  Nickel oxide, in particular in its doped, semiconducting form, is an
important component of several optoelectronic devices. Doping NiO is commonly
achieved either by incorporation of lithium, which readily occupies Ni sites
substitutionally, producing the Li$_{\text{Ni}}$ acceptor, or by supplying
reactive oxygen species during NiO film deposition, which leads to the
formation of Ni vacancies (V$_{\mathrm{Ni}}$). However, the energetic position
of these acceptors in the NiO band gap has not been experimentally determined
until today. In this work, we close this knowledge gap by studying rectifying
n$^{++}$p heterojunctions of NiO on top of fluorine-doped tin oxide. These
structures show sufficient rectification to perform electric characterization
by defect spectroscopic techniques, specifically capacitance-voltage and
thermal admittance spectroscopy. Using these methods, the (0/-) charge
transition levels are determined to be 190meV and 409meV above the valence band
edge for the Li$_{\text{Ni}}$ and the V$_{\text{Ni}}$ acceptor, respectively.
