Photovoltaic Effect Of Ferroelectric Heterojunction Films Modulated By The External Field

Ferroelectric photovoltaic effect has attracted attention since 2010,which can be attributed to the abnormal open-circuit voltage(V_oc)beyond the limited of optical bandgap.Meangwhile,spontaneous polarization of ferroelectric material could be adjusted by external electric field,whose directioncan be artificially controlled arbitrary switching between two or more states.This switch-ability has been exploited in ferroelectric-based non-volatile memories and logic devices.Compared with ferroelectric ceramics,ferroelectric thin films have faster photoelectric response and easier coupling of micro semiconductor nanodevices,which brings more extensive application scenarios to ferroelectric materials.Therefore,Photovoltaic effect of ferroelectric films has become one of important research directions for ferroelectric materials.It is unavoidable that the photoelectric conversion efficiency of ferroelectric materials is still very low,which is attributed to the extremely low photocurrent generated by the photovoltaic effect of ferroelectric materials.The main work of this paper is based on two lower ferroelectric materials with effective optical band gaps:Bi₅Ti₃FeO₁₅（BFTO）and BiFeO₃（BFO）.The photocurrent of the BFTO thin film was improved by introducing a CuO buffer layer and applying a pulsed electric field.Photocurrent of BFO/ZnO NWs heterojunction is regulated by an external stress field method.The main contents of the study are as follows:First,BFTO is selected as the basis for investigating the polarization-dependent photovoltaic effect of ferroelectric thin films,due to its good anti-polarized fatigue character and high resistivity..The sol-gel method is used to fabricate BFTO films by virtue of its low cost and simplicity.Fundamental measurements such as XRD,macroscopic ferroelectric testing,UV-visible absorption spectroscopy and piezoelectric microscopy have shown that the BFTO film has good crystallinity,excellent ferroelectric property.The BFTO film was irradiated with 405 nm violet light.After applying a pulse voltage of±10 V and a pulse width of 1 ms to the BFTO film,the photovoltaic effect of the BFTO film irradiated with 405 nm violet light showed two different states,namely,the state of upward polarization(P_up)and the state of downward polarization(P_down).Compared with the initial state in the P state,the open circuit voltage of the BFTO film is increased from-0.13V to-0.95V,and the short-circuit current density is increased from+7.64μA/cm² to+43.88μA/cm².The photovoltaic effect is greatly improved in the state of P_upp due to the excellent ferroelectric properties of the BFTO film.In the state of P_down,the direction of open circuit voltage and short-circuit current density of the BFTO film are changed,whose values changs to+0.11V and-6.26μA/cm²,respectively.The reason for the above situation is that the direction of the spontaneous polarization field of the BFTO film has been changed under the influence of an external pulsed electric field.In this paper,the BFTO/CuO heterojunction films were constructed by introducing a low-gap,high-conductivity semiconductor CuO as a buffer layer to further improve the photovoltaic effect of the BFTO film.The P-E and PFM tests of the BFTO/CuO heterojunction films shown that the ferroelectric property of the BFTO/CuO heterojunction is slightly lower than that of BFTO.According to the test of Morte-Schottky,the BFTO film ascribed to n-type semiconductor,which forms a heterojunction with the p-type CuO film and can promote the separation of photogenerated carriers.Compared with BFTO film,the open circuit voltage and short-circuit current of BFTO/CuO heterojunction films increase to-0.16V and+76.20μA/cm²,respectively,in the initial state.After the pulsed electric field was applied to the BFTO/CuO heterojunction film,whose open-circuit voltage was increased to-0.23V and the short-circuit current density was increased to+124.75μA/cm² in the P_upp state.In the state of P_down,the variation trend of photovoltaic properties of the BFTO/CuO heterojunction films are consistent with the BFTO film.That is,the direction of V_occ and the direction of the J_scc are changed.Although the photomodulation ratio of BFTO/CuO heterojunction film is lower than that of BFTO film under polarization,photovoltaic properties of the heterojunction film can still be modulated by the applied electric field.The rise and development of piezoelectric optoelectronics have provided an efficient way to modulate the photovoltaic effect of ferroelectric thin films.The purpose of adjusting the energy band structure of the contact surfaces of the heterojunction composed of a nanowire array with a piezoelectric effect and a ferroelectric material is to adjust the photovoltaic effect of the ferroelectric thin film.This article described in detail the growth method for the growth of ZnO thin films grown along the C-axis direction by magnetron sputtering and the preparation of uniform dense ZnO nanowire arrays by the hydrothermal method.The piezoelectric potential distribution of a single ZnO nanowire under compression and tension was calculated by finite element analysis.The calculation shows that when the ZnO nanowires are subjected to a tensile force along the C-axis direction,the direction of the piezoelectric potential in the ZnO NW is opposite to that in the C-axis direction,which is represented by the negative potential at the lower end and the positive potential at the upper end.In contrast,when the ZnO NW is subjected to pressure in the C-axis direction,the direction of the piezoelectric potential on the ZnO NW is the same as the direction of the C-axis,showing a positive potential at the lower end and a negative potential at the upper end.The principle of photovoltaic effect of heterojunction is modulated in two different states of compression and tension,and a brief analysis is made from the perspective of energy band bending.In experiments,it was found that the photovoltaic effect of the ZnO NWs/BFO heterojunction was reduced under tension.