Dark-Bright Exciton Splitting Dominates Low-Temperature Diffusion in Halide Perovskite Nanocrystal Assemblies

Abstract

Semiconductor nanocrystals could replace conventional bulk materials completely in displays and light-emitting diodes. However, the organic ligands enabling their unique optical properties, prevent current flow in nanocrystal films, leaving energy transfer as the only means of injecting or extracting carriers. Here, we investigate exciton diffusion in halide perovskite superlattices - nearly perfect 3D nanocrystal assemblies. This high degree of order is not as crucial as the individual nanocrystal size, which affects transport differently depending on temperature. Up to 70 K, a confinement-induced splitting of excitonic energies, especially for the smallest nanocrystals, traps excitons into dark levels, suppressing diffusion. At intermediate temperatures, the distance of individual FRET steps corresponding to nanocrystal size enhances diffusion in the larger nanocrystals. This trend is reversed, as exciton dissociation and carrier trapping in less strongly confined nanocrystals become dominant up to room temperature. Our results reveal that transport must be factored strongly into nanocrystal design strategies for future optoelectronic applications.

Size‐ and Temperature‐Dependent Lattice Anisotropy and Structural Distortion in CsPbBr3 Quantum Dots by Reciprocal Space X‐ray Total Scattering Analysis

“…[ 60 ] This is a hot topic in the field of LHP QDs, as demonstrated by the flourishing number of reports on low‐T spectroscopic experiments and related fundamental works. [ 38,41,97–101 ]…”

“…[60] This is a hot topic in the field of LHP QDs, as demonstrated by the flourishing number of reports on low-T spectroscopic experiments and related fundamental works. [38,41,[97][98][99][100][101] Indeed, the NC response to cooling was shown to increase the lattice anisotropy of CsPbI 3 NCs. To check whether such a hypothesis is valid also for CsPbBr 3 NCs, we further performed variable-T measurements, from 300 K down to 6 K, on dried NCs for the 5 nm and 9 nm didodecyldimethyl ammonium bromide (DDAB)-capped samples (see Experimental Section).…”