Studies On The Preparation And Ferroelectric Photovoltaic Properties Of PZT-BFCO Nanocomposite Films

When ferroelectric materials with non center-symmetry are under homogeneous illumination, anomalous photovoltaic effect is produced, meaning that the open-circuit voltage(Voc) can exceed the limit of energy band gap. Therefore, the photoelectric conversion efficiency of ferroelectric films may exceed that of traditional semiconductor solar cells. For the past few years, ferroelectric photovoltaic effect has attracted a lot of attention. However, the short-circuit current(Jsc) induced by illumination in ferroelectric films is still very small, the photoelectric conversion efficiency needs to be improved, and the conversion mechanism of ferroelectric photovoltaic is not yet completely understood at present. In this work, PZT thin films have been selected due to its high remanent polarization, and BFCO with narrow band gap was co-sputtered with PZT in order to increase the minor carrier density. The main contents of this article are as follows:1、PZT target was successfully sintered at about 1100℃ by conventional solid state reaction method. Then we prepared ITO film as the transparent conductive top electrode. In order to improve the performance of ITO electrode, we have systematically studied different process conditions including sputtering power, substrate temperature,Ar pressure and so on. The optimized ITO electrode has a square resistance of 1.5Ω/□and light transmittance better than 80% when the incident light wavelength is larger than 400 nm.2、Epitaxial PZT thin films with a composition of Pb(Zr0.52Ti0.48)O3 were grown on the(100)-oriented SrTiO3(STO) single crystal substrate by a 90 o off-axis RF sputtering method. The processing conditions affecting the phase structure of the film have been studies in details, such as substrate temperature, deposition pressure, and power. XRD analysis confirmed that the PZT thin films have a good epitaxial structure.Ferroelectric test results show that the saturated polarization and double remnant polarization are about 30μC/cm2 and 20μC/cm2, respectively.3、PZT-BFCO nanocomposite films were then prepared by a co-sputtering method. XRD analysis showed that PZT peak intensity decreased upon increasing the BFCO volume fraction, and impure phases were found when the BFCO content is high.The morphology analysis by AFM confirmed that the PZT-BFCO grains became uneven and defects also increased upon increasing the BFCO volume fraction.Meanwhile, BFCO dopant also leads to a decrease of the remnant polarization and the saturation polarization, and an increase of the leakage current.4、The photovoltaic properties of ITO/PZT-xBFCO/Nb : STO were measured under the illumination of an AM1.5 solar simulator using a Kethley 2400 source meter.It was found that the BFCO doping can effectively enhance the short-circuit current density and open-circuit voltage. The best Voc and Jsc are approximately 0.41 V and 0.72?A/cm2. The Jsc is one order of magnitude higher than that of pure PZT films, which is about 0.087 ?A/cm2. The reason was attributed to the narrow band gap(1.79eV) of BFCO, which can provide more non-equilibrium minor carriers. Detailed photovoltaic test results of PZT-BFCO films with different film thickness as well as polarization states indicates that the interfacial Schottky barrier between ITO/PZT-BFCO plays a dominant role in separating the electron-hole pairs, but not the depolarization field due to the low remanent polarization of the PZT-BFCO film.