Atomic force microscopy imaging of polycrystalline CuInSe2 thin films

In this study atomic force microscopy (AFM) imaging has been used to study the structural properties of polycrystalline crystalline CuInSe2 films, whi ch are widely used as absorber materials in thin film solar cell devices. T his technique demonstrated an excellent capability for the reproducible ima ging of these rough polycrystalline materials. AFM imaging in combination w ith statistical analysis revealed distinct differences in the structural pr operties (i.e. grain width and height distributions, root-mean-square (RMS: ) and peal; to valley (R(p-v)) roughness values) as a function of the speci fic growth technique used and the hulk composition of the films. In the cas e of Cu-rich films, prepared by the H2Se/Ar treatment of Cu/In/Cu alloys, r ough surface structures were in general observed. Statistical analysis reve aled two distinct distribution of grains in these samples (1.0-2.5 mu m and 3-5.5 mu m) with large RMS and R(p-v) roughness values of 380 nm and 2.6 m u m, respectively In-rich films were characterized by the presence of much smaller, roughly circular clusters with a significant reduction in both the width and height distributions as well as RMS and R(p-v) roughness values. The most successful growth techniques, in terms of producing homogeneous a nd dense films, were in the cases of H2Se/Ar treated metallic InSe/Cu/InSe alloys and the coevaporation of all materials to form CuInSe2. Both these t echniques produced absorber films with very narrow grain width and height d istributions as well as small roughness values. It was possible to establis h that high efficiency devices are associated with the use of absorber film s with narrow width distributions between 0.5 and 2 mu m and small RMS (> 3 00 nm) roughness values. These values are used as a figure of merit in our laboratories to evaluate the structural properties of our CuInSe2 thin film s.