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

Abstract

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 (10 GHz). Variable frequency EDMR allows us to establish the role of spin-orbit coupling in spin-dependent processes whereas pulsed EDMR allows for the observation of 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, in which monomer units are twisted with respect to each other, and an ordered (β) phase, where all monomers lie within one plane. In thin films of organic light-emitting diodes (OLEDs) the appearance of a particular phase can be controlled by deposition parameters and solvent vapor annealing, and is verified by electroluminescence spectroscopy. Under bipolar charge carrier 2 injection conditions 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 coupling of the carrier spins to nearby nuclei spins. 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. In both morphologies, we observe the presence of at least two different spin-dependent recombination processes. 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. This additional contribution arises since triplet lifetimes are increased at lower temperatures. We tentatively conclude that spectral broadening induced by hyperfine coupling is slightly weaker in the more ordered β-phase than in the glassy-phase, since protons are more evenly spaced, whereas broadening effects due to spin-orbit coupling, which impacts the distribution of g-factors, appears to be somewhat more significant in the β-phase. PACS

Correlation between bias-dependent ESR signals and magnetic field effects in organic light emitting diodes

“…Particularly, in the present result, the existence of a trion and a bipolaron was not necessary to assume for explaining the bias-dependent MC and MEL curves. Such a trion could be present at low temperature, 28,29 but we emphasize that the greatest contribution to the MFE-magnitude around room temperature is the conversion efficiency between SP and TP.…”

“…This result is consistent with the recent report that such half-field EDMR signals were observed only at low temperature in polymer OLEDs. 28,29 Related to the trion generated via the bipolaron, the weak EDMR signal observed in the hole-only device could be a result of spin-dependent reaction of positive polaron carriers into the bipolaron. Then, if the EDMR and ELDMR signals of OLED occur via the trion, the trion should be formed by the spin-dependent reactions of the positive bipolaron and the negative countercarrier.…”

Nanotesla magnetoresistance in π-conjugated polymer devices

“…Since our devices show sensitivity to fields two orders of magnitude weaker than the Earth's field, one may speculate that even longer spin coherence times may be responsible for the observed effect. However, such long times have not been observed in direct measurements of spin coherence in organic semiconductors by electrically detected spin-echo spectroscopy, which typically yield pure decoherence times (T 2 ) of order 1 μs [42][43][44]. This discrepancy may be linked to the fact that EDMR measurements, such as spin-echo spectroscopy, solely monitor mobile resonant charge species, whereas it is precisely this hopping mobility of carriers in the inhomogeneous hyperfine fields which induces spin dephasing [42].…”

Innovation of Materials, Devices, and Functionalized Interfaces in Organic Spintronics

“…The spin relaxation time can also be obtained using pulsed electrically detected magnetic resonance spectroscopy based on OLEDs, [ 94,95 ] by which the coherent spin motions can be observed. Using this method, the spin relaxation time of super yellow poly‐phenylene‐vinylene (SY‐PPV) was measured to be ≈29 µs, while that of P3HT was ≈0.2 µs.…”

“…[ 23 ] Additionally, polyfluorene (Figure 2) has been used to investigate spin transport properties using ISHE measurements in spin pumping devices [ 94,117 ] and spin‐Rabi oscillations in OLEDs. [ 95 ]…”