Spiro‐Configured Self‐Assembling Molecule Modulates Phase Distribution at the Buried Interface for Efficient Sky‐Blue Perovskite LEDs

Quasi‑two‑dimensional (quasi‑2D) perovskites are promising candidates for efficient blue perovskite light‑emitting diodes (PeLEDs), but their complex low‑dimensional phase distributions often result in non‑radiative recombination and emission broadening. Herein, we design 3‐(4‐(10H‐spiro[acridine‐9,9′‐fluoren]‐10‐yl)phenyl)propanoic acid (SAF‐PPA), a spiro‐configured self‐assembling molecule (SAM), to modify the hole‐injection interface. The steric hindrance of SAF‑PPA suppresses the disordered aggregation typically observed in linear SAMs, thereby promoting a more ordered interfacial structure. Moreover, the terminal carboxyl groups of SAF‐PPA passivate uncoordinated Pb 2+ defects and interact with organic spacer cations. This interaction reduces the accumulation of low‑dimensional phases at the interface and promotes a more uniform vertical phase distribution. The SAF‐PPA‐modified films also exhibit reduced parasitic emission from minority phases and weaker exciton‐phonon coupling. As a result, quasi‑2D sky‑blue PeLEDs achieve an external quantum efficiency of 21.9%, a turn‑on voltage as low as 76% of the bandgap voltage, and an ultra‐narrow electroluminescence linewidth of 15.4 nm. A representative SAF‐PPA device further exhibits a T 50 operational lifetime of 901.8 min under constant‐current operation at an initial luminance of 500 cd m −2 . This work overcomes the long‑standing efficiency‐spectral purity trade‑off in quasi‐2D blue PeLEDs and provides a novel interfacial engineering strategy for high‑performance optoelectronic devices.