Stéphane Bastide

Design of porous silicon antireflection coatings for silicon solar cells

A porous silicon (PS) layer formed electrochemically in the outer part of the n emitter of p-n Si junctions can be used as an efficient antireflection coating (ARC). A two-step procedure is presented which can determine the electrochemical parameters leading to the formation of an optimized single-layer PS ARC. Single-layer PS ARCs achieving: 7% effective reflectance between 400 and 1000 nm are obtained on shallow p‐n junction solar cells. To reduce the reflectance further, the design of double-layer ARCs based on PS is investigated. PS layers with different porosities can be realized in a single experiment by modulating the current density during the electrochemical process. It is shown theoretically and experimentally that such PS structures can lead to an effective reflectance below 3%.

Chemical synthesis of molybdenum disulfide nanoparticles in an organic solution

Molybdenum disulfide, MoS2, nanoparticles can be synthesized either at high temperature as mono- and polycrystalline materials or at low temperature using various (electro)chemical routes. In the work presented, MoS2 nanoparticles were obtained using a low temperature (140 °C) method via a chemical solution reaction route between the organometallic precursor Mo(CO)6 and sulfur in p-xylene. As obtained, the MoS2 nanoparticles of 10–30 nm diameter were mostly amorphous with a rounded shape. Upon annealing at 550 °C under vacuum, the nanoparticles lost their rounded shape and became slightly crystallized with curved (002) basal van der Waals planes (2H hexagonal structure). An increase of 2–4% in the d(002) spacing of the annealed MoS2 nanoparticles compared to polycrystalline MoS2 was observed. The size and shape of these nanoparticles play an important role for their properties, e.g. in catalysis and lubrication.

Formation and characterization of porous silicon layers for application in multicrystalline silicon solar cells

Abstract The electrochemical formation of porous silicon (PS) layers in the n+ emitter of silicon p–n+ homojunctions for solar energy conversion has been investigated. During the electrochemical process under constant polarization, a variation of the current density occurs. This effect is explained by considering the doping impurity gradient in the emitter and by TEM characterization of the PS layer structure. Optical transmission measurements indicate that modifications of the refractive index and absorption coefficient of PS are mainly related to the porosity value. Reflectivity measurements, spectral response and I–V characteristics show that PS acts as an efficient antireflection coating layer. However, beyond a critical layer thickness, i.e. when PS reaches the p–n+ interface, the junction properties are degraded.

Influence of the electrochemical deposition parameters on the microstructure of MoS2 thin films

Abstract Thin films molybdenum dichalcogenide, MoS 2 , were prepared by cathodic electrochemical deposition from aqueous and non-aqueous solutions of tetrathiomolybdate ions, at different temperatures. The films were X-ray amorphous as deposited. They consist of an amorphous matrix in which quantum sized nanocrystallites or clusters were embedded. Upon annealing at high temperatures, the films obtained from aqueous solutions become crystalline and highly texturized having their van der Waals planes oriented parallel to the substrate, whereas, those obtained from ethylene glycol solutions kept on the amorphous matrix, with slightly larger sizes MoS 2 nanoparticles embedded, than before annealing. Difference in the mechanism of the electrodeposition in aqueous and ethylene glycol solutions is discussed.

Morphology of porous n-type silicon obtained by photoelectrochemical etching II: Study of the tangled Si wires in the nanoporous layer

Abstract Visible luminescence observed from the nanoporous layer of the two (100)-orinted low doped and highly doped PEC-etched n-type Si is explained as being due to the existence of single crystal silicon quantum wires within their structure. The nanometer-size tangled Si structure is contained and attached to a regular geometric Si macroarray. TEM studies also reveal subtle variations in morphology between the two layers studied, which could explain the blueshift in the spectrum of the low-doped specimen — thinner and more rigid irregular wires — as compared to the highly doped specimen.

Variable Optical Properties and Effective Porosity of CdSe Nanocrystalline Films Electrodeposited from Selenosulfate Solutions

CdSe films electrodeposited from a selenosulfate bath are normally made up of dense (nonporous) assemblies of nanocrystals (4-5 nm). At cathodic deposition currents higher than a certain value, the properties of the filmschange. While the CdSe crystal size remains the same, the apparent bandgap of the films increases by 0.2-0.3 eV, a change visually observable by a change in film color from brown to orange. In contrast to the low-current films, there is considerable strain in the high-current films, as evidenced by the tendency of the films to crack with increasing thickness and either peel off the substrate or roughen. This variation in morphology from smooth to rough films allows control over the effective porosity of the films. Several explanations for the different optical behaviors of the two types of films are suggested, including size quantization modified by overlap interaction between the tightly connected crystals and in situ hydrogen absorption on the crystal surfaces due to simultaneous hydrogen evolution at higher current densities.

First Evidence of Rh Nano-Hydride Formation at Low Pressure.

Rh-based nanoparticles supported on a porous carbon host were prepared with tunable average sizes ranging from 1.3 to 3.0 nm. Depending on the vacuum or hydrogen environment during thermal treatment, either Rh metal or hydride is formed at nanoscale, respectively. In contrast to bulk Rh that can form a hydride phase under 4 GPa pressure, the metallic Rh nanoparticles (∼2.3 nm) absorb hydrogen and form a hydride phase at pressure below 0.1 MPa, as evidenced by the presence of a plateau pressure in the pressure-composition isotherm curves at room temperature. Larger metal nanoparticles (∼3.0 nm) form only a solid solution with hydrogen under similar conditions. This suggests a nanoscale effect that drastically changes the Rh-H thermodynamics. The nanosized Rh hydride phase is stable at room temperature and only desorbs hydrogen above 175 °C. Within the present hydride particle size range (1.3-2.3 nm), the hydrogen desorption is size-dependent, as proven by different thermal analysis techniques.

Electrochemical Preparation of H 2 S and H 2Se

generation were also brieflystudied for comparison. The mechanisms of hydride formation are discussed. Both the reaction of nascent hydrogen with the freeelement and direct reduction of the element are considered. The latter is believed to be the dominant mechanism.© 2005 The Electrochemical Society. @DOI: 10.1149/1.1852441# All rights reserved.Manuscript submitted March 31, 2004; revised manuscript received August 3, 2004. Available electronically January 24, 2005.

Sonochemical synthesis of Fe3O4@NH2-mesoporous silica@Polypyrrole/Pd: A core/double shell nanocomposite for catalytic applications

There is a growing interest in sonochemistry for either the controlled design of nanostructured materials or for the synthesis of polymers and polymer composites. It is fast and highly efficient method that provides materials with exceptional and enhanced structural and chemical properties. Herein, we take advantage of the versatile sonochemical process in order to design core/double layered shell nanocomposite denoted by Fe3O4@NH2-mesoporous silica@ PPy/Pd. This magnetic, multicomponent material was designed in a three-step sono-process: (i) synthesis of magnetic core, (ii) cure of mesoporous silica, and (iii) sonochemical deposition of PPy/Pd. This last step was achieved within 1 h, a much shorter duration compared to conventional routes which usually take several hours to few days. The final nanocomposite can be recovered with a simple magnetic stick. X-ray diffraction patterns highlighted the presence of zerovalent palladium on the surface of the magnetic nanocomposite. The catalytic activity of the solid support was investigated by the study of the p-nitrophenol (p-NP) reduction and the Methyl Orange (MO) degradation in aqueous media. Results showed a very high catalytic efficiency, a high conversion yield of p-NP into 4-aminophenol (more than 94%) and an almost entire degradation of MO (99%) with a fast kinetics fitting to the first order model. This work demonstrates conclusively the benefits of sonochemistry in the design of metal nanoparticle-decorated inorganic/polymer hybrid system with outstanding performances.

In Situ PL and SPV Monitored Charge Carrier Injection During Metal Assisted Etching of Intrinsic a-Si Layers on c-Si.

Although hydrogenated amorphous silicon is already widely examined regarding its structural and electronic properties, the chemical etching behavior of this material is only roughly understood. We present a detailed study of the etching properties of intrinsic hydrogenated amorphous silicon, (i)a-Si:H, layers on crystalline silicon, c-Si, within the framework of metal assisted chemical etching (MACE) using silver nanoparticles (Ag NPs). The etching processes are examined by in situ photoluminescence (PL) and in situ surface photovoltage (SPV) measurements, as these techniques allow a monitoring of the hole injection that takes place during MACE. By in situ PL measurements and SEM images, we could interpret the different stages of the MACE process of (i)a-Si:H layers and determine etch rates of (i)a-Si:H, that are found to be influenced by the size of the Ag NPs. In situ PL and in situ SPV measurements both enable researchers to determine when the Ag NPs reach the (i)a-Si:H/c-Si interface. Furthermore, a preferential MACE of (i)a-Si:H versus c-Si is revealed for the first time. This effect could be explained by an interplay of the different thermodynamic and structural properties of the two materials as well as by hole injection during MACE resulting in a field effect passivation. The presented results allow an application of the examined MACE processes for Si nanostructuring applications.