title: Absence of singlet fission and carrier multiplication in a model conjugated polymer: tracking the triplet population through phosphorescence.
abstract: Singlet fission, or multiple exciton generation, has been purported to occur in a variety of material systems. Given the current interest in exploiting this process in photovoltaics, we search for the direct signature of singlet fission, phosphorescence from the triplet state, in a model polymeric organic semiconductor for which photoinduced absorption experiments have implied a tripling of the intersystem crossing yield at the onset of fission. Fluorescence and phosphorescence are clearly discriminated using a picosecond gated photoluminescence excitation technique, at variable temperature. At low excitation densities, in a quasi-steady-state experiment, we detect no change of the relative triplet yield to within 4% for photon energies of almost three times the triplet energy of 2.1 eV. Identical results are obtained under nonlinear two-photon excitation. We conclude that assignments of singlet fission based on induced absorptions alone should be treated with caution and may substantially overestimate excited-state intersystem crossing yields, raising questions with regards to the applicability of the process in devices.
authors.affiliation: Institut für Experimentelle und Angewandte Physik, Universität Regensburg, Universitätsstrasse 31, 93053 Regensburg, Germany.
authors.forename: Sebastian
authors.initials: S
authors.lastname: Bange
authors.forename: Ullrich
authors.initials: U
authors.lastname: Scherf
authors.forename: John M
authors.initials: JM
authors.lastname: Lupton
mesh_terms: D005453:Fluorescence; D008163:Luminescent Measurements; D008958:Models, Molecular; D015394:Molecular Structure; D011108:Polymers / Q000737:chemistry*
chemical_list: D011108:Polymers