Revivification of nickel oxide-perovskite interfaces via nickel nitrate to boost performance in perovskite solar cells

Nickel oxide (NiO_x) is a prospective hole transporting layer (HTL) material for inverted (p-i-n) perovskite solar cells (PSCs); however, inevitable surface defects on the NiO_x surface interfere with the realization of high-performance PSCs. Herein, nickel nitrate is used to eliminate defect sites and hydroxyl groups on the NiO_x surface. For the first time, it is found that a uniform and ultra-thin nickel nitride (Ni_xN) layer can be formed on the NiO_x surface through the spin coating of the nickel nitrate solution at low temperatures. Promisingly, this new approach allows to overcome interface losses originating from defect sites and hydroxyl groups on the NiO_x surface and to achieve high efficiency and long-term operational stability in PSCs. Furthermore, Ni_xN serves better energy level alignment between NiO_x HTL and perovskite layer, resulting in improved charge extraction and transportation and reduced charge recombination. The Ni_xN-modified NiO_x-based PSC demonstrates an outstanding efficiency of 20.45% and it shows long-term operational stability about 82% of the initial efficiency after 1000 h under ambient air conditions. Thus, this simple approach using nickel nitrate can be a promising and universal strategy to revive nickel oxide-perovskite interfaces and boost the performance and operational stability of PSCs.