Interface Effect Between NiO_x and Perovskite and Their Impact on Solar Cells—A Comparative Study of Sputtered and Nanoparticle NiO_x

In perovskite solar cells with inverted structure, Nickel oxide (NiO_x) has been used as a hole transport layer because of its high chemical stability, excellent energy level alignment with the absorber layer, cost-effectiveness, and ease of fabrication via various deposition methods -spin coating, sputtering, ALD, etc.-. However, NiO_x can react with the perovskite depending on the valency of nickel ions. Reactants acting as a barrier during hole extraction at the NiO_x-perovskite interface were provided by the deprotonation of cationic ammonium and oxidation of iodide species. Despite the importance of this interface, a fundamental understanding of the interface effect in NiO_x and perovskite, especially in terms of device performance, has not been thoroughly explored. In this study, we investigated material properties of two different NiO_x thin films using nanoparticle NiO_x precursor (NP NiO_x) and RF sputtering method (SP NiO_x) and their effect on interface effect with perovskite, device performance, and stability. In order to elucidate the difference between NP NiO_x and SP NiO_x, we investigated the interface of NiO_x/perovskite by XPS, UPS, Raman, and electrical measurements. These NiO_x films significantly differed in the Ni³⁺/Ni²⁺ ratio, consequently affecting their conductivity. Experimental results showed that large and smooth grains are observed in perovskite on top of SP NiO_x. Additionally, the interfacial interaction between NiO_x and perovskite was verified through I-V measurements. The device using SP NiO_x showed a severe current drop, which means there is severe hole depletion between SP NiO_x and perovskite but not in NP NiO_x. To further explain the underlying mechanisms, we performed XPS and UPS measurements on NiO_x/perovskite samples with varying perovskite thicknesses. The results revealed notable shifts in the energy levels of Pb 4f and I 3d orbitals in SP NiO_x-based devices, while NP NiO_x-based devices showed no significant shifts. Also, we identified a unique bonding configuration in NP NiO_x films, likely formed during the synthesis process, which may help suppress hole depletion at the interface. As a result, NP NiO_x-based PSCs demonstrated better hole extraction and higher fill factor (FF) compared to SP NiO_x-based devices. This study suggests an insight into the optimization of NiOx-based HTLs for achieving higher-efficiency PSCs, and the origin of the interface effect between NiO_x and perovskite.