyl-3,5-benzene (mCP), and 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) layers. The device structure was ITO/TAPC (x nm)/mCP:FIrpic (5 nm, 10%)/BCP (55 nm)/LiF/Al. The 4 nm TAPC device shows current efficiency and power efficiency of 34 cd/A and 15.1 lm/W, respectively, at a luminance of 1540 cd/m². We investigate the effect of TAPC thickness on the current efficiency, power efficiency, driving voltage, and electroluminescence characteristics.     

Exciplex emission and energy transfer in white light-emitting organic electroluminescent device

We report a white light organic electroluminescent (EL) device achieved through exciplex formation between organic materials as a green color light source. Exciplex formation at 500 nm originated from poly(N-vinylcarbazole) (PVK) and 2,5-bis(5-tert-bytyl-2-benzoxazolyl)thiophene (BBOT), exciplex formation at 550 nm between from N,N′-diphenyl-N,N′-bis(3-metylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD) and BBOT was assumed to correspond to the complex formation covering the green color region. In the mixed emitting materials, the PL decay lifetimes of BBOT and poly(3-hexylthiophene) (P3HT) were increased as much as 35–68 and 1.5–19 ns, respectively. This indicates that the energy transfer occurred from PVK to BBOT and P3HT, enhancing the quantum efficiency of these materials. We achieved white light emission with brightness as great as 12.3 μW/cm² at 20 V, corresponding to CIE coordinates of x=0.29 and y=0.353. The EL spectrum of the device was changed with a function of the applied voltage.     

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.     

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.     

Efficient blue-to-violet organic light-emitting diodes

Organic light-emitting diodes emitting in the range of 400 nm (violet) to 460 nm (blue) are reported. The basic device structure consists of indium–tin oxide/N,N′-diphenyl-N,N′-bis-(3-methylphenyl)-1,1′-biphenyl-4,4′-diamine (TPD)/2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP)/lithium fluoride (LiF)/aluminum. Offset of the energy levels at the TPD/BCP interface favors blocking of holes on the TPD side of the interface. Voltage-induced color change is observed and explained in terms of a switching from emission dominated by interfacial exciplex-induced recombination at low applied bias to one dominated by bulk exciton-induced recombination at high applied bias. With the addition of copper(II) phthalocyanine (CuPc) as an anode buffer layer and tris-8-(hydroxyquinoline) aluminum (Alq₃) as a cathode buffer layer, external quantum efficiencies as high as 0.5% at blue emission and 0.4% at violet emission have been obtained.     

Solution processable organic electro-phosphorescent iridium complex based on a benzothiazole derivative

We have synthesized a new solution processable iridium complex, di[2-(4′-octyloxyphenyl) benzothiazole]iridium(III)acetoacetone, [(OPBT)₂Ir(acac)], based on benzothiazole derivative for organic electro-phosphorescent devices. The synthesized molecule was identified by ¹H NMR and ¹³C NMR, and readily soluble in common organic solvents such as chlorobenzene. The UV–visible absorption and photoluminescence properties of pristine [(OPBT)₂Ir(acac) thin film as well as poly(N-vinylcarbzole) (PVK) thin film doped with the iridium complex were studied. The maximum UV–visible absorption and photoluminescence (PL) spectra are found to be at 337 nm and 547 nm, respectively. We have fabricated phosphorescent organic light-emitting devices using the ITO/PEDOT:PSS (40 nm)/PVK:(OPBT)₂Ir(acac) (40 nm)/Balq (40 nm)/LiF (1 nm)/Al (80 nm) configuration with the iridium complex as a triplet emissive dopant in poly(N-vinylcarbazole) (PVK) host. The electroluminescence (EL) devices showed greenish yellow light emission with maximum peak at 551 nm. Especially, the maximum external quantum and current efficiency of 1 mol% doped device were 1.74% and 4.89 cd/A, respectively.     

Organic tunable electroluminescent diodes from polyfluorene derivatives

We report on the electroluminescent properties of recently synthesized fluorine-based π-conjugated polymers. The spectral emission varies from blue to yellow depending on the composition of the alternated copolymers containing thiophene or phenylene moieties. The luminance of the devices can be enhanced by adequate balancing of the hole and electron injection/transport. Incorporation of a hole transporting molecule in the polymer and insertion of an insulating buffer layer in the device resulted in enhancement of the luminous efficiency. A 30-fold enhancement of the luminance was obtained by inserting an electron transporting layer. The highest luminance reached was 1640 cd/m² at 17 V and was obtained with a green emitter, poly(2,2′-(5,5′-bithienylene)-2,7-(9,9-dioctylfluorene)) (PBTF).     

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.     

White organic light-emitting devices with a bipolar transport layer between blue fluorescent and yellow phosphor-sensitized-fluorescent emitting layers

We report white organic light-emitting devices (WOLEDs) based on 4,4′-bis(2,2′-diphenylvinyl)-1,1′-biphenyl (DPVBi) and phosphorescence sensitized 5,6,11,12-tetraphenylnaphthacene (rubrene). By introducing a bipolar transport 4,4′-N,N′-dicarbazole-biphenyl (CBP) layer between the fluorescent and the phosphor-sensitized-fluorescent layers, additional light emission from the phosphorescence sensitized layer is observed. This can be attributed to the elimination of the Dexter energy transfer between these two emitters. White emission with Commission International de L’Eclairage coordinates of (0.22,0.33) and a maximum luminance of 22,360 cd/m² were obtained. The maximum current efficiency can reach 10.7 cd/A.     

Highly efficient bilayer blue phosphorescent organic light-emitting devices

We have developed highly efficient blue phosphorescent organic light-emitting devices comprising of two organic layers. Hole transporting 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) was used as an emitting host for iridium(III)bis[(4,6-di-fluorophenyl)-pyridinato-N,C²′]picolinate (FIrpic) guest. In our bilayer system, the host–guest energy transfer process leads to a low optimal doping concentration of 2 wt%, while the better charge balance is achieved by the better electron injection into the host layer from electron transport layer. Using these bilayer structures, we demonstrate a maximum current efficiency of 34 cd/A in the device structure of ITO/TAPC: FIrpic (30 nm, 2 wt%)/3-(4-biphenyl-yl)-4-phenyl-5-(4-tert-butylphenyl)-1,2,4-triazole (50 nm)/LiF/Al.     

Synthesis and characterization of new blue light emitting material with tetraphenylsilyl

The new amorphous blue light emitting material, which is composed of biphenylenevinylene with α-phenyl as main unit and bulky triphenylsilyl as side units, is designed, synthesized and characterized. The tetraphenylsilane groups of tetrahedral molecular skelecton makes the material have high glass transition temperature of 130 °C and good film quality. Blue organic light emitting device (OLED) employing this compound as the emitter exhibit the maximum luminescence of 4100 cd/m², the maximum external quantum efficiency of 0.7% and current efficiency of 1.67 cd/A at 12.8 V.     

Control of efficiency characteristics in green phosphorescent organic light-emitting devices

The efficiency characteristics in green phosphorescent devices at low luminance as well as high luminance were investigated by using single, host-mixed and multiple emitting layer (EML) structure. 4,4′-Bis(N-carbazolyl)-1,1′-biphenyl (CBP) was used as a host suppressing electron injection and transport, whilst a host (GH) with a spirobifluorene-type backbone was used as a host enhancing electron injection and transport. When two hosts were optimally mixed (CBP:GH = 3:1) or a triple EML structure with GH–EML sandwiched between two CBP–EMLs were used, the improved current efficiency at high luminance was obtained and at the same time, the current efficiency at low luminance was maintained low.     

Multilayer blue polymer light-emitting devices with spin-coated interlayers

Employment of multilayer heterostructures is a common approach to achieve efficiency and stable organic light emitting diodes (OLEDs). In this work, we report multilayer blue polymer light-emitting devices (PLEDs) by using spin-coated fluorene-triarylamine copolymers as interlayers between the conductive polymer poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT) and the emitting layer. A blue PLED with stepped hole injection profile yields an external quantum efficiency of 6.0% at a luminance of 9500 cd/m² at 5.5 V and an extrapolated lifetime of more than 18,000 h from 100 cd/m².     

Highly efficient blue electroluminescence based on a new anthracene derivative

A novel blue-light-emitting material, 2,3,6,7-tetramethyl-9,10-dinaphthyl-anthracene (TMADN), was synthesized and characterized. Organic light-emitting diode (OLED), which has a double-layer structure, has been fabricated. In this OLED, the homemade TMADN was used as the light-emitting material and 4,7-diphenyl-1,10-phenanthroline (DPA) was used as the hole blocking/electron transporting material, N,N′-biphenyl-N,N′-bis-(1-naphenyl)-[1,1′-biphenyl]-4,4′-diamine (NPB) was used as the hole transporting material. The peak emission of electroluminescence (EL) is at about 456 nm and the CIE coordinates are (0.171, 0.228). The brightness of the device is up to 5600 cd/m² at 17 V with the maximum EL efficiency of 2.2 cd/A.     

Highly efficient energy transfer to a novel organic dye in OLED devices

The neutral 4,4-difluoro-8-(2,2′:6′,2″-terpyridine-4′-yl)-1,3,5,7-tetramethyl-2,6-diethyl-4-bora-3a,4a-diaza-s-indacene (Boditerpy) molecule was synthesized and incorporated as dopant (<1%) in double-layer organic light emitting diodes (OLEDs) with the configuration ITO/α-NPD(60 nm)/Alq₃(60 nm):Boditerpy (0.4 nm)/LiF(0.02 nm)/Al(80 nm). This device exhibits green emission with a brightness of 545 cd/m² at 8 V and a maximum power efficiency of 0.9 lm/W. A full quantitative energy transfer process is indicated by a complete quenching of light emission from Alq₃ in photoluminescence. However, I–V characteristics indicate some losses during the charge transfer processes in OLED configuration