Elizabeth, A.; Conradi, H.; Sahoo, S.K.; Kodalle, T.; Kaufmann, C.A.; Kühne, T.D.; Mirhosseini, H.; Abou-Ras, D.; Mönig, H.: Correlating facet orientation, defect-level density and dipole layer formation at the surface of polycrystalline CuInSe2 thin films. Acta Materialia 200 (2020), p. 463-470
10.1016/j.actamat.2020.09.028
Open Access Version
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 μm 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 cells.