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

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 CsPbBr₃ 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 CsPbI₃ 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.