Singlet Fission - The Effects of Solvent Polarity and Conformation

Licentiate thesis, 2022

Singlet fission (SF) is a process in which one initial high energy singlet excited state is converted into two triplet excited states of roughly half the energy of the initial state. This process has the potential to increase the efficiency of conventional solar cells by modifying the energy of the incident solar radiation to better match the energy input required to generate electricity. However, the SF technology is still in an early stage of its development and a clear picture of the mechanism has not yet emerged. The aim of this thesis is to shed some light on both the mechanistic aspect and in addition investigate the integration of SF materials with semiconductors in a model system.

In the work presented herein we have demonstrated that the relative orientation of the molecules involved in the SF process governs both the rate of formation and decay of the formed triplet states in an intramolecular SF system. Transient absorption studies have revealed that it is possible to selectively excite different conformations and observe orders of magnitude different SF rates for the same molecule by changing the excitation wavelength. Furthermore, conformational changes in the excited state have been utilized to increase the lifetime of the triplet pair which could be of importance in future device implementation. Additionally, we have investigated an intermolecular SF system attached to the surface of mesoporous semiconductors. Here, we found that the surrounding solvent polarity plays a crucial part in deciding what photophysical process is favored on the surface. The study suggests that highly polar solvents are detrimental to SF and triplet injection efficiency for this system due to stabilization of charge separated states.