From sunlight to green hydrogen. Metal oxides as extremely thin absorbers for photoelectrochemical water splitting

The global greenhouse gas emissions have to be drastically reduced within the next years to limit the anthropogenic climate impact. The largest emissions in the sectors energy, industry, traffic and buildings are driven by the use of fossil fuels, which therefore have to be replaced by renewable sources. A promising route to achieve this is the utilization of solar energy in the concept of photoelectrochemical (PEC) water splitting, which generates solar fuels. For this a photoelectrode is immersed in an aqueous electrolyte which drives electrochemical water splitting at its surface by photogenerated charge carriers. A variety of material requirements for the photoelectrode arises, e.g. a suitable bandgap and chemical stability. The material class of complex metal oxides offers a wide variety of possible photoelectrode materials coupled with the intrinsic stability of oxides. A bottleneck of these oxides is their usually moderate optical absorption and weak charge carrier transport properties. It is either necessary to work with nanostructured samples or with high quality thin films to address this mismatch. Two thin film deposition techniques to individually tackle these challenges are atomic layer deposition (ALD) and pulsed laser deposition (PLD). These two are used in this thesis to deposit extremely thin photoabsorbers. The growth processes, the film properties and the suitability of materials and processes are studied and evaluated. The first part investigates ALD of Bi${}_2$O${}_3$ from [Bi(tmhd)${}_3$]. Thermal and plasma-enhanced (PE) deposition process are established with growth rates of 0.24 and 0.35 Å/cycle, respectively. The PE process does not only show an increased growth rate but also results in less carbon contaminations, 4.3 at.% compared to 9.4 at.% for the thermal process. Both effects are explained by incomplete ligand removal in the thermal process but complete removal in the PE process, which is shown by real-time spectroscopic ellipsometry (RT-SE). An improvement of the SE-model by fitting two layers allows to simultaneously follow the surface Bi(tmhd)${}_x$ layer as well as the bulk Bi${}_2$O${}_3$ film in the PE process. The second part investigates ALD of Mn${}_2$V${}_2$O${}_7$ as a possible ternary absorber material. Individual binary processes of MnO and VO${}_x$ are optimized at 200°C with growth rates of 0.91 Å/cycle and 0.25 Å/cycle, respectively. Both are combined in a supercycle to deposit the ternary Mn${}_2$V${}_2$O${}_7$. Post-deposition annealing in argon leads to crystallization into the desired β-phase with an indirect bandgap of 1.83 eV. While the material does show a photoresponse in a PEC cell, the photocurrents are very low and correspond to absorbed-photon-to-current efficiency (APCE) values below 1%. The last part investigates PLD of CuBi${}_2$O${}_4$, especially the influence of the deposition parameters laser fluence, substrate temperature and oxygen background pressure. All thin films exhibit excellent quality in terms of phase purity, composition, bandgap (1.85 eV), charge carrier transport and photocurrents. As final quality criteria unmatched APCE values of up to 76% demonstrate the suitability of PLD to deposit high quality absorber materials. Low laser fluences and high substrate temperatures slightly improve the material properties while an oxygen background pressure should be avoided.