Understanding Optical Properties and Electronic Structures of High‐Entropy Alloyed Perovskite Nanocrystals

High‐entropy alloying (HEA) has emerged as a prominent strategy to modulate physiochemical properties of nanomaterials. Nevertheless, this approach is underexplored in luminescent semiconductor nanocrystals (NCs) due to the lack of understanding into the HEA‐induced electronic effect and photophysical behaviors. Herein, harnessing the defect tolerance of metal halide perovskite NCs, we systematically synthesized and characterized high‐entropy halide perovskite (HEP) NCs containing multiple B‐site elements (Pb 2+ , Sr 2 ⁺, Ca 2 ⁺, Cd 2 ⁺, and Mg 2 ⁺). High‐resolution transmission electron microscopy, transient photoluminescence and absorption spectroscopy, X‐ray photoemission spectroscopy, and density functional theory simulations are employed to unravel the evolution of electronic structures with respect to the alloying degree and link them to the spectral signatures and photostability. Counterintuitively, although the HEP NCs exhibit lateral sizes smaller than the Bohr diameter of CsPbBr 3 NCs (∼7 nm), HEA reduces the band dispersion and broadens the conduction band, thereby vanishing the excitonic feature by forming near band‐edge shallow states. We show that these HEA‐induced shallow states foster rapid radiative recombination and improve photostability, accompanied by a significantly reduced lead content by up to 70%. These findings pioneer the understanding of the correlation between HEA‐induced electronic effect and photophysical properties, highlighting the versatility of HEA for band structure engineering and stabilization of metal halide perovskites NCs.