Kinetics of singlet fission in organic semiconductors: Specific features of T-exciton migration effects

Abstract

Kinetics of singlet fission (SF) in organic semiconductors, associated with spontaneous splitting of the excited singlet state (S1) into a pair of triplet (T) excitons, is known to be strongly affected by geminate annihilation of generated TT-pairs. In this work, we analyze in detail the SF-kinetics within lattice-migration (hopping), diffusion-migration, and exponential-kinetics two-state models (TSMs), which allow us to accurately describe the effects of relative T-exciton migration in TT-pairs. In the proposed TSMs, the migration effects are treated within the approximation of kinetic coupling of two states: [TT]-state of interacting TT-pairs and [T + T]-state of freely migrating T-excitons. The TSMs are applied to study some important specific properties of the kinetics of SF-processes in the external magnetic field. In our work, we concentrate on the analysis of T-exciton-migration effects on SF-kinetics. It is demonstrated, in particular, that the anisotropy of T-exciton migration strongly manifests itself in SF-kinetics, especially in its (inverse-power type) long time part. High migration anisotropy also leads to a substantial decrease in the rate of T-exciton escape from [TT]-state and thus to TT-caging which can result in crossing of SF-kinetic curves corresponding to different magnetic fields. The analysis of this effect is shown to provide important information on specific features of the kinetics of SF-processes.