Pulsed laser deposition of efficient ternary metal oxide photoelectrodes

Metal oxides are a promising class of photoelectrode materials for photoelectrochemical PEC water splitting because they are in general cheaper and more stable in aqueous solutions than conventional III V semiconductors. Since only few if any of the simple binary oxides show the desired properties, recent efforts in the field have shifted towards investigating the more complex multinary oxides. To study the fundamental properties and performance limitations of such novel photoelectrode materials, one needs to be able to deposit thin and compact films of high electronic quality. Pulsed laser deposition PLD is a versatile physical vapor deposition technique that meets these demands. Therefore, in this thesis, this powerful tool is used to i deposit dense and high quality BiVO4 photoelectrodes and ii to comprehensively evaluate the new and promising material amp; 945; SnWO4. In the first part, the complex PLD process of BiVO4 films by ablating a BiVO4 target is systematically elucidated with a special focus on the deviations from an ideal stoichiometric target to substrate transfer. By correlating the V Bi ratio of the films with their charge carrier transport properties and PEC performance, remarkable AM1.5 sulfite oxidation photocurrents of 2.4 0.2 mAcm 2 at E 1.23 V vs. RHE with stoichiometric films are achieved without any doping or nanostructuring. BiVO4 photoelectrodes with similar PEC performance are additionally prepared for the first time by the alternating ablation of Bi2O3 and V2O5 targets. This approach is shown to be an attractive alternative route to control the cation stoichiometry and lays the foundation for the future growth of epitaxial BiVO4 films. The second part of this thesis contains a comprehensive evaluation of amp; 945; SnWO4 as a novel photoelectrode material. amp; 945; SnWO4 has recently attracted attention in the field due to the combination of a nearly ideal bandgap 1.9 eV and a very early photocurrent onset potential 0 V vs. RHE . Using phase pure pulsed laser deposited films, the close to optimum band alignment and bandgap is confirmed, and other important parameters such as the charge carrier mobility, lifetime, diffusion length, and the PEC stability are reported for the first time. A high temperature treatment is shown to enhance the charge carrier mobility of amp; 945; SnWO4 films by more than two orders of magnitude, as measured with time resolved microwave conductivity TRMC . This results in one of the highest effective charge carrier diffusion lengths ever measured in a metal oxide photoelectrode 200 nm . A complimentary analysis by time resolved terahertz spectroscopy TRTS shows that this improvement can be attributed to larger grain domain sizes with increasing heat treatment temperature. In other words, grain boundaries significantly limit the charge carrier transport in amp; 945; SnWO4. In addition, a hole conductive NiOx protection layer is introduced to prevent self passivation of the surface of the amp; 945; SnWO4 films formation of a thin SnO2 layer , which drastically increases the sulfite oxidation photocurrent by a factor of 100 setting a new benchmark AM1.5 photocurrent density 0.75 mA cm 2 at E 1.23 V vs. RHE and IPCE 38 at amp; 955; 355 nm for amp; 945; SnWO4. These findings provide important insights into the key PEC properties and performance limitations of amp; 945; SnWO4, and allow the identification of strategies to further improve the performance of this promising photoanode materia

Gepulste Laserabscheidung von effizienten ternären Metalloxid-Photoelektroden

Climate change, mainly driven by the combustion of fossil fuels releasing the greenhouse gas CO2, is a major threat to humanity and the ecosystems on our planet. To limit global warming, a transition towards a sustainable energy infrastructure based on renewable energies is essential. The storage of solar energy in the form of chemical bonds, in so-called solar fuels, is thereby believed to be a key technology. This approach solves the problem of the intermittent nature of solar power–the only renewable energy source capable of meeting the still growing world energy demand on a TW scale. One possible pathway towards solar fuels is the generation of hydrogen via photoelectrochemical (PEC) water splitting using semiconducting photoelectrodes immersed in an aqueous electrolyte. However, the material that meets all the stringent requirements for efficient and commercially viable solar water splitting is still elusive. Metal oxides are a promising class of photoelectrode materials because they are in general cheaper and more stable in aqueous solutions than conventional III-V semiconductors. Since only few—if any—of the simple binary oxides show the desired properties, recent efforts in the field have shifted towards investigating the more complex multinary oxides. To study the fundamental properties and performance limitations of such novel photoelectrode materials, one needs to be able to deposit thin and compact films of high electronic quality. Pulsed laser deposition (PLD) is a versatile physical vapor deposition technique that meets these demands. Therefore, in this thesis, this powerful tool is used to (i) deposit dense and high-quality BiVO4 photoelectrodes and (ii) to comprehensively evaluate the new and promising material a-SnWO4. In the first part, the complex PLD process of BiVO4 films by ablating a BiVO4 target is systematically elucidated with a special focus on the deviations from an ideal stoichiometric target-to-substrate transfer. By correlating the V:Bi ratio of the films with their charge carrier transport properties and PEC performance, remarkable AM1.5 sulfite oxidation photocurrents of 2.4 ± 0.2 mAcm-2 at E = 1.23 V vs. RHE with stoichiometric films are achieved without any doping or nanostructuring. BiVO4 photoelectrodes with similar PEC performance are additionally prepared for the first time by the alternating ablation of Bi2O3 and V2O5 targets. This approach is shown to be an attractive alternative route to control the cation stoichiometry and lays the foundation for the future growth of epitaxial BiVO4 films. The second part of this thesis contains a comprehensive evaluation of a-SnWO4 as a novel photoelectrode material. a-SnWO4 has recently attracted attention in the field due to the combination of a nearly ideal bandgap (~1.9 eV) and a very early photocurrent onset potential (~0 V vs. RHE). Using phase-pure pulsed laser deposited films, the close-tooptimum band alignment and bandgap is confirmed, and other important parameters such as the charge carrier mobility, lifetime, diffusion length, and the PEC stability are reported for the first time. A high-temperature treatment is shown to enhance the charge carrier mobility of a- SnWO4 films by more than two orders of magnitude, as measured with time-resolved microwave conductivity (TRMC). This results in one of the highest effective charge carrier diffusion lengths ever measured in a metal oxide photoelectrode (~200 nm). A complimentary analysis by time-resolved terahertz spectroscopy (TRTS) shows that this improvement can be attributed to larger grain/domain sizes with increasing heat-treatment temperature. In other words, grain boundaries significantly limit the charge carrier transport in a-SnWO4. In addition, a hole-conductive NiOx protection layer is introduced to prevent self-passivation of the surface of the a-SnWO4 films (formation of a thin SnO2 layer), which drastically increases the sulfite oxidation photocurrent by a factor of ~100 setting a new benchmark AM1.5 photocurrent density (~0.75 mA cm-2 at E = 1.23 V vs. RHE) and IPCE (~38% at λ = 355 nm) for a-SnWO4. These findings provide important insights into the key PEC properties and performance limitations of a-SnWO4, and allow the identification of strategies to further improve the performance of this promising photoanode material.Der Klimawandel, welcher hauptsächlich durch die Verbrennung fossiler Brennstoffe zur Energiegewinnung und den daraus resultierenden Emissionen des Treibhausgases CO2 vorangetrieben wird, ist eine immense Bedrohung für die Menschheit und Ökosysteme unserer Erde. Um die globale Erwärmung zu begrenzen, ist ein Übergang von fossilen zu erneuerbaren Energien essentiell. Dabei wird der Speicherung von solarer Energie in Form von chemischen Bindungen in sogenannten solaren Brennstoffen eine bedeutende Rolle zugeschrieben. Dieses an die Photosynthese in Pflanzen angelehnte Konzept löst das Problem der diskontinuierlich verfügbaren Solarenergie, welche unverzichtbar ist um den stetig wachsenden Weltenergiebedarf auf einer TW-Skala zu decken. Eine Möglichkeit solare Brennstoffe herzustellen ist die Produktion von Wasserstoff mittels photoelektrochemischer Wasserspaltung mit Hilfe von in wässrigen Lösungen eintauchenden halbleitenden Photoelektroden. Das Photoelektrodenmaterial, das alle strikten Anforderungen erfüllt um hoch-effizient und wirtschaftlich Wasserstoff zu erzeugen, wurde jedoch noch nicht gefunden. Metalloxide stellen eine vielversprechende Materialklasse für Photoelektroden dar, weil sie generell günstiger und stabiler sind als konventionelle III-V Halbleiter. Da nur wenige der binären Oxide geeignete Eigenschaften besitzen, werden in diesem Forschungsfeld nun auch komplexere multinäre Oxide untersucht. Um die fundamentalen Eigenschaften und limitierenden Faktoren zu erforschen, werden kompakte Filme von hoher elektronischer Qualität benötigt. Die gepulste Laserabscheidung („pulsed laser deposition“, PLD) ist ein vielseitiges physikalisches Gasphasenabscheidungs-Verfahren, das diese Anforderungen erfüllt. Daher wird es in dieser Arbeit genutzt, um zum einen (i) kompakte und qualitativ hochwertige BiVO4 Photoelektroden abzuscheiden und zum anderen (ii) das neue und vielversprechende Material a-SnWO4 umfassend zu untersuchen. Im ersten Teil dieser Arbeit wird der komplexe gepulste Laserabscheidungsprozess von BiVO4 Filmen mittels Ablation eines BiVO4 Targets systematisch mit besonderem Hinblick auf Abweichungen von einem idealen stöchiometrischen Target-zu-Substrat Transfer untersucht. Dabei wird das V:Bi Verhältnis der Filme mit deren Ladungsträgertransport und photoelektrochemischen Aktivität korreliert und so ohne jegliche Dotierung oder Nanostrukturierung relativ hohe AM1.5 Photoströme mit stöchiometrischen Filmen erzielt (2.4 ± 0.2 mAcm-2 bei E = 1.23 V vs. RHE mit Na2SO3 als Lochfänger). Darüber hinaus werden BiVO4 Photoelektroden mit einer ähnlich hohen Photoaktivität zum ersten Mal durch die alternierende Ablation von Bi2O3 und V2O5 Targets hergestellt. Dieser Ansatz stellt sich als eleganter alternativer Weg heraus, die kationische Stöchiometrie von Metalloxiden zu kontrollieren und legt die Grundlage für das zukünftige Abscheiden von epitaktischen Schichten. Im zweiten Teil der Arbeit wird a-SnWO4 ausführlich als Photoelektrodenmaterial evaluiert. Dieses Material erregte Aufmerksamkeit aufgrund seiner nahezu idealen Bandlücke (~1.9 eV) und seines bei sehr negativen Potentialen einsetzenden Photostroms (~0 V vs. RHE). Mittels gepulster Laserabscheidung hergestellte a-SnWO4 Filme werden benutzt, um die schon in der Literatur bekannte Bandanordnung und –lücke zu bestätigen und zum ersten Mal weitere essentielle Eigenschaften wie die Mobilität, Lebensdauer und Diffusionslänge der Ladungsträger sowie die photoelektrochemische Stabilität zu untersuchen. Es wird gezeigt, dass die mittels zeit-aufgelöster Mikrowellen Spektroskopie gemessene Ladungsträgermobilität durch eine Hochtemperatur-Behandlung der Schichten um mehr als zwei Größenordnungen erhöht werden kann. Daraus ergibt sich für a-SnWO4 eine der größten je gemessenen Diffusionslängen in Metalloxid Photoelektroden (~200 nm). Eine ergänzende Analyse mittels zeit-aufgelöster Terahertz Spektroskopie zeigt, dass diese Verbesserung auf größere Korngrößen in den hochtemperaturbehandelten Schichten zurückgeführt werden kann und liefert die wichtige Erkenntnis, dass Korngrenzen den Ladungstransport in a-SnWO4 limitieren. Zusätzlich wird eine lochleitende NiOx Schutzschicht auf die a-SnWO4 Filme aufgetragen, die die Selbstpassivierung (Oxidation der Oberfläche zu SnO2) verhindert. Dadurch wird der Photostrom (mit Na2SO3 als Lochfänger) um einen Faktor von ~100 erhöht und so neue Maßstäbe für den AM1.5 Photostrom (~0.75 mA cm-2 bei E = 1.23 V vs. RHE) und die Quanteneffizienz (~38% bei λ = 355 nm) für dieses Material gesetzt. Diese Ergebnisse liefern wichtige Erkenntnisse über Schlüssel-Parameter von a-SnWO4 und zeigen Strategien auf, die Performance von diesem vielversprechenden Photoelektrodenmaterial weiter zu verbessern

Efficiency Gains for Thermally Coupled Solar Hydrogen Production in Extreme Cold

Hydrogen produced from water using solar energy constitutes a sustainable alternative to fossil fuels, but solar hydrogen is not yet economically competitive. A major question is whether the approach of coupling photovoltaics via the electricity grid to electrolysis is preferential to higher levels of device integration in ‘artificial leaf’ designs. Here, we scrutinise the effects of thermally coupled solar water splitting on device efficiencies and catalyst footprint for sub-freezing ambient temperatures of -20C. These conditions are found for a significant fraction of the year in many world regions. Using a combination of electrochemical experiments and modelling, we demonstrate that thermal coupling broadens the operating window and significantly reduces the required catalyst loading when compared to electrolysis decoupled from photovoltaics. Efficiency benefits differ qualitatively for double- and triple junction solar absorbers, which has implications for the general design of outdoor-located photoelectochemical devices. Similar to high-efficiency photovoltaics that reached technological maturity in space, application cases in polar or alpine climates could support the scale-up of solar hydrogen at the global scale

Climatic response of thermally coupled solar water splitting in Antarctica

Hydrogen is a versatile energy carrier. When produced with renewable energy by water splitting, it is a carbon neutral alternative to fossil fuels. The industrialization process of this technology is currently dominated by electrolyzers powered by solar or wind energy. For small scale applications, however, more integrated device designs for water splitting using solar energy might optimize hydrogen production due to lower balance of system costs and a smarter thermal management. Such devices offer the opportunity to thermally couple the solar cell and the electrochemical compartment. In this way, heat losses in the absorber can be turned into an efficiency boost for the device via simultaneously enhancing the catalytic performance of the water splitting reactions, cooling the absorber, and decreasing the ohmic losses.[1,2] However,integrated devices (sometimes also referred to as “artificial leaves”), currently suffer from a lower technology readiness level (TRL) than the completely decoupled approach

The annual-hydrogen-yield-climatic-response ratio: evaluating the real-life performance of integrated solar water splitting devices

Integrated solar water splitting devices that produce hydrogen without the use of power inverters operate outdoors and are hence exposed to varying weather conditions. As a result, they might sometimes work at non-optimal operation points below or above the maximum power point of the photovoltaic component, which would directly translate into efficiency losses. Up until now, however, no common parameter describing and quantifying this and other real-life operating related losses (e.g. spectral mismatch) exists in the community. Therefore, the annual-hydrogen-yield-climatic-response (AHYCR) ratio is introduced as a figure of merit to evaluate the outdoor performance of integrated solar water splitting devices. This value is defined as the ratio between the real annual hydrogen yield and the theoretical yield assuming the solar-to-hydrogen device efficiency at standard conditions. This parameter is derived for an exemplary system based on state-of-the-art AlGaAs//Si dual-junction solar cells and an anion exchange membrane electrolyzer using hourly resolved climate data from a location in southern California and from reanalysis data of Antarctica. Moreover, the advantage of devices operating at low current densities over completely decoupled PV-electrolysis is discussed. This work will help to evaluate, compare and optimize the climatic response of solar water splitting devices in different climate zones