Evolution of CuInSe2 (112) surface due to annealing: XPS study

Although CuInSe2-based solar cells are amongst the most promising thin film photovoltaic devices the level of fundamental knowledge on this material is low. This is reflected in the wide gap between the achieved record efficiency (18.8% [1]) and the theoretical limit of about 30% for a single junction solar cell. The evolution of the elemental composition and chemical bonding of the (112) surface of CuInSe2 single crystals, cleaned by 4 keV N+ and then annealed at temperatures from 200 to 750 DGC in vacuum, was analysed using X-ray photoelectron spectroscopy. After annealing at temperatures over 500 DGC but below 700 DGC the elemental composition approached to CuIn3Se5. Following annealing at 650 DGC the surface structure was examined using X-ray photoelectron diffraction. Experimental 2*p-projection maps were constructed from the full hemispherical intensity distributions of In, Cu photo- and Se Auger-electron emission. These maps, the elemental composition, chemical bonding and the Cu2Se-In2Se3 phase diagram suggested the following three options for the surface compound (each with a sphalerite-based structure): (1) the single-phase state CuIn3Se5, (2) a mixture of In2Se3 and CuInSe2, (3) a mixture of In2Se3 and Cu2 Se.