Suppressing the naturally ultrafast nucleation and growth rates of perovskite nanocrystals is a big challenge to develop high-performance deeply blue perovskite light-emitting diodes. Here, a cryogenic temperature thermodynamically suppressed synthetic strategy using liquid nitrogen is designed to obtain ultrasmall CsPbBr3 quantum dots (QDs; ≈3 nm). Due to its strong confinement effect, the as-obtained CsPbBr3 QDs present strong deeply blue emission (≈460 nm) with a high quantum yield value of up to 98%, a large exciton binding energy of 301.6 meV, and excellent spectra stability for 60 d under atmosphere environment. This unprecedented regime indicates that cryogenic temperature can eliminate pre-existing trap states and suppress th  nonradiative process. Besides, the resultant perovskite light-emitting diodes based on ultrasmall CsPbBr3 QDs show deeply blue emission (≈ 460 nm) with a Commission Internationale de l’Eclairage (CIE) color coordinate of (0.145, 0.054), better than the blue National Television Standards Committee (NTSC) standard. Evidently, this cryogenic temperature synthetic strategy will pave the way for the large-scale synthesis of the strongly confined ultrasmall quantum dots systems and open the door for the development of next

generation solid-state lighting and displays.