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 CdCl₂ passivation step prior to halide exchange provides a simple solution. Treating CsPbCl₃ NCs with CdCl₂ 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 CsPbCl₃/CsPbI₃ 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.