Non-Radiative Energy Transfer for Photovoltaic Solar Energy Conversion: Lead Sulfide Quantum Dots on Silicon Nanopillars

This dissertation comprises a study aimed at understanding the competing dynamics of energy and charge transfer in quantum dot (QD) solids and from QDs to crystalline semiconductor substrates to assess a new type of hybrid solar cell that is based on non-radiative resonant energy transfer (NRET) from light absorbers such as nanocrystal QDs to high mobility charge carrier transport channels such as silicon nanopillars.;As a platform to investigate a NRET solar cell we employed lead sulfide nanocrystal QDs as light absorbers and silicon as the acceptor transport channel for the NRET generated electrons and holes. Given NRET as the basic physical process at the core of the new type of solar cell the dissertation focused on examining: (1) synthesis of and surface ligand exchange for high quantum efficiency lead sulfide quantum dots, (2) studies of inter-QD NRET and competing inter-QD charge transfer as a function of inter-QD average separation and temperature, (3) structural and optical characteristics of lead sulfide quantum dots adsorbed on crystalline silicon surfaces, and (4) fabrication and examination of prototype colloidal PbS QD - silicon nanopillar array solar cell.;The work in these four areas has each provided insights into and new results for the field of quantum dots, QD-based solids, and QD based opto-electronic devices that are of generic value. The need for maintaining the high quantum efficiency (QE) of the as-synthesized PbS QDs while exchanging the surface ligands with new ones better suited for the device lead us to introduce a new approach to ligand exchange that employs pre-conjugated lead cation -- ligand complexes as units that replace the lead cations bound to their as-grown ligand, thus maintaining the Pb-rich stoichiometry that suppresses defect formation while gaining the ability to control the length of the ligands.;The ability to control the length of the ligands allowed control over the QD-QD separation in densely packed films referred to as QD-solids. These QD solids of controlled and experimentally determined average inter-QD separation enabled the first systematic study of exciton decay dynamics involving competition between separation-dependent QD to QD NRET and QD to QD charge transfer as a function of temperature and quantum dot size. Our principal findings are : (1) the NRET rate from smaller to larger QDs increases with decreasing QD-QD average separation as the inverse sixth power, as expected; (2) reduction in temperature enhances the inter-QD NRET rate (3) exciton decay in the largest QDs is dominated by thermally activated tunneling of charge and (4) a consistent understanding of both inter QD energy and charge transfer is obtained by accounting for the ligand-length dependent effective medium nature of QD solid dielectric constant and postulating the presence of gap states.;As transfer of energy from the QDs adjacent to the acceptor channels is an integral step in the NRET-based photovoltaic solar cell paradigm we under took examination of the structural nature of the PbS QD- crystalline Si interface as formed upon deposition of PbS QDs on Si. To directly image the QD-substrate interface in a transmission electron microscope (TEM) we introduced a new approach to TEM specimen preparation that enabled the first simultaneous high resolution imaging of a silicon nanopillar with thickness less than 100 nm and PbS QDs adsorbed on it, and thus the interfacial region as well. The separation between the crystalline core of the PbS QD and the crystalline silicon was found to be substantially less than the length of the ligands that coat the PbS QD.;The dissertation concludes with a report on progress towards fabricating and characterizing PbS QD - silicon nanowall solar cell devices. Prototype devices were made by etching high aspect ratio and high density of trenches in silicon having an existing p-n junction. The walls of silicon between the trenches thus act as the charge transport channels for the electrons and holes generated by the NRET from QD absorbers in the trenches. (Abstract shortened by UMI.).