Photophysics of charge transfer in a polyfluorene/violanthrone blend

We present a study of the photophysical and photovoltaic properties of blends of violanthrone in poly[9,9-bis(2-ethylhexyl)-fluorene-2,7-diyl] (PF2/6). Photoluminescence quenching and photocurrent measurements show moderate efficiencies for charge generation, characteristic of such polymer/dye blends. Pump-probe measurements on blend films suggest that while similar to47% of the total exciton population dissociates within 4 ps of photoexcitation, only similar to32% subsequently results in the formation of dye anions. We attribute the discrepancy to the likely formation of complex species with long lifetimes, such as stabilized interface charge pairs or exciplexes. This conclusion is supported by the appearance of a long lifetime component of 2.4 ns in the dynamics of the photoinduced absorption signal associated to polarons in photoinduced absorption bands centered at 560 nm.