The effect of multilayer quantum well structure on the characteristics of OLEDs

Organic light-emitting diodes (OLEDs) with multilayer quantum well (MQW) structure were fabricated, which consisted of alternate organic materials 2-(4-biphenylyl)-5(4-tert-butyl-phenyl)-1,3,4-oxadiazole (PBD) and tris(8-hydroxyquinoline) aluminum (Alq₃). PBD is used as potential barrier layer, Alq₃ used as potential well layer and emitting layer. Compared with double-layer structure, the luminescent characteristics of the devices with MQW structure were investigated. MQW structures conduce to energy transfer between wells and barriers, which is attributed to good overlap and the decrease of the distance between layers. The MQW structures make electrons and holes distribute in different wells and then increase the number of the formation of excitons to further enhance their recombination efficiency. Hence, such device achieves the maximum brightness and efficiency of 3630 cd/m² and 3.28 cd/A, respectively.     

Improvement of brightness and efficiency in organic light-emitting diodes using 1,3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl) phenylene as the hole buffer layer

We have reported the efficient Poly (2-methoxy, 5-(2-ethylhexoxy)-1,4-phenylene vinylene) (MEHPPV) organic light-emitting diodes based on 3-bis(4-tert-butylphenyl-1,3,4-oxadiazoyl) phenylene (OXD-7) and ITO bilayer anodes. OXD-7 was inserted between ITO anode and MEHPPV and acted as a hole buffer layer. The results demonstrated that the efficiency and brightness of the devices with ITO/OXD-7/MEHPPV/LiF/Al were four factors higher than that of devices with ITO/MEHPPV/LiF/Al. However, the efficiency and brightness of devices with ITO/OXD-7/MEHPPV/Al were worse than that of devices with ITO/MEHPPV/Al. Our results suggested that the better performance of devices should have not only the balance of hole and electron injection, but also higher hole and electron density in the emissive layer. OXD-7, as an electron-transporting material, was demonstrated to be a good candidate for hole injection buffer layer.     

Organic light-emitting diodes with a nanostructured TiO2 layer at the interface between ITO and NPB layers

Organic light-emitting diodes (OLEDs) with a nanostructured TiO₂ layer at the interface between indium tin oxide and a-naphtylphenyliphenyl diamine layers were fabricated using a vacuum evaporation method. The nanostructured TiO₂ layer was achieved by the Sol–Gel method. Compared to the different thickness of the buffer layer, the OLEDs with the 6 nm buffer layer showed the highest efficiency. The enhancements in efficiency result from an improved balance of hole and electron injections and a more homogeneous adhesion of the buffer layer inserted.     

Effect of PEDOT:PSS vs. MoO3 as the hole injection layer on performance of C545T-based green electroluminescent light-emitting diodes

In order to understand the effect of hole injection on the performance of organic light-emitting diodes (OLEDs) with electron- and hole-type hosts used in the emissive layer, we fabricated OLEDs based on a green fluorescent 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H-(1)benzopyropyrano(6,7-8-I,j)quinolizin-11-one (C545T) with poly(3,4-ethylenedioxythiophene):poly(styrenesulfonate) (PEDOT:PSS) and MoO₃ as the hole injection layer, respectively. Here, N,N′-di(naphthalene-1-yl)-N,N′-diphenyl-benzidine (NPB) and tris-(8-hydroxyquinoline) aluminum (Alq3) are, respectively, used as the host in the emissive layer for comparison. It is clearly found that different hole injection layers play different roles in the adjustment of the electron/hole injection and the transport balance, thus the different hosts are needed in the emissive layer for high electroluminescence efficiency OLEDs. This means that the selection of appropriate hole injection layers for OLEDs according to the different hosts in the emissive layer is especially important in the fabrication of high efficiency single emissive layer OLEDs.     

Low operating-voltage and high power-efficiency OLED employing MoO3-doped CuPc as hole injection layer

Effects of doping molybdenum oxide (MoO₃) in copper phthalocyanine (CuPc) as hole injection layer in OLEDs are studied. A green OLED with structure of ITO/MoO₃-doped CuPc/NPB/10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H, 11H-(1)-benzopyropyrano(6,7,8-i,j) quinolizin-11-one (C545T): tris(8-hydroxyquinoline) aluminum (Alq₃)/Alq₃/LiF/Al shows the driving voltage of 4.4 V, and power efficiency of 4.3 lm/W at luminance of 100 cd/m². The charge transfer complex between CuPc and MoO₃ plays a decisive role in improving the performance of OLEDs. The AFM characterization shows that the doped film owns a better smooth surface, which is also in good agreement with the electrical performance of OLEDs.     

Efficient organic light-emitting diodes with C60 buffer layer

Organic light-emitting diodes (OLEDs) with C₆₀ buffer layer were fabricated. The effect of C₆₀ buffer layer on the performance of the devices was investigated by inserting C₆₀ buffer layer at the interface between the electrode and organic layers. The device structures were (1) ITO/C₆₀ (0.0, 0.4, 0.7 and 1.0 nm)/NPB/Alq₃/LiF/Al and (2) ITO/NPB/Alq₃/C₆₀ (0.0, 0.4, 0.7 and 1.0 nm)/LiF/Al. The highest brightness and efficiency of the device (1) with 0.7 nm-thick C₆₀ layer reached 6439 cd/m² at 16 V and 1.80 cd/A at 6.4 V, respectively. The enhancements in brightness and efficiency are attributed to an improved balance of hole and electron injections due to C₆₀ layer blocking parts of the injected holes. On the contrary, the brightness and efficiency of the devices with the structure (2) had been hardly enhanced.     

Metal (IV) tetras (8‐hydroxyquinoline) (M = Zr,Hf) used as electroluminescent material and electron‐transport layer in OLEDs

Abstract— In this paper, we report on the utilization of zirconium (IV) tetras (8‐hydroxyquinoline), Zrq₄, and hafnium (IV) tetras (8‐hydroxyquinoline), Hfq₄, as an electroluminescent material in fluorescent organic light‐emitting diodes (OLED) and as electron transport layer (ETL) for high‐efficiency electrophosphorescent organic light‐emitting diodes (PHOLEDs). Structural studies show that the metal tetraquinolates (Mq₄) have a very low dipole moment (<0.1 D), in contrast to Alq₃ which has an estimated dipole moment of 4.7 D. Mobility measurements show that Mq₄ complexes give mobilities of (3.5 ± 0.5) × 10^?6 cm²/V‐sec, which are close to the values reported for Alq₃, i.e., (2.3–4.3) × 10^?6 cm²/V‐sec. OLEDs were prepared with the structure ITO/NPD (400 Å)/Mq_n (500 Å)/LiF/Al (NPD = 4‐4′‐bis[N‐(1‐naphthyl)‐N‐phenyl‐amino]bi phenyl, Mq_n = Alq₃, Zrq₄, Hfq₄. The Mq₄‐based OLEDs gave external efficiencies of 1.1%, while the Alq₃‐based devices of the same structure gave efficiencies of 0.7%. PHOLEDs have been fabricated with the structure ITO/NPD (500 Å)/CBP‐8% Ir(ppy)³ (250 Å)/BCP (150 Å)/Mq_n (250 Å)/LiF/Al (CBP = N,N′‐dicarbazolyl‐4‐4′‐biphenyl, Ir(ppy)³ = fac‐tris(2‐phenylpyrridine)iridium, BCP = bathocruprione). PHOLEDs with Mq₄ ETLs showed a greatly improved efficiency, when compared to Alq₃‐based PHOLEDs. The Zrq₄‐based PHOLEDs gave a peak external quantum efficiency of 14% at 0.3 mA/cm² (150 cd/m²), while the Hfq₄ based PHOLED gave a peak external quantum efficiency of 15% at 0.6 mA/cm² (300 cd/m²). Comparable PHOLEDs with an Alq₃ ETL give peak external quantum efficiencies of 8.0% at 0.5 mA/cm². The devices gave an electroluminescence (EL) spectrum consisting only of fac‐tris(2‐phenylpyrridine)iridium (Ir(ppy)³) dopant emission (CIE coordinates of 0.26, 0.66), with no Mq₄ emission observed at any bias level.     

Efficiency enhancement of solution‐processed single‐layer blue‐phosphorescence organic light‐emitting devices having co‐host materials of polymer (PVK) and small‐molecule (SimCP2)

Abstract— A new type of single‐layer blue‐phosphorescence organic light‐emitting devices (OLEDs) containing poly(9‐vinylcarbazole) (PVK) and small‐molecule‐based amorphous ambipolar bis(3,5‐di(^9H‐carbazol‐9‐yl)phenyl) diphenylsilane (SimCP2) as the co‐host material have been demonstrated. All active materials [PVK, SimCP2, Flrpic (blue‐phosphorescence dopant), and OXD‐7 (electron transport)] were mixed in a single layer for solution processing in the fabrication of OLEDs. The SimCP2 small‐molecule host has adequate high electron and hole‐carrier mobiltieis of ～10^?4 cm²/V‐sec and a sufficiently large triplet state energy of ～2.70 eV in confining emission energy on FIrpic. Based on such an architecture for single‐layer devices, a maximum external quantum efficiency of 6.2%, luminous efficiency of 15.8 cd/A, luminous power efficiency of 11 lm/W, and Commision Internale de l'Eclairage (CIE^x,y) coordinates of (0.14,0.32) were achieved. Compared with those having PVK as the single‐host material, the improvement in the device performance is attributed to the balance of hole and electron mobilities of the co‐host material, efficient triplet‐state energy confinement on FIrpic, and the high homogeneity of the thin‐film active layer. Flexible blue‐phosphorescence OLEDs based on solution‐processed SimCP2 host material (withou PVK) have been demonstrated as well.     

A design for improving the sensitivity of a Mach–Zehnder interferometer to chemical and biological measurands

Integrated optical Mach–Zehnder interferometers (MZIs) composed of graded-index channel waveguides are often used as chemical/biological sensors. Such MZIs have a relatively low sensitivity because the graded-index active arms have a weak evanescent field. To improve the sensitivity, a channel-planar composite optical waveguide (COWG) is proposed as a substitute for the graded-index active arm. An actual channel-planar COWG was fabricated by sputtering a tapered TiO₂ film onto a straight glass channel waveguide prepared by the potassium ion exchange method. Measurement of the evanescent absorption of the dye solution demonstrated a significantly enhanced evanescent field over the TiO₂ film region caused by adiabatic transition of the guided mode between the channel waveguide and TiO₂ film. Theoretical calculations show that the sensitivity of a glass-based MZI can increase 71 times when the COWG active arm contains a 25 nm thick and 5 mm long tapered TiO₂ film. The use of a COWG as the active arm of a glass-based MZI also allows for elimination of the low-index buffer layer because of a large difference in evanescent field between the COWG and channel waveguide.     

Electrical and morphological characterization of platinum thin-films with various adhesion layers for high temperature applications

In this paper we report on the morphological and electrical properties of platinum (Pt) thin-films with Titanium (Ti) and, alternatively, Titanium dioxide (TiO₂) as adhesion layers for high temperature applications. All films were sputter deposited on silicon substrates and afterwards annealed in air up to 800 °C. The results show that Ti diffuses into Pt grain boundaries forming oxide precipitates (TiO_x) in the Pt grain boundaries. The resistivity of Pt/Ti thin-films increased continuously with annealing temperature up to 500 °C and decreases again continuously above 500 °C. In contrast, TiO₂ demonstrates a dense stable oxide layer after annealing. Pt/TiO₂ thin-films show a continuous decrease in the sheet resistance with increasing the annealing temperature. Accordingly, TiO₂ thin-film is the preferable adhesive layer for Pt over Ti thin-films for high temperature applications.

     

High‐efficiency p‐i‐n organic light‐emitting diodes with long lifetime

Abstract— High‐performance organic light‐emitting diodes (OLEDs) are promoting future applications of solid‐state lighting and flat‐panel displays. We demonstrate here that the performance demands for OLEDs are met by the PIN (p‐doped hole‐transport layer/intrinsically conductive emission layer/n‐doped electron‐transport layer) approach. This approach enables high current efficiency, low driving voltage, as well as long OLED lifetimes. Data on very‐high‐efficiency diodes (power efficiencies exceeding 70 lm/W) incorporating a double‐emission layer, comprised of two bipolar layers doped with tris(phenylpyridine)iridium [Ir(ppy)₃], into the PIN architecture are shown. Lifetimes of more than 220,000 hours at a brightness of 150 cd/m² are reported for a red PIN diode. The PIN approach further allows the integration of highly efficient top‐emitting diodes on a wide range of substrates. This is an important factor, especially for display applications where the compatibility of PIN OLEDs with various kinds of substrates is a key advantage. The PIN concept is very compatible with different backplanes, including passive‐matrix substrates as well as active‐matrix substrates on low‐temperature polysilicon (LTPS) or, in particular, amorphous silicon (a‐Si).     

α-MoO3/TiO2 core/shell nanorods: Controlled-synthesis and low-temperature gas sensing properties

Crystalline α-MoO₃/TiO₂ core/shell nanorods are fabricated by a hydrothermal method and subsequent annealing processes under H₂/Ar flow and in the ambient atmosphere. The shell layer is composed of crystalline TiO₂ particles with a diameter of 2-6 nm, and its thickness can be easily controlled in the range of 15-45 nm. The core/shell nanorods show enhanced sensing properties to ethanol vapor compared to bare α-MoO₃ nanorods. The sensing mechanism is different from that of other one-dimensional metal oxide core/shell nanostructures due to very weak response of TiO₂ nanoparticles to ethanol. The enhanced sensing properties can be explained by the change of type II heterojunction barrier formed at the interface between α-MoO₃ and TiO₂ in the different gas atmosphere. The present results demonstrate a novel sensing mechanism available for gas sensors with high performance.     

Organic light‐emitting diodes with enhanced out‐coupling efficiency using high‐refractive‐index glass frit

We have fabricated a novel type of substrate for organic light‐emitting diodes (OLEDs) to improve the light out‐coupling efficiency. It was fabricated by forming an excellent flat layer using a high‐refractive‐index B₂O₃‐SiO₂‐Bi₂O₃ frit glass on the light diffusive glass substrate. By using this substrate, we have sufficiently reduced the total internal reflection of OLEDs, and we successfully obtained more than 1.9 times higher light out‐coupling efficiency without spectral changes and viewing angle dependency. Furthermore, we have also successfully demonstrated 50 × 50 mm large‐area white OLEDs with this novel substrate.     

Improved performance of organic light‐emitting devices with ultra‐thin hole‐blocking layers

Abstract— Tris‐(8‐hydroxyqunoline) aluminum (Alq₃)‐based organic light‐emitting devices (OLEDs) using different thickness of 2,9‐Dimethyl‐4,7‐diphenyl‐1,110‐phenanthorline (BCP) as a hole‐blocking layer inserted both in the electron‐ and hole‐transport layers have been fabricated. The devices have a configuration of indium tin oxide (ITO)/m‐MTDATA (80 nm)/BCP (X nm)/NPB (20 nm)/Alq₃ (40 nm)/BCP (X nm)/Alq₃ (60 nm)/Mg: Ag (200 nm), where m‐MTDATA is 4, 4′, 4″‐Tris(N‐3‐methylphenyl‐N‐phenyl‐amino) triphenylamine, which is used to improve hole injection and NPB is N,N′‐Di(naphth‐2‐yl)‐N,N′‐diphenyl‐benzidine. X varies between 0 and 2 nm. For a device with an optimal thickness of 1‐nm BCP, the current and power efficiencies were significantly improved by 47% and 43%, respectively, compared to that of a standard device without a BCP layer. The improved efficiencies are due to a good balance between the electron and hole injection, exciton formation, and confinement within the luminescent region. Based on the optimal device mentioned above, the NPB layer thickness influences the properties of the OLEDs.     

Alpha-1-fetoprotein antibody functionalized Au nanoparticles: Catalytic labels for the electrochemical detection of α-1-fetoprotein based on TiO2 nanoparticles synthesized with ionic liquid

A novel strategy for the preparation of amperometric immunosensor for rapid determination of α-1-fetoprotein (AFP) in human serum has been developed. TiO₂ nanoparticles (NPs) were prepared by solvothermal reaction using TiCl₄ as raw materials and the mixture of ionic liquids and doubly distilled water as solvent. α-1-fetoprotein antibody (AFP Ab) was mixed with TiO₂ NPs/chitsotan (CHIT) solution and immobilized onto the surface of a glassy carbon electrode. AFP (Ab) functionalized Au NPs were used as catalytic labels for the amperometric detection of AFP by means of the electrocatalyzed reduction of Au NPs to H₂O₂. The electrochemical behavior of the immunosensor was studied. Other experimental conditions such as pH, immunoreactions temperature and time were also studied. The prepared immunosensor offers an excellent amperometric response for AFP ranging from 1.0 to 160.0 ng/mL with a detection limit of 0.1 ng/mL. The result shows that the immunosensor displays rapid response, high sensitivity, good reproducibility and favorable stability.     

Efficiency enhancement in blue organic light emitting diodes with a composite hole transport layer based on poly(ethylenedioxythiophene):poly(styrenesulfonate) doped with TiO2 nanoparticles

Blue color organic/polymeric light emitting diodes are very important because they can be used for tri-color display applications, fluorescence imaging, and exciting yellow phosphor for generating white light for general illumination. But the efficiency of blue organic/polymeric light emitting diodes is considerably low due to their large band gap that requires higher energy for effective emission. In this paper we report the enhancement in polyfluorene blue organic light emitting diodes with a polymer nano-composite hole transport layer. Blue light emitting diode based on polyfluorene as an emissive layer and poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate)–titanium dioxide nanocomposite as the hole transport layer were fabricated and studied. Different concentrations of titanium dioxide nanoparticles were doped in poly(3,4 ethylenedioxythiophene):poly(styrenesulfonate) in the hole transport layer and the performance of the devices were studied. Significant enhancement in the blue peak at 430 nm of polyfluorene has been observed with increase in concentration of TiO₂ nanoparticles in the hole transport layer. The turn on voltage of the device has also been found to improve significantly with the incorporation of titanium dioxide nanoparticles in the hole transport layer. The optimized concentration of titanium dioxide in the hole transport layer for most efficient device has been found to 15 wt.%.     

Conformational studies by molecular mechanics and molecular orbital methods on the antiamoebic drug 1-(4-imidazolylsulfonyl)-4-phenylimidazole

Conformational attributes of the antiamoebic drug 1-(4-imidazolylsulfonyl)-4-phenylimidazole have been studied by molecular mechanics and M.O. methods. It is found that an electronic transition is possible from the HOMO to the LUMO, thus making the SO₂ functional group more available for an electrophylic attack. No definite indication is found that conformational factors may play a role in determining the antiamoebic activity of this compound.     

Solution-processed high-performance organic light-emitting diodes containing a green-emitting multiresonant thermally activated delayed fluorescent dendrimer

A multi-resonance thermally activated delayed fluorescence (MR-TADF) dendrimer emitter and a related reference MR-TADF compound were designed, synthesized, and characterized for use as narrowband emitters in solution-processed OLEDs. The 1 wt% doped films in PMMA film revealed that the compounds MR-D1 and MR-D2 showed narrowband green emission at λ_PL of 490 and 495 nm and with FWHM of 23 and 29 nm, respectively. The 50 wt% doped films in mCP still show narrowband green emission at λ_PL of 495 and 499 nm and with FWHM of 28 nm for MR-D1 and MR-D2 , respectively, while conserving the small ΔE_ST of 0.14 and 0.13 eV, respectively. OLEDs containing an emissive layer consisting of 50 wt% MR-D1 and MR-D2 in mCP showed high EQE_max of 27.7% and 21.0%, respectively, and low efficiency roll-off of 19% and 30% at a luminance of 2000 cd/m⁻².     

Thermodynamic properties of sodium pyrovanadate (Na4V2O7) at high temperature (298.15–873 K)

The sodium pyrovanadate (Na₄V₂O₇) powder was synthesized by solid-state reaction using sodium carbonate (Na₂CO₃) and vanadium pentoxide (V₂O₅) as raw materials. X-ray powder diffraction (XRD), scanning electron microscope (SEM), and differential scanning calorimeter (DSC) were used to accurately characterize the synthesized sample. The solid-state phase transformation from α-Na₄V₂O₇ to β-Na₄V₂O₇ occurs at the temperature 696 K and the enthalpy is equals to 1.03 ± 0.01 kJ/mol, the endothermic effect at 931 K and the enthalpy is equals to 31.35 ± 0.31 kJ/mol, which is related to the melting of Na₄V₂O₇. The high-temperature heat capacity of Na₄V₂O₇ was measured using a Multi-high temperature calorimeter 96 line and DSC. The obtained high-temperature heat capacity of Na₄V₂O₇, as a function of temperature, was modeled as: Cp=314.62+0.05T-5494390T^-2 J·mol^-1·K^-1 (298.15-873 K). The temperature dependence on heat capacity was then used for computing changes in the enthalpy, entropy, and Gibbs free energy at the specific temperature internal.     

Electrical response characterization of PVA–P(AA/AMPS) IPN hydrogels in aqueous Na2SO4 solution

Interpenetrating polymer networks (IPN) hydrogels composed of poly(vinyl acohol) (PVA) and poly(acrylic acid-co-2-acrylamido-2-methyl propyl sulfonic acid) (P(AA/AMPS)) were synthesized by solution polymerization. The IPN hydrogels were characterized using Fourier-transform infrared (FT-IR) and X-ray diffraction (XRD). The results indicated that strong hydrogen bond was formed between PVA and P(AA/AMPS), and the crystallinity of PVA in IPN hydrogels was declined significantly. The swelling/deswelling properties of IPN hydrogel in aqueous Na₂SO₄ solution were studied. When a sheet of water swollen IPN hydrogel (all the samples were swollen in deionized water) was placed in aqueous Na₂SO₄ solution, the IPN hydrogel shrunk. However, if a voltage was applied, the IPN hydrogel shrunk at high concentration of Na₂SO₄ solution, but swelled at low concentration. The results show that the critical concentration of Na₂SO₄ solution is about 0.005 mol/L. The stimuli response of the IPN hydrogel in electric field was also investigated. When water swollen samples were placed between a pair of electrodes in aqueous Na₂SO₄ solution, the IPN hydrogel showed significant bending behavior upon the application of an electric field. The bending direction of the IPN hydrogel depends on the concentration of Na₂SO₄ solution. At high concentration the IPN hydrogel bended toward anode, contrarily, at low concentration the IPN hydrogel bended toward cathode. The critical concentration of Na₂SO₄ solution is also about 0.005 mol/L. Further investigation showed that the gel component, concentration of aqueous Na₂SO₄ solution and the applied voltage influenced the bending speed of IPN hydrogel.