The Photovoltaic Effect Of Bi_3.15Nd_0.85Ti₃O₁₂ Thin Films

Ferroelectric thin films have become a promising candidate material for future photovoltaic applications. Recently, the photovoltaic effect of ferroelectric materials has aroused intensive research interest for their distinct pathway that leads to the separation of electrons and holes and their high open circuit voltage. In the past few decades, many efforts have been made to study the photovoltaic effect of ferroelectric thin film, while the photovoltaic mechanism of the ferroelectric thin film is still controversial. In this thesis, Bi3.15Nd0.85Ti3O12(BNT) ferroelectric thin films were fabricated by sol-gel method, and the annealing conditions were controlled to achieve the desired ferroelectric and dielectric properties. The UV light source, scanning electron microscope, semiconductor test system and ferroelectric analyzer were used to study the BNT thin films. The current-voltage behaviors of the BNT thin films under different conditions, such as annealing conditions, polarization state, light intensity, light wavelength and illumination time, were investigated. The main research work and results can be summarized as follows:1. Dependence of photovoltaic effect on annealing conditionsThe BNT ferroelectric thin films were prepared under different annealing temperatures(600 °C, 650 °C and 700 °C) and atmospheres(N2, O2 and Air), and the microstructure, electrical properties and photovoltaic response were characterized. The results show that the surfaces of the BNT thin films with different annealing conditions are uniform, compact, smooth and crack-free. With the increase of the annealing temperature, the grain size becomes larger and the open circuit voltage(Voc) of the BNT thin film decreases from-1 V to-0.5 V. With the increase of oxygen concentration, the ferroelectric performance of the BNT thin film becomes better, the short circuit current density(Jsc) increases from 2.5 μA/cm2 to 4 μA/cm2, and the Voc decreases from-0.35 V to-0.43 V. These phenomena are resulted from the inhibition of the oxygen vacancies of the BNT thin film.2. Dependence of photovoltaic effect on polarization stateTo study the effect of the polarization state on the photovoltaic response of BNT ferroelectric thin films, the J-V curves of the BNT thin films were tested at different polarization states. The results indicate that the direction of the short-circuit current(Isc) is always opposite to the polarized electric field. The contribution from the ferroelectric polarization can be estimated by separating the photovoltaic response into two independent components which are Vp from the ferroelectric polarization and Vbi from the internal bias field. The results show that the contribution from the polarization is much larger than that from the internal field.3. Dependence of photovoltaic effect on the light intensity and wavelengthThe effects of light intensity and light wavelength on the photovoltaic response of BNT ferroelectric thin films were investigated. Jsc is observed to increase linearly with the increase of the illumination intensity, and Voc becomes saturate at high illumination intensity. The photocurrent of the BNT thin film was measured with the function of light wavelength. Jsc starts to increase significantly when the photon energy is larger than 2.8 eV. And Jsc reaches the maximum when the photon energy is 3.37 eV which is closely corresponding to the measured band gap 3.3 eV of the BNT thin film. The result confirms that the photocurrent is different from the current of pyroelectric effect which is nearly independent with the light wavelength.