  Non-stoichiometric nickel oxide (NiOx) is the only metal oxide successfully
used as hole transport material in p-i-n type perovskite solar cells (PSCs).
Its favorable opto-electronic properties and facile large-scale preparation
methods are potentially relevant for future commercialization of PSCs, though
currently low operational stability of PSCs containing NiOx hole transport
layers are reported. Poorly understood degradation reactions at the interface
to the perovskite are seen as cause for the inferior stability and a variety of
interface passivation approaches have been shown to be effective in improving
the overall solar cell performance. To gain a better understanding of the
processes happening at this interface, we systematically passivated possible
specific defects on NiOx with three different categories of organic/inorganic
compounds. The effects on the NiOx and the perovskite (MAPbI3) were
investigated using x-ray photoelectron spectroscopy (XPS), X-ray diffraction
(XRD), and scanning electron microscopy (SEM) where we find that the structural
stability and film formation can be significantly affected. In combination with
Density Functional Theory (DFT) calculations, a likely origin of
NiOx-perovskite degradation interactions is proposed. The surface passivated
NiOx was incorporated into MAPbI3 based PSCs and its influence on overall
performance, particularly operational stability, was investigated by
current-voltage (J-V), impedance spectroscopy (IS), and open circuit voltage
decay (OCVD) measurements. Interestingly, we find that a superior structural
stability due to an interface passivation must not relate to high operational
stability. The discrepancy comes from the formation of excess ions at the
interface which negatively impacts all solar cell parameters.
