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Understanding the excited-state dynamics of nonfullerene electron acceptors is essential for further improvement of organic solar cells as they are responsible for near-IR light absorption. Herein, we investigated the singlet and triplet excited-state dynamics in Y6, a novel nonfullerene acceptor, using transient absorption spectroscopy. We found that, even at low excitation fluences, pristine Y6 films show biphasic singlet exciton decay kinetics with decay constants of ∼220 ps and ∼1200 ps. The majority of the Y6 singlet excitons decayed with the faster (∼220 ps) component, whereas a clear photoluminescence with the slower (∼1200 ps) component was observed, which is the origin of the large discrepancies in the previously reported exciton lifetimes of Y6 in the solid state. At high excitation fluences, on the other hand, Y6 singlet excitons are more likely to decay via singlet–singlet exciton annihilation due to fast exciton diffusion in crystalline domains, after which we observed ultrafast triplet formation, assigned to singlet fission from higher excited singlet states.
Natsuda et al. (Thu,) studied this question.