Origin of the Black Color of NiO Used as Photocathode in p-Type Dye-Sensitized Solar Cells

We report herein on the origin of the black color of NiO, the widespread p-type semiconductor used as photocathode in p-type dye-sensitized solar cells (DSSCs). Clearly, the presence of nickel metal (Ni-0) was revealed by X-ray diffraction analyses and X-ray photoelectron spectroscopy in all NiO samples we have prepared according to chemical routes usually employed for DSSC applications. Thus, the black color of the cathodes is correlated to the existence of Ni-0 and not to the increased amount of Ni2+/Ni3+ mixed valence as commonly suggested in the literature. Surprisingly, the presence of elemental nickel does not seem to affect drastically the photovoltaic performances of DSSCs that is rather controlled by the amount of chemisorbed dye. Sintering of NiO samples at higher temperature (i.e., from 450 to 900 degrees C) triggers the progressive oxidation of Ni-0 and ultimately leads to pale green NiO samples, but to larger particles size that results in lower conversion efficiencies.