Bandgap tuning in samarium-modified bismuth titanate by site engineering using iron and cobalt co-doping for photovoltaic application

Abstract

Multiferroic materials have faced great interest for solar energy conversion applications due to the charge separation caused by the effective ferroelectric polarization and the photovoltage generated across the bandgap, which in principle can lead to highly efficient energy conversion. However, ferroelectric materials may show poor visible light absorption, mainly due to the large bandgap. In this work, a novel approach is adopted to effectively reduce the bandgap of a multiferroic material by doping transition metal (TM) ions, namely (Co/Fe) into the Ti site of the Bi3.25Sm0.75Ti3O12 (BSmT) lattice. The structural optimization results show that there is a difference in the lengths of the Δ(Ti(1)-O1) and Δ(Ti(2)-O5) bonds and that their value increases with increasing Co/Fe concentration, resulting in structural distortions. The optical studies showed that the BSmT band gap was successfully reduced from 3.22 eV to 2.02 eV with Co/Fe co-doping. The structural distortion explains this reduction in bandgap due to the modulation of orbital overlap and compensation mechanism. In trivalent (Fe3+/Co3+) co-doping at the titanium (Ti4+) site, oxygen vacancies are formed to maintain electroneutrality. To prove the multiferroelectricity behavior in this study, the ferromagnetism and ferroelectricity ordering of the sintered BSmT:FC2 sample was studied at room temperature. The results show unsaturated leaky P-E hysteresis loops due to domain pinning caused by the accumulation of oxygen vacancies near the domain boundaries. At the same time, distinct S-type M-H hysteresis loops were obtained, indicating typical ferromagnetic ordering. The results obtained in this work can be considered promising to promote the building of intrinsic multiferroics to develop photovoltaic devices for next-generation applications.