Replacing cesium with formamidinium cation in colloidal perovskite nanoplatelets: Synthetic alloying and post-synthetic cation exchange towards efficient blue emission

Blue-emitting perovskite materials have attracted remarkable attention in recent years owing to their potential in full-color display and lighting applications. Two-dimensional (2D) perovskite nanoplatelets (NPls) are considered as competitive blue emitters due to high carrier recombination probability and good spectrum stability compared with their mixed-halide counterparts. However, the 2D nature and high defect density of perovskite NPls generally result in nonradiative carrier recombination, making it problematic for achieving efficient blue emission. Herein, we reported a novel way to enhance the blue emission efficiency of perovskite NPls. We found that by using formamidinium cation (FA⁺) to replace the cesium cation (Cs⁺) during NPls synthesis, the photoluminescence (PL) of the NPls can be significantly enhanced. The PL quantum yield (PLQY) of NPls exhibited a monoclinic rising tendency with increased FA⁺ content, and the FAPbBr₃ NPls finally show efficient deep blue emission at 433 nm with PLQY up to 70% as well as narrow emission bandwidth of only 12 nm. We further demonstrated that efficient blue emission can be realized not only by FA⁺ alloying but also by post-synthetic treatment of the as-synthesized CsPbBr₃ NPls using FA⁺, where the FA⁺ was capable to replace the Cs⁺ in perovskite NPls through cation exchange. Resultantly, PLQY as high as 90% was achieved at a pure blue wavelength of 457 nm. Our work provides a promising strategy for achieving efficient and color-pure blue-emitting perovskite materials, which could potentially further promote their light-emitting applications.