Effects Of Non-stoichiometry And Electrode Interfaces On The Performance Of BaTiO₃-based Ferroelectric Photovoltaic Devices

Ferroelectric materials,such as BaTiO₃,BiFeO₃ and Pb?Ti,Zr?O₃,are characterized by a spontaneous polarization,which could create a steady photocurrent under light irradiation,named ferroelectric photovoltaic?FPV?effect.This effect has attracted much attention because of its ultra-high photovoltaic voltage,but its specific origin is still controversial.Generally,the intrinsic FPV effect is entangled with other photovoltaic effects,such as the Schottky junction effect at the electrode interface,which makes the observed photovoltaic phenomena extremely complex and difficult to understand.In view of the above,this study systematically elaborates the relationship between intrinsic FPV effect and electrode interface effect under different conditions by adjusting the intrinsic FPV effect through non-stoichiometry induced microstructural changes,and adjusting the photovoltaic effect at the electrode interfaces by changing the electrode materials.The main research contents and results are as follows:1.Photovoltaic properties and mechanisms of non-stoichiometric BaTiO₃ ceramics?1?A series of BaTiO₃ ceramics?Ba/Ti=0.92¹.05?were fabricated by tape casting method.The basic physical properties show that the Ba/Ti ratio has a significant effect on the microstructure of the materials.Under the same sintering conditions,the grain size decreases from 2³?m?Ba-excess?to less than 1?m?Ti-excess?,but the effect of non-stoichiometry on the ferroelectric properties and optical absorption properties of the materials is relatively small.The photovoltaic performance test shows that the Ba/Ti ratio has a significant impact on the photovoltaic performance.When the deviation of Ba/Ti ratio from stoichiometric ratio is less than 1%,the ferroelectric photovoltaic performance is improved by more than10 times.?2?The phenomenon of photovoltaic inversion induced by non-stoichiometry was observed for the first time.TEM studies showed that this phenomenon was closely related to abnormal grain boundaries?GB?in Ba-excess samples.A competition model between intrinsic FPV effect and Schottky junction effect was proposed.Compared with Ti-excess samples with 1-2 nm wide GB,the GB of Ba-rich samples reach 10-15 nm.The increase of the width of GB raises the potential step at GB,which leads to the deteriorating of intrinsic FPV effect,resulting in photovoltaic inversion.2.Study on electrode interface effect in BaTiO₃-based ferroelectric photovoltaic devices?1?The photovoltaic effect at electrode interface and its characteristics:In the unpolarized state,the photovoltaic devices with Al/BTO/Al and Au/BTO/Au structures have stable photovoltaic output,and the photovoltaic output direction changes with the change of illumination direction.Under the same illumination direction,the photovoltaic output direction of Al/BTO/Al is opposite to that of Au/BTO/Au.Based on the above characteristics,a double-interface effect model is proposed.Because of the different illumination intensity,the photovoltaic output at the interface of the two electrodes partly cancels with each other and produces a stable output.?2?Collaborative design of interface effect and intrinsic FPV effect:Firstly,a photovoltaic device with Au/BTO/Al structure is designed to make the photovoltaic output at the interface of two electrodes synergy with each other.The experimental results show that the energy conversion efficiency of the device has been increased by more than 10 times.Secondly,the device is polarized by electric field with a specific direction,which further synergizes the interface effect with the intrinsic FPV effect.The test results show that the energy conversion efficiency of the device is further improved by 3 times.