M. C Lopez, D. Leinen, F. Martin and J. R. Ramos-Barrado

Laboratorio de Materiales y Superficie. Departamentos de Fisica Aplicada & ingenieria

Quimica. Universidad de Malaga. E29071 Malaga. SPAIN.

Abstract:

ZnO/ZnS bilayers to antireflection coatings for solar cells have been prepared by spray pyrolysis using mixed aqueous solutions of zinc acetate dehydrated and thiourea or zinc chloride and thiourea. The structure, surface morphology, chemical composition and optical properties of the bilayer are investigated as a function of the initial solution. X-Ray photoelectron spectroscopy (XPS) analysis and Ar ion-beam sputter etching was carried out to obtain XPS depth profile of bilayer. Neither carbon nor others by-products which could change the refractive index of the bilayer have been found in the interface. Some differences between the bilayer with the ZnS film obtained from ZnCl2 or Zn acetate can be observed.

1. Introduction:

The efficiency of solar cells dramatically depends on light that can arrive to the active layer. Due to reflection losses, the photogenerated current density (and consequently, the efficiency) reaches a level much lower than he maximum for the standard terrestrial spectrums of light coming from the sun [1-3]. To reduce the reflection of incident light at the surface of the cell, an antireflecting coating (ARC) can be used over the cell [2-6]. This coating can range from a simple layer to a multilayer system of many layers having almost zero reflectance over a wide range of wavelengths [2].

In general, the design of solar cells, follows a step-down interference coating structure: ns>n1>n2>nm, where ns and nm are the substrate and incident medium refractive indices, respectively [4,6]. This choice is based on the spectral stability of the coating and for low reflectance. Stability means that the low-reflectance spectrum changes very slightly with thickness and refractive index variation [4,6].

Zinc oxide and sulphide can be used to form a bilayer because both compounds have appropriate electrical and optical properties.

Zinc sulphide (ZnS) is an n-type semiconducting. ZnS thin film is a promising material for its use in various application devices due to its wide band gap of 3.7eV at

room temperature, its high refractive index (2.35) [7-11], and its low absortion over a broad wavelength range, from 400 to 1400nm [11].

Zinc oxide (ZnO), is and n-type semiconducting [12]. It is a widebandgap (3.24eV) semiconductor material with high optical transparency in the visible and near-infrarred region of electromagnetic spectrum and high refractive index (1.9). Due to these properties, ZnO is a promising material for solar cell applications such as antireflection coating [13].

Spray pyrolysis is a useful alternative to the traditional methods to obtain antireflection coatings for solar cells. It is particular interesting because of its simplicity, low cost and minimum waste production. The spray pyrolysis allows coat big surfaces and it is easy to insert in an industrial line of production. This method of deposition allows obtain thin films with different chemical, physical and morphological properties; the characteristics of the films depend on the spray rate, temperature of the substrate and high of the nozzle values. ZnO/ZnS bilayer obtained by spray pyrolysis technique shows a smooth and homogeneous surface without by-products.