  A large body of literature reports that both bismuth vanadate and haematite
photoanodes are semiconductors with an extremely high doping density between
10^18-10^21 cm^-3. Such values are obtained from Mott-Schottky plots by
assuming that the measured capacitance is dominated by the capacitance of the
depletion layer formed by the doping density within the photoanode. In this
work, we show that such an assumption is erroneous in many cases because the
injection of electrons from the collecting contact creates a ubiquitous
capacitance step that is very difficult to distinguish from that of the
depletion layer. Based on this reasoning, we derive an analytical resolution
limit that is independent of the assumed active area and surface roughness of
the photoanode, below which doping densities cannot be measured in a
capacitance measurement. We find that the reported doping densities in
literature lie very close to this value and therefore conclude that there is no
credible evidence from capacitance measurements that confirms that bismuth
vanadate and haematite photoanodes contain high doping densities.
