Synergistic Regulation of Additive's Steric Hindrance and Amino Group Flexibility Enables All-Bromide Pure Blue Perovskite Light-Emitting Diodes.

Quasi-2D (q-2D) perovskite materials can construct a distribution of multiple n phases by incorporating spacer organic cation additives and thus have been widely applied in blue perovskite light-emitting diodes (PeLEDs). However, the lack of research on the regulation of low--value q-2D phases by different additives has hindered the development of pure-blue PeLEDs. Herein, by investigating a series of organic cation additives, we identified the key factors governing the assembly of n phases: the additive's steric hindrance and amino group flexibility. Additives with high steric hindrance can effectively segment the 3D perovskite lattice, facilitating the formation of low- phases. However, increased flexibility of the amino groups in additives that coordinate with the perovskite lattice induces the further assembly of low- phases into high- phases during crystallization, whereas low flexibility can anchor low- phase components and inhibit their further assembly. By utilizing additives with different steric hindrance and flexibility, we achieved the regulation of electroluminescence spectra of all-bromide q-2D perovskites from green (517 nm) to deep blue (449 nm). It was confirmed that the p-F-PEABr additive, featuring high steric hindrance and low amino group flexibility, can effectively control the uniform distribution of low- phases. Thus, the corresponding q-2D PeLEDs achieved high-quality pure blue electroluminescence at 468 nm with an external quantum efficiency (EQE) of 2.72%, and no obvious spectral shift was observed. This work uncovers the mechanism underlying the effect of spacer additives' steric hindrance and amino group flexibility on the precise regulation of q-2D phase distribution, paving the way for the development of pure blue PeLEDs.