Room‐Temperature Ion‐Exchange‐Mediated Self‐Assembly toward Formamidinium Perovskite Nanoplates with Finely Tunable, Ultrapure Green Emissions for Achieving Rec. 2020 Displays

Delicate engineering of chromaticity is required to faithfully reproduce colors in a backlit display, this is extremely difficult for green downconverters because the human eye is highly sensitive to green colors. The central challenge is to achieve finely tunable green emissions in the narrow range of 525–535 nm while keeping the full width at half maximum (FWHM) <25 nm at the same time. Here, a room‐temperature ion‐exchange‐mediated self‐assembly strategy for preparing FAPbBr 3 (FA = CH(NH 2 ) 2 + ) nanoplates (NPs) to fulfill this goal is introduced. 2D layered OA 2 PbBr 4 (OA is octadecylamine) NPs are first synthesized by spontaneous reprecipitation, and are then transformed into FAPbBr 3 NPs through a OA + ‐to‐FA + exchange induced self‐assembly of HP monolayers. A c ‐axis contraction in this process makes a relative large thickness variation in OA 2 PbBr 4 NPs, which can be realized by simply varying the precursor concentration, only result in a small thickness change in subsequent FAPbBr 3 NPs, thereby enabling finely tunable emissions in the range of 525–535 nm along with FWHM <25 nm and a quantum yield up to 85%. As a downconverter, the FAPbBr 3 NPs realize an ultrapure green backlight that covers ≈95% Rec. 2020 standard in the CIE 1931 color space.