DOI: https://doi.org/10.29363/nanoge.emlem.2024.018
Publication date: 13th July 2024
Colloidal quantum dots (QDs) have attracted great interest for fundamental studies of exciton and charge dynamics in semiconductor nanostructures, as well as for their applications in various devices. Consequently, extensive studies have provided insights into the exciton and charge carrier dynamics of colloidal QDs. However, exciton and charge carrier transport in QD solids present a different dynamics, as it is dictated by the packing structure of particles and the concomitant coupling between dots. Additionally, the desired exciton transport dynamics of QD solids differs between applications; for example, a photovoltaic cell requires fast exciton transport to the charge-separating interfaces, whereas in a light-emitting diode, this can lead to undesired quenching of luminescence. Therefore, comprehensive understanding of exciton transport physics in QD solids is needed, with the eventual aim of controlling these transport properties in order to optimize device performance.
Despite its importance, true charge carrier transport in QD solids has hardly been reported due to limitations in time- and space-resolved techniques. Here we directly probe the initial exciton dynamics in QD solids at femtosecond (fs) timescales following photo-generation, using a novel integrated time- and space-resolved technique called transient absorption microscopy (TAM). TAM offers dual capabilities: femtosecond time resolution and nanometer-scale spatial resolution. Surprisingly, we find that when the material has a Bohr radius much larger than the QD size, excitons first undergo very fast transport (diffusivity of ~102 cm2 s−1) within ~300 fs after photoexcitation and then switch into a much slower transport regime (~10−1–1 cm2 s−1). Intriguingly, reducing the interdot distance in the QD solids only enhances transport in the slower regime, while it unexpectedly diminishes the initial fast regime. Both QD packing density and heterogeneity have great impacts on these transport regimes and the transition between them. These findings suggest routes to control the optoelectronic properties of QD solids.
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