Appareillage automatisé de mesure simultanée du pouvoir thermoélectrique et de la conductivité électrique. Application à l'étude de couches polymères semi-conductrices

Nous présentons dans cet article un système de mesure simultanée de la conductivité σ, et du pouvoir thermoélectrique S : il permet des mesures en fonction de la température (entre 130 K et 360 K) dans le cas de couches semi-conductrices relativement fragiles. A titre d'application, nous indiquons les résultats que nous avons obtenus dans le cas de couches polymères (PPP) implantées avec des ions sodium: alors que seule une semi-conduction par défaut est générée par de fortes énergies d'implantation (E = 250 keV ), il apparaît une semiconduction induite par le dopage n lors d'implantations à basse énergie (E = 30 keV )

Transport mechanisms in 8-tris-hydroxyquinoline aluminium (Alq3) electronic layers: a study by photodipolar absorption

International audienceThis paper describes the role of traps in the electronic conductivity of 8-tris-hydroxyquinoline aluminium in a conventional sandwich structure with indium tin oxide and aluminium electrodes. New results obtained by photodipolar absorption techniques and impedance spectroscopy are presented. The former method acts as a probe to highlight the role of traps. It is shown that optical pumping of electrons to trap levels gives a clear increase in dielectric absorption due to the reorientation of dipoles associated with trapped charges. The trap depth is estimated to be around Et= 0.19 eV, a value in good agreement with theoretical calculations and thermoluminescence measurements. The latter method permits a representation of the sample in terms of a circuit composed of a parallel capacitor (Cp) and resistor (Rp) both in series with a resistor Rs≈ 50 Ωlocated on the anode side. A logarithmic plot of Rp as a function of the dc bias voltage gives a linear law that is recognized, for the first time, to be a consequence of a trapped charge limited (TCL) current. The linearity can be improved by the introduction of a field-dependent mobility

Up-conversion injection in Rubrene/Perylene-diimide-heterostructure electroluminescent diodes

We implement and demonstrate a scheme that permits to drive electroluminescence with an extremely low turn-on voltage. The device behaves like compound semiconductors, in which the electroluminescence turn-on voltage is about the same as the open circuit voltage for the photovoltaic effect. However, the electroluminescence turn-on voltage is about half of the band gap of the emitting material, that cannot be explained using current models of charge injection in organic semiconductors. We hereby propose explanation through an Auger-type two-step injection mechanism (Auger-fountain).Comment: we report the first evidence of an Auger-fountain electroluminescence in a semiconductor heterojunctio

Etude par microanalyse ionique de films de poly(paraphénylène) et des phénomènes induits par implantation ionique

L'étude par microanalyse ionique de films de polyparaphénylène nous a permis de mettre en évidence la présence d'atomes d'azote et d'oxygène ; à partir de l'implantation préliminaire d'isotopes 15N et 18O nous avons pu évaluer les concentrations atomiques en azote et oxygène qui sont respectivement inférieures à 1 % et de l'ordre de 4 %. Ces atomes d'impuretés sont localisés dans une zone de défauts, située à l'arrière de la couche implantée et dont l'importance croît avec la valeur des paramètres d'implantation

High Temperature Thermopower in La_{2/3}Ca_{1/3}MnO_3 Films: Evidence for Polaronic Transport

Thermoelectric power, electrical resistivity and magnetization experiments, performed in the paramagnetic phase of La_{2/3}Ca_{1/3}MnO_3, provide evidence for polaron-dominated conduction in CMR materials. At high temperatures, a large, nearly field-independent difference between the activation energies for resistivity (rho) and thermopower (S), a characteristic of Holstein Polarons, is observed, and ln(rho) ceases to scale with the magnetization. On approaching T_c, both energies become field-dependent, indicating that the polarons are magnetically polarized. Below T_c, the thermopower follows a law S(H) prop. 1/rho (H) as in non saturated ferromagnetic metals.Comment: 10 pages, 5 .gif figures. Phys. Rev B (in press

Recent advances in solid-state organic lasers

Organic solid-state lasers are reviewed, with a special emphasis on works published during the last decade. Referring originally to dyes in solid-state polymeric matrices, organic lasers also include the rich family of organic semiconductors, paced by the rapid development of organic light emitting diodes. Organic lasers are broadly tunable coherent sources are potentially compact, convenient and manufactured at low-costs. In this review, we describe the basic photophysics of the materials used as gain media in organic lasers with a specific look at the distinctive feature of dyes and semiconductors. We also outline the laser architectures used in state-of-the-art organic lasers and the performances of these devices with regard to output power, lifetime, and beam quality. A survey of the recent trends in the field is given, highlighting the latest developments in terms of wavelength coverage, wavelength agility, efficiency and compactness, or towards integrated low-cost sources, with a special focus on the great challenges remaining for achieving direct electrical pumping. Finally, we discuss the very recent demonstration of new kinds of organic lasers based on polaritons or surface plasmons, which open new and very promising routes in the field of organic nanophotonics

Acetylene-Based Materials in Organic Photovoltaics

Fossil fuel alternatives, such as solar energy, are moving to the forefront in a variety of research fields. Organic photovoltaic systems hold the promise of a lightweight, flexible, cost-effective solar energy conversion platform, which could benefit from simple solution-processing of the active layer. The discovery of semiconductive polyacetylene by Heeger et al. in the late 1970s was a milestone towards the use of organic materials in electronics; the development of efficient protocols for the palladium catalyzed alkynylation reactions and the new conception of steric and conformational advantages of acetylenes have been recently focused the attention on conjugated triple-bond containing systems as a promising class of semiconductors for OPVs applications. We review here the most important and representative (poly)arylacetylenes that have been used in the field. A general introduction to (poly)arylacetylenes, and the most common synthetic approaches directed toward making these materials will be firstly given. After a brief discussion on working principles and critical parameters of OPVs, we will focus on molecular arylacetylenes, (co)polymers containing triple bonds, and metallopolyyne polymers as p-type semiconductor materials. The last section will deal with hybrids in which oligomeric/polymeric structures incorporating acetylenic linkages such as phenylene ethynylenes have been attached onto C60, and their use as the active materials in photovoltaic devices