Thermodynamic Control of Excitation Transport Regimes in Nanocrystal Solids

Abstract Controlling excitation transport is central to the operation of light-harvesting and light-emitting devices based on nanocrystal films. Exciton diffusion is typically described as hopping through a homogeneous medium; however, even highly monodisperse nanocrystal ensembles exhibit energetic disorder arising from size variation. We demonstrate that such disorder couples exciton diffusivity in real space to energetic relaxation within the density of states. Using spectrally and temporally resolved photoluminescence reabsorption spectroscopy of CsPbBr3 nanocrystal films, we separately track exciton diffusion and energetic relaxation across a range of temperatures and nanocrystal sizes. We find that at low temperature, excitons preferentially migrate downhill toward larger, lower-energy nanocrystals, causing a transient collapse of exciton diffusivity that is particularly pronounced for smaller nanocrystals exhibiting higher energetic disorder. At room temperature, these effects are effectively countered by thermally activated uphill transfer, restoring normal diffusion. Nanocrystal solids thus offer tunable transport properties governed by the interplay between disorder and thermal energy.