The photovoltaic effect has been one of the interesting properties in ferroelectric materials, because they produce a greater steady-state voltage than a band gap under illumination in the absence of an external field. Its potential usefulness for optical storage, photostriction, and power conversion have developed the research on the photovoltaic properties mainly in the ferroelectric single crystals as well as ceramics. Recently, the effect of additives on photovoltaic properties has been carried out since it was reported that the additives have improved on the bulk photovoltaic effect in ferroelectric materials.
In Previous studies, photovotaic current and photovoltage were investigated as dopant contents and doping concentration in PLZT. Nonaka et al. observed the changes of the photovoltaic effect in PZT ceramics singly doped with 0-3 mol% of Ta and they reported that both photovoltaic current and photovoltage were markedly increased by the doping of Ta up to 1 mol% of doped Ta. They concluded that the formation of Pb vacancies onto the perovskite lattice has a fundamental significance on the photovoltaic properties in PZT ceramic but there were no discussion about the conductivity changes as the doping concentrations. PZT ceramics have P-type conductivity and donor doping may decrease the coinductivity and it is thought that it comes into effect in the poling process or under illumination. But it is not obvious becouse of Pb evaporation during the sintering. But it is not obvious becouse of Pb evaporation during the sintering.
In this paper, we investigated the effect of the conductivity on the photovoltaic current in Nb doped $BaTiO_3$. There have been a number of studies of the defect chemistry of donor doped $BaTiO_3$. In barium titanate, donors are compensated by both cation vacancies and electrons, the relative distribution between which is determined by doping level, temperature, and oxygen activity. As temperature is lowered, the relative proportion of cation vacancies increases. In low Nb doped $BaTiO_3$, the compensating defect mode is purely varium vacancy $(Nb_{total}=[Nb^{·}_{Ti}]=n+2[V'_{Ba}])$. And it has been proposed barium vacancies are formed at the grain boundaries to compensate donors Considering the barium vacancy diffusion coefficient obtained by Wernicke we became aware of that heat-treatment(1200℃) time after sintering(1300℃ 30min) affected the conductivity to control the relative distribution of $V'_{Ba}$ & electrons. The immutability of grain size was convinced by SEM and $[Nb^{·}_{Ti}]$ was constant because of the same doping concentration of Nb. The photovoltaic current of the samples sintered after a heat-treatment in $H_2$ gave certainty to that the photovoltaic current is changed by the change of the conductivity not the concentration of A-site vacancy$(V'_{Ba})$.
