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.

Understanding exciton diffusion properties is essential for organic optoelectronics such as organic solar cells (OSCs) because excitons in semiconducting polymer solids need to migrate as quasiparticles to a donor/acceptor interface to dissociate into charges in OSCs. Previous studies mainly focused on the diffusion length of excitons. In addition, the dimensionality of exciton diffusion is an important property because OSCs usually consist of semi-crystalline conjugated polymers wherein anisotropic exciton diffusion is expected. Herein we investigate the dimensionality of exciton diffusion in a naphthobisoxadiazole-based low-bandgap polymer PNOz4T through transient absorption spectroscopy. We track the time evolution of the excitation intensity dependent transient signals caused by singlet-singlet exciton annihilation. By analyzing the time dependence of the annihilation rate coefficient, we find that excitons generated in PNOz4T crystalline domains show anisotropic two-dimensional diffusion along the backbone and π-stacking directions, which can be rationalized by comparable intra-and interchain exciton couplings.

The replacement of benzene with naphthalene in the central core of an acceptor achieved a longer singlet lifetime and a higher power conversion efficiency.