Singlet fission represents a promising way to downconvert high-energy solar photons and allow single-junction photovoltaic cells to surpass the Shockley-Queisser efficiency limit. Organic crystals composed of polyacene molecules like anthracene, tetracene and pentacene are useful model systems as well as potential components of functional organic and hybrid solar cells based on this phenomenon. We will describe efforts to understand the different photophysical steps involved in singlet fission using time-resolved transient absorption and photoluminescence measurements. The initial fission reaction depends sensitively on the energy separation of the singlet and triplet pair states. Comparison of fission dynamics in pentacene and tetracene crystals mixed with inert spacer molecules provides a way to probe how the fission mechanism changes as the reaction transitions from endo- to exothermic. After the triplet pair is produced, the lifetime and structure of the <sup>1</sup>(TT) state determines the next stage of evolution. The nonequilibrium spin state distributions produced by the singlet fission event can play an important role in both the ultimate triplet yield and the observation of experimental quantities like delayed fluorescence. Temperature dependent quantum beat measurements of the <sup>1</sup>(TT) state suggest that single crystal tetracene supports an activated fission process that produces triplet excitons that can diffuse independently while maintaining spin coherence. At low temperatures, triplets are produced by a different mechanism, possibly defect mediated, that does not appear to produce spin coherence.

Electronic and spin coherence effects on singlet fission in polyacene single crystals and blends

ACS Spring Meeting 2021

The official archive and repository for all ACS Meetings & Expositions abstracts and select posters or presentations.