Exciton Self‐Trapping Enables Broadband Emission in Two‐Monolayer Colloidal CdSe Quantum Wells

Achieving intrinsic broadband emission in colloidal quantum wells (CQWs) via self‐trapped excitons (STEs) is highly desirable for solid‐state lighting, yet remains challenging due to their rigid lattices and inherently weak exciton‐phonon coupling. In this work, we have linked the broadband emission from two‐monolayer CdSe CQWs with extreme quantum confinement to the radiative recombination of STEs. This observation unlocks efficient intrinsic STE emission without extrinsic dopants or stacking formations, characterized by an exceptionally broad bandwidth (354 meV centered at 548 nm), high quantum yield (∼90%), and shallow trapping barriers (∼14.8 meV) enabled by strong exciton‐phonon coupling (Huang‐Rhys factor ∼36.7). Ultrafast spectroscopic studies confirm rapid exciton self‐trapping (∼170 fs) and negligible exciton‐exciton annihilation, reflecting the localized STE state. These atomically thin CdSe CQWs thus provide a chemically simple, highly efficient, and stable foundation for next‐generation white‐light emitters.