V. A. Kolesnikov

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.

Bright blue–green electroluminescence from aromatic polyimides

New polyimide type of polymers, aromatic polyimides (APIs) containing sulphur atoms in the backbone, were investigated as electron-hole transporting and light-emitting materials for use in unilayer electroluminescent diodes. The simplicity of synthesis, high thermal stability, organic solvent solubility together with excellent film-forming properties make the APIs potentially of interest for technological applications.

Aromatic Polyimides as Efficient Materials for Organic Electroluminescent Devices

Derivatives of newly synthesized aromatic polyimides containing sulphur atoms in the backbone were studied as electron-hole-transporting and light-emitting materials for use in combination with tris(8-hydroxyquinoline)-aluminium in multilayer organic electroluminescent devices. Efficient bipolar charge transport has been revealed in the polymers. Bright electroluminescence (15 000 cd m -2 at 14V) was observed in the visible range. The possible role of intermolecular donor-acceptor interactions in the mechanism of electroluminescence is discussed.

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.

Generation and transport of charge carriers in polyvinylcarbazole

The processes of generation and transport of excess charge carriers in polyvinylcarbazole at room temperature and at 353 K were experimentally studied. The polymer was charged with pulses of electrons at energies of 7 and 50 keV, which differed in both linear energy transfer and track structure. A universal method of investigation based on the combination of the time-of-flight technique in the both (surface and bulk) modes with the measurement of radiation-induced conductivity was used. The radiation-chemical yield of free charges was measured using two independent procedures. It was shown that the radiation chemical yield of free charges at 293 K was somewhat smaller for 7-keV than for 50-keV electrons; in the latter case, G = 1.1 in an electric field of 2 × 107 V/m. The parameters of the generalized physical model were determined for polyvinylcarbazole.

Effect of hole scavenging and preirradiation on electron transport in polyvinylcarbazole

The results of experimental and theoretical studies of the effects of hole (majority carrier) scavengers, such as aromatic hydrazone, and preliminary irradiation on electron transport in polyvinylcarbazole are presented. It is shown that the scavenging effects can satisfactorily be described in terms of the multiple trapping model and the transport parameters of electrons—minority charge carriers—can simultaneously be determined (although approximately). The description of preirradiation effects required this model to be extended, in particular, with allowance for radiation-induced alteration in the electronic structure of the polymer at doses starting from the order of 100 Gy.

Photovoltaic devices based on palladium(II) meso-tetra(benzo-15-crown-5)porphyrinate

The properties of photovoltaic cells containing an MEH-PPV composition with palladium(II) meso-tetra(benzo-15-crown-5)porphyrinate as an electron-conduction layer and copper(II) phthalocyanine as a hole transport layer were studied. The influence of the environment and a top-electrode metal on the operation efficiency was also examined. It was found that oxygen at the fabrication steps affects in different manners the formation of open-circuit voltage and the performance of the device. A certain correlation between the properties of electrodes and the photoelectromotive force was established. The highest conversion efficiency of 0.02 electrons per incident photon was obtained for samples that additionally contained an amorphous selenium layer applied over the aforementioned conducting layers.

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.

The conduction switching effect in thin polymer layers

The effect of pressure-induced and field-induced conduction switching in thin polymer layers was experimentally studied. Conduction switching was observed in thin layers of poly(diphenylene phthalide), polyvinylcarbazole, some aromatic polyimides, and methoxypoly(phenylene-vinylene). It was found that conduction switching depends in a critical manner on the formation mode of the metal/polymer contact.

ELECTROLUMINESCENCE OF POLY(HYDROXYAMINO ESTERS) : ALUMINUM COMPLEXES

Abstract We demonstrate that the synthesis of polymer complexes (PHAE-A1) occurs between poly(hydroxyamino esters) (PHAEs) and aluminum by using the conventional vacuum deposition method in the process of thin metal layer formation on the surface of the polymer. These complexes exhibit electroluminescence similar in spectral characteristics to the well-known low-molecular tris(8-quinolinolato) aluminum complex (Alq 3 ). Spectral and electrical data for the polymer light-emitting diodes A1/PHAE-A1/PHAE/PANI with PHAEs of various chemical structure are reported.