Study On Double-Perovskite Bi₂FeCrO₆ Epitaxial Films For Ferroelectric Photovoltaic Applications

In recent years,the anomalous photovoltaic effect in ferroelectric materials has attracted extensive research interests.Ferroelectric materials with anomalous photovoltaic effects can not only yield open circuit photovoltages exceeding the band gap of the material,but also allow the control of the open-circuit photovoltage and short-circuit photocurrent of the devices by modifying ferroelectric polarization.However,the band gap of traditional perovskite ferroelectric materials is typically 3-4 eV,which is far greater than the optimal band gap of solar cells?1.4 eV?,making them unsuitable for absorbing sunlight and limiting the improvements of the power conversion efficiency.Lowering the band gap while maintaining ferroelectric properties is a promising route to improve the power conversion efficiency.Double-perovskite Bi₂FeCrO₆?BFCO?is a valuable candicate for ferroelectric photovoltaic solar cells.Density-functional theory calculations have predicted the coexistence of a large remanent polarization?P_r?of 80?C/cm² and ferrimagnetism with a saturation magnetization of 160 emu/cc and a suitable band gap?Eg?of 1.48 eV,making BFCO a potential candidate for solar cells,magnetoelectric coupling,spintronics,and semiconductor devices.In view of this,this dissertation has carried out research on the double-perovskite BFCO.In this dissertation,a pulsed-laser deposition technique was used for prepareation of BFCO epitaxial films.The deposition parameters,magnetic,optical,ferroelectric properties,and the structure-property relationships of BFCO epitaxial films were investigated in depth.Finally,the BFCO epitaxial films were used to fabricate ferroelectric photovoltaic solar cells.The detailed work is as following:1?BFCO epitaxial films were grown on SrTiO₃ single-crystal substrates by pulsed laser deposition?PLD?.The influences of oxygen pressure,substrate temperature,and laser frequency on the phase formation and surface morphology of the BFCO films were studied in details.The deposition parameters were optimized to a substrate temperature700°C,oxygen pressure 0.8Pa and laser pulse frequency 6Hz.The optimized BFCO epitaxial thin films show a narrow rocking curve of 002 Bragg peak with a FWHM about0.08°,and the surface roughness 0.5 nm,indicating high epitaxial quality of the films.In addition,the?-2?XRD pattern of BFCO films exhibits two strong superlattice peaks,which were labeled as 1/2 1/2 1/2 and 3/2 3/2 3/2.Moreover,the relative intensity of the superlattice peaks can be easily adjusted by changing the substrate temperature and the laser frequency,and the highest intensity reaches 4.4%.X-ray photoelectron spectroscopy shows that the valence states of Fe and Cr are mainly+3,and the ordering of Fe/Cr was discussed from the perspective of ionic valence states and ionic radii.2?In previous works,the 1/2 1/2 1/2 and 3/2 3/2 3/2 superlattice peaks were seen as a proof of the B-site ion ordering of the double-perovskite materials.The PPMS and UV-VIS-NIR measurements were performed to investigate the magnetic and optical properties of the BFCO films.It is found that the saturation magnetization of BFCO films with very strong superlattice peaks?R=4.4%?and with no superlattice peaks?R=0?are both smaller than 6 emu/cc,much below the predicted value?160 emu/cc?.In addition,the band gaps of the BFCO films with different superlattice peaks intensities?R=0-4.4%?are about 2.5 eV,which is larger than the theoretically calculated value of the ordered BFCO?1.48 eV?.These results evidence that the Fe/Cr ions in BFCO films with strong superlattice peak intensity are still disordered.The observation of superlattice peaks in itself is not a sufficient proof of chemical order in double-perovskite BFCO films.3?The phase transition of the BFCO films were investigated by using temperature-dependent HRXRD and Reciprocal Space Mapping.It was found that BFCO films undergo a structural phase transition from a low-temperature rhombohedral phase to a high-temperature tetragonal phase near 600 K.Ongoing through such a structural pahse transition the intensity of the superlattice peaks decreased and vanished above 600K completely,and during the cooling process,the superlattice peaks reappeared.In addition,the temperature-dependent Raman spectroscopy showed that BFCO has a characteristic spectrum similar to that of BiFeO₃ at room temperature,and the structure is rhombohedral.As the temperature increases,the Ag-like mode undergoes a blue shift at 573-623 K,and the A₁?LO?mode disappeares,which is consistent with the results of the HRXRD.These results obviously rule out the possibility of the ordered arrangement of B site Fe/Cr.4?The probed ferroelectricity confirmed that the spontaneous polarization direction of BFCO films lies close to the rhombohedral 111 direction.The measured polarization of BFCO films with weak superlattice peaks is about 80?C/cm²,close to the theoretical calculated value.However,BFCO films with strong superlattice peaks show a large polarization about 91?C/cm²,which is enhanced about 15%.The strong superlattice peaks related to the additional displacements of Bi³⁺,Fe³⁺,and Cr³⁺cations along the 111direction.These additional displacements contribute to and enhance ferroelectric polarization.5?In view of the large ferroelectric polarization and relatively narrow band gap,the applications of BFCO films in ferroelectric photovoltaic devices were explored.BFCO films were deposited on conductive Nb-doped SrTiO₃ single-crystal substrates and utilized transparent conductive ITO films were used as the top electrode.The measured open-circuit photovoltage is 1.1 V,the short-circuit photocurrent is 1.6 mA/cm²,and the power conversion efficiency is about 0.9%under the AM 1.5G illumination.In addition,upon applying an external electric field,the photovoltaic output of the solar cell can be effectively regulated due to the change of ferroelectric state of the BFCO films.The optimal photovoltaic output was obtained by applying a sufficiently strong electric voltage on the ITO top electrode to turn the ferroelectric polarization down completely.The photovoltage,the photocurrent density,and the power conversion efficiency were enhanced to 2.8 V,2.3 mA/cm²,and 3.9%,respectively.These results demonstrate that Bi₂FeCrO₆ is a promising candidate for photovoltaic applications.