Multiple-exciton generation is a phenomenon underpinning the design of next-generation photovoltaic devices, by allowing solar cells to exceed the thermodynamic single-junction efficiency limit. In organic materials, a singlet exciton can generate a pair of triplet excitons, allowing for high-yield multiple-exciton generation known as “singlet fission.” The spin dynamics of triplet excitons is an important mechanism that dictates the efficiency of singlet-fission-based devices, but experimentally characterising these dynamics in a given material has proven challenging.
Much of the study of organic triplet excitons has been carried out via EPR, which allows one to distinguish the populations of each spin-sublevel, as well as observe how these populations evolve over time. While most such studies of singlet fission use continuous-wave EPR, pulsed EPR has also proven valuable, for instance, in identifying triplet-exciton pairs in strongly coupled spin states, such as the spin-2 “quintet” state. In this work we perform both cw- and pulsed EPR on a molecular dimer capable of singlet fission, and demonstrate that the two methods probe two different regimes of triplet-triplet interaction.
We use the Phase-inverted Echo-Amplitude detected NUTation (PEANUT) pulse sequence and find that this experiment detects only some of the species detected by cw-EPR. Since PEANUT is sensitive to the coherence lifetime of the observed spins, we draw on earlier theoretical results to conclude that the two experiments probe two different formation mechanisms for the quintet biexciton. Furthermore, where previous studies have assumed that triplet polarisation dynamics are dominated by anisotropic decay, our results suggest that anisotropic triplet generation via the quintet state plays a significant role. Hence, we demonstrate that combining cw- and pulsed EPR provides a unique probe for characterising multiexciton dynamics, and screening potential materials for use in singlet fission-based devices.