Surface analysis of CdTe thin film solar cells

The surface properties of CdTe thin film solar cells prepared by ANTEC usin g the close-space sublimation - (CSS) - technique have been analyzed by X-r ay diffraction (XRD), atomic force microscopy (AFM), photoelectron emission microscopy (PEEM), high-resolution scanning electron microscopy (HRSEM) an d photoelectron spectroscopy (XPS) after different pretreat ment conditions . Exposure of the CdTe films to air leads to surface oxidation with the for mation of TeO2 and CdO. The amount of surface oxides depends on the CdCl2 a ctivation process. Activated surfaces are less oxidized than non-activated surfaces. Due to that surface oxidation, the surface is more n-type, indica ting the formation of a surface barrier. The surface oxide can be removed b y mild sputtering. The results suggest that no extra surface defects are in troduced by this procedure. Before sputtering, Cl is found on the surface o f the activated material, although no such contamination is found in the st oichiometric bulk material using XPS. A variation in the Fermi level positi on is observed for the non-activated to the activated CdTe material from we akly to higher p-doped levels. This type of conversion is evidently restric ted 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, t he electronic properties of the film also depend on the different pretreatm ent steps. (C) 2001 Elsevier Science B.V. All rights reserved.