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 CsPbCl_3- Br :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 CsPbCl_3- Br 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 CsPbCl_3- Br host band states and predict the temperature- and composition-dependent optical properties of Mn²⁺-doped halide perovskite NCs.