Singlet fission in acene blends

Singlet fission (SF), the photophysical process converting an excited singlet exciton into two triplet excitons, is a promising approach to boost solar cell efficiencies. It is controlled by various parameters such as intermolecular interactions, energetics, entropy and vibronic coupling and a controlled modification of these parameters is key to a fundamental understanding. Blends of organic semiconductors present an interesting alternative to established methods of chemical functionalization and their potential for the study of SF pathways will be discussed using acene blends as example. Mixed thin films of SF chromophores and weakly interacting, high-bandgap spacer molecules allow one to study the impact of a replacement of nearest neighbors on the SF rates. While the SF rate in pentacene, for which SF is a coherent process, is unaffected by the introduction of spacer molecules into the film, we observe a significant decrease in the SF rate in tetracene, indicating incoherent SF. Mixing the two SF chromophores pentacene and tetracene with low pentacene concentrations leads to heterofission of a singlet on pentacene into two triplets on pentacene and tetracene, respectively, when selectively exciting pentacene. This heterofission process is outcompeted by pentacene homofission if the pentacene concentration exceeds 5%. Photoexcitation above the tetracene band gap additionally allows for energy transfer from tetracene to pentacene and results in complex dynamics.