We examined the mechanisms underlying the free carrier generation in a very topical PM6/Y6 organic solar cell. We observed slow yet efficient spatial charge dissociation driven by downhill energy relaxation through the interfacial energy cascade.

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.

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The large voltage loss is a significant disadvantage of organic solar cells (OSCs) compared with inorganic and perovskite solar cells and should be reduced to further improve the power conversion efficiency of OSCs. Herein, the voltage loss in OSCs with a systematically controlled energy difference between the excited and charge transfer states is discussed. The voltage loss is compared between two types of OSCs: narrow‐bandgap donors paired with wide‐bandgap acceptors and narrow‐bandgap fused‐ring nonfullerene acceptors (NFAs) paired with wide‐bandgap donors, to elucidate whether there are any essential advantages in the latter. The first advantage of narrow‐bandgap‐NFA‐based OSCs is their higher photoluminescence quantum yield, owing to their rigid fused‐ring architecture. The second advantage is the ability to achieve efficient charge separation with a small voltage loss.