  Despite showing great promise for optoelectronics, the commercialization of
halide perovskite nanostructure-based devices is hampered by inefficient
electrical excitation and strong exciton binding energies. While transport of
excitons in an energy-tailored system via F\"orster resonance energy transfer
(FRET) could be an efficient alternative, halide ion migration makes the
realization of cascaded structures difficult. Here, we show how these could be
obtained by exploiting the pronounced quantum confinement effect in two
dimensional CsPbBr3 based nanoplatelets (NPls). In thin films of NPls of two
predetermined thicknesses, we observe an enhanced acceptor photoluminescence
(PL) emission and a decreased donor PL lifetime. This indicates a FRET-mediated
process, benefitted by the structural parameters of the NPls. We determine
corresponding transfer rates up to k_FRET=0.99 ns^-1 and efficiencies of nearly
\eta_FRET=70%. We also show FRET to occur between perovskite NPls of other
thicknesses. Consequently, this strategy could lead to tailored, energy cascade
nanostructures for improved optoelectronic devices.
