Nanoscale Characterization for Understanding Performance Limitations in Water Splitting Photoelectrodes

<br/>Photoelectrochemical water splitting is a promising route for efficient conversion of solar energy into chemical fuels. In this context, economically viable photosystems are often based of semiconductor thin films which are polycrystalline or nanostructured with highly complex architectures e.g. with boundaries between differently orientated grains, different crystal facet orientations, as well as locally varying composition or phase. These nano- to micrometer properties often control critical processes, such as efficiency and stability, of the macroscale system. To understand the impact of nanoscale properties on the macroscopic performance, we aim at resolving local structural, chemical, and optoelectronic heterogeneities and at elucidating their effect on light-driven processes.<br/><br/>In this work, we use a correlative approach to elucidate the nanoscale properties of polycrystalline BiVO₄ thin film photoelectrodes. Mapping of the local charge transport properties by photoconductive atomic force microscopy reveals the tolerance to grain boundaries.[1] Interestingly, scanning nearfield infrared microscopy shows strongly varying absorption from VO₄ stretching modes between grains and at grain boundaries which correlate with the heterogeneities in the local photoconductivity across the polycrystalline thin films. Furthermore, local temperature-dependent current-voltage spectroscopy show that the low intrinsic bulk conductivity of BiVO₄ limits the electron transport through the film, and that the transport mechanism can be attributed to space charge limited current in the presence of trap states.[1] Performing the same measurements in-situ in controlled gas-phase environment reveals the influence of chemical interactions of adsorbed oxygen and water on charge transport and interfacial charge transfer.[2] For BiVO₄, we demonstrate that adsorbed oxygen acts as a surface trap state for electrons, which enhances the built-in potential and depletes the BiVO₄ layer. Overall, combining insights from different nanoscale techniques generates a comprehensive picture of charge separation, transport, and transfer at the nanoscale. The gained nanoscale understanding of energy materials enables the rational design of durable and efficient solar fuel devices.<br/><br/>[1] Eichhorn et al. Nanoscale imaging of charge carrier transport in water splitting photoanodes,<i> Nat. Commun. </i>9, 2597 (2018).<br/>[2] Eichhorn et al. Disentangling the role of surface chemical interactions on interfacial charge transport at BiVO₄ photoanodes, <i>ACS Appl. Mater. Interfaces</i>, 10, 41, 35129 (2018).