Small Organic Molecules for Efficient Singlet Fission: Role of Silicon Substitution

Singlet fission (SF) has emerged as an important mechanism for enhancing the efficiency of organic solar cells. In search for new molecules for SF, silicon substituted oligoacenes are shown to be excellent candidates. Here we show that monosilicon substitution in the central ring of anthracene is found to be the smallest closed shell molecule predicted to exhibit SF. The crystal structure of 10-cyano-9-silaanthracene (10-CN-9-SA) shows the molecules in slipped parallel stacked orientations with small intermolecular distances (d_{center-center} = 4.13 Å). We have performed calculations using the Marcus electron transfer theory to calculate the SF rate in a chromophoric pair. Our calculation indicates that the lowest energy CT state mediates as a real intermediate in a SF pathway maximizing the SF rate. Short intermolecular contacts and low-lying charge transfer (CT) states lead to an anticipated triplet yield of ∼200% in the SF process for these crystals. An indirect one-electron integral mechanism through a CT state predominates over the direct two-electron integral mechanism for this extremely efficient SF.