Effect of regulation of defect ion on ferroelectric photovoltaic characteristics of BiFeO<sub>3</sub>-BaTiO<sub>3</sub> based perovskite materials

Abstract

Ferroelectrics materials, as a candidate of materials, have recently received attention, for they possess applications in photovoltaic devices and can couple the light absorption with other functional properties. In these materials, the strong inversion symmetry is broken, which is because the spontaneous electric polarization promotes the desirable separation of photo-excited carriers and allows voltages higher than the band-gap, thus permitting efficiency beyond the maximum possible value in a conventional p-n junction solar cell. Much effort has been made to study the ferroelectric photovoltaic effect in several families of ferroelectric perovskite oxides, such as Pb(Zr,Ti)O<sub>3</sub>, LiNbO<sub>3</sub>, BaTiO<sub>3</sub>, KNbO<sub>3</sub>, Na<sub>0.5</sub>Bi<sub>0.5</sub>TiO<sub>3</sub>-BaTiO<sub>3</sub>, AgNbO<sub>3 </sub>and BiFeO<sub>3</sub>. However, their photo-electric conversion efficiency is now still very low though this field is being studied. The observed output photocurrent is very low due to the negative influence of a wide band-gap and small absorption coefficient, which is caused by the wide band-gaps for most of ferroelectric materials such as Pb(Zr,Ti)O<sub>3 </sub>(~3.5 eV), and BaTiO<sub>3</sub> (~3.3 eV), especially. Although the BiFeO<sub>3</sub> system with low band-gap (2.7 eV), which can absorb most visible light for electron transition, is considered as a potential photovoltaic material, it is difficult to synthesize pure perovskite structure. The BiFeO<sub>3</sub>-BaTiO<sub>3</sub> (BF-BT) ferroelectric material with excellent piezoelectric and ferroelectric properties has been widely concerned by researchers in recent years. However, it is still unclear whether this system has the same advantages as BiFeO<sub>3</sub> materials with excellent photovoltaic properties. In this work, the Bi(Fe<sub>0.96</sub>Mg<sub>0.02–</sub><i><sub>x</sub></i>Ti<sub>0.02+<i>x</i></sub>)O<sub>3</sub>-0.3BaTiO<sub>3</sub> ferroelectric ceramics are prepared by the conventional synthesis method to uncover the piezoelectric and ferroelectric properties, as well as the photovoltaic performance with different ratios of Mg<sup>2+</sup>/Ti<sup>4</sup>. Because of the electronic production caused by replacing Mg<sup>2+</sup> ions with Ti<sup>4+</sup> ions, the conductivity of sample increases, and thus its piezoelectric and ferroelectric properties deteriorate. The piezoelectric coefficient <i>d</i><sub>33</sub> decreases from 195 pC/N at <i>x</i> = 0 to 27 pC/N at <i>x</i> = 0.02. Conversely, the range of absorption spectrum increases when the Mg<sup>2+</sup> ions are replaced by Ti<sup>4+</sup> ions. The band gap of sample decreases from 1.954 eV at <i>x</i> = 0 to 1.800 eV at <i>x</i> = 0.02. The photocurrent of sample increases from 3.71 nA/cm<sup>2</sup> at <i>x</i> = 0 to 32.45 nA/cm<sup>2</sup> at <i>x</i> = 0.02 because of the combined action of reducing the band gap and internal bias field.