Tuning Interfacial Charge-Transfer Excitons at Polymer-Polymer Heterojunctions under Hydrostatic Pressure

We present photoluminescence spectroscopy of blend thin films of poly(9,9-di-n-octylfluorene-0ex0exalt-benzothiadiazole) (F8BT) with electron-donating copolymers under hydrostatic pressure. The photoluminescence spectrum of F8BT redshifts by 530±60 meV over pressures of 0.1 MPa--8.8 GPa. That of the interfacial charge-transfer exciton redshifts by 270--370 meV over a similar pressure range. Despite the relatively small shift of the charge-transfer exciton, the excited state dynamics at the molecular heterojunction are strongly enhanced due to the decreasing intermolecular distance, leading to favored exciplex emission.