M.E. Rincón

Semiconductor thin films by chemical bath deposition for solar energy related applications

In this paper we present the basic concepts underlying the chemical bath deposition technique and the recipes developed in our laboratory during the past ten years for the deposition of good-quality thin films of CdS, CdSe, ZnS, ZnSe, PbS, SnS, Bi2S3, Bi2Se3, Sb2S3, CuS, CuSe, etc. Typical growth curves, and optical and electrical properties of these films are presented. The effect of annealing the films in air on their structure and composition and on the electrical properties is notable: CdS and ZnS films become conductive through a partial conversion to oxide phase; CdSe becomes photosensitive, SnS converts to SnO2, etc. The use of precipitates formed during deposition for screen printing and sintering, in polymer composites and as a source for vapor-phase deposition is presented. Some examples of the application of the films in solar energy related work are presented.

Effect of Organic and Inorganic Passivation in Quantum-Dot-Sensitized Solar Cells.

The effect of semiconductor passivation on quantum-dot-sensitized solar cells (QDSCs) has been systematically characterized for CdS and CdS/ZnS. We have found that passivation strongly depends on the passivation agent, obtaining an enhancement of the solar cell efficiency for compounds containing amine and thiol groups and, in contrast, a decrease in performance for passivating agents with acid groups. Passivation can induce a change in the position of TiO2 conduction band and also in the recombination rate and nature, reflected in a change in the β parameter. Especially interesting is the finding that β, and consequently the fill factor can be increased with the passivation treatment. Applying this strategy, record cells of 4.65% efficiency for PbS-based QDSCs have been produced.

Visible light-induced degradation of blue textile azo dye on TiO2=CdO-ZnO coupled nanoporous films

Abstract The photodegradation of a typical textile blue azo dye, followed by UV–VIS spectra analysis, has been carried out successfully under white light illumination on TiO 2 /CdO–ZnO nanoporous coupled thin films. A relatively fast degradation occurs in dye solutions with concentrations of 100 mg/l (pH=3), at temperatures of 85°C, and with the aid of 400 mg/l hydrogen peroxide. Photodegradation also occurs on nanoporous TiO 2 films but with significant lower efficiency than on TiO 2 /CdO–ZnO coupled nanoporous films. Dye photodegradation does not occur on TiO 2 /CdO or TiO 2 /ZnO nanoporous films, suggesting that both CdO and ZnO components are required on the sensitization of TiO 2 nanoporous films. A combined effect of new sensitizing interband states (response to white illumination) and/or rectification phenomena (improved charge separation) may be responsible of the higher photocatalytic activity of the TiO 2 /CdO–ZnO nanoporous films. Similarly, the alternative route for visible degradation, the photosensitized degradation mechanism, could also benefit from the coupled nanoporous films due to a higher driving force for electron injection (dye oxidation).

Thermal treatment effects in the photovoltaic conversion of spray-painted TiO2 coatings sensitized by chemically deposited CdSe thin films

Abstract We have investigated the photovoltaic conversion of spray “painted” (spray deposited) titanium dioxide coatings sensitized with chemically deposited cadmium selenide thin films. The structural, optical and photoelectrochemical characterization of the composite films indicate the importance of thermal treatments in improving the photocurrent quantum yields. Up to 400°C, the effect of air annealing is to shift the onset of absorption to longer wavelengths and to improve the photocurrent substantially. Thermal treatment above 450°C converts the heterosystem into a cadmium titanate with poor photoelectrochemical properties. Due to the thin nature of the CdSe film it is used as sensitizer, the presence of the TiO 2 matrix helps to improve the performance of the chemically deposited films. That is, higher photocurrents and a red shifted spectral response were obtained in the sensitized films, when compared to the spectra of the corresponding CdSe used as sensitizer. This behavior, along with the good photostability of the coatings, is promising for the use of the sensitized film in photocatalytic as well as photovoltaic applications.

Photoelectrochemical behavior of chemically deposited CdSe and coupled CdS/CdSe semiconductor films

Abstract Photoelectrochemical effects at chemically deposited CdSe thin films (2000xa0A) coupled with as-prepared and air annealed (250°C) CdS films have been investigated by monitoring open-circuit voltage (Voc) and short-circuit current density (Isc) at varying incident light intensities and for different heat-treatments temperatures. Two consecutive chemical baths were used in the coupled system. Each bath has been optimized in earlier studies for the deposition of highly photosensitive CdS and CdSe thin films. The photoelectrochemical behavior of single and coupled films was investigated in ferricyanide redox couples. The enhanced short-circuit photocurrent of the as-deposited CdS/CdSe system, despite their lower photosensitivity, indicated that charge separation improved in the coupled system. The role of post-deposition thermal treatments in improving the photoelectrochemical cell characteristics and stability of coupled semiconductors was investigated. Excellent I–V properties were obtained for CdSe and CdS250/CdSe photoelectrodes annealed at 280°C. For the coupled system: Voc=960xa0mV; Isc=8.6xa0mA/cm2; fill factor (ff)=0.53 and cell efficiency (η)=4.2%. The linearity of Voc/ln(IL) and Isc/IL plots supports the Schottky–Mott model for these interfaces. The stability of the coupled photoanode is superior to that of the CdSe only-film for the initial 3xa0h.

A comparison of the various thermal treatments of chemically deposited bismuth sulfide thin films and the effect on the structural and electrical properties

Abstract The physicochemical transformations of chemically precipitated bismuth sulfide powder have been studied by differential scanning calorimetry (DSC) and X-ray diffraction analysis (XRD). The structural changes have been correlated to the electrical properties of chemically deposited bismuth sulfide thin films annealed at different temperatures ( T ) and pressures ( P t ). Two irreversible exothermic transformations are observed at 210 and 253°C in the powder, which correspond to the crystallization of Bi 2 S 3 , and to the crystallization and decomposition of local defects and their reaction to form Bi 2 S 3 . As in the case of the precipitate, the conductivity of chemically deposited bismuth sulfide thin films upon annealing is due to the formation of multicomponent phases assisted by further sulfur loss. The complex mechanism that enhances the film conductivity shows a compensating effect between P t and T . The temperature and pressure regime at which conductive films were obtained with Bi 2 S 3 as the only component of the crystalline phase were: 190°C under vacuum, and 250°C in the P t range of 10–1000 mbar. Under these conditions, the dark conductivity and photoconductivity of the films are in the range of 10 −2 to 3 Ω −1 cm −1 , mainly determined by the charge transport through Bi 2 S 3 crystals.

Inclusion of Bi2S3 nanoparticles in polypyrrole thin films electropolymerized on chemically deposited bismuth sulfide electrodes: synthesis and characterization

The successful electropolymerization of pyrrole and its codeposition with Bi2S3 nanoparticles on chemically deposited bismuth sulfide substrates is described. The materials were designed to explore new approaches to improve light-collection efficiency in polymer photovoltaics. We report the effect of bismuth sulfide nanoparticles on the electropolymerization of pyrrole and on the morphology and optoelectronic properties of the composite film. The differences observed on the optical band gaps, photoaction spectra and open-circuit photovoltages, led to the conclusion that polymerization under the presence of nanoparticles produces tighter and thinner polymeric coatings than the ones obtained without nanoparticles. Under illumination, the composite polymeric coating has low absorption and high electronic conductivity, typical of the oxidized form of polypyrrole. This suggests a photoinduced charge transfer reaction compatible with the electron-accepting nature of bismuth sulfide nanoparticles. When subjected to long time illumination, atomic force microscopy, optical characterization and photoelectrochemical studies showed that the presence of Bi2S3 nanoparticles in the polymeric film improves the efficiency of the coating against photocorrosion.

Effect of Bi2S3 nanoparticles in the protection mechanism of polypyrrole thin films

Abstract Polycrystalline Bi2S3 electrodes coated with thin films of polypyrrole (PPy) and polypyrrole with inclusions of bismuth sulfide nanoparticles (PPy-Bi2S3(np)) were subjected to long time illumination in polysulfide media. Atomic force microscopy (AFM), optical characterization and photoelectrochemical (PEC) studies showed that the presence of Bi2S3 nanoparticles in the polymeric film improves the efficiency of the coating against photocorrosion. The data also suggest different protection mechanisms for single and composite PPy films.

TiO2/clinoptilolite composites for photocatalytic degradation of anionic and cationic contaminants

The present work aims to study the bulk and surface properties of the TiO2/clinoptilolite composite on the crystalline structure, superficial area, bandgap energy, zeta potential, particle size distribution, and chemical composition; in order to analyze the effect of the clinoptilolite proportion in the photocatalytic degradation of pollutants. TiO2/clinoptilolite composites were prepared by adding different mass proportions of clinoptilolite to a sol–gel bath containing TiCl4 as the titania precursor. Surface charge studies explain the larger sensitivity to composite ratio observed in the photocatalytic degradation of anionic pollutants than in cationic dyes. An optimum TiO2/clinoptilolite ratio of 90/10 was found to be the most efficient in terms of lower tendency to agglomeration, largest surface area, and increased crystallite size. Improvement in composite surface area occurs only at low clinoptilolite wt% and seems to be caused by lower agglomeration of nanometric TiO2 and acid-induced porosity in the zeolite.

Carbon nanotubes/carbon xerogel-nafion electrodes: a comparative study of preparation methods

We report the preparation and electrochemical characterization of carbon nanotubes (CNT)/carbon xerogel-nafion (CXN) electrodes obtained by casting carbon nanotube inks on carbon xerogel-nafion matrixes under terrestrial (g) and enhanced (13u2009g) gravity. The impregnated electrodes were compared with composites prepared by mixing CXN dispersions with CNT inks. For casted CNT, alternate current scanning electrochemical microscopy studies along the film-electrode area showed differences that can be correlated with the position of the electronic conducting CNT phase on the more resistive matrix. It revealed the transition from a conductive to dielectric surface when impregnation takes place at terrestrial and enhanced gravity, respectively. Although the addition of CNT enhances the capacitance and mechanical properties of CXN in all preparation methods, the largest specific capacitance was observed in electrodes impregnated at 1u2009g. Electrodes prepared by mixing and those casted at 13u2009g show similar capacitance values regardless of contrasting conductivity. A mechanism explaining the microstructural, electrical, and adsorptive differences brought out by the various preparation methods is proposed.