G. Razzini

Electrochemical Solar Cells with Layer‐Type Semiconductor Anodes: Chemical Treatments of the Crystal Surface

Photoelectrochemical cells based on layer‐type semiconductors (i.e., transition metal sulfides and selenides) have gained substantial interest due to their reasonably high solar efficiency and good stability against photocorrosion. However, the performance of this type of cell may be limited by irregularities in the semiconductor, since edges of the van der Waals surface exposed to the electrolyte may act as recombination centers. To control this effect, chemical surface treatments, based on specific agents which perform selectively on the transition metal atoms at the edge sites, have been studied. In particular, the effect of the disodium salt of the ethylene diamine tetracetic acid (EDTA) molecule (as well as that of many other organic molecules) on the performance of an , sample cell has been investigated. The treatment with EDTA generally improved both short‐circuit current and power output, even if a considerable variation in the response from crystal to crystal was observed. The stability of the EDTA effect under prolonged cell operation was also examined.

Photoelectrochemical visualization in real-time of hydrogen distribution in plastic regions of low-carbon steel

The in-situ scanning photoelectrochemical microscopy is a new technique giving images of hydrogen diffusion in the metals in real time with a good resolution. This paper presents some photoelectrochemical images of hydrogen distribution in a low-carbon steel ahead of a crack tip and in the regions submitted to a high deformation such as in the case of the metal bending.

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.

The photoelectrochemical behavior of CuIn/sub 5/S/sub 8/

The photoelectrochemical behavior of n-CuIn/sub 5/S/sub 8/ a chalcogenide having a spinel-type structure, has been studied in various electrolytes by measuring the spectral response, the current-potential characteristics, an the differential capacitance. The best results have been obtained in an aqueous sulfide-polysulfide solution, where the CuIn/sub 5/S/sub 8/ semiconductor electrode shows an excellent stability. The flatband potential of this semiconductor electrode has been found to depend on the solution composition. The most negative values have been observed in the sulfide-polysulfide solution, owing to specific adsorption of S/sup 2 -/ ions on the electrode surface. Preliminary results on the output power characteristic of the CuIn/sub 5/S/sub 8/ photoelectrochemical cell are also discussed.

The scanning photoelectrochemical microscopy of diffusing hydrogen into metals

Abstract A new photoelectrochemical (PEC) technique is reported to image in real time the hydrogen diffusion into a metal in situ and continuously by means of a scanning laser spot. When a metal such as iron, stainless steel or nickel covered by a passive film is loaded with hydrogen and illuminated, the photoresponse of the passive film (the electrode is anodically polarized in an alkaline solution) is improved owing to the hydrogen photooxidation. This photoresponse is proportional to the hydrogen concentration in the metal. Scanning the metal surface by a laser spot in real time a well-detailed image is obtained of the diffusing hydrogen into the metal at a microscopic level. The present report describes the scanning photoelectrochemical microscopy (SPEM) of hydrogen diffution into an AISI 304 stainless steel foil.

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 treatments of n-CuInSe2 polycrystalline electrodes for photoelectrochemical applications

Abstract Literature results indicate that copper indium selenide (CuInSe2) photoelectrodes show excellent stability when coated by an indium oxide layer and maintained in junction with a “protective” electrolyte, formed by an aqueous solution of the I−/I−3 couple with addition of Cu+ and In3+ ions. Using voltammetric, capacitance, complex impedance and scanning electron microscopy studies, we have investigated further the role of semiconductor surface treatments and of electrolyte composition in controlling the efficiency and the stability of polycrystalline n-CuInSe2 electrodes.

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.