characterization

 AFM

Standard photoelectrochemical characterization such as cyclic voltammetry or onset potential measurements are large-scale measurements using macroscopic electrodes, which do not provide information about local performance and structural variations within the semiconductor thin film. However, finer resolution is necessary to understand the fundamental mechanisms dominating performance at the nanometer scale, which is especially interesting for systems where grains, grain boundaries and crystal facets may dominate the performance and efficiency of the whole device.

To elucidate the performance of PEC material at the nanoscale, we use conductive and electrochemical atomic force microscopy (AFM). (Photo)conductive AFM is a powerful tool to reveal correlations between topographic features and its functional properties via current mapping as well as to identify the underlying charge transport mechanism via local IV-spectroscopy. Complementary, electrochemical AFM provides insight into structural changes under photoelectrochemical operation condition.

Synchrotron - STXM

While pc-AFM provides insights into functional variations and EC-AFM resolves structural changes during operation, changes in the electronic structure and chemical composition within the sample still needs to be addressed. Therefore, we use scanning transmission X-ray microscopy (STXM) to probe local functional variations of the electronic structure with a spatial resolution of 10 nm. The chemical sensitivity of the absorption-contrast in STXM measurements arises from the sensitivity of near edge x-ray absorption fine structure (NEXAFS) spectroscopy, which can be accessed through spatially resolved spectra, images at specific X-ray energies, or image-sequences across absorption edges in x-ray microscopes. Since the materials properties change in the course of the reaction due to due to (photo)corrosion of the material, as-prepared as well as ex-situ degraded samples are investigated.

TEM

The PEC performance is not only determined by the bulk properties of the light absorber material, but it also strongly depends on the charge transport across the interfaces such as light absorber/catalyst or catalyst/electrolyte interface. To image and analyze the atomic structure at the semiconductor/catalyst interface we use cutting-edge transmission electron microscopies.

Spatial collection efficiency extraction

The spatial collection efficiency (SCE) of photovoltaic and photoelectrochemical devices has a critical influence on their performance. Defined as the fraction of photogenerated charge carriers created at a specific point within the device that eventually contribute to the collected photocurrent, measurement of the spatial collection efficiency can shed new light on different charge transport mechanisms and aid in device optimization. We have developed a non-destructive empirical method to extract the spatial collection efficiency by combining Incident Photons to Current Efficiency (IPCE) measurements with optical modeling.  The block diagram bellow shows the main steps in the SCE extraction process. Optical models and IPCE measurements of the material under study are used to produce the optical generation profile, and photoaction spectrum, which are inserted into a regularization problem. Next, screening algorithms are used in order to identify the physical SCE profile. Analysis of the SCE profile can reveal significant information on charge transport and loss mechanism in the material and distinguish between bulk and surface losses.