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Explaining the impact of oxygen incorporated into SrTiO3 single crystals in strongly reducing H2/H2O containing atmospheres under UV illumination at 350 °C.

The interaction of light with solids has been of ever-growing interest for centuries, even more so since the quest for sustainable utilization and storage of solar energy became a major task for industry and research. With SrTiO3 being a model material for an extensive exploration of the defect chemistry of mixed conducting perovskite oxides, it has also been a vanguard in advancing the understanding of the interaction between light and the electronic and ionic structure of solids. In the course of these efforts, many phenomena occurring during or subsequent to the illumination of SrTiO3 have been investigated. Here, we give an overview of the numerous photoinduced effects in SrTiO3 and their inherent connection to electronic structure and defect chemistry. In more detail, advances in the fields of photoconductivity, photoluminescence, photovoltages, photochromism and photocatalysis are summarized and their underlying elemental processes are discussed. In light of recent research, this review also emphasizes the fundamental differences between illuminating SrTiO3 either at low temperatures (200 °C), where in addition to electronic processes, also photoionic interactions become relevant. A survey of the multitude of different processes shows that a profound and comprehensive understanding of the defect chemistry and its alteration under illumination is both vital to optimizing devices and to pushing the boundaries of research and advancing the fundamental understanding of solids., Illumination of SrTiO3 induces a multitude of electronic and ionic effects. Recent advances in photoconductivity, photoluminescence, photovoltage, photochromism and photocatalysis are highlighted.

Advanced functional materials 32(38), 2202226 (2022). doi:10.1002/adfm.202202226, Different SrTiO$_3$ thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO$_3$ films prepared by pulsed laser deposition. Impedance spectroscopy reveals electronically pseudo-intrinsic conductivities for a broad range of different dopants (Fe, Al, Ni) and partly high dopant concentrations up to several percent. Using inductively-coupled plasma optical emission spectroscopy and reciprocal space mapping, a severe Sr deficiency is found and positron annihilation lifetime spectroscopy revealed Sr vacancies as predominant point defects. From synchrotron-based X-ray standing wave and X-ray absorption spectroscopy measurements, a change in site occupation is deduced for Fe-doped SrTiO$_3$ films, accompanied by a change in the dopant type. Based on these experiments, a model is deduced, which explains the almost ubiquitous pseudo-intrinsic conductivity of these films. Sr deficiency is suggested as key driver by introducing Sr vacancies and causing site changes (Fe$_{Sr}$ and Ti$_{Sr}$) to accommodate nonstoichiometry. Sr vacancies act as mid-gap acceptor states, pinning the Fermi level, provided that additional donor states (most probably Ti$_{Sr}^{\bullet\bullet}$) are present. Defect chemical modeling revealed that such a Fermi level pinning also causes a self-limitation of the Ti site change and leads to a very robust pseudo-intrinsic situation, irrespective of Sr/Ti ratios and doping., Published by Wiley-VCH, Weinheim

Nominally undoped SrTiO

The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition. Structural information was obtained by reciprocal space mapping while solution-based inductively-coupled plasma optical emission spectroscopy and positron annihilation lifetime spectroscopy were employed to reveal the cation composition and the predominant point defects of the thin films, respectively. A severe cation non-stoichiometry with Sr vacancies was found in films deposited from stoichiometric targets. The across plane electrical conductivity of such epitaxial films was studied in the temperature range of 250–720 °C by impedance spectroscopy. This revealed a pseudo-intrinsic electronic conductivity despite the substantial Fe acceptor doping, i.e. conductivities being several orders of magnitude lower than expected. Variation of PLD deposition parameters causes some changes of the cation stoichiometry, but the films still have conductivities much lower than expected. Targets with significant Sr excess (in the range of several percent) were employed to improve the cation stoichiometry in the films. The use of 7% Sr-excess targets resulted in near-stoichiometric films with conductivities close to the stoichiometric bulk counterpart. The measurements show that a fine-tuning of the film stoichiometry is required in order to obtain acceptor doped SrTiO3 thin films with bulk-like properties. One can conclude that, although reciprocal space maps give a first hint whether or not cation non-stoichiometry is present, conductivity measurements are more appropriate for assessing SrTiO3 film quality in terms of cation stoichiometry., The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition.

High temperature solid oxide solar cells based on SrTiO3 (STO) are investigated. Schottky contacts between STO and different materials, including La1-xSrxCrO3, La0.8Sr0.2MnO3, La0.6Sr0.4CoO3 and metals like Au and Pt are illuminated by UV light with a wavelength of 365 nm at 350 °C in air and the resulting voltages and currents are measured. With certain material combinations, e.g. LaCrO3/STO or Au/STO heterojunctions, high photovoltages of more than 1.0 V are obtained. Current measurements reveal changes in the bulk STO due to stoichiometry polarization, leading to a self-enhancement effect of the photovoltaic cell. Mechanistic insight in the processes under UV light is gained by electrochemical impedance spectroscopy, especially regarding the space charge region at the heterojunction and bulk changes due to stoichiometry polarization. In addition to the electronic effects upon illumination, also photo-ionic effects of oxygen transport caused by chemical potential changes were observed.

The interplay of structure, composition and electrical conductivity was investigated for Fe-doped SrTiO3 thin films prepared by pulsed laser deposition. Structural information was obtained by reciprocal space mapping while solution-based inductively-coupled plasma optical emission spectroscopy and positron annihilation lifetime spectroscopy were employed to reveal the cation composition and the predominant point defects of the thin films, respectively. A severe cation non-stoichiometry with Sr vacancies was found in films deposited from stoichiometric targets. The across plane electrical conductivity of such epitaxial films was studied in the temperature range of 250–720 °C by impedance spectroscopy. This revealed a pseudo-intrinsic electronic conductivity despite the substantial Fe acceptor doping, i.e. conductivities being several orders of magnitude lower than expected. Variation of PLD deposition parameters causes some changes of the cation stoichiometry, but the films still have conductivities much lower than expected. Targets with significant Sr excess (in the range of several percent) were employed to improve the cation stoichiometry in the films. The use of 7% Sr-excess targets resulted in near-stoichiometric films with conductivities close to the stoichiometric bulk counterpart. The measurements show that a fine-tuning of the film stoichiometry is required in order to obtain acceptor doped SrTiO3 thin films with bulk-like properties. One can conclude that, although reciprocal space maps give a first hint whether or not cation non-stoichiometry is present, conductivity measurements are more appropriate for assessing SrTiO3 film quality in terms of cation stoichiometry.