|Title:||Interface effect on the photovoltaic properties of BiFeO₃|
|Advisors:||Huang, H. T. (AP)|
Choy, S. H. (AP)
Hong Kong Polytechnic University -- Dissertations
|Department:||Department of Applied Physics|
|Pages:||139 pages : color illustrations|
|Abstract:||Ferroelectric-photovoltaic (FePV) devices employing a homogeneous ferroelectric material as the light absorbing layer, have been investigated intensively during the past decades. The physical mechanism governing the FePV effect is distinctly different from that of a typical p-n junction type photovoltaic (PV) effect, whose open-circuit voltage is limited by the bandgap of the light absorbing material. Therefore, FePV material is regarded as the potential candidate for above-bandgap solar cells. Among all the FePV materials, BiFeO₃ (BFO), possessing simultaneous ferroelectric and antiferromagnetic orders at room temperature, is most widely studied in the last decade. In this thesis, systematic experimental work has been carried out to investigate the FePV effect in BFO. First, the influence of secondary phase on FePV of BFO thin film was examined. Pulsed laser deposition (PLD) was used to fabricate epitaxial BFO thin films. Three different targets were used for laser ablation and different ablation parameters (oxygen pressure and laser pulse frequency) were tried to obtain phase pure BFO thin film. Our results demonstrated that the photocurrent and photoconductivity of BFO thin films are closely related to Bi₂O₃ secondary phase and bismuth vacancies. A large photocurrent density of 12 mA/cm², a large remnant polarization of 60 μC/cm² and good switching behaviour were observed in optimized epitaxial BFO films.|
Second, the influence of metal-semiconductor interface effect on BFO FePV was studied. Titanium (Ti) and gold (Au) metals are used as symmetric in-plane electrodes, respectively, on optimized BFO thin films. Since the work functions of Ti and Au are different, which are above and beneath the Fermi level of BFO, respectively, Ohmic contact is formed at the Ti/BFO interface and Schottky contact is formed at the Au/BFO interface. The contact types were confirmed by Kelvin probe force microscopy. The photovoltaic behaviours of these two types of electrode BFO thin films were performed and the photocurrent is larger in a BFO thin film with an Ohmic contact. Last, but not least, the strain effect on BFO FePV behaviour was studied. Epitaxial BFO thin films are deposited on LaAlO₃ (LAO) [1 0 0] and 0.67Pb(Mg₁/₃Nb₂/₃)O3-0.33PbTiO₃ (PMN-PT) [1 1 1] substrates. No obvious increase in short circuit photo-current was detected in BFO/LAO sample which has a tetragonal-like BFO phase, as confirmed by Raman spectroscopy. However, large photoconductivity at large voltage bias was obtained. A small but non-trivial photovoltaic effect was observed in BFO/PMN-PT sample with a rhombohedral structure. It is concluded that the FePV effect is also closely related to the symmetry group of the lattice structure. A prominent photo-response was observed in strain-free electrospun BFO nanofibers, which may have potential applications due to the flexible nature of the nanofibers.
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