Study On Preparation And Photovoltaic Cells Of Double-perovskite Bi₂F_eMnO₆ Epitaxial Oxide Thin Film

Multiferroic materials are two kinds of ferroelectric materials with ferroelectric,ferromagnetic and ferroelastic properties.Because of its ferroelectric and spontaneous polarization characteristics,multiferroic materials usually have ferroelectric photovoltaic effect.As we all know,the open circuit voltage of the traditional PN junction photovoltaic cell is limited by the semiconductor band gap.However,the characteristic of the ferroelectric photovoltaic effect makes its open circuit voltage break through the limit of band gap and can reverse of the built electric field,that is to realize the reversal regulation of the photovoltaic effect.BiFeO₃?BFO?is a typical multiferroics with room-temperature ferroelectricity,antiferromagnetism and ferroeletric photovoltaic effect.But BFO is a wide band gap?2.7 e V?semiconductor with a low absorption coefficient of visible light and large leakage current,which makes its photocurrent very small and easy to depolarize.BiMnO₃?BMO?is a ferromagnetic and low-temperature ferromagnetic material at room temperature.It has a smaller optical band gap?1.0¹.2 e V?,but BMO is a metastable phase,and it is not easy to prepare a thick film.This drawback limits the application in the photovoltaic field.In this dissertation,Bi₂FeMnO₆?BFMO?double perovskite thin films are chosen as the research object.The growth,electrical properties and photovoltaic cell properties of BFMO thin films are systematically studied.It is found that BFMO keeps the ferroelectric properties of BFO while remains the high absorption coefficient of BMO to visible light region.Compared with BFO,the short-circuit current of BFMO photovoltaic cells is enhanced significantly.The specific research results are as follows:1.Using Pulsed Laser Deposition?PLD?method,we fabricated BFMO epitaxial thin film on the?001?Sr Ti O₃?STO?substrate with the conductive Sr Ru O₃?SRO?layer.Furthermore,Fe-Mn ions shows a high degree of ordering along the [111] direction.By PLD method we prepared BFMO thin films on the substrate STO?001?withthe conductive SRO layer.Our work reveals that the growth of SRO thin film are sensitive to the deposited temperature and direction of substrate.We found that BFMO surface is rougher and exists the impure phase with increasing the thickness of BFMO.However,oxygen pressure has little effect on the morphology of the film.-2,Phi X-ray diffraction?XRD?scans and Atomic Force Microscope?AFM?measurenmts have confirmed that BFMO is a good-quality epitaxial thin film.Reciprocal Space Mapping?RSM?further shows that BFMO is epitaxial.The asymmetry XRD scan demonstrates that Fe-Mn ion exhibits a high degree of ordering along the [111] direction.2.The electrical property of BFMO epitaxial thin film is studied.we confirm that BFMO epitaxial thin film has downward self-polarization and good ferroelectric retention.However,it is found that BFMO has a large leakage current,and the dominant leakage mechanism is analyzed.The electrical property of BFMO epitaxial thin film was investigated by Au/BFMO/SRO/STO?001?capacitor structure.At first,we used Piezoelectric Force Microscopy?PFM?to confirm BFMO ferroelectricity and a good ferroelectric retention.Both domain flipping and rectifying effect demonstrate that BFMO has the downward self-polarization.Because of the large leakage of BFMO,we analysis five leakage mechanisms in the film,involving Ohmic Conduction,space-charge-limited conduction,Poole-Frenkel emission,Schottky emission and Fowler-Nordheim tunneling.Ohmic conduction works under 80 k V/cm.From 80 to 1350 k V/cm at negative bias,the leakage current is mainly governed by PF emission;at positive bias,it might be subject to the combination of SCLC and PF emission.Above 1350 k V/cm,Schottky emission dominates the leakage current.We fitted and calculated the Schottky barrier height at the Au/BFMO and BFMO/SRO interface are 0.55 e V and 0.53 e V,respectively.The zero field trap ionization energy of BFMO is calculated to be 0.4 e V based on the analysis oftemperature leakage current.The energy band alignment of the capacitor was constructed by Ultroviolet Photoelectron Spectrometer?UPS?,and the conduction type of BFMO is n type.3.The optical property of BFMO epitaxial thin film is also studied.BFMO has a large absorption coefficient of the visible light region.Compared with BFO,BFMO thin-film photovoltaic cells have larger photocurrent.Transmitted spectra of BFMO thin films show that the absorption behavior of nearinfrared light is superior to that of BFO.BFMO has a wide absorption spectrum and solve partly the problem of relatively high band gap and low absorption of BFO.It is possible to generate larger photocurrent.We fabricate ITO/BFMO/SRO/STO?001?as a prototype ferroelectric cell and investigate its ferroelectric photovoltaic effect.We construct the energy band alignment of ferroelectric cell.It is found that the BFMO film exhibits a rectifying characteristic of the backward diode affected by the Schottky barrier,which is different from the rectification characteristic of the forward diode of the capacitor structure with selfpolarization effect.BFMO has a good ferroelectric photovoltaic effect.The measurement shows that the short-circuit current density of its photovoltaic cells is about 20 /,two orders of magnitude higher than that of BFO?⁰.1 /?.