From Bulk to Surface: Sodium Treatment Reduces Recombination at the Nickel Oxide/Perovskite Interface

Abstract The effect of sodium doping in NiO as a contact layer for perovskite solar cells is investigated. A combined X‐ray diffraction and X‐ray photoelectron spectroscopy analysis reveals that Na + mostly segregates as NaO x /NaCl species around NiO crystallites, with the effect of reducing interface capacitance as revealed by impedance spectroscopy. Inspired by this finding, the NiO/perovskite interface in perovskite solar cells is modified via insertion of an ultrathin NaCl interlayer, which increases the NiO work‐function by 0.3 eV. This leads to an increase of power conversion efficiency, approaching 18%, and open‐circuit voltage due to a remarkable suppression of surface recombination, as revealed by photoluminescence analysis and light intensity–dependent electrical measurements.