Influence of stoichiometry on the electrochromic cerium-titanium oxide compounds

CeO₂-TiO₂ finds use as passive counter-electrode in electrochromic devices. Thin films were produced by dc-sputtering in a wide range of compositions. Influence of total pressure and oxygen partial pressure on the optical constants of TiO₂ was investigated. Slightly substoichiometric TiO₂ films exhibit a red-shift of the bandgap. The TiO₂ content in the compound essentially determines the degree of cathodical coloring upon Li⁺ intercalation. However, pure TiO₂ films with comparable visible transmittance in the clear state behave differently during electrochemical cycling depending on oxygen stoichiometry. Films that are deposited at higher total pressure are more oxygen rich and require initial formatting until current voltage cycles become stable. CeO₂-TiO₂ films of intermediate compositions have the relatively highest charge capacity. Comparison with atomic force microscopy indicates a correlation of small grain size with high charge capacity.