  F\"orster Resonant Energy Transfer (FRET)-mediated exciton diffusion through
artificial nanoscale building block assemblies could be used as a new
optoelectronic design element to transport energy. However, so far nanocrystal
(NC) systems supported only diffusion length of 30 nm, which are too small to
be useful in devices. Here, we demonstrate a FRET-mediated exciton diffusion
length of 200 nm with 0.5 cm2/s diffusivity through an ordered, two-dimensional
assembly of cesium lead bromide perovskite nanocrystals (PNC). Exciton
diffusion was directly measured via steady-state and time-resolved
photoluminescence (PL) microscopy, with physical modeling providing deeper
insight into the transport process. This exceptionally efficient exciton
transport is facilitated by PNCs high PL quantum yield, large absorption
cross-section, and high polarizability, together with minimal energetic and
geometric disorder of the assembly. This FRET-mediated exciton diffusion length
matches perovskites optical absorption depth, opening the possibility to design
new optoelectronic device architectures with improved performances, and
providing insight into the high conversion efficiencies of PNC-based
optoelectronic devices.
