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The transport properties of electrons and holes in a poly(p-phenylene vinylene)-based polymer light-emitting diode (PLED) have been investigated. Using admittance spectroscopy, we are able to simultaneously obtain the electron and hole mobility, μ=μ0 exp(γE), in a single PLED. The dynamics of the electron and hole transport are separated in the frequency domain as a result of the different transit times. At room temperature, we find μ0=5.6×10−11 m2/V s and γ=5.0×10−4 (m/V)1/2 for holes. For electrons μ0=1.0×10−12 m2/V s and γ=8.0×10−4 (m/V)1/2 are obtained.

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The interplay between inter-and intrachain charge transport in bulk polythiophene in the hopping regime has been clarified by studying the conductivity s as a function of frequency v͞2p (up to 3 THz), temperature T, and doping level c. We present a model which quantitatively explains the observed crossover from quasi-one-dimensional transport to three-dimensional hopping conduction with increasing doping level. At high frequencies the conductivity is dominated by charge transport on onedimensional conducting chains. PACS numbers: 71.20.Rv, 71.55.Jv, 72.60. + g, 72.80.Le The charge transport mechanisms in conjugated polymers, although extensively studied over the past two decades, are still far from being completely understood. Neither the behavior around the insulator-to-metal transition (IMT), which can be induced in several polymer materials upon appropriate doping, nor the nature of hopping transport in the deeply insulating regime is yet resolved. While some studies indicate that transport is dominated by hops between three-dimensional (3D), well conducting regions [1,2], in other cases the strongly one-dimensional (1D) character of the polymer systems appears to be a crucial factor [3][4][5].In investigating the nature of hopping transport in conjugated polymers, studying the temperature and doping level dependence of the dc conductivity is an important tool. Since the dc conductivity is determined by the weakest links in the conducting path spanning the sample, the study of s dc ͑T ͒ gives insight in the slowest relevant transport processes in the system.On the insulating side of the IMT, the dc conductivity is predicted by many models to follow the well-known hopping expressionwhere the value of g and the interpretation of T 0 depend on the details of the model. The original Mott theory for 3D variable range hopping with a constant density of states (DOS) at the Fermi energy predicts g 1͞4 [6], while several modifications of the model have been proposed to describe the frequently observed value g 1͞2. Studying the dependence of g and T 0 on doping level c provides the opportunity to discriminate between the various hopping models and extract parameters determining the conductive properties such as the DOS and the localization length.While the dc conductivity is sensitive to the slowest transport processes, the ac conductivity s͑v͒ provides information about processes occurring at time scales t ഠ v 21 . Especially in conjugated polymers, where intrachain and interchain transition rates may differ by orders of magnitude, knowledge of s͑v͒ at high frequencies can help to clarify the properties of charge transport on a polymer chain.In this Letter, we present a systematic study of the charge transport in a conjugated polymer far away from the IMT, as a function of frequency, temperature, and doping level. By selecting a polymer system with very low interchain mobility, a separation of interchain and intrachain contributions to the conductivity can be made when the applied frequency is varied over 12 decade...

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-chargelimited 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.

The unusual 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 and plasma frequency. This relation excludes percolation models of the metallic state. Instead, the carrier dynamics can be understood in terms of the low density of delocalized states around the Fermi level, which arises from the competion between disorder-induced localization and interchaininteractions-induced delocalization.The finite conductivity of a metal at zero Kelvin is a consequence of the lattice-periodicity and the finite density of states at the Fermi level (E F ). By breaking translational symmetry, disorder localizes chargecarriers. Upon growing disorder, eventually a metalinsulator transition (MIT) occurs [1], the effect being the stronger the lower the dimensionality. In one dimension (1D) any disorder localizes the electronic states. The MIT in quasi-1D conducting polymers and single-walled carbon nanotubes is also disorder-driven, but its exact nature is under severe debate. Many authors claim the presence of a "heterogeneous" state in which the relevant disorder length-scale is large compared to the electronic correlation-length. In this case, the MIT corresponds to a percolation transition of metallic islands embedded in an amorphous matrix. [2][3][4] Other studies suggest that the MIT is of the Anderson type [1] with disorder occurring on length-scales equal or less than the electronic correlation-length. [5,6] Then, extended and localized states are separated in energy by the mobility edge (E c ), and the MIT occurs when E F crosses E c .

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