Study On Ferroelectric Domain Structure And Photovoltaic Effect Of Bi₂FeCrO₆ Epitaxial Oxide Thin Film

Multiferroics,defined as materials that simultaneously exhibit two or more properties of ferroelectricity,ferroelasticity and ferromagnetism in a single phase,have attracted extensive interest during recent years.Under the action of external field such as light and stress,it will produce many unique physical phenomena.Compared with traditional semiconductor solar cells,ferroelectric photovoltaic cells generate open circuit voltage which can break through the material gap width.Moreover,the ferroelectric material has ferroelectric polarization,and the polarization direction can be reversed under an external electric field,and the photo-generated current and voltage can also be reversed and changed accordingly.These advantages show that ferroelectric materials have great application potential in the photovoltaic.Bi Fe O₃(BFO)has attracted more attentions for its ferroelectricity(T_C?1100 K)and antiferromagnetism(T_N?640 K)at room temperature(RT).However,the potential applications of BFO on multiferroic devices have been limited by its weak antiferromagnetism and large optical band gap(2.7 e V).In 2014,Nechache et al.prepared Bi₂Fe Cr O₆(BFCO)double perovskite films with a high degree of Fe-Cr order along the[111]direction.By adjusting the laser pulse frequency,the order of Fe-Cr can be adjusted,thereby adjusting the band gap of BFCO film.The most remarkable finding was that BFCO film,as the light absorber,greatly improved the performance of BFCO ferroelectric photovoltaics device and achieved a world-record efficiency of 8.1%.However,as a new multiferroic oxide film material,the ferroelectric domain structure of BFCO is still unclear.This thesis takes BFCO epitaxial film as the research object,and mainly discusses the growth process,domain structure,electrical and optical properties of BFCO epitaxial film.The study found that the domain structure of BFCO is similar to that of BFO,and the domain structure can be controlled by changing the substrates.The BFCO keeps the ferroelectric properties of BFO while broadening the absorption of visible light.The specific research results are as follows:1.The high-quality BFCO films were epitaxially grown on SrRuO₃(SRO)/SrTiO₃(STO)(001)and STO(001)substrates by pulsed laser deposition.The influence of deposition temperature,energy,and frequency conditions on the growth of BFCO film was systematically explored,and the best deposition condition of BFCO film was comprehensively obtained.The X-ray diffraction and Atomic Force Microscopy scanning showed that the BFCO film with a typical step-flow surface morphology was epitaxially grown along the surface of the STO(001)substrate.By using Reciprocal Space Mapping(RSM)measurement,it is confirmed that the out-of-plane lattice parameter of epitaxial BFCO film calculated by fitting is3.974(?).And it was found that the BFCO(103)diffraction peaks split,which implies that the BFCO film has a multi-domain structure.2.The ferroelectric properties and domain structure of BFCO films epitaxially grown on SRO/STO(001)and STO(001)substrates were investigated.Using piezoelectric force microscopy(PFM)and RSM,it was determined that the domain structure of the 200 nm thick BFCO thin film grown on SRO/STO(001)has eight polarization vectors.This is due to the combination of the flexural electrical effect and Sr O self-termination of the SRO layer,which causes the BFCO to have both upward and downward spontaneous polarization.In order to eliminate the influence of the bottom electrode of SRO,PFM was used to determine the domain structure of a 200 nm thick BFCO film directly grown on STO(001).It is confirmed that the spontaneous polarization direction is only upward,and the in-plane domain structure have four polarizations in some regions and degenerates to two polarizations in some regions.It is speculated that this phenomenon is due to the combination of Ti O₂-terminated surface of STO(001)substrate and the nonuniform distribution of Fe and Cr elements,resulting in the degeneration of the polarization vectors of BFCO.The PFM was used to characterize the local ferroelectricity of the BFCO films.At the same time,the P-E hysteresis loop of Au/BFCO/SRO/STO(001)film capacitor was characterized at room temperature by a ferroelectric tester,which proved that BFCO is room temperature ferroelectric material.3.A photovoltaic device with ITO/BFCO/SRO/STO(001)structure was prepared.It was found that compared with BFO,BFCO broadens the absorption range of sunlight and has better carrier separation efficiency,which can generate larger photocurrent.By comparing the absorption spectra of BFCO and BFO films,it is found that BFCO broadens the absorption in the near-infrared band and improves the absorption rate of light.At the same time,ITO/BFCO/SRO/STO(001)and ITO/BFO/SRO/STO(001)photovoltaic devices were prepared.Under standard sunlight,violet light and green light,BFCO broadens the absorption range of sunlight and has better carrier separation efficiency,which can generate larger photocurrent and open-circuit voltage than BFO.By applying voltage field,we control the polarization of the BFCO film,which increases the photocurrent and achieves a enhancement of the photovoltaic effect.