Reghu Menon

Polymer light-emitting diodes with polyethylene dioxythiophene–polystyrene sulfonate as the transparent anode

Abstract Polyethylene dioxythiophene–polystyrene sulfonate (PEDT–PSS) thin films spin-cast from aqueous dispersion have been used as semi-transparent anodes for polymer light-emitting diodes (LEDs). An external quantum efficiency of 2.0% was achieved with poly(2-methoxy,5-(2′-ethyl-hexyloxy)-1,4-phenylene vinylene) (MEH–PPV), as the semiconducting luminescent polymer. LEDs fabricated with the PEDT–PSS anode exhibit an enhanced stress life in excess of 500 h at 70 mA/cm2 under constant-current conditions (initial brightness 600 cd/m2, reduced to 450 cd/m2 after 500 h).

Electronic transport in doped poly (3,4-ethylenedioxythiophene) near the metal-insulator transition

Abstract The temperature dependences of the conductivity and magnetoconductivity of poly(3,4-ethylenedioxythiophene) doped with PF6, BF4 and CF3SO3 in the metallic and critical regimes have been studied. Doped films exhibit a weak temperature dependence of the resistivity, ϱ(T), with the characteristic resistivity ratio ϱr = ϱ(1.4 K)/ϱ(291 K) = 1.5–20; i.e. close to the metal-insulator transition. For metallic samples (ϱr 2.1. In both cases (negative and positive TCR), the low-temperature conductivity of metallic samples is well described by a T1/2 dependence, both in the presence of a magnetic field and with the magnetic field equal to zero. The magnetoconductance of samples in metallic and critical regimes is negative, Isotropic, and, for metallic samples, exhibits H2 and H1/2 dependences at low and high magnetic fields, respectively. The results for metallic samples are explained as resulting from the influence of electron-electron interactions on the lowtemperature conductivity.

Correlation of morphology and charge transport in poly(3,4-ethylenedioxythiophene)–polystyrenesulfonic acid (PEDOT–PSS) films

A wide variation in the charge transport properties of poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT-PSS) films is attributed to the degree of phase segregation of the excess insulating polyanion. The results indicate that the charge transport in PEDOT-PSS can vary from hopping to the critical regime of the metal-insulator transition, depending on the subtle details of morphology. The extent of electrical connectivity in the films, directly obtained from a temperature-dependent high-frequency transport study, indicates various limiting factors to the transport, which are correlated with the phase separation process. The low temperature magnetotransport further supports this morphology-dependent transport scenario.

Electrical characterization of conducting polypyrrole by THz time-domain spectroscopy

Using an optoelectronic THz beam system for THz time-domain spectroscopy (THz TDS), we have measured the absorption and index of refraction of a conducting polypyrrole film from low frequencies to 2.5 THz. From these measurements, the dc conductivity of 215/(Ω cm) and the complex conductance were obtained over this frequency range. All of the results were well fit by Drude theory, which gives a carrier scattering time of only 12.6 fs, less than 1/10 that of the semiconductors, thereby illustrating the disorder and low mobility of the polymer.

Nature of the Metallic State in Conducting Polypyrrole

O WILEY-VCH Verlag GmbH, D-69469 Weinheim, 1998 0935-9648/98/0604-0456

Metallic state in disordered quasi-one-dimensional conductors

17.50+.50/0 Adv. Mater. 1998, 10, No. 6[14] a) M. Binggeli, C. M. Mate, Appl. Phys. Lett. 1994, 65, 415. b) D.A.Grigg, P.E. Russell, J.E. Griffith, J. Vac. Sci. Technol. A 1992, 10, 680.c) A. L. Weisenhorn, P. K. Hansma, T. R. Albrecht, C. F. Quate, Appl.Phys. Lett. 1989, 2651.

Interfaces and traps in pentacene field-effect transistor

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.

Correlation between morphology and ambipolar transport in organic field-effect transistors

The equivalent circuit parameters for a pentacene organic field-effect transistor are determined from low frequency impedance measurements in the dark as well as under light illumination. The source-drain channel impedance parameters are obtained from Bode plot analysis and the deviations at low frequency are mainly due to the contact impedance. The charge accumulation at organic semiconductor–metal interface and dielectric–semiconductor interface is monitored from the response to light as an additional parameter to find out the contributions arising from photovoltaic and photoconductive effects. The shift in threshold voltage is due to the accumulation of photogenerated carriers under source-drain electrodes and at dielectric–semiconductor interface, and also this dominates the carrier transport. The charge carrier trapping at various interfaces and in the semiconductor is estimated from the dc and ac impedance measurements under illumination.

Synthesis, electrical, and optical properties of conjugated polymers

Attaining ambipolar charge transport in organic field-effect transistors (OFET) is highly desirable from both fundamental understanding and application points of view. We present the results of an approach to obtain ambipolar OFET with an active layer of organic semiconductor blends using semiconducting polymers in composite with fullerene derivatives. Clear features of forming the super position of both hole and electron-enhanced channels for an applied gate field are observed. The present studies suggest a strong correlation of thin-film nanomorphology and ambipolar transport in field-effect devices.

Space-charge limited conduction in doped polypyrrole devices

Publisher Summary This chapter examines important aspects of the synthesis, electrical, and optical properties of conducting polymers. There are a variety of routes toward the synthesis of polyacetylene. These syntheses can be classified into four categories: catalytic polymerization of acetylene, noncatalytic polymerization of acetylene, catalytic polymerization of monomers other than acetylene, and, finally, precursor methods. Many chemical reaction schemes have been proposed for polyaromatics and they can be classified as—(1) direct oxidation of benzene, (2) organometallic coupling, (3) precursor methods, and (4) electrochemical syntheses. There are several approaches toward the preparation of poly(p-phenylene vinylene) (FPV), which can be classified as direct and precursor methods. Several of these methods have been applied to synthesize heterocyclic derivatives, alkoxy-substituted poly(thienylene vinylene), and poly(furylene vinylene). Several types of π-electron systems have shown very interesting features in the electrical transport properties. The charge-transfer complexes consisting of electron-donating and electron-withdrawing organic molecules have a wide range of exciting electronic properties of low-dimensional systems. The typical sp2-carbon-rich systems such as intercalated graphite, carbon-60, and carbon nanotubes display a wide range of behavior from metals to insulators.