Jean Rousset

Structure and catalytic activity of palladium-platinum aggregates obtained by laser vaporisation of bulk alloys

Abstract The European emission legislation concerning the pollution abatement of the exhaust gas from light cars imposes a reduction of the concentration of aromatics in gasoline, down to 2%. The hydrogenation of these hydrocarbons can be performed on Pd or Pt catalysts. However, these catalysts are poisoned by traces of sulphur — 100–300 ppm — remaining in the fuels. It is claimed in the literature that bimetallic Pd–Pt catalysts are less affected by this poisoning than the pure metals. To verify this point, both Pd–Pt and pure metal aggregates were synthesised by laser vaporisation of bulk alloys and deposited on alumina. Analytical microscopy and EXAFS have shown that these clusters have a very narrow composition distribution and form alloys. The study of their activity in the hydrogenation of Tetralin, a model molecule, in the presence of variable amounts of H2S, has shown that Pt is more active than Pd at low sulphur concentration, whereas Pd becomes more active for the highest H2S content. Contrary to what is claimed in the literature, no synergetic effect has been found found by alloying these two metals; actually their activity is the simple additivity of that of the two metals.

Raman Characterization of a New Te-Rich Binary Compound: CdTe2

Structural characterization by Raman spectroscopy of CdTe thin films electrodeposited in acidic conditions is considered in this work. This study focuses on the evolution of material properties as a function of the applied potential and the film thickness, demonstrating the possibility to obtain a new Te-rich compound with a II/VI ratio of 1/2 under specific bath conditions. Raman measurements carried out on etched samples first allow the elimination of the assumption of a mixture of phases CdTe + Te and tend to confirm the formation of the CdTe(2) binary compound. The signature of this phase on the Raman spectrum is the increase of the LO band intensity compared to that obtained for the CdTe. The influence of the laser power is also considered. While no effect is observed on CdTe films, the increase of the incident irradiation power leads to the decomposition of the CdTe(2) compound into two more stable phases namely CdTe and Te.

Influence of a Boron Precursor on the Growth and Optoelectronic Properties of Electrodeposited Zinc Oxide Thin Film

Highly transparent and conductive materials are required for many industrial applications. One of the interesting features of ZnO is the possibility to dope it using different elements, hence improving its conductivity. Results concerning the zinc oxide thin films electrodeposited in a zinc perchlorate medium containing a boron precursor are presented in this study. The addition of boron to the electrolyte leads to significant effects on the morphology and crystalline structure as well as an evolution of the optical properties of the material. Varying the concentration of boric acid from 0 to 15 mM strongly improves the compactness of the deposit and increases the band gap from 3.33 to 3.45 eV. Investigations were also conducted to estimate and determine the influence of boric acid on the electrical properties of the ZnO layers. As a result, no doping effect effect by boron was demonstrated. However, the role of boric acid on the material quality has also been proven and discussed. Boric acid strongly contributes to the growth of high quality electrodeposited zinc oxide. The high doping level of the film can be attributed to the perchlorate ions introduced in the bath. Finally, a ZnO layer electrodeposited in a boron rich electrolyte was tested as front contact of a Cu(In, Ga)(S, Se)2 based solar cell. An efficiency of 12.5% was measured with a quite high fill factor (>70%) which confirms the high conductivity of the ZnO thin film.

Slow Diffusion and Long Lifetime in Metal Halide Perovskites for Photovoltaics

Metal halide perovskites feature excellent absorption, emission, and charge carrier transport properties. These materials are therefore very well suited for photovoltaic applications where there is a growing interest. Still, questions arise when looking at the unusual long carrier lifetime that, regarding the micrometric diffusion length, would imply a very low diffusion coefficient as compared to the commonly used photovoltaic absorbers. In this paper, we point out the slow in-depth diffusion mechanism in the time-resolved photoluminescence decay interpretation by using an experimental setup with a wide-field excitation condition and an appropriate model of the carrier dynamics. The model includes charge carrier transport, photon recycling, and trap dynamics. It is verified on various excitation intensities, different interface properties, and various perovskite absorbers such as mixed alloys with the cesium content. For most of the perovskite-based materials we analyzed, the band-to-band recombination r...

Electrodeposition of ZnO-doped films as window layer for Cd-free CIGS-based solar cells

The Cu(In,Ga)Se2 (CIGS) thin film solar cell technology has made a steady progress within the last decade reaching efficiency up to 22.3% on laboratory scale, thus overpassing the highest efficiency for polycrystalline silicon solar cells. High efficiency CIGS modules employ a so-called buffer layer of cadmium sulfide CdS deposited by Chemical Bath Deposition (CBD), which presence and Cd-containing waste present some environmental concerns. A second potential bottleneck for CIGS technology is its window layer made of i-ZnO/ZnO:Al, which is deposited by sputtering requiring expensive vacuum equipment. A non-vacuum deposition of transparent conductive oxide (TCO) relying on simpler equipment with lower investment costs will be more economically attractive, and could increase competitiveness of CIGS-based modules with the mainstream silicon-based technologies. In the frame of Novazolar project, we have developed a low-cost aqueous solution photo assisted electrodeposition process of the ZnO-based window layer for high efficiency CIGS-based solar cells. The window layer deposition have been first optimized on classical CdS buffer layer leading to cells with efficiencies similar to those measured with the sputtered references on the same absorber (15%). The the optimized ZnO doped layer has been adapted to cadmium free devices where the CdS is replaced by chemical bath deposited zinc oxysulfide Zn(S,O) buffer layer. The effect of different growth parameters has been studied on CBD-Zn(S,O)-plated co-evaporated Cu(In,Ga)Se2 substrates provided by the Zentrum für Sonnenenergie-und Wasserstoff-Forschung (ZSW). This optimization of the electrodeposition of ZnO:Cl on CIGS/Zn(S,O) stacks led to record efficiency of 14%, while the reference cell with a sputtered (Zn,Mg)O/ZnO:Al window layer has an efficiency of 15.2%.