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.     

Deep blue organic light-emitting diodes based on triphenylenes

We report about new easy-to-synthesize deep blue light-emitting organic materials. Various substituted low-molecular-weight triphenylene-derivatives have been prepared in a one-step procedure and are easily available on large scale and high purity. Furthermore, the synthesis of an oxetane functionalized, photo-crosslinkable triphenylene-based emitter material with enhanced film-forming properties is described. The low-molecular-weight emitters were vacuum-deposited, whereas the photo-crosslinkable emitter material derivative was processed from solution. The optical and electrical properties of the compounds were investigated. The corresponding photoluminescence emission spectra exhibit λ_max,ems values around 400 nm. Organic light-emitting multi layer devices were fabricated and characterized. OLED devices from these molecules emit deep blue light of 436–456 nm.     

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).     

Organic light-emitting diodes and organic light-emitting electrochemical cells based on silole–fluorene derivatives

Silole groups are known to present a high electron affinity. Initially, copolymerization of siloles with fluorene was aimed at improving electron injection into the polymer layer and so improving the electroluminescent properties of organic light-emitting diodes (OLED's) made from fluorene. But it also provides the ability to turn the light emission colour to the green part of the spectrum and to stop the well-known spectral shift degradation occurring in fluorene-based materials. In this paper we report the synthesis and the characterisation of 1,1-dimethyl-2,5-bis(fluoren-2-yl)-3,4-diphenylsilole 4, and of two soluble conjugated random copolymers derived from 9,9-ditetradecylfluorene and 1,1-dialkyl-2,5-diphenylsilole, where the alkyl group is either methyl 11a or n-hexyl 11b. Silole 4 crystallizes in the triclinic P-1 space group with a = 9.8771(8), b = 10.6240(10), c = 16.585(2) Å, α = 95.775(8), β = 97.025(7), and γ = 111.738(8)°. The results obtained with this molecule, operating in a single-layer OLED (luminance ≈450 Cd/m² at 12 V; η_max = 0.2 Cd/A), give evidences for the complementarity of the silole and the fluorenyl moieties in the improvement of the charge injection processes when compared with 1,1-dimethyl-2,3,4,5-tetraphenylsilole. The results obtained from organic light-emitting electrochemical cells (LEC's) made from silole–fluorene copolymers 11a, 11b and molten salts show an improvement of both the device lifetime and the spectral stability when compared with polyfluorene. To explain devices performances electrical characterisation data and atomic force microscope (AFM) imaging were combined.     

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.     

High color rendering white organic light-emitting diodes fabricated using a broad-bandwidth red phosphorescent emitter for lighting applications

High color rendering white organic light-emitting devices (WOLEDs) were developed using a broad-bandwidth red phosphorescent iridium complex, bis[2-(1-naphthyl)benzothiazolato-N,C²′]iridium(III) acetylacetonate [Ir(absn)₂(acac)]. The red phosphorescent emitter Ir(absn)₂(acac) was used to fabricate blue–red and blue–green–red WOLEDs by combining blue-emitting bis[2-(4,6-difluorophenyl)pyridinato-N,C²′]iridium(III) picolinate (FIrpic) and green-emitting tris-fac-(2-cyclohexenylpyridine) iridium (III) [Ir(chpy)₃] in multiple-emissive layers. Mixed host emissive layers were employed using a hole-transport-type host 4,4′,4″-tris(N-carbazolyl)triphenylamine (TCTA) and an electron-transport-type host 2,6-bis[3-(carbazol-9-yl)phenyl]pyridine (DCzPPy) for efficient charge carrier injection. Di-[4-(N,N-ditolyl-amino)-phenyl]cyclohexane (TAPC) and 1,3-bis(3,5-dipyrid-3-yl-phenyl)benzene (BmPyPB) were used as the hole and electron transporting layers, respectively. The effects of the emissive layer thickness and the doping ratios of different color dopants on WOLED performances were investigated. The WOLED based on ITO/TAPC/TCTA:FIrpic (10%):Ir(absn)₂(acac) (4%)/TCTA:Ir(chpy)₃ (9%, 6 nm)/DCzPPy:FIrpic (13%):Ir(absn)₂(acac) (4%)/BmPyPB/LiF/Al exhibited an external quantum efficiency of 10.7%, a power efficiency of 23.0 lm/W, a very high color rendering index (CRI) of 88.1, and a correlated color temperature (CCT) of 2629 K at 1000 cd/m².     

Undoped yellow-emitting organic light-emitting diodes from a phenothiazine-based derivative

The single layer and multilayer undoped light-emitting devices were fabricated using a new soluble phenothiazine-based derivative, poly(3,7-N-octyl phenothiozinyl terephthalylidene) (POPTP). Through the optimization of device structures, the multilayer device has a maximum luminance of 1203 cd/m² at the bias voltage of 9.3 V, using 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) as a hole-blocking layer and tris-(8-hydroxyquinoline)aluminium (Alq₃) as a electron-injection/transporting layer. The Commision International de L’Eclairage (CIE) coordinates stabilized at (x, y) = (0.46, 0.53) at various bias voltages. Additionally, the dominant wavelength (λ_D) of around 575 nm and the color purity of approximately 100% indicated a pure yellow emission property. Therefore, POPTP is a stable candidate material with a pure yellow emission for the undoped organic light-emitting diodes (OLEDs).     

Organic light-emitting diodes based on multilayer structures

We study various organic structures and explain the obtained improvements by the use of protective layer or hole transport layer in multilayer OLEDs based on Alq₃ as emitter.     

Adjustable electroluminescence: blue-green to red organic light-emitting diodes based on novel poly-non-conjugated oligomers

Two new polymers, poly-9,10(1,3-bis(4-ethynylphenoxy)propane)anthracene and poly-1,2(tetra-2,5-thienylene-1,2-vinylene)dimethylsilyslethane, based on conjugated chromophores that are interconnected via non-conjugated spacers, were prepared and characterized in terms of their photo- and electroluminescence (PL and EL, respectively) properties in pure films and in solid solutions. The application of solid solutions of the two polymers in PVK:PBD (polyvinyl carbazole:2-(4-biphenyl)-5-(4-tert-buthyl phenyl)-1,3,4-oxadiazole) matrices as active layers in adjustable blue-green to red OLED is presented.     

White organic light-emitting diodes via mixing exciplex and electroplex emissions

The authors report the fabrication of white organic light-emitting devices and discuss their electroluminescence (EL) properties. The device structure is ITO/TPD (50 nm)/BCP (8 nm)/Rubrene (0.5 nm)/BCP (10 nm)/Alq₃ (20 nm)/LiF (1 nm)/Al. In the EL spectra of this device, two new emissions peaking at 590 and 630 nm have been observed. These two emissions should be attributed to triplet exciplex and electroplex occurring at TPD/BCP interface. White emission was obtained based on this device under 12 V driving voltage, the Commission Internationale de l’Eclairage (CIE) coordinates arrives to (0.31, 0.33).     

Microcavity effects on emissive color and electroluminescent performance in organic light-emitting diodes

Microcavity organic light-emitting diodes having a top metal mirror and a bottom dielectric mirror, which was distributed Bragg reflectors (DBR) fabricated by using TiO₂–SiO₂ alternative dielectric multilayer with a central stop-band and two sub-stop-bands, were fabricated. In the devices, the active layers consisted of a hole-transporting layer N,N′-di(naphthalene-l-yl)-N,N′-diphenylbenzidine (NPB) and an electron-transporting/emitting layer tris(8-hydroxy-quinoline) aluminum (Alq₃). The relationship of the electroluminescent (EL) spectrum and efficiency with the thickness of the active layer and metal layer was studied. It was found that the EL emissive color did not strongly depend on the thickness of the organic layer and metal layer, which was attributed to the excellent photon confinement role of the narrow stop-band of the used dielectric mirror. Thus, high efficiency microcavity organic light-emitting diodes were achieved, and the peak wavelength and color purity were not obviously changed, via optimizing the thickness of organic layer and metal electrode.     

Transient electroluminescence in multilayer organic light-emitting diodes: experiment and theory

We have investigated a multilayer organic light-emitting diode (OLED) with 1,3,5-tris(N,N-bis-(4,5-methoxy-phenyl)-aminophenyl)-benzene (TAPB) acting as hole transporting layer (HTL) and tris(8-hydroxy-quinolinolato) aluminium (Alq₃) as electron transporting layer (ETL). Positive charge carriers in the HTL were detected optically as a function of the applied bias. Furthermore, we investigated the DC-characteristics of current and brightness as well as the onset behaviour of the electroluminescence (EL) as a function of the applied bias. An analytical model is developed to describe the observed carrier concentrations as well as the current–voltage characteristics and the transient EL measurements quantitatively.     

Investigation of double emissive layer structures on phosphorescent blue organic light-emitting diodes

The performances of blue phosphorescent organic light-emitting diodes (PHOLEDs) at high current densities have been investigated with double emissive layer structures (D-EMLs). The D-EMLs are comprised of two emissive layers with a hole-transport-type host of N,N′-dicarbazolyl-3,5-benzene (mCP) and an electro transport-type ultrawide band-gap host of m-bis-(triphenylsilyl)benzene (UGH3) both doped with a blue electro-phosphorescent dopant of iridium(III)bis(4,6-difluorophenyl-pyridinato-N,C^2′) picolinate (FIrpic). The expansion of hole/electron recombination zone in D-EMLs has been successfully achieved by controlling of each EML properties, therefore external quantum efficiency, especially at high current density region was significantly enhanced. Moreover, the blue PHOLED with D-EMLs showed substantially reduced roll-off with the external quantum efficiency of 10.0% at 5000 cd/m².     

Hole blocking layer free phosphorescent organic light-emitting diodes by using a charge confining structure

Phosphorescent organic light-emitting diodes without hole blocking layer were developed by using a charge confining device structure which can confine excitons within light-emitting layer. Narrow triplet bandgap host material was sandwiched between wide triplet bandgap host materials and excitons could be effectively confined in narrow bandgap host light-emitting layer. The charge confining devices showed excellent device performances even without hole blocking layer because excitons were confined inside light-emitting layer.     

High efficiency deep blue phosphorescent organic light-emitting diodes using a double emissive layer structure

High efficiency deep blue phosphorescent organic light-emitting diodes (PHOLEDs) have been developed with a double emissive layer structure (D-EML). Using bis(4′,6′-difluorophenylpyridinato)tetrakis(1-pyrazolyl)borate (FIr6) as an electro-phosphorescent dopant, we achieved a maximum external quantum efficiency of 14.8% in the deep blue PHOLEDs with D-EMLs, which is 50% higher value than that of 9.76% with single emissive layer structure (S-EML). Moreover, the external quantum efficiency at high current density region of 10 mA/cm² was maintained up to 11.3% in this D-EML device. We attributed this enhancement to the expansion of carrier recombination region and the effective confinement of exciton within the emissive layer.     

Thermal aging of single-layer polymer light-emitting diodes composed of a carbazole and oxadiazole containing copolymer doped with singlet or triplet emitters

An electron-transporting monomer was synthesized that was structurally and energetically similar to the small molecule 2-biphenyl-4-yl-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (tBu-PBD). The monomer was copolymerized with 2-(9H-carbazol-9-yl)ethyl 2-methylacrylate and the resulting copolymer was utilized in organic light emitting devices which employed fluorescent coumarin 6 (C6) or phosphorescent tris(2-phenylpyridine)iridium(III) [Ir(ppy)₃] emitters. The copolymer devices exhibited a mean luminance of ca. 400 and 3552 cd/m² with the C6 and Ir(ppy)₃ emitters, respectively, that was stable with thermal aging at temperatures ranging from 23 °C to 130 °C. Comparable poly(9-vinyl-9H-carbazole)/tBu-PBD blend devices exhibited more pronounced variations in performance with thermal aging.     

Solution-processed white organic light-emitting diode based on a single-emitting small molecule

A pure-white organic light-emitting diode (WOLED) with a simple device architecture of indium tin oxide (ITO)/poly-(3,4-ethylenedioxythiophene):poly-(styrenesuphonic acid) (PEDOT:PSS)/L-Z-390/1,3,5-tri(N-phenylbenzimidazol-2-yl)benzene (TPBi)/LiF/Al was realized by using a single small molecule 3,3′-(2,7-Dis((9-(2,3,5,6-tetrafluorophenyl)-9H-carbazol-3-yl)ethynyl)-9H-fluorene-9,9-diyl)bis(9-heptyl-9H-carbazole) (L-Z-390) as the light-emitting material. The WOLED exhibited efficient white emission with a turn-on voltage of 6 V, a maximum luminous efficiency of 3.6 cd/A at 6 V, a maximum luminous of 204 cd/m² at 13 V and Commission Internationale de l’Eclairage (CIE) coordinates of (0.33, 0.34). The device performance characteristics are among the best ever reported for solution-processed WOLEDs with a single-emitting small molecule. The white light comes from the combination of the blue emission originating from the excimer of L-Z-390 and the long-wavelength orange emission originating from the electromer of L-Z-390 under a high electric field. The double-layer device based on 3,3′-(2,7-Dis((9-(2,3,5,6-tetrafluorophenyl)-9H-carbazol-3-yl)ethynyl)-9H-fluorene-9,9-diyl)bis(9-(2,3,5,6-tetrafluorophenyl)-9H-carbazole) (L-Z-398), the other molecule with two more electron-withdrawing 2,3,5,6-tetrafluorophenyl units than L-Z-390, shows green EL under the high voltages. That L-Z-398 cannot form electromer under a high voltage may be because the L-Z-398 cannot stably form L-Z-398⁺ component and L-Z-398^? component simultaneously due to the too strong electron-withdrawing ability of the four 2,3,5,6-tetrafluorophenyl units connected to each carbazole.     

High-efficiency red polymer light-emitting diodes based on optimizing blend polymer host

High efficiency stable red light-emitting diodes have been realized employing red copolymer (PFO-DHTBT15) from 9,9-dioctylfluorene (DOF) and 4,7-di(3-hexylthien-2-yl)-2,1,3-benzothiadiazole (DHTBT) blending into green copolymer (PFO-BT15) from 9,9-dioctylfluorene and 2,1,3-benzothiadiazole (BT) as a novel fluorescent emitting layer. The external quantum and luminous efficiency of device from blend film (PFO-DHTBT15:PFO-BT15 = 10:90) reached 5.2% and 3.16 cd/A at the current density of 35 mA/cm², respectively. The corresponding Commission Internationale de l’Eclairage coordinates is (0.64, 0.36). In comparison with the devices from phenyl-substituted poly [p-phenylene vinylene] derivative (P-PPV) as host of PFO-DHTBT15, the device from PFO-DHTBT15/PFO-BT15 blend film shows higher luminous efficiency and better stability under high current density at the same blend weight ratio. The improved device performance is mainly attributed to the effective energy transfer from PFO-BT15 copolymer to PFO-DHTBT15 copolymer and better carrier confinement.     

Thermally stable fluorescent blue organic light-emitting diodes using spirobifluorene based anthracene host materials with different substitution position

Thermally stable blue organic light-emitting diodes (OLEDs) were developed using anthracene based host materials with a spirobifluorene group. 4-Bromospirobifluorene and 2-bromospirobifluorene were attached to the anthracene core and the effect of the substitution position on the physical properties and device performances of the blue fluorescent OLEDs was investigated. The 4-spirobifluorene substitution was better than the 2-spirobifluorene substitution in terms of thermal stability and widened the bandgap of the anthracene based host material due to the geometrical structure of the material. However, the wide bandgap of the host material with 4-spirobifluorene had negative effect on the current density and efficiency of the blue devices.     

Field effect transistor behavior of organic light-emitting diodes with a modified configuration of ITO anode

We have developed the organic light-emitting diodes (OLEDs) with a modified configuration of ITO anode in which a thin channel was etched to form a bottom-contact field effect transistor (FET) using ITO and MgAg as a source/drain electrode and a gate electrode, respectively. The hole injection layer in OLEDs functioned as an active layer of FET and the other organic layers as insulator-like layer. The devices were found to exhibit a behavior of FET due to horizontal charge migration between source and drain, and an electro-optical transfer characteristic due to vertical charge transport and recombination. We have investigated the dependence of drain current on the channel length from 5 to 30 μm and found that the modified channel length could change drain current directionally and quantitatively.