M.T. Gutiérrez

Deposition of transparent and conductive Al-doped ZnO thin films for photovoltaic solar cells

Abstract The effect of the substrate temperature on the optoelectronic properties of ZnO-based thin films prepared by rf magnetron sputtering has been studied. Three different targets (Zn/Al 98/2 at%, ZnO:Al 98/2 at% and ZnO:Al2O3 98/2 wt%) have been investigated in order to compare resulting samples and try to reduce the substrate temperature down to room temperature. From the ZnO:Al2O3 target, transparent conductive zinc oxide has been obtained at 25°C with the average optical transmission in the 400–800 nm wavelength range, T = 80–90% and resistivity, ϱ = 3−5 × 10−3 Ωcm. In Al:Zn0 layers, the spatial distribution of the electrical properties across the substrate placed parallel to the target has been improved by depositing at high substrate temperatures, above 200°C. Besides, owing to diffusion processes of CuInSe2 and CdS take place at 200°C, an AI:ZnO/CdS/CuInSe2 polycrystalline solar cell made with the Al:ZnO deposited at 25°C as the transparent conductive oxide, has shown a more efficient photovoltaic response, η = 6.8%, than the one measured when the aluminium-doped zinc oxide has been prepared at 200°C, η = 1.8%.

Preparation and characterization of sol-gel TiO2 antireflective coatings for silicon

Sol–gel polymeric TiO2 films have been obtained by dip coating. These films showed antireflective properties on silicon substrates as well as suitable adherent properties. The film thickness varied from 55 to 122 nm, and the refractive index presented values between 1.95 and 2.10. Both these parameters can be easily tailored by varying the withdrawal rate and the [TbuTi]/[Ethanol] ratio in the solution. The structural properties of the films showed that amorphous films are obtained by sintering at 450°C, meanwhile, the nanocrystalline titania anatase phase is formed after heating the films at 850°C for 5 s. By using these films, the hemispherical reflectance of polished silicon wafers is reduced in all the wavelength range and particularly, it is decreased from 37 to 1.5% at approximately 600 nm. At the same time, the solar averaged reflectance of the uncoated silicon is reduced from 0.38 to 0.12 after the sol–gel TiO2 coating.

SnO2 substrate effects on the morphology and composition of chemical bath deposited ZnSe thin films

ZnSe thin films are deposited by the chemical bath deposition (CBD) method onto different glass and SnO2 coated glass substrates. Three SnO2 substrates with different grain size, roughness, conductivity and optical transmission are used. The roughness of the surfaces increases after CBD deposition of ca 0. l μm thick ZnSe films and further upon heating at 200°C due to recrystallisation of the deposit. This effect seems more intense the bigger the grain size of the SnO2 substrate. Above 400°C annealing temperature, the roughness of the films diminishes below that of bare SnO2 substrates, after conversion of ZnSe into a ZnO film. For films deposited on glass substrates, the annealing process gives rise to the roughest surfaces, due to a weaker film-substrate interaction and more important recrystallisation. The optical characterisation shows a decrease in the integral transmittance by about 15% for all the substrates studied after deposition of the ZnSe film. On the glass substrate, the annealing treatment shifts the absorption edge to longer wavelengths reflecting the important recrystallisation. This effect does not take place with SnO2 substrate due to the better crystallinity of the initial film and the stronger film-substrate interaction. Transmittance increases after annealing at 400°C due to the formation of the ZnO film. Compositional analysis by XPS shows that films are composed of a mixture of ZnSe and ZnO, the proportion differing among substrates. In general, films deposited on SnO2 have higher proportion of ZnO than on glass substrate. We attribute this effect to differences in the chemical deposition process induced by the substrate surface.

Photovoltaic windows by chemical bath deposition

Abstract The paper presents a scope of different studies performed on thin-film materials, commonly used as window layers in polycrystalline thin-film solar cells, and prepared by the chemical bath deposition (CBD) method. The presented studies try to offer an approach to some key points of the chemical preparation that are directly related to the final quality and properties of the films. Results on cadmium sulphide (CdS), cadmium sulphide-transparent conductive oxides interfaces (CdS/TCO (ZnO and ITO)), zinc selenide (ZnSe) and indium hydroxy-sulphide In(OH) x S y , are presented and discussed.

Sol–gel TiO2 antireflective films for textured monocrystalline silicon solar cells

Abstract The aim of this study is validate the sol–gel method as a procedure to prepare antireflective films (AR) for textured monocrystalline solar cells. In this way, sol–gel TiO 2 films were deposited on both microscope slides and textured monocrystalline silicon solar cells by dip coating. The solutions were prepared by mixing tetrabutyl ortotitanate, water and ethanol using a basic compound as catalytic agent. The sintering of the samples was performed in the 400–800 °C range, by using a heating rate of 30°/min. It was observed that the sintering atmosphere influenced the morphological and antireflective properties of the films. The film thickness was tailored by varying the withdrawal rate. GAXRD analysis showed that nanocrystalline titania anatase and rutile were formed and the average crystallite size increased with the sintering temperature. The solar averaged reflectance of the textured monocrystalline solar cells was reduced from 0.09 to 0.02 after the sol–gel TiO 2 deposition.

Morphological and compositional study of CBD-ZnSe thin films by microscopy techniques and angle resolved XPS

Abstract A morphological and compositional study is carried out on ZnSe thin films obtained by the chemical bath deposition (CBD) method. SEM, TEM and AFM images are shown for the morphological characterisation. Angle-resolved XPS (ARXPS) measurements are used for the study of surface and subsurface composition of the films. The composition underneath is studied with XPS analysis of films eroded by sputtering. It is found that films have a mixed ZnSe-ZnO (or Zn(OH)2) composition. The Zn/Se ratio in the film increases with depth, indicating that Zn is preferentially as ZnO-Zn(OH)2 close to the film-substrate interface, and that the ZnSe proportion increases above. Such composition inhomogeneity is attributed to a change in the deposition mechanism during film growth. At first, it proceeds via reaction of adsorbed Zn and Se precursors, and then by deposition of ZnSe clusters formed in the bulk of the solution. Apparently the first mechanism is less efficient for the formation of a pure ZnSe film, at least under the experimental conditions used here, hence gives rise to higher concentration of Zn oxides close to the film-substrate interface. At longer times during the deposition process, the cluster precipitation mechanism predominates and, consequentially, the top layers of the film become richer in ZnSe but less compact. Other compounds detected by ARXPS are Se0 occluded in the bulk of the film, and some SeO2 at the surface. Annealing at 300°C results in structural and compositional changes which involve compaction of the films, the loss of the occluded Se0, the increment of the SeO2 overlayer and the transformation of Zn(OH)2 into ZnO and/or Zn(O, Se) compounds.

Optimisation of indium tin oxide thin films for photovoltaic applications

Abstract The influence of deposition parameters on optoelectronic and structural properties of Sn-doped In2O3 thin films grown by r.f. magnetron sputtering has been investigated. Two different targets, In/Sn (95/5 at.%) and In2O3:SnO2 (95/5wt.%) have been studied in order to compare resulting samples and try to reduce the substrate temperature down to room temperature. By using the In/Sn target, transparent conductive indium tin oxide has been obtained at a substrate temperature of 400 °C, with T = 80–90% and p~ 10−4 Ω cm. Meanwhile, low sheet resistance, 5–15 Ω/□, and high transmittance in the visible range, 80–90%, have been measured for ITO coatings made at room temperature with the oxidized target, by introducing very low O2 mass-flow rates in the sputtering chamber.

Properties of RF sputtered zinc oxide based thin films made from different targets

Abstract The effect of deposition parameters on optoelectronic and properties of ZnO based thin films prepared by RF magnetron sputtering have been studied. Different targets (pure Zn, ZnO, ZnAl (98/2 at 2%), ZnOAl (98/2 at%), and ZnOAl2O3 (98/2 wt%)) have been investigated to compare resulting samples and establish the best target composition. From reactive sputtering, using a ZnAl target, transparent conductive zinc oxide has been obtained at 380°C with E g = 3.25–3.35 eV and ϱ = 4.8 × 10 −4 ω cm. Reduction of substrate temperature at 200°C has been possible by nonreactive sputtering from ZnOAl and ZnOAl2O3 targets. The values of the energy gap and resistivity under these conditions are 3.30–3.35 eV and 1 × 10−3 ω can respectively.

Post-deposition annealing effects in RF reactive magnetron sputtered indium tin oxide thin films

Abstract Indium tin oxide films have been grown by RF reactive magnetron sputtering. The influence of the deposition parameters on the properties of the films has been investigated and optimized, obtaining a value for the figure of merit of 6700 (Ω cm)−1. As-grown indium tin oxide films were annealed in vacuum and O2 atmosphere. After these heat treatments the electro-optical properties were improved, with values for the resistivity of 1.9 × 10−4 Ω cm and the figure of merit of 26700 (Ω cm)−1.

Quartz-crystal microbalance study of the growth of Zn(Se,O) thin-films in a chemical bath. A sequential electroless-chemical process

Abstract The chemical bath deposition of Zn(Se,O) thin films is studied with a quartz crystal microbalance (QCM). The deposition velocity is measured with QCM under different experimental conditions, including substrate properties, bath temperature and bath composition. The plots of the growth velocity vs. time reflect the sequence of processes for the deposition of the film. At the beginning, an induction period takes place, which ends up with the formation of first ZnSe particles. These particles activate an electroless reaction induced by a reducer, hydrazine or hydroxide, which gives rise to deposition of a film with ZnO and ZnSe composition. This step is necessary to attain compact and thicker films with appropriate conditions for solar cell application. The electroless mechanism is decelerated after deposition of some nanometers of film, hence, a second growth mechanism starts to predominate consisting in the chemical reaction of soluble selenide with hydroxide and Zn2+ cations on the substrate surface. The layers resulting from this second mechanism have higher proportion of ZnSe than the previously electroless generated layers. At longer times, the predominant growth mechanism is the deposition of ZnSe clusters formed in the solution, which produces less compact and poorly adherent top layers. This sequence of mechanisms gives rise to films with heterogeneous morphology and composition in depth.