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 (CuxNi1−xO1+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.