Characterization of TiO x films and their application as antireflection coatings for silicon solar cells

Abstract

Antireflection coatings of TiO x for silicon solar cells were prepared by the reactive evaporation of TiO and were characterized by several different techniques. Optical parameters were determined by an ellipsometric method and a proper computer program was set up in order to obtain the complex refractive index and the thickness of the films from various ellipsometer readings. In addition, the reflectance was measured directly and, for transparent substrates, was associated with the transmittance for optical evaluations. The structure of the deposited films was analysed by transmission electron microscopy and electron diffraction. The stoichiometry of the films was determined by means of electron microprobe analysis and the Rutherford backscattering technique; the ratio x = O/Ti ranged from 1.7 to 2.2. The effects of the evaporation parameters were tested, and a linear dependence on substrate temperature was found for the refractive and absorption indices, whereas only a minor irregular influence of oxygen partial pressure and of deposition rate was detected. The structure of the films changed from amorphous (substrate at room temperature) to crystalline TiO 2 (200 °C); various types of titanium oxides were identified. Moreover, the crystallite size was found to depend not only on the temperature of the substrate but also on its crystallographic orientation.