Preparation Of BiFeO₃ Thin-Film Heterostructure Based On Fe-Site Substitution Strategy And Corresponding Study Of Optoelectronic Properties

Ferroelectric oxide BiFeO₃(BFO)plays an important role in various fields including random storage,photoelectric detection,photoluminescence and so on.Compared with other ferroelectric oxides,BFO has a relatively narrow band gap(E_g<3.0 e V),so it can be a potential candidate for photovoltaic applications.At present,the bottleneck of the development of BFO materials lies in the leakage current caused by oxygen vacancies and the changeable oxidation state of iron ions.Ion doping technology can effectively improve the photoelectric properties of BFO films,which is expected to break through the above-mentioned bottleneck.In this study,BFO thin films were prepared by sol-gel method,and the corresponding thin-film heterojunctions were constructed by matching hole transport layer(HTL)NiO thin films to BFO.In addition,the Fe sites of BFO were doped with Cr or Zn,Bi Fe_1-xCr_xO₃(BFCO)and Bi Fe_1-xZn_xO₃(BFZO)(x=0.2?0.8)were synthesized respectively,and photoelectric response characteristics of BFX(X=Cr,Zn)O/NiO heterostructures in the visible range(Vis)were investigated.The content of this paper consists of three parts as follows:(1)BFO thin films were prepared on FTO substrates by sol-gel method,and the structure,morphology,optical absorption and leakage current of BFO thin films were systematically studied.The BFO/NiO heterostructure was constructed by combining BFO with p-type NiO hole transport layer.The photoelectric tests showed that the heterostructure can effectively reduce the leakage current of the device and improve its photoelectric response speed.(2)BFCO thin films were synthesized by sol-gel method,and BFCO/NiO heterojunction was constructed by using NiO as HTL.The effect of Cr doping ratio on BFCO films was systematically studied.It was proved that Cr doping can effectively inhibit the reduction of Fe³⁺and the generation of oxygen vacancy,so it can effectively reduce the leakage current density of BFCO films.The response time of BCO/NiO heterojunction in Vis can reach the order of milliseconds,which was nearly two orders-of-magnitude lower than that of pure BFO/NiO film and one order-of-magnitude lower than that of single-layer BFCO film.The results showed that the functional interface structure of BFCO-NiO can promote the separation of photogenerated electron hole pairs and improve the carrier transport rate,which would consequently realize the high-speed Vis detection based on BFCO films.(3)BFZO thin films were also synthesized by doping zinc ions in BFO according to the strategy of Fe-site substitution.By optimizing the doping ratio of Zn ions,BFZO bandgap can be reduced from 2.54 e V to 2.0 e V.BFZO/NiO heterojunction was prepared on FTO substrate to further accelerate carrier transportation,so that the heterojunction would achieve a high optical response speed to visible light,and the response time constant can reach 0.36 ms.Such performance was better than similar BFO-based photodetectors reported in the literature.