Fig. 1. (Color online) (a) Sol-gel preparation of NiO_x nanoparticles. Reproduced with permission^[8], Copyright 2023, Wiley. (b) Possible degradation mechanism of NiO_x-perovskite heterojunction. Reproduced with permission^[10], Copyright 2022, the Royal Society of Chemistry.

Fig. 2. (Color online) (a) The oxidation state and ionic radius for several metals. Reproduced with permission^[19], Copyright 2020, the Royal Society of Chemistry. (b) Synthesis of NiO_x nanoparticles. Reproduced with permission^[22], Copyright 2022, Wiley. (c) NiO_x modified by TTTS. Reproduced with permission^[23], Copyright 2022, Wiley. (d) The preparation of NiO_x/carbon heterostructure. Reproduced with permission^[24], Copyright 2021, Wiley. (e) SAM-modified NiO_x at the interface. Reproduced with permission^[27], Copyright 2021, Wiley. (f) Flexible PSCs with bridging molecules. Reproduced with permission^[28], Copyright 2022, Nature. (g) The energy level diagram for device with PTAA. Reproduced with permission^[30], Copyright 2021, Elsevier. (h) TMSBr buffer layer inhibiting perovskite degradation. Reproduced with permission^[10], Copyright 2022, the Royal Society of Chemistry.