Bandgap narrowing of Ba0.92Na0.04Bi0.04TiO3 ferroelectric ceramics by transition metals doping for photovoltaic applications

Abstract A wide bandgap is one of the greatest issues that restrain the photoelectric application of classical ferroelectric oxides in the visible spectrum. Typical approaches to reduce their bandgap, as well as to tune other physical properties (dielectric, piezoelectric …), is to use specific dopants. This work reports the variation by doping of the structural and optical bandgap properties of the Ba0.92Bi0.04Na0.04Ti0.96M0.04O3 (BNBT-M); (M = Ni, Fe, Co) ceramics. Structural refinement shows that all samples are crystallized with a tetragonal phase with a P4mm space group. Detailed analysis of the bond lengths, bond angles, octahedral distortions demonstrates the cooperative effect induced by the Co, Fe, and Ni doping. UV–Vis absorption spectra showed a decrease in the bandgap and increased absorption for longer wavelengths, induced by transitions metal (TM) ions doping. The bandgap narrowing is attributed to the development of new M − O bonds and states in the electronic structure, as the TM ions are incorporated on the Ti site, with simultaneous oxygen vacancies creation. The formation of oxygen vacancies via metal ions doping in BNBT was proven by ESR study. The present investigation can provide useful insight into the understanding and ferroelectrics bandgap tuning for visible-light photovoltaic applications.