  Nickel oxide is a versatile p-type semiconducting oxide with many
applications in opto-electronic devices, but high doping concentrations are
often required to achieve necessary electrical conductivity. In contrast to
many other transparent oxide semiconductors, even moderate levels of doping of
NiO can lead to significant optical absorption in the visible spectral range,
limiting the application range of the material. This correlation has been
reported extensively in literature, but its origin has been unknown until now.
This work combines experimental data on optical properties from a variety of
NiO samples with results from hybrid density functional theory calculations. It
shows that strong electron-phonon interaction leads to a significant blue shift
(0.6-1 eV) of electronic transitions from the valence band maximum to defect
states by light absorption with respect to the thermodynamic charge transition
levels. This essentially renders NiO a narrow-gap semiconductor by defect band
formation already at moderate doping levels, with strong light absorption for
photon energies of approximately 1 eV. The calculations are also shown to be
fully consistent with experimental data on defect-related light emission in
NiO.
