Matthias Marescaux

Inhomogeneous luminance in organic light emitting diodes related to electrode resistivity

In organic light emitting diodes (OLEDs) with transparent electrodes, the luminance usually becomes inhomogeneous if the size of the pixel increases above 10mm. A theoretical model for inhomogeneous voltage and luminance in OLEDs is provided together with an approximate analytical solution for the problem in case of cylindrical symmetry. Experimental observations of inhomogeneous luminance are compared with numerical simulations based on the theoretical model, proving the applicability of the approximations made in the theoretical model.

Conductor grid optimization for luminance loss reduction in organic light emitting diodes

The recently increased efficiency of organic light emitting devices (OLED) brings lighting applications within reach. If the area of the OLED is of the order a cm2, voltage and brightness losses related to the square resistance of the transparent electrode become important. The homogeneity of the voltage and brightness can be improved by contacting the transparent electrode from all edges and by adding a metallic grid to the transparent electrode. This grid should have narrow lines to minimize transmission losses and improve the total light emission from the OLED. The voltage losses in grids with different shapes (triangular, square, and hexagonal) are evaluated and the grid parameters are optimized to maximize the total light emission. It turns out that a hexagonal grid has lower loss than a square grid with the same line width.

Diffuse double layer charging in nonpolar liquids

The presence of charges in nonpolar liquids is usually neglected. However, in some applications, they have important effects. In this work, an analytical description is given for the dynamics of diffuse double layers when a voltage step of the order of kT∕e is applied. Part of the result is an expression for the current, which is consistent with current measurements on nonpolar liquids and with other works. In measurements for higher voltages, we observe and briefly discuss nonlinear effects. From measurements both at high and at low voltages, we can obtain the same properties of the liquid.

Charge transport and current in non-polar liquids

Surfactant molecules in a non-polar liquid form charged and uncharged inverse micelles. When a potential difference is applied over the mixture, the charged inverse micelles drift towards the electrode with the opposite polarity. The motion of charges is associated with a transient current, which can be measured in an external circuit. In this paper, transient currents and steady state charge densities are described analytically in different ranges of parameter values (applied voltage, charge density, device thickness, mobility,...). The generation of additional charged inverse micelles and the electrophoretic motion of colloidal particles in the mixture is modelled and measured experimentally.

Near-infrared spectroscopy of AGB star candidates in Fornax, Sculptor, and NGC 6822 ,

Context. The asymptotic giant branch (AGB) phase is characterised by substantial mass loss that is accompanied by the formation of dust. In extreme cases this will make the star no longer visible in the optical. For a better understanding of AGB evolution it is important to identify and characterise these very red AGB stars. Aims. The first aim of this article is to improve the census of red AGB stars in three Local Group galaxies, based on near-IR spectroscopic observations of new candidates with red 2MASS (J − K) colours. The opportunity is taken to compare the near-IR spectra with those of Milky Way stars. Methods. We used ISAAC on the ESO VLT to take J and H-band spectra of 36 targets in Fornax, Sculptor and NGC 6822. Results. Twelve new C-stars are found in Fornax, and one is confirmed in Sculptor. All C-stars have (J − K) > 1.6, and are brighter than −3.55 in bolometric magnitude. Ten new oxygen-rich late-type giant stars are identified in Fornax, but none is extremely red or very luminous. Five luminous O-rich AGB stars are identified in NGC 6822, of which 3 show water absorption, indicative of spectral type M. Again, none is as red as Milky Way OH/IR stars, but in this galaxy the list of candidate AGB stars is biased against very red objects. In some C-stars with (J − K) > 2 an extremely strong 1.53 μm absorption band is found. These stars are probably all Mira variables and the feature is related to the low temperature, high density chemistry that is a first step towards dust formation and mass loss.

Electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles

We have compared optical tracking and electric current measurements to study the electrokinetics of colloidal particles in nonpolar media containing charged inverse micelles. Particle trajectories are measured in response to a voltage step, revealing spatial and temporal variations of the electric field when space-charge layers are created by charged inverse micelles. The particle trajectories and current measurements are in good agreement with simulations and analytical approximations based on drift and diffusion of charges. Electrohydrodynamic effects observed at high concentrations of charged inverse micelles are explained by injection of charged inverse micelles from the bulk into the space-charge layers.

Micellization and adsorption of surfactant in a nonpolar liquid in micrometer scale geometries

Mixtures of nonpolar liquid and surfactant are used increasingly in applications with microscopic dimensions. However, most methods to characterize them are performed on bulk solutions. We measure electrical transient currents in thin layers of nonpolar liquid with surfactant and derive several properties from these measurements. This paper reports the results for different liquid layer thicknesses and surfactant concentrations. We observe a dependence on the layer thickness of the inverse micelle concentration, which cannot be explained by bulk micellization alone. A model including surface adsorption is proposed that describes surfactant behavior in microscale geometries.