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 NaYF₄:Yb,Tm/Cs₄PbBr₆ heterostructures with tunable architecture-from core/shell to UCNP-in-LHP solids. Crucially, the Cs₄PbBr₆ layer acts not merely as a passive energy acceptor, but as a functional sub-energy level that facilitates back energy transfer to Tm³⁺ 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.