Polarisation Effects of Exciton Migration and Singlet Fission in TIPS-Pentacene Nanoparticles

Abstract

Converting solar energy into electricity using photovoltaic (PV) cells is a renewable and environmentally friendly way to meet the world’s energy requirements. One of the major shortcomings of conventional solar cells is their 34% theoretical efficiency limit. Singlet fission (SF) may be exploited to increase this limit to 46%. SF can split the energy of a high-energy photon by producing two triplet excited states (excitons) from one singlet exciton. This process enables the excess energy above the band gap of the PV material to be harvested rather than being lost through thermal relaxation. However, many aspects of the SF process are still heavily debated and further research is required to exploit its full potential. Molecular arrangement, or morphology, affects the electronic coupling between molecules and is known to be critical in obtaining a high SF yield. Morphology also influences exciton migration, which is important because singlet migration can limit SF, and triplet migration to a donor-acceptor interface is essential for harvesting triplets. Therefore, the morphology of a SF layer must be optimised for favourable coupling between chromophores to allow for complete SF as well as efficient exciton migration. Previous research has focused on exploring the effect of average interchromophore separation on the SF rate and yield by studying amorphous 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-Pn) nanoparticles (NPs). By embedding TIPS-Pn in an amorphous polymer matrix, the mass ratio of TIPS-Pn to the host polymer can be varied to change the average intermolecular TIPS-Pn separation. Here, we present ultrafast time-resolved fluorescence and transient absorption (TA) polarisation anisotropy to investigate exciton migration in this system. We also developed a Monte Carlo (MC) simulation to offer insight into singlet migration and SF in these NPs. Our analyses show that diffusion-limited SF acts to increase (or suppress the decay of) the fluorescence anisotropy in an amorphous system. Furthermore, we find that with the SF model employed in our MC simulation, the experimental fluorescence and anisotropy data can only be reproduced by assuming that the TIPS-Pn molecules form amorphous clusters within the NP systems. The data presented in this thesis highlight the applications of time-resolved polarisation anisotropy which, along with a MC simulation, provides a means to model and analyse the dependence of morphology on SF and exciton migration in amorphous systems.