The Investigation of Ultrafast Charge Dynamics within Conjugated Organic Ladder Semiconducting Materials for Optoelectronic Applications

Abstract

This dissertation is a collection of research investigations that focus on understanding the ultrafast charge dynamics of organic semiconducting materials designed for optoelectronic applications. Optoelectronic applications are devices that utilize energy or light to generate charges that will create electricity, transport charges, or emit light. Examples of optoelectronic applications are photovoltaics for solar energy, transistors, or light emitting diodes. The main objective of optoelectronic applications is developing devices that are highly efficient, effective and low-cost to manufacture. One approach to developing low-cost devices is to use materials that are relatively cost efficient and have the ability of utilizing energy, such as organic semiconducting materials. In order for devices of optoelectronic application to work, the organic semiconducting materials used to create these devices must have the ability to generate or transport charges throughout the devices. Ultrafast charge dynamics are the motion or driving force behind the ability of materials to generate and transport charges. Depending on the structure of the material used in these devices, the ultrafast dynamics will be different and affected by the structure of the organic semiconducting materials used in these devices.

My PhD work focused on investigating ultrafast charge dynamics of organic conjugated ladder semiconducting materials for optoelectronic applications. Based on the organic semiconducting materials studied, the types of ultrafast charge dynamics that I investigated concerning these organic semiconducting materials were: electron-phonon interaction; reorganization relaxation; exciton; intramolecular charge transfer; fluorescence; and radial formation. In doing so, I utilized femtosecond transient absorption (fs-TA) technology, a pump-probe laser experiment that provides information on excited state dynamics on the molecules.

Chapter III address the one of my PhD research investigation that focuses on investigating the ultrafast dynamics of organic ladder oligomers used in transistor applications. The results indicated that the longer conjugated length oligomer (seven fused rings) had slower decay times of the ultrafast dynamics due to the decrease in electron-phonon (electron-vibrational) interaction. As a result, the charge transport mechanism of the longer oligomer was therefore more efficient than the shorter oligomer, a key discovery.

Chapter IV address another one of my PhD research investigation that focuses the exciton dynamics and the intramolecular charge generation mechanism of acceptor-donor-acceptor small molecules that are designed for solar cell applications. These molecules varied in the type of donor unit (fused-ring or non-fused ring) used within the molecular structure design. In addition, these molecules varied in having a π bridge to connect the donor and acceptor units within the structure of these molecules. The results from this investigation provide insight into which molecular structure of an acceptor-donor-acceptor small molecule will best lead to the discovery of an efficient solar cell device. Chapters V and VI of this dissertation discusses the research investigations where I collaborated with my colleagues on other research investigations concerning the ultrafast charge dynamics of other organic polymeric semiconducting materials designed for optoelectronic applications.

The main objective of all the research investigations discussed in this dissertation is to increase the knowledge of ultrafast charge dynamics of organic semiconducting materials and to understand the structure-functional relationship between charge dynamics and these materials. Ideally, the information in this dissertation will lead to finding the optimal structural design of organic semiconducting materials that will lead to highly efficient and effective devices for optoelectronic applications.