Enrique Barrera

A new cobalt oxide electrodeposit bath for solar absorbers

Abstract A study was carried out in a Hull cell in order to optimize the deposition conditions of cobalt xide (black cobalt) in an electrolytic bath, which uses cobalt nitrate for direct obtention of black cobalt. Thermal stability of the material was surveyed on several samples of black cobalt prepared on stainless-steel with a thickness of approximately of 2.5 μm. It was found that the optical properties change, in respect to the initial values, with time of treatment until an equlibrium is reached. This equilibrium depends on the substrate and the temperature of the treatment used.

Formation Mechanisms and Characterization of Black and White Cobalt Electrodeposition onto Stainless Steel

Cobalt electrodeposition onto a stainless steel substrate from 1.17 M Co(II) aqueous solution containing 0.98 M H 2SO4, 0.56 M KCl, and 0.2 M H3BO3 was evaluated in the absence (i) and presence (ii) of 0.1 M KNO3. Cobalt electrodeposited from the electrolytic bath (i) was white-gray colored, whereas deposition from bath ( ii) formed a black-colored surface. SEM-WDX, AFM, and XRD analysis of the steel surfaces covered with these two deposits revealed distinct characteristics for black and white cobalt fil ms. Although both deposits were composed of metallic cobalt, the white cobalt deposit was a smooth, 2D film while the black deposit consisted of many dispersed, nano-sized clusters of 150 to 250 nm in diameter. Analysis of potentiostatic current transients ( I-t curves) indicated that formation of white cobalt was carried out by multiple 3D nucleation limited by lattice incorporation of cobalt a datoms to the growth centers. Formation of black cobalt was shown to involve the simultaneous processes of 3D nucleus formation and growth, limited by mass transfer, and the reduction of nitrates in the medium onto the surfaces of these nuclei. It is shown that , beside this cobalt-nitrate interaction, NO 3 ions in solution can block active sites for cobalt reduction and the effect of this phenomenon

Cobalt oxide films grown by a dipping sol-gel process

Cobalt oxide thin films were prepared by the dipping sol-gel process, using two different inorganic precursors: cobalt chloride and cobalt nitrate salts. Also, samples with a different number of dipping-annealing cycles (3, 5, and 7) where prepared. Composition, structure, surface morphology and optical properties of such films have been characterised by means of X-ray diffraction, differential thermogravimetric analysis (DTA), transmittance spectra and atomic force microscopy (AFM). The results show that starting from distinct precursors leads to different properties: film water contents, surface roughness, crystallite size, total film transmittance, absorption coefficient and refractive index. Absorption coefficients higher than 104 cm−1 where found for all the samples. Refractive indices vary from n ~ 1.9–2.8 in the near infrared region. Our study shows that using a relatively simple preparation method like the sol-gel process, cobalt oxide films with specific properties, can be made.

Improving the contact properties of CdS-decorated TiO2 nanotube arrays using an electrochemical/thermal/chemical approach

Decoration of TiO2 nanotube films (TiO2 nanotube arrays (TNAs)) with CdS nanoparticles has been pursued for a broad range of applications that goes from solar cells to biological sensors. In most synthesis methods, the scale-up of devices has been challenging due to the poor contact at the chalcogenide/oxide interface. In this work, we validate the electrochemical/thermal/chemical route as a superior strategy to sensitize TNAs with CdS nanoparticles when compared with conventional methods. The process consisted of (i) electrodeposition of cadmium on TNAs to ensure strong bonding between TiO2 and Cd precursor particles, (ii) air annealing of Cd-decorated TNAs to thermally oxidize cadmium to cadmium oxide, and (iii) total sulfurization of cadmium oxide to obtain CdS in an hexagonal phase matching that of TNAs. X-ray diffraction (XRD), scanning electron microscopy (SEM), and X-ray photoelectron spectroscopy (XPS) analyses indicated the complete transformation of cadmium precursor particles into CdS and a good surface coverage of the internal/external walls of TNAs. When compared to samples prepared by successive ionic layer adsorption and reaction (SILAR), electrochemical impedance spectroscopy data revealed the improvement of the electrical properties of the TNA matrix due to the sulfurization process and a lower contact resistance at the CdS/TNA interface. These improvements explain the superior photoelectrochemical response of CdS/TNA photoelectrodes obtained by the electrochemical/thermal/chemical route.

Towards cost-effective antireflective-coating and surface-texturing

Methods for reducing surface reflectance in crystalline-silicon (c-Si) based solar cells is briefly reviewed. In general, an antireflection coating (ARC) is designed for normal light incidence, which consists of a single quarter-wavelength (λ/4) layer with a proper optical transparency. Theoretical and experimental comparisons were carried out for both, single and double ARC using silicon nitride (SiNx) and silicon oxide (SiOx). Double ARC for MgF/CeO, ZnS, TiOx is also considered to compare with the previous coatings. Solar cell surface texturization and its effects are also discussed. Concepts of reactive ion etching (RIE) and its performances on c-Si based surfaces are viewed. Finally, graded-index ARC performances as a function of light incidence angles are discussed.