Studies on Pb Free Ferroelectric Materials for Photovoltaic Applications
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Ferroelectrics have shown a promising alternative material system for photovoltaic applications. High open circuit voltages in ferroelectric thin films have generated considerable interest in the field of ferroelectric photovoltaics in last the few years. The physical mechanism of photovoltaic effect in ferroelectrics is not completely understood compared to the semiconductor based photovoltaic technology. This dissertation investigates photovoltaic characteristics of ferroelectric films in an attempt to enhance the understanding of ferroelectric photovoltaics. This dissertation presents photovoltaic properties in ferroelectric materials in pure, doped and composite BiFeO3 (BFO) thin films. Thin films were fabricated by Pulse laser deposition (PLD) and RF magnetron sputtering techniques. The growth parameters of the films were optimized to get a pure phase of the thin films for achieving good ferroelectric characteristics for photovoltaic (PV) applications. A measurement setup was established to study PV characteristics of the devices with a light source. The photovoltaic characteristics of the cells on parameters, such as electrical poling, nature of top electrodes, and intensity of illumination, were investigated. The lead free ferroelectric materials with high remnant polarization have been characterized for photovoltaic applications. However the materials have shown high leakage characteristics due to Fe+2 ions, leaving oxygen vacancies into structures. The photovoltaic response was improved by applying low work function electrodes and separated charge due to light can be dragged with the external load. We studied photovoltaic effect in both configurations of electrodes in ferroelectric semiconductor materials. The more efficient light passing through electrodes was considered by utilizing transparent oxide electrodes of 2% Al doped ZnO. Alternative electrodes of mono/bilayer graphene were also utilized as suitable electrodes for collecting charges when exposed to the light on the sample. Apart from conventional (p-n) solar cells, we have proposed two mechanisms to understand the charge separation in ferroelectric materials in our dissertation for understanding of photovoltaic effect: (1). The photo-excited carriers are separated at nano-scale steps of the electrostatic potential at ferroelectric domain walls, and the photo-excited carriers are separated by macroscopic polarization over the whole film thickness. (2) The photogenerated electrons and holes are driven by the polarization-induced internal electric field (depolarization field) in the opposite directions toward the cathode and anode, respectively, and thus contribute to the photovoltaic output. A simple consideration of the polarity of the photovoltaic effect will not be able to distinguish between the two mechanism. Confirmation of the polarity of Voc and Jsc of the present cells after the application of the electric field revealed that the present photovoltaic effect is ascribed to the depolarization effect due to the incomplete screening. The transparent conductive oxides as a top electrode were carried out for the improvement of the photovoltaic properties in a ferroelectric film. Recent experimental investigations have provided strong evidences that ferroelectricity is able to exist in ultrathin film of a few nanometers with high quality ferroelectric films, it is therefore possible that the significant photovoltaic efficiency can become comparable or even exceed that of the currently dominant semiconductor-based photovoltaic materials.