Since the discovery of electroluminescence in conjugated polymers it has been recognized that charge transport is a key ingredient for the efficiency of the polymer light-emitting diodes (PLEDs). This review focuses on the charge transport properties of these materials. From temperature dependent current density–voltage characteristics it has been obtained that the hole transport in poly(dialkoxy-p-phenylene vinylene) (PPV) is governed by a combination of space-charge effects and a field- and temperature-dependent mobility. The origin of the hole mobility, which seems to be generic for a large class of disordered materials, arises from hopping in a system with both energetic and structural disorder. The response time of PPV-based PLEDs is governed by the dispersive transport of holes towards the cathode. Based on the results of the electron- and hole-transport a device model for PLEDs is proposed in which the light generation is due to bimolecular recombination between the injected electrons and holes. The unbalanced electron and hole transport gives rise to a bias dependent efficiency. By comparison with experiment it is found that the bimolecular recombination process is of the Langevin-type, in which the rate-limiting step is the diffusion of electrons and holes towards each other. The occurrence of Langevin recombination explains why the conversion efficiency of current into light of a PLED is temperature independent. The understanding of the device operation of PLEDs indicates directions for further improvement of the performance.

Charge transport in poly(p-phenylene vinylene) light-emitting diodes

Ni/polyaniline (PANi) nanocomposites were prepared by chemical polymerization, and electromagnetic characteristics were then studied at 2–18GHz. The permittivity of the Ni/PANi nanocomposite presents dual dielectric relaxations with increasing content of PANi to over 15.6wt%, which is ascribed to a cooperative consequence of the core/shell interfaces and the dielectric PANi shells. Additionally, the permeability presents a strong natural resonance around 2–8GHz, which is dominant among microwave magnetic loss. The proper matching of the permittivity and the permeability contributes to enhanced microwave absorption.

Enhanced microwave absorption in Ni/polyaniline nanocomposites by dual dielectric relaxations

Polymeric electro-optic materials have recently been developed that, when fabricated into devices such as Mach-Zehnder interferometers, permit drive ( Vπ) voltages of less than 1 V to be realized at the telecommunications wavelength of 1.3 and 1.55 microns. Operation of polymeric electro-optic modulators to frequencies (bandwidths) of greater than 100 GHz has been demonstrated. The total insertion loss of polymeric electro-optic modulators has been reduced to values as low as 5 dB, which is competitive with values obtained for lithium niobate modulators and is much lower than that obtained for gallium arsenide electro-absorptive modulators. Polymeric electro-optic modulators can be operated for long periods of time at temperatures on the order of 100 °C. Techniques have been developed for seamlessly integrating polymeric electro-optic circuitry with passive low loss optical circuitry (e.g., silica long-haul transmission fiber and medium-range fluoropolymer fibers) and with very large scale integration (VLSI) semiconductor electronics. These advances have created a considerable interest in the commercialization of polymeric electrooptic materials. Polymeric electro-optic materials are now being evaluated for applications such as phased array radar, satellite and fiber telecommunications, cable television (CATV), optical gyroscopes, electronic counter measure (ECM) systems, backplane interconnects for high-speed computers, ultrafast (100 Gbit/s) analog-to-digital (A/D) converters, land mine detection, radio frequency (rf) photonics, and spatial light modulators. This review discusses the structure-function relationships that had to be defined and the synthesis and processing advances achieved before materials appropriate for commercialization could be produced. Topics discussed include the design and synthesis of chromophores that simultaneously exhibit large molecular hyperpolarizability, low optical absorption, processability (e.g., solubility in various processing media), and the prerequisite (thermal, chemical, electrochemical, and photochemical) stability to survive conditions encountered in the fabrication and operation of polymeric electro-optic devices. Chromophore-chromophore intermolecular electrostatic interactions have been shown to be the most serious problem impeding the optimization of electro-optic activity in organic materials. The quantitative theoretical treatment of such interactions by equilibrium and Monte Carlo statistical mechanical methods is discussed. Rules for designing chromophores with shapes leading to maximum obtainable electro-optic activity are discussed and the synthetic realization of such structures is reviewed. The role of electrostatic interactions in influencing the choice of processing conditions is also discussed. A number of processing steps, including spin casting, electric-field poling, lattice hardening, fabrication of buried channel waveguides (including deposition of cladding layers), electrode deposition and connection with electronic circuitry, and integration of active and passive optical circuitry, are required. Each of these steps can affect device performance (e.g., influence optical loss, electro-optic activity, and stability). The systematic optimization of each of these steps is reviewed. Finally, device design and operation are reviewed and speculation on the future of the field is expressed.

Nonlinear Optical Polymeric Materials: From Chromophore Design to Commercial Applications

Thermally stimulated discharge of electrets

A review of the corona charging of polymers is presented. After providing a brief account of the corona discharge process and of its application for charging materials, the paper focuses on the evolution of corona triodes that allow the sample surface potential to be monitored during the charging process. Particular emphasis is given to the main contributions arising from research, basically on Teflon FEP and ferroelectric polymers, in which a constant-current corona triode was used. Such a triode system has become a powerful tool in the study of charge transport and ferroelectric properties of polymers, because keeping the charging current constant facilitates matching of the experimental results to a number of theoretical models. >

Corona charging of polymers

A series of measurements carried out with the point‐to‐plane symmetry has shown that positive corona confirms the Townsend relation while for the negative corona this happens only at large point‐to‐plane distances (greater than 1.5 cm). In this case, the ratio of the prefactors in Townsend relation, for the same geometry, approaches the expected constant (the ratio of the mobilities) at larger distances, around 6 cm. For distances below 1.5 cm, the square root of the current is linear with the voltage, with an intercept of ∼1.4 kV. The results are interpreted as giving evidence of an electronic component at small distances, as suggested by Sigmond. The characteristics change to Townsend at higher distances when the electronic component becomes small as compared with the ionic one.

Point-to-plane corona: current-voltage characteristics for positive and negative polarity with evidence of an electronic component

The laser-induced pressure pulse method (LIPP) is employed to investigate the electric field distribution in soda-lime glass samples after application of a dc potential (dc poling). The modifications of near-surface composition resulting from the dc polarization procedure were determined by means of three nuclear techniques for near-surface analysis: Rutherford backscattering spectrometry (RBS), elastic recoil detection analysis (ERD) and nuclear reaction analysis. An alkali-depleted layer is formed near the anode. This depletion layer persists for a long time after the dc poling. The electric field in this region is of the order of the intrinsic dielectric breakdown, and its configuration depends on the penetration, or not, of hydrogen. Based on these results, the conduction mechanism in the alkali-depleted layer is discussed. It is suggested that when hydrogen blocking electrodes are used ionized electrons are probably the charge carriers in the alkali-depleted layer.

Electric field distribution and near-surface modifications in soda-lime glass submitted to a dc potential

This paper reports results of measurements of radiation‐induced conductivity of Teflon (Teflon is a registered trademark of the E. J. du Pont de Nemours and Co.). FEP (polyfluorethylene propylene) foils irradiated by x rays at atmospheric pressure. The current induced by irradiation of the poled dielectric at exposure rates of the order of 102 R/s, initially increases with time for 10–20 s, reaches a maximum, and subsequently decreases. After an irradiation time of about 1 h, a steady‐state value is attained which is of the order of 25% of the maximum current. Measurements are reported of the dependence of the current amplitudes on exposure rate, of the delayed component of induced conductivity, and of the behavior of samples which had been irradiated in short‐circuit before poling. Radiation‐induced polarization effects or space‐charge formation could not be detected. The behavior of the current‐time curves is interpreted in terms of a theory of unipolar (hole) conduction.

Radiation‐induced conductivity in Teflon irradiated by x rays

The ac complex conductivity σ*(f) of polyaniline (PAN) films at different doping levels and different temperatures, in the 1–100 KHz frequency range, are reported The results are typical of a disordered medium where the real component of ac conductivity is frequency independent at low frequencies, rising for higher values of frequencies In order to interpret both the real and the imaginary components of σ*(f), we developed a model which considers the doped PAN as a disordered insulating matrix, sprinkled with conductive islands generated by doping, as indicated by energy dispersed x-ray microanalysis The conduction through the insulating matrix obeys the random free energy barrier model, while in the conductive islands a metallic frequency-independent conductivity is considered From the fittings we obtained the activation energy value of the maximum energy barrier of the doping mechanism and estimated the concentration of hopping sites

G. F. Leal Ferreira

Papers

Point-to-plane corona: current-voltage characteristics for positive and negative polarity with evidence of an electronic component

Electric field distribution and near-surface modifications in soda-lime glass submitted to a dc potential

Radiation‐induced conductivity in Teflon irradiated by x rays

Alternating electrical conductivity of polyaniline

Charge Storage and Transport in Electron-Irradiated and Corona-Charged Dielectrics