There has been a growing interest in applying CsPbX3 (X = Cl, Br, I) nanocrystals (NCs) for optoelectronic application. However, research on doping of this new class of promising NCs with optically active and/or magnetic transition metal ions is still limited. Here we report a facile room temperature method for Mn2+ doping into CsPbCl3 NCs. By addition of a small amount of concentrated HCl acid to a clear solution containing Mn2+, Cs+, and Pb2+ precursors, Mn2+-doped CsPbCl3 NCs with strong orange luminescence of Mn2+ at ∼600 nm are obtained. Mn2+-doped CsPbCl3 NCs show the characteristic cubic phase structure very similar to the undoped counterpart, indicating that the nucleation and growth mechanism are not significantly modified for the doping concentrations realized (0.1 at. % – 2.1 at. %). To enhance the Mn2+ emission intensity and to improve the stability of the doped NCs, isocrystalline shell growth was applied. Growth of an undoped CsPbCl3 shell greatly enhanced the emission intensity of Mn2+ and resulted in lengthening the radiative lifetime of the Mn2+ emission to 1.4 ms. The core–shell NCs also show superior thermal stability and no thermal degradation up to at least 110 °C, which is important in applications.

Doping nanocrystals (NCs) with luminescent activators provides additional color tunability for these highly efficient luminescent materials. In CsPbCl3 perovskite NCs the exciton-to-activator energy transfer (ET) has been observed to be less efficient than in II–VI semiconductor NCs. Here we investigate the evolution of the exciton-to-Mn2+ ET efficiency as a function of composition (Br/Cl ratio) and temperature in CsPbCl3–xBrx:Mn2+ NCs. The results show a strong dependence of the transfer efficiency on Br– content. An initial fast increase in the relative Mn2+ emission intensity with increasing Br– content is followed by a decrease for higher Br– contents. The results are explained by a reduced exciton decay rate and faster exciton-to-Mn2+ ET upon Br– substitution. Further addition of Br– and narrowing of the host bandgap make back-transfer from Mn2+ to the CsPbCl3–xBrx host possible and lead to a reduction in Mn2+ emission. Temperature-dependent measurements provide support for the role of back-transfer as the highest Mn2+-to-exciton emission intensity ratio is reached at higher Br– content at 4.2 K where thermally activated back-transfer is suppressed. With the present results it is possible to pinpoint the position of the Mn2+ excited state relative to the CsPbCl3–xBrx host band states and predict the temperature- and composition-dependent optical properties of Mn2+-doped halide perovskite NCs.

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 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 CsPbCl3 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 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.

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