Photoelectrochemical Spectroscopy Research Articles

Surface morphology control of Cu2ZnSnS4 by addition of cellulose in solution process for high performance superstrate solar cell

Hydroxypropyl cellulose is added into the precursor solution to deposit Cu2ZnSnS4 (CZTS) thin film for superstrate solar cell. The crystallinity and morphologies of CZTS thin films can be manipulated by adding a small amount of hydroxypropyl cellulose. With addition of 1.0 wt% hydroxypropyl cellulose, the crystallinity and grain size of CZTS are obviously improved, and the light-to-electric conversion efficiency of CZTS thin film solar cell is enhanced from 2.07% to 3.92%. The photoelectric performance enhancement mechanism is analyzed by the measurements of electrochemical impedance spectroscopy and intensity modulating photoelectrochemical spectroscopy of CZTS thin film solar cells and certificated to be formation of the bigger CZTS grains due to addition of hydroxypropyl cellulose, which reduces the boundary defects and makes the film surface be more continuous. These results suggest that addition of hydroxypropyl cellulose into the precursor solution is an effective way to obtain high performance CZTS thin film solar cells.

Template Electrochemical Growth and Properties of Mo Oxide Nanostructures

This work is aimed at studying the growing process of nanostructures electrodeposited from molybdate aqueous solutions at different pH values into pores of polycarbonate membrane templates. The challenging issue was the opportunity to investigate a rather complex deposition process in a confined ambient, where electrochemical conditions are quite different from those usually established for deposition on a flat substrate. Nanostructures were grown from a bath containing Mo7O246– (NH4)6Mo7O24·4H2O) at different concentrations (50–100 g/L), at a constant cathodic current density of 2 mA/cm2 (electrodeposition area ∼8 cm2). Nanostructured deposit was characterized by XRD, EDS, Raman, XPS, and photoelectrochemical spectroscopy. Several parameters have been investigated, such as electrodeposition time, concentration of Mo precursor, and pH. These parameters do not influence the nature of deposit that always consists of mixed valence molybdenum oxides, whereas the nanostructure morphology changes with pH. Nanostructures were found amorphous and consisting mainly of MoO2 underneath α-MoO3. Photoelectrochemical spectroscopy revealed n-type conductivity and two linear regions in the (Iph·hν)0.5 vs hν plot, giving optical gaps of 2.5 and 3.2 eV, typical of MoO2 and α-MoO3, respectively. To our best knowledge, this is the first paper dealing with the fabrication of a uniform array of MoO2/MoO3 core–shell nanowires by template electrosynthesis.

Study of ion adsorption at passive iron by using the impedance and photoadmittance methods

Intensity-modulated photocurrent (photoadmittance) and electrochemical impedance of anodicoxidized iron electrode in neutral nitrate solutions and in the presence of Ba2+, Ca2+, Cl−, and C6H5COO− (benzoate) are studied. It is shown that the ion adsorption at passive iron affects but slightly the system’s impedance; by contrast, it affects the photocurrent value significantly: when adsorbing, the anions increase the photoeffect, while the cations decrease it. These effects are associated with the potential drop redistribution in the Helmholtz layer and the film. The dissimilar changes of the generation current in the presence of similarly charged ions at their equal concentration evidence their different adsorption activity. The correlation between the generation current and surface-active ion concentration in solution is found. The photoelectrochemical spectroscopy allows evaluating qualitatively the surface-active ion adsorption at the passive iron and judging on the ion adsorption by the dependence of the generation current on the ion concentration.

Semiconductor properties and protective role of passive films of iron base alloys

Semiconductor properties of passive films formed on the Fe–18Cr alloy in a borate buffer solution (pH = 8.4) and 0.1 M H 2SO 4 solution were examined using a photoelectrochemical spectroscopy and an electrochemical impedance spectroscopy. Photo current reveals two photo action spectra that derived from outer hydroxide and inner oxide layers. A typical n-type semiconductor behaviour is observed by both photo current and impedance for the passive films formed in the borate buffer solution. On the other hand, a negative photo current generated, the absolute value of which decreased as applied potential increased in the sulfuric acid solution. This indicates that the passive film behaves as a p-type semiconductor. However, Mott–Schottky plot revealed the typical n-type semiconductor property. It is concluded that the passive film on the Fe–18Cr alloy formed in the borate buffer solution is composed of both n-type outer hydroxide and inner oxide layers. On the other hand, the passive film of the Fe–18Cr alloy in the sulphuric acid consists of p-type oxide and n-type hydroxide layers. The behaviour of passive film growth and corrosion was discussed in terms of the electronic structure in the passive film.

Capacitance and photoelectrochemical studies for the assessment of anodic oxide films on aluminium

Photoelectrochemical spectroscopy and capacitance measurements were used in this work to assess the electronic properties of the oxide films formed on 99.5% aluminium and 2024-T3 aluminium alloy by anodising in a sulphuric-boric bath. The morphology of these films was also studied by transmission electron microscopy cross-section observations. The results obtained indicate that the oxide films formed on aluminium show a n-type semiconductive behaviour, with bandgap energies that are identical for the oxides studied, despite their different characteristics. It was found out that capacitance measurements may be used as a valuable technique for the assessment of the quality of anodised layers, allowing the distinction between an efficient and an inefficient sealing. Therefore, they may be used to predict the corrosion resistance of these materials.

Porphyrin dyes for TiO2 sensitization

A series of six new free base porphyrins were synthesized for use as photosensitizers in TiO2 dye-sensitized photo-electrochemical cells. The porphyrin sensitizers are attached to the TiO2 photoelectrode by phosphonic or carboxylic acid anchoring groups. These anchoring groups were placed on different substitution positions on the porphyrin moiety. The new dyes were fully characterized by absorption and emission spectroscopies, electrochemistry and photo-electrochemical spectroscopy. The photo-electrochemical performances of the sensitizers are discussed and compared to the known 5,10,15,20-tetra(4-carboxyphenyl)porphyrin sensitizer. In this study, we show that the nature of the anchoring group (phosphonic or carboxylic acids) has little impact on the photo-electrochemical performance of the cell. However, the substitution position of the anchoring group on the porphyrin strongly influences the monochromatic photon-to-electron conversion efficiency of the resulting cell. The results indicate that the electronic coupling of this type of dye with the d-band of the semiconductor is one of the key parameters in the design of efficient sensitizers.

Photoelectrochemical study on the passive film on Fe

Electronic properties of the passive film formed on Fe in pH 8.5 buffer solution were examined by the photoelectrochemical spectroscopy. Photocurrent responses for the passive film on Fe with film formation potential are explained by the modified band structure model developed on the basis of that for crystalline spinel (γ-Fe 2O 3) involving two types of electronic excitation processes. i.e., the p–d and the d–d transition. The photocurrent spectra for the passive film on Fe measured in a wide range of photon energy could be resolved into two components of photocurrent spectrum reflecting effects of each electronic transition on the photocurrent spectra. The band gap energy of each electronic transition process was determined from the resolved photocurrent spectrum; 2.87 eV for the d–d transition, and 3.67 eV for the p–d transition. A small photocurrent peak at 2.7 eV appeared at film formation potentials higher than 400 mV was associated with the formation of γ-FeOOH layer on γ-Fe 2O 3 film.

Correlation between surface properties and photocatalytic and photoelectrochemical activity of In 2O 3 nanocrystalline films and powders

Nanocrystalline In 2O 3 thin films and powders have been prepared by sol–gel procedure and characterized by various bulk and surface techniques (X-ray diffraction, X-ray photoelectron spectroscopy and photoelectrochemical spectroscopy). Photocatalytic reactivity for rhodamine 6G photodestruction and photoelectrochemical (PEC) properties have been studied for the In 2O 3 samples heated at different temperatures. The photocatalytic activity of the powder grows initially with increasing annealing temperature from 200 to 300°C and then drops sharply with further increase in temperature. A strikingly large difference in photocurrent onset potentials and photocurrent quantum yields has been revealed for the films heated at 200 and 400°C. The susceptibility of the oxide surface to hydroxylation appears to be a key factor responsible for the sharp change in photoreactivity and PEC properties.

Characterization and photoelectrochemical properties of nanocrystalline In 2O 3 film electrodes

The photoelectrochemical properties of nanocrystalline In 2O 3 film electrodes prepared by sol-gel method have been studied. The surface and bulk characterization of sol-gel-derived In 2O 3 films and powders heated at different temperatures has been performed using X-ray diffraction analysis, X-ray photoelectron, IR-absorption and photoelectrochemical spectroscopies. A significant shift of the photocurrent onset potential in the positive direction and a rise in the photocurrent quantum yield are observed as the temperature of the electrode heat treatment increases from 200 to 400°C. A tentative explanation for this effect has been provided, taking into account the formation of the finest hydroxide layer at the nanocrystallite surface of slightly heated In 2O 3 films.

Photoelectrochemical Studies of CdS Nanoparticle-Modified Electrodes

The kinetics of charge transfer at an electrode modified with a submonolayer of CdS nanoparticles is considered using intensity modulated photoelectrochemical spectroscopy (IMPS). First, the method of preparing an electrochemically addressable layer of CdS nanoparticles is detailed. Then a model of charge transfer from nanoparticle-modified electrodes is developed, and equations for the AC photocurrent are derived. Excellent agreement between the experimental and theoretical responses for an electrode coated with nanoparticles of approximately 5-nm radius is obtained. It is demonstrated how rate constants for both hole and electron transfer from semiconductor nanoparticles may be determined.

Comparison of CdS thin films prepared by different techniques for applications in solar cells as window materials

CdS thin films as window materials for solar cells have been prepared by three procedures; chemical bath deposition, electrodeposition in an aqueous medium at 80 °C and electrodeposition in a non-aqueous medium at 170 °C. As deposited films along with those obtained after annealing in air at 400 °C for 15 min were studied using X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES), scanning electron microscopy (SEM), optical absorption spectra and photoelectrochemical spectroscopy (PEC) techniques under identical experimental conditions. X-ray diffraction data indicate the formation of hexagonal CdS as the predominent phase, but the SEM studies show that their textures are widely dependent on the conditions employed. GDOES profiling indicates the incorporation of Na and Si into CdS films prepared by all three techniques. Annealing of chemical bath deposited films causes a red shift of the absorbance edge and also a shift in the maxima of the photocurrent action spectra towards the low energy side. However, this effect was comparatively negligible for the samples prepared by the other two techniques. PEC studies indicate that CdS materials grown by all three techniques are all n-type. All studies indicate that the films grown at 170 °C using non-aqueous solutions are of better crystallinity and of improved electrical properties. © 1998 Kluwer Academic Publishers

Photoelectrochemistry of the insertion compounds Na xInSe and Li xInSe

The insertion reaction of Li and Na in lamellar InSe single crystal was performed electrochemically in a non-aqueous electrolyte at room temperature. The insertion product was, at the electrode surface, an intercalation compound (Li x InSe, Na x InSe) or the result of the chemical decomposition of the electrode material (Li 2Se, Na 2Se). The development of the reaction was followed by photoelectrochemical spectroscopy and by cyclic photocurrent voltammetry performed in situ in the reaction electrolyte. A Fermi level shift in the intercalation compound was deduced by a flatband shift to lower potential and a small increase of the bandgap energy resulted from the intercalation reaction. The Na-inserted samples have been analyzed by grazing angle incidence X-ray diffraction and an increase in the interlayer distance was observed.

Electrochemically intercalated MxC60 thin films in a solid state cell (M = Li, K): Optical and photoelectrochemical characterization

Solid-state electrochemical cells have been prepared with C60 vacuum-evaporated thin films, a Li- or K-source counter electrode and a polymer PEO-LiClO4 (PEO-KClO4) electrolyte. The electrochemical intercalation in C60 of Li+ (or K+) ions has been performed under constant current conditions up to a formal stoichiometry of the fulleride film equal to Li12C60 (K5C60). A complete charge-transfer pocess from the intercalated alkali to the alkali-metal compound has been assumed. Several quasi-equilibrium potential plateaux were observed during intercalation, that we associate with the coexistence of phases with different intercalant concentration. The electrochemical intercalation process is irreversible to a large extent. Optical and photoelectrochemical spectroscopy of the fulleride films was done “in-situ” at different moments of the intercalation reaction by illuminating the film electrodes through the transparent and conducting glass substrates. The photo electrochemical spectral response agrees well with the optical absorption spectra, both indicating a bandgap of 2.2 eV. The photoelectrochemical response shows a minimum forx = 3 (in the KxC60 compound) and a maximum atx = 4 (in both KxC60 andLixC60 compounds), in agreement with previous conductivity results. NIR diffuse reflectance spectra of the solid-state cell show absorption bands in the fulleride films at a wavelength of 1100 nm. A band-energy diagram has been proposed for the MxC60/PEO-MClO4/M electrochemical cell.

Properties of a new, low bandgap photoconducting phase of lead oxide

Atomic sputtering has been employed to fabricate a new, thin ( ⩽ 900 nm) n-type semiconducting phase of lead oxide of approximate stoichiometry PbO 1.50. Films 120 to 880 nm in thickness deposited in a 100% O 2 plasma and vacuum annealed for 8 h reproducibly exhibited indirect bandgaps of ~1.18 eV, while films of ~ 320 nm and vacuum annealed for ~1 h displayed higher bandgaps of ~1.30 eV. Films deposited in a 50:50 mixture of O 2 and N 2 resulted in an oxide displaying a bandgap of ~1.44 eV. All films thicker than 400 nm required vacuum annealing at ~ 250° C for periods of time > 2 h before any photoelectrochemical response was observed. The lower bandgap oxide phases are stable in aqueous alkaline media over a narrow potential range of ~ 0.20 V both in the dark as well as under chopped white light illumination (~ 200 mW/cm 2). These films have been characterized using a variety of techniques including XPS, Auger, X-ray diffraction, impedance and photoelectrochemical spectroscopies.

Cathodic generation of CdSe xTe 1− x at Te film electrodes

CdSe x Te 1− x films have been electrodeposited by the co-reduction of a tellurium substrate (to telluride) and soluble SeSO −2 3 (to selenide) in the presence of cadmium complexes. The tellurium film is itself initially electroplated in acidic media from tellurite. The necessary sequence of electrode reactions to allow CdSe x Te 1− x compositions to be formed is regulated by the potentials maintained in the pH 9.6 ammonia buffer-ethylene diamine tetraacetic acid solution. Photoelectrochemical spectroscopy and X-ray diffraction of annealed films were used to characterize the products. In the absence of cadmium ion, the method allows generation of known fluxes of telluride and selenide downstream of the generating electrode surface for other synthetic purposes.

Spectroscopic studies of semiconductor/electrolyte interfaces

In this paper, several new spectroscopic techniques are described which can provide important information on semiconductor/electrolyte interfaces. These are stress modulated, photomodulated, and potential modulated photoelectro-chemical spectroscopies. Two-beam spectroscopy is also used to investigate the behaviour of several semiconductor/electrolyte systems as a function of applied potential and redox couple.

Photomodulated and One‐Beam and Two‐Beam Potential Modulated Photoelectrochemical Spectroscopies

Three techniques for measuring interband transitions in semiconductor electrodes in electrolytes are described. These are photomodulated and one‐beam and two‐beam potential modulated spectroscopies. The usefulness of the techniques is demonstrated by using each technique to measure the direct band gap transition for a single crystal electrode in a electrolyte. The difference between a two‐beam potential modulated photoelectrochemical spectrum and a corresponding one‐beam spectrum is interpreted to be a measure of minority carrier recombination inside and at the surface of the semiconductor electrode. Peaks in the modulated photocurrent spectra at the direct bandedge of are attributed to field induced absorption effects.