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Low photoluminescence quantum yield (PLQY) and spectra instability, the two most difficult challenges in blue‐emitting CsPbBrxCl3−x NCs, have not yet been solved. Quickly controlling the reaction thermodynamics is crucial to enhance crystallinity, thus PLQY and spectra stability, but it has been ignored until now. An ultrafast thermodynamic control (UTC) strategy is designed by utilizing liquid nitrogen to instantaneously freeze the superior crystal lattices of CsPbBrxCl3−x NCs formed at high temperature. The average cooling rate exhibits a 33‐fold increase compared to conventional ice‐water cooling (from 1.5 to 50 K s−1). This UTC can make the reaction thermodynamic energy of the system lower than the threshold very quickly. Therefore, abrupt termination of further crystal growth can be achieved, which also avoids additional nucleation at low temperature. With the assist of defect passivation, the final blue‐emitting CsPbBrxCl3−x NCs exhibit an absolute PLQY of 98%, representing the highest value in Pb‐based blue perovskites to date. More importantly, they exhibit superior spectra instability. This UTC strategy not only represents a new avenue to synthesize perovskite NCs with excellent crystal quality and ultrahigh PLQY, but also provides a good reference to deal with the recognized bottleneck of spectra instability. [ABSTRACT FROM AUTHOR]

All-inorganic perovskite nanocrystals have held great promise as incredibly effective luminescent materials and also have been synthesized efficiently by room temperature approaches. Toluene, a high toxicity solvent, is often used as a valid antisolvent in ligand-assisted reprecipitation strategy, which badly restricts the commercial application of this convenient method. Here, aiming to address the toluene toxicity issue, the low toxic tetraethyl orthosilicate (TEOS) is employed as a new alternative antisolvent to achieve high uniform and composition dependent luminescent all-inorganic perovskite Cs 4 PbX 6 (X = I, Br, Cl) combined with CsPbX 3 (X = I, Br, Cl) nanocrystals with tunable emission large-span wavelengths (626–437 nm) and remarkably narrow full width at half-maximum (FWHM) monochromaticity (down to 19 nm). Meanwhile, the ratio of CsBr to PbBr 2 is proved to be a critical factor to control the crystalline phase of the resulting perovskite nanocrystals. Additionally, one monochromatic light-emitting diode (LED) lamp with brightly pure green emission is fabricated based on bromine-based perovskite nanocrystals. This newly developed low toxicity antisolvent synthesis (LTaS) is suitable candidate for commercial production in an environmentally-friendly way, and the as-obtained perovskite materials with superior optical merits will be brought to the forefront of lighting and displays. [ABSTRACT FROM AUTHOR]