Exciton Diffusion in 3D Halide Perovskite Nanocrystal Assemblies

Halide perovskite nanocrystals (PNC) have attracted the scientific community for over a decade, but there are still open questions addressing research not only for academic purposes. Among these are PNC stability, maximizing quantum yields, and improving transport performance in thin films. Especially excited-state transport is of tremendous importance for light emission and light-harvesting devices. Surrounded by organic ligands, charge transport is nearly nonexistent in PNC films, rendering energy transfer of excitons the dominant transport mechanism.<br/>We investigate exciton diffusion in films of PNCs cubes and quantum-confined nanoplatelets around room temperature (15-50 °C). Here, diffusion takes advantage of the individual NC size, yielding threefold diffusivity for the largest NCs compared to the smallest ones. Surprisingly, the diffusion process worsens with increasing temperature (up to 50 °C) for all NCs systems. This rate at which this happens differs among the NCs; consequently, the 3 ML NPLs outperform the other NC systems for high temperatures. These two phenomena can be explained by different exciton/free charge carrier ratios present in the NCs. Larger free electron/hole pair ratios decrease the transport efficiency with charge transport irrelevant.<br/>Furthermore, we present unprecedented exciton diffusion results in perovskite nanocrystal films. We investigate an unordered film consisting of 14 nm cubes and highly ordered superlattices comprising either 5 nm or 9 nm large cubes within the range of 9-220 K. Surprisingly, exciton diffusion is higher in the larger PNCs film, irrespective of order for almost all temperatures. Within this temperature range, initially, the diffusivity increases with increasing temperature, reaches a maximum, and drops with increasing temperature for both film types.² We detect three effects whose interplay is responsible for this novel trend. 1) Thermally activated FRET hopping, 2) size-dependent exciton fine level splitting between dark and bright states, and 3) exciton/free charge carrier ratios varying with exciton binding energy (PNC size). This study offers deeper insights into exciton diffusion processes, especially at low temperatures.