Ni-doped SrBi₂Nb₂O₉ – Perovskite oxides with reduced band gap and stable ferroelectricity for photovoltaic applications

In this work, Ni-doped SrBi₂Nb₂O₉ (Sr_1-xBi_2+xNb_2-xNi_xO_9-x, or SBNN) ceramics were successfully fabricated by using a solid state reaction method, and their band gaps were determined by UV–vis–NIR absorption spectrum. It is found that Ni doping could significantly reduce the band gap of SrBi₂Nb₂O₉. The Sr_0.91Bi_2.09Nb_1.91Ni_0.09O_8.91 (SBNN9) ceramics show the lowest band gap of 2.25 eV with a relatively high remanent polarization of 2.5 μC/cm². The band-gap reducing effect upon Ni doping might be attributed to the formation of the intermediate gap states generated by the Ni²⁺ cations. Despite oxygen vacancies were unavoidably introduced into the systems after Ni doping, the remanent polarizations of the SBNN ceramics remain almost the same or even larger than the pure SrBi₂Nb₂O₉. Our experimental results are also confirmed by the theoretical analysis based on the first-principle calculations. Novel ferroelectric photovoltaic effects were observed in these SBNN compounds. The switchable photovoltaic outputs induced by electric field were detected along different poling directions in the SBNN9 sample. The SBNN ceramics reported in the present work display narrow band gaps and relatively large polarizations, and show promise for ferroelectric photovoltaic device applications.