Hollow Cathode Gas Flow Sputtering of Nickel Oxide Thin Films for Hole‐Transport Layer Application in Perovskite Solar Cells

Nickel oxide (NiO1+δ) is a versatile material used in various fields such as optoelectronics, spintronics, electrochemistry, and catalysis which is prepared with a wide range of deposition methods. Herein, for the deposition of NiO1+δ films, the reactive gas flow sputtering (GFS) process using a metallic Ni hollow cathode is developed. This technique is distinct and has numerous advantages compared to conventional sputtering methods. The NiO1+δ films are sputtered at low temperatures (100 ºC) for various oxygen partial pressures during the GFS process. Additionally, Cu‐incorporated NiO1+δ (Cu x Ni1−x O1+δ) films are obtained with 5 and 8 at% Cu. The thin films of NiO1+δ are characterized and evaluated as a hole‐transporting layer (HTL) in perovskite solar cells (PSCs). The NiO1+δ devices are benchmarked against state‐of‐the‐art self‐assembled monolayers (SAM) ([2‐(3,6‐dimethoxy‐9H‐carbazol‐9‐yl)ethyl]phosphonic acid (also known as MeO2PACz)‐based PSCs. The best‐performing NiO1+δ PSC achieves an efficiency (η) of ≈16% without a passivation layer at the HTL interface and demonstrates better operational stability compared to the SAM device. The findings suggest that further optimization of GFS NiO1+δ devices can lead to higher‐performing and more stable PSCs. This study introduces a novel hollow cathode reactive gas flow sputtering (GFS) technique for fabricating nickel oxide (NiOx) and copper‐incorporated NiOx (NiOx:Cu) thin films. These films serve as hole‐transporting layers (HTL) in perovskite solar cells (PSCs). NiOx and NiOx:Cu exhibit enhanced operational stability compared to organic self‐assembled monolayer HTLs, highlighting the potential of GFS in advancing PSC technology.