Ion-Induced Formation of Charge-Transfer States in Conjugated Polyelectrolytes

We use time-resolved optical spectroscopy to demonstrate that the luminescence quenching observed when ions are incorporated in films of conjugated polymers can be explained by the formation of charge-transfer (CT) states that are stabilized by the Coulomb field of ions. Our investigation is focused on a conjugated polyelectrolyte (CPE) derived from F8BT (poly(9,9'-dioctylfluorene-a/t-benzothiadiazole)). The statistical copolymer contains tetra-alkyl ammonium moieties and BF4- counteranions attached to a moderate (similar to 7%) density of polymer alkyl side chains, providing a film morphology comparable to F8BT but with ions distributed on the length scale of exciton diffusion. The ionic substituents have little influence over the polymers electronic absorption and emission properties in solution, however photoluminescence (PL) quantum efficiency (similar to 6%) is considerably lower for the polyelectrolyte compared with F8BT (similar to 60%) in thin films. Time-resolved PL spectroscopy reveals that the primary exciton lifetime is shortened in the polyelectrolyte and a red-shifted CT emission peak with a longer lifetime emerges. Transient absorption spectroscopy of thin films enables us to detect CT states that persist beyond the primary decay and are found to be immobile. The PL intensity of the partially ionic film is found to increase with decreasing temperature, consistent with thermally activated exciton hopping (E-act = 28 meV) prior to formation of CT states at ionic regions. We suggest that ion-induced stabilization of CT states is a general phenomenon in CPEs, which raises the possibility that ions might be arranged to direct the flow of excitons toward charge-separating interfaces in polymer photovoltaic devices.