Charge generation kinetics and transport mechanisms in blended polyfluorene photovoltaic devices

We report a compositional analysis of blended hole-accepting and electron-accepting polyfluorene related materials, poly(9,9'-dioctylfluoreneco-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) [PFB] and poly(9,9'-dioctylfluorene-co-benzo-thiadiazole) [F8BT], in films and in photovoltaic devices. We find that photoluminescence quenching is insensitive to blend composition but the photovoltaic quantum yield is strongly composition dependent. This indicates that charge transport, and not charge generation, is the factor limiting device performance. We demonstrate that a meso-length scale phase separation optimizes charge transport properties.