PZT ceramics is one of the most useful ferroelectric materials and shows different electric and piezoelectric properties by doping of various impurities. It was expected that acceptor and donor impurity doping would also affect on photovoltaic property differently in PZT. Tetragonal phase $Pb(Zr_x Ti_{1-x})O_3$ (X=0.3, 0.4, 0.5) samples were prepared by conventional oxide mixing method and measured photovoltaic currents to determine the undoped PZT composition. The photovoltaic current decreased as Zr content increased. This is explained by reduction of the spontaneous polarization due to decrease of the tetragonality with increasing Zr content.
In PZT ceramics, $Mg^{2+}$ is a B-site acceptor and $W^{6+}$ is a B-site donor. Various amounts of MgO and $WO_3$ (0.5 ~ 3.0mol%) were doped into PZT(4/6) to investigate the impurity doping effect on the photovoltaic property.
Photovoltaic currents were measured from intercepts of Y-axis of current-voltage(I-V) curves under illumination. In MgO-doped PZT(4/6) the photovoltaic current increased with increasing MgO content to 2mol% and decreased at more MgO content. In $WO_3$-doped PZT(4/6) the photovoltaic current increased with increasing $WO_3$ content to 1.5mol% and decreased at more $WO_3$ content. In PZT ceramics, $Mg^{2+}$ and $W^{6+}$ form acceptor and donor levels in the energy band gap respectively and photocarriers can be excited via these levels and contribute to the extrinsic photovoltaic current and at more than certain MgO, $WO_3$ content the reduction of the life time of the carriers and tetragonality with increasing doping content brings about decrease of the photovoltaic current.
Photovoltages were measured from intercepts of the X-axis of the I-V curves under illumination. In MgO-doped PZT, photovoltage decreased from 11V/cm to several volts/cm with doping content and in $WO_3$-doped PZT, photovoltage increased by 40 times than undoped PZT at maximum value. Because photovoltage($E_{st}$), photovoltaic current($J_{ph}$) and conductivity under illuminationσ(σ) has the relationship $E_{st}=J_{ph}σ$ , the variation of the photovoltage can be explained by the conductivity under illumination.
Conductivities under illumination(σ) were determined from slopes of the I-V curves. In MgO-doped PZT, σ increased with increasing doping content and was saturated from 1.5mol%. In $WO_3$-doped PZT, σ decreased with doping content and saturated from 1,0mol%. Undoped PZT has p-type conductivity, and $Mg^{2+}$ ions supply holes. It is thought that these excess hole carriers contribute to enhancement of the σ. W6+ions supply electrons and reduce the hole carrier concentration. This is thought to be the reason of the reduction of the σ.
Photovoltaic currents were measured with illumination time. MgO-doped PZT showed current fluctuation in short time period and average current didn't change with illumination time. In contrast to MgO-doped PZT, $WO_3$-doped PZT didn't show the fluctuation and current increased with illumination time. Such aspects of photovoltaic current can be explained by domain switching due to localized space charge field arisen from trapped electrons at grain boundaries. Space charge field can switch domains and change of spontaneous polarization brings about transient current. In MgO-doped PZT space charge field can be screened fast because of the high conductivity, and domain wall cannot overcome strain energy and return to the initial position. These serial procedures are thought to be the reason of the current fluctuation. In $WO_3$-doped PZT the space charge field can be strong and maintained long enough for domain wall to be settled and photovoltaic current continuously increases because of the increased spontaneous polarization.
