A kinetic and spectroscopic analysis of distance-dependent singlet fission in TIPS-pentacene

Abstract

Singlet exciton fission (SF) is a process with the potential to extend the maximum theoretical efficiency of solar cells from 34% to 46%. By generating two triplet excitons from one singlet exciton, the process effectively splits the energy of a high-energy photon in two, reducing efficiency loss by thermal relaxation. While the process has a strong theoretical grounding, the mechanistic details of SF and practicalities of implementation in photovoltaic devices are insufficiently understood to exploit its full potential. In this thesis the effect of intermolecular distance on SF is studied by embedding 6,13-bis(triisopropylsilylethynyl) pentacene (TIPS-pentacene) in an amorphous polymer matrix in the form of aqueous nanoparticle dispersions. By varying the mass ratio of TIPS-pentacene to the host polymer, the average intermolecular separation between TIPS-pentacene molecules is varied systematically from approximately 1 to 5 nm, resulting in a range of SF quantum yields. We study this system using both steady-state and ultrafast time-resolved spectroscopic techniques, and fit the results to a kinetic model to decipher the observed behaviour. The quantum yield of SF is shown to decrease with intermolecular separation, which is explained by diffusion-limited SF and an increase in loss pathways through exciton trap sites. We additionally identify an intermediate species in the SF process, and show that a significant proportion of this species decays non-radiatively without dissociating to form separated triplets, revealing another major loss pathway that has important implications for future research and applications.