The efficiency of CdTe based solar cells is strongly enhanced by a thermal treatment in HCF2Cl ambient. CdTe thin films deposited on CdS/ ZnO/ITO/glass by Closed Space Sublimation before and after the annealing are characterised. The CdTe morphology is studied by atomic force microscopy and scanning electron microscopy. In the treated films the non-homogeneous distribution of the grain size disappears, in addition an increasing of the dimensions of the grains is observed. Cathodoluminescence analyses show a remarkable difference in the spectra between the treated and untreated structures. A strong increase in the intensity of the 1.4 eV band is observed by increasing the HCF2Cl content. A model of the electronic levels inside the CdTe band gap, due to incorporation of Cl (or F) is proposed. (C) 2007 Elsevier B.V All rights reserved.