Controlling the Efficiency of Singlet Fission in TIPS-Pentacene/Polymer Composite Nanoparticles

Abstract

Singlet fission (SF) is a process with the potential to increase the efficiency of solar cells by reducing losses from thermal relaxation of hot carriers. By generating two triplet excitons from one singlet exciton, the process effectively splits the energy of high-energy photons into two, providing a means to circumvent the Shockley–Queisser limit. Although the applications of SF are promising, questions remain about the mechanistic details and practicalities of implementation in photovoltaic devices that must be resolved to exploit its full potential. In this study, we present a way to investigate the effect of average intermolecular distance on SF by embedding 6,13-bis(triisopropylsilylethynyl)pentacene (TIPS-Pn) in an amorphous polymer matrix in the form of aqueous nanoparticle dispersions. By controlling the mass ratio of TIPS-Pn to the host polymer, we systematically tune the concentration of TIPS-Pn molecules in a nanoparticle and in turn, the average intermolecular separation, leading to 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 behavior. The quantum yield of SF is shown to decrease with average intermolecular separation, which is explained by diffusion-limited SF and an increase in loss pathways through isolated sites. Additionally, we identify an intermediate species in the SF process and show that a significant proportion of this species decays nonradiatively without dissociating to form separated triplets, revealing a major loss pathway that has important implications for future research and applications.