It is well known that ferroelectric materials possess the photovoltaic effect that produces a greater steady-state voltage than a band gap under illumination in the absence of an external field. The photovoltaic effect has been investigated due to its potential usefulness for optical storage, photostriction, and power conversion. Especially, the photovoltaic effect can be applied to the NDRO FRAM (Non-Destructive Read Out Ferroelectric Random Access Memory) and photostrictive actuator. FRAM with optical read-out has merits such as NDRO, fast response time, high efficiency of write access, and reduce the fatigue effect. Photostrictive actuator has also advantages of remote control, low power solution, electrode-less structure, enhanced reliability, and micro-displacement controllable. To apply the photovoltaic effect to these devices, the basic studies are needed; coincidence explanation of the photovoltaic phenomena, and improvement and measurement of the photovoltaic properties. In this study, the photovoltaic properties were investigated in $(Pb_{1-x}La_x)TiO_3$ ceramics and $Pb(Zr_{0.4]Ti_{0.6})O_3$ thin films in a point of a basic study.
$(Pb_{1-x}La_x)TiO_3$ ceramics were prepared by a conventional mixed oxide method, and domain instability and grain size dependence were investigated.
The photovoltaic current increased with illumination time in $(Pb_{0.85}La_{0.15})TiO_3$ ferroelectric ceramics under illumination in the absence of external field. The acoustic emission (AE) signals was also detected during illumination. Both non-steady state photovoltaic current and AE signals had maximum at the poling field of 1.5kV/mm, which was slightly lower than the coercive field. It was explained to be associated with the increase in remanent polarization due to increase in spontaneous polarization and photo-induced domain switching, both of which resulted from the space charge field established by photo-excited non-equilibrium electrons trapped at the grain boundaries.
The effect of grain size on the photovoltaic current was considered in $(Pb_{1-x}La_x)TiO_3$ ferroelectric ceramics. The photovoltaic current had a peak at a critical grains size which increased with a decreasing in the concentration of lanthanum. The origin of the effects was an electro-potential barrier, which was built by photo-excited electrons trapped at the grain boundary, and impeded the movement of the following photo-excited electrons. At a critical grain size, an electro-potential barrier plays a minimum role in the movement of photo-excited electrons, and maximum photovoltaic current with the grain size can be obtained.
$Pb(Zr_{0.4}Ti_{0.6})O_3$ [PZT] thin films were fabricated by a sol-gel method, and transparent top electrodes of ITO were deposited by a RF sputtering. In ITO/PZT/Pt capacitor, the dependence of the photovoltaic properties on the remanent polarization, and measurement of the photovoltaic voltage were investigated.
The relationship between the photovoltaic current and the remanent polarization was not linear, but its asymmetric was very large due to the leakage current. The hysteresis of photoconductivity with poling field was another clue of the leakage current. Therefore, to obtain the symmetric photovoltaic current, the interface between electrode and PZT thin film should be controlled. The photovoltaic voltage was also asymmetric due to the internal field, but its degree was small. Repeated access of poling and illumination reduced the photovoltaic properties, and suppressed the P-E hysteresis loop. Strong external field with illumination restored both the P-E hysteresis loop and the photovoltaic properties.
The photovoltaic voltages were measured by the two methods; conventional method of photo I-V curves and the photovoltaic voltage vs. illumination time curves by a voltmeter. The photovoltaic voltage was formed due to the charging of the crystal capacitance. When the photovoltaic current was large, the photovoltaic voltages measured by the two methods had the same value. However, when small photovoltaic current flowed, the photovoltaic voltages had different values. The photovoltaic voltage obtained by photo-IV curve was the maximum voltage that can be obtained in the ferroelectrics. To obtain the maximum photovoltaic voltage measured by a voltmeter directly, the photovoltaic current should be enough large to reach the Maxwellian relaxation by larger illumination area or higher illumination intensity.
