Metal oxide/Si heterostructures make up an exciting design route to high performance electrodes for photo-electrochemical (PEC) water splitting. By monochromatic light sources, contributions of the individual layers in WO3/n-Si heterostructures are untangled.It shows that band bending near the WO3/n-Si interface is instrumental in charge separation and transport, and in generating a photovoltage that drives the PEC process. A thin metal layer inserted at the WO3/n-Si interface helps establishing the relation among the band bending depth, the photovoltage, and the PEC activity. This discovery breaks with the dominant Z-scheme

We evaluate the impact of defects in WO3 thin films on the photoelectrochemical (PEC) properties during water splitting. We study physical defects, such as micro holes or cracks, by two different deposition techniques: sputtering and atomic layer deposition (ALD). Chemical defects, such as oxygen vacancies, are tailored by different annealing atmospheres, i.e. air, N2, and O2. The results show that the physical defects inside the film increase the resistance for the charge transfer and also result in a higher recombination rate which inhibits the photocurrent generation. Chemical defects yield in an increased adsorption of OH groups on the film surface and enhance the PEC efficiency. Excess amount of chemical defects can also inhibit the electron transfer, thus decreasing the photocurrent generation. In this study, the highest performance was obtained for WO3 films deposited by ALD and annealed in air, which have the least physical defects and an appropriate amount of oxygen vacancies.

Inspired by the fascinating class of hybrid organic-inorganic halide perovskite materials, halide double perovskites have emerged as non-toxic Pb-free contender for the application as active layers in optoelectronic devices. Heterovalent substitution of Pb 2+ by non-toxic metal cations yield the double perovskite structure which allows for compositional flexibility. In fact, the compositional space is large given that multiple cations and halides can be combined resulting in >10 6 perovskite combinations. A starkly increasing number of stable halide double perovskite compositions are theoretically predicted. The synthesis however, lacks behind and many double perovskites are primarily synthesized as powder samples instead of thin films. The latter however, are needed for thin film devices such as solar cells, light-emitting devices, and thin film transistors. When comparing the synthetic approaches successfully applied to hybrid perovskites to methods used for the fabrication of double perovskites, the latter is clearly in its very infancy. The question is whether solution engineering and compositional modification strategies can be exploited to match the exceptional optoelectronic properties of hybrid perovskites. This review is motivated by a text mining effort that not only illustrates the prevalence of powder over thin film synthesis but also the discrepancy between the number of compositions experimentally realized and studied as compared to the many predicted compositions. Here we summarize the synthesis aspects of halide double perovskites, and in particular of thin films, including deposition techniques and synthetic modifications to alter film properties.

The photoelectrochemical (PEC) activity of microstructured electrodes remains low despite the highly enlarged surface area and enhanced light harvesting. To obtain a deeper understanding of the effect of 3D geometry on the PEC performance, well-defined WO 3 /n-Si and WO 3 /pn-Si micropillar arrays are fabricated and subjected to a quantitative analysis of the relationship between the geometry of the micropillars (length, pitch) and their PEC activity. For WO 3 /n-Si micropillars, it is found that the photocurrent increases for WO 3 /n-Si pillars, but not in proportion to the increase in surface area that results from increased pillar length or reduced pillar pitch. Optical simulations show that a reduced pillar pitch results in areas of low light intensity due to a shadowing effect. For WO 3 /pn-Si micropillar photoelectrodes, the p-n junction enhances the photocurrent density up to a factor of 4 at low applied bias potential (0.8 V vs RHE) compared to the WO 3 /n-Si. However, the enhancement in photocurrent density increases first and then decreases with reduced pillar pitch, which scales with the photovoltage generated by the p-n junction. This is related to an increased dead layer of the p-n junction Si surface, which results in a decreased photovoltage even though the total surface area increases.