Capture and control of excitations

Abstract

Fundamental understanding of the capture and control of excitations, including photons and excitons, in optoelectronic devices is important to optimizing their performance. Devices such as luminescent solar concentrators and signal concentrators that increase the efficiency of solar energy production and the speed of point-to-point communication, respectively, will be crucial for maximizing the sustainability and connectedness of the world going forward. Behind the workings of these devices are micro-scale interactions of excitations with the device materials that must be carefully modeled and well understood. In this thesis, I model the performance of both luminescent solar concentrators and signal concentrators using the Monte Carlo method to predict the efficiency from the average results of many trials of quantum behavior. For each of these devices, I propose a path to improved performance. For luminescent solar concentrators, this is the use of tandem fluorophores. In this approach, the addition of a second fluorophore material increases the amount of sunlight that can be absorbed without interfering with the efficiency at which the first fluorophore collects solar photons. For signal concentrators, this is a mutli-aggregate fluorophore with <100 ps fluorescence lifetime that does not re-absorb its own emission because of the introduction of an artificial Stokes' shift. In addition, in this thesis I model the photophysical properties of the C8S3 J-aggregate to understand two of its properties: the long exciton migration it exhibits, and its ability to be irreversibly photobrightened and reversibly photodarkened under continuous illumination. I show the exciton migration distance is due to strong nearest-neighbor coupling along a helical direction that is aligned close to the axis of the aggregate tube, while the photobrightening and photodarkening behaviors are due to changes in two different types of disorder.