Pseudohalide Anion\nSurface Engineering for Mixed Cation\nPerovskite Nanocrystals

Lead halide perovskite nanocrystals (NCs) have undergone\ngreat\ndevelopment in the optoelectrical field. The mixed cation perovskite\nCs<sub>1–<i>x</i></sub>FA<i><sub>x</sub></i>PbI<sub>3</sub> NCs exhibit great potential in photoelectric devices\nbecause of their improved structural stability and increased light\nabsorption range. However, achieving homogeneous Cs<sub>1–<i>x</i></sub>FA<i><sub>x</sub></i>PbI<sub>3</sub> NCs\nwith fewer surface defects remains challenging. In this work, we employ\na two-step exchange-passivation method for the preparation of Cs<sub>1–<i>x</i></sub>FA<i><sub>x</sub></i>PbI<sub>3</sub> NCs and adopt a surface repair strategy based on pseudohalide\nions to decrease the surface trap density. Owing to the similar radius\nof the pseudohalide ions to that of the iodine ions and their strong\nelectronegativity from F atoms, the pseudohalide ions can fill the\nsurface iodine vacancies of the NCs and form Hydron bonds with FA<sup>+</sup>, resulting in improved photoelectric properties and suppressed\nnonrecombination of carriers. This work highlights the importance\nof surface chemistry in perovskite NCs and may provide insights for\nsynthesizing high-performance mixed A-site perovskite NCs for future\napplications.