Silicon quantum dots mesomaterial for improved optical properties and charge dynamics in third generation photovoltaic composites

Abstract

Designing innovative architectures for silicon quantum dot (SiQD) mesomaterials is the key to achieving their promise for implementation in the third generation solar cells. To this end, we have developed a computational methodology that predicts the optical properties and charge dynamics for these hybrid nanoscale structures. Specifically, a way of computationally estimating the effective energy level alignment at heterojunctions was derived to characterize junction types and interfacial charge separation character. Next, a full quantum treatment was developed to calculate transition rates within the setting of incoherent, phonon-assisted carrier hopping. This model was applied to consider transport within the organic/inorganic SiQD/P3HT system, and it was found to accurately capture the charge transfer and photoluminescence rates. In particular, with dangling bonds accounted for, the method recovers the charge recombination and electron hopping rates observed experimentally. The analysis pinpointed the source of the poor photo-conversion efficiency as being due to dangling bond defects. With the appropriate computational tools in hand, we then considered new nanostructure architectures intended to overcome the drawbacks of those already proposed. Three general strategies for designing future photovoltaic devices were developed. The first is based on the concept of short bridge networks, a new type-II quantum dot mesomaterial in which the dots are connected by covalently bonded, short-bridge molecules. There we found that both polaron dissociation and subsequent carrier hopping are significantly enhanced by a double superexchange mechanism in which the electronic coupling of both carrier types is aided by the presence of mediating, non-resonant material. The second strategy that we developed is associated with the harvesting of low energy photons via direct generation of spatially separated excitons. To achieve this goal, we found that two criteria need to be satisfied between quantum dots and their functionalizing termination group: they have to have a type-II energy level alignment; and they need to be joined via a conjugating vinyl bond. In principle, the absorption edge can be made arbitrarily small and is no longer dependent on quantum dot size. We were able to create a proof-of-concept assembly demonstrating this concept. The third design strategy developed in this dissertation research was to optimize dot size to achieve high stability in ambient environments. Specifically, we predict that appropriately terminated silicon quantum dots with diameters in the range of 1.2-2 nm will be extremely resistant to oxidation. This is because of the absence of both surface defects and geometry-related vulnerabilities allowing even very short passivating ligands to generate a large barrier to oxidation processes.