Metal-insulator transition in PF6 doped polypyrrole: Failure of disorder-only models

The free-carrier scattering time and plasma frequency in PF6 doped polypyrrole near the metal-insulator transition (MIT) is obtained by dielectric spectroscopy (8-700 GHz) down to 2 K. The specific T-dependence allows us to quantify the distance to the MIT for both metallic and insulating samples. The evolution of the free-carrier parameters when going deeper into the metallic regime conflicts with both the homogeneous and heterogeneous disorder models. We suggest this failure to stem from the interplay between electronic correlations and interchain charge transfer, which is not explicitly considered in these theories

Frequency dependent carrier mobility in polymer LED’s

The charge mobility in polymer LED’s depends on the polymer and its packing. In the latter various length scales play a role. To study this aspect, we performed dielectric spectroscopy as function of electric field and temperature on OC1C10-poly(p-phenylene vinylene). At low frequencies the mobility (μ) is mainly determined by the energetic disorder of the weakest link. Around 100 GHz μ has increased by 6 orders of magnitude and the field dependence has changed significantly. At these high frequencies, which in this case are representative for the pair limit, the field dependence shows that still appreciable energy barriers have to be overcome. Together these data allow a detailed characterization of the charge transport in the LED.

Crossover from space-charge-limited to recombination-limited transport in polymer light-emitting diodes

By performing admittance spectroscopy as a function of frequency on polymer light-emitting diodes, inductive and capacitive charge-relaxation processes with different characteristic time scales are separated. The inductive contributions arise from the finite transit time of injected carriers, while the capacitive contributions stem from dielectric redistribution of charge density in the device. The crossover from inductive charge relaxation at low bias to capacitive charge relaxation at high bias marks the transition from space-charge-limited to recombination-limited current flow. This unexpected result shows that, while the individual carrier mobilities are strongly enhanced by the applied field, the recombination mobility remains unaffected.

Admittance spectroscopy on polymer light-emitting diodes

From admittance spectroscopy measurements on poly(p-phenylene vinylene) based light-emitting diodes various relaxation processes can be observed. At low bias inductive contributions due to the transit of charge carriers dominate, at high bias capacitive contributions as a result of charge redistribution appears. These processes reveal a transition from space-charge limited to recombination limited behavior, as is also indicated by the current-voltage characteristics.</p

Metallic state in disordered quasi-one-dimensional conductors

The metallic state in conjugated polymers and single-walled carbon nanotubes: is studied by dielectric spectroscopy (8-600 GHz). We have found an intriguing correlation between scattering time (tau) and plasma frequency (omega (p)):tau (proportional to)omega (-1.3)(p). This relation conflicts with the usually applied models that only consider disorder. Based on the observed parallels with doped semiconductors, we argue that the interchain coupling t(perpendicular to) plays a role comparable to the doping level and that the unusual free-carrier dynamics in the metallic state can be explained when including the role of t(perpendicular to) in the conventional models

Unusual carrier dynamics in disordered quasi-1D systems: not so unusual

Charge transport in conducting polymers and single-walled carbon nanotubes is studied by means of complex dielectric spectroscopy down to 2 K. The negative dielectric constants, low plasma freqeuncies, and anomalously long relaxation times can be understood in terms of a low density of delocalized states near the Fermi level, which stems from the competition between disorder-driven localization and interchain charge transfer

The Drude parameters for metallic PF6 doped polypyrrole

We measured the two components of the complex dielectric function of insulating and metallic PF6 doped polypyrrole up to 4 meV simultaneously. These data are used as an input performed Kramers-Kronig analysis on the higher energy reflection data. This has helped to clarify those results, which were previously deduced from the FIR and UV/VIS data, that could be artifacts

Carrier Dynamics in Conducting Polymers: Case of PF6PF_6 Doped Polypyrrole

The carrier dynamics in $PF-^{6}$ doped polypyrrole has been probed by dielectric spectroscopy (from $10-^{4 }$to 4 eV), down to 4.2 K. The phase-sensitive sub-THz data have assisted to resolve the discrepancies in Kramers-Kronig analysis in earlier studies. Even in metallic samples, just 1% of the carriers are delocalized, at 300 K; the fraction drops down considerably as a function of disorder, carrier density, and temperature. This subtle metallic feature and the anomalies in carrier dynamics are attributed to coherent and incoherent transport between short conjugated segments