L.Peraldo Bicelli

Electrodeposition and heat treatment of CuIn Se2 films

Thin polycrystalline CuInSe 2 films to be employed in solar cells have been prepared by electrodeposition on a titanium substrate. n-type films were obtained by thoroughly controlling the bath composition (an aqueous solution containing CuCl 2 , InCl 3 and SeO 2 , pH around 1.5) and the operating conditions. These films were then submitted to heat treatment at temperatures up to about 500 °C both in an O 2 - and in an H 2 -containing atmosphere. The as-grown and treated samples were examined by X-ray diffraction, electron microprobe analysis and scanning electron microscopy. Hot probe analyses and resistivity measurements were also performed. The results obtained provided evidence that the quality of the films improves as a result of heat treatments and, in particular, that the mean crystallite size increases. X-ray investigation showed the films to have a high degree of (111) F , (112) U preferred orientation and to contain small quantities of binary phases, possibly indium selenides, which were oxidized at temperatures above 300 °C. Furthermore, macroscopic oxidation of the CuInSe 2 films to In 2 O 3 was detected at the highest temperature in the O 2 -containing atmosphere.

Hydrogen: A clean energy source

Abstract In this review, hydrogen has been considered as a “clean” energy source and carrier and discussed in comparison with present day energy sources mainly based on fossil fuels and nuclear energy. In particular, the environmental and safety issues connected with both nuclear power and coal gasification plants, such as those due to nuclear wastes, acid rain and carbon dioxide, have been considered. Conventional as well as advanced methods of hydrogen production have been examined, attention drawn to direct hydrogen production from alternative energy sources, and an overview provided on the state of the art. A brief insight into hydrogen storage and distribution as well as into conversion and utilization concludes the review.

Electrochemical solar cells with layer-type semiconductor anodes. Performance of n-MoSe2 cells

Abstract The output characteristics and the long-term performances of n -MoSe 2 (I − , I 2 ) electrochemical solar cells have been investigated. It has been confirmed that, by analogy with other layer-type, d-band transition metal dichalcogenide systems, the surface state of the semiconductor plays a key role in the behaviour of the cell. With ‘smooth’ crystal samples, fill factor and efficiency values of the order of 0.6 and 6%, respectively, have been obtained under AM1 illumination. Such performances are, however, drastically reduced if ‘irregular’ crystal samples are used. Control of these undesirable surface state effects has been attempted by chemical treatments specific to the unsaturated transition metal atoms exposed to the electrolyte at the edge sites. Finally, the stability of n -MoSe 2 I − , I 2 ) cells under long time operation, has also been evaluated.

Electrochemical lithiation of Pb3O4

Abstract Pb 3 O 4 has been investigated as a cathode material for voltage compatible, nonaqueous lithium cells and its good performance has been demonstrated. The mechanism of the process has been elucidated by undertaking X-ray and XPS analyses of the discharge products at significant points of the discharge curve. Pb(II) was unambiguously recognized by the latter technique, thus evidencing a two-step reduction mechanism en route to Pb(O) during lithiation.

Study of CdTe electrodeposition from a nonaqueous bath

A detailed investigation has been carried out on the processes occurring during the electrodeposition of cadmium telluride on a nickel electrode from a special bath containing CdCl2, TeCl4 and KI in ethylene glycol at 160 °C. KI was added to the bath both to improve the deposit adhesion on the substrate and to hinder ion complexation. Steady-state current-potential and cyclic voltammetry experiments were performed along with electrolyte ohmic resistance measurements. The experimental results evidenced the fairly good electrochemical behaviour of this organic electrolyte bath and its negligible ohmic resistance. Cyclic voltammetry showed that the reduction of Te(IV) ions occurs via a four-electron process as well as a six-electron process followed by a chemical reaction. Moreover, during slow-scan cyclic voltammetry and steady-state current-potential studies, the underpotential deposition of Cd(II) ions was observed. This behaviour has been attributed to the negative Gibbs free energy change involved in the CdTe-forming reaction.

Thermodynamic evaluation of the n-AgIn5Se8 and n-CuIn5S8 stability in photoelectrochemical cells

The thermodynamic stability of n-AgIn5Se8 photoelectrodes in several electrolytes was evaluated and the results were compared to those recently published concerning their behaviour in photoectrochemical cells. The bond cleavage photoreactions and the oxidative photodecomposition reactions possibly taking place in aqueous solutions containing either the iodide-polyiodide, the sulphide-polysulphide or the Ce3+/Ce4+ redox systems were examined. Reactions forming chalcogen atoms (eventually solvated to sulphide or polysulphide ions in the sulphide-polysulphide electrolyte) plus cations and/or chalcogenides, oxides and iodides were considered. Their Gibbs free energy changes and their decomposition potentials in standard conditions reffered to the normal hydrogen electrode were evaluated. Experimental data were utilised for constructing the compounds energy diagram in open-circuit conditions in order to compare the hole quasi-Fermi level at the surface with the Fermi level relevant to the above processes and to predict the materials stability in different electrolytes. In aqueous sodium sulphate solutions, AgIn5Se8 results to be highly unstable even in rest conditions, in agreement with experimental findings, whereas CuIn5S8 photodecomposes at high reverse bias only. In the I−/I3− containing electrolyte, the AgIn5Se8 situation is even worse since many photocorrosion reactions are thermodynamically possible, while the Fermi level of the redox system is located below the hole quasi-Fermi level at the electrode surface. On the contrary, owing to its favourable energy position, the redox process may positively compete with the photodecomposition reactions of CuIn5S8 in the sulphide-polysulphide electrolyte, though thermodynamics by itself is not able to protect the material. Hence, the observed behaviour depends on the kinetic activation barriers and therefore, on the parameters individually related to the sample quality, surface pretreatment, electrolyte composition, etc. In the Ce3+/Ce4+ solution, CuIn5S8 has a more positive flatband potential than in the other electrolyte studied, and several photodecomposition processes are more probable than the redox process.

Surface behaviour of n-MoSe2 photoanodes in photoelectrochemical solar cells

Abstract This is a brief study of the principles behind photoelectrochemical solar cells and the problems relative to them; we also examined the various types of semiconductive material that are used in these cells. We then focused our attention on monocrystalline electrodes of n-MoSe2 in aqueous solutions containing the iodine-iodide couple. The results of our experiments revealed the importance of the orientation of the electrode surface in this layer-type crystalline material. In fact, electrodes having surfaces which are different from these parallel to the close-packed layers give limited performance. We tried to avoid this phenomenon by intervening with the appropriate chemical complexing agents. Photoelectrochemical measurements were also conducted by illuminating only a limited area of the surface and simultaneously observing the electrode with a metallographic microscope. It was thus possible to identify areas of the surface in which the photoelectrochemical performance was high (active areas) and those where it was low (inactive areas). In addition, when the surface was illuminated, it was noted that even at open-circuit conditions there is photo-oxidation of the iodine-iodide ion in the active areas, with the surface taking on intense colours. We studied this complex phenomenology, and in particular the lack of photocatalytic processes in the inactive regions, in the hope of obtaining material that could offer better performance.

Photoelectrochemical performance of anodic n-TiO2 films submitted to hydrogen reduction

Abstract The photoelectrochemical behaviour of semiconducting n-TiO2 films prepared by anodic oxidation of titanium plates in concentrated alkaline solutions at very high current densities, and subsequently cathodically reduced or thermally reduced in a hydrogen atmosphere, was investigated. The original and reduced films were examined by scanning electron microscopy and X-ray diffraction analysis. Hydrogen reduction improved the photoelectrochemical performance of the oxide giving the best results when reducing the films at 600°C for 2 h. Preliminary water photoelectrolysis experiments showed that hydrogen could be produced at the counter electrode even without applying an external bias voltage, in agreement with the proposed energy diagram for the TiO2 films.

Photoelectrochemical behaviour and XPS characterization of a (Ti,Al,V)O2 film obtained by non-conventional anodic oxidation of a commercial TiAlV alloy

Abstract With the aim to improve the photoelectrochemical behaviour and to reduce the optical band gap of TiO 2 without sacrificing its stability, special n -type (Ti,Al,V)O 2 films have been prepared by anodically oxidizing a commercial titanium alloy following a non-conventional technique. Such alloy contained 6 w/o aluminium and 4 w/o vanadium. The spectral response of (Ti,Al,V)O 2 was dramatically shifted toward the visible region with respect to the response of the parent TiO 2 , the band-gap energy being about 2 V. The photocurrent-voltage characteristics have also been examined, and the flat-band potential determined both from the photocurrent onset potential and from the material bulk electronegativity. X-ray Photoelectron Spectroscopy (XPS or ESCA) coupled with argon ion sputtering has been used to obtain information about the in-depth concentration profile of dopants (Al, V) in the oxidized films. The lack of vanadium ions and the lower content of aluminium ions than that expected on the basis of bulk composition have been clearly evidentiated. The photoelectrochemical behaviour is then discussed both in the light of XPS results and on considering previous works in the field.

Surface characterization of anodic titanium dioxide films for photoelectrochemical solar cells

Abstract Three different titanium dioxide films have been prepared by anodic oxidation of titanium, and their morphology, structure and surface composition are examined, the latter by ESCA analysis. A mutual relationship has been found between the presence of metal atoms, such as sodium, in the bulk of the material and its response in photoelectrochemical cells.