title: Role of bad dihedral angles: methylfluorenes act as energy barriers for excitons and polarons of oligofluorenes.
abstract: "Defects" are one of the main obstacles for the use of organic conjugated molecules in efficient organic photovoltaics, but the definite origins of these defects are elusive to experiments and even in concept. Bad dihedral angles in conjugated molecules produced by adjacent units are considered to act as defects for excitons and polarons, slowing down their transports. While such defects are discussed, their properties were not well-understood. As a model system for such defects, we synthesized oligofluorenes incorporating methylfluorene(s) that can create large dihedral angles between adjacent fluorenes due to steric hindrance, mimicking bad dihedral angles presumably produced in polyfluorenes. Experimental measurements find that singlet excitons are substantially more sensitive to such bad dihedral angles than triplet excitons or negative or positive polarons. The barrier heights for singlets are about three times higher than the barriers for electrons, holes, or triplets. For all four species, the large dihedrals act as energy barriers, not traps.
authors.affiliation: Chemistry Department, Brookhaven National Laboratory , Upton, New York 11973-5000, United States.
authors.forename: Tomoyasu
authors.initials: T
authors.lastname: Mani
authors.forename: John R
authors.initials: JR
authors.lastname: Miller