Shining Light on Light-Matter States: Fundamental Studies of Electronic Strong Coupling in Organic Materials

Abstract

Electronic strong coupling presents an enticing new method to control the function and reactivity of a molecule by introducing states at new energy levels in a system. The new states are hybrid light-matter states called polaritons which have a variety of unique properties unavailable with purely molecular states. In this thesis, I present spectroscopic studies of polaritons aimed at understanding and ultimately engineering polariton properties for the improvement of the efficiency of organic optoelectronic devices. In Chapter 1, I provide an overview of polariton dynamics across the timescales of tens of femtoseconds through to the observation of steady-state applications. In Chapter 2, I provide a detailed study of the transient spectral features and dynamics of a cavity system in the ultrastrong coupling regime ultimately demonstrating that the unique properties of polaritons, including the property of exceptional delocalization, influence the rate and likelihood of transitions between a polariton and a non-polariton state. Chapter 3 explores the advantages of the delocalized polariton states in the transport of excitations to the donor-acceptor interface in the manufacture of an efficient bilayer organic solar cell. In Chapter 4, I present results investigating the role of the properties of the molecule, namely the molecular orientation and packing in a thin film, in determining the spectroscopic properties of the polaritons. Chapter 5 presents an in-depth spectroscopic study of the molecule from Chapter 4 and relates the packing and molecular orientation to the performance of the molecule as an acceptor in an organic photovoltaic cell. In Chapter 6, I provide experimental details regarding the fabrication and spectroscopic characterization of the Fabry-Perot cavity systems presented in this thesis. Chapter 7 concludes this thesis with a comment on the state of electronic strong coupling, namely that a deeper understanding of the interplay between the properties of the molecule and the properties of the polaritons will provide the necessary platform to begin to engineer polaritons for applications in chemistry and materials science.