Exciton Dynamics in Multi-Chromophore Materials

Increased access to renewable energy is vital for continued human existence and development while minimising the impact of climate change. Over the past several decades photovoltaic (PV) technology has evolved to produce a variety of functional organic and inorganic PVs. Single junction silicon PV devices have come to dominate solar energy conversion due to their, efficiency, relative simplicity, and affordability. However, the efficiency of silicon PV is approaching the Shockley-Quiesser limit of efficiency for single junction PV devices. To exceed this limit and increase the portion of energy generated, new device architectures are required that can exceed the aforementioned limit. Singlet Fission (SF) is a spin-allowed process of exciton fission that transforms one spin-singlet excited state into two spin-triplet excited states localised on separate molecules. SF can potentially be harnessed to increase the efficiency of silicon photovoltaics by converting one absorbed high energy photon into two smaller energy excitons. The transformation of exciton energy provides a promising pathway for reducing losses via thermalisation which account for a large intrinsic loss mechanism of single junction PV. Chapter 2 of this thesis investigates the mechanisms of SF in solution to understand the role of intermediate states such that SF efficiency can be maximised in PV applications. Solutions were studied to better control concentrations and slow down reaction time scales compared to the solid state. This chapter specifically challenges the idea of red-shifted excimer intermediates in SF that facilitate high triplet yields. Solubilised acenes are probed with multiple spectroscopic techniques including transient absorption (TA) and time and spectrally resolved photoluminescence measurements (TSRPL). A kinetic model of SF in solubilised tetracene is constructed from data obtained from TSRPL and is used to show that triplets form directly from singlets and vice versa. Rigorous analysis of TA data of solubilised tetracene, ...