I. Sakellis

Universal frequency-dependent ac conductivity of conducting polymer networks

A model based on the aspect of the distribution of the length of conduction paths accessible for electric charge flow reproduces the universal power-law dispersive ac conductivity observed in polymer networks and, generally, in disordered matter. Power exponents larger than unity observed in some cases are physically acceptable within this model. A saturation high frequency region is also predicted, in agreement with experimental results. There exists not a “universal fractional power law” (and it is useless searching for a unique common critical exponent) but a qualitative universal behavior of the ac conductivity in disordered media.

Hopping charge transport mechanisms in conducting polypyrrole: Studying the thermal degradation of the dielectric relaxation

Isotherms of the imaginary part of the permittivity from 10−2to2×106Hz from liquid nitrogen to room temperature for fresh and thermally aged specimens of conducting polypyrrole reveal a dielectric loss peak, which is affected by the reduction of conducting grains with aging. Charge trapping at the interfaces separating the conductive islands seems invalid. Thermal aging indicates that macroscopic conductivity and short range one have different aging evolution. The first (dc conductivity) is dominated by the tunneling of the carriers between neighboring grains through the intermediate insulating barriers, though the second (ac conductivity) is due to a backward-forward movement of the carriers and is controlled by the intrachain transport of them and their hopping between the chains.

Surface passivation effect by fluorine plasma treatment on ZnO for efficiency and lifetime improvement of inverted polymer solar cells

Zinc oxide (ZnO) is an important material for polymer solar cells (PSCs) where the characteristics of the interface can dominate both the efficiency and lifetime of the device. In this work we study the effect of fluorine (SF6) plasma surface treatment of ZnO films on the performance of PSCs with an inverted structure. The interaction between fluorine species present in the SF6 plasma and the ZnO surface is also investigated in detail. We provide fundamental insights into the passivation effect of fluorine by analyzing our experimental results and theoretical calculations and we propose a mechanism according to which a fluorine atom substitutes an oxygen atom or occupies an oxygen vacancy site eliminating an electron trap while it may also attract hydrogen atoms thus favoring hydrogen doping. These multiple fluorine roles can reduce both the recombination losses and the electron extraction barrier at the ZnO/fullerene interface improving the selectivity of the cathode contact. Therefore, the fabricated devices using the fluorine plasma treated ZnO show high efficiency and stable characteristics, irrespective of the donor : acceptor combinations in the photoactive blend. Inverted polymer solar cells, consisting of the P3HT:PC71BM blend, exhibited increased lifetime and high power conversion efficiency (PCE) of 4.6%, while the ones with the PCDTBT:PC71BM blend exhibited a PCE of 6.9%. Our champion devices with the PTB7:PC71BM blends reached a high PCE of 8.0% and simultaneously showed exceptional environmental stability when using the fluorine passivated ZnO cathode interlayers.

Low frequency dielectric relaxation phenomena in conducting polypyrrole and conducting polypyrrole-zeolite composites

The dielectric properties of polypyrrole-zeolite composites up to 50%w∕w zeolite are studied in the frequency range from 10−2to2×106Hz from room temperature to liquid nitrogen temperature. The complex permittivity formalism reveals a temperature dependent relaxation in all samples except for the 25%w∕w zeolite composite. The frequency fmax where a maximum of a loss peak is located varies with temperature by the Williams-Lander-Ferry law. The values of the activation energy of the relaxation process (which are of the order of polaronic dc conductivity) have the tendency to reach a minimum in the 25%w∕w composition, which is a loss-free composite. The 50%w∕w zeolite behaves as a dielectric where ionic relaxation dominates. The temperature variation of the strength of the dielectric mechanism follows a Curie law, apart from 50%w∕w zeolite where the dielectric strength is practically constant. The frequencies, where loss peaks are maximum, as well as dc conductivity follow qualitatively the same temperature l...

Correlation of the scaling exponent γ of the diffusivity-density function in viscous liquids with their elastic properties

Fundamental thermodynamical concepts and a solid-state point defect elastic model are used to formulate a diffusivity-density scaling function for viscous liquids. It is proved in a straightforward manner that the scaling exponent gamma describing the density scaling of the diffusivity is related with the pressure derivative of the isothermal bulk modulus.

Migration volume for polaron dielectric relaxation in disordered materials

A theoretical study of the influence of pressure on the dielectric relaxation related with polaron tunneling and phonon-assisted hopping in disordered solids is developed. The sign and absolute value of the migration volume, which is obtained by employing the present formulation, evidence the nature of the relaxation. As a paradigm, positive and negative values of migration volume are evaluated by analyzing recently published dielectric loss measurements under pressure in semiconducting polypyrrole. A straightforward relation between the value of the migration volume and the nature of short-range polaron flow and the size of polaron distortion is revealed.

An insight into the localization of charge carriers in conducting polyaniline by analyzing thermally stimulated depolarization signals

A dielectric relaxation mechanism, which is attributed to the localized motion of trapped polarons, has been recorded in conducting polyaniline by employing the thermally stimulated depolarization current technique. The signal was analyzed within the frame of the normal distribution in the activation energy value. The experimental dielectric relaxation results were manipulated in order to evaluate the length of the jump distance that the trapped polarons transfer along and the concentration of trap centers. The concentration of trapped carriers is calculated from two different viewpoints: the pair approximation that assumes phonon-assisted tunneling through the barrier separating two adjacent sites and the pinning model that considers the trapped polaron oscillating around its pinning point. Both models provide compatible results.

Separation of electric charge flow mechanisms in conducting polymer networks under hydrostatic pressure

To distinguish between different electric charge flow mechanisms in conducting polymer networks, the authors performed ac conductivity and complex permittivity measurements in conducting polypyrrole at various hydrostatic pressure values. Pressure influences capacitive coupling of chains or conducting grains. The measurement of ac conductivity and complex permittivity in the frequency domain at different hydrostatic pressures seems to be a tool for resolving different conductivity modes in conjugated polymer networks and understanding electric signal propagation in random networks.

On the origin of time-temperature superposition in disordered solids

In this paper by employing various experimental facts for different classes of materials, two critical conditions are derived, which lead to the empirically used scaling relations describing the ac response of disordered nonmetals. In parallel the origin of time-temperature superposition is explored while the scenario of time-pressure (or /temperature) superposition is introduced. Moreover, a unified equation for the entire frequency range of ac conductivity is proposed. The results are physically visualized within the frame of percolation theory, attempting to establish a link to the dominant approaches of Hunt [Philos. Mag. B 81, 875 (2001)] and Dyre [J. Appl. Phys. 64, 2456 (1988)].

Scaling and universality of ac conductivity and dielectric response in disordered materials under pressure

Scaling of physical quantities describing the ac response is a common feature of disordered matter, while in some cases, empirical functions have been proposed. The exact forms of analytic functions that scale complex conductivity and permittivity are derived in this work by exploring the effect of pressure at various temperatures on the ac conductivity of polypyrrole. The scaling behavior is visualized within the frame of percolation and self-similarity.