Materials For Organic Light-emitting Devices Research Articles

Comparative study of photoelectric properties of a copolymer and the corresponding homopolymers based on poly(3-alkylthiophene)s

As semiconducting materials in organic light-emitting devices (OLEDs), a novel, highly soluble poly[(3-octylthiophene)-co-(3-(2-ethyl-1-hexylthiophene))] (P3OTIOT) and the corresponding homopolymers (poly(3-octylthiophene) (P3OT) and poly(3-isooctylthiophene) (P3IOT)) were prepared by an FeCl 3-oxidative approach to compare their photoelectric properties. Characterization of the polymers included FT-IR, 1H NMR, gel permeation chromatography (GPC), thermo-gravimetric analysis (TGA), UV–vis spectroscopy, photoluminescence (PL) and electroluminescence (EL). P3OTIOT and P3OT depicted excellent solubility in common organic solvents. TGA studies showed that all of the materials exhibited very good thermal stabilities, losing 5% of their weight on heating to 300 °C. The optical property investigations showed that the band-gap energy of P3OTIOT was similar to that of P3OT (2.43 eV) at 2.45 eV and 6% lower than that of P3IOT (2.6 eV) in CHCl 3 solution. In PL spectra, the emission maxima of P3OTIOT and P3IOT were 50 nm and 130 nm blue-shifted with respect to that of P3OT, respectively. However, the PL intensity of P3OTIOT was seven times higher than that of P3OT. Single layer polymer light-emitting devices (PLEDs) with the ITO/polymer/Ag configurations were fabricated by the spin-coating method with P3OT, P3IOT and P3OTIOT as the EL materials, which exhibited red (650 nm), orange-red (610 nm) and yellow-green (525 nm) EL, respectively. The external EL quantum efficiencies (QE) of P3IOT and P3OTIOT devices are 6.4 × 10 −3% and 5.1 × 10 −3% which are about five and four times higher than that of the P3OT device (1.2 × 10 −3%), respectively. The turn-on voltage of the P3OTIOT device (5 V) is between that of the P3OT (4.5 V) and P3IOT (6 V) devices. These results indicated that the P3OTIOT combined the photoelectric properties of P3OT and P3IOT with excellent solubility, processability, low band-gap energy, high QE and low turn-on voltage in the PLEDs, and they might be excellent polymeric materials for applications in organic light-emitting diodes, light-emitting electrochemical cells and polymer solar cells.

Optical materials for organic light-emitting devices

The article provides a brief review of typical classes of optical materials for organic light-emitting-devices (LEDs) that includes fluorescent materials, phosphors, electron donor–acceptor and line-like emitting molecular systems. The emphasis is on low-molecular weight materials though the role of polymers is mentioned. A great deal of progress in improving efficiency and spectral properties of organic LEDs is demonstrated based on analyses of electronic processes in organic optical materials.

Syntheses and Properties of Novel Quarterphenylene-based Materials for Blue Organic Light-emitting Devices

Abstract A series of quarterphenylene-based compounds were prepared and investigated as the hole-transport layer and the host materials in organic light-emitting devices (OLEDs). These compounds have wide HOMO–LUMO energy gaps (ca. 3.57 eV) due to the twisted backbone. A maximum external efficiency of 17% was achieved for blue organic light-emitting device using iridium(III)–bis[2-(4,6-difluorophenyl)pyridinate-N,C2′]picolinate (FIrpic) as an emitting material.

Novel Electron-transport Material Containing Boron Atom with a High Triplet Excited Energy Level

Abstract Tris-[3-(3-pyridyl)mesityl]borane (3TPYMB) with high triplet excited energy level was synthesized and investigated as an electron-transport material for organic light-emitting devices (OLEDs). Larger current density and higher luminance were observed for the OLED using 3TPYMB, compared with the OLED with a conventional electron-transport material, tris(8-quinolinolato)aluminum (Alq3).

Solution-processable dendric triphenylamine nonamers as hole-transporting and hole-injection materials for organic light-emitting devices

We demonstrated two solution-processable triphenylamine dendric nonamers: the N-atom-centered nonamer (TPA9-1) and the phenyl-centered nonamer (TPA9-2). The materials were found to have high glass transition temperature ( T g) up to almost 200 °C. The fractional difference of the central units of the molecular structures caused different adaptability due to the different ionization potentials ( I p). TPA9-1 (N-atom-centered), whose I p was smaller than that of TPA9-2 (phenyl-centered), was suitable as hole-injection layer material, and TPA9-2 was suitable as hole-transporting layer material. Computational chemistry provides a ready explanation for the I p difference between the two materials.

True Blue: Blue-Emitting Aluminum(III) Quinolinolate Complexes

Blue-emitting heteroleptic aluminum(III) bis(2-methyl-8-quinolinolate)phenolate complexes were synthesized. A tunable, blue-to-green emission is achieved by attaching electron-withdrawing modulators to the emisssive quinaldinate ligand. The electronic nature of modulator substituents attached to the position of the highest HOMO (highest occupied molecular orbital) density is used to modulate ligand HOMO levels to achieve effective emission tuning to obtain blue-emitting materials. Optical and electrochemical properties of the resulting complexes were investigated and compared to the results of density functional theory (DFT/B3LYP/6-31G*) studies. The resulting materials may find application as organic light-emitting device materials.

Synthesis and fluorescence studies on novel complexes of Tb(III) and Eu(III) with 4-(9 H-carbazol-9-yl) benzoic acid

The complexes of Tb(III), Eu(III) with 4-(9 H-carbazol-9-yl) benzoic acid ( p-CZBA) have been synthesized, and the compositions and structural characteristics of them have been performed by the element analysis, infrared spectra, thermal analysis, 1H NMR and fluorescence spectra. Their compositions are Tb( p-CZB) 3(H 2O) 2 and Eu( p-CZB) 3(H 2O) 2, respectively. Photoluminescence measurements indicated that p-CZBA itself is a blue emitter under UV radiation. When the effective energy transfer from the organic ligand to the central Tb 3+ ions occurs in the binary complex, strong and characteristic green emission appears due to the 5D 4 → 7F j transitions of the 4f electrons of the central Tb 3+ ions. The Tb(III) binary complex is an excellent green-light-emitter, and would be considered as a candidate material for organic light-emitting devices (OLEDs). Eu( p-CZB) 3(H 2O) 2 hardly has fluorescence property, this is because the energy of the triplet state of the ligand is not matchable with the energy of the lowest excited state of Eu 3+.

AFM studies on formation of new phase responsible for enhanced photoluminescence in light-emitting small molecular thin films

Tris(8-hydroxyquinoline) metal complex (AlQ 3) is a widely used light-emitting material in organic light-emitting devices (OLEDs). The environmental stability is still a major problem with OLEDs and needs further improvements. In this report, we examine the optical properties of the thin films of this material and the effect of environment on it. The annealing in different ambients were done with the aim to understand the physical phenomenon involved in AlQ 3 thin films. An enhanced photoluminescence intensity is observed for the samples annealed in oxygen near 100 °C. Sudden change in roughness which may be as a consequence of change in surface morphology due to phase change and fraction of new phase is estimated by phase images taken by Atomic Force Microscopy (AFM). The enhanced photoluminescence is understood in terms of formation of a new phase.

5.3: New Triazole Derivatives as Hole-Blocking and Electron-Transporting Materials for Organic Light-Emitting Devices

We have demonstrated bipyridyl substituted triazole derivatives (Bpy-TAZs) as an electron-transporting material for organic light-emitting devices (OLEDs). Hybridization of bipyridyl to triazole is a good way to improve electron-transporting ability of triazoles with keeping good hole-blocking ability, which is a useful property of triazole derivatives. By employing a new Bpy-TAZ (Bpy-TAZ-03) as a hole-blocking and electron-transporting material for phosphorescent OLEDs, lower operation voltage was achieved with keeping the same external quantum efficiency of electroluminescence (almost 10%) as compared with the conventional hole-blocking and electron-transporting bilayer consisting of bathocuproine and tris (8-hydroxyquinolinato) aluminum.

Bipyridyl oxadiazoles as efficient and durable electron-transporting and hole-blocking molecular materials

We have demonstrated new and efficient electron transporting (ET) and hole blocking (HB) materials for organic light-emitting devices (OLEDs). The bipyridyl-substituted oxadiazole derivatives can form a stable glassy state with glass transition temperatures greater than 100 °C. The bipyridyl-substituted oxadiazole derivatives have excellent ET, HB and electron accepting ability and also exhibit electrical operation durability. Bipyridyl-substituted oxadiazoles are a promising candidate for ET and HB materials for OLEDs.

Investigation of a Novel Light-blue-emitting Fluorescent Guest/Host System with Excellent Lifetime

AbstractIn this work, organic light emitting devices (OLEDs) based on a blue-emitting fluorescent guest/host-system from Merck OLED Materials GmbH is investigated. OLEDs comprising a hole transport layer (HTL), the emissive film Merck Blue Host:Merck Blue Guest (MBH:MBG), a hole-blocking film and the electron transport layer (ETL) were prepared by vacuum thermal evaporation. The hole-blocking capabilities of aluminum(III)bis(2-methyl-8-quinolinato)4-phenylphenolate (BAlq) and the host material MBH were investigated. By employing an additional HBL, the current efficiency could be increased from 5.7 to 7.4 cd/A. Furthermore, 2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane (F4-TCNQ) doping of the HTL was investigated. Devices with 4,7-diphenyl-1,10-phenanthroline (BPhen) or 1,3,5-Tris-(N-phenylbenzimidazol-2-yl)benzene (TPBI) as alternative ETLs were fabricated and conclusions were drawn regarding the charge balance in the devices. It was found that employing tris-(8-hydroxyquinoline) aluminum (Alq3) as ETL leads to the best lifetimes of about 2000 hours at a constant current of 20 mA/cm2 while p-doping in combination with BPhen as ETL leads to the highest efficiency of 5.7 lm/W max. and 4.4 lm/W at 1000 cd/m2.

A High Tg Carbazole-Based Hole-Transporting Material for Organic Light-Emitting Devices

A new branched carbazole derivative, 1,3,5-tris(2-(9-ethylcarbazyl-3)ethylene)benzene (TECEB), was prepared as a hole-transporting material for organic light-emitting devices (OLEDs). TECEB is comparable to 1,4-bis(1-naphthylphenylamino)biphenyl (NPB) in terms of highest-occupied molecular orbital/lowest-unoccupied molecular orbial energy levels, hole-drift mobility, as well as device performance (maximum luminence of about 10 000 cd m-2 and current efficiency of 3.27 cd A-1) in a standard hole-transporting layer/tris-(8-hydroxyquinoline)aluminum double-layer device, but is superior to NPB in terms of its higher glass-transition temperature (Tg, 130 °C) and ease of synthesis. The latter features suggest that TECEB can be a potential alternative material to NPB especially for high-temperature applications in OLEDs and other organic electronic devices.

Novel Starburst Molecule as a Hole Injecting and Transporting Material for Organic Light-Emitting Devices

We report the synthesis of a novel starburst molecule, 4,4‘,4‘ ‘-tris(N-3-methylphenyl-N-(9-ethylcarbazyl-3)amino) triphenylamine (PCATA), and its application in organic light-emitting devices (OLEDs). The introduction of PCATA into the standard NPB/Alq3 OLED as the hole injecting and transporting layer dramatically enhanced the device efficiency to 5.7 cd/A and 2.2 lm/W, which are a factor of 2 higher than those of the standard OLED without the PCATA layer. The performance enhancement is attributed to a better balance of hole and electron injection in the PCATA-added OLED.

A novel electron-transport material for organic light-emitting devices

Journal of the Society for Information DisplayVolume 8, Issue 1 p. 1-1 Introduction Andras I. Lakatos, Andras I. Lakatos EditorSearch for more papers by this author Andras I. Lakatos, Andras I. Lakatos EditorSearch for more papers by this author First published: 18 June 2012 https://doi.org/10.1889/1.1828693Citations: 12AboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL No abstract is available for this article.Citing Literature Volume8, Issue1March 2000Pages 1-1 RelatedInformation

55.2: Bipyridyl Oxadiazoles as a New Class of Durable and Efficient Electron-Transporting Materials

We demonstrate a new class of high-performance electron transporting (ET) molecular materials for organic light-emitting devices (OLEDs.) The bipyridyl-substituted oxadiazole derivatives potentially have excellent ET and electron-accepting ability and also exhibit good thermal stability. We demonstrate for the first time that pure organic compounds, composed of only carbon, hydrogen, nitrogen, and oxygen, can provide good ET properties with practical operational durability in OLEDs.

Combinatorial Fabrication and Screening of Organic Light-Emitting Device Arrays

AbstractStudies of combinatorial fabrication and screening of organic light-emitting devices (OLEDs) are reviewed. These studies include screening of luminescent materials, electron and hole transport layers, lower-gap emitting guest dopants in small molecular emitters, and electronically doped polymeric anodes. The review focuses on screening of 2-dimensional (2-d) small molecular UV/violet arrays, 1-d blue-to-red arrays, and 1-d intense white OLED libraries, and briefly describes arrays fabricated to study Förster energy transfer in guest-host OLEDs. It demonstrates that combinatorial fabrication of OLEDs has become a powerful tool for screening OLED materials and configurations, and for studying their basic properties.

Blue organic electroluminescent device with tetra(β-naphthyl)silane as hole blocking materials

A new emitting material, tetra(β-naphthyl)silane (TNS), was synthesized and used as hole blocking material in organic light-emitting devices (OLEDs). Devices with a configuration of ITO/α-NPB/TNS/Alq3/Al have been constructed, where ITO is indium–tin-oxide, α-NPB is (4,4′-bis[N-(1-naphthyl)-N-phenylamino] biphenyl), and Alq3 is (8-quinolinolato) aluminum. A high performance blue light OLED was obtained, which emits blue light with a peak at 439 nm and starts to emit light at 6 V and the luminance can reach to 3800 cd/m2 at 13 V and 250 mA/cm2.

Tris-(8-hydroxyquinoline)aluminum-modified indium tin oxide for enhancing the efficiency and reliability of organic light-emitting devices

Tris-(8-hydroxyquinoline)aluminum (Alq3), which is typically used as an electron transport material for organic light-emitting devices (OLEDs), was used in this study for OLED anode modification. The electronic structure at the indium tin oxide (ITO)/organic interface for improvement of carrier injection was studied using ultraviolet photoelectron spectroscopy. The interfacial analysis reveals that the barrier height at the ITO/organic interface can be varied from ∼0.6–1.08eV. It is demonstrated that the barrier for hole injection from an ITO anode to a hole transporting layer can be engineered by inserting an ultrathin interlayer of Alq3, a few nanometers thick. The presence of an Alq3 interlayer is shown to improve the current balance, leading to an enhancement in the electroluminecent efficiency and operational stability of OLEDs.

Electronic properties of silole-based organic semiconductors

We report on a detailed quantum-chemical study of the geometric structure and electronic properties of 2,5-bis(6(')-(2('),2(")-bipyridyl))-1,1-dimethyl-3,4-diphenylsilole (PyPySPyPy) and 2,5-di- (3-biphenyl)-1,1-dimethyl-3,4-diphenylsilole (PPSPP). These molecular systems are attractive candidates for application as electron-transport materials in organic light-emitting devices. Density Functional Theory (DFT), time-dependent DFT, and correlated semiempirical (ZINDO/CIS) calculations are carried out in order to evaluate parameters determining electron-transport and optical characteristics. Experimental data show that PyPySPyPy possesses an electron-transport mobility that is significantly greater than PPSPP, while PPSPP has a significantly larger photoluminescence quantum yield; however, the theoretical results indicate that the two systems undergo similar geometric transformations upon reduction and have comparable molecular orbital structures and energies. This suggests that intermolecular interactions (solid-state packing, electronic coupling) play significant roles in the contrasting performance of these two molecular systems.

Structural Design of a Passive Matrix (PM) Color Organic Light-Emitting Device

We use a variable angle incidence spectroscopic ellipsometer (VASE) to characterize several materials for organic light-emitting devices (OLEDs): indium tin oxide (ITO), N, N′-bis-(1-naphthyl)-N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine (NPB), copper phthalocyanine (CuPc) and poly (3,4-ethylenedioxythiophene) (PEDOT). We consider that CuPc is not a good buffer layer to full-color OLEDs. We use PEDOT to replace CuPc as a buffer layer, and we use the software Wvase32 to analyze the effects of absorption and interference on the external quantum efficiency (ηe) in a PM color OLED by using side-by-side red (R), green (G), blue (B) OLEDs. The best structural parameters of three subpixels for realizing a PM color display are obtained.