Study On The Photovoltaic Effect Of Ferroelectric-based Composite Films And Its Manipulation

Due to the characteristic of reversible spontaneous polarization, ferroelectric materials have been widely used in application areas such as sensors, actuators and non-volatile memories. Also, the ferroelectricity has become one of the hot topics in condensed matter physics and developing novel functional materials around the world. Recent researches have witnessed another potential value of ferroelectrics in solar energy conversion area, which can be attributed to the abnormal open-circuit voltage (Voc) unlimited by the optical band gaps and polarization-related separation mechanism of photo-induced carriers. Compared with bulk ceramics, the ferroelectric films exhibit faster response times and higher energy conversion efficiencies. Moreover, it is easy to integrate ferroelectric films with existing semiconductor technologies, which makes them become the focus research of ferroelectric photovoltaic effect (FePVE) and related exploitation of new functional devices.However, the physical mechanism about FePVE keeps unclear and a qualitative explanation accepted by most researchers is indicated below. In the typical electrode/ferroelectric/electrode heterostructural film, the ferroelectrics produce abundant of photo-induced carriers under the incident light. Subsequently, these carriers are separated by the combination of ferroelectric polarization-induced depolarization field and the internal electric fields formed by the interfacial Schottky barriers. Since most ferroelectrics share wide bandgaps and poor conductivity, the short-circuit current (Isc) of FePVE is quite low in spite of the high open-circuit voltage. In addition, the back-to-back interfacial electric fields formed by the film and two electrodes inevitably hinder the effective separation of photo-induced carriers, accompanied with the low energy conversion efficiency. It is still worth mentioning that the multiple physical properties of ferroelectrics make it possible to couple with other order parameters and the manipulation of FePVE by multi-physical fields is helpful for developing novel functional devices.Considering about the questions emerged at the physical mechanism, efficiency promotion and multiple manipulation of photovoltaic effect in ferroelectric films, we compose this thesis and propose a series of feasible way to enhance and manipulate ferroelectric photovoltaic effect. We systematically explore the manipulation of ferroelectric photovoltaic effect by cycling electric polarization, interfacial polarization coupling and magnetoelectric coupling interactions. The main contents and results are presented as follows.We prepare a typical Pt/PZT/Pt heterostrutural film by sol-gel method and systematically study the influence of cycling electric polarization treatment on the photovoltaic effect. First, this ferroelectric PZT film is characterized successively by the XRD, SEM, AFM and ferroelectric analyzer to confirm its microstructure and ferroelectric properties. Further, we build a self-made photovoltaic apparatus and carry out the photovoltaic J-V curve measurement. Through the experiments of photovoltaic parameters under different polarization states with the opposite directions and "eliminating polarization" treatment, we conclude that the polarization nearly has no effect on the photovoltaic output and the interface effect dominates the carrier separation. Then we apply a sequence of cycling electric polarization on this PZT film and continue to find out its manipulation of photovoltaic parameters. After 1.03×108 numbers of cycling polarization treatments, both Voc and Isc reduce to nearly half of initial states. We eventually conduct a series of experiments on ferroelectric remnant polarization, capacitance distribution and energy band structures under different cycling polarization, and attribute the manipulation of cycling electric polarization on the ferroelectric photovoltaic effect to the long-range migration of defect charges (e.g. oxygen vacancies) from inside the film to the electrodes.By inserting a ZnO semiconductor layer with spontaneous polarization into the ferroelectric ITO/PZT/Au film, we develop a new approach to manipulate the ferroelectric photovoltaic effect by introducing the interface polarization coupling. This layered composite film is characterized by a series of experiments to verify microstructure and ferroelectric properties. The subsequent asymmetric hysteresis tests of both polarization and photovoltaic parameters to the applied voltage confirm the existence of interface polarization coupling effect. Then the effects of polarization coupling strength, ferroelectric polarization switching, ZnO layer thickness and the layered heterostructural structure on the photovoltaic effect of the PZT-ZnO bilayer films are systematically investigated and clearly presented. It is concluded that after inserting a ZnO layer the power conversion efficiency of the PZT-ZnO heterostructural film is improved by nearly two orders of magnitude and the polarization modulation ratio is increased about four times. Finally, according to the these experiments and analysis of the whole energy band structures, it is demonstrated that the enhancement and manipulation of ferroelectric photovoltaic effect can be ascribed to the accumulation or depletion of free carriers in response to switchable polarization charges, not only tuning both depletion layer width and interfacial barrier height but also changing the net electric field distribution within the whole heterostructure.For the first time, we realize the magnetic manipulation of photovoltaic effect in the 0-3 type multiferroic composite films and obtain the maximum magnetic modulation ratios of Isc and Voc to be as high as 13.7% and 12.8% upon the application of 6 kOe DC magnetic field. Firstly, a series of experiments are performed to characterize the microstructural and magnetoelectric properties of the film. We subsequently carry out the photovoltaic measurements in our self-made apparatus and verify the inhibiting effect of magnetic fields on the photovoltaic output of composite film by eliminating the influence of thermal effect. Then the effects of magnetic field strength and incident light intensities on the magnetic manipulation of photovoltaic effect are systematically investigated. Through remnant polarization measurements by PUND method under various magnetic fields and analysis of the whole energy band structures, we elucidate the mechanism of manipulating photovoltaic effect by magnetic fields and attribute it to the magnetoelectric coupling interaction via interfacial stress transferring at nanoscale. In detail, when applying a magnetic field, the stress is transferred to the PZT matrix by the magnetostrictive effect of CFO, not only suppress the depolarization field but also reconstruct the energy band structures at CFO-PZT interfaces.