Pure‐Red Light‐Emitting Diodes Based on Lead Halide Perovskite Nanoplatelets Obtained by Ostwald Ripening and Two‐Step Anion Exchange

Abstract Metal halide perovskite nanoplatelets (NPls) are promising materials for the fabrication of light‐emitting diodes (LEDs) with emission tunable across the whole visible spectral range. However, synthesis of relatively thick perovskite NPls with a well‐controlled thickness (measured in monolayers, ML) is challenging. In this study, transformations between CsPbBr 3 NPls of different thickness are studied, which occur through the Ostwald ripening mechanism. Under optimal conditions, post‐synthetic lateral growth of NPls facilitates self‐purification of the colloidal solution, leading to the formation of a monodisperse ensemble of 5ML NPls at the expense of the undesired 2ML NPl by‐products. The subsequent two‐step Br − ‐to‐I − anion exchange enables synthesis of pure‐red emitting CsPbI 3 NPls with a near‐unity photoluminescence quantum yield. LEDs based on these NPls show external quantum efficiency reaching 17.3%, and possess a narrow (26 nm) electroluminescence centered at 637 nm, which fully meets the requirements of the Rec 2020 standard for pure‐red emission for ultra‐high definition displays.