Martin A. Abkowitz

Carrier deep‐trapping mobility‐lifetime products in poly(p‐phenylene vinylene)

In order to estimate the deep‐trapping mobility‐lifetime (μτ) product for holes and electrons in poly(p‐phenylene vinylene) (PPV), trap‐free space‐charge‐limited current, and time‐of‐flight measurements were performed on bilayer devices comprising of a PPV layer and a trap‐free molecularly doped polymer layer. μτ products of about 10−9 and 10−12 cm2/V were found for holes and electrons respectively, corresponding to an average range of 1 μm for holes and 10 A for electrons under an electric field of 105 V/cm. The low μτ value for electrons is attributed to severe deep trapping of electrons in PPV, which effectively reduces their range. Implications on the efficiency of electroluminescence devices are discussed.

Pyroelectricity in polyvinylidene fluoride

The pyroelectric behavior of as‐received 2‐mil polyvinylidene fluoride films was studied as a function of the poling treatment. It is shown that poling induces a thermally reversible electric polarization in addition to an electric polarization which vanishes upon the first heating of the sample. Experimental evidence is given that the reversible polarization involves charge injection from the electrodes in addition to a field‐induced structural change (probably the formation of oriented crystalline form I). For short poling times ([inverted lazy s] seconds) the mechanism of charge injection appears to be dominant. Initial results of the temperature and voltage dependence of the dc conductivity and of the temperature dependence of the dielectric loss at 50 and 100 Hz for unpoled and poled 2‐mil polyvinylidene fluoride (PVF2) films are presented.

Dipole reorientation in polyvinylidene fluoride

The thermally stimulated current (TSC) from polarized polyvinylidene fluoride is studied below room temperature. The experimental results indicate that the maximum of the TSC observed at ∼ −50°C is due to reorientation of dipoles in a process associated with the glass transition. The TSC peak is inhomogeneously broadened by a distribution in relaxation frequencies. The results are analyzed in terms of both Arrhenius, and Williams‐Landel‐Ferry descriptions of the temperature dependence of the relaxation processes and the distribution function for the relaxation frequencies is calculated from the temperature dependence of the TSC. It is shown that a distribution in preexponential factors or in activation energies can account for the observed width of the TSC peak. For the latter case a distribution varying exponentially with activation energy seems most likely. From the data we obtain (3.6±0.4)×104 °K for the apparent activation energy and ∼2×10−3 sec−1 for the relaxation frequency of the dominant realignme...

ESR and electronic spectral investigation of the self‐association of phthalocyanine dyes in solution

The degree of molecular association of a copper phthalocyanine dye in benzene and tetrahydrofuran has been studied by electronic spectroscopy and ESR. Electronic spectroscopy in the visible spectral region demonstrates the existence of monomer‐dimer equilibria in the 10−6–10−3 M concentration range. The dimerization constants Keq=Cd/Cm2 are (1.58± 0.09)× 104M−1 and (1.41± 0.50)× 102M−1 at room temperature in benzene and tetrahydrofuran, respectively. The ESR envelope spectra are found to vary systematically in the same concentration range as a function of solvent at fixed temperature. To deduce the manner in which aggregation in these solvents influences the detailed features of the ESR envelope spectrum, it is found useful to follow the evolution of envelope lineshape in copper phthalocyanine‐metal‐free phthalocyanine solid solutions as the concentration of copper phthalocyanine is progressively increased. The principal effect is found to be the perturbing influence of intermolecular copper exchange inte...

Evolution in the charge injection efficiency of evaporated Au contacts on a molecularly doped polymer

Injection efficiency from evaporated Au contacts on a molecularly doped polymer (MDP) system has been previously observed to evolve from blocking to ohmic over time. In the present article this contact forming phenomenon is analyzed in detail. The initially blocking nature of the Au contact is in contrast with that expected from the relative workfunctions of Au and of the polymer which suggest Au should inject holes efficiently. It is also in apparent contrast to a differently prepared interface of the same materials. The phenomenon is not unique to this interface, having been confirmed also for evaporated Ag and mechanically made liquid Hg contacts on the same MDP. The MDP is a disordered solid state solution of electroactive triarylamine hole transporting TPD molecules in a polycarbonate matrix. The trap-free hole-transport MDP provides a model system for the study of metal/polymer interfaces by enabling the use of a recently developed technique that gives a quantitative measure of contact injection eff...

Microwave Dielectric Constant and Conductivity Measurements in the Phthalocyanines

Dielectric constant and loss measurements have been made on α, β, and X metal free and copper phthalocyanine at microwave frequencies. Measurements were carried out on pressed pellets of polycrystalline material. Dielectric constant data is analyzed in terms of a powder dielectric model. The relative dielectric conductivities of these phthalocyanines are examined along with similar measurements on various β transition metal phthalocyanines. Dielectric conductivities of these metal phthalocyanines appear to be consistent with dc conductivities reported in the literature.

Photogeneration and Photoluminescence Studies in Bilayer Structures Containing Poly(P-Phenylene Vinylene) and Molecularly Doped Polymers

Abstract The photogeneration efficiency for photocarriers (holes and electrons) in poly(p-phenylene vinylene) (PPV) is measured in the presence of high electric fields, for bilayer sandwich devices containing molecularly doped polymers (MDP) and PPV. A hole transport diamine molecule (TPD) and an electron transport malononitrile molecule (BCFM) each dispersed in polycarbonate were used to study holes and electrons respectively. By performing photoluminescence (PL) experiments in the bilayer structures and hole collection experiments in single layers without the TPD:PC overcoat, we have concluded that generation of photocarriers in PPV is mainly a bulk mediated process, assisted by the applied electric field, and not sensitized by the presence of the MDP. The electric field dependence of photogeneration suggests that photoexcitation results in a strongly correlated electron-hole pair dissociated by the electric field.

Photoelectronic Properties of Acid‐Doped Poly (N‐Vinylcarbazole): o‐Dinitrobenzene Charge Transfer Complex Films

The light intensity dependence of the buildup and the dark decay of the current through the poly(N‐vinylcarbazole):o‐dinitrobenzene charge transfer complex doped with trichloroacetic acid has been studied at room temperature. The weight ratio of these components was (24:1:1). We find that the buildup and decay of the current can be described by Is tanhβt and the second order rate law, Is/(αt+1), respectively, where Is is the saturation current and t the time. α and β depend upon the square root of the light intensity whereas Is increases in a manner roughly proportional to it. The acid greatly enhances Is, and reduces the second order decay constant α by orders of magnitude as compared to acid free systems. It is shown that the slow dark decay of the current is closely associated with the presence of a photogenerated radical species which gives rise to an ESR signal. Experiments on multilayer films indicate that the current measured following illumination arises predominantly from hole flow. The carrier g...

Direct evaluation of injection efficiency from metals into trap-free small-molecule-based transport layers: probing the details of interface formation

For devices based on the use of electronic polymers there is practical interest in exploring the variations in contact behavior that might result under realistic manufacturing conditions like multilayer device assembly based on solution coating technology. Small molecule doped polymers (MDPs) developed principally as large area coatings for electrophotographic use are now finding wider device applications. These polymers are insulators capable of transporting excess injected charge with a unipolar drift mobility which can be tuned over a wide range by varying the concentration of transport active species. Most significant in the present context, MDPs can be rendered trap free by molecular design. These unique characteristics of MDPs make it possible to analyze the relative injection efficiencies of their interfaces with various contacts simply by a direct comparison of current voltage characteristics with time of flight drift mobility measurements carried out on the same film coatings. Measurements were carried out by measuring dark hole injection into the MDP film TPD/polycarbonate and a polymeric analog, PTPB, from various preformed metal substrates as well as evaporated top contacts. For preformed metal substrates under fully relaxed conditions, it was found that while injection efficiency nominally scaled with the estimated interfacial energy step there was significant variance that in some cases could be clearly associated with the specific details of interfacial chemistry. Time and temperature dependent phenomenon were also delineated and analyzed. Here contact injection efficiencies were observed to increase monotonically, following initial formation, from an emission limited to final steady state which could be ohmic.

Photoinduced Hole and Electron Transfer from Poly(phenylene Vinylene) into Molecularly Doped Polymers

Abstract Photoinduced hole transfer between photoexcited π-conjugated polymer, poly(p-phenylene vinylene) (PPV) and an adjacent trap-free hole-transport molecularly doped polymer containing TPD in a polycarbonate binder in an electric field, is analyzed. Also analyzed and described, under similar electric field conditions, is photoinduced electron transfer between PPV and a trap-free electron transport polymer containing BCFM and a polycarbonate binder. Transfer of carriers, both holes and electrons, to the respective transport layers is fast and coherent, without delay. The collection efficiency at an electric field of 4 × 105Vcm−1 is 25% for photoinduced holes and near 0.2% for photoinduced electrons.