No abstract

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.

The voltage loss incurred by nonradiative charge recombination should be reduced to further improve the power conversion efficiency of organic solar cells (OSCs). This work discusses the nonradiative voltage loss in OSCs with systematically controlled energy offset between optical bandgap and charge transfer (CT) states. It is demonstrated that the nonradiative voltage loss is a function of the energy offset; it drops sharply with decreasing energy offset. By measuring the quantum yields of electroluminescence from OSCs and decay kinetics of CT states, it is found that the radiative decay rate of CT states becomes larger when the energy offset is negligible compared with those in conventional OSCs with sufficient energy offset. This behavior is rationalized by hybridization between CT and local excited states, resulting in a considerable enhancement of the oscillator strength of CT states. Based on a trend observed in this study, the precise mechanism by which the energy offset affects the nonradiative voltage loss is discussed.