• Mainz, R.; Sanli, E.S.; Stange, H.; Azulay, D.; Brunken, S.; Greiner, D.; Hajaj, S.; Heinemann, M.D.; Kaufmann, C.A.; Klaus, M.; Ramasse, Q.M.; Rodriguez Alvarez, H.; Weber, A.; Balberg, I.; Millo, O.; van Aken, P.A.; Abou-Ras, D.: Annihilation of structural defects in chalcogenide absorber films for high-efficiency solar cells. Energy & Environmental Science 9 (2016), p. 1818-1827

Open Access Version

In polycrystalline semiconductor absorbers for thin-film solar cells, structural defects may enhance electron-hole recombination and hence lower the resulting energy conversion efficiency. To be able to efficiently design and optimize fabrication processes that result in high-quality materials, knowledge of the nature of structural defects as well as their formation and annihilation during film growth is essential. Here we show that in co-evaporated Cu(In,Ga)Se2 absorber films the density of defects is strongly influenced by the reaction path and substrate temperature during film growth. A combination of high-resolution electron microscopy, atomic force microscopy, scanning tunneling microscopy, and X-ray diffraction shows that Cu(In,Ga)Se2 absorber films deposited at low temperature without a Cu-rich stage suffer from a high density of - partially electronically active - planar defects in the {112} planes. Real-time X-ray diffraction reveals that these faults are nearly completely annihilated during an intermediate Cu-rich process stage with [Cu]/([In] + [Ga]) > 1. Moreover, correlations between real-time diffraction and fluorescence analysis during Cu-Se deposition reveal that rapid defect annihilation starts shortly before the start of segregation of excess Cu-Se at the surface of the Cu(In,Ga)Se2 film. The presented results hence provide direct insights into the dynamics of the film-quality-improving mechanism.