Concentration quenching effect of organic light‐emitting devices using DCM1‐doped tetraphenylgermole

We examine the concentration quenching of a 4‐(dicyanomethylene)2‐methyl‐6‐(p‐dimethylaminostyryl)‐4H‐pyran (DCM1)‐doped 1,1‐bis(2‐phenylethynyl)‐2,3,4,5‐tetraphenylgermole (HPAG)‐based light‐emitting diode. Originally, HPAG emits in the ~500‐nm (green) region, which can be converted to a red‐emission material by using DCM1 doping. As the DCM1 concentration increased from 1 to 10 wt%, the electroluminescence peak positions are red‐shifted from 604 to 644 nm, respectively. Increasing doping density not only shows the red‐shift but also shows decreasing luminance efficiency. Förster energy transfer between the HPAG host material and the DCM1 guest material is responsible for the strong red‐emission behavior. The calculated Förster radius (4.0 nm) for excellent Förster energy transfer characteristics with increasing doping concentration of DCM1 is consistent with experimental results. The maximum luminance efficiency was 6.64 cd/A at 11.0 mA/cm². The HPAG germole compound shows excellent red‐emission host–guest system properties for red organic light‐emitting device applications. Copyright © 2011 John Wiley & Sons, Ltd.     

Silsesquioxane-based nanocomposite dendrimers with photo-luminescent and charge transport properties

The synthesis and characterization of octavinylsilsesquioxane (OVS)-based nanocomposite dendrimers with luminescent and charge transport properties are reported. The nanocomposite dendrimers were prepared in high yield using mild Heck chemistry of mono-haloaromatic compounds with the peripheral vinylsilane groups of OVS. Attachment of 2-naphthalene, 2-(9,9-dimethyl)fluorene, and 2-(4-phenyl)-5-(1-naphthyl)-1,3,4-oxadiazole resulted in materials with blue-violet emission (360-380 nm) and photo-luminescent quantum efficiencies (PLQEs) from 1 to 26%. Blue-green emission was observed for attachment of 1-pyrene, 9-anthracene, and N1-(4-phenyl)-N1,N4,N4-triphenylbenzene-1,4-diamine with PLQEs ranging from 23 to 50%. Despite the planar characteristics of the organic dendrons, the nanocomposite dendrimers are completely amorphous and have high glass transition temperatures (Tg) ranging from 115 to 186 degrees C with decomposition temperatures (Td) exceeding 450 degrees C. Matrix-assisted laser desorption ionization-time of flight shows that unlike traditional Heck chemistry, haloaromatic compounds are adding twice across the vinylsilane groups. Finally, organic light emitting diodes using the aromatic amine-based dendrimer as hole injection layers show 55% improvement in device efficiency over traditional materials (5.16 vs. 3.24 cd A(-1)) with brightness levels exceeding 40,000 cd m(-2).     

基于新型连接层的有机叠层电致发光器件

制备了一种基于新的电荷生成层m-MTDATA∶MoO3的叠层有机电致发光器件。叠层器件与单发光层器件相比,发光亮度和电流效率均有成倍的提高。叠层器件的最大电流效率达到了30.06 cd/A,最大亮度为83 210 cd/m2,分别约为普通器件的2倍。除此之外,叠层器件在整个电流密度范围内的电流效率都很稳定。结果表明:m-MTDATA∶MoO3可以作为高效率的叠层有机电致发光器件的电荷生成层。     

Effect of change in pulsed DC frequency on indium-tin oxide anode on J-V-L performance of built-up organic light emitting diode during facing-target sputtering

Investigations were carried out on the changes in the electrical and optical properties and surface roughness of the indium-tin oxide (ITO) anode as a function of DC pulse frequency during facing-target sputtering. The current density-voltage-luminescence (J-V-L) characteristics of organic light emitting diodes (OLEDs) developed on the anodes were measured and analyzed in relation to the properties of ITO. When the pulsed DC frequency was less than 120 kHz, the resistivity of ITO was maintained well below 4.3 × 10⁻⁴ Ω cm and the optical energy band gap was greater than 4.1 eV, but these properties changed abruptly at 150 kHz with the morphological transition from columnar to equi-axed. Meanwhile, the surface roughness decreased continuously with increasing pulsed DC frequency up to 150 kHz. The J-V characteristics of the built-up OLED deteriorated slightly as the pulsed DC frequency increased to 120 kHz and then deteriorated rapidly at 150 kHz. The L-V curves, however, showed an improvement of luminescence as the frequency increased up to 120 kHz. These J-V-L characteristics imply that ITO which is more conductive and with a higher band gap can be obtained at the lower pulsed DC frequencies, which is desirable for higher current flow; however, better luminescence is closely related to smoother surfaces. Therefore, for the optimized J-V-L performance of OLEDs, a moderate pulse DC frequency, below the morphological transition of ITO, is desirable.     

Synthesis of poly(N‐9‐ethylcarbazole‐exo‐norbornene‐5,6‐dicarboximide) for hole‐transporting layer in hybrid organic light‐emitting devices

We synthesized new polynorbornene dicarboximide (PCaNI) functionalized with hole‐transporting carbazole moieties and its copolymer (PCaNA) by ring‐opening metathesis polymerization (ROMP), where the PCaNA was further reacted with 3‐amino‐triethoxysilane to prepare PCaNI/silica hybrid. We also investigated the feasibility of PCaNI and PCaNI/silica hybrid (PCaSi) as a hole‐transporting material for hybrid organic light emitting devices (HOLEDs). To improve the performance of the PCaNI‐based HOLEDs, N,N′‐diphenyl‐N,N′‐(3‐methylphenyl)‐[1,1′‐biphenyl]‐4‐4′‐diamine (TPD) was also introduced into the PCaNI matrix. Results showed that PCaNI exhibited high glass transition temperature (～260 °C) and high optical transparency in the visible region. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) of PCaNI were measured as 5.6 and 2.2 eV, while the TPD‐doped PCaNI showed 5.7 eV (HOMO) and 2.6 eV (LUMO). The PCaNI/silica hybrid nanolayers showed excellent solvent resistance due to the formation of covalent bonds between ITO and PCaNI. The HOLEDs with PCaNI/TPD or PCaSi/TPD hybrid nanolayers exhibited relatively higher luminance (～10,000 cd/m²), lower operating voltage (～6.5 V at 300 cd/m²), and higher current efficiency (～2.7 cd/A). © 2010 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem, 2010     

电场对单层有机电致发光二极管复合发光的影响

以电流连续性方程为基础，用易测量的注入电流密度来确定载流子浓度的边界值，得出载流子浓度和电流密度的解析表达式。计算并讨论了电子密度和电子电流密度在器件中的分布，电场对它们的影响以及电场与势垒对复合效率的影响。该模型较好地解释了有关实验现象。     

UV–VIS and mid-IR ellipsometer characterization of layers used in OLED devices

We report on the analysis of layer materials commonly used for the fabrication of organic light-emitting diodes (OLEDs) by means of spectroscopic ellipsometry operating in the ultraviolet–visible (UV–VIS) and in the mid-infrared (mid-IR) range of the electromagnetic spectrum. On covering a wide spectral range, these non-destructive, non-contact techniques offer a huge potential in characterizing thin films in terms of layer thickness values, optical indices, absorption properties modified by incorporating optically active dopant molecules, and, finally, electrical layer properties. Individual absorption bands of single organic films can significantly affect the luminance and performance efficiency of OLEDs when building multi-layer stacks. Using mid-IR spectroscopic ellipsometry, the resistance of as-deposited and annealed indium–tin oxide (ITO) layers on glass is determined and compared with values obtained by four-point probe techniques.     

Monocyclometalated Gold(III) Monoaryl Complexes—A New Class of Triplet Phosphors with Highly Tunable and Efficient Emission Properties

Highly tunable and rich phosphorescent emission properties based on the stable monocyclometalated gold(III) monoaryl structural motif are reported. Monochloro complexes of the type cis‐[(N^C)Au(C₆H₂(CF₃)₃)(Cl)] N^C=2‐phenylpyridine (ppy)] ( 1 ), [N^C=benzo[h]quinoline (bzq)] ( 2 ), [N^C=2‐(5‐Methyl‐2‐thienyl)pyridine (5m‐thpy)] ( 3 ) were successfully prepared in modest to good yields by reacting an excess of 2, 4, 6‐tris(trifluoromethyl)phenyl lithium (LiFmes) with the corresponding dichloride complexes cis‐[(N^C)AuCl₂]. Subsequent replacement of the chloride ligand in 1 with strong ligand field strength such as cyanide and terminal alkynes resulted in complexes of the type cis‐[(ppy)Au(Fmes)(R)] R=CN ( 4 ), I ( 5 ), C?C?C₆H₅ ( 6 ) and C?C?C₆H₄N(C₆H₅)‐p ( 7 ). Single crystal X‐ray diffraction studies of all the complexes except 7 were performed to further corroborate their chemical identity. Thermogravimetric analysis (TGA) studies of the uncommon cis configured aryl alkyne complex 7 confirmed the high stability of this complex. Detailed photophysical investigations carried out in solution at room temperature, at 77 K (2‐MeTHF) in rigidified media, solid state and 5 wt % PMMA revealed the phosphorescent nature of emission in these complexes. Additionally, their behavior was found to be governed based on both the nature of the cyclometalated ligand and the electronic properties of the ancillary ligands. Highly efficient interligand charge transfer in complex 7 provides access to a wide range of emission colors (solvent‐dependent) from deep blue to red with phosphorescence emission quantum yield of 30 % (441 nm) and 39 % (622 nm) in solution and solid state, respectively, and is the highest reported for any Au^III complexes. DFT and TDDFT calculations carried out further validated the observations and assignments based on the photophysical experimental findings.     

Theoretical Study of Xanthenone and Phenothiazine Derivatives for Blue TADF Emitter

Two novel thermally activated delayed fluorescence (TADF) materials (PTZ-XTN and 2PTZ-XTN), with phenothiazine (PTZ) as an electron donor and xanthenone derivatives (XTN) as electron acceptors, were designed and theoretically investigated as blue OLED emitters. We used density functional theory (DFT) and time dependent DFT (TD-DFT) calculations to determine the electron distribution of HOMO and LUMO and the energy of the lowest singlet (S₁) and the lowest triplet (T₁) excited states. The large dihedral angle between the electron donor and the electron acceptor imparted a small spatial overlap between HOMO and LUMO in all the materials. This charge separation of the HOMO and LUMO leads to a small energy gap between the S₁ state and T₁ state, thereby leading to TADF emission. Among the materials studied, PTZ-XTN has the most suitable properties for a blue TADF OLED emitter, even though 2PTZ-XTN has the smallest energy gap between the S₁ and T₁ states.     

Heat revolution on photophysical properties and electroluminescent performance of Ir(ppy)3-doped bipolar host of oxadiazole derivatives attaching with inert group of tert-butyl moiety

Two bipolar materials,2,5-bis(2-(9H-carbazole-9-yl)phenyl)-1,3,4-oxadiazole(o-CzOXD)and 2,5-bis(2-(3',6'-di-tert-butyl-9H-carbazole-9-yl)phenyl)-1,3,4-oxadiazole(tBu-o-CzOXD),were synthesized according to reported methods.In parallel study,it was demonstrated that introduction of inert tert-butyl group improved material thermal stability,even though this modification only had a slight influence to the photophysical and electrochemical properties of these materials.A comparative study focusing on effects of heat treatment was carried out on the quartz glass substrates with vacuum deposited films containing one of the bipolar host doped with 6 wt%fac-tris(2-phenylpyridinato-N,C2’)iridium(Ir(ppy)3).Results show that when the two samples were heated,the absorption,emission,and photo images of the host:dopant system changed,with the o-CzOXD suffering more severe degradation under high temperature,which is consistent with their thermal stability.In addition,it was proved that the high temperature-annealed host:dopant system can enhance the emission of the dopant.This finding was used as a guideline to improve our device performance.We fabricated two types of phosphorescent organic light-emitting devices(PhOLEDs),one was based on o-CzOXD,the other was based on tBu-o-CzOXD.They had analogous structure.We investigated the effect of heat on device performance by selectively annealing.Although these two freshly prepared devices exhibited similar performance,when annealed at 90°C for 10 min,the OLEDs based on tBu-o-CzOXD showed significant performance enhancement,which can be attributed to the observation that annealing Ir(ppy)3 doped host can change film morphology and enhance the dopant emission.The maximum efficiencies of the freshly prepared tBu-o-CzOXD device were 25.8 cd A-1,23.1lm W-1,and 9.3%;whereas those for annealed device were 47.0 cd A-1,42.2 lm W-1,and 13.4%.