Tuning Exciton-Mn2+ Energy Transfer in Mixed Halide Perovskite Nanocrystals

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 BC content. An initial fast increase in the relative Mn2+ emission intensity with increasing BC 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.