Preparation And Photoelectric Properties Of Bismuth Ferrite Thin Films

Bismuth ferrite(BiFeO₃)not only has large residual polarization but also has a relatively narrow bandgap in ferroelectric materials,and was considered as a promising ferroelectric material.Nowadays it was proved that BiFeO₃ thin films can produce the open circuit voltage higher than its bandgap as the absorbed layer of solar cells,and this makes it become a hot spot in the field of ferroelectric photovoltaics.However,it still faces the main problems caused by the complicated preparation of BiFeO₃ thin films,and unclear mechanism of the ferroelectric photovoltaic effect.The objectives of this thesis are to prepare BiFeO₃ thin films by radio frequency magnetron sputtering,and to investigate the influence of annealing temperature and sputtering time on the morphology of BiFeO₃ thin films,as well as absorption,ferroelectric properties and photovoltaic effect.In addition,it is found that the concentration of Oxygen element in BiFeO₃ thin films decreases over time in different gas environments(air,oxygen and vacuum).The research works in this thesis are summarized as followings:1.BiFeO₃ thin films were deposited on the silicon wafer and then annealed under different temperature.The BiFeO₃ thin films'morphology was obtained by scanning electron microscope,and BiFeO₃ thin films with better quality was obtained after optimizing the preparation processes.Moreover,the composition of BiFeO₃ thin films was analyzed by EDAX,and the effect of annealing on the composition of BiFeO₃ thin films was studied.The changes in the composition of the BiFeO₃ thin films in air,oxygen and vacuum were discussed.The experimental results show that the changes in the composition of BiFeO₃ thin films is in close correlation with temperature and gaseous environment.2.The absorption of BiFeO₃ thin films was measured by optical test system.Experimental results show that the bandgap of BiFeO₃ thin films is about 2.4 electron volts before annealing,and 2.27 electron volts after annealing at 450 degrees temperature.It indicates that annealing can narrow the bandgap of BiFeO₃ .Furthermore,the hysteresis loops of the BiFeO₃ thin films were obtained as annealed at 450 degrees temperature and without annealing.According to the experimental results,it shows that annealing the thin films at 450degrees temperature can increase the residual polarization of BiFeO₃ and reduce the coercive field of BiFeO₃ .3.The influence of electric-field prepolarization on the photovoltaic effect of BiFeO₃ thin film solar cells was investigated.The experimental results show that open circuit voltage and short circuit current of BiFeO₃ thin film solar cells in the condition of both electric-field prepolarization and light conditions are higher than those one only under the light,and electric-field prepolarization can enhance photovoltaic effect of BiFeO₃ thin film solar cells.In addition,it is found that both the open circuit voltage and the short circuit current gradually decrease and then tend to be steady after electric-field prepolarization.Moreover,the results are interpreted through polarization deflection of electric dipoles in ferroelectric.