Strong Quantum Confinement Enhanced Exciton‐to‐Mn 2+ Energy Transfer in Perovskite Nanocrystals

ABSTRACT Mn 2+ doping in perovskite nanocrystals introduces a new exciton decay pathway, enabling tailored optoelectronic properties for applications in scintillators through suppressed self‐absorption. However, their performance is limited by inefficient exciton‐to‐Mn 2 + energy transfer. Enhancing this energy transfer efficiency relies on a deeper understanding of the underlying energy transfer mechanisms. Herein, a thoroughly comparative spectroscopic investigation on the Mn 2+ ‐doped perovskite nanocrystals with different degrees of quantum confinement is conducted. In weakly confined nanocrystals, energy transfer occurs via longitudinal‐optical‐phonon‐assisted formation of self‐trapped excitons. The large energy of longitudinal optical phonons (∼ 46 meV) resulting its low occupation probability at low temperatures (<200 K, k B T <17 meV), leading to nearly zero energy transfer efficiency at cryogenic temperatures. Conversely, in a strongly confined system, excitons become delocalized across the entire structure. This enables direct Dexter‐type energy transfer to Mn 2 + ions via exchange interaction, achieving near 100% efficiency. Additionally, strong confinement and spin‐orbit coupling hybridize singlet and triplet exciton states, allowing long‐lived dark singlets (τ = 240 ns at 10 K) to efficiently drive the spin‐forbidden Mn 2 + transition (ΔS = 1). The mechanistic understanding of confinement‐dependent exciton‐dopant interactions presented here provides a materials design paradigm for engineering high‐efficiency spectral‐converting nanocrystals, with immediate application potential in scintillators.