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Photoelectrocatalysis takes advantage of electrocatalytic and photocatalytic processes [...]

International audience; To reduce the energy cost and environmental impact of biological nitrogen removal in wastewater treatment plants, it would be advantageous to treat urea contained in urine at the source. In this perspective, FTO/Ti-Fe2O3 (nanorods) photoelectrodes decorated with Ni as catalyst are developed and tested for urea photoelectrocatalytic oxidation under solar illumination. Gains up to 0.50 V in oxidation onset potential vs. metallic Ni are obtained thanks to a Ni photoelectrodeposition method. In situ transmission measurements (based on NiOOH light absorption) during electrochemical cycling allowed to evaluate the state of active Ni sites and confirmed that urea oxidation mechanism is of EC type. Photoelectrolyses give faradaic efficiencies of 10–18% and 9-35% for N2 and O2 formation, respectively. A significant and unexpected NO2− production (∼65%) is detected indicating another or incomplete reaction pathway. The photoelectrocatalytic removal of nitrogen from urea solutions is demonstrated but requires catalysts with higher selectivity towards N2.

To shift towards the greener city, photocatalytic urban infrastructures have emerged as a promising solution for pollution remediation. To reach this goal, the large bandgap semiconductors, such as nontoxic Zinc Oxide (ZnO), already proved their excellent photocatalytic performances. However, integrating and developing cost-effective and greener photocatalytic surfaces with an easily scaled-up synthesis method and without energy and chemical product overconsumption is still challenging. Therefore, this work proposes to develop a depolluting Zinc (Zn) roof covered by ZnO nanostructures (NSs) using a one-step seedless hydrothermal growth method in 2 h. The feasibility of this synthesis was firstly studied on small areas of Zn (1.25 cm2) before being scaled up to medium-sized areas (25 cm2). The efficiency of this functionalization route for ZnO NSs grown without seed layer was attributed to the presence of Zn2+ sites and the native oxide film on the Zn surface. Their photocatalytic efficiency was demonstrated by removing in less than 3 h the Methylene Blue (MB) and Acid Red 14 (AR14) in both DI water and rainwater under UV-light. Promising results were also recorded under solar light. Therefore, the photocatalytic Zn roof functionalized by ZnO NSs is a promising route for rainwater purification by photocatalysis.

Olive pit powder particles were impregnated with copper and nickel nitrates and pyrolyzed at 400 °C. The resulting material consists of bimetallic CuNi-decorated biochar. CuNi nanocatalysts were found to be as small as 10 nm and very well dispersed over biochar. XRD results evidenced zero valent copper and nickel and formation of copper-nickel solid solutions.The biochar@CuNi was found to be efficient catalyst of the reduction of methyl orange (MO) dye, taken as model pollutant. To sum up, the one pot method devised in this work provided unique CuNi-decorated biochar and opens new horizons for the emerging topic of biochar-supported nanocatalysts.

International audience

Photocatalysis is proven to be the most efficient and environmentally friendly method for the degradation of organic pollutants in water purification. To meet the requirement of large-scale water treatment, there are two important points: One is the lifetime and chemical stability of the photocatalyst material, especially in the complex and harsh aqueous conditions. The other is the ease of synthesis of such photocatalysts with specific nano-morphology. In this work, two common photocatalyst materials, zinc oxide (ZnO) and titanium dioxide (TiO2), are selected to form more sustainable photocatalysts with high chemical stability. This involves the combination of both TiO2 and ZnO in a two-step simple synthesis method. It appears advantageous to exploit the conformal deposition of atomic layer deposition (ALD) to achieve nanometer-thick TiO2 coating on ZnO nanowires (NWs) with a high aspect ratio, which are firmly anchored to a substrate and exhibit a large specific surface area. The high chemical stability of the ALD TiO2 coating has been investigated in detail and proven to be effective under both strong acid and strong alkaline aqueous solutions. In addition, photocatalysis experiments with organic dyes show that via this simple two-step synthesis method, the produced ZnO/TiO2 tandem photocatalysts does indeed exhibit improved chemical stability in a harsh environment, while allowing efficient photodegradation.

International audience; Developing micro- and nanomaterials for environmental pollution remediation is currently a pertinent topic. Among the plethora of strategies, designing supported nanocatalysts for the degradation of pollutants has achieved prominence. In this context, we are addressing one of the UN Sustainable Development Goals by valorizing agrowaste as a source of biochar, which serves as a support for bimetallic nanocatalysts. Herein, olive pit powder particles were impregnated with copper and nickel nitrates and pyrolyzed at 400 °C. The resulting material consists of bimetallic CuNi-decorated biochar. CuNi nanocatalysts were found to be as small as 10 nm and very well dispersed over biochar with zero valent copper and nickel and the formation of copper–nickel solid solutions. The biochar@CuNi (B@CuNi) exhibited typical soft ferromagnet hysteresis loops with zero remanence and zero coercivity. The biochar@CuNi was found to be an efficient catalyst of the reduction in methyl orange (MO) dye, taken as a model pollutant. In sum, the one-pot method devised in this work provides unique CuNi-decorated biochar and broadens the horizons of the emerging topic of biochar-supported nanocatalysts.

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.

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 Fe

The hydrogen sorption properties of palladium particles synthesized with different sizes (2, 6 and 18 nm) dispersed on high surface area graphite powders are studied by both solid/gas and electrochemical methods. We demonstrate that we can directly evaluate by cyclic voltammetry the influence of the particle size on Pd hydride phase formation despite the ohmic losses and capacitive currents associated with powder electrodes. The size-dependent Pd hydride compositions calculated from electrochemical signals and pressure-composition-isotherms under ambient conditions are in good agreement and show the same trend: as the Pd particle size is reduced, the hydrogen content increases in the α phase and decreases in the β phase. Changes in electrochemical signals during cycling are observed when the potential domain covers the Pd oxide region, corresponding to an increase of the mean size of the smallest particles from 2 to ∼12 nm (as confirmed by ex situ X-ray diffraction) whereas no modification occurs if it is restricted to the hydrogen region. This highlights that voltammograms can be used to readily detect in situ size modifications of Pd nanomaterials.

The hydrogen sorption properties of nanosized palladium particles dispersed in high surface area graphite powders are studied by both solid/gas and electrochemical methods. Using either classical powder electrodes or a cavity microelectrode we show that cyclic voltammetry peaks coming from the hydrogen sorption/evolution, that are usually strongly overlapped in powder electrodes, can be resolved at low scan rates. With the cavity microelectrode, we could measure voltammograms over a 3-decade range of potential scan rates and thus describe the sorption processes in this hybrid material.

A simple and effective method for producing Ag nanoparticles at room temperature and on the time scale of several minutes on the surface of hydrogenated Si nitride (SiN x :H) films is presented. Depending on the stoichiometry of SiN x :H layers, Ag nanostructures with different sizes and densities can be produced. The structural properties of the deposited Ag nanoparticles are described in details with respect to the stoichiometries of SiN x :H films.

Macropore formation by photoelectrochemical (PEC) etching of N-type crystalline Si under front side illumination has been investigated. We report on two important parameters that influence the pore morphology: the applied anodic bias and the spectral distribution of the illumination source. On (100) c - Si, the pore morphology changes from a round shape at low bias (0 V/NHE) to a conical shape at high bias (2 V/NHE). In the latter case we show that, by adjusting the spectrum of the white light source with longpass filters of different cut-on wavelengths, we can control the cone angle of the pores from 50° with white light to 10° with infrared light. Optical measurements indicate that macropores formed with a narrow cone angle (20°) could be of interest for Si solar cell texturization since they result in a very low reflectivity (2.6% at 800 nm) and in an efficient light trapping.

Metal-assisted chemical etching (MAE) was used as an alternative route to texturise the front surface of multicrystalline Si solar cells. Ag nanoparticles deposited by an electroless method on the Si surface acted as catalysts for Si etching in a solution containing HF and H2O2, an oxidizing species. MAE formed a duplex structure at the surface of the Si wafer consisting in a mesoporous Si layer that after dissolution in NaOH revealed the second structure, which is a macrotexturised surface. After texturisation, the effective reflectivity (AM1.5G, 400-1000 nm) ranged between 12% and 15% depending on the duration of the dissolution treatment of the mesoporous layer. MAE texturised solar cells were compared to NaOH texturised solar cells made with similar wafers (∼25% in effective reflectivity). MAE texturisation leads to a gain in fill factor (FF) but not in short-circuit current (JSC) (from average values). The best MAE cell exhibited higher JSC (+2%) and efficiency (16.1%) compared to the best NaOH cell (15.7%). (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A conventional flatbed scanner equipped with an additional diffusor is used for rapid measurements of an important figure of merit for optical surfaces, the effective reflectivity of devices such as solar cells. The application of the technique to multicrystalline silicon wafers with light-trapping structures obtained by electrochemical etching is shown. The use of this method for rapid quality control in production environments as well as in the lab is envisaged: even with a non-optimized diffusor, a spatial resolution of ∼0.1×0.1 mm2 can be achieved, with an accuracy of the reflectivity measurements of ∼1% and data-acquisition times around 10 s per wafer.

Metal-assisted etching of silicon in HF/H2O2/H2O solutions with Ag nanoparticles as catalyst agents was investigated. SEM observations and etch rate measurements were carried out as a function of the etching solution composition. Depending on the relative amount of HF and H2O2, different regimes of dissolution take place and a strong similarity with the etching in HF–HNO3 solution is evidenced, for the first time. Formation of meso- and macroporous Si, etched craters and polished Si are observed as the HF/H2O2 ratio decreases. The dissolution mechanisms are discussed on the basis of a localized hole injection from the Ag nanoparticles into Si and in terms of the well known chemistry of Si dissolution in HF-based chemical and electrochemical systems. At high HF/H2O2 ratio, there is no formation of oxide at the surface. Hole injection and Si dissolution occur at the level of the Ag nanoparticle only, resulting in the formation of meso and macropores depending on the Ag nanoparticle size. At low HF/H2O2 ratio, the Si surface is oxidized, the injected holes are homogeneously distributed and thus polishing occurs. There is an intermediate range of composition in which injected holes diffuse away from the Ag nanoparticles and cone-shaped macropores, several tens of nm in diameter are formed.

Taking advantage of the electrochemical formation of macropores in aqueous HF under illumination, we have developed an efficient photoelectrochemical (PEC) texturization on n-type multicrystalline Si (mc-Si) wafers which leads to a decrease in the reflectivity of solar cells. An optimized mc-Si texturized surface exhibits an 8.5% effective reflectivity (λ=400-1000 nm, AM1.5 standard solar illumination).

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.

Des cellules solaires extremely thin absorber (eta) ZnO/CdSe/CuSCN a base de nanofils de ZnO ont ete realisees avec succes en utilisant des techniques faciles d'acces de depot en solution et par voie electrochimique. Une couche constituee de nanofils de ZnO monocristallin de plusieurs microns de hauteur et de 100-200 nm de diametre a ete deposee sur un substrat de verre conducteur (SnO 2 :F) recouvert d'une couche de ZnO formee par pyrolyse de spray agissant comme couche electronique bloquante. Une couche mince de 30-40 nm de CdSe jouant le role d'un absorbeur a ete electrodeposee de maniere conforme le long des nanofils de ZnO. Les espaces entre les nanofils composite de ZnO/CdSe ont ete remplis par du CuSCN semiconducteur de type p ; l'ensemble formant une jonction p-i-n. La couche de nanofils composite de ZnO/CdSe presente un important effet de confinement optique de la lumiere, avec une absorption effective de ∼ 89% et une reflectivite effective de ∼ 8% dans la region 400-800 nm du spectre solaire (AMI.5). L'influence du recuit sur la taille des grains de CdSe et sur le rendement de conversion de l'energie de la cellule eta-solar cell a ete analysee. Le rendement de conversion observe des cellules apres recuit de la couche de CdSe est, pour la meilleure d'entre elles, de 2,3%, demontrant ainsi que la nanoheterostructure ZnO/CdSe/CuSCN constitue une option interessante pour developper un nouveau type de cellules photovoltaiques.

The ZnO/CdTe/CuSCN heterostructure was analyzed as a candidate to act as an inorganic eta -solar cell. A ZnO film consisting of single crystal nanocolumns was electrodeposited on a transparent conducting substrate which acts as n-type material. As absorber material we used CdTe, which was deposited on the ZnO columnar film by Metal Organic Chemical Vapor Deposition. In order to complete the eta -solar cell we deposited a CuSCN layer by chemical solution deposition. A conformal and uniform CdTe coverage of the ZnO columns was achieved, producing a very efficient light trapping effect. The effective absorption (∼87%) and effective reflectance (∼10%) of the complete heterostructure in the 400–800 nm (AM1.5) solar spectrum range are very favorable for the use of this heterostructure in photovoltaic devices. Preliminary current–voltage characterization made on this ZnO/CdTe/CuSCN heterostructure shows rectifying behavior and a photovoltaic effect.

MX2 nanoparticles, with M = Mo, W and X = S or Se, were synthesized via a chemical reaction between molybdenum carbonyl (or tungsten carbonyl) and chalcogen dissolved in para-xylene solution. X-ray diffraction and high-resolution transmission electron microscopy analysis showed that the MoS2, WS2, MoSe2 and WSe2 powders were constituted of rounded amorphous nanoparticles. Upon annealing at 550 °C the nanoparticles crystallized, losing their round shape. The four materials synthesized under similar conditions showed that the annealed selenide nanoparticles (MoSe2 and WSe2) are well crystallized, with aligned stacks of 5–10 layers (002 van der Waals (vdW) planes), while the annealed sulfide nanoparticles (MoS2 and WS2) exhibit curved and tangled 002 planes, randomly distributed in the particle.

The formation of porous Si (PSi) antireflection coating (ARC) on n + -p multicrystalline Si solar cells has been investigated in view of its implementation at the last stage of the solar cell processing. PSi layers were formed either by electrochemical (EC) etching or by chemical stain (CS) etching and led to effective reflectivity around 13% (AM1.5). Solar cells with solar energy conversion efficiency of 13.3% and 12.6% were obtained with PSi ARC formed by EC- and CS-etching, respectively, the difference originating from a 2-3% larger short-circuit current for the EC-etched cells. Additionally, the FF was higher by 1-2% (a.v.) after PSi formation, which demonstrates that both methods are non-deleterious for the front contact quality and hence suitable for a standard solar cell process.

Conditions of macropore formation on multicrystalline medium doped silicon were deduced from a systematic study of the macroporous morphology of (100) and (111) oriented monocrystalline silicon in function of the porous etching parameters and silicon doping. A new promising method to texturize the surface of multicrystalline silicon wafers has been developed which leads to a macroporous surface with a very low effective reflectivity ∼9%. Macroporous multicrystalline silicon solar cells exhibit high efficiency larger than 13.5%.

First results of CdTe deposition on ZnO films consisting of free standing single crystalline columns of several micrometers height and ∼100– 200 nm diameter are presented. The ZnO films have been obtained by electrochemical deposition on conductive glass. Morphology, structural and optical properties of the nanostructured columnar CdTe/ZnO composite films have been studied. The cadmium telluride ( 40 nm thick) deposited by vapor-phase epitaxy under dynamical vacuum is lining the ZnO columns as a continuous smooth thin film with conformal coverage. The polycrystalline CdTe layer is a mixture of cubic and hexagonal phases and is semiconductor with a direct band gap optical transition of 1.56 eV . The nanocomposite CdTe/ZnO film shows strong light trapping effect and has considerable potential for use in photoelectric thin film devices.

A promising anode material for Li-ion batteries based on MgH₂ with around 5 nm average particles size was synthesized by a bottom-up method. A series of several composites containing MgH₂ nanoparticles well dispersed into a porous carbon host has been prepared with different metal content up to 70 wt%. A narrow particle size distribution (1-10 nm) of the MgH₂ nanospecies with around 5.5 nm average size can be controlled up to 50 wt% Mg. After a ball milling treatment under Ar, the composite containing 50 wt% Mg shows an impressive cycle life stability with a good electrochemical capacity of around 500 mA h g(-1). Moreover, the nanoparticles' size distribution is stable during cycling.

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.

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%. © 2000 Elsevier Science S.A. All rights reserved.

We present a simple, compact, and robust arrangement for surface photovoltage measurements of free semiconductor surfaces immersed in liquids. It is based on the classical Kelvin probe arrangement, where the semiconductor sample is put in a liquid-containing, electrically insulating vessel, with an optically transparent window, situated between the sample and the Kelvin probe. At the price of permitting relative, rather than absolute, contact potential difference values, this modification enables easy, routine surface photovoltage measurements of semiconductors in any kind of liquid ambient. The validity and efficiency of this approach are demonstrated by surface photovoltage spectra obtained from the p-InP(100) surface in various liquid etchants.

The antireflection properties of electrochemically formed porous silicon (PS) layers in the 0.3 μm thick n+ emitter of Si p–n+ junctions, have been optimized for application to commercial silicon photovoltaic cells. The porosity and thickness of the PS layers are easily adjusted by controlling the electrochemical formation conditions (current density and anodization time). The appropriate PS formation conditions were determined by carrying out a two steps experiment. A first set of samples allowed to determine the optimal porosity and a second one to adjust the thickness of the PS layers, by evaluating the interference features of the reflectance produced by the layers. A PS layer with optimal antireflection coating (ARC) characteristics was obtained in 30% HF in only 3.5 s. The effective reflectance is reduced to 7.3% between 400 and 1150 nm which leads to a gain of up to 33% in the theoretical short circuit current of a p–n+ shallow junction compared to a reference junction without a PS layer. The effective reflectance with optimized PS layers is significantly less than that obtained with a classical TiO2 ARC on a NaOH pretextured multicrystalline surface (11%).

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.

Control of the work function of GaAs single crystals, under ambient conditions, was achieved by chemisorption of a series of benzoic acid derivatives with varying dipole moments. Quantitative Fourier transform infrared spectroscopy shows that the benzoic acid derivatives bind as carboxylates, via coordination to oxidized Ga or As atoms, with a surface coverage of about one layer and a binding constant of 2.1 104 M-1 for benzoic acid. Contact potential difference measurements reveal that molecules affect the work function by changing the electron affinity while band bending is not affected significantly. The direction of the electron affinity changes depends on the direction of the dipole moments, and the extent of the change increases linearly with the dipole's magnitude. Investigation of the surface composition by X-ray photoelectron spectroscopy shows that the etched surface, onto which the molecules adsorb, is covered by an oxide layer. This may prevent the molecules from affecting band bending.

We report photoelectrochemical measurements of diffusion lengths in p-type multicrystalline silicon used for photovoltaic solar cells. We show that, under some conditions, this method provides a very practical means for rendering an account of the quality nature of this material through diffusion length variations. The results obtained during two years of use of this method in the industrial processing of photovoltaic cells are presented and discussed

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.

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.

We have prepared absorbing structures for photovoltaic cells with different nano-texturization, obtained by means of a femtosecond laser, without the use of corrosive gas (i.e. under vacuum). To take in account the 3D structured front surface, the emitter doping has been realized by using Plasma Immersion Ion Implantation (so-called PULSION). The results show a photocurrent increase up to 60 % in the laser texturized zones.

We report on a method based on photoelectrochemistry which allows the measurement of diffusion lengths of multicrystalline silicon before fabricating the photovoltaic device. The results obtained with this method after two years experience on a production line, by the photovoltaic manufacturer Photowatt Int. are presented. It concerns the variation of the diffusion length within the ingots and its correlation with the short-circuit current of the final cells, and the effect of industrial gettering. We also present diffusion length maps on a 10/spl times/10 cm/sup 2/ cell.

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Formation of Si nanowire (SiNW) arrays using top-down methods based on metal-assisted chemical etching of Si is reported. Three different approaches were tested: i) a two-step process consisting in the electroless deposition of Ag nanoparticles followed by HF/H2O2 etching; ii) simultaneous electroless Ag deposition and etching in HF and highly concentrated AgNO3; iii) a multi-step process using an Au replica mask obtained by Au evaporation on Si samples covered with polystyrene spheres, followed by HF/H2O2 etching. The morphologies of the Si structures obtained in each case are discussed in terms of the influence of different parameters. The third approach led to well controlled etching. Depending on the polystyrene sphere size, Si pillars 1 µm or SiNWs 200 nm in diameter could be obtained.

Bi 2 S 3 thin films with different morphology and crystallite size were electrodeposited on conducting glass substrates from a dimethylsulfoxide solution of bismuth chloride and elemental sulfur at various temperatures. Porous Bi 2 S 3 films, constituted of open-packed nanoparticles ( 500 nm) that constitute a microstructured film. The stoichiometry of the Bi 2 S 3 films was confirmed by chemical analysis. However, X-ray diffraction indicated distortions in Bi 2 S 3 lattice. Deposition temperature was found to influence the distortion, with the smallest distortion observed for the highest temperature (150°C). Optical gaps in the 1.31-1.45 eV range were estimated for the obtained Bi 2 S 3 films. The influence of the morphology and structural defects on the optical properties of Bi 2 S 3 films was also discussed.

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.

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.

Optical and structural properties of thin porous silicon (PS) layers electrochemically formed in the outermost part of 0.3 μm thin emitters of commercial silicon shallow junctions were studied by spectroscopic ellipsometry (SE). The SE data were evaluated with a multiparameter fitting procedure based on the Bruggeman effective medium approximation by using a mixture of fine‐grain polycrystalline Si and voids. The fit results show that the thicknesses of PS films grown at a constant current density of in 30% hydrofluoric acid (HF) with etching times of 1.5 and 3.5 s are 54 and 105 nm, respectively. When the formation time is less than 2.5 s, the porosity is a constant 60% throughout the entire PS layer. A porosity gradient occurs across the layer for formation times greater than 2.5 s, resulting in a layer of up to 85% porosity near the surface after 3.5 s. This increasing porosity is due to chemical dissolution by the HF electrolyte, occurring as a simultaneous reaction to the electrochemical PS formation. All the PS layers grow at a rate of , which is independent of the PS formation time and is not affected by the phosphorus doping gradient in the emitter. The calculated number of elementary charges, z, needed to dissolve one Si atom from the bulk is 3.3. Comparison of the SE fit with total reflectance measurements confirms the validity of the SE modeling. PS layer thicknesses were measured independently of the SE experiments by combining secondary ion mass spectroscopy measurements and surface profiling. © 2000 The Electrochemical Society. All rights reserved.