  Favorable optoelectronic properties and ease of fabrication make NiO a
promising hole transport layer for perovskite solar cells. To achieve maximum
efficiency, the electronic levels of NiO need to be optimally aligned with
those of the perovskite absorber. Applying surface modifiers by adsorbing
species on the NiO surface, is one of the most widespread strategies to tune
its energy levels. Alkali halides are simple inorganic surface modifiers that
have been extensively used in organic optoelectronics, however, rarely studied
in perovskite solar cells. Using density functional theory (DFT) calculations,
we investigate the effect of single layer adsorption of twenty different alkali
halides on the electronic levels of NiO. Our results show that alkali halides
can shift the position of the valence band maximum (VBM) of NiO to a
surprisingly large extend in both directions, from -3:10 eV to +1:59 eV. We
interpret the direction and magnitude of the shift in terms of the surface
dipoles, formed by the adsorbed cations and anions, where the magnitude of the
VBM shift is a monotonic function of the surface coverage. Our results indicate
that with alkali halide surface modifiers, the electronic levels of NiO can be
tuned robustly and potentially match those of many perovskite compositions in
perovskite solar cells.
