Search

Different SrTiO3 thin films are investigated to unravel the nature of ultra-low conductivities recently found in SrTiO3 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 SrTiO3 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 TiSr center dot center dot\[{\rm{Ti}}_{{\rm{Sr}}}{ \bullet \bullet }\]) 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., Advanced Functional Materials, 32 (38), ISSN:1616-3028, ISSN:1616-301X

The defect chemistry in the mid-band gap semiconductor SrTiO3 (STO) plays a crucial role for its properties. These can be tuned by temperature, doping, p(O2), and UV irradiation. Since UV exposure leads to the formation of photoelectrons and –holes the oxygen chemical potential of oxygen is changed. This results in an enhanced incorporation of oxygen into Fe-doped STO at elevated temperatures. In this work different approaches to investigate this phenomena are introduced. i) Oxygen compositional changes can be probed by measuring the light induced weight change using GaPO4 microbalances. ii) UV exposure and oxygen incorporation also cause a photo and a battery voltage, which can be measured in-situ. iii) Inplane conductivity measurements reveal an increase of the electron hole conductivity, as the result of Fe3+ oxidation to Fe4+. iv) This change of the Fe oxidation state additionally leads to a change in the color of Fe-doped STO from brownish/transparent to black. Two broad additional absorption bands in the region of 440 nm and 590 nm occur.

In the last decades Fe-doped SrTiO3 (Fe:STO) has become one of the best investigated mixed conducting materials. Nevertheless, the effect of UV irradiation at elevated temperatures on the material’s composition, more precisely on its oxygen content, has hardly been examined so far. UV illumination in oxygen causes an enhanced oxygen incorporation rate and therefore a decreased oxygen vacancy concentration of SrTiO3. Consequently, Fe3+ in Fe:STO is oxidized to Fe4+ to preserve charge neutrality. This oxidation may cause color changes in Fe:STO and a decline in the AC and DC resistivity, which can be investigated by UV/VIS spectroscopy, inplane impedance spectroscopy, van der Pauw measurements respectively. Oxygen can also be released again from Fe:STO by annealing without UV exposure. Therefore, reversibly switching between these described states can be obtained.

The impact of UV irradiation on Fe‐doped SrTiO3 (Fe:STO) single crystals is investigated at elevated temperatures. Illumination leads to incorporation of oxygen into the single crystals and thus to a decreasing oxygen vacancy concentration and oxidation of Fe3+ to Fe4+. The Fe4+ ions cause a color change from transparent/brownish to black. This photochromic blackening due to stoichiometry changes at elevated temperatures is irreversible at room temperature, but annealing at high temperatures, for example at 700 °C, can restore the original stoichiometry and color. Absorbance changes due to UV irradiation are monitored by ex situ and in situ UV/VIS spectroscopy experiments and changes in electrical properties are measured by van der Pauw measurements and in‐plane electrochemical impedance spectroscopy. After 1140 min of illumination at 440 °C, for example, electrical measurements reveal a conductivity increase by more than a factor of 5 due to the enhanced hole concentration in blackened Fe:STO. In addition, UV illumination increases the oxygen chemical potential up to a calculated p(O2) of more than 109 Pa in Fe:STO. Hence, UV light can be used to tune the color, but also electrical properties of Fe:STO by directly impacting the bulk defect concentrations.

Der Wechsel von Kathoden in Festoxidbrennstoffzellen (SOFC) mit Oberfl��chenpfadkinetik zu gemischt leitenden Kathoden mit Volumenspfad war eine der wichtigsten Errungenschaften innerhalb des letzten Jahrzehnts in der Festoxidbrennstoffzellenforschung. Auch im Fall der Anoden wird mit dem ��bergang zu gemischt leitenden Systemen eine weitere Erh��hung der Leistung der Festoxidbrennstoffzellen erwartet. Ein vielversprechendes Material f��r diese Anwendung ist (La,Sr)(Cr,Mn)O3-d (LSCrM), das die M��glichkeit bietet, die elektronischen Eigenschaften auf zwei Arten zu ver��ndern: die Sr-Dotierung am A-Platz, das Cr:Mn Verh��ltnis am B-Platz In dieser Arbeit wurden LSCrM D��nnschichten mit sowohl variierendem Sr Gehalt als auch mit variierendem Cr:Mn Verh��ltnis mittels gepulster Laserabscheidung (pulsed laser deposition, PLD) hergestellt. Leitf��higkeitsmessungen an diesen Schichten wurden in 1% O2, 100 % O2 und H2/H2O (1:1) Atmosph��ren mittels Van-der-Pauw Methode durchgef��hrt. Die erhaltenen Leitf��higkeitsmessdaten werden im Zusammenhang mit der Defektchemie des Material diskutiert. Niedrige Mn Konzentrationen f��hren beispielsweise zu einer drastischen Verringerung der Leif��higkeit, h��chstwahrscheinlich durch Trapping der Ladungstr��ger (Elektronenl��cher) auf den Mn Pl��tzen. Mit h��heren Mn Konzentrationen kommt es zur Perkolation der Mn Trap-Niveaus und weitaus h��here Leitf��higkeiten k��nnen erreicht werden. Die Aktivierungsenergie f��r die Leitf��higkeit jedes einzelnen Materials wird mit Hilfe von Arrhenius-Diagrammen bestimmt und im Zusammenhang mit der zugeh��rigen Leitf��higkeit diskutiert. Dabei wurde ein Zusammenhang zwischen Leitf��higkeit und Aktivierungsenergie erkannt, wobei h��here Aktivierungsenergien im Fall von niedrigeren Leitf��higkeiten zu finden waren. Weiters blieben die hergestellten Proben w��hrend der gesamten Messzeit sowohl unter oxidierenden als auch unter reduzierenden Bedingungen stabil. Im zweiten Teil dieser Arbeit wurde von ausgew��hlten Zusammensetzungen auch die elektrochemische Aktivit��t f��r die H2-Oxidation untersucht. Diese Experimente wurden an D��nnschicht-Modellelektroden auf Yttriumoxid stabilisierten Zirkoniumdioxid-Elektrolyten in H2/H2O Atmosph��re durchgef��hrt, wobei eine por��se Ni/YSZ Gegenelektrode verwendet wurde. Das elektrochemische Verhalten wurde mittels Impedanzspektroskopie charakterisiert und die grundlegenden Eigenschaften wie Elektrodenpolarisationswiderstand werden im Zusammenhang mit der Leitf��higkeit des Materials diskutiert. Der Elektrodenwiderstand h��ngt dabei nur schwach von der Gesamtleitf��higkeit ab, allerdings wurde ein niedriger Widerstand f��r La0.9Sr0.1Cr0.2Mn0.8O3-d beobachtet, was durch eine Abh��ngigkeit des Elektrodenpolarisationswiderstandes vom Mn Gehalt erkl��rt werden konnte., The change from solid oxide fuel cell (SOFC) cathodes with surface path kinetics to mixed conducting cathodes exhibiting a bulk path was one of the most important achievements within the last decade of SOFC research. Also in case of anodes the transition to mixed conducting systems is expected to further increase the performance of SOFCs. A promising material for this application is (La,Sr)(Cr,Mn)O3-d (LSCrM), which provides the opportunity of two possible ways of changing its electronic properties: the Sr-doping on the A-site, the Cr:Mn ratio on the B-site. In this study, LSCrM thin films with both varying Sr content and Cr:Mn ratio were prepared by pulsed laser deposition (PLD). Conductivity measurements on these thin films were carried out in 1 % O2, 100 % O2 as well as H2/H2O (1:1) atmospheres using the Van-der-Pauw-method. The obtained conductivity data are discussed in terms of the material-s defect chemistry. Low Mn contents, for example, decrease the conductivity drastically, most likely due to trapping of charge carriers (electron holes) on the Mn sites. With higher Mn contents, site percolation occurs and much higher conductivities can be obtained. The activation energies for each material-s conductivity are calculated from Arrhenius plots and are discussed referring to the corresponding conductivity. A connection between conductivity and activation energy was found. Higher activation energies correspond to lower conductivities at the probed temperatures. Furthermore, the prepared samples demonstrated stability during the time of the measurement under oxidizing as well as reducing conditions. In the second part of the study selected compositions were also investigated as model-type thin film electrodes on yttria stabilized zirconia (YSZ) electrolytes in H2/H2O atmosphere, using a porous Ni/YSZ counter electrode. Electrochemical behavior was characterized by means of impedance measurements and elementary properties such as electrode polarization resistance are discussed referring to the material-s conductivity. The polarization resistance in humid H2 is less dependent on the total conductivity, however, a rather low resistance was observed for La0.9Sr0.1Cr0.2Mn0.8O3-d electrodes. Therefore, the Mn concentration appears to affect the polarization resistance to a higher extent.

Wide band gap semi-conductors such as SrTiO3 (STO) have attracted high interest for photoelectrochemical water splitting and as part of high temperature photovoltaic cells. However, the impact of UV irradiation on the composition (e.g. the oxygen content) of STO and Fe doped STO (Fe:STO) at elevated temperatures has hardly been studied. In Fe:STO, UV exposure causes an enhanced oxygen incorporation rate and, therefore, a decreased oxygen vacancy concentration in air at elevated temperatures. In this contribution we discuss such bulk stoichiometric changes in terms of optical and electrochemical (e.g. AC conductivity) properties. Moreover, the kinetics behind the incorporation were examined, more precisely chemical diffusion coefficients obtained from UV/VIS measurements were calculated.

Wide band gap semi-conductors such as SrTiO3 (STO) have attracted high interest for photoelectrochemical water splitting and as part of high temperature photovoltaic cells. However, the impact of UV irradiation on the composition (e.g. the oxygen content) of STO and Fe doped STO (Fe:STO) at elevated temperatures has hardly been studied. In Fe:STO, UV exposure causes an enhanced oxygen incorporation rate and, therefore, a decreased oxygen vacancy concentration in air at elevated temperatures. In this contribution we discuss such bulk stoichiometric changes in terms of optical and electrochemical (e.g. AC conductivity) properties. Moreover, the kinetics behind the incorporation were examined, more precisely chemical diffusion coefficients obtained from UV/VIS measurements were calculated.

SrTiO3 (STO), a perovskite-type oxide, shows many interesting effects under UV light illumination, such as an increase in oxygen incorporation rate, an enhancement in conductivity and a change in color in Fe doped STO single crystals. The increased oxygen uptake under UV light has also been used for a battery-type voltage in STO under UV light irradiation. With a top layer, e.g. (La,Sr)CrO3 (LSCr), much higher photovoltages can be realized. Such high temperature solid oxide photovoltaic (PV) cells are promising due to their possible combination with solid oxide electrolyzer cells (SOEC). By using a common electrode of a PV cell and an underlying SOEC it is possible to directly transfer light to chemical energy. However, optimization of voltage and power of such high temperature PV cells is still required. In this study, different materials (e.g. (La, Sr)(Cr,Mn)O3 (LSCrM), (La,Sr)CoO3 (LSC), (La,Sr)FeO3 (LSF) and NdNiO3 (NNO),…) were tested for their applicability in oxide solar cells and mechanistic insight in processes under UV is gained.

SrTiO3 (STO), a perovskite-type oxide, shows many interesting effects under UV light illumination, such as an increase in oxygen incorporation rate, an enhancement in conductivity and a change in color in Fe doped STO single crystals. The increased oxygen uptake under UV light has also been used for a battery-type voltage in STO under UV light irradiation. With a top layer, e.g. (La,Sr)CrO3 (LSCr), much higher photovoltages can be realized. Such high temperature solid oxide photovoltaic (PV) cells are promising due to their possible combination with solid oxide electrolyzer cells (SOEC). By using a common electrode of a PV cell and an underlying SOEC it is possible to directly transfer light to chemical energy. However, optimization of voltage and power of such high temperature PV cells is still required. In this study, different materials (e.g. (La, Sr)(Cr,Mn)O3 (LSCrM), (La,Sr)CoO3 (LSC), (La,Sr)FeO3 (LSF) and NdNiO3 (NNO),…) were tested for their applicability in oxide solar cells and mechanistic insight in processes under UV is gained.