Stress-modulated bulk photovoltaic effect in polar oxide ceramics


CALL: 2013

DOMAIN: MS - New Functional and Intelligent Materials and Surfaces


LAST NAME: Granzow




KEYWORDS: Multiferroics; Photoferroelectrics; Bulk Photovoltaic Effect; Stress-induced phenomena; Domain wall engineering

START: 2014-08-01

END: 2017-07-31


Submitted Abstract

The present project investigates the stress-dependence of the bulk photovoltaic effect, merging aspects of solid state physics and materials engineering. Its goal is to understand light-induced charge transport mechanisms in ferroelectrics and the interplay between stress, electronic excitation, phonon spectrum and domain structures. Ferroelectrics are functional materials par excellence, and photoferroelectric properties are one of the emerging hot topics in the large field of ferroelectrics research. This knowledge can be used to develop new intelligent functional materials based on stress-modulated photovoltaic ferroelectrics.The ‘normal’ photovoltaic (PV) effect in semiconductors relies on the separation of charge carriers at boundaries such as p-n-junctions. It is an interface effect and the voltage can never exceed the band gap energy. In contrast, the ‘anomalous’ or ‘bulk’ photovoltaic effect (BPVE) in ferroelectrics can create a voltage that is several orders of magnitude higher and scales with the sample thickness. It has been used in the field of photorefractive optics, but its highly efficient energy conversion in domain-engineered ferroelectric structures has only very recently been discovered. However, the exact charge transport mechanism, its dependence on parameters such as local structure and deformations, and the effect of uncharged or charged domain walls remains poorly understood.This project focuses on the piezo-photovoltaic effect, i.e. the response of the BPVE to a compressive mechanical stress, particularly in bulk lithium niobate (LiNbO3, LN) single crystals, a prototypical material for studies of the BPVE. Both stoichiometric and congruently melting LN doped with shallow or deep donor centres such as Fe, Ce or Cu are considered. The concentration of traps is controlled by annealing in different reducing / oxidizing atmospheres. The stress-dependence of the BPVE is quantified by measuring the piezo-photovoltaic tensor: cuboid samples are submitted to a uniaxial compressive stress and illuminated with linearly polarized light with the wavelength ranging from the near-UV to the NIR. An electric field is applied and the resulting current measured, recording the current-voltage (I-V) curves. They are then used to calculate the elements of the piezo-photovoltaic tensor. The absolute values and the sign of the different tensor elements indicate the physical mechanism underlying the BPVE. Apart from monodomain samples, polydomain structures with charged and uncharged domain walls are investigated to elucidate the effect of these walls as sources or drains of electric charges. Particular attention is paid to the change of bound charge density under stress due to the piezoelectric effect. It is conceivable that the project results could be applied within the field of photovoltaic energy generation or small scale photosensors. The project is thus connected to the broader research environment in the field of photosensitive materials, both academic and industrial.

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