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Photovoltaic Studies on Ferroelectric Oxides with Scanning Optical Probes

Photovoltaic Studies on Ferroelectric Oxides with Scanning Optical Probes
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The ferroelectric photovoltaics has recently regained much interest due to the scientific importance as well as potential applications in optoelectronics and solar cell devices. The ferroelectric photovoltaic effect are mainly characterized by very large photovoltage output, which is observed to be linearly proportional to ferroelectric crystal length along the ferroelectric polarization, not limited by the band gap of ferroelectric materials. Despite the advantage of large photovoltage, the ferroelectric photovoltaics has never been thought as a viable alternative to conventional solar cell devices because of low power conversion efficiency, which is mainly due to poor electrical transport properties of ferroelectric materials. Furthermore, most of ferroelectric materials have a wide band gap of 3~4eV, limiting their use as solar cell devices due to low absorption of solar radiation. On the contrary, ferroelectric BiFeO3 has a relatively small band gap of 2.7eV and large polarization of about 100μC/cm2, suggesting a possibility of new concept of solar cell devices. Since the early observation of photovoltaic effect in ferroelectric materials, many researchers tried to find a true origin for the ferroelectric photovoltaic effect, which includes asymmetry in carrier excitation from a defect level to the conduction band, the asymmetry of elementary processes such as photoexcitation and scattering in a noncentrosymmetric medium, DC field created by the optical-rectification effect and depolarization field. Recent theoretical calculations indicate that an electrostatic potential step is present across the nanometer-scale ferroelectric domain walls, such as 71°, 109°, 180° domain walls of BiFeO3 and 90° domain wall of PbTiO3, producing intense internal field at the domain walls. The photogenerated carriers are thought to be effectively separated at the domain walls due to the presence of internal electric field, as in the depletion region of conventional semiconductor p-n junction-based photovoltaic devices. The electrostatic potential at the domain walls is additive. As a result, open circuit voltage is observed to be linearly proportional to the number of domain walls between the electrodes, producing large open circuit voltage, much larger than the band gap of ferroelectric BiFeO3. Besides ferroelectric domain wall, the ferroelectric domain itself also contributes to the photovoltaic response of BiFeO3, as observed in monodomain BiFeO3 single crystal. That is, the ferroelectric photovoltaic response is thought to be closely related to the ferroelectric domain structure. In order to investigate the interaction of ferroelectric ordering with light at the microscopic level, the scanning photocurrent microscopy, which is able to map the local photocurrent responses at the sub-micrometer scale, was used. The photovoltaic current of epitaxial (001) BiFeO3 film deposited on (001) SrTiO3 substrate was reversibly changed upon the ferroelectric switching. The sign of local photovoltaic current was found to be in close relation with the local polarization orientation and its magnitude was spatially nonuniform. Specifically, the local photovoltaic response of BiFeO3 was enhanced near charged domain walls, which were produced as a result of domain pinning during ferroelectric switching. Through the spectrally resolved photovoltaic current measurement, the sub-band gap excitation, besides band-to-band transition, was observed near the charged domain walls, suggesting the enhancement of photovoltaic current is related to defect levels inside the band gap, which are able to locally lower the effective band gap of BiFeO3 near charged domain walls. Ionized oxygen vacancies can respond to an electric field, resulting in electromigration of oxygen vacancies toward negative electrode. The electromigrated oxygen vacancies are assumed to be accumulated near charged domain walls, producing defect levels. In other words, the increased photo-generation rate due to locally reduced effective band gap results in the enhancement of photovoltaic current near charged domain walls. The single ferroelectric domain contribution to the ferroelectric photovoltaic effect was also experimentally demonstrated using monodomain (110) BiFeO3 film. The photovoltaic current of monodomain BiFeO3 was reversibly switched upon the ferroelectric domain reversal, as in the case of multidomain (001) BiFeO3 film. The understanding of the microscopic interaction of ferroelectric order with light will provide an opportunity of using ferroelectric materials as a new concept of solar cells and optoelectronic devices. Furthermore, multiferroic character of BiFeO3, where the ferroelectric, ferroelastic and antiferromagnetic order parameters all interact with each other, suggest a possibility of multifunctional devices, in which the photovoltaic effect can be controlled by the electric, magnetic and/or stress fields.
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