The performance of thin film chalcopyrite solar cells based on a p-Cu(In,Ga)(S,Se)(2)/n-ZnO pn-junction can be improved significantly by inserting a buffer at the interface. Besides the widely used CdS, ZnSe is a promising alternative. Such a layer can be most successfully grown by halogen supported chemical vapour deposition (H-CVD) at temperatures of approximately 300degreesC. With buffers grown at higher temperatures, the solar cells show lower efficiencies, possibly caused by thermally activated atomic interdiffusion. Heavy ion elastic recoil detection analysis (HI-ERDA) has been used to study depth-dependent atomic concentration profiles. These measurements revealed diffusion of indium from the Cu(In,Ga)(S,Se)(2) absorber material into the ZnSe buffer. However. a quantitative determination of the diffusion parameters is difficult. since surface and interface roughness of the polycrystalline material system affect the measured energy spectra of the sample constituents. Therefore, the diffusion coefficient has been calculated from separate measurements of evaporated indium layers on ZnSe single crystals, annealed in argon atmosphere. Since the results were still largely influenced by the sample's surface roughness, additionally, atomic force microscopy (AFM) measurements have been performed to determine the surface morphology. With these additional information, it was possible to extract the diffusion coefficient of indium in ZnSe. So for the first time, it is possible to analyse qualitatively and quantitatively atomic diffusion processes by ERDA for the material system and the temperatures used for solar cell fabrication.