Evaluation of Pb0.95La0.05Zr0.54Ti0.46O3 thin film based capacitors for photovoltaic applications: using top electrodes of varying work function

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Ferroelectricity and optical transparency are the two properties of lanthanum doped lead zirconate titanate (PLZT) that are utilized to explore its use for photovoltaic (PV) applications. A ferroelectric material sandwiched between two electrodes forms a MFM (metal/ferroelectric/metal) capacitor structure and can generate photocurrent and photovoltage under illumination. The PV effect in these structures is a combination of bulk PV effect and metal/ferroelectric interface effect. Both the material and the electrode properties are critical to improve the PV parameters obtained from these structures. A careful selection of electrode material can enhance the interface photovoltaic effect by modifying the Schottky barrier formed at the interface between electrode and ferroelectric material. This work investigates the effect of top electrodes of varying work functions on the photovoltaic properties of ferroelectric Pb0.95La0.05Zr0.54Ti0.46O3 (PLZT) thin films. PLZT thin films were prepared using a chemical solution deposition method and post annealed in the temperature range of 550-750 ˚C. A variety of characterization methods including x-ray diffraction, Raman, UV-visible and x-ray spectroscopy were used to understand the material properties of the grown films. A detailed analysis of ferroelectric measurements of the films was carried out to study their electrical behavior. Films annealed at annealing temperature (Ta) of 750 ˚C exhibit perovskite peaks, and higher chemical valence states and polarization compared to films annealed at lower temperatures. Thus, Ta of 750 ˚C was chosen to design capacitors studied in this work. Metal/PLZT/Pt (MFM) symmetric and asymmetric capacitor structures with various metals (Pt, Au, and Al) as top electrodes were fabricated. The asymmetric structure designed with Au top and Pt bottom electrode showed an improvement in the PV parameters, while retaining the ferroelectric properties. On the other hand, the symmetric structure designed with Pt top and bottom electrode showed lower PV parameters, but higher polarization. This work further investigated the ferroelectric and photovoltaic properties of PLZT thin film capacitors in two different electrode configurations, coplanar and interplanar, using Au electrodes. A simulation PV model was designed for the symmetric structure, which was helpful in understanding the charge transport properties. This work was extended to design a simulation model for asymmetric structure using ITO as top and Pt as bottom electrode. This work suggests that asymmetric structure results in higher charge transport, which results in higher PV parameters of PLZT thin film based capacitors.

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Electrical engineering