  Halide perovskites have emerged as disruptive semiconductors for applications
including photovoltaics and light emitting devices, with modular optoelectronic
properties realisable through composition and dimensionality tuning. Layered
Ruddlesden-Popper perovskites of the form BA2MAn-1PbnI3n+1, where n is the
number of lead-halide and methylammonium (MA) sheets spaced by longer
butylammonium (BA) cations, are particularly interesting due to their unique
two-dimensional character and charge carrier dynamics dominated by strongly
bound excitons. However, long-range energy transport through exciton diffusion
in these materials is not understood or realised. Here, we employ local
time-resolved luminescence mapping techniques to visualise exciton transport in
high-quality exfoliated flakes of the BA2MAn-1PbnI3n+1 perovskite family. We
uncover two distinct transport regimes, depending on the temperature range
studied. At temperatures above 100 K, diffusion is mediated by thermally
activated hopping processes between localised states. At lower temperatures, a
non-uniform energetic landscape emerges in which exciton transport is dominated
by energy funnelling processes to lower energy states, leading to long range
transport over hundreds of nanometres even in the absence of exciton-phonon
coupling and in the presence of local optoelectronic heterogeneity. Efficient,
long-range and switchable excitonic funnelling offers exciting possibilities of
controlled directional long-range transport in these 2D materials for new
device applications.
