Photovoltaic performance and morphology of polyfluorene blends: A combined microscopic and photovoltaic investigation

The interplay between phase separation in polyfluorene blends which show photoinduced charge transfer and photovoltaic performance in photodiodes has been investigated. Phase separation length scales have been varied from several microns to tens of nanometers by limiting the time allowed for solvent-enhanced self-organization through several different processing routes. Concurrent with the decrease in feature size, an increase in maximum photovoltaic efficiency of nearly 1 order of magnitude was observed in photodiodes incorporating the phase-separated blends as the active layer. The structure of the blend films was investigated using fluorescence microscopy, fluorescence scanning near-field optical microscopy, and atomic force microscopy. In some cases, a hierarchy of micron- and nanometer-scale phase separation was observed which may explain the unexpectedly high photoresponse in devices with up to micron-scale phase separation structure. This result along with in situ fluorescence microscopy studies of the transformation process highlights the complex, multistage nature of the conjugated polymer blend formation process which generally exhibits spinodal behavior.