Tuning Exciton–Mn(2+) Energy Transfer in Mixed Halide Perovskite Nanocrystals

[Image: see text] 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.