Bernard Ratier

UV: visible and infrared characterization of poly(p-xylylene) films for waveguide applications and OLED encapsulation

Abstract UV–visible and infrared transmission of three sorts of poly( p -xylylene) films obtained by vapor-deposition polymerization have been studied in order to appraise their opportunity in two optical applications: waveguide achievement by reactive ion beam etching (RIBE) and organic light emitting diode (OLED) encapsulation. Optical changes induced by etching or thermal annealing (needed for the former application) are reported. While the high visible or near infrared transmission (∼95%) of the films is suitable for OLED encapsulation, the crystalline nature of these materials revealed by annealing could be a handicap for waveguide application.

Solid-state dye-sensitized solar cells based on ZnO nanocrystals

We report on the development of solution-processed ZnO-based dye-sensitized solar cells. We fabricate mesoporous ZnO electrodes from sol-gel processed nanoparticles, which are subsequently sensitized with conventional ruthenium complexes and infiltrated with the solid-state hole transporter medium 2, 2, 7, 7-tetrakis-(N, N-di-p-methoxyphenylamine)-9, 9-spirobifluorene (spiro-OMeTAD). Starting from ZnO nanorods synthesized from solution, we investigate the porous ZnO film morphology using various precursor formulations. The nature of the polymeric additive used in the initial ZnO formulation, as well as the ZnO electrode sintering treatment, is varied and its influence on device performance and charge dynamics, probed by transient perturbation techniques, is discussed. We show that using ethyl-cellulose in the initial ZnO formulation is responsible for an improved dye loading on the ZnO porous electrode, while a gradual sintering step at 350 degrees C is suitable for the proper removal of the organic phases that can be found in the ZnO films after their deposition by spin-coating. Using only 800 nm thick porous ZnO electrodes sensitized by N719, the best performing device exhibits a short-circuit current density of 2.43 mA cm(-2) under simulated solar emission of (100 mW cm(-2)), associated with an overall power conversion efficiency of 0.50%.

Optical modeling of organic solar cells based on CuPc and C60.

We have investigated the influence of the poly(3,4-ethylenedioxythiophene)-blend-poly(styrene-sulfonate) (PEDOT:PSS) layer on the short-circuit current density (J(sc)) of single planar heterojunction organic solar cells based on a copper phthalocyanine (CuPc)-buckminsterfullerene (C(60)) active layer. Complete optical and electrical modeling of the cell has been performed taking into account optical interferences and exciton diffusion. Comparison of experimental and simulated external quantum efficiency has allowed us to estimate the exciton diffusion length to be 37 nm for the CuPc and 19 nm for the C(60). The dependence of short-circuit current densities versus the thickness of the PEDOT:PSS layer is analyzed and compared with experimental data. It is found that the variation in short-circuit current densities could be explained by optical interferences.

Field‐effect transistors based on poly(p‐phenylene vinylene) doped by ion implantation

We have fabricated metal‐insulator‐semiconductor field‐effect transistors (MISFETs), with thin films of polycrystalline poly(p‐phenylene vinylene) (PPV) as the semiconducting layer and report here the successful operation of a PPV MISFET based on the p‐type doping of the polymer layer by ion implantation of iodine. The measured field‐effect mobility of the charge carriers in this ion‐ implanted PPV is in the range of 10−7 to 10−8 cm2/Vu2009s. These values are in the same range as those obtained from a PPV MISFET in which the PPV was doped from the gas phase.

Nanoscale control of the network morphology of high efficiency polymer fullerene solar cells by the use of high material concentration in the liquid phase

Despite the constant improvement of their power conversion efficiencies, organic solar cells based on an interpenetrating network of a conjugated polymer as donor and fullerene derivatives as acceptor materials still need to be improved for commercial use. In this context, we present a study on the optimization of solar cells based on poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61 butyric acid methyl ester (PCBM) by varying a specific cell parameter, namely the concentration of the active layer components in the liquid phase before blend film deposition, in order to improve device performance and to better understand the relation between morphology and device operation. Our study shows a significant increase of the short-circuit current, open-circuit voltage and cell efficiency by properly choosing the formulation of the initial blend before film deposition. We demonstrate that the active layer morphology, which is strongly dependent on the initial material concentrations and the processing conditions, can greatly impact the electronic characteristics of the device, especially regarding charge recombination dynamics at the donor-acceptor interface. Our optimized P3HT:PCBM device exhibits both slow recombination and high photocurrent generation associated with an overall power conversion efficiency of 4.25% under 100 mW cm(-2) illumination (AM1.5G).

Vapor deposition polymerization and reactive ion beam etching of poly(p-xylylene) films for waveguide applications

Vapor deposition polymerization (VDP) of parylene-n and parylene-c films has been studied in terms of the initial evaporated monomer mass: controlled and homogeneous film thicknesses are obtained provided that the vapor flow is low. In order to use these materials for ribbon waveguide applications, reactive ion beam etching of the films has been tried, giving very low etching speed.

N-type doping and thermoelectric properties of co-sublimed cesium-carbonate-doped fullerene

Organic semiconductors exhibit a large Seebeck coefficient and a poor thermal conductivity allowing them to become strong candidates for thermoelectric applications. These materials have been widely used in organic electronics with the fabrication of organic light-emitting diodes, organic solar cells, and transistors. However, few studies have reported on thermoelectric properties of organic materials even though they offer specific advantages such as cost-effectiveness and flexibility. In this article, we discuss the fabrication and characterization of fullerene C60 doped with cesium carbonate (Cs2CO3). The evolution of the morphology, electrical conductivity, and Seebeck coefficient was analyzed as a function of the dopant concentration. An optimal power factor of 28.8xa0μWm−1xa0K−2 was obtained at room temperature for a molar ratio of 15.2xa0%. Thus far, this power factor value constitutes the best thermoelectric performance achieved with N-type organic materials.

Single-Step Preparation of TiO2/MWCNT Nanohybrid Materials by Laser Pyrolysis and Application to Efficient Photovoltaic Energy Conversion

This paper presents the continuous-flowand single-step synthesis of a TiO2/MWCNT (multiwall carbon nanotubes) nanohybrid material. The synthesis method allows achieving high coverage and intimate interface between the TiO2particles and MWCNTs, together with a highly homogeneous distribution of nanotubes within the oxide. Such materials used as active layer in theporous photoelectrode of solid-state dye-sensitized solar cells leads to a substantial performance improvement (20%) as compared to reference devices.

Influence of Nitrogen Doping on Device Operation for TiO2-Based Solid-State Dye-Sensitized Solar Cells: Photo-Physics from Materials to Devices

Solid-state dye-sensitized solar cells (ssDSSC) constitute a major approach to photovoltaic energy conversion with efficiencies over 8% reported thanks to the rational design of efficient porous metal oxide electrodes, organic chromophores, and hole transporters. Among the various strategies used to push the performance ahead, doping of the nanocrystalline titanium dioxide (TiO2) electrode is regularly proposed to extend the photo-activity of the materials into the visible range. However, although various beneficial effects for device performance have been observed in the literature, they remain strongly dependent on the method used for the production of the metal oxide, and the influence of nitrogen atoms on charge kinetics remains unclear. To shed light on this open question, we synthesized a set of N-doped TiO2 nanopowders with various nitrogen contents, and exploited them for the fabrication of ssDSSC. Particularly, we carefully analyzed the localization of the dopants using X-ray photo-electron spectroscopy (XPS) and monitored their influence on the photo-induced charge kinetics probed both at the material and device levels. We demonstrate a strong correlation between the kinetics of photo-induced charge carriers probed both at the level of the nanopowders and at the level of working solar cells, illustrating a direct transposition of the photo-physic properties from materials to devices.

Thermo Electric Power Versus Temperature in Implanted Polyparaphenylene (PPP)

Abstract Polyparaphenylene (PPP) samples were implanted with alkali metal ions using different energies. They were characterized and the evolution of thermo electric power versus temperature was followed. The results may be analyzed in terms of a variable range hopping process between localized states induced near the Fermi level through implantation. Furthermore with samples implanted at low energy, a transition toward values characteristic of the expected doping shows up (n type doping with alkali metals).