Investigating the Photophysics of New and Stable Singlet Fission Materials

Abstract

In this thesis, we describe our investigations into the mechanism of singlet exciton fission, examining the effects of morphology, aromaticity, and photoluminescence yield on the fission process, and developing generalised principles to aid the design of new materials. Singlet fission is the process of forming two triplet excitations from one photon and is apparent in only a handful of organic molecules due to the requirement for the lowest excited triplet state to have approximately half the energy of the lowest excited singlet state. Through this process, there is potential to improve the efficiency of solar cells by reducing losses to thermalisation. Consequently, new singlet fission materials, particularly those with better photostability, are of great interest. We have drawn our conclusions by studying three novel singlet fission systems: TIPS-tetrabenzo[a,c,l,n]pentacene (TTBP), a family of indolonaphthyridine derivatives (INDTs), and a series of tetracene derivatives with different solubilising groups. In each system, we apply ultrafast optical techniques in combination with optically pumped magnetic resonance experiments to characterise the physics of photoexcited states in our materials. These characterisations then allows us to deduce structure-function relationships and further our mechanistic understanding of the fission process. The first two systems expand the narrow library of known singlet fission materials and introduce two new pathways to more photostable fission materials: via benzannulation of acenes in TTBP or via excited state aromaticity in INDTs. Our characterisation of TTBP leads to a new design principle for reducing non-radiative voltage losses by maximising photoluminescence yield. The final study on tetracene derivatives demonstrates that differences in intermolecular packing induced by small changes to molecular structure have a drastic effect on singlet fission. Our findings indicates that a delicate balance must be struck whereby orbital overlap and inter-triplet coupling are neither too weak nor too strong.