EXPLORING EXCITED STATE DYNAMICS AND SINGLET FISSION IN ORGANIC SEMICONDUCTORS

Abstract

The ongoing shift towards sustainable energy solutions has increased focus on solar technologies, with organic photovoltaics (OPVs) and organic semiconductors emerging as vital innovations for future solar energy generation. Organic photovoltaics (OPVs) and organic semiconductors present a compelling opportunity to produce solar energy at lower costs, with greater flexibility, and through favourable manufacturing processes. This thesis focuses on the dynamics of excited states within organic semiconductors, emphasizing singlet fission (SF), a mechanism that can break the Shockley-Queisser (SQ) efficiency limit of 32% for single-junction solar cells. By addressing the core limitations of organic materials and examining the interactions in organic-polymeric and small molecule systems, this study contributes to advancing OPV technologies. Chapter 1 introduces the background of solar energy, discussing the constraints of conventional photovoltaic devices and the Shockley-Queisser (SQ) limit, which caps single-junction solar cell efficiency at 32%. Achieving efficiency beyond this limit is challenging due to energy losses that occur when absorbed photons either remain unconverted or are lost as heat. SF has emerged as a promising mechanism to mitigate these losses, as it generates two excitons from a single high-energy photon, allowing OPVs to utilize high-energy photons more effectively. Organic semiconductors, particularly those based on π-conjugated structures with tunable electronic properties, are ideal candidates for SF applications due to their flexibility, low-cost processing, and customizable band gaps. However, despite these advantages, organic photovoltaics currently underperform compared to their inorganic counterparts. Chapter 1 introduces the structure, behavior, and potential of organic semiconductors and sets up the study’s focus on materials and mechanisms that could push OPVs toward higher efficiencies. To address this, SF materials capable of producing two excitons from one photon have shown potential to enhance the power conversion efficiency (PCE) of OPVs.