Mechanism and characteristics of photovoltaic responses in sandwiched ferroelectric plzt thin film devices

Abstract

Photocurrent and photovoltage can be generated in ferroelectric materials under near-ultraviolet illumination which is known as ferroelectric photovoltaic effect. At present, research interest is being drawn by the exciting possibility of using ferroelectric thin films for the photovoltaic applications of optical sensor, actuator and energy transducer. With regard to these promising applications, photovoltaic properties in ferroelectric thin film need to be studied carefully and the photovoltaic power conversion efficiency needs to be enhanced. Theoretical model is also necessary to understand the photovoltaic generation mechanism. The objective of this work is to study photovoltaic properties of PLZT-based ferroelectric thin films and to improve the photovoltaic power conversion efficiency for potential ferroelectric-based photovoltaic applications. Some effects and issues that are closely related to ferroelectric photovoltaics, such as Schottky effect, thickness effect, screening effect and the stability issue of photovoltaic response, were also investigated in detail to reveal the inherent photovoltaic properties in ferroelectrics. In this work, the PLZT-based ferroelectric thin films were fabricated using chemical solution deposition (CSD) and physical vapor deposition (PVD). Photovoltaic effects in PLZT thin films were systematically studied through experimental and theoretical investigations.

First, it was found that interfacial Schottky barriers significantly influence the magnitude and polarity of photovoltaic outputs in ferroelectric thin films. Asymmetric Schottky barriers at the two ferroelectric-electrode interfaces cause non-zero photovoltaic output in the unpoled films and asymmetric outputs in different poling directions in the poled films.

Secondly, as for the thickness effect, both short circuit photocurrent and photovoltaic efficiency showed exponential-like increase with the decrease in film thickness. Photovoltage in the poled films showed a linear dependence on film thickness.

Thirdly, when it comes to the screening effect, the dielectric constant of the electrodes substantially influences the photovoltaic output of the sandwiched ferroelectric thin film in between electrodes. A low-dielectric-constant electrode showed more severe screening effect than the high-dielectric-constant electrode. As a result, the use of electrodes with high dielectric constant will give rise to dramatically enhanced magnitude of photocurrent.

Furthermore, an unprecedented high power conversion efficiency of 0.28% was demonstrated in our sputtered nanoscale ferroelectric epitaxial PLZT thin films with the thickness of tens of nanometres. It significantly exceeds the so-far-reported data and theoretically predicted limit of the photovoltaic efficiency in ferroelectrics. Our theoretical analysis predicted that an even higher efficiency may exist in high quality ferroelectric ultrathin films.