Correlating facet orientation, defect level density and dipole layer formation at the surface of polycrystalline CuInSe2 thin films

Abstract

Individual grains of chalcopyrite solar cell absorbers can facet in different crystallographic directions at their surfaces. To gain a deeper understanding of the junction formation in these devices, we correlate variations in the surface facet orientation with the defect electronic properties. We use a combined ana lytical approach based on scanning tunneling spectroscopy STS , scanning electron microscopy, and elec tron back scatter diffraction EBSD , where we perform these experiments on identical surface areas as small as 2 2 amp; 956;m 2 with a lateral resolution well below 50 nm. The topography of the absorber sur faces indicates two main morphological features micro faceted, long basalt like columns and their short nano faceted terminations. Our STS results reveal that the long columns exhibit spectral signatures typ ical for the presence of pronounced oxidation induced surface dipoles in conjunction with an increased density of electronic defect levels. In contrast, the nano faceted terminations of the basalt like columns are largely passivated in terms of electronic defect levels within the band gap region. Corresponding crystallographic data based on EBSD experiments show that the surface of the basalt like columns can be assigned to intrinsically polar facet orientations, while the passivated terminations are assigned to non polar planes. Ab initio calculations suggest that the polar surfaces are more prone to oxidation and resulting O induced defects, in comparison to non polar planes. Our results emphasize the correlation between morphology, surface facet orientations and surface electronic properties. Furthermore, this work aids in gaining a fundamental understanding of oxidation induced lateral inhomogeneities in view of the p n junction formation in chalcopyrite thin film solar cell