Regulating solution epitaxy of PbTiO₃ film by Ni ions for enhanced visible-light photovoltaic current

Bulk photovoltaic effect of perovskite ferroelectric oxides has been widely explored because of its ability to obtain the above-bandgap photovoltage. However, the photovoltaic current in these materials remains low at nA level in the visible-light range, severely limiting the device applications due to a wide bandgap. Herein, we report a Ni ions-assisted coprecipitation-hydrothermal method to regulate growth of single-crystal PbTiO₃ film with a controlled thickness from 0.7 μm to 2.2 μm. The epitaxial relationship between the tetragonal perovskite film and cubic Nb:SrTiO₃ substrate has been characterized to be {001}_film || {100}_substrate. The film adopts a single-domain structure with a polarization direction pointing to the substrate. Interestingly, the film exhibits a large photovoltaic current under 405 nm irradiation, with values reaching 3.6 mA/cm², which is ∼3.6 times higher than those of the reported ferroelectric materials. Introducing Ni ions as an additive into the precursor solution was investigated to effectively mediate the competitive nucleation and growth processes between the film and the by-product powder, thereby enabling a tunable thickness of the films. An intriguing Ti-vacancy composition-gradient was revealed throughout the film and its coupling with the spontaneous polarization generates a polarization-gradient and thus built-in electric field, accounting for the excellent photovoltaic performance reported here.