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 CuInSe2 films, which are widely used as absorber materials in thin film solar cell devices. This technique demonstrated an excellent capability for the reproducible imaging of these rough polycrystalline materials. AFM imaging in combination with statistical analysis revealed distinct differences in the structural properties (i.e. grain width and height distributions, root-mean-square (RMS) and peak to valley (R(p-v)) roughness values) as a function of the specific growth technique used and the bulk composition of the films. In the case of Cu-rich films, prepared by the H2Se/Ar treatment of Cu/In/Cu alloys, rough surface structures were in general observed. Statistical analysis revealed two distinct distribution of grains in these samples (1.0-2.5 &mgr;m and 3-5.5 &mgr;m) with large RMS and R(p-v) roughness values of 380 nm and 2.6 &mgr;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 and 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 techniques produced absorber films with very narrow grain width and height distributions as well as small roughness values. It was possible to establish that high efficiency devices are associated with the use of absorber films with narrow width distributions between 0.5 and 2 &mgr;m and small RMS (> 300 nm) roughness values. These values are used as a figure of merit in our laboratories to evaluate the structural properties of our CuInSe2 thin films.