Sintering‐driven effects on the band gap of (Pb,La)(Ti,Ni)O₃ photovoltaic ceramics

Abstract

In this work, the influence of the sintering temperature on the physical properties of (Pb0.8La0.2)(Ti0.9Ni0.1)O3 (PLT‐Ni) ceramics is reported. The experimental data revealed that the energy band gap of PLT‐Ni ceramics could be tailored from approximately 2.7 to 2.0 eV by changing the sintering temperature from 1100°C to 1250°C. It is demonstrated that the simple substitution of Ti4+ by Ni2+ cations is effective to decrease the intrinsic band gap while increasing the tetragonality factor and the spontaneous polarization. However, the additional red‐shift observed in the absorption edge of the PLT‐Ni with increasing the sintering temperature was associated with a continuous increase in the oxygen vacancies (VO2‐) amount. It is believed that the impact of the creation of these thermally induced VO2‐ is manifold. The presence of VO2‐ and Ni2+ ions generate the Ni2+‐VO2‐ defect‐pairs that promoted both a decrease in the intrinsic band gap and an additional increase of the tetragonality factor, consequently, increasing the spontaneous polarization. The creation of Ni2+‐VO2‐ defects also changed the local symmetry of Ni2+ ions from octahedral to a square pyramid, thus lifting the degeneracy of the Ni2+ 3d orbitals. With the increase in the sintering temperature, lower‐energy absorbing intraband states were also formed due to an excess of VO2‐, being responsible for an add‐on shoulder in the absorption edge, extending the light absorption curve to longer wavelengths and leading to an additional absorption in “all investigated” spectrum as well.