Kee, TakHuang, DavidStuart, Alexandra Nicole2024-09-242024-09-242024https://hdl.handle.net/2440/142555Singlet fission (SF) is a process in which two triplet excited states are generated from one singlet excited state. It has the potential to increase the efficiency of solar cells through reducing energy losses from the thermalisation of high-energy photons, but significant enhancements in solar cell performance using SF have yet to be realised. There are still many challenges associated with the implementation of SF in devices, including the accurate identification and quantification of SF, the photostability of materials that undergo SF, and loss pathways associated with SF intermediate species. This thesis presents a series of spectroscopic studies aimed at addressing these challenges. We focus in particular on polyacenes, a class of organic semiconductors commonly studied for SF in addition to a range of other applications. The first study in this thesis (Chapter 3) demonstrates the importance of distinguishing SF from intersystem crossing (ISC), another process that produces triplet excitons. We identified several discrepancies in ISC rate constants previously reported in the literature for two commonly studied polyacenes, 5,14-bis(triisopropylsilylethynyl) tetracene (TIPS-Tn) and 6,13- bis(triisopropylsilylethynyl) pentacene (TIPS-Pn). We resolved these discrepancies by using transient absorption spectroscopy to quantify an upper bound on the ISC rate constants. Our results demonstrate that SF can contribute to triplet formation even in ostensibly dilute solutions, which has previously been overlooked, presumably leading to the overestimation of ISC rate constants. We also illustrated how errors in ISC rate constants can propagate to errors in estimating SF rates and yields, highlighting the importance of distinguishing the two processes accurately. In Chapter 4, we investigated the role of oxygen on the photophysics and photodegradation of polyacenes. We again used transient absorption spectroscopy to study the photophysics of TIPS-Tn and TIPS-Pn in solution with and without oxygen. We also characterised the photodegradation of these polyacenes both in solution and in the solid state in the form of aqueous suspensions of nanoparticles (NPs). By modelling both the transient absorption and photodegradation data of the solution and the solid state simultaneously, we were able to determine the predominant mechanism of photodegradation in these polyacenes, which has previously been unclear. We found that both polyacenes sensitise and undergo photooxidation with singlet oxygen. Interestingly, our results show that TIPS-Pn predominantly reacts with singlet oxygen from the singlet excited state, whilst TIPS-Tn from the triplet excited state. This results in SF suppressing photodegradation in TIPS-Pn, but enhancing it in TIPS-Tn, which raises a new challenge for the implementation of this material in SF devices. This chapter also demonstrates the various different ways oxygen can influence the photophysics in these materials, providing important context for future studies. Whilst the results of this study were primarily interpreted in the context of SF and solar cells, the implications are significant for the study and application of polyacenes in any real-world environment. Finally, in Chapter 5, we used the knowledge developed in the previous chapters to address a major loss pathway in SF, namely the inability to effectively separate the resultant triplet excitons. We investigated the efficacy of an energy gradient in a disordered solid-state system in the form of aqueous suspensions of polyacene NPs. This involved blending TIPS-Tn, a higher triplet-energy molecule, with TIPS-Pn, a lower triplet-energy molecule, to promote the separation of triplets generated via SF on TIPS-Tn to TIPS-Pn. We took a novel approach of using the photodegradation of the blend NPs to characterise the photophysics of the system, in particular probing the extent of triplet energy transfer from TIPS-Tn to TIPS-Pn. We found that the photodegradation of TIPS-Tn was suppressed when TIPS-Pn was added, indicating exciton transfer from TIPS-Tn to TIPS-Pn. Modelling the photodegradation showed that some triplet energy transfer did occur from TIPS-Tn to TIPS-Pn, but singlet energy transfer was also prevalent, which acted to suppress SF rather than assist it. This study demonstrates that specific and precise molecular arrangements, or otherwise chromophores blends with different energetics that are designed to avoid singlet energy transfer, will be necessary for an energy gradient to be beneficial to SF.enpolyacenessinglet fissionphotodegradationorganic electronicsspectroscopyorganic semiconductorsexcitonsphotovoltaicsThesis