Synthesis And Reversible Photovoltaic Effect Of BiFeO₃Based Ferroelectric Thin Films

BiFeO3(BFO) is the only known multiferroic material that has bothferroelectric and anti-ferromagnetism properties in room temperature, andits multiferroic properties allow its wide application probability in storage,modulator, sensor and actuator. Meanwhile, as a semiconductingferroelectric material, BFO has a relatively narrow optical band gap: about2.7eV, which is smaller than other ferroelectrics, giving BFO greaterapplication potentials in visible range. Hence, the study of photovoltaiceffect and it’s coupling with ferroelectric properties for BFO basedferroelectric thin films is a hot issue. Those researches have great scientificvalue for the development and application of novel photoelectron andphotovoltaic materials.In this paper, a series of BFO-based ferroelectric thin films wereprepared on FTO conductive glasses using sol-gel method. Then, themethods of improving the photovoltaic properties of BFO ferroelectric thinfilms and their mechanisms were studied using various means includingsolid solution method, doping modification and introducing heterostructure.The primary research results are as follows:(1) Pure phase perovskite structure BiFeO3-(Na0.5Bi0.5)0.94Ba0.06TiO3(BFO-NBT) solid solution polycrystalline thin films were successfullyprepared on FTO substrate using sol-gel method. The thin film has a flatand dense surface, and its grains show a uniform size with a diameter ofaround100nm; The introduction of NBT inhibited the formation ofsecondary phases and improved the (100)-orientation; The band gap (Eg)of the film increased slightly when the amount of NBT was increased continuously, while the corresponding maximum conversion efficiencyshowed a deduction trend and the photovoltaic property kept deteriorating.The open-circuit voltage and short-circuit current density of pure BFO thinfilm are0.32V and11.6μA/cm2, respectively. Moreover, polarization haslittle impact on the photovoltaic effect and diode effect of pure BFOsample.(2) Sr-doped BFO thin films (BSFO) were prepared on FTO substratesusing sol-gel method. Slight peak separation was observed in BSFO thinfilm at the degree of32°according the XRD characterization, indicatingSr-doping contributed to the presence of rhombohedral perovskite stucture;the film has good surface quality. Its RMS is about7nm in an area of2μm2μm, and the grains show a uniform size with a diameter of about60nm; while the Sr doping has little impact on the band gap (Eg), itimproved the photovoltaic property of the film significantly. Theopen-circuit voltage and short-circuit current density of BSFO thin film are0.4V and37μA/cm2respectively, and the maximum light electricityconversion efficiency when film was doped is about4times that withoutdoping; The doped film showed a switchable but unstable photovoltaiceffect and diode effect. Based on the PFM experiment result, we give thedetailed description of the photovoltaic effect and its switchablemechanism of polycrystalline BFO based thin films, for the first time.(3)(Na0.5Bi0.5)0.94Ba0.06TiO3/BiFeO3heterostructure thin films(NBT-BT/BFO) were prepared on FTO substrates using sol-gel method.Pure phase perovskite structures were discovered in those thin films for theheterostructures with XRD characterization; the film also has good surfacequality. Its RMS is olny about5.4nm in an area of2μm2μm, and thegrains size is about100nm; its band gap (Eg) is fitted to be about2.76evusing Trauc equation, indicating the introduction of NBT-BT bufferheterogeneous layer has little impact on Eg. Compared to samples withoutNBT-BT buffer heterogeneous layer, the open-circuit voltage andshort-circuit current density of Au/BFO/NBT-BT/FTO heterostructure showed significant increase, reaching Voc=0.5V, Jsc=18.9μA/cm2,respectively. A qualitative analysis on the photovoltaic mechanism of theheterostucture was given and the barrier height was estimated by fitting thevariable temperature J-V curves; When ITO top electrode was used, theopen-circuit voltage and short-circuit current density of the structure canreach Voc=0.71V，Jsc=45μA/cm2, respectively, and the light electricityconversion efficiency can be3.4times of that when Au top electrode wasused.