Negative differential resistance in doped poly(3-methylthiophene) devices

The current density-voltage (J-V) characteristics of poly(3-methylthiophene) devices show a negative differential resistance (NDR) at room temperature with a large peak to valley current ratio (similar to 507). This NDR can be tuned by two orders of magnitude by controlling the carrier density due to the variation of the space-charge region in the device. The temperature and scan rate dependent J-V measurements infer that the NDR is mainly driven by the trapping and de-trapping of carriers. The photo-generation of carriers is observed to reduce the NDR effect.

Carrier density-dependent transport in poly(3-methylthiophene): from injection-limited to space-charge-limited current

Current-voltage (I-V) and impedance measurements were carried out in doped poly(3-methylthiophene) devices by varying the carrier density. As the carrier concentration reduces the I-V characteristics indicate that the conduction mechanism is limited by metal-polymer interface, as also observed in impedance data. The temperature dependence of I-V in moderately doped samples shows a trap-controlled space-charge-limited conduction (SCLC); whereas in lightly doped devices injection-limited conduction is observed at lower bias and SCLC at higher voltages. The carrier density-dependent quasi-Fermi level adjustment and trap-limited transport could explain this variation in conduction mechanism. Capacitance measurements at lower frequencies and higher bias voltages show a sign change in values due to the significant variations in the relaxation behaviour for lightly and moderately doped samples. The electrical hysteresis increases as carrier density is reduced due to the time scales involved in the de-trapping of carriers.

NMR relaxation studies in doped poly-3-methylthiophene

NMR relaxation rates (1/T-1), magnetic susceptibility, and electrical conductivity studies in doped poly-3-methylthiophene are reported in this paper. The magnetic susceptibility data show the contributions from both Pauli and Curie spins, with the size of the Pauli term depending strongly on the doping level. Proton and fluorine NMR relaxation rates have been studied as a function of temperature (3-300 K) and field (for protons at 0.9, 9.0, 16.4, and 23.4 T, and for fluorine at 9.0 T). The temperature dependence of T-1 is classified into three regimes: (a) For T < (g mu(B) B/2k(B)), the relaxation mechanism follows a modified Korringa relation due to electron-electron interactions and disorder. H-1-T-1 is due to the electron-nuclear dipolar interaction in addition to the contact term. (b) For the intermediate temperature range (g mu(B) B/2k(B)) < T < T-BPP (the temperature where the contribution from the reorientation motion to the T-1 is insignificant) the relaxation mechanism is via spin diffusion to the paramagnetic centers. (c) In the high-temperature regime and at low Larmor frequency the relaxation follows the modified Bloembergen, Purcell, and Pound model. T-1 data analysis has been carried out in light of these models depending upon the temperature and frequency range of study. Fluorine relaxation data have been analyzed and attributed to the PF6 reorientation. The cross relaxation among the H-1 and F-19 nuclei has been observed in the entire temperature range suggesting the role of magnetic dipolar interaction modulated by the reorientation of the symmetric molecular subgroups. The data analysis shows that the enhancement in the Korringa ratio is greater in a less conducting sample. Intra-and interchain hopping of charge carriers is found to be a dominant relaxation mechanism at low temperature. Frequency dependence of T-1(-1) on temperature shows that at low temperature T < (g mu(B) B/2k(B))] the system shows three dimensions and changes to quasi one dimension at high temperature. Moreover, a good correlation between electrical conductivity, magnetic susceptibility, and NMR T-1 data has been observed.

Injection barrier induced deviations in space charge limited conduction in doped poly(3-methylthiophene) based devices

The carrier density dependent current-voltage (J V) characteristics of electrochemically prepared poly(3-methylthiophene) (P3MeT) have been investigated in Pt/P3MeT/Al devices, as a function of temperature from 280 to 84 K. In these devices, the charge transport is found to be mainly governed by different transport regimes of space charge limited conduction (SCLC). In a lightly doped device, SCLC controlled by exponentially distributed traps (Vl+1 law, l > 1) is observed in the intermediate voltage range (0.5-2 V) at all temperatures. However, at higher bias (> 2 V), the current deviates from the usual Vl+1 law where the slope is found to be less than 2 of the logJ-logV plot, which is attributed to the presence of the injection barrier. These deviations gradually disappear at higher doping level due to reduction in the injection barrier. Numerical simulations of the Vl+1 law by introducing the injection barrier show good agreement with experimental data. The results show that carrier density can tune the charge transport mechanism in Pt/P3MeT/Al devices to understand the non-Ohmic behavior. The plausible reasons for the origin of injection barrier and the transitions in the transport mechanism with carrier density are discussed. (C) 2015 AIP Publishing LLC.

CHRONOAMPEROMETRIC STUDIES ON MIGRATION AND DIFFUSION OF COUNTERION IN POLY(3-METHYLTHIOPHENE)

Small amplitude potential step experiments were carried out to study the counterion transfer process in oxidized poly(3-methylthiophene) (PMT) film. The results demonstrate that anion transfer process in PMT film is migration rather than diffusion. A porous metal electrode model-single hole model, which takes into account both the ionic resistance of the film and the uncompensated solution resistance, was found suitable to describe the potential step experiments. According to this model, the ionic resistivity of oxidized PMT film was calculated to be 5.0 x 10(4) OMEGA.cm, and, in turn, the diffusion coefficent of ClO4- ion in PMT film 3.7 x 10(-9) cm2/s.

DOPING EFFECTS ON THE POLYMER-CHAIN STRUCTURE OF POLY(3-METHYLTHIOPHENE)

The C=C stretching Raman shifts and photoluminescence (PL) for poly(3-methylthiophene) (P3MT) are measured at various doping levels by in situ electrochemical Raman and PL spectroscopic techniques. It is found that the doping for P3MT induces the nonlinear excitations (soliton, Polaron, bipolaron), but also affects the polymer-chain structure, including the conjugated length and the interchain distance.

INSITU CONDUCTIVITY ELECTROCHEMISTRY OF CONDUCTIVE POLYMERS - POLY(3-METHYLTHIOPHENE) AND POLYTHIOPHENE

In situ monitoring of conductivity and potential response of conductive polymers during electrochemical process had been described. A renewable carbon fibre array ring-glassy carbon disk electrode was used for this purpose. Poly(3-methylthiophene) and polythiophene were investigated with this method, and some 5 orders in magnitude of conductivity changes were observed during the electrochemical redox process.

ELECTROCHEMICAL STABILITY AND ELECTROCATALYSIS BEHAVIORS OF POLY(3-METHYLTHIOPHENE) THIN-FILM MODIFIED ELECTRODES

The Electrochemical stability of poly(3-methylthiophene) (PMT) thin film modified glassy carbon electrodes was investigated experimentally with successive cyclic voltammetry(CV) The effects of electrolyte solutions on the stability were studied. In the presence of small hydrated anions (less-than-or-equal-to 3.5nm) in the solution, the electroactivity of PMT films decreased with the characteristics of second order kinetics. In a solution with large hydrated anions (greater-than-or-equal-to 4 nm), PMT films have good stability. PMT/GO electrode can electrocatalyse the oxidation of Br- and Cl- anions, and loses its electroactivity rapidly. X-ray photoelectron spectra (XPS) have demonstrated that chlorine has bonded covalently onto the PMT structure after OV cycles in NaCl solutions.

Weak ferromagnetism in poly(3-methylthiophene)(PMTh)

Ferromagnetic behaviour at 300 K has been observed from SQUID experiments in ClO4- doped PMTh, the samples were prepared electrochemically at 25 degreesC in acetonitrile with 0.1M LiClO4 and then partially reduced. Atomic absorption analysis discards magnetic particles contamination. Hysteresis curves were observed for pressed pellets in ail range of temperatures (300K-2K). The remanence at 300 K and pressed at 250 bar was around 8.06x10(-4) emu/g with coercitivity of 130 Oe. The influence of water content in the solvent during the sample synthesis and the pressure is shown. We discuss a model that explains our data in terms of the anisotropic superexchange Dziatoshinski-Moriya interaction giving rise to weak ferromagnetism.

Room temperature ferromagnetic behavior in pressed pellets of doped poly (3-methylthiophene)

Room temperature ferromagnetic behavior has been observed in pressed pellets of doped poly(3-methylthiophene). In this work we show that thermoremance data taken in two different ways favours the interpretation of data in terms of the Dzialoshinski-Moriya anisotropic superexchange interaction of the polarons via dopant anions giving rise to weak ferromagnetism.

Huge values of room-temperature dielectric constants in pellets of poly(3-methylthiophene)

Room temperature data of impedance and phase angle in pellets of electrochemically synthesized ClO4- doped poly(3-methylthiophene) (P3MT) were analyzed assuming the sample being represented by a parallel resistor-capacitor (RC) circuit or by a series RC circuit. The last assumption proved to be the correct one, and to confirm it we use the sample as the RC component of a resistor-capacitor-inductor series resonator. We discuss the possibility of this RC series behavior to be due to a charge-density wave characteristic also evidenced from the huge values of the low-frequency dielectric constant of the system.

GIGAHERTZ CONDUCTIVITY OF PRESSED PELLETS OF CLO4--DOPED POLY(3-METHYLTHIOPHENE) OBTAINED FROM ELECTRON-SPIN-RESONANCE MEASUREMENTS

Gigahertz conductivity of pressed pellets of ClO4--doped poly( 3-methylthiophene) can be readily obtained from the asymmetry ratio (A / B) of the electron spin resonance line using Dyson's theory. The measurements were performed in three different frequencies, 1.3, 9.4, and 35 GHz. The temperature dependence of the gigahertz conductivity is sensitive to the heating rate, probably due to the ordering of the randomly assembled anions. (C) 1994 Academic Press, Inc.

Field-dependent conductivity at low electric fields in pressed pellets of doped poly (3-methylthiophene): evidence of charge-density wave depinning

Field-dependent conductivity at low electric fields was observed from low to room temperature in pressed pellets of doped poly(3-methylthiophene). The room temperature data showed good agreement with Bardeen's theory of charge-density wave depinning and the values of the parameters obtained are consistent with a strong electron-phonon interaction as expected for quasi-one dimensional systems. (C) 2003 Elsevier B.V. Ltd. All rights reserved.

Two-phase separation observed in thermally treated pellets of ClO4- doped poly(3-methylthiophene) from electron spin resonance measurements

A pressed pellet of CIO (-)(4) poly (3-methylthiophene) (P3MT) was heated for two hours at 85 degrees C and suddenly dropped in liquid nitrogen. A change was observed around 220 K in the Electron Spin Resonance (ESR) spectra when the sample was slowly cooled from room temperature. ESR line asymmetry parameter (A/B) showed two spatially separated phases. One was identified as a small metallic-like phase. The other phase, the larger one, makes a transition to a semiconducting Charge-Density Wave (CDW) state.

Crystallization observed from the spin behavior in poly(3-methylthiophene)

Electron Spin Resonance (ESR) data in pressed pellets of doped poly(3-methylthiophene) (P3MT) show a single ESR line that changes into a broad and a narrow superimposed lines below 225K. D.C. susceptibility measurements using a SQUID susceptometer didn't reveal any special feature at this temperature. Assuming crystallization of the polymer, we attribute the broad line to the crystalline fraction of the polymer and the narrow line to the amorphous one.