Paper detail

Morphology effects on spin-dependent transport and recombination in polyfluorene thin films

We have studied the role of spin-dependent processes on conductivity in polyfluorene (PFO) thin films by conducting continuous wave (c.w.) electrically detected magnetic resonance (EDMR) spectroscopy at temperatures between 10 K and 293 K using microwave frequencies between about 100 MHz and 20 GHz as well as pulsed EDMR at X-band. Variable frequency EDMR allows us to establish the role of spin-orbit coupling in spin-dependent processes, pulsed EDMR probes coherent spin motion effects. We used PFO for this study in order to allow for the investigation of the effects of microscopic morphological ordering since this material can adopt two distinct intrachain morphologies: an amorphous (glassy) phase, and an ordered (beta) phase. In thin films of organic light-emitting diodes (OLEDs) the appearance of a particular phase can be controlled by deposition parameters, and is verified by electroluminescence spectroscopy. We conducted multi-frequency c.w. EDMR, electrically detected Rabi spinbeat experiments, Hahn-echo and inversion-recovery measurements. Coherent echo spectroscopy reveals electrically detected electron spin-echo envelope modulation (ESEEM) due to the precession of the carrier spins around the protons. Our results demonstrate that while conformational disorder can influence the observed EDMR signals, including the sign of the current changes on resonance as well as the magnitudes of local hyperfine fields and charge carrier spin-orbit interactions, it does not qualitatively affect the nature of spin-dependent transitions in this material. At 293 K and 10 K, polaron-pair recombination through weakly spin-spin coupled intermediate charge carrier pair states is dominant, while at low temperatures, additional signatures of spin-dependent charge transport through the interaction of polarons with triplet excitons are seen in the half-field resonance of a triplet spin-1 species.