Mn²⁺ Mediated Spin‐Exchange Enables CsPbBr₃ Nanoplatelets to Function as Highly Efficient Triplet Sensitizers for Triplet‐Triplet Annihilation Upconversion

AbstractPerovskite nanoplatelets (NPls) have emerged as a new generation of nanomaterials that are promising for optoelectronic applications due to their large absorption and fast yet efficient emission. Particularly, they are also potential triplet sensitizers for triplet‐triplet annihilation upconversion (TTA‐UC), provided that the fast intrinsic decay of exciton can be circumvented. Herein, doping the CsPbBr3 NPls with Mn2+ (Mn2+: CsPbBr3 NPls) can efficiently convert fast‐decay triplet exciton to a long‐lived excited state of Mn2+ through Dexter‐type spin‐allowed energy transfer, enabling simultaneous large absorption and long‐lived excited state are demonstrated. Additionally, similar to the exciton‐to‐Mn2+ energy transfer, Mn2+‐to‐triplet state of annihilator energy transfer is also spin‐allowed and thus highly efficient. These pave the way for using Mn2+: CsPbBr3 NPls as triplet sensitizers. The superior triplet sensitization ability of Mn2+: CsPbBr3 NPls is demonstrated by using 9,10‐diphenylanthracene molecules (DPA) as representative annihilators. 93% quenching of Mn2+ emission and considerably accelerated decay of Mn2+ are observed after the addition of a trace amount of DPA. TTA‐UC measurement demonstrates that benefitted from the long‐lived excited state of Mn2+, the normalized upconversion photoluminescence quantum yield is ≈10% with an energy threshold being as low as ≈0.35 W cm−2. The results shed new light on using Mn2+: CsPbBr3 NPls as a triplet sensitizer.