Halide‐Exchange Arrest Enables Reabsorption‐Free CsPbCl ₃ /CsPbI ₃ Perovskite Core/Shell Nanocrystals

Expanding the Stokes shift of lead‐halide perovskite nanocrystals (NCs) without compromising their sharp, fast excitonic emission has remained elusive, as high halide mobility erases the compositional gradients required for stable core/shell architectures. Here, it is shown that introducing a CdCl 2 passivation step prior to halide exchange provides a simple solution. Treating CsPbCl 3 NCs with CdCl 2 eliminates halide‐vacancy traps, enhances emission yield, and crucially blocks inward diffusion of I − , arresting the Cl − → I − exchange after just a few monolayers. This produces CsPbCl 3 /CsPbI 3 core/shell NCs that absorb at 3.14 eV from the core and emit at 1.91 eV from the shell, achieving an apparent Stokes shift of ≈1.2 eV. The heterostructures exhibit ≈70% photoluminescence quantum yield, fast emission lifetime (≈10 ns) and complete suppression of reabsorption losses, as confirmed by liquid‐waveguiding experiments. Transient absorption spectroscopy and DFT modeling reveal an inverted type‐I band alignment with ultrafast (≈60 ps) core‐to‐shell exciton transfer. This fully solution‐processed chemistry enables heterostructuring‐based wavefunction engineering – long employed to expand the capabilities of conventional quantum dots – now realized in perovskite NCs, which provides a practical route to reabsorption‐free perovskite emitters for advanced photonic and quantum technologies.