NiOx passivation in perovskite solar cells: from surface reactivity to device performance

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.