High efficiency CdTe/CdS thin film solar cells with a good stability can be obtained if a low gap. high conductivity p-type semiconductor, such as Sb2Te3, is used as a back contact for CdTe. However, even though Sb2Te3 is used as a back contact, some cells can exhibit a low fill factor. This, at first sight, could suggest that the back contact is not the right one. Here we would like to show that a low fill factor could depend not only on the back contact but also on the front contact and on the amount of Na-diffusion from the soda lime glass substrate.