Imaging the Fluorescence Decay Lifetime of a Conjugated-Polymer Blend By Using a Scanning Near-Field Optical Microscope

摘要

Understanding the structure of phase-separated conjugat-ed-polymer blends is of both fundamental and practical interest. Such blends can be produced by spin-casting or ink-jet printing a solution containing a mixture of polymers, with the resultant micro- and nanoscale structures acting as self-assembled molecular-scale heterojunctions. These hetero-junctions can act as sites for geminate electron-hole pairs to disassociate into uncorrelated charge-pairs or to undergo radiative relaxation from exciton or exciplex states. For this reason, conjugated-polymer blends have been used as the active semiconductor material in both thin-film light-emitting diodes (LEDs) and photovoltaic devices (PVs).rnA number of recent reports have demonstrated that the micro- and nanoscale structure of spin-cast polymer blends can be highly nonequilibrium in nature, an effect resulting from the fast quenching of phase separation that occurs as a result of the rapid removal of the casting solvent (by evaporation). As a consequence, the phase-separated domains that form are not pure but often contain some fractions of both polymers. We note that measurements on the bulk conductivity values of polymer blends have shown that the carrier mobility can be reduced; an effect arising from the presence of the permanent dipole moments of the polymeric components. This suggests that the structures formed through phase-separation may exhibit a significant variation in local electronic properties. A characterization of the local electronic properties of such conjugated-polymer blends provides a potential means to understand their operation in optoelectronic devices — a task that has recently been addressed via a number of innovative scanning probe microscopy studies.