Multiple Exciton Generation in Small Optical Gap Silicon Quantum Dots Allotropes
Mark T. Lusk
A promising long-term approach for converting solar photons to either electrical or chemical free energy is to utilize the unique properties of semiconductor quantum dots and rods to modify the relaxation pathways of their excited states. Manipulation of the relative relaxation rates can produce greatly enhanced power conversion efficiency by generating multiple electron-hole pairs (excitons) from single photons, known as Multiple Exciton Generation (MEG). It is particularly attractive to consider silicon (Si) nanostructures for MEG since Si is the basis of current integrated circuit technology and more than 90% of all photovoltaic cell production. However, the optical gap of the common, diamond Si-I phase is 1.12 eV, and Si NCs have gaps that are even higher. Since MEG requires photons that are at least double a material's optical gap, MEG with Si-I NCs can only occur for photons that are not well represented in the solar spectrum. The proposed investigation therefore seeks to design, synthesize and test Si NCs with nonconventional crystal structures that carry out MEG with dramatically improved efficacy. A computationally led investigation will carry out closed loops of design, synthesis and testing of Si-III and Si-XII NCs, and other promising theoretically predicted structures will also be considered as well. In NC form, these allotropes would have optical gaps in the ideal range for MEG. However, such high-pressure NCs have not been stabilized under ambient conditions, even though they are stable as bulk materials, and their MEG properties have never been studied. NCs, encapsulated within an amorphous shell or matrix, will be computationally studied to identify structures capable of efficient MEG and exciton transport. Simulations will also be used to find loading paths that cause more common Si NCs to be converted to these structures.
PARSEC, SIESTA, NWChem, RGWBS and the NANOPSE suite.