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Intrachain and Interchain Exciton-Exciton Annihilation in Donor-Acceptor Copolymers.

PM6 is a donor-acceptor copolymer widely used in high-efficiency organic solar cells (OSC). Here, we report the observation of exciton-exciton annihilation (EEA) in both PM6 copolymer solution and film. We find that the EEA mechanisms are distinct in these two systems. In the PM6 solution, EEA occurs via the long-range dipole-dipole interaction between intrachain excitons, while in the PM6 film it takes place between the interchain polaron pairs (spatially indirect excitons) via the short-range Coulomb interaction. The quantum yield of free polaron formation in the film is also quantified, which indicates that EEA leads to extra interchain polaron-pair dissociation, accounting for the enhanced free polaron formation.