Solution-processed NiO_x nanoparticles with a wide pH window as an efficient hole transport material for high performance tin-based perovskite solar cells

The development of tin-based perovskite solar cells (PSCs) is a promising approach to meet the demand of eco-friendly, lead-free perovskite photovoltaics. Limited to the poor chemical stability of tin perovskites, tin-based PSCs usually have to be fabricated with an inverted device structure and both the device efficiency and stability are highly dependent on the selection of hole transport materials (HTMs). Here, we report the synthesis of inorganic nickel oxide (NiO_x) nanoparticles (NPs) via a soft base precipitation method, which enables us to obtain highly dispersed NiO_x NPs over a wide pH window. The as-prepared NiO_x NPs are employed as the HTM for formamidinium tin iodide (FASnI₃) PSCs, resulting in excellent device efficiency (∼8%) and stability (1200 h). Our study offers a facile strategy for mass production of NiO_x NPs and easy access to efficient and robust inorganic HTMs that could further boost the development of high performance tin-based PSCs.