Polarization Imaging of Emissive Charge Transfer States in Polymer/FullereneBlends.

Photoexcitation of conjugated polymer–fullereneblends resultsin population of a local charge transfer (CT) state at the interfacebetween the two materials. The competition between recombination anddissociation of this interfacial state limits the generation of fullyseparated free charges. Therefore, a detailed understanding of theCT states is critical for building a comprehensive picture of theorganic solar cells operation. We applied a new fluorescence microscopymethod called two-dimensional polarization imaging to gain insightinto the orientation of the transition dipole moments of the CT states,and the associated excitation energy transfer processes in TQ1:PCBMblend films. The polymer phase was oriented mechanically to relatethe polymer dipole moment orientation to that of the CT states. CTstate formation was observed to be much faster than energy transferin the polymer phase. However, after being formed an emissive CT statedoes not exchange excitation energy with other CT states, suggestingthat they are spatially and/or energetically isolated. We found thatthe quantum yield of the CT emission is smaller for CT states spatiallylocated in the highly oriented polymer domains, which is interpretedas the result of enhanced CT state dissociation in highly orderedstructures. [ABSTRACT FROM AUTHOR]