Long-Lived Dark Exciton Emission in Mn-Doped CsPbCl Perovskite Nanocrystals.

The unusual temperature dependence of exciton emission decay in CsPbX perovskite nanocrystals (NCs) attracts considerable attention. Upon cooling, extremely short (sub-ns) lifetimes were observed and were explained by an inverted bright-dark state splitting. Here, we report temperature-dependent exciton lifetimes for CsPbCl NCs doped with 0-41% Mn. The exciton emission lifetime increases upon cooling from 300 to 75 K. Upon further cooling, a strong and fast sub-ns decay component develops. However, the decay is strongly biexponential and also a weak, slow decay component is observed with a ∼40-50 ns lifetime below 20 K. The slow component has a ∼5-10 times stronger relative intensity in Mn-doped NCs compared to that in undoped CsPbCl NCs. The temperature dependence of the slow component resembles that of CdSe and PbSe quantum dots with an activation energy of ∼19 meV for the dark-bright state splitting. Based on our observations, we propose an alternative explanation for the short, sub-ns exciton decay time in CsPbX NCs. Slow bright-dark state relaxation at cryogenic temperatures gives rise to almost exclusively bright state emission. Incorporation of Mn or high magnetic fields enhances the bright-dark state relaxation and allows for the observation of the long-lived dark state emission at cryogenic temperatures.