Geminate Pairs and Their Role in Radiation-Induced Conductivity of Poly(methyl methacrylate)

Non-steady-state radiation-induced conductivity of poly(methyl methacrylate) was studied at room and elevated temperatures. It was shown that the conductivity is due to electric polarization of geminate pairs. The evolution of geminate pairs was rationalized in terms of the Rose–Fowler–Weisberg generalized model. The conclusions by the model were compared with the results of previous studies on the kinetics of formation and decay of geminate pairs upon pulse radiolysis or flash photolysis of doped poly(methyl methacrylate).     

Electric Conductivity of Sandwich Structures with Films of Poly-N-epoxypropylcarbazole Doped with Organic Dyes of Various Ionicity

The conducting characteristics of samples of a sandwich structure with films based on the photoconducting polymer poly-N-epoxypropylcarbazole doped with cationic, anionic, cationic–anionic, neutral, and bipolar organic dyes were investigated. It was shown that the conductivity of the samples is caused by ion drift, thermofield generation of holes from uncontrollable impurity centers, thermofield generation of electrons and holes from the dopant molecules, and thermofield injection of electrons and holes from the electric contacts. The contribution from the injection currents of the charge carriers increases in the transition from a cationic dye to anionic, cationic–anionic, and intraionic dyes.     

Generation of excess charge carriers in molecularly doped polymers by electron-beam irradiation

The processes of generation and transport of excess charge carriers in molecularly doped polycarbonate and polystyrene were experimentally studied at room temperature. The polymers were ionized by 7-and 50-keV electrons. The radiation-chemical yield of free ions was determined by means of the universal method based on the combination of the time-of-flight technique in two carrier generation modes (surface and bulk generation) with the measurement of nonsteady-state conductivity. It was shown that the radiation-chemical yield of free ions under irradiation by 7-keV electrons is almost the same as that in the case of 50-keV electrons, despite the substantially different values of the linear energy transfer for these electrons.     

The effect of temperature on carrier transport in molecularly doped polycarbonate

Hole transport in low-polarity polystyrene (PS) doped with 10 wt % tritolylamine was studied. The radiation-induced mode of the time-of-flight technique (TOF) with the carrier generation zone of a variable thickness was used. A theoretical treatment of the data in terms of the Gaussian disorder model has shown the mobility value to be fundamentally inconsistent with the flat shape of the plateau, a contradiction that cannot be resolved within the framework of this model. It has been shown that hole transport is actually dispersive, rather than quasi-equilibrium. The contribution of radiation conductivity of the polymer matrix to the TOF signal was evaluated.     

Radiation-Induced Conductivity of Plasticized Flexible-Chain Polymers

The radiation-induced conductivity of cured and transformer-oil extended polybutadiene and polyisoprene rubbers was studied. It was shown that the extender ratio and degree of cure of a polymer system have a much greater effect on the radiation conductivity than the addition of donor–acceptor dopants trapping mobile charge carriers. It was concluded that the molecular mobility is the major factor determining the electron transport properties of the systems of interest.     

Field-dependent photogeneration in molecularly doped polymers

The photogeneration of holes in triphenylamine- doped polycarbonate, polystyrene, polyvinyl butyral has been investigated. The field dependence of the photogeneration efficiencies can be explained by the Onsager theory of geminate recombination. The results show that the number of bound electron-hole pairs that are formed per absorbed photon is strongly dependent on the polymer composition whereas the pair disassociation probability is independent of the polymer.     

Time resolved observation of geminate recombination in metal-free phthalocyanine

Delayed-collection-field measurements of carrier generation and delayed fluorescence in metal-free phthalocyanine have been shown to exhibit the same time dependence. This indicates the existence of geminate electron—hole pairs with survival times extending to tens of milliseconds.     

Photogeneration of holes and electrons in amorphous molecular semiconductors with neutral and ionic organic dyes

Photoconducting properties of amorphous molecular semiconductors based on polystyrene films doped with tetranitrofluorenone and merocyanine or an anionic polymethine dye, or with epoxypropylcarbazole and merocyanine or a cationic polymethine dye were studied. The former type of the films is characterized by electron conductivity, whereas the latter type by hole conductivity. The activation energy for photogeneration of mobile charge carriers increases on passing from a merocyanine dye to ionic dyes and decreases with a growth in quantum energy of excitation light for the films of the former type, but does not depend on the light quantum energy for the films of the latter type. It was concluded that the activation energy of photogeneration is determined by electrostatic interaction of a photogenerated charge carrier with the ionized photogeneration site or a counterion for neutral and ionic dyes, respectively. At low dissipation rates of the excess thermal energy of excited dye molecules via electron-nucleus interaction, photogenerated electrons have a possibility to travel over a long distance from the photogeneration site as compared with holes.__________Translated from Khimiya Vysokikh Energii, Vol. 39, No. 3, 2005, pp. 195–203.Original Russian Text Copyright © 2005 by Davidenko, Derevyanko, Zabolotnyi, Ishchenko, Kuvshinskii, Studzinskii.     

Charge Carrier Transport in Disordered Organic Matrices

Various aspects of charge carrier transport in solid disordered matrices are considered. It is shown that the field dependence of mobility is defined by correlation properties of the material, specifically, by the law of spatial decay of the correlation function energy–energy. The well known Poole–Frenkel dependence of mobility is observed in polar materials and is connected with an exceedingly slow decay of the correlation function in dipole glass. The true behavior of mobility in nonpolar materials differs from the Poole–Frenkel dependence but is barely distinguishable from it in a narrow range of fields studied experimentally. The charge transport in media with short-range and charged traps is considered. It is shown that the electrode roughness affects not only the efficiency of injection of charge carriers but their transport characteristics as well.     

A way for evaluating parameters of electron transport in non-polar molecular liquids derived from analysis of the trapped electron recombination kinetics

The geminate recombination kinetics of electron-ion pairs produced by high energy radiation in liquid hydrocarbons is considered in the two state model of electron transport. The purpose of the study is to relate the trapped electron transient optical absorption, observed in the pulse radiolysis experiments, to fundamental parameters of electron transport in liquid. It is shown that measurements of the half-life time and amplitude of the trapped electron decay curve allow one to find the electron life time in a localized state.     

A tandem mechanism of charge-carrier photogeneration in disordered organic materials

A model of tandem charge-carrier photogeneration in disordered organic materials at low excess photon energies above the absorption edge is formulated. It suggests that relaxed singlet excitons dissociate into strongly bound metastable short off-chain geminate pairs. In parallel, triplet excitons can be generated from both primary singlets and short geminate pairs. Owing to a long intrinsic lifetime of triplets, most of them are quenched by geminate pairs and the latter can gain additional energy that facilitates transformation of short pairs into longer ones. This reduces the Coulomb binding energy such that field-assisted full dissociation into free carriers becomes feasible.     

Specific features of physical chemistry and electronic structure of lower dicarboxylic acids

The static and complex dielectric permittivities and dc electrical conductivities of solid oxalic, malonic, and succinic acids were measured. The relaxation times of single crystal blocks (3–5 days) and mean relaxation frequencies of molecules and low-molecular-weight associates of oxalic acid dihydrate (5 MHz) and malonic acid (160 kHz) at 19°C were estimated. Aqueous oxalic acid can absorb electromagnetic energy owing to the presence of solvated electrons and holes. The electrical conductivity of this acid is protonic, and that of the anhydrous acids, electronic; the electron-hole pairs are mainly generated in these acids by lattice vibrations. The applied alternating electric field caused desorption of active gases from the acids. In some experiments with malonic acid, fast isothermal rearrangements of hydrogen bond networks and avalanche ionization of molecules were observed at a slight increase in temperature. The diagrams of charge distribution in the free acid molecules were constructed on the basis of the previous dipole moment measurements and IR data.Translated from Zhurnal Obshchei Khimii, Vol. 74, No. 11, 2004, pp. 1828–1834.Original Russian Text Copyright © 2004 by Ponomarenko, Borovikov, Sivachek.This revised version was published online in April 2005 with a corrected cover date.     

Radiation-induced conductivity in polymers during long-term irradiation

Radiation-induced conductivity (RIC) in polystyrene, poly(ethylene terephthalate), polyvinylcarbazole, and low-density polyethylene during long-term (to 3.6 × 10³ s) irradiation with 50-keV electrons (dose rate of 6–830 Gy/s) was experimentally and theoretically studied. It was shown that the nonmonotonic RIC kinetics in the polymers is a direct consequence of the generation and the subsequent transport of charge carriers in them in the presence of traps distributed over a broad energy range almost according to the exponential law. This phenomenon has no relation to degradation and crosslinking processes that occur in irradiated polymers. The nonmonotonic RIC kinetics in polymers is a universal phenomenon, and it is described satisfactorily in terms of the Rose-Fowler-Vaisberg model.     

共轭聚合物薄膜中光生载流子的产生及输运机制

对共轭聚合物光生载流子的产生机制进行了初步探讨,分析了由最初产生的电子 空穴对经过晶格驰豫之后形成极化子 激子的热离化过程,认为同一链上的激子会迅速复合,只有链间激子对光电流作出贡献.研究了共轭聚合物中载流子的输运机制,导出了共聚物的电导率公式,其计算值与实验结果符合,我们认为是极化子的链间跃迁实现了整个共聚物的电导和光致发光,较好地解释了实验事实.     

Picosecond geminate charge pair recombination in liquid hexane

We review recent measurements of the recombination kinetics of geminate cation-electron pairs produced by two-photon ionization of anthracene and benzene in liquid hexane at temperatures from 191 to 296 K. Two techniques, infrared-stimulated conductivity and transient absorption by geminate electrons are compared and found to agree reasonably well. The time-dependent diffusion theory of geminate charge pair recombination is shown to provide a satisfactory fit to the decay kinetics.     

Determination of the effective concentration of free charge carriers in solid proton-conducting electrolytes

A method for the theoretical determination of the effective concentration of free charge carriers in solid proton-conducting electrolytes with the use of experimentally measured values of the electrical conductivity has been proposed. Polyantimonic acid has been considered as an example. It has been shown that the high electrical conductivity of solid proton-conducting electrolytes in comparison to other compounds with hydrogen bonds is due to the high concentration of effective free charge carriers and the low activation energy for the mobility of a proton.Translated from Teoreticheskaya i éxperimental'naya Khimiya, Vol. 24, No. 1, pp. 111–114, January–February, 1988.     

Composite Nickel–Polystyrene Electrodes: Surface State of Nickel and Ion Transfer

Nickel–polystyrene composite materials produced by the chemical deposition of nickel on polystyrene beads exhibit high conductivity at a nickel concentration of less than one percent by volume. These composites exhibit a higher sensitivity to Ni²⁺ions (RT/zF) than bulk nickel. According to X-ray photoelectron spectroscopy data, the increased sensitivity of nickel–polystyrene electrodes is due to the formation of a modified surface layer of the composite containing a nickel–polystyrene complex and to direct Ni²⁺ion exchange at the interface between the solution and this layer.     

Electron transport of holes in molecularly doped polycarbonate and its radiation-induced conductivity

The salient features of charge transport in a typical molecularly doped polymer (polycarbonate + 30 wt % DEH hydrazone) were studied by time-of-flight and nonsteady-state radiation-induced conductivity measurements. It was shows that the mobility of holes (major carriers) is due to dispersive transport in the temperature range 296–353 K covering the glass transition temperature at an observation time of up to a few seconds. The appearance of a plateau on the current transient, presumably manifesting the establishing of quasiequilibrium (Gaussian) transport, is the artifact of the time-of-flight technique when the charge carrier generation takes place at the sample surface. All of the obtained results can be satisfactorily rationalized in terms of the Rose-Fowler-Weisberg model with a uniform set of parameters of the model. Such an approach is compatible with the basic concepts of the radiation chemistry of condensed phase (the Onsager theory and the Langevin recombination mechanism), structural features of a disordered medium (transport zone, structural traps), and rotational diffusion of small molecules or their molecular groups in vitrified polymers.     

Exciton dissociation in conjugated polymers

In conjugated polymers, a majority of photogenerated charges form metastable geminate pairs (GPs), of which only some fraction can dissociate completely. Both the yield of GP photogeneration and the probability of further dissociation of GPs into free charges depend upon an external electric field. In the present article we discuss several experimental methods to detect the existence of geminate pairs such as delayed field collection of charges, field quenching of fluorescence, and field-assisted photoinduced optical absorption. It is shown that the field dependences of the exciton dissociation into GPs and of the free carrier photogeneration yield are rather similar. This is in contrast with the traditional Onsager theory, which assumes field-independent yield of primary photoionization and disregards the field dependence of the initial separations between carriers in GPs.     

Direct observation of f-pair magnetic field effects and time-dependence of radical pair composition using rapidly switched magnetic fields and time-resolved infrared methods

A rapidly switched (<10 ns) magnetic field was employed to directly observe magnetic fields from f-pair reactions of radical pairs in homogeneous solution. Geminate radical pairs from the photoabstraction reaction of benzophenone from cyclohexanol were observed directly using a pump-probe pulsed magnetic field method to determine their existence time. No magnetic field effects from geminate pairs were observed at times greater than 100 ns after initial photoexcitation. By measuring magnetic field effects for fields applied continuously only after this initial geminate period, f-pair effects could be directly observed. Measurement of the time-dependence of the field effect for the photolysis of 2-hydroxy-4-(2-hydroxyethoxy)-2-methylpropiophenone in cyclohexanol using time-resolved infrared spectroscopy revealed not only the presence of f-pair magnetic field effects but also the ability of the time dependence of the MARY spectra to observe the changing composition of the randomly encountering pairs throughout the second order reaction period.