Spectroscopic Studies of Organic Photovoltaic Non-Fullerene Acceptors

Abstract

The aim of this thesis is to better understand the photophysics of a class of functional organic materials called ‘non-fullerene acceptors’ (NFAs). They have become well-established as a component in the bulk heterojunction active layer of organic photovoltaic (OPV) cells, and are credited with facilitating a rapid increase in the power conversion efficiencies of these devices in recent years. However, despite their clear importance to device performance, their detailed photophysics remains poorly understood. Herein, we perform both steady-state and transient spectroscopic studies in order to advance structure-photophysics relationships for the Y-family of next-generation NFAs. We propose theoretically that the primary photoexcitation in thin films of the archetypal NFA Y6 is a manifold of hybrid Frenkel-charge transfer (HFCT) dimer states. This manifold gives rise to the characteristic steady-state absorption band in the visible and near-infrared regions. Through an excitation-dependent transient absorption study, we experimentally confirm this hypothesis. In addition, we show that polaron pairs are generated upon photoexcitation in Y6. Both of these species are absent in a ‘control’ molecule, IEICO. Through dilution in an inert, solid-state host, we highlight the importance of both the dielectric environment, and the size of microcrystalline domains, in facilitating the HFCT interaction and polaron-pair generation in Y6. We confirm, with a Strickler-Berg analysis, the origin of the reported inverse temperature dependence of the integrated photoluminescence emission of Y6. It is due primarily to the reduction in radiative rate with redshifted emission, and not an increase in the population of a low-energy non-emissive state, upon cooling. A systematic study of other members of the Y-family, modifying the acceptor moieties in the core and at the periphery of the molecule, draws attention to their role in scaffolding the intermolecular interactions. In addition, we show that direct local morphology, as controlled through film fabrication conditions, also has an impact on the observed photophysics. The understanding gleaned from these investigations will prove invaluable to the further development of NFAs, and the continued improvement in OPV device performance.