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 CsPbCl 3 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-Mn 2+ ET efficiency as a function of composition (Br/Cl ratio) and temperature in CsPbCl 3- x Br x :Mn 2+ NCs. The results show a strong dependence of the transfer efficiency on Br - content. An initial fast increase in the relative Mn 2+ 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-Mn 2+ ET upon Br - substitution. Further addition of Br - and narrowing of the host bandgap make back-transfer from Mn 2+ to the CsPbCl 3- x Br x host possible and lead to a reduction in Mn 2+ emission. Temperature-dependent measurements provide support for the role of back-transfer as the highest Mn 2+ -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 Mn 2+ excited state relative to the CsPbCl 3- x Br x host band states and predict the temperature- and composition-dependent optical properties of Mn 2+ -doped halide perovskite NCs.