M.A. Martínez

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%.

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.

Morphological and structural studies of CBD-CdS thin films by microscopy and diffraction techniques

Abstract The influence of cadmium salt and thiourea concentrations on the morphological and structural properties of chemical bath-deposited CdS thin films has been investigated. Two different feature regimes have been distinguished: an inner continuous layer grown directly on the glass and independent on the deposition conditions, and other porous overlayer, more dependent on the chemical concentrations. Root mean square, RMS, and average roughnesses, Ra, as quantified by AFM, are about 10–13 nm and 7–11 nm, respectively, for all CdS samples tested. These films are sulphur-poor, decreasing S/Cd atomic ratio from 0.82 at low cadmium salt, 1 mM, and high thiourea concentrations, 100 mM, down to 0.76 at higher [Cd2+], 5 mM, and lower [TU], 10 mM.

Accurate control of thin film CdS growth process by adjusting the chemical bath deposition parameters

The influence of cadmium salt and thiourea concentrations and deposition time on the optical and electrical properties of chemical bath deposited CdS thin films has been studied. The decrease in cadmium salt and increase in thiourea concentrations, to the order of 1 mM and 100 mM respectively, have been found to be optimal for raising the optical transmission without conductivity deterioration and allowing a suitable thickness control less dependent on the deposition time. Additionally, 200 and 400°C air-annealing treatments for 20 min have been carried out. Their analysis has demonstrated the optical absorption edge shifts towards lower photon energies as the air-annealing temperature raises. Besides, dark and light resistivities decrease in comparison with as-grown CdS data.

Cadmium sulphide growth investigations on different SnO2 substrates

Abstract Optical, structural and morphological properties of chemical bath deposited cadmium sulphide, CdS, on different commercial SnO2/glass substrates have been analysed. Although CdS–SnO2 bilayer optical behaviour is directly determined by tin oxide characteristics, cadmium sulphide optical features remain independent on substrates. CdS covers the layer underneath in a continuous and homogeneous way, having a grain size between 100 and 300 nm, dependent on substrate morphology. 400°C, 20 min air-annealing treatment has produced only a light decrease in the average visible transmission of CdS–SnO2 bilayers.

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.

CuInSe2 thin films obtained by a novel electrodeposition and sputtering combined method

Abstract The combination of electrodeposited Cu–Se alloy and sputtered In precursors, followed by annealing in Se atmosphere, was found successful to obtain CuInSe 2 thin films with Cu/In composition ratios varying between 0.25 and 2.75. Structural and morphological properties were observed to be dependent on annealing temperature and Cu/In ratio. When Cu/In>1, both a CuInSe 2 crystallinity improvement and a smooth surface were achieved. Apart from the ternary CuInSe 2 , two different binaries were detected: Cu 2 Se crystallises after 400°C annealing in samples with Cu/In>1, and In 2 Se 3 does after annealing at 500°C for the distinct Cu/In ratios were considered.

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.