Ricerc@Sapienza

The power conversion efficiency (PCE) of NiO based perovskite solar cells has recently hit a record 22.1%
with a hybrid organic–inorganic perovskite composition and a PCE above 15% in a fully inorganic
configuration was achieved. Moreover, NiO processing is a mature technology, with different industrially
attractive processes demonstrated in the last few years. These considerations, along with the excellent
stabilities reported, clearly point towards NiO as the most efficient inorganic hole selective layer for lead
halide perovskite photovoltaics, which is the topic of this review. NiO optoelectronics is discussed by
analysing the different doping mechanisms, with a focus on the case of alkaline and transition metal
cation dopants. Doping allows tuning the conductivity and the energy levels of NiO, improving the
overall performance and adapting the material to a variety of perovskite compositions. Furthermore, we
summarise the main investigations on the NiO/perovskite interface stability. In fact, the surface of NiO is
commonly oxidised and reactive with perovskite, also under the effect of light, thermal and electrical
stress. Interface engineering strategies should be considered aiming at long term stability and the highest
efficiency. Finally, we present the main achievements in flexible, fully printed and lead-free perovskite
photovoltaics which employ NiO as a layer and provide our perspective to accelerate the improvement
of these technologies. Overall, we show that adequately doped and passivated NiO might be an ideal
hole selective layer in every possible application of perovskite solar cells.