Characterization of Semiconductor Nanocrystal Assemblies as Components of Optoelectronic Devices

Abstract

This dissertation presents new insight into the ability of small molecule passivated NCs to achieve intimate approach distances, despite being well passivated, while developing guiding principles in the area of ligand mediated microstructure control and the resulting macroscopic optical and electronic properties that close packing of high quality NCs enables. NC ligand coverage will be characterized quantitatively through thermogravimetric analysis (TGA), and qualitatively by photoluminescence and electroluminescence, in the case of functional devices; illustrating the importance of practitioner dependent control of ligand coverage through variations in the dispersion precipitation purification procedure. A unique examination of the relative contribution of energy and charge transfer in NC LEDs will demonstrate the ability to achieve charge transfer, at a level competitive with energy transfer, to well passivated NCs at various wt% loading in a polymer matrix. The observation of potential dependent recombination zones within an active layer further suggest novel, NC surface passivation mediated control of blend microstructure during solution processing towards the development of a bi-continuous network. Next, NC self-assembly and resulting microstructure dependent optical and electronic properties will be examined through electroluminescence and high-resolution transmission electron microscopy (TEM) micrographs of functional NC/polymer bulk heterojunction LEDs. The joint characterization of NC optical properties, and self-assembly microstructure provide a deeper understanding of the significant and inseparable effects of minimal changes in NC surface passivation on structure and function, and emphasize the potential to rely on strongly passivating ligands to control physical properties and processing parameters concurrently towards higher efficiency devices via low cost processing. Finally, micro-contact printing of blazed transmission gratings, using stable dispersions of core and core/shell NCs will be shown to produce close packed assemblies of NCs forming near-wavelength luminescent superstructures separated in space. We show the dominant contribution of a two-monolayer thick sharp interface CdS shell to the diffraction efficiency, and necessarily the refractive index, of the NCs, independent of core size. Utilization of these gratings as in-coupling elements at various positions within a device architecture are also examined. These new observations were achieved by unprecedented control of NC architecture during dispersion processing, while maintaining high luminescence, made possible by optimized NC surface passivation. These studies enable the formation of new LED architectures, and new optoelectronic devices based on angle resolved, monochromatic fluorescence from diffraction gratings prepared from simple solution processing approaches. Further, the novel observation of angle amplified interfering fluorescence from these features is argued to be a result of long range radiative coupling and superradiance enabled by the monodispersity and high-quality NC surface passivation described herein.