Preparation Of Ca And/or Mn Doped BiFeO₃ Films And Study On Their Photovoltaic Properties

Facing the increasingly serious energy crisis and environmental problems,it is urgent to develop alternative green energy.Solar energy is widely used because of its environmental protection,pollution-free and inexhaustible characteristics.In recent years,more and more attention has been paid to ferroelectric materials due to the breakthrough of photovoltaic applications.The open circuit voltage of ferroelectric photovoltaic technology is not limited by the optical band gap of ferroelectric materials,and the photovoltaic performance can be adjusted by adjusting the depolarization field in ferroelectric materials.BiFeO₃(BFO)is one of the most attractive ferroelectric photovoltaic materials because of its large remanent polarization(?60?C/cm²)and relatively narrow band gap(<2.7 e V).The biggest obstacle to the development of BFO based photovoltaic devices is the high leakage current density of materials.At present,the photocurrent of ferroelectric materials is basically between n A/cm²??A/cm²,which results in the low conversion efficiency of BFO thin film photovoltaic cells.At present,the photovoltaic performance can be improved by means of ion doping,band regulation,polarization regulation and other methods,in which ion doping is the most direct and effective way.In this work,BFO is modified by A-site Ca ion,B-site Mn ion and co doping of Ca and Mn ion.High quality epitaxial BFO based films are prepared by pulse laser deposition.The influence of ion doping on optical properties,ferroelectric properties and photovoltaic properties of BFO is studied.The research contents and conclusions of this work are as follows:1.Optimization of the preparation process of BFO epitaxial thin films:taking pure BFO ceramics as the target material,the(100)-oriented single-crystal Sr Ti O₃(STO)substrate was used to grow the thin films.The effects of the substrate temperature and the deposition oxygen pressure on the structure and morphology of BFO thin films were studied.The results show that the growth temperature window of BFO thin film is narrow.When the deposition temperature is 700?and the oxygen pressure is 3.0 Pa,the BFO thin film with pure(100)orientation and rhombohedral perovskite structure is successfully prepared,and the film is the most compact and smooth.The effect of oxygen pressure on the structure and morphology of the films is small,but if the deposition oxygen pressure is too high,the(110)crystal surface appears in the thin film.2.A series of Bi_1-xCa_xFe O₃(x=0,0.10,0.20,0.40)thin films and photovoltaic devices were grown on SRO/STO substrates with Sr Ru O₃(SRO)as the bottom electrode and ITO as the top electrode by pulse laser deposition.The effects of A-site Ca doping on the structure,optical properties,ferroelectric properties and photovoltaic properties of BFO were studied.The results show that all the films have(100)orientation and rhombohedral perovskite structure,but when the Ca content is high,bismuth rich phase appears in the film;Bi_0.9Ca_0.1Fe O₃film has a high-quality epitaxial,and the out-plane relationship with the substrate surface is Bi_0.9Ca_0.1Fe O₃(100)//SRO(100)//STO(100),and the in-plane relationship is Bi_0.9Ca_0.1Fe O₃[110]//SRO[110]//STO[110];The optical band gaps of BFO,Bi_0.9Ca_0.1Fe O₃,Bi_0.8Ca_0.2Fe O₃,Bi_0.6Ca_0.4Fe O₃films are 2.68 e V,2.57 e V,2.56 e V and 2.70 e V,respectively,that is to say,the introduction of Ca ion in A-site can reduce the optical band gaps of BFO to some extent;the remanent polarization of Bi_0.9Ca_0.1Fe O₃films after annealing is 16.74?C/cm²is more than three times of the remanent polarization value of pure BFO,the introduction of Ca ion and annealing process can improve the ferroelectricity of BFO film;Accordingly,Bi_0.9Ca_0.1Fe O₃film device has the maximum short-circuit current density and open circuit voltage(J_SC=0.21 m A/cm²,V_OC=-0.2 V),which are significantly higher than those of pure BFO film device(J_SCand Voc are 0.08 m A/cm²and-0.11 V,respectively).In particular,the introduction of Ca can significantly improve the short-circuit current density of BFO.3.Taking Bi Fe_0.95Mn_0.05O₃?Bi_0.98Ca_0.02Fe_0.95Mn_0.05O₃ceramics as targets and SRO/STO as substrates,Mn ions doped B-site?Ca and Mn ions co-doped Bi Fe_0.95Mn_0.05O₃(BFMO)?Bi_0.98Ca_0.02Fe_0.95Mn_0.05O₃(BCFMO)thin film and its photovoltaic devices were prepared by pulsed laser deposition method.The results of characterization test show that the films are all high quality epitaxial films with compact and smooth surface.The optical band gap of BFMO and BCFMO films were 2.55 e V and 2.41 e V,respectively,and the remanent polarization values were 79.5?C/cm²and 92.5?C/cm²,respectively.It can be seen that the co-doped of Ca and Mn ions can reduce the optical band gap of BFO films more effectively and improve the remanent polarization strength more effectively than the single doping of Mn ions.Accordingly,the V_OCand J_SCof BCFMO thin film devices are-0.29 V and 0.26 m A/cm²,respectively,which are significantly higher than those of BFMO thin film devices(V_OCand J_SCare-0.16 V and 0.16 m A/cm²).That is to say,the co-doped of Ca and Mn ions can improve the photovoltaic performance of BFO more effectively than the single doping of Mn ions.The narrow band gap and large remanent polarization in BCFMO thin film at the same time are the reasons for the significant improvement of its photovoltaic performance.At the same time,the results show that the depolarization field in the BCFMO film can regulate its photovoltaic performance.When the polarization direction is upward,the ITO/BCFMO/SRO film photovoltaic device shows the best performance,and when the polarization direction is downward,the photovoltaic performance of the film photovoltaic device is the worst.In this work,the results show that the co-doped BFO with Ca and Mn ions is expected to obtain ferroelectric photovoltaic materials with both narrow band gap and large remanent polarization,and its good photovoltaic performance parameters show its great application potential in the field of thin film photovoltaic technology.