Influence of Feature Size, Film Thickness, and Silicon Doping on the Performance of Nanostructured Hematite Photoanodes for Solar Water Splitting

Photoanodes consisting of nanostructured hematite prepared by atmospheric pressure chemical vapor deposition (APCVD) have previously set a benchmark for solar water splitting. Here, we fully investigate this promising system by varying critical synthetic parameters and probing the photoanode performance to determine the major factors that influence operation. By varying the film thickness, we show film growth to be linear with an incubation time. We find no concern with electron transport for films up to 600 nm, but a higher recombination rate of photogenerated carriers in the hematite near the interface with the fluorine-doped tin oxide, as compared to the bulk section of the film. The mechanism for the formation of the thin film's nanoporous dendritic structure is discussed on the basis of the results from varying the substrate growth temperate. The observed feature sizes of the film are found to depend strongly on this temperature and the presence of silicon dopant precursor (TEOS). Raman and Mössbauer experiments reveal how temperature and doping affect the crystallinity and ultimately the photoperformance. We also use impedance spectroscopy to find evidence for an unusually high donor density, which allows the formation of a space-charge field inside the nanosized features of the polycrystalline hematite photoanode.