Long-Lived Dark Exciton Emission in Mn-Doped CsPbCl3 Perovskite Nanocrystals

The unusual temperature dependence of exciton emission decay in CsPbX3 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 CsPbCl3 NCs doped with 0-41% Mn2+. 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 similar to 40-50 ns lifetime below 20 K. The slow component has a similar to 5-10 times stronger relative intensity in Mn-doped NCs compared to that in undoped CsPbCl3 NCs. The temperature dependence of the slow component resembles that of CdSe and PbSe quantum dots with an activation energy of similar to 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 CsPbX3 NCs. Slow bright-dark state relaxation at cryogenic temperatures gives rise to almost exclusively bright state emission. Incorporation of Mn2+ 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.