$PbTiO_3$ ceramics doped with 10, 15, and 20 mol% La+3 were fabricated by solid state sintering. $BaTiO_3$ ceramics doped with 0, 0.1, 0.3, and 0.5 wt.% FeO were fabricated by hot isostatic pressure(HIP). Photovoltaic current was measured under 300W Xenon lamp radiation in the absence of applied field.
Glass has proposed a one dimensional model demonstrating a bulk photovoltaic effect also occurred in pure-ferroelectrics. But it was found that not a one dimensional model but a three dimensional model could illuminate that photo-excited electrons moved toward the positive end of the spontaneous polarization in pure-ferroelectrics.
Photo-excited electrons were trapped at grain boundary. Electrons trapped at grain boundary induced electro-potential barrier and space charge field. The dependence of Photovoltaic current(J) on the grain size in ferroelectric ceramics was defined by
$J(d)= α d exp(-γd^{2})$ + Jion
where d is grain size and Jion is ionic photovoltaic current which has no relationship with grain size. This was explained by the number and height of electro-potential barriers. Under the critical grain size in which photovoltaic current is maximum, the number of electro-potential barriers increased with the grain size increased. Over the critical grain size, the height of electro-potential barriers increased with the grain size increased.
Therefore the maximum photovoltaic current was obtained in the critical grain size where the two effects were optimized. It was also found that photovoltaic current in ceramics increased during illumination. This phenomenon was explained by space charge field which induced increasing of spontaneous polarization in grain. This was similar to photorefractive effect.
Donor impurity raised Glass constant(κ), but depressed spontaneous polarization. Therefore the maximum photovoltaic current could be obtained in doping concentration where the two effects were optimized. In Fe-doped $BaTiO_3$ ceramics, the maximum photovoltaic current was obtained in 0.1wt.% FeO doping concentration.
