Influence of the fluorine doping on the optical properties of CdS thin films for photovoltaic applications

The performance of CdTe/CdS thin film solar cells is strongly dependent on the properties of the CdS layer. In particular the electrical properties of the heterojunction between n-type CdS and p-type CdTe are influenced by the recombination activity at the grain boundaries. It is found that introducing fluorine in the sputtering chamber during CdS growth, the solar cell efficiency is improved. In order to study the influence of fluorine on the optical properties of CdS, undoped and F-doped thin films are investigated by cathodoluminescence and spectroscopic ellipsometry. Cathodoluminescence analyses reveal an improvement of the luminescence efficiency of the F-doped material, with respect to the undoped one, through the observed increase of the intensity of the near band edge emission at about 2.42 eV, accompanied by a blue shift of the center of the band. An analogous improvement of the optical response is confirmed by the ellipsometry results, showing an increase of the optical bandgap and a steeper absorption edge for the F-doped films. A model based on the reaction of the incorporated fluorine with the interstitial Cd atoms is presented, in order to explain the differences in the luminescence behavior between the undoped and F-doped samples and the observed passivation of the grain boundaries. Spectroellipsometric measurements reveal that the presence of fluorine reduces the growth rate of CdS and increases the bandgap and reduces the refraction and extinction indexes.