Surface analysis of CdTe thin film solar cells

The surface properties of CdTe thin film solar cells prepared by ANTEC using the close-space sublimation — (CSS) — technique have been analyzed by X-ray diffraction (XRD), atomic force microscopy (AFM), photoelectron emission microscopy (PEEM), high-resolution scanning electron microscopy (HRSEM) and photoelectron spectroscopy (XPS) after different pretreatment conditions. Exposure of the CdTe films to air leads to surface oxidation with the formation of TeO2 and CdO. The amount of surface oxides depends on the CdCl2 activation process. Activated surfaces are less oxidized than non-activated surfaces. Due to that surface oxidation, the surface is more n-type, indicating the formation of a surface barrier. The surface oxide can be removed by mild sputtering. The results suggest that no extra surface defects are introduced by this procedure. Before sputtering, Cl is found on the surface of the activated material, although no such contamination is found in the stoichiometric bulk material using XPS. A variation in the Fermi level position is observed for the non-activated to the activated CdTe material from weakly to higher p-doped levels. This type of conversion is evidently restricted to the near surface area as further in the bulk, weakly p-doped CdTe is found again. The results indicate that, besides the surface composition, the electronic properties of the film also depend on the different pretreatment steps.