Triplet excitons have been the focus of considerable attention with regards to the functioning of polymer solar cells, because these species are long-lived and quench subsequently generated singlet excitons in their vicinity. The role of triplets in poly(3-hexylthiophene) (P3HT) has been investigated extensively with contrary conclusions regarding their importance. We probe the various roles triplets can play in P3HT by analyzing the photoluminescence (PL) from isolated single-chain aggregates and multi-chain mesoscopic aggregates. Solvent vapor annealing allows deterministic growth of P3HT aggregates consisting of ~20 chains, which exhibit red-shifted and broadened PL compared to single-chain aggregates. The multi-chain aggregates exhibit a decrease of photon antibunching contrast compared to single-chain aggregates, implying rather weak interchain excitonic coupling and energy transfer. Nevertheless, the influence of triplet-quenching oxygen on PL and a photon correlation analysis of aggregate PL reveal that triplets are quenched by intermolecular interactions in the bulk state.
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