Combinatorial fabrication and screening of organic light-emitting device arrays

The combinatorial fabrication and screening of 2-dimensional (2-d) small molecular UV-violet organic light-emitting device (OLED) arrays, 1-d blue-to-red arrays, 1-d intense white OLED libraries, 1-d arrays to study Förster energy transfer in guest-host OLEDs, and 2-d arrays to study exciplex emission from OLEDs is described. The results demonstrate the power of combinatorial approaches for screening OLED materials and configurations, and for studying their basic properties.     

Atomic fluorescence mapping of optical field intensity profiles issuing from nanostructured slits, milled into subwavelength metallic layers

In this work, we report on the fabrication and characteristics of light-emitting diodes based on p-GaN/i-ZnO/n-ZnO heterojunction. A 30 nm i-ZnO layer was grown on p-GaN by rf reactive magnetron sputtering, then a n-ZnO was deposited by the electron beam evaporation technique. The current-voltage characteristic of the obtained p-i-n heterojunction exhibited a diode-like rectifying behavior. Because the electrons from n-ZnO and the holes from p-GaN could be injected into a i-ZnO layer with a relatively low carrier concentration and mobility, the radiative recombination was mainly confined in i-ZnO region. As a result, an ultraviolet electro-emission at 3.21 eV, related to ZnO exciton recombination, was observed in a room-temperature electroluminescence spectrum of p-i-n heterojunction under forward bias.     

Electroluminescence and its excitation mechanism of SiOx films deposited by electron-beam evaporation

Blue electroluminescence from SiO_x films deposited by electron beam evaporation was observed. This blue emission blueshifted from 450 to 410 nm with increasing applied voltage. The dependences of blue emission on applied voltage, frequency and conduction current were studied. Our experimental data support that blue emission from SiO_x films is the result of both recombination of charge carriers injected from opposite electrodes and impact excitation of hot electrons, the recombination of carriers injected is dominant in low and medium electric fields but hot electron impact excitation is dominant under high electric fields.     

Influence of Dopant Concentration on Electroluminescent Performance of Organic White-Light-Emitting Device with Double-Emissive-Layered Structure

A novel phosphorescent organic white-light-emitting device （WOLED） with contiguration of ITO/NPB/CBP： TBPe：rubrene/Zn（BTZ）2：Ir（piq）2（acac）/Zn（BTZ）2/Mg：Ag is fabricated successfully, where the phosphorescent dye bis （1-（phenyl）isoquinoline） iridium （Ⅲ） acetylanetonate （Ir（piq）2 （acac）） doped into bis-（2-（2-hydroxyphenyl） benzothiazole）zinc （Zn（BTZ）2） （greenish-blue emitting material with electron transport character） as the red emitting layer, and fluorescent dye 2,5,8,11-tetra-tertbutylperylene （TBPe） and 5,6,11,12-tetraphenyl-naphthacene （rubrene） together doped into 4,4＇-N,N＇-dicarbazole-biphenyl （CBP） （ambipolar conductivity material） as the blue-orange emitting layer, respectively. The two emitting layers are sandwiched between the hole-transport layer N ,N＇-biphenyl-N , N＇-bis （1-naph thyl）-（1,1＇-biphenyl）-4, 4 Cdiamine （NP B） and electron-transport layer （Zn（BTZ）2 ） The optimum device turns on at the driving voltage of 4.5 V. A maximum external quantum efficiency of 1.53%. and brightness 15000 cd/m^2 are presented. The best point of the Commission Internationale de 1＇Eclairage （CIE） coordinates locates at （0.335, 0.338） at about 13 V. Moreover, we also discuss how to achieve the bright pure white light through optimizing the doping concentration of each dye from the viewpoint of energy transfer process.     

On the improvement of the electroluminescent signal of organic light emitting diodes by the presence of an ultra‐thin metal layer at the interface organic/ITO

The electroluminescent (EL) signal of organic light emitting diodes (OLEDs) based on simple “hole transporting layer/electron transporting layer” (HTL/ETL) structures has been studied as a function of the anode/HTL interface, the anode being an indium tin oxide (ITO) film. It is shown that the electroluminescent (EL) signal increases when a metal ultra‐thin layer is introduced between the anode and the HTL. Experimental results show that the work function value of the metal is only one of the factors which allow improving the EL signal via better hole injection efficiency. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Infrared electroluminescence from a Si MOS structure with Ge in the oxide

We report on room-temperature infrared electroluminescence (EL) from metal-oxide-semiconductor devices made from Si. We compare the luminescence from RF sputtered oxide films containing SiO₂ with and without Ge by using a composite target and luminescence from a SiO₂ layer made by rapid thermal oxidation. The sputtered films were annealed in the temperature range 600-900 °C. This densifies the films and is likely to reduce the concentration of defects. A luminescence peak located around 1150-1170 nm is observed at current densities as low as 0.1 A/cm². The corresponding photon energy is close to that of the Si band gap. In addition, we observe several broad luminescence bands in the range 1000-1750 nm. These bands get stronger with Ge in the SiO₂ film. Some of these bands have previously been suggested and are directly associated with Ge. Since we observe that the intensity is correlated with the presence of Ge while the mere presence of the bands is not, we discuss the EL bands being due to defects which concentration is influenced by Ge in the oxide.     

Efficient Solution-Processed Blue Electrophosphorescent Devices Based on a Novel Small-Molecule Host

Efficient blue small molecular phosphorescent fight-emitting diodes with a blue phosphorescent dye bis（3,5- difluoro-2-（2-pyridyl）-phenyl-（2-carboxypride） iridium （Ⅲ） （Flrpic） doped into a novel small-molecule host 9,9- bis[4-（3,6-di-tert-butylcarbazol-9-yl）phenyl] fluorene （TBCPF） as the light-emitting layer have been fabricated by spin-coating. The host TBCPF can form homogeneous amorphous films by spin-coating and has triplet energy higher than that of the blue phosphorescent dye Flrpic. All the devices with different Flrpic concentration in the emitting layer give emission from Flrpic indicating complete energy transfer from TBCPF to Flrpic. The device shows the best performance with a peak brightness of 8050 cd/m^2 at 10.2 V and the maximum current efficiency up to 3.52 cd/A, when the Flrpic doped concentration is as high as 16%.     

Electroluminescence from Multilayered Diamond/CeF3/SiO2 Films

We report a thin film electroluminescent device with a three-layer structure （diamond/CeF3/SiO2 films）, which has a luminance of 1.5 cd/m^2 at dc voltage 215 V. The electroluminescence spectrum at room temperature shows that the main peaks locate at 527 and 593nm, which are attributed to isolated emission centers of Ce^3＋ ions.     

Optimized Performances of Thick Film Organic Lighting-Emitting Diodes

The performance of organic light-emitting diodes (OLEDs) with thick film is optimized. The alternative vanadium oxide (V₂O₅) and N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) layers are used to enhance holes in the emissive region, and 4,7-dipheny-1,10-phenanthroline (Bphen) doped 8-tris-hydroxyquinoline aluminium (Alq₃) is used to enhance electrons in the emissive region, thus ITO/V₂O₅ (8nm)/NPB (52nm)/V₂O₅ (8nm)/NPB (52nm)/Alq₃ (30 and 45nm)/Alq₃:Bphen (30wt%, 30 and 45nm)/LiF (1nm)/Al (120nm) devices are fabricated. The thick-film devices show the turn-on voltage of about 3V and the maximal power efficiency of 4.5lm/W, which is 1.46 times higher than the conventional thin-film OLEDs.     

Effect of Precursor Molar Ratio of [S^2-]/[Zn^2＋] on Particle Size and Photoluminescence of ZnS：Mn^2＋ Nanocrystals

We investigate the influence of precursor molar ratio of [S^2-]/[Zn^2＋] on particle size and photoluminescence （PL） of ZnS：Mn^2＋ nanocrystMs. By changing the [S^2-]/[Zn^2＋] ratio from 0.6 （Zn-rich） to 2.0 （S-rich）, the particle size increases from nearly 2. 7nm to about 4.Ohm. The increase in the ratio of [S^2-]/[Zn^2＋] cadses a decrease of PL emission intensity of ZnS host while a distinct increase of Mn^2＋ emission. The maximum intensity for the luminescence of Mn^2＋ emission is observed at the ratio of [S^2-]/[Zn^2＋] ≈ 1.5. The possible mechanism for the results is discussed by filling of S^2- vacancies and the increase of Mn^2＋ ions incorporated into ZnS lattices.     

Effect of laser-etched indium tin oxide on optoelectrical properties of organic light-emitting diodes

This study investigated how laser and wet etching methods of ITO substrates affect the optoelectrical properties of OLEDs. Experimental results indicated that the OLED with a laser-etched ITO substrate has a lower driving voltage than that with a wet-etched ITO substrate. According to scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements, the ITO etching methods yielded different surface morphologies of ITO pattern edges. The lower luminance of the OLED with a laser-etched ITO substrate is attributable to the fringe at the edge of ITO pattern, which causes a high local electric field resulting in the recrystallization of organic materials.     

Improved electroluminescence performance of ZnS:Cu,Cl phosphors by ultrasonic treatment

ZnS:Cu,Cl electroluminescence (EL) phosphors were prepared by high-temperature (1150 °C) solid-state reaction, subsequent ultrasonic treatment (t=0-60 min) and final low-temperature annealing process at 750 °C. The as-synthesized phosphors were characterized by X-ray powder diffraction (XRD), UV-vis absorbance spectra, electron probe microanalyzer (EPMA) and photoluminescence (PL) spectra. EL performance was investigated on an EL lamp fabricated by screen-printing at 100 V and 400 Hz. Ultrasound irradiation leads to intensity reductions and width increases of some XRD diffraction peaks, and results in a slight red-shift of UV-vis absorption edge. It also exhibits strong influences on PL and EL properties of the phosphors. Generally, PL performance monotonically declines with the increase of ultrasonic time, while EL performance benefits from the ultrasonic treatment and is superior to that of the commercial ones. The defects in the microstructure induced by the ultrasonic treatment are the fundamental reason for the change of PL and EL performances.     

Crystallographic and luminescent characterizations of blue-emitting BaAl2S4 : Eu electroluminescent thin films

We characterized the crystallization and luminescence of blue-emitting BaAl₂S₄ : Eu electroluminescent thin films, prepared using switching electron-beam evaporation with two targets. From the photoluminescence intensity and decay profile of the activated Eu²⁺ ions in the BaAl₂S₄ : Eu, we found that the optimum annealing conditions for preparing highly luminescent thin films are a temperature of around 900°C and an annealing time of 2 min. We analyzed the crystalline properties using cross-sectional transmission electron microscope images. Evaluation of the cathodoluminescence spectra in the cross-sections showed that the BaAl₂S₄ : Eu emitting layer was luminously inhomogeneous on the depth of the layer and that the main luminescent area was near the surface of the emitting layer. We discuss here the relationship between the crystalline and luminescent properties.     

Synthesis and optical properties of tetragonal KTa0.6Nb0.4O3 nanoparticles

Tetragonal phase KTa_0.6Nb_0.4O₃ (KTN) nanoparticles have been prepared by hydrothermal method. The obtained particles were characterized by X-ray powder diffraction, transmission electron microscopy, energy dispersive X-ray spectroscopy and UV-vis absorption spectrum techniques. A systematic change in crystal structure from cubic to tetragonal is observed with the increase of reaction temperature and KOH concentration. Room temperature UV-vis absorption spectrums of KTN particles show that the band gap changes from 3.24 to 3.34 eV with grain size diminished, which reveals the existence of blue-shift phenomenon of absorption bands.     

Annealing effect on soft magnetic properties and magnetoimpedance of Finemet Fe73.5Si13.5B9Nb3Au1 alloy

Effect of annealing on the soft magnetic properties of Fe_73.5Si_13.5B₉Nb₃Au₁ amorphous ribbon has been investigated by means of structure examination, magnetoimpedance ratio (MIR) and incremental permeability ratio (PR) spectra measured in the frequency range of 1–10 MHz at a fixed current of 10 mA X-ray diffraction analysis showed that the as-cast sample was amorphous and it became nanocrystalline under a proper heat treatment. When annealing amorphous alloy at 530 °C for 30, 60, 90 min, soft magnetic properties have been improved drastically. Among the samples investigated, the sample annealed at 530 °C for 90 min showed the softest magnetic behavior. The MIR and PR curves revealed the desirable changes in anisotropy field depending upon annealing.     

Near-Infrared Emission from Organic Light-Emitting Diodes Based on Copper Phthalocyanine with a Periodically Arranged Alq3：CuPc/DCM Multilayer Structure

We demonstrate near-infrared organic light-emitting devices with a periodically arranged tris（8-quinolinolato）aluminum （Alq3）：copper phthalocyanine （CuPc）/4-（dicyanomethylene）-2-methyl-6-（4-dimethylaminost-yry）-4H-pyran （DCM） multilayer structure. DCM and Alq3 doped with CuPc were periodically deposited. Room-temperature electrophosphorescence was observed at about 1.1 μm due to transitions from the first excited triplet state to the singlet ground state （T1 - S0） of CuPc. In this device, we utilize the overlap between the Q band πr - π^＊ at about 625nm of the absorption spectra of CuPc and the PL spectra of the DCM. The near-infrared emission intensity of the CuPc-doped Alq3 device with DCM increases about 2.5 times larger than that of the device without DCM. We attribute the efficiency enhancement to the better overlap between the PL spectra of DCM and the absorption spectra of CuPc.     

Effect of OH on the upconversion luminescent efficiency of Y2O3:Yb, Er nanostructures

In this work, we used the hydrothermal method to synthesize Yb³⁺ and Er³⁺ ions doped cubic Y₂O₃ nanostructures, which is an upconversion luminescent material. Three distinct shapes such as nanotubes, nanospheres and nanoflakes formed in the products by adjusting the pH value of reacting solution. Powder X-ray diffraction analyses indicate that all the three nanostructures were pure cubic phase, while electron microscopy measurements confirm the formation of different morphologies. These nanostructures exhibit strong visible upconversion luminescence under the excitation of a 978-nm diode laser. The FTIR and fluorescent decay measurements at the size and morphology of sample changed from tubes to flakes indicate that the OH⁻ group is the origin of luminescent efficiency change. OH⁻ ions with high vibration frequency provide an efficient means to quench the luminescence. However, the relative intensity and pump-power dependence of the green and red emissions varies with the three nanostructures possessing different size and morphology.     

Dependence of band structures on stacking and field in layered graphene

Novel systems of layered graphene are attracting interest for theories and applications. The stability, band structures of few-layer graphite films, and their dependence on electric field applied along the c-axis are examined within the density functional theory. We predict that those of Bernal type and also rhombohedral type tri- and tetra-layer graphite films exhibit stability. Rhombohedral-type systems, including AB-bilayer, show variable band gap induced by perpendicular electric fields, whereas the other systems, such as the Bernal-type films, stay semi-metallic.     

Silicon–germanium nanostructures for on-chip optical interconnects

Silicon–germanium epitaxially grown on silicon in the form of two-dimensional (quantum wells) and three-dimensional (quantum dots) nanostructures exhibits photoluminescence and electroluminescence in the technologically important spectral range of 1.3–1.6 μm. Until recently, the major roadblocks for practical applications of these devices were strong thermal quenching of the luminescence quantum efficiency, and a long carrier radiative lifetime. This paper summarizes recent progress in the understanding of carrier recombination in Si/SiGe nanostructures and presents a potential new route toward CMOS compatible light emitters for on-chip optical interconnects.     

Pulsed Nd:YAG laser depositions of ITO and DLC films for OLED applications

Indium-tin oxide (ITO) films deposited on heated and non-heated glass substrates by a pulsed Nd:YAG laser at 355 nm and ∼2.5 J/cm² were used in the fabrication of simple organic light-emitting diodes (OLEDs), ITO/(PVK + Alq₃ + TPD)/Al. The ITO was deposited on heated glass substrates which possessed resistivity as low as ∼3 × 10⁻⁴ Ω cm, optical transmission as high as ∼92% and carrier concentration of about ∼5 × 10²⁰ cm⁻³, were comparable to the commercial ITO. Substrate heating transformed the ITO microstructure from amorphous to polycrystalline, as revealed by the XRD spectrum. While the polycrystalline ITO produced higher OLED brightness, it was still lower than that on the commercial ITO due to surface roughness. A DLC layer of ∼1.5 nm deposited on this ITO at laser fluence of >12.5 J/cm² improved its device brightness by suppressing the surface roughness effect.