3 research outputs found
Intermolecular CT excitons enable nanosecond excited-state lifetimes in NIR-absorbing non-fullerene acceptors for efficient organic solar cells
State-of-the-art Y6-type molecular acceptors exhibit nanosecond excited-state
lifetimes despite their low optical gaps (~1.4 eV), thus allowing organic solar
cells (OSCs) to achieve highly efficient charge generation with extended
near-infrared (NIR) absorption range (up to ~1000 nm). However, the precise
molecular-level mechanism that enables low-energy excited states in Y6-type
acceptors to achieve nanosecond lifetimes has remained elusive. Here, we
demonstrate that the distinct packing of Y6 molecules in film leads to a strong
intermolecular charge-transfer (iCT) character of the lowest excited state in
Y6 aggregates, which is absent in other low-gap acceptors such as ITIC. Due to
strong electronic couplings between the adjacent Y6 molecules, the iCT-exciton
energies are greatly reduced by up to ~0.25 eV with respect to excitons formed
in separated molecules. Importantly, despite their low energies, the iCT
excitons have reduced non-adiabatic electron-vibration couplings with the
electronic ground state, thus suppressing non-radiative recombination and
allowing Y6 to overcome the well-known energy gap law. Our results reveal the
fundamental relationship between molecular packing and nanosecond excited-state
lifetimes in NIR-absorbing Y6-type acceptors underlying the outstanding
performance of Y6-based OSCs
Assessing intra- and inter-molecular charge transfer excitations in non-fullerene acceptors using electroabsorption spectroscopy
Abstract Organic photovoltaic cells using Y6 non-fullerene acceptors have recently achieved high efficiency, and it was suggested to be attributed to the charge-transfer (CT) nature of the excitations in Y6 aggregates. Here, by combining electroabsorption spectroscopy measurements and electronic-structure calculations, we find that the charge-transfer character already exists in isolated Y6 molecules but is strongly increased when there is molecular aggregation. Surprisingly, it is found that the large enhanced charge transfer in clustered Y6 molecules is not due to an increase in excited-state dipole moment, Δμ, as observed in other organic systems, but due to a reduced polarizability change, Δp. It is proposed that such a strong charge-transfer character is promoted by the stabilization of the charge-transfer energy upon aggregation, as deduced from density functional theory and four-state model calculations. This work provides insight into the correlation between molecular electronic properties and charge-transfer characteristics in organic electronic materials