Oxygen vacancy migration and dispersive photoconductivity in Bi4Ti3O12-d

We investigated the influence of photoexcited nonequilibrium electrons in Bi4Ti3O12-d which contains a high density of oxygen vacancies. With increasing oxygen deficiency, the photocurrent undergoes a rapid enhancement, accompanied by a marked change in kinetics. In heavily oxygen-deficient Bi4Ti3O12-d (d = 0.45), we observed a nonexponential relaxation of photogenerated carriers with an increasingly slow decay after termination of visible illumination with energy lower than the band gap. This phenomenon is similar to what has been reported for persistent photoconductivity effects in the layered perovskite YBa2Cu3O7-d and Nd2Ti3O9, both of which resulted from the space charge field established by photoexcited electrons trapped at oxygen vacancies. We argue that oxygen vacancies acting as trapping centers for electrons are linked to dispersive diffusion in the perovskite blocks, which control carrier recombination.