Multilayer Organic Light-emitting Diodes Research Articles

Modified Julolidine-Containing Emitters for Red Organic Light-Emitting Diodes

The compounds 2.1.1-(2-(4-pentylbicyclo[2.2.2]octan-1-yl)-6-(2-(1,1,7,7-tetramethyl-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)vinyl)-4H-pyran-4-ylidene)malononitrile (Red 1) and 2-(2-(2-(1,1-dimethyl-7,7-bis((trimethylsilylmethyl)-1,2,3,5,6,7-hexahydropyrido[3,2,1-ij]quinolin-9-yl)vinyl)-6-(4-pentylbicyclo[2.2.2]octan-1-yl)-4H-pyran-4-ylidene)malononitrile (Red 2) with modified julolidine moieties were developed and synthesized. To determine the electroluminescence properties of these materials, multilayered organic light-emitting diodes (OLEDs) were fabricated with a device structure of indium tin oxide (ITO)/N,N'-diphenyl-N,N'-(1-napthyl)-(1,1'-phenyl)-4,4'-diamine (50 nm)/tris(8-quinolinolato)-aluminum (Alq3):dopants Red 1 and Red 2 (30 nm, 1–5%)/Alq3 (50 nm)/8-hydroxyquinolatolithium (2 nm)/Al. All devices exhibited efficient red emission. In particular, a device using Red 2 as the dopant material showed a maximum luminance of 1455 cd/m2 at 12.0 V, and maximum luminous and power efficiencies of 1.71 cd/A and 1.32 lm/W, respectively. The Commission Internationale de l'Eclairage coordinates of this device were (0.64,0.36) at 7.0 V, which indicated stable color chromaticity at various voltages.

Multi-layer organic light-emitting diodes processed from solution using phosphorescent dendrimers in a polymer host

A uniform dispersion of highly soluble phosphorescent dendrimer emitters is achieved by blending with a polymer host poly(9-vinylcarbazole) (PVK) containing N,N′-diphenyl- N,N′-(bis(3-methylphenyl)-[1,1-biphenyl]-4,4′-diamine (TPD) and 2-(4-biphen-4′-yl)-5-(4- tert-butylphenyl)-1,3,4-oxadiazole (PBD). No visible aggregation or self-quenching was observed for guest-to-host weight ratios of up to 33:67. The dendrimers contain a fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy) 3] core, first generation biphenyl-based dendrons, and 2-ethylhexyloxy surface groups. The guest–host blend is used for all solution processed organic light-emitting diodes. A maximum external and current efficiency of 10.2% and 38 cd/A (at 5 V and a brightness of 50 cd/m 2), and a maximum brightness of 27,000 cd/m 2 (at 14.5 V), were obtained when a CsF/Al cathode was used. Blade coating was used to fabricate a multi-layer structure that also contained an electron-transport layer. The device that had a LiF/Al cathode had a maximal efficiency of 40 cd/A corresponding to an external quantum efficiency of 10.8% (at 5 V and a brightness of 19 cd/m 2). The maximum brightness of the second device was 17,840 cd/m 2 at 14 V.

Electronic transport properties through thiophenes on switchable domains

The electronic transport of electrons and holes through stacks of $\ensuremath{\alpha},\ensuremath{\omega}$-dicyano-$\ensuremath{\beta},{\ensuremath{\beta}}^{\ensuremath{'}}$-dibutyl-quaterthiophene (DCNDBQT) as part of a novel organic ferroic field-effect transistor is investigated. The novel application of a ferroelectric instead of a dielectric substrate provides the possibility to switch bitwise the ferroelectric domains and to employ the polarization of these domains as a gate field in an organic semiconductor. A device containing very thin DCNDBQT films of around 20-nm thickness is intended to be suitable for logical as well as optical applications. We investigate the device properties with the help of a phenomenological model called multilayer organic light-emitting diodes, which was extended to transverse fields. The results showed that space-charge and image-charge effects play a crucial role in these organic devices.

Dependence of charge trapping of fluorescent and phosphorescent dopants in organic light-emitting diodes on the dye species and current density

This paper utilizes multilayer organic light-emitting diodes with a thin layer of dye molecules to study the mechanism of charge trapping under different electric regimes. It demonstrates that the carrier trapping was independent of the current density in devices using fluorescent material as the emitting molecule while this process was exactly opposite when phosphorescent material was used. The triplet–triplet annihilation and dissociation of excitons into free charge carriers was considered to contribute to the decrease in phosphorescent emission under high electric fields. Moreover, the fluorescent dye molecule with a lower energy gap and ionized potential than the host emitter was observed to facilitate the carrier trapping mechanism, and it would produce photon emission.

Analysis of metal-oxide-based charge generation layers used in stacked organic light-emitting diodes

We study electron and hole injection in MoO3 charge generation layers (CGLs) commonly used for establishing balanced injection in multilayer stacked organic light-emitting diodes (SOLEDs). A compound CGL consisting of 100-Å-thick MoO3 and Li-doped 4,7-diphenyl-1,10-phenanthroline in a 1:1 molar ratio is demonstrated to have a high electron generation efficiency. Charge injection from the compound CGL is modeled based on a two-step process consisting of tunneling-assisted thermionic emission over an injection barrier of (1.2±0.2) eV and a trap level due to oxygen vacancies at (0.06±0.01) eV above the MoO3 valence band edge. Peak external quantum efficiencies (EQEs) of (10.5±0.2)%, (10.1±0.2)%, (8.6±0.2)%, and (8.9±0.2)% are obtained for tris-(phenylpyridine)iridium-based electrophosphorescent OLEDs with indium tin oxide (ITO) anode/CGL cathode, CGL anode/CGL cathode, CGL anode/Al cathode, and ITO anode/Al cathode contacts, respectively. Based on our analysis, a three-element green emitting electrophosphorescent SOLED is demonstrated with a peak forward-viewing EQE=(24.3±1.0)% and a power efficiency of (19±1) lm/W.

The effect of alkaline metal chlorides on the properties of organic light-emitting diodes

The multilayer organic light-emitting diodes (OLEDs) have been fabricated with a thin alkaline metal chloride layer inserted inside an electron transport layer (ETL), tris (8-hydroxyquinoline) aluminum (Alq 3). The alkaline metal chloride layer was inserted inside 60 nm Alq 3 at d=0, 10, 20 and 30 nm positions ( d is the distance of the interlayer away from the Al cathode). The devices, with alkaline metal chlorides inserted at the Alq 3/Al interface, showed electron injection and electroluminescence (EL) intensity improvements. When the alkaline metal chlorides were inserted inside the Alq 3 layer at 10, 20 or 30 nm position apart from the Al cathode, both EL intensity and efficiency were enhanced for the devices with a thin potassium chloride (KCl) or rubidium chloride (RbCl) layer. On the contrary, the improvements were not observed for the OLEDs with a thin sodium chloride (NaCl) layer. A proposed insulator buffer layer model is employed to explain these characteristics of the devices.

Organic light-emitting diodes using potassium chloride as efficiency and stability enhancers

A multilayer organic light-emitting diode (OLED) has been fabricated with a thin (1 nm) potassium chloride (KCl) layer inserted inside an electron transport layer (ETL), tris (8-hydroxyquinoline) aluminum (Alq 3). The structure of device was ITO/NPB/Alq 3/KCl/Alq 3/Al. The KCl layer was inserted inside 60 nm Alq 3 at 10, 20 and 30 nm positions away from the Alq 3/Al interface. The device shows the optical power and electroluminescence (EL) efficiency enhancements. The highest optical power density of devices with KCl at different positions is more than twice as high as that of the device without KCl. The EL efficiency is enhanced by more than 50% by inserting the thin KCl insulating layer. The mechanism of KCl EL efficiency enhancer is that the thin KCl layer induces carrier trap sites and gives better recombination in the device. After air-exposure 42 h, the efficiency of the devices with KCl is enhanced but not for the device without KCl. The insertion of KCl inside Alq 3 may improve the stability of OLED.

Modeling Disorder in the Crystal Structure of the α Polymorph of Alq3

Alq3, tris(quinolin-8-olato)aluminum(III) is used in the electron transport and/or electron-injecting layer in multilayer organic light-emitting diode device structures. In recent years, five crystalline phases (α, β, γ, δ, and e) of Alq3 have been identified. A structure of the α form, containing the meridional isomer, has been reported in literature based on powder XRD data. Single crystals of α Alq3 have been found to have essentially the same unit cell parameters [triclinic, space group $$P \bar {1}$$ , a = 6.2455(10) A, b = 12.8710(18) A, c = 14.739(3) A, α = 69.890(6)°, β = 89.464(5)°, and γ = 82.520(13)°] as reported previously from powder XRD: triclinic, space group $$P \bar {1}$$ , a = 6.2586(8) A, b = 12.914(2) A, c = 14.743(2) A, α = 109.66(1)°, β = 89.66(1)°, and γ = 97.68(1)°. The single-crystal XRD structure for α Alq3, however, appeared to be present mostly as the facial isomer, with a high degree of disorder. Although the structure obtained by single-crystal XRD appears to be predominantly or entirely facial, various spectroscopy results (particularly NMR) are more consistent with a meridional composition. To resolve the α Alq3 isomerism contradiction, we have reconsidered the structural conclusions that were drawn from the single crystal XRD. Based on modeling results, the hypothesis is that α Alq3 could be a disordered analog of e Alq3, with meridional conformation. Disorder in the crystal structure of the α polymorph of Alq3 was modeled with help from various spectroscopic techniques.

The effect of rubidium chloride on properties of organic light-emitting diodes

A multilayer organic light-emitting diode (OLED) was fabricated with a thin rubidium chloride (RbCl) layer inserted inside an electron transport layer (ETL), tris (8-hydroxyquinoline) aluminum (Alq 3). Here, we set d is the distance of the RbCl layer away from the Alq 3/Al interface. The rubidium chloride layer was inserted inside 60 nm Alq 3 at d = 0.0, 2.5, 5.0, 7.5, 10, 20 and 30 nm positions. When the RbCl layer is positioned closer to the Al cathode, both the current density and EL efficiency are enhanced due to the enhanced electron injection. The devices show the electroluminescent (EL) efficiency improvement without an enhanced injection if the value of d is lager than 5.0 nm. The suggested mechanism of RbCl EL efficiency enhancer is carrier trap sites induced by the thin RbCl layer. The trapped charges alter the distribution of the field inside the OLED and, consequently, give better recombination in the device.

All-solution-processed blue small molecular organic light-emitting diodes with multilayer device structure

All-solution-processed multilayer blue small molecular organic light-emitting diodes are fabricated by blade coating method. Fluorescent blue host,1-(7-(9,9′-bianthracen-10-yl)- 9,9-dioctyl-9H-fluoren-2-yl)pyrene, and blue dopant, 4,4′-(1E,1′E)-2,2′-(naphthalene-2,6-diyl)bis(ethene-2,1-diyl)bis(N,N-bis(4-hexylphenyl)aniline), are used to achieve good solubility and pinhole-free thin film by solution process. The multilayer device structure with hole/electron transport layer is achieved by blade coating method without the dissolution problem between layers. The efficiency of the all-solution-processed device is 4.8 cd/A at 1200 cd/m 2, close to that by thermal deposition in high vacuum chamber. The device performance is optimized with the annealing temperature of TPBi layer at 50 °C.

Energy level alignment at the interfaces in a multilayer organic light-emitting diode structure

Received 2 March 2009; revised manuscript received 4 May 2009; published 11 June 2009We use photoelectron spectroscopy to study the electronic structure and energy level alignment throughoutan organic light-emitting diode. The structure under investigation is a state-of-the-art long-living red phospho-rescent device composed of doped charge-injection layers, charge-blocking layers, and an emission layer. Byconsecutively building up the whole device, the key parameters of every interface are measured. Our resultsshow that the doped layers have a significant influence on the device energetics, especially in controlling thebuilt-in potential, and that there are mostly only small dipoles present at the interfaces of the intrinsic organiclayers.DOI: 10.1103/PhysRevB.79.245308 PACS number s : 79.60.Fr, 85.60.Jb

ELECTROLUMINESCENT PROPERTIES OF CERTAIN POLYAROMATIC COMPOUNDS: PART 1–CHARACTERISTICS OF OLED DEVICES BASED ON FLUORESCENT POLYAROMATIC DOPANTS

Organic electroluminescent devices recently have been focus of attention. Since the first demonstrations in this field, rapid developments have occurred in the usage of organic polyaromatic compounds and their derivatives for the purpose of multi-layer structures, multi-color, and full-color organic light-emitting diode displays. In the part-1 of the present short review, recent technical developments in the preparation and structure of organic light emitting diodes with special attention to organic emitters involving polyaromatic kernels have been considered.

Synthesis, characterization and electrophosphorescent properties of mononuclear platinum(II) complexes based on 2-phenylbenzoimidazole derivatives

The rational design, synthesis and characterization of five phosphorescent platinum complexes [(C^N)Pt(acac)] [Hacac = acetylacetone, HC^N = 1-methyl-2-(4-fluorophenyl)benzoimidazole (H-FMBI), 1-methyl-2-phenylbenzoimidazole (H-MBI), 1,2-diphenyl-benzoimidazole (H-PBI), 1-(4-(3,6-di- t-butylcarbazol-9-yl))phenyl-2-phenylbenzoimidazole ( t-BuCz-H-PBI), and 1-(4-(3,6-di-(3,6-di- t-butyl-carbazol-9-yl))carbazol-9-yl)phenyl-2-phenylbenzoimidazole ( t-BuCzCz-H-PBI)] have been discussed. The crystal structure of (MBI)Pt(acac) shows a nearly ideal square planar geometry around Pt atom and the weak intermolecular interactions with π–π spacing of 3.55 Å. All of the complexes emit green phosphorescence from the metal-to-ligand charge-transfer (MLCT) excited state with high quantum efficiency (0.08–0.17) at room temperature. A multilayer organic light-emitting diode (OLED) with (MBI)Pt(acac) as phosphorescent dopant was fabricated using the method of high-vacuum thermal evaporation, which gives a maximum brightness, luminous and power efficiency of 13 605 cd/m 2, 15.1 cd/A and 4.3 lm/W, respectively. In contrast, the comparable performance can be achieved in the solution-processed OLED based on ( t-BuCzPBI)Pt(acac) with a peak brightness, luminous and power efficiency of 13 606 cd/m 2, 17.5 cd/A and 8.4 lm/W, respectively. The better device efficiency results from the good square plane of central Pt coordination unit and the inhibition of the aggregates due to bulky and rigid t-butylcarbazole dendrons.

Enhancement of current-voltage characteristics of multilayer organic light emitting diodes by using nanostructured composite films

With the aim of improving the photonic efficiency of an organic light emitting diode (OLED) and its display duration, both the hole transport layer (HTL) and the emitting layer (EL) were prepared as nanostructured thin films. For the HTL, nanocomposite films were prepared by spin-coating a homogeneous solution of low molecular weight poly(4-styrenesulfonate) (PEDOT-PSS) and surfactant-capped TiO2 nanocrystals onto low resistivity indium tin oxide (ITO) substrates; for the EL, nancrystalline titatium oxide (nc-TiO2)-embedded Poly[2-methoxy-5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV+nc-TiO2) conjugate polymers were spin-coated onto the HTL. Also, for a shallow contact of Al/LiF/MEH-PPV instead of Al/MEH-PPV a super LiF thin film was deposited onto the EL by vacuum evaporation. The resulting multilayer OLED had the following structure of Al/LiF/MEH-PPV+nc-TiO2/PEDOT-PSS+nc-TiO2/ITO. Characterization of the nanocomposite films showed that both the current-voltage (I-V) characteristics and the photoluminescent properties of the nanocomposite materials were significantly enhanced in comparison with the standard polymers. OLEDs made from these layers would exhibit a large photonic efficiency.

Multifunctional Crosslinkable Iridium Complexes as Hole Transporting/Electron Blocking and Emitting Materials for Solution‐Processed Multilayer Organic Light‐Emitting Diodes

AbstractHere, a new series of crosslinkable heteroleptic iridium (III) complexes for use in solution processed phosphorescent organic light emitting diodes (OLEDs) is reported. These iridium compounds have the general formula of (PPZ‐VB)2Ir(CˆN), where PPZ‐VB is phenylpyrazole (PPZ) vinyl benzyl (VB) ether; and the CˆN ligands represent a family of four different cyclometallating ligands including 1‐phenylpyrazolyl (PPZ) (1), 2‐(4,6‐difluorophenyl)pyridyl (DFPPY) (2), 2‐(p‐tolyl)pyridyl (TPY) (3), and 2‐phenylquinolyl (PQ) (4). With the incorporation of two crosslinkable VB ether groups, these compounds can be fully crosslinked after heating at 180 °C for 30 min. The crosslinked films exhibit excellent solvent resistance and film smoothness which enables fabrication of high‐performance multilayer OLEDs by sequential solution processing of multiple layers. Furthermore, the photophysical properties of these compounds can be easily controlled by simply changing the cyclometallating CˆN ligand in order to tune the triplet energy within the range of 3.0–2.2 eV. This diversity makes these materials not only suitable for use in hole transporting and electron blocking but also as emissive layers of several colors. Therefore, these compounds are applied as effective materials for all‐solution processed OLEDs with (PPZ‐VB)2IrPPZ (1) acting as hole transporting and electron blocking layer and host material, as well as three other compounds, (PPZ‐VB)2IrDFPPY (2), (PPZ‐VB)2IrTPY(3), and (PPZ‐VB)2IrPQ(4), used as crosslinkable phosphorescent emitters.

Molecular Modification of 2,7-Diphenyl[1]benzothieno[3,2-b]benzothiophene (DPh-BTBT) with Diarylamino Substituents: From Crystalline Order to Amorphous State in Evaporated Thin Films

Abstract Introduction of diarylamino substituents on 2,7-diphenyl[1]benzothieno[3,2-b]benzothiophene (DPh-BTBT) caused morphological change in the evaporated thin-film state: from highly crystalline films with edge-on molecular orientation for DPh-BTBT to amorphous films for the diarylamino derivatives. The former was unsuitable as a hole-transporting layer in organic light-emitting diodes (OLEDs), whereas the latter acted as a superior hole-transporting layer in multilayered OLEDs.

Highly-efficient solution-processed phosphorescent multi-layer organic light-emitting diodes investigated by electromodulation spectroscopy

We report on electromodulation (EM) spectroscopy studies of phosphorescent multi-layer organic light-emitting diodes (OLEDs) that are processed from solution. Compared to conventional single-layer OLEDs, they comprise an additional layer of a crosslinkable, oxetane-functionalized triphenylamine-dimer (XTPD) that is inserted between the PEDOT:PSS anode and the emissive layer. Devices with optimized stack architecture feature reduced operating voltages and reach a current efficiency approaching 40 cd/A—twice as much as the corresponding single-layer device. Using EM measurements, we quantify the electric field in the XTPD layer and the emissive layer of such a multi-layer OLED and also measure the average electric field in a single-layer reference device. By comparing the dependence of the internal field on the applied voltage for devices with and without the XTPD layer, we find that in the device containing the XTPD layer there is an increased accumulation of electrons at the anode side of the emissive layer. This accumulation enhances the recombination probability and supports the injection of holes into the emissive layer which explains the observed efficiency improvement and reduction in operating voltage compared to conventional single-layer OLEDs.

Theoretical Investigation of Organic Amines as Hole Transporting Materials: Correlation to the Hammett Parameter of the Substituent, Ionization Potential, and Reorganization Energy Level

Theoretical calculations on organic amines widely used as hole-transporting materials (HTMs) in multilayer organic light-emitting diodes have been performed. The calculated Ip and the reorganization energy for hole transport (λ+) of triphenylamine (TPA), 9-phenyl-9H-carbazole (PC), and their derivatives, are found to be related to their Hammett parameter (σ). In this study, the density functional theory (DFT) calculation is used to optimize 82 TPA and PC derivatives. Electronic structures of these compounds in the neutral and the radical-cation states are obtained based on calculations on optimized geometrical structures. The Ip and λ+ values are derived from calculated heats of formation (or total energy) of the neutral and the radical-cation states. In particular, the calculated Ips for these derivatives correlate well with the experimental data. The substitution effect for the mono-substituted TPA and PC is displayed in that the Ips of the TPA and PC derivatives with electron-donating and -withdrawing substituents are lower and higher than those of TPA and PC, respectively. For the effect of substitution position, the para-substituted TPA derivatives have higher Ip and –EHOMO than those of meta-substituted TPAs. The substitution effects in di- and tri-substituted TPAs are more pronounced than that of mono-substituted ones. According to the results, the calculated Ips shows an excellent agreement with the experimental oxidation potentials (EP/2) in these TPA derivatives. Furthermore, these calculation results can be employed to predict electro-luminescent properties for new and improved HTMs.

Synthesis, photophysical and electrophosphorescent properties of mononuclear Pt(II) complexes with arylamine functionalized cyclometalating ligands

Abstract Four cyclometalated Pt(II) complexes, i.e., [(L2)PtCl] (1b), [(L3)PtCl] (1c), [(L2)PtC CC6H5] (2b) and [(L3)PtC CC6H5] (2c) (HL2 = 4-[p-(N-butyl-N-phenyl)anilino]-6-phenyl-2,2′-bipyridine and HL3 = 4-[p-(N,N′-dibutyl-N′-phenyl)phenylene-diamino]-phenyl-6-phenyl-2,2′-bipyridine), have been synthesized and verified by 1H NMR, 13C NMR and X-ray crystallography. Unlike previously reported complexes [(L1)PtCl] (1a) and [(L1)PtC CC6H5] (2a) (HL1 = 4,6-diphenyl-2,2′-bipyridine), intense and continuous absorption bands in the region of 300–500 nm with strong metal-to-ligand charge transfer (1MLCT) (dπ(Pt) → π∗(L)) transitions (e ∼ 2 × 104 dm3 mol−1 cm−1) at 449–467 nm were observed in the UV–Vis absorption spectra of complexes 1b, 1c, 2b and 2c. Meanwhile, with the introduction of electron-donating arylamino groups in the ligands of 1a and 2a, complexes 1b and 2b display stronger phosphorescence in CH2Cl2 solutions at room temperature with bathochromically shifted emission maxima at 595 and 600 nm, relatively higher quantum yields of 0.11 and 0.26, and much longer lifetimes of 8.4 and 4.5 μs, respectively. An electrochromic film of 1b-based polymer was obtained on Pt or ITO electrode surface, which suggests an efficient oxidative polymerization behavior. An orange multilayer organic light-emitting diode with 1b as phosphorescent dopant was fabricated, achieving a maximum current efficiency of 11.3 cd A−1 and a maximum external efficiency of 5.7%. The luminescent properties of complexes 1c and 2c are dependent on pH value and solvent polarity, which is attributed to the protonation of arylamino units in the C^N^N cyclometalating ligands.

Photoluminescence and Electroluminescence Characteristics of 3,3′-Dicyano-[7,7′-biindolizine]-1,1′,2,2′-tetracarboxylic Acid 1,2-Diethyl 1′,2′-Dimethyl Ester

Abstract We investigated photoluminescence (PL) and electroluminescence (EL) characteristics of 3,3′-dicyano-[7,7′-biindolizine]-1,1′,2,2′-tetracarboxylic acid 1,2-diethyl 1′,2′-dimethyl ester (Et2Me2-biInd). We found that Et2Me2-biInd has efficient blue PL and a multilayer organic light-emitting diode doped with Et2Me2-biInd exhibits a relatively high EL efficiency of 1.8±0.1%.