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.     

The nature of plateau on time-of-flight curves in molecularly doped polymers

Transient current curves in a polar molecularly doped polymer for surface and bulk generations of charge carriers are compared. No transformation of the horizontal plateau on the former to the curve predicted by the diffusion and drift theory was found, contrary to expectations for quasi-equilibrium transport conditions. This behavior indicates a nonequilibrium character of the transport of charge carriers. The plateau appearance is explained in our case by the depleted subsurface layer effect, as we already pointed out earlier.     

The nature of the field dependence of drift mobility in molecularly doped polymers

A direct experimental comparison is performed for the field dependences of drift and effective mobilities of holes in a polar molecularly doped polymer (polycarbonate containing 30 wt % p-diethylaminobenzaldehyde diphenylhydrazone), which are measured by the time-of-flight method during the bulk irradiation of polymer samples with a pulse of fast electrons. A numerical simulation of the time-of-flight experiments is conducted with the use of the multiple-trapping model with the Gaussian energy trap distribution. The parameters of the model are determined during independent measurements. It is shown that, because of the nonequilibrium state of the transport of holes under the conditions of the time-of-flight experiment, the true Poole–Frenkel constant (0.27 (μm/V)^1/2) makes up only a part of the experimentally measured value (0.39 (μm/V)^1/2).     

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.     

Organic electroluminescent devices with molecularly doped polymers and anode interface phthalocyanine layer

We studied the properties of organic electroluminescent devices using molecularly doped polymers as a hole transport layer and having a metal-free phthalocyanine (H₂Pc) layer between anode and hole transport layer. A vacuum-deposited H₂Pc metastable layer was converted to a more stable microcrystalline layer by dichloromethane solvent treatment. The devices exhibited good current–voltage and luminance properties. Because the activation energy of carrier transport for the devices with H₂Pc was almost the same as that for the devices without it, it is considered that the H₂Pc layer in this device is electrically inert. This means that improved contact efficiency may be obtained at the anode interface due to the introduction of the H₂Pc layer. © 1997 John Wiley & Sons, Ltd.     

Universality of charge-carrier transport in molecularly doped polymers

The charge-carrier transport model based on the multiple-trapping quasi-band theory with the Gaussian or exponential energy distributions of traps for the two-layer structure of the molecularly doped polymer sample is used to explain the experimentally observed constancy of the flat plateau on time-of-flight curves in a wide electric-field range. The constant shape of the time-of-flight curve under the condition of nonequilibrium transport is observed for the exponential, rather than Gaussian, energy trap distribution. This observation may be used to distinguish between these trap distributions. Finding the true Poole–Frenkel constant requires that the nonequilibrium transport in molecularly doped polymers be taken into account during treatment of the data on the field dependence of mobility.     

Formation of a plateau on time-of-flight curves of molecularly doped polymers

A program for numerical computation of the curves of the transient current in samples of molecularly doped polymers has been developed with allowance for the presence of a defective surface layer in them. Time-of-flight curves are calculated with the multiple-trapping model with a Gaussian energy distribution of traps. The model parameters are determined from the results of independent measurements. Numerical computations are in qualitative agreement with experimental data for a typical molecularly doped polymer. The features of the formation of a flat plateau in samples of different thickness are discussed in terms of the proposed model.     

Theoretical description of electron transport in homo-and molecularly doped polymers

The general aspects of electron transport in disordered organic media (polymers, organic glasses, molecularly doped polymers) are considered. A critical analysis of experimental results and their interpretation in terms of basic theoretical models and experimental methods used in research in this area is presented. Particular attention is given to the comparative analysis of the data obtained by the time-of-flight technique and in studying the radiation conductivity of polymers, in particular, to the possibility of describing the conductivity data in terms of the Gaussian disorder model.     

Dispersive transport in molecularly doped polymers: Theory and experiment

Theoretical and experimental studies of the carrier transport in molecularly doped polymers (MDPs) have been reported. Theoretical analysis uses the multiple trapping (MT) model with an exponential and Gaussian trap distributions. Experimental technique is based on an electron gun technology enabling one to conduct time of flight measurements using the surface and the bulk carrier generation. The list of MDPs tested includes both polar and non-polar systems, some with varying dopant concentration. Experimental results are compared to the MT model predictions as well as the mainstream theories of the hopping conduction in MDPs.     

An organic electroluminescent device with a molecularly doped polymer hole transport layer

Electroluminescent(EL) devices have been fabricated using four different polymers with different glass transition temperatures (T_g) dispersed with N,N′-bis-(3-methylphenyl)-N,N′-diphenyl-1,1′-biphenyl-4,4′-diamine (TPD) as a hole transport layer and tris(8-hydroxyquinoline) aluminum (Alq₃) as an emitting layer. It was found that the higher the T_g of the polymer, the longer the lifetime of the device. From observations of TPD-doped polymer films with optical microscope and atomic force microscope, dispersing TPD in the polymers was found to suppress the crystallization that causes the roughness of the film surface. It was also observed that the higher the T_g of the host polymers, the more difficult TPD crystallization was. The property of the EL device with polyethersulfone (PES) dispersed with TPD was also investigated. The lifetime of EL device with the TPD doped PES film was improved more than five times at a current density below 10 mA/cm² compared with the device with a conventional TPD hole transport layer. © 1997 John Wiley & Sons, Ltd.     

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 field dependence of mobility in molecularly doped polymers under the conditions of nonequilibrium transport

A multiple-trapping model has been used to numerically analyze the effects of the nonequilibrium transport of holes on the field dependence of drift mobility as obtained in a time-of-flight experiment. The intrinsic field dependence has been assigned to the frequency factor in the form of the Poole-Frenkel law. Strongly dispersive transport, as well as a nonequilibrium-charge-transfer regime given by the Gaussian disorder model, has been considered. It has been established that the currently accepted approach to the analysis of the field dependence of mobility needs to be reexamined.     

The effect of charged centers on the transport of charge carriers in molecularly doped polymers

The effect of charged centers on the mobility of charge carriers in molecularly doped polycarbonate is experimentally studied by the method of accelerated electrons for the generation of charge carriers in the surface layer and in the polymer bulk. The hypersensitivity of mobility toward the presence of charged sites that is predicted by the theory of correlated disorder (dipolar glass) is not found. With accumulation of charged centers, transformation of the time-of-flight curves with a well-pronounced plateau region, which is indicative of quasiequilibrium transport according to theory, proceeds in full agreement with the classical concepts on the role of a space charge or bimolecular recombination during high-signal irradiation. Transport of charge carriers in the molecularly doped polymer is not quasiequilibrium but dispersive.     

Organic electroluminescent devices containing phosphorescent molecules in molecularly doped hole transporting layer

We demonstrate high-efficient simple electrophosphorescent devices comprised of tris{4-[N-(3-methylphenyl)anilino]phenyl}amine (m-MTDATA) dispersed in a polycarbonate (PC) matrix as a hole-transporting layer (HTL), and 2-(biphenyl-4-yl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole (PBD) as an electron-transporting layer (ETL). The HTL doped with a complex phosphor fac-tris(2-phenylpyridine)iridium, [Ir(ppy)₃], and/or 5,6,11,12-tetraphenyltetracene (rubrene) fluorescent dye is shown to act as an emitter. Devices containing [Ir(ppy)₃] as a single HTL dopant show the highest external quantum efficiency (QE) reaching 9 % (photon/electron) due to direct electron-hole recombination on phosphorescent [Ir(ppy)₃]. A decrease in QE of one order of magnitude at high current densities is observed in all devices. Addition of rubrene to [Ir(ppy)₃]-doped devices shifts the maximum QE towards larger current densities.     

Focused ultrasound and molecularly imprinted polymers: a new approach to organotin analysis in environmental samples

There is a high interest in speciation of organotin compounds (OTCs) in biota and marine sediment samples, due to their influence in the transmission of the contamination in the trophic chain. Sample treatment is still the most "compromising" step of speciation analysis. Extraction methods are in general time-consuming due to long extraction times and several analytical steps involved. In addition, in most cases there are problems of low recovery, especially for MBT. These drawbacks, added to the high matrix effects generally present in biota samples, make the sample treatment for organotin analysis a serious challenge for environmental issues. Here we present a novel, fast and efficient two steps method for organotin speciation in mussel and oyster tissue as well as in marine sediments. The first step based on the use of ultrasonic probe extraction for species leaching allowed us to quantitatively extract these compounds in a few minutes. Matrix interferences drastically decreased by applying a clean-up step based on the use of an imprinted polymer especially designed for tributyltin (TBT). This procedure increased accuracy and precision of the GC-FPD analysis and improving the limit of detection, Besides, this new method prevents the use of standard addition calibration method, which is mandatory without the clean-up step. The optimization and validation has been performed by using three reference materials: mussel tissue CRM-477, oyster candidate T-38 and sediment PACS-2.     

Drift mobility of excess charge carriers in molecularly doped polymers upon photo-and radiation-induced charge generation

Comparative experiments on molecularly doped (with aromatic hydrazone) polycarbonate were performed to measure the drift mobility of excess charge carriers using carrier generation by exposure to light and ionizing radiation. Measurements showed practically complete coincidence of both mobility values and signal shapes. Compared were the results of measurements on polymer specimens with various thicknesses supported on substrates with different surface roughness, as well as for samples with or without an injecting amorphous-selenium layer. Dispersive carrier transport was observed in all cases. However, current transients sometimes displayed a plateau, depending on the sample preparation conditions. As the magnitude of generated (injected to the polymer layer) charge increased, a current spike appeared by the end of the transit time, thus indicating the emergence of local space-charge-limited current.     

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.     

The features of hole transport in molecularly doped polystyrene

The hole transport in polystyrene doped with 15 wt % tritolylamine is studied via the time-of-flight technique in both the surface carrier generation mode and the bulk carrier generation mode. The hole transport is shown to be dispersive with a dispersion parameter value of 0.99 ± 0.03 at room temperature. The results contradict the widely held view that hole transport in this molecularly doped polymer is quasi-equilibrium. The hole mobility is found to be almost independent of the electric-field intensity.