Engineering Full‐Spectrum Upconversion Through Coherent Energy Recycling in NaYF ₄ :Yb,Tm/Cs ₄ PbBr ₆ Heterostructure

Hybrid heterojunctions integrating lanthanide‐doped upconversion nanoparticles (UCNPs) and lead‐halide perovskites (LHPs) offer compelling opportunities for multimodal luminescence and tailored photon conversion. However, the synthesis of well‐defined UCNP/LHP heterostructures is hindered by the rapid crystallization kinetics and ionic character of perovskites, while the energy transfer mechanisms—particularly the role of LHPs in modulating upconversion—remain poorly understood. Here, we demonstrate a thermodynamics‐controlled epitaxial growth strategy that enables the fabrication of high‐quality NaYF 4 :Yb,Tm/Cs 4 PbBr 6 heterostructures with tunable architecture—from core/shell to UCNP‐in‐LHP solids. Crucially, the Cs 4 PbBr 6 layer acts not merely as a passive energy acceptor, but as a functional sub‐energy level that facilitates back energy transfer to Tm 3+ and redirects its excited‐state population. This coherent energy recycling enables broadly tunable upconversion emission across the entire visible spectrum. Our findings provide a robust route to high‐quality UCNP/LHP heterostructures and establish LHPs as active participants in the engineering of upconversion pathways.