Annealing atmosphere effect on Ni states in the thermal-decomposed NiO_x films for perovskite solar cell application

NiO_x films with different Ni states are prepared by decomposing the solution-derived Ni(OH)₂ films in varies of atmospheres. All the NiO_x films exhibit full coverage on the substrates with rough surfaces. Both the air annealing and O₂ annealing result in similar film compositions, which are composed of NiO, Ni₂O₃ and NiOOH. While additional metallic nickel is observed in the N₂-annealed sample, which is different from the pure NiO powders annealed in N₂, indicating a complicated decomposition process of Ni(OH)₂ in N₂ atmosphere. The as-prepared NiO_x films were employed as the hole transport layer of inverted planar perovskite solar cells. Owing to the existence of metallic nickel, an obvious performance degradation is observed for device based on the N₂-annealed sample. Finally, the champion power conversion efficiency (PCE) of 14.55% is achieved on the device based on the air-annealed NiO_x film, which should be attributed to the slightly higher content of NiOOH compared to the O₂-annealed sample.