  Hot electrons play a crucial role in enhancing the efficiency of
photon-to-current conversion or photocatalytic reactions. In semiconductor
nanocrystals, energetic hot electrons capable of photoemission can be generated
via the upconversion process involving the dopant-originated intermediate
state, currently known only in Mn-doped cadmium chalcogenide quantum dots.
Here, we report that Mn-doped CsPbBr3 nanocrystals are an excellent platform
for generating hot electrons via upconversion that can benefit from various
desirable exciton properties and the structural diversity of metal halide
perovskites (MHP). 2-dimensional Mn-doped CsPbBr$_3$ nanoplatelets are
particularly advantageous for hot electron upconversion due to the strong
exciton-dopant interaction mediating the upconversion process. Furthermore,
nanoplatelets reveal evidence for the hot electron upconversion via long-lived
dark exciton in addition to bright exciton that may enhance the upconversion
efficiency. This study not only establishes the feasibility of hot electron
upconversion in MHP host but also demonstrates the potential merits of
2-dimensional MHP nanocrystals in hot electron upconversion.
