Population dynamics of multiple triplet excitons revealed from time-dependent fluorescence quenching of single conjugated polymer chains (original) (raw)

The advent of multiple exciton harvesting schemes and prolonging exciton lifetimes to improve performance attributes of solar cells based on conjugated organic materials presents some interesting challenges that must be overcome in order to realize the full potential of these strategies. This is especially important for applications involving multi-chromophoric conjugated polymers where interactions between multiple spin-forbidden triplet excitons can be significant and are mediated by chain conformation. We use single molecule spectroscopic techniques to investigate interactions between multiple triplet excitons and emissive singlets by monitoring time-dependent fluorescence quenching on time scales commensurate with the triplet lifetime. Structurally related conjugated polymers differing by heteroatom substitution were targeted and we use a stochastic photodynamic model to numerically simulate the evolution of multi-exciton populations following photoexcitation. Single chains of poly(3-hexylthiophene) (P3HT) exhibit longer-lived triplet dynamics and larger steady-state triplet occupancies compared to those of poly(3-hexylselenophene) (P3HS), which has a larger reported triplet yield. Triplet populations evolve and relax much faster in P3HS which only becomes evident when considering all kinetic factors governing exciton population dynamics. Overall, we uncover new guidelines for effectively managing multi-exciton populations and interactions in conjugated polymers and improving their light harvesting efficiency. Conjugated organic polymers have demonstrated promise in solar cell applications but extreme heterogeneity and efficient intrinsic loss mechanisms 1,2 , such as rapid non-radiative excitation energy dissipation, are responsible for large disparities between measured and predicted efficiencies. There is now widespread interest for mitigating performance losses by generating multiple excitons per photon absorbed or extending exciton lifetimes 3-5. Singlet fission-the generation of two triplet excitons from one singlet exciton-and heavy atom substitution to increase triplet character, respectively, have attracted the most attention 4,6-11 , but applications involving conjugated polymers are limited 12,13. In fact, mechanistic studies of singlet fission have concentrated on crystalline solids and small molecule arrays with well-defined chromophore orientations. Furthermore, efforts to increase triplet exciton character and tune triplet interactions in polymers may be complicated by contributions from large vibrational displacements along high frequency modes that modulate spin-orbit coupling strength 14. Perhaps the most significant obstacle for effectively utilizing multi-exciton generation and harvesting strategies can be traced to the multi-chromophoric nature of polymers (i.e., many conjugated segments of varying length) and variable inter-chromophore coupling due to conformational heterogeneity 15. For example, the longer lifetimes of triplets creates complex photophysical scenarios due to the presence of multiple excitonic states of different spin on many chromophore segments that interact over a broad range of time scales (e.g., ~10 −12-10 −3 s). Interestingly, previous pulse radiolysis studies found that isolated conjugated polymer chains can support a large number of triplets (~30) simultaneously 16. While much of the current focus has emphasized elucidating triplet formation mechanisms on ultrafast time scales, relatively little is known about how populations of multiple triplet excitons evolve on longer time scales (i.e., comparable to triplet lifetimes). Unfortunately, resolving multi-excitonic interactions at the materials level in polymers is complicated from a myriad of competing decay channels arising from intermolecular interactions and aggregation 12,17. However, by