Electron mobilities have been measured in N,N′-bis(1,2-dimethylpropyl)-1,4,5,8-naphthalenetetracarboxylic diimide doped poly(styrene) containing a series of acceptor traps: 4-(cyanocarboethoxymethylidene)-2-methyl-1,4-naphthoquinone (MNQ), 3,5-dimethyl-3′,5′-diisopropyl-4,4′-diphenoquinone (DPQ), 4H-1,1-dioxo-2,6-di-tert-butyl-4-(dicyanomethylidene)thiopyran (TBS), N,N′-dicyano-2-tert-butyl-9,10-anthraquinonediimine (DCAQ), and 4H-1,1-dioxo-4-dicyanomethylidene-2-p-tolyl-6-phenylthiopyran (PTS). From reduction potential measurements, the trap depths of MNQ, DPQ, TBS, DCAQ, and PTS are 0.19, 0.19, 0.20, 0.35, and 0.40 eV, respectively. The mobilities decrease with increasing trap depth and trap concentration. The results are discussed within the framework of the Hoesterey-Letson formalism and the recent simulations of Wolf and co-workers and Borsenberger and co-workers.

Hole mobilities have been measured in polystyrene films doped with a mixture of two very similar triphenylamine derivatives at an overall concentration of 20% by weight. The data were analyzed in the framework of a formalism based on disorder due to Bässler and coworkers. The key predictions of the formalism concern the field and temperature dependencies of the mobility from which the key parameters σ, the energy width of the hopping site manifold, and ∑, the positional disorder parameter, can be determined. The experimental results from this study are in good agreement with the predictions of the formalism, although some of them are difficult to explain on arguments based on disorder. This is especially true for the energetic disorder parameter σ that was found to be smaller for films that contained 5% of the higher oxidation potential compound (an antitrap thus) compared to polystyrene layers doped with 20% of the pure compounds. But even the trap case cannot be fully explained on the basis of trapping arguments or the dipolar disorder model. Therefore, the results confirm earlier findings that doubly-doped polymers show some other contribution to charge transport than merely disorder related effects.

The low-temperature thermally stimulated luminescence (TSL) technique has been applied for the first time for probing the energetic disorder of localized states in molecularly doped polymers (MDPs) as substituted triphenylamines doped into poly(styrene) (PS). TSL of both the neat MDPs, as tri-p-tolylamine (TTA) and tri-p-anisylamine (TAA) doped PS, and the doubly doped polymers, as TTA doped PS containing small concentrations of di-p-anisyl-p-tolylamine (DAT), was studied. The results are described in terms of the Gaussian disorder model and the energetic relaxation of photogenerated charge carriers, that provide reasonable understanding of all observed trends in the TSL. Analysis of both the energetic position of the TSL peak maximum and the shape of its high-energy wing allowed extraction of a parameter characterizing the energetic disorder in MDPs, which agreed well with the width of the density-of-states determined from charge transport measurements. The effect of extrinsic trapping because of DAT on TSL properties can be reasonably interpreted in terms of the effective energetic disorder and the TSL results are in good agreement with those obtained earlier by charge transport studies.

The purpose of the present work is to delineate similarities as well as differences concerning the charge transporting properties of π-conjugated polymers as well as of discotic liquid crystals. Materials investigated are a (i) ladder-type poly-paraphenylene (LPPP), (ii) a member of the phenylenevinylene family, and (iii) several asymmetrically substituted triphenylenes. The disorder formalism explains the field and temperature dependence of the mobility adequately provided that the disorder, which is controlled by the sample topology and random dipolar electric fields, is sufficiently large. Differences are noted in highly ordered LPPP and in symmetric discotic crystals. It is conjectured that finite size effects in the case of LPPP and dynamic effects in the liquid crystals overcompensate the effect of energetic disorder.

Measurement of the drift mobility defined as the proportionality constant of the mean velocity (v) to the electric field strength (E) is necessary for understanding of carrier transport. However, it is difficult to obtain v from the time-of-flight transients. Thus the velocity obtained from the transit time has been analyzed instead of the mean velocity. The most common measurement of the mobility (μ_{k_ex}) is obtained from the time derived from the intersection of the asymptotes to the plateau and tail of the transients. Because the long tail of photocurrent transients for molecularly doped polymers indicates anomalous dispersion of carrier transit times, the difference between ν/E and μ_{k_ex} is not negligible. Recently, a theoretical photocurrent transients equation (PTE) has been introduced. Fitting of the PTE to nondispersive transients gives the values of ν and the diffusion coefficient (D) simultaneously. In this article, using the PTE, both the mobility (μ^k_cal), obtained from a kink in the photocurrent transient, and the tail-broadening parameter (W_cal) were derived as functions of ν, D and sample thickness. We have tried to explain the anomalous behavior of μ_{k_ex} and the tail-broaden-ing parameter (W_ex) in order to verify the PTE. The dependences of μ_{k_ex} and W_cal on the electric field and the sample thickness satisfactory agreed with those of μ_{k_ex} and W_ex. These verify the PTE and suggest that fitting of the PTE to photocurrent transients is suitable way to obtain the drift mobility.

Transient time-of-flight methods with voltage pulse injection have been adapted to determine the field dependent mobility of holes in the electroluminescent polymer, MEH-PPV. The time-resolved current response confirms that gold forms an Ohmic contact to MEH-PPV and that there are very few traps in the polymer. These conclusions are in agreement with earlier interpretation of steady-state current-voltage measurements.

The carrier transport properties of a molecularly diluted smectic liquid crystalline photoconductor, 2-(4′-octylphenyl)-6-dodecyloxynaphthalene (8-PNP-O12) and 2-(4′-hexyoxy)-6-octypbiphenyl (6O-BP-8) system, were investigated by time-of-flight technique, in order to clarify the nature of electronic conduction in the liquid crystalline mesophases. The mobility in the diluted liquid crystals was ambipolar, independent of both electric field and temperature in SmA and SmB phase as in the pure 8-PNP-O12, and continuously reduced with an increase in the diluent concentration. The reduction, however, remained within a small range of one third of that of pure material even in 60 mol%. The carrier transport in the diluted liquid crystals was described by the relation of a μ/² ∝ exp(-2 ρ/ρ₀), where μ is the mobility, ρ the average hopping distance, and ρ₀ a wavefunction decay constant of molecular orbital, indicating the 2-dimentional random hopping mechanism. The fairly large ρ₀ of 2.3 ∼ 2.4 A characterizes a fast mobility gently decreasing with an increase in the diluent concentration. The molecular ordering within a smectic layer did not affect the carrier transport properties at all except the initial difference of the mobility, as far as comparison of those in SmA and SmB phases were concerned. In addition, the effect of self-organization of hopping site is discussed in terms of carrier transport in disorded materials system.

We previously reported that contact injection efficiencies are amenable to direct measurement in thin trap-free hole transport polymer films by a technique combining time-of-flight bulk mobility measurements with steady state current densities measured at the contact under test. In the present article, films of a trap-free molecularly doped polymer, TPD/polycarbonate, are solution coated onto a carbon-filled polymer substrate that is demonstrably ohmic for hole injection. Thermally evaporated Au and Ag as well as liquid Hg form the top contacts. Field dependent contact injection efficiencies are computed from the combined measurements and monitored over time. A persistent pattern in the evolution of contact injection efficiency with time is revealed. Invariably contact injection efficiencies evolve by orders of magnitude from initially blocking to ohmic or to an equilibrium value dependent on the nature of the metal. For thermally evaporated Au contacts, coating studies suggest that the slow stage in the observed two-stage evolution of contact formation represents a process of recovery from damage to the transport layer's surface caused by the accumulating hot Au atoms. Such a process is not observed for substrate Au. Comparisons of the evolution in injection efficiencies of evaporated Ag contacts with substrate Ag, as well as of liquid Hg contacts, demonstrate that the initially blocking nature of the contact and a fast evolution process are not associated with recovery of the interface from thermal damage but are probably a more general aspect of contact formation.

The photoconduction mechanism in a metal-free phthalocyanine pigment dispersed in a polymer matrix was investigated. The charging potential started to decay remarkably after a threshold light exposure. The threshold exposure increased as the initial potential increased and as the thickness of the photoconductive layer decreased. This result may indicate that the threshold exposure depends on the quantity of charge. The temperature dependence of the threshold exposure was also investigated. The threshold exposure decreases with increasing temperature. The activation energy was estimated to be 0.049eV at an electric field of 4.5 × 10⁵V/cm. This value is almost equal to that of the photogeneration process in phthalocyanine. The photoinduced decay rate after the induction period increased and the activation energy decreased with increasing field intensity. The anticipated field dependent phenomenon was not found in these results. Therefore, we think there is a possibility that the mechanism is different from the prevalent trap theory.