Marie-Noëlle Bussac

An optimisation method for separating and rebuilding one-dimensional dispersive waves from multi-point measurements. Application to elastic or viscoelastic bars

When using a classical SHPB (split Hopkinson pressure bar) set-up, the useful measuring time is limited by the length of the bars, so that the maximum strain which can be measured in material testing applications is also limited. In this paper, a new method with no time limits is presented for measuring the force and displacement at any station on a bar from strain or velocity measurements performed at various places on the bar. The method takes the wave dispersion into account, as must inevitably be done when making long time measurements. It can be applied to one-dimensional and single-mode waves of all kinds propagating through a medium (flexural waves in beams, acoustic waves in wave guides, etc.). With bars of usual sizes, the measuring time can be up to 50 times longer than the time available with classical methods. An analysis of the sensitivity of the results to the accuracy of the experimental data and to the quality of the wave propagation modelling was also carried out. Experimental results are given which show the efficiency of the method.

Numerical model for organic light-emitting diodes

An extensive numerical model recently developed for the multilayer organic light-emitting diode is described and applied to a set of real devices. The model contains a detailed description of electrical contacts including dipolar layer formation, thermionic and tunneling injection, space charge effects, field dependent mobilities and recombination processes. The model is applied to simulate several single layer devices and the family of bilayer devices made in our group. It provides insight into the energy level shifts, internal electric fields and charge distribution (and consequently recombination) throughout the device. Finally, the analysis is extended to the optimization of bilayer device.

Simulation of charge injection enhancements in organic light-emitting diodes

We have investigated by numerical simulation of real devices the reasons for electron injection enhancement due to lithium fluoride (LiF) and for hole injection enhancement due to copper phtalocyanine (CuPc) in organic light-emitting diodes (OLEDs). The reference data introduced in the code were obtained from Kelvin probe and charge transport measurements. In the case of LiF, the reduction of the injection barrier is mainly due to a static dipolar charge distribution across the ionic layer, while in CuPc the space charge which lowers the barrier results from a large hole accumulation at the CuPc/hole-transmitting layer interface, during injection.

Ultrathin organic transistors on oxide surfaces

We have built a model organic field-effect transistor that is basically composed of a single layer of pentacene crystal in interaction with an oxide surface. Drain and source contacts are ohmic so that the pentacene layer can carry a current density as high as 3000 A cm−2 at a gate voltage of –60 V. Four-probe and two-probe transport measurements as a function of temperature and fields are presented in relation with structural near-field observations. The experimental results suggest a simple two-dimensional model where the equilibrium between free and trapped carriers at the oxide interface determines the OFET characteristics and performance.

Identification de la relation de dispersion dans les barres

Dispersion and attenuation of longitudinal waves in elastic or weakly viscoelastic rods are measured by analysing the resonant frequencies present in the strain spectrum due to an unknown loading. The method takes the finite measuring time of the test into account. It is applied to an aluminium bar, in which the dispersion relation is identified very accurately at frequencies up to 60 kHz.

Image force effects at contacts in organic light-emitting diodes

The effect of an image force on the barrier for the injection of the carrier from the electrode into the organic layer is shown to be essential for proper understanding of the metal/organic contact and, in particular, of recent results of internal photoemission experiments for metal/Alq3 interfaces. While the calculation presented does not depend on the details of the carrier transport in the organic layer, the resulting value of the injection barrier is expected to determine the conduction regime of the device.

Numerical model for injection and transport in multilayers OLEDs

We present a recently developed numerical code for OLED simulation. This code contains a detailed description of contacts, charge transport and recombination. Its efficiency is briefly shown through examples of single- and bilayer devices and discussed in more detail through the investigation of the action of a LiF thin film on injection. In particular, we show how the code can help to discriminate between several possible explanations for this phenomenon.

Charge transport mechanisms in microcrystalline silicon

A heterogeneous charge transport model for microcrystalline silicon based on fluctuation-induced tunneling is presented that fits the low-temperature saturation observed in dark conductivity measurements and accounts for the film microstructure. Excellent agreement is found when the model is applied to data reported in the literature, particularly for highly crystalline samples, which produce the highest performance transistors. Values obtained for the three fitting parameters are consistent with typical measurements of microcrystalline silicon film morphology and the conduction band offset between amorphous and crystalline silicons.

Séparation et reconstruction des ondes dans les barres élastiques et viscoélastiques à partir de mesures redondantes

La duree du chargement habituellement utilisee avec le systeme des barres de Hopkinson (ou SHPB pour Split Hopkinson Pressure Bar) est limitee par la necessite de separer les ondes se propageant dans des sens opposes. En utilisant le principe du maximum de vraisemblance, et a partir de N mesures de deformation et P mesures de vitesse materielle (N+P⩾2), nous construisons une nouvelle methode de separation des ondes dans les barres. Cette methode permet de calculer, sans limitation de duree, les contraintes, les deformations, les deplacements et les vitesses materielles en tout point de la barre. Elle tient compte des effets dispersifs aussi bien dans les barres elastiques que dans les barres viscoelastiques. Elle est illustree sur une barre libre pour laquelle le deplacement mesure est 30 fois superieur a celui qui serait atteint par la methode classique.

Numerical model for simulation of transport and recombination in OLEDs

We present a recently developed numerical code for OLED simulation, This code contains a detailed description of contacts, charge transport and recombination. The route towards the simulation of a complex multilayer device is detailed through examples of single-layer and bilayer devices simulations as well as through the investigation of the action of a LiF thin film on injection.