E. D. Pozhidaev

Charge carrier transport in polyvinylcarbazole

A critical analysis of the existing time-of-flight (TOF) data in poly(N-vinylcarbazole) (PVK) proves that these are highly controversial with claims and counterclaims about charge carrier transport (dispersive versus Gaussian). It is felt that the TOF method taken alone is incapable of resolving the standing dilemma. As a final means to resolve it, we propose a combination of two varieties of the TOF technique using both sheet-like and uniform carrier generation modes in conjunction with radiation-induced conductivity measurements. All three techniques are realized using the ELA-50 electron gun facility. To demonstrate the effectiveness of our approach we report experimental data for PVK, which show that carrier transport in this polymer is indeed dispersive. Evidence is presented substantiating the gross interference the surface traps could exert on the shape of a TOF transient. As a result, a preflight part of the TOF signal should not be used for parameter evaluation.

Time of flight results for molecularly doped polymers revisited

We examine the published data concerning the shape of the current transients obtained by the time of flight (TOF) technique for molecularly doped polymers and polyvinylcarbazole (PVK) and compare it with the predictions from the existing theories of charge carrier transport in disordered organic solids. We show that TOF current shapes frequently run contrary to theoretical predictions. Plateau appearances on TOF curves may or may not mean the equilibration of the charge carrier transport. Using both TOF and TOF-2 (uniform generation of charge carriers) techniques we demonstrate that hole transport in polycarbonate doped with p-diethylaminobenzaldehyde-diphenylhydrazone (DEH) and in PVK is indeed dispersive despite the fact that samples of these polymers equipped with an a-Se generation layer produce TOF curves with a plateau.

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.

Analysis of the time-of-flight transients in molecularly doped polymers using the Gaussian disorder model

Using published data for four molecularly doped polymers, which exhibit flat plateaus on the time-of-flight transients, we compared theoretical curves with experimental ones. The numerical calculations as well as parameter values were based on the Gaussian disorder model. In no case were flat plateaus predicted to appear. According to theory carrier transit should proceed in the non-equilibrium regime. We saw close agreement for the transit times in weakly polar polymers even at high fields but only at elevated temperatures, while in highly polar polymers similar agreement occurred only at low fields. In addition, the Gaussian disorder model does not account for the current shape universality (regarding field variation) frequently observed experimentally in polar molecularly doped polymers.

Charge Carrier Transport in a Molecularly Doped Polymer: Dispersive versus Gaussian

A typical molecularly doped polymer (bisphenol-A-polycarbonate containing 30 mass% of hydrazone DEH) has been put to thorough experimental examination by both time of flight and radiationinduced conductivity methods. A much improved measurement technique incorporating a computerassisted registration scheme that allows one to record a complete current transient over up to five decades in time in one single shot was employed. Studies of radiation-induced conductivity (conductivity proper as well as transit effects) uniquely prove the dispersive rather than the Gaussian transport of holes (majority carriers) in this polymer. The shape of the time-of flight current transients is strongly influenced by some extraneous factor, presumably surface traps, whose role in radiation-induced conductivity could hardly be detected. We describe the phenomenon quantitatively using a multiple trapping formalism. A critical discussion of the present situation in the field of charge carrier transport in disordered solids is also included.

Experimental and Theoretical Studies of Radiation-Induced Conductivity in Spacecraft Polymers

We have reviewed the basic results in the radiation-induced conductivity and the bulk charging of polymers obtained by our group during the past 30 years, which are mostly unknown in the West. Special attention is given to a new experimental technique extensively used in our investigations based on an electron-gun technology allowing combined induced conductivity and carrier mobility measurements. Quasi-band and hopping theories of the carrier transport are critically discussed.

Effect of charged centers on electron transport in molecularly doped polymers: Theory and experiment

The effect of charged centers on charge-carrier mobility in polycarbonate, a polar molecularly doped polymer, is studied. The nature of this effect is revealed, and a simplified physicomathematical model is proposed to describe it. The performed numerical calculations are qualitatively consistent with experimental results. Preliminary studies are conducted to elucidate the nature of the defective surface layer in molecularly doped polymer samples.

Theoretical analysis of the Rose-Fowler-Vaisberg model

Results of numerical calculations based on the Rose-Fowler-Vaisberg model of radiation-induced conductivity in a case polymer upon long (104 s) irradiation at doses of 5 × 105–107 Gy are reported. Two irradiation modes were considered: (1) preliminary irradiation and irradiation repeated at variable times after the end of the first irradiation and (2) probing the virgin and irradiated polymer with a standard pulse of ionizing radiation. It was shown that the properties of radiation-induced conductivity, such as its overshoot kinetics, a considerable difference between current transients for the initial and the repeated irradiation, extremely long annealing times of dose effects, and the absence of a steady state, are naturally explained in terms of this model (with allowance for the generation of radiation-induced traps as regards the last property). The Rose-Fowler-Vaisberg theory should be considered at present a well-approved semiempirical model of radiation-induced conductivity in polymers.

Hole Transport in Bisphenol-A Polycarbonate Doped with N,N′-Diphenyl-N,N′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine

The time-of-flight (TOF) transients of solution-cast, free-standing films of N,N′-diphenyl-N,N-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′diamine (TPD) in bisphenol A polycarbonate (PC) have been studied using electron gun induced charge generation. This molecularly doped polymer (MDP) has been shown to exhibit perfectly flat plateaus on its time-of-flight curves with optical excitation. Our TOF results with continuously changing electron energies, as well as numerical calculations using a multiple trapping model with a Gaussian trap distribution (MTMg), suggest that charge carrier transport in this molecularly doped polymer is nonequilibrium and the flat plateaus can be explained by the presence of a thin surface layer depleted of transport material. The depleted surface layers on samples of this molecularly doped polymer are extremely thin (less than 0.12 μm), with those relating to the release side (contacting a substrate during coating/drying procedure) being much smaller than for the free side exposed t...

The universal nature of dispersive transport in molecularly doped polymers

The time-of-flight technique is used to measure hole mobility in molecularly doped polycarbonate and polystyrene that contain both polar and weakly polar additives. The two versions of the technique with the bulk and surface generation of charge carriers under small-signal conditions are employed. Numerical calculations show that the time dependence of the transient-current curves obtained with the first version of the technique is in agreement with the theory of multiple trapping for an exponential energy distribution of traps. In the case of time-of-flight curves with surface generation, the run of the post-transit branch is likewise consistent with the theory, whereas this consistency is often violated for the pretransit branch of the curves. This result is due to the effect of the defective surface layer of a polymer, which is not taken into account in numerical calculations. The results show that the hole transport in the studied molecularly doped polymers is dispersive. An increase in the polarity of the polymer matrix and the dopant drastically decreases the hole mobility and, at the same time, increases its field and temperature dependence.