| Author: | Wong, Ki Hei |
| Title: | Study of influence of oxygen vacancies on photovoltaic effect in lead-free ceramics |
| Advisors: | Kwok, K. W. (AP) |
| Degree: | M.Phil. |
| Year: | 2022 |
| Subject: | Ceramic materials -- Electric properties Photovoltaic effect Ferroelectricity Hong Kong Polytechnic University -- Dissertations |
| Department: | Department of Applied Physics |
| Pages: | xvi, 114 pages : color illustrations |
| Language: | English |
| Abstract: | The ferroelectric photovoltaic effect has received considerable research interest due to its fascinating features, including the above bandgap voltage and switchable polarization-dependent photocurrent. However, during the fabrication process of the common ABO3 perovskite ferroelectric oxides, the formation of defects such as oxygen vacancies inside the ceramic is unpreventable. It has been reported that the defects (oxygen vacancies) would affect the photovoltaic response in ferroelectric photovoltaic devices. For example, the direction of the photocurrent response of the device can be independent of its polarization. In addition, the photocurrent may depend on the distribution of oxygen vacancies inside the ferroelectric material. In this study, we aim to study the influence of oxygen vacancies on the photovoltaic effect in ferroelectric ceramic oxide. The weak photocurrent generated by the ferroelectric material may even be enhanced by modifying the distribution of oxygen vacancies inside the ceramics. In this work, Barium titanate-based ferroelectric ceramic Ba0.9Ca0.1TiO3 (BCT) has been chosen as the based material due to its ferroelectric photovoltaic response and easy doping with other chemicals. In addition, Ca2+ ions in BCT ceramics can act as an acceptor inhibitor to reduce the possibility of the formation of the hexagonal structure. Therefore, the tetragonal structure of the BCT ceramics will be more stable. It is suggested that the formation of oxygen vacancies in BCT –ceramics can be promoted through acceptor doping. Since Fe3+ and Ti4+ share similar radii, Fe3+ has been chosen as the dopant to BCT ceramics. 0.025 mol and 0.1 mol Fe3+ is doped in BCT ceramic to fabricate the Ba0.9Ca0.1Ti0.975Fe0.025O3 (BCT-Fe-0.025) and Ba0.9Ca0.1Ti0.9Fe0.1O3 (BCT-Fe-0.1) ceramics. Ba0.9Ca0.1Ti0.9Fe0.1O3 (BCT-Fe-0.1) and Ba0.9Ca0.1Ti0.975Fe0.025O3 (BCT-Fe-0.025) have been fabricated by conventional solid-state reaction method. To study the influence of oxygen vacancies on the photocurrent response of the BCT-Fe-0.1 and BCT-Fe-0.025 ceramics, they are sintered in different atmospheres (O2, N2 and air) and their oxygen vacancies content are analyzed with the XPS O1s spectrums. The ferroelectric and dielectric properties of the BCT-Fe-0.1 and BCT-Fe-0.025 ceramics sintered in different atmospheres are also examined. It is revealed that the BCT-Fe-0.025 ceramic exhibits ferroelectric properties at room temperature, while the BCT-Fe-0.1 remains non-ferroelectric. In addition, the ferroelectric properties of the BCT-Fe-0.025 ceramics are also affected by the sintering atmosphere. The BCT-Fe-0.025 ceramics sintered in Air (BCT-Fe-0.025-Air) have the largest remnant polarization (4.5 µC/cm2) and saturated polarization (18 µC/cm2), while the BCT-Fe-0.025 ceramics sintered in O2 (BCT-Fe-0.025-O2) have the lowest remnant polarization (2.5 µC/cm2) and saturated polarization (10 µC/cm2). The remnant polarization and saturated polarization of the BCT-Fe-0.025 sintered in N2 (BCT-Fe-0.025-N2) are 3.5 µC/cm2 and 15 µC/cm2 respectively. The photocurrent effect of non-ferroelectric BCT-Fe-0.1 ceramics have been investigated using vertical symmetric ITO and Au electrodes configuration. Photocurrent can be observed in non-ferroelectric BCT-Fe-0.1 oxides, and this should be attributed to the accumulation of oxygen vacancies which generate diffusion current and drift current that contribute to the photovoltaic response. The BCT-Fe-0.1 sintered in O2, N2 and Air have a significantly different photocurrent response. For the BCT-Fe-0.1 ceramics with ITO electrodes, for 30 seconds of A.M. 1.5 solar light illumination, the average photocurrent generated by the as-fabricated BCT-Fe-0.1-Air and BCT-Fe-0.1-O2 ceramics are 2.4 nA (4.8 nA/cm2) and 3.5 nA (6.9 nA/cm2) respectively. However, steady photocurrent cannot be observed in the BCT-Fe-0.1-N2 ceramic and only sharp current peaks (0.38 nA or 0.75 nA/cm2) are observed once the light source is turned on / off. Similar results can also be observed in ceramics with Au electrodes. The distribution of oxygen vacancies of the BCT-Fe-0.1 can be modified by applying an external field at a high temperature. As a result, the photocurrent of the BCT-Fe-0.1 ceramics can be switchable. Furthermore, the photocurrent response of the BCT-Fe-0.1 ceramics can even be enhanced by applying appropriate external electric fields. The ferroelectric photovoltaic effect of BCT-Fe-0.025 ceramics sintered in O2, N2 and Air are also investigated with vertical symmetric Au electrode configuration. The ceramics are poled at 1kV/mm at room temperature. The steady photocurrent generated by the BCT-Fe-0.025-Air, BCT-Fe-0.025-N2 and BCT-Fe-0.025-O2 are 3.34 nA (6.68nA/cm2), 3.2 nA (6.4 nA/cm2) and 2.2 nA (4.4 nA/cm2) respectively. It illustrates that the sintering atmosphere would affect the ferroelectric properties of the BCT-Fe-0.025 ceramics and influence the ferroelectric photovoltaic response. As illustrated in BCT-Fe-0.1 ceramics, the distribution of oxygen vacancies inside the ceramics can be modulated by applying external electric fields at high temperatures. To study the influence of the distribution of oxygen vacancies on the photovoltaic response of the BCT-Fe-0.025 ceramics, the ceramics are poled with 1kV/mm at 100°C. The electric field is removed after the ceramic is cooled down for preventing depolarization. The enhancement of the photocurrent responses of BCT-Fe-0.025-O2 and BCT-Fe-0.025-N2 cannot be observed. For the BCT-Fe-0.025-Air, a significant enhancement of photocurrent response is resulted after poling at 100°C. Compared with the ceramic poled at room temperature, the average short-circuit photocurrent of the ceramic is 3 times larger (14.5 nA vs 3.34 nA). It is suggested that the accumulation of oxygen vacancies near the bottom electrodes contributes additional photocurrent that enhances the photocurrent performance of the BCT-Fe-0.025-Air ceramics. It is proposed that the distribution of oxygen vacancies can be modulated to enhance the ferroelectric photovoltaic response. However, further investigations are still required to study deeper the mechanisms behind the influence of the distribution of oxygen vacancies towards the ferroelectric photovoltaic effect. |
| Rights: | All rights reserved |
| Access: | open access |
Copyright Undertaking
As a bona fide Library user, I declare that:
- I will abide by the rules and legal ordinances governing copyright regarding the use of the Database.
- I will use the Database for the purpose of my research or private study only and not for circulation or further reproduction or any other purpose.
- I agree to indemnify and hold the University harmless from and against any loss, damage, cost, liability or expenses arising from copyright infringement or unauthorized usage.
By downloading any item(s) listed above, you acknowledge that you have read and understood the copyright undertaking as stated above, and agree to be bound by all of its terms.
Please use this identifier to cite or link to this item:
https://theses.lib.polyu.edu.hk/handle/200/11842