Mitigation of Defect Formation at the NiO_x/Perovskite Interface in p-i-n Perovskite Solar Cells

Nickel oxide (NiO_x) is widely utilized as an inorganic hole transport layer (HTL) in inverted metal halide perovskite (MHP) solar cells due to its high bandgap, transparency, stability, and scalability. However, its high surface reactivity and the presence of interfacial defects at the NiO_x/MHP interface negatively impact the device performance. To address these issues, the community has explored ultraviolet ozone (UVO) post-treatment of NiO_x and the use of organic molecules for surface passivation. Nevertheless, the individual effects of these processes and their influence on the bulk and surface characteristics of NiO_x, as well as the NiO_x/MHP interface, have not been thoroughly investigated and understood. This study based on photoemission analyses reveals that the UVO process increases the NiO_x reactivity and introduces defects. We identify the nature of defect states at the interface of pristine and UVO-treated NiO_x with MHP and demonstrate that the implementation of MeO-2PACz (M2P) as an organic interlayer mitigates this issue. Additionally, we find that neither UVO treatment nor M2P molecule anchoring significantly impacts the bulk properties of NiO_x.