Use this URL to cite or link to this record in EThOS: https://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.752510
Title: Photoelectric processes in ferroelectric/multiferroic materials
Author: Yang, Mingmin
ISNI:       0000 0004 7425 6404
Awarding Body: University of Warwick
Current Institution: University of Warwick
Date of Award: 2018
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Abstract:
Photoferroelectrics, which is defined as the interaction of ferroelectric materials with light, has attracted renewed attention recently and emerged as a topic of both fundamental interest and technological importance. It not only provides potential applications in sensors and photovoltaic devices but also offers a fertile playground to gain insight into the physics of ferroelectricity. As a prominent example, the bulk photovoltaic effect manifested in the ferroelectric materials under illumination gives rise to an anomalous open-circuit photovoltage exceeding the bandgap as well as a light polarisation-dependent photocurrent, offering an alternative approach to boost the solar energy conversion efficiency. Although it has been established for decades, the field is still in its fancy and many fundamental issues remain to be resolved to fully exploit its potential. In the first part of this thesis, we focus on the photoelectric processes in the bulk photovoltaic effect of bismuth ferrite to unravel respectively the essential role of the sub-bandgap levels, its correlation with ferroelectric polarization and role of domain walls in conduction of photovoltaic current. Results demonstrate the sub-bandgap levels is at the electronic origin of the bulk photovoltaic effect in bismuth ferrite. The activity of the sub-bandgap levels in the photoelectric processes can be effectively utilized to tailor the ferroelectric photovoltaic performance. Also, contrary to the common intuition, we prove the independence of the bulk photovoltaic effect on the ferroelectric polarization. We also found that the ferroelectric domain walls can facilitate the conduction and collection of the photocurrent originated in the bulk photovoltaic effect despite its adverse effect on the photovoltage. Inspired by the abundant phenomena in the photoferroelectric field, we explored the light-induced reversible manipulation of the ferroelectric polarization in a deterministic way. This interesting issue is successfully addressed in this thesis by utilizing a combination of the bulk photovoltaic effect and a nanoscale electrode. The collection of photocurrent by an atomic force microscope tip generates a giant electric field locally, enabling ferroelectric switching. By tuning the direction of the photocurrent via either illumination areas or light polarization, the ferroelectric polarization can be reversibly controlled. At the last part of the thesis, we creatively generalised the bulk photovoltaic effect, which was originally constrained to the non-centrosymmetric materials, to a universal effect allowed in all the semiconductors irrespective of their symmetry by the mediation of the flexoelectric effect. This new photovoltaic effect, termed as flexo-photovoltaic effect, may offer a new mechanism to enhance solar cell efficiency. The research works studied in this thesis not only provide fundamental insights into the interactions of ferroelectrics with light but also largely expand the scope of photoferroelectrics into centrosymmetric materials.
Supervisor: Not available Sponsor: University of Warwick
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID: uk.bl.ethos.752510  DOI: Not available
Keywords: QC Physics
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