Charge injection in an LED with a hybrid composite as the emissive layer

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)

Photo and electroluminescence behavior of Tb(ACAC)(3)phen complex used as emissive layer on organic light emitting diodes

This work shows the luminescence properties of a rare-earth organic complex, the Tb(ACAC)(3)phen. The results show the (5)D(4)->(7)F(3,4,5,6) transitions with no influence of the ligand. The photoluminescence excitation spectrum is tentatively interpreted by the ligands absorption. An organic light emitting diode (CLED) was made by thermal evaporation using TPD (N,N`-bis(3-methylphenyl)N,N`-diphenylbenzidine) and Alq3 (aluminum-tris(8-hydroxyquinoline)) as hole and electron transport layers, respectively. The emission reproduces the photoluminescence spectrum of the terbium complex at room temperature, with Commission Internationale de l`Eclairage - CIE (x,y) color coordinates of (0.28,0.55). No presence of any bands from the ligands was observed. The potential use of this compound in efficient devices is discussed. (C) 2008 Elsevier B.V. All rights reserved.

Doppler lidar measurements of oriented planar ice crystals falling from supercooled and glaciated layer clouds

The properties of planar ice crystals settling horizontally have been investigated using a vertically pointing Doppler lidar. Strong specular reflections were observed from their oriented basal facets, identified by comparison with a second lidar pointing 4° from zenith. Analysis of 17 months of continuous high-resolution observations reveals that these pristine crystals are frequently observed in ice falling from mid-level mixed-phase layer clouds (85% of the time for layers at −15 °C). Detailed analysis of a case study indicates that the crystals are nucleated and grow rapidly within the supercooled layer, then fall out, forming well-defined layers of specular reflection. From the lidar alone the fraction of oriented crystals cannot be quantified, but polarimetric radar measurements confirmed that a substantial fraction of the crystal population was well oriented. As the crystals fall into subsaturated air, specular reflection is observed to switch off as the crystal faces become rounded and lose their faceted structure. Specular reflection in ice falling from supercooled layers colder than −22 °C was also observed, but this was much less pronounced than at warmer temperatures: we suggest that in cold clouds it is the small droplets in the distribution that freeze into plates and produce specular reflection, whilst larger droplets freeze into complex polycrystals. The lidar Doppler measurements show that typical fall speeds for the oriented crystals are ≈ 0.3 m s−1, with a weak temperature correlation; the corresponding Reynolds number is Re ∼ 10, in agreement with light-pillar measurements. Coincident Doppler radar observations show no correlation between the specular enhancement and the eddy dissipation rate, indicating that turbulence does not control crystal orientation in these clouds. Copyright © 2010 Royal Meteorological Society

Effect of ion concentration of ionomer in electron injection layer of polymer light-emitting devices

Polymer light-emitting devices (PLEDs) with poly(2-methoxy-5-hexyloxy)-p-phenylenevinylene (OC1OC6-PPV) as the emissive layer were studied with an electron injection layer of ionomers consisting of copolymers of styrene and methylmethacrylate (PS/PMMA) with 3, 6 and 8 mol% degree of sulfonation. The ionomers were able to form very thin films over the emissive layer, with less than 30 nm. Additionally, the presence of ion pairs of ionomer suppresses the tendency toward dewetting of the thin film of ionomer (similar to 10 nm) which can cause malfunction of the device. The effect of the ionomers was investigated as a function of the ion content. The devices performance, characterized by their current density and luminance intensity versus voltage, showed a remarkable increase with the ionomer layer up to 6 mol% of ionic groups, decreasing after that for the 8 mol% ionomer device. The study of the impedance spectroscopy in the frequency range from 0.1 to 10(6) Hz showed that the injection phenomena dominate over the transport in the electroluminescent polymer bulk. (c) 2006 Elsevier B.V. All rights reserved.

Polymer light emitting devices with Langmuir-Blodgett (LB) films: Enhanced performance due to an electron-injecting layer of ionomers

Polymer light-emitting devices (PLEDs) have been produced with Langmuir-Blodgett (LB) films from poly(2-methoxy-5-hexyloxy)-p-phenylenevinylene (OC1OC6-PPV) as the emissive layer and an ionomer of a copolymer of styrene and methylmethacrylate (PS/PMMA) as an electron-injection layer. The main features of such devices are the low operating voltages, obtainable firstly due to the good quality of the ultrathin LB films that allows PLEDs to be produced reproducibly and secondly due to the improved electrical and luminance properties brought by the electron-injection layer. Also demonstrated is the superior performance of an all-LB device compared to another one produced with cast films of the same materials. Published by Elsevier B.V.

Modeling of organic light-emitting diodes with graded concentration in the emissive multilayer

We model the electrical behavior of organic light-emitting diodes whose emissive multilayer is formed by blends of an electron transporting material, tris-(8-hydroxyquinoline) aluminum (Alq(3)) and a hole transporting material, N,N-'-diphenyl-N,N-'-bis(1,1(')-biphenyl)-4,4-diamine. The multilayer is composed of layers of different concentration. The Alq(3) concentration gradually decreases from the cathode to the anode. We demonstrate that these graded devices have higher efficiency and operate at lower applied voltages than devices whose emissive layer is made of nominally homogeneous blends. Our results show an important advantage of graded devices, namely, the low values of the recombination rate distribution near the cathode and the anode, so that electrode quenching is expected to be significantly suppressed in these devices. (C) 2004 American Institute of Physics.

A method for estimating the turbulent kinetic energy dissipation rate from a vertically pointing Doppler lidar, and independent evaluation from balloon-borne in situ measurements

A method of estimating dissipation rates from a vertically pointing Doppler lidar with high temporal and spatial resolution has been evaluated by comparison with independent measurements derived from a balloon-borne sonic anemometer. This method utilizes the variance of the mean Doppler velocity from a number of sequential samples and requires an estimate of the horizontal wind speed. The noise contribution to the variance can be estimated from the observed signal-to-noise ratio and removed where appropriate. The relative size of the noise variance to the observed variance provides a measure of the confidence in the retrieval. Comparison with in situ dissipation rates derived from the balloon-borne sonic anemometer reveal that this particular Doppler lidar is capable of retrieving dissipation rates over a range of at least three orders of magnitude. This method is most suitable for retrieval of dissipation rates within the convective well-mixed boundary layer where the scales of motion that the Doppler lidar probes remain well within the inertial subrange. Caution must be applied when estimating dissipation rates in more quiescent conditions. For the particular Doppler lidar described here, the selection of suitably short integration times will permit this method to be applicable in such situations but at the expense of accuracy in the Doppler velocity estimates. The two case studies presented here suggest that, with profiles every 4 s, reliable estimates of ϵ can be derived to within at least an order of magnitude throughout almost all of the lowest 2 km and, in the convective boundary layer, to within 50%. Increasing the integration time for individual profiles to 30 s can improve the accuracy substantially but potentially confines retrievals to within the convective boundary layer. Therefore, optimization of certain instrument parameters may be required for specific implementations.

Impact of anthropogenic heat emissions on London's temperatures

We investigate the role of the anthropogenic heat flux on the urban heat island of London. To do this, the time-varying anthropogenic heat flux is added to an urban surface-energy balance parametrization, the Met Office–Reading Urban Surface Exchange Scheme (MORUSES), implemented in a 1 km resolution version of the UK Met Office Unified Model. The anthropogenic heat flux is derived from energy-demand data for London and is specified on the model's 1 km grid; it includes variations on diurnal and seasonal time-scales. We contrast a spring case with a winter case, to illustrate the effects of the larger anthropogenic heat flux in winter and the different roles played by thermodynamics in the different seasons. The surface-energy balance channels the anthropogenic heat into heating the urban surface, which warms slowly because of the large heat capacity of the urban surface. About one third of this additional warming goes into increasing the outgoing long-wave radiation and only about two thirds goes into increasing the sensible heat flux that warms the atmosphere. The anthropogenic heat flux has a larger effect on screen-level temperatures in the winter case, partly because the anthropogenic flux is larger then and partly because the boundary layer is shallower in winter. For the specific winter case studied here, the anthropogenic heat flux maintains a well-mixed boundary layer through the whole night over London, whereas the surrounding rural boundary layer becomes strongly stably stratified. This finding is likely to have important implications for air quality in winter. On the whole, inclusion of the anthropogenic heat flux improves the comparison between model simulations and measurements of screen-level temperature slightly and indicates that the anthropogenic heat flux is beginning to be an important factor in the London urban heat island.

Heat and moisture budgets from airborne measurements and high-resolution model simulations

High-resolution simulations with a mesoscale model are performed to estimate heat and moisture budgets of a well-mixed boundary layer. The model budgets are validated against energy budgets obtained from airborne measurements over heterogeneous terrain in Western Germany. Time rate of change, vertical divergence, and horizontal advection for an atmospheric column of air are estimated. Results show that the time trend of specific humidity exhibits some deficiencies, while the potential temperature trend is matched accurately. Furthermore, the simulated turbulent surface fluxes of sensible and latent heat are comparable to the measured fluxes, leading to similar values of the vertical divergence. The analysis of different horizontal model resolutions exhibits improved surface fluxes with increased resolution, a fact attributed to a reduced aggregation effect. Scale-interaction effects could be identified: while time trends and advection are strongly influenced by mesoscale forcing, the turbulent surface fluxes are mainly controlled by microscale processes.

Orographic drag associated with lee waves trapped at an inversion

The drag produced by 2D orographic gravity waves trapped at a temperature inversion and waves propagating in the stably stratified layer existing above are explicitly calculated using linear theory, for a two-layer atmosphere with neutral static stability near the surface, mimicking a well-mixed boundary layer. For realistic values of the flow parameters, trapped lee wave drag, which is given by a closed analytical expression, is comparable to propagating wave drag, especially in moderately to strongly non-hydrostatic conditions. In resonant flow, both drag components substantially exceed the single-layer hydrostatic drag estimate used in most parametrization schemes. Both drag components are optimally amplified for a relatively low-level inversion and Froude numbers Fr ≈ 1. While propagating wave drag is maximized for approximately hydrostatic flow, trapped lee wave drag is maximized for l_2 a = O(1) (where l_2 is the Scorer parameter in the stable layer and a is the mountain width). This roughly happens when the horizontal scale of trapped lee waves matches that of the mountain slope. The drag behavior as a function of Fr for l_2 H = 0.5 (where H is the inversion height) and different values of l2a shows good agreement with numerical simulations. Regions of parameter space with high trapped lee wave drag correlate reasonably well with those where lee wave rotors were found to occur in previous nonlinear numerical simulations including frictional effects. This suggests that trapped lee wave drag, besides giving a relevant contribution to low-level drag exerted on the atmosphere, may also be useful to diagnose lee rotor formation.

Improving a convection-permitting model simulation of a cold air outbreak

A convection-permitting local-area model was used to simulate a cold air outbreak crossing from the Norwegian Sea into the Atlantic Ocean near Scotland. A control model run based on an operational configuration of the Met Office UKV high-resolution (1.5 km grid spacing) NWP model was compared to satellite, aircraft and radar data. While the control model captured the large-scale features of the synoptic situation, it was not able to reproduce the shallow (<1.5 km) stratiform layer to the north of the open cellular convection. Liquid water paths were found to be too low in both the stratiform and convective cloud regions. Sensitivity analyses including a modified boundary-layer diagnosis to generate a more well-mixed boundary layer and inhibition of ice formation to lower temperatures improved cloud morphology and comparisons with observational data. Copyright © 2013 Royal Meteorological Society and British Crown Copyright, the Met Office

Electrical properties of polymer/metal interface in polymer light-emitting devices: electron injection barrier suppression

The electrical characterization of a high efficient multilayer polymer light emitting diode using poly[(2-methoxy-5-hexyloxy)-p-phenylenevinylene] as the emissive layer and an anionic fluorinated surfactant as the electron transport layer was performed. For the sake of comparison, a conventional single layer device was fabricated. The density current vs. voltage measurements revealed that the conventional device has a higher threshold voltage and lower current compared to the surfactant modified device. The effective barrier height for electron injection was suppressed. The influence of the interfaces and bulk contributions to the dc and high frequencies conductivities of the devices was also discussed. (c) 2006 Springer Science + Business Media, Inc.

Síntese e caracterização de argilas pilarizadas com pilares mistos Al/Co

Two pillaring methods were tested to synthesize pillared clays containing mixed Al/Co pillars. Using the first method, based on the traditional procedure, were obtained materials containing different Co concentrations: 10, 25, 50, 75 and 100 % of Co in the pillaring solution. Just the experiments with low concentrations (10 and 25 % of Co) has formed pillared clays, whereas the sample with 25 % of cobalt showed best results compared with the one obtained just using Al as pillaring agent (basal spacing higher than 18 Å and surface area bigger than 300 m²/g). The 27Al NMR results pointed out the formation of mixed Al/Co pillars due to decreased between the intensities of AlVI/AlIV signals, indicating that the AlIV content decreased while Co content increased, suggesting the isomorphic substitution of Al atoms for Co in the Keggin ion structure (pillaring agent). For the samples containing 75 and 100 % of cobalt, it was verified the formation of others materials, which could be identified as hydrotalcite like compounds. The second pillarization method was named mixed layers, because the objective was to intercalate clay layers with hydrotalcite layers. Thus, after calcination, the hydrotalcite layers would dehydroxylate, resulting just in the metals oxides, intercalated between the clay sheets, thus generating, a pillared clay. For this purpose, were tested 4 synthesis procedures: physical mixture, mixture in water, ionic exchange under reflux and in situ synthesis. Of these, the method which showed the best results was the in situ synthesis, in which basal spacings of 14 Å (after calcination) were obtained, indicating that the samples are intercalated with metal oxides (Mg and Al). This procedure was reproduced with a Co-Al LDH (layered double hydroxide) and similar results were obtained, testifying the method reproducibility

Electroluminescence of phenylene-vinylene random copolymers with different conjugation lengths

PPV random derivates were synthesized and characterized. Polymer light emitting diodes (PLEDs) were assembled using the random copolymers as emissive layer and showed EL in the blue-green region in function of the method of preparation. The increase in the average conjugation degree in the polymer chain led to the reduction of the turn-on voltage of the device. The addition of Alq3 as ETL increased tenfold the luminescence efficiency. (C) 2009 Elsevier B.V. All rights reserved.