Localized electronic states related to O2 intercalation and photoirradiation on C60 films and C70 films

Abstract

Deep localized electronic states are created by O2 intercalation into C60 films and C70 films, which causes the Fermi level to shift down to the middle of gap. The states act as a trap level for charge carriers and as nonradiative recombination centers. It seems that prepared C60 films and C70 films have a shallow localized state. The shallow state is located at ∼0.2 eV under the conduction band and affects the electrical and optical properties. Furthermore, the photoirradiation of C60 films and C70 films causes polymerization of the O2-free sample and oxidization of the O2-intercalated sample. The quasistable electronic states at room temperature are created as a result of photo-oxidization of C60 films. C60 oxides create deep localized electronic states which cannot disappear under thermal annealing. The photoluminescence intensity of O2-free samples increases with photoirradiation for 1 h. It is found for the first time that this increase occurs along with a decrease of localized state density.