Research On The Preparation And Photovoltaic Effect Of BiFeO₃ Thin Films

BaTiO₃（BTO） and Pb（Zr,Ti）O₃（PZT） are classical ferroelectric materials, they all have photovoltaic effect. While their photovoltaic efficiencies are small because of the wide band gap. In recent, a new kind of ferroeletic material BiFeO₃（BTO） attracted extensive attentions. It exhibits antiferromagnetic ordering（TN=650 K） and ferroelectric behaviour with a high ferroelectric Curie temperature（TC =1110 K）. It has a rhombohedrally distorted perovskite structure with oxygen octahedrons diagonal axis rotation of angle around the axis of body diagonal. Compared with the single crystal block and epitaxial films, polycrystalline films are easy to process and cheap. It owns obvious advantages in practical applications, such as photoelectric detector, optical sensors, optical modulator and converter. In this dissertation, BFO films were synthesized by sol-gel method, and A site substitution was doped to improve the properties of the films. We studied the ferroelectricity, UV-Vis properties and photovoltaic properties at room temperature. The major work can be summarized as the following:Porous BFO films were prepared on FTO/glass substrates by a sol-gel method annealing at 450-600 °C. XRD results show that the films are highly（100） preferred oriented. the pore-radius of the films annealed at 450°C-600 °C are 2 μm, 2 μm, 6 μm and 1 μm, respectively. The band gaps are 2. 31 e V, 2. 50 e V, 2.51 e V and 2.62 e V for the films annealed at 450 °C, 500 °C, 550 °C and 600 °C, respectively.Significant ferroelectric photovoltaic effect was observed in the porous BFO film annealed at 600 °C, the short circuit current density is 3.67×10-8A /cm2 and open circuit voltage is-0.262 V.Na substituted BFO film structures have been prepared on FTO/glass substrates by a sol-gel method. XRD results showed that all films adopt random orientation and R3c perovskite structure. UV-Vis absorption results indicated that the films exhibit intense absorbance around 450 nm. The band gaps are 2.59 e V, 2.63 e V, 2.62 e V and 2.52 e V for the films substituted with 0%, 5%, 10% and 20% Na, respectively. Substantially enhanced photovoltaic effect was observed in the 20% Na substituted BFO film. The short circuit current density is 1.224 μA/cm2 and the open circuit voltage is-0.65 V for the 20% Na substituted BFO film.K substitited BFO films have been fabricated on FTO/glass substrates using the sol-gel method. XRD results show that the films are randomly oriented. UV-Vis absorption results indicate that the porous films exhibit intense absorbance around 500 nm. The band gaps are 2.52 e V, 2.59 e V, 2.64 e V and 2.62 e V for the films substituted with 0%, 5%, 10% and 20% K, respectively. By 20% K substituted, the photovoltaic effect was substantially enhanced. Bulk photovoltaic effect was observed in the 20% K substituted BFO film. The Bi0.8K0.2Fe O3 film exhibits a short circuit current density of 1.32 μA/cm2 and open circuit voltage of-0.45 V. The Voc can be switched by poling with a voltage larger than coercive voltage.