From bulk to surface: sodium treatment reduces recombination at the nickel oxide/perovskite Interface

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 NaOx/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.