John S. Facci

Charge transport in electroactive polymers consisting of fixed molecular redox sites

Abstract Electrochemically reactive polymers of molecular sites that act as discrete positions of donor and acceptor electron transfer activity are electrical conductors by the mechanism of electron self-exchange reactions between the molecular positions. The electrochemically reactive materials are also ionically conducting. This paper discusses some current issues related to the polymeric electron self-exchange dynamics, including separation of electron from ion mobility, theory that accounts for currents limited by concentration polarized and by electrical potential gradients, and the consequences of site dilution as found in differing degrees of mixed valency and when non-electroactive sites are added. Recent transport data are presented that compare electron transport dynamics under concentration polarized and electrical gradient-driven conditions.

The Electrochemistry Group Medal Lecture: Electron self-exchange dynamics between redox sites in polymers

Using microelectrochemical techniques, our laboratory has explored self-exchange-based electron transport in a variety of mixed-valent polymeric media. The transport rate is measured as the electron diffusion coefficient, De, or the self-exchange rate constant kex. The basic variables for electron transport in mixed-valent polymer materials include: (a) the physical mobility of the counterions of the polymer that migrate due to electroneutrality requirements, (b) the physical diffusion coefficient, Dphys, of the monomeric or polymeric oxidized and reduced molecular sites or ions relative to the rate of electron hopping or tunnelling between donor/acceptor pairs, (c) the observational timescale relative to these mobilities which provides the distinction between transient and steady-state experiments, and (d) the chemical environment of the polymer, whether dry and solvent-free or contacted by solvent vapour or liquid. Experimental strategies and results are presented for the measurement of rates of ion diffusion, Di, in N2-dry and solvent-wetted mixed valent polymers. In a dry, mixed-valent osmium complex polymer, the electron-transport rate measured under steady-state conditions, where no ion transport occurs concurrently, is much faster than the diffusion rate of the ion as estimated in a transient electrolysis experiment. In a solvent-wetted osmium complex polymer, the electron-transport rate measured under transient conditions is much slower than that of the ion which was measured under steady-state conditions. These circumstances allow isolation of individual processes and are interpreted as giving electron-transport rates not strongly influenced by macroscopic ion-transport rates. Cyclic voltammetry of [Co(bpy)3]2+ and of Li+TCNQ– in dry poly(ethylene oxide) polymer electrolyte solvents exhibits differing measured diffusion coefficients, Dapp, for the oxidation vs. the reduction of each compound, reflecting the coupling of physical diffusion and electron self-exchange transport. Microdisc electrode voltammetry of solutions of a synthesized ferrocene mono-tagged poly(ethylene oxide) in a polymer solvent of comparable molecular weight gives Dapp values smaller than those for ferrocene monomer dissolved in the same polymer solvent. The Dapp in the former case measures the self-diffusion rate of a linear chain polymer within a linear chain polymer solvent. Measurability of this rate has implications for assumptions about diffusive mobility of redox molecules attached to polymer chains.

Behavior of an ideal injecting contact on a trap‐free polymer

Solution coatable insulators capable of unipolar photoinjected carrier transport with negligible loss of transiting charge to deep traps are required for the fabrication of organic electrophotographic receptors. Molecularly designed polymeric insulators with these characteristics can now be routinely synthesized. Such trap‐free polymer films provide a unique venue for the study of contact and interface behavior. A test for distinguishing ohmic from emission limited contact behavior which exploits the availability of these polymers is described. The test involves direct comparison of dark injection transients excited by application of a voltage step to the contact under investigation, with small signal time‐of‐flight transients photoexcited by laser pulse irradiation through a semitransparent blocking contact on the opposite face of the same specimen film.

EMISSION LIMITED INJECTION BY THERMALLY ASSISTED TUNNELING INTO A TRAP-FREE TRANSPORT POLYMER

We have measured the steady‐state current supported by a variety of contacts on a polytetraphenylbenzidine hole transport polymer. When glassy carbon is used as a hole injecting contact to the polymer, the current is found to exhibit prototypical emission limited behavior. Unique features of the electric field and temperature dependence of the emission limited current cannot be explained by injection theories appropriate to conventional band type semiconductors. A model of thermally assisted tunneling from carriers at the Fermi level of the contact to localized states in the polymer has been formulated. This model is able to critically account for key features in the experimental data.

Silanization and non-aqueous electrochemistry of two oxide states on platinum electrodes

Abstract When the surface of a Pt electrode is oxidized in aqueous 1 M H2SO4 at +0.8 to +2.0 V vs. SSCE for a few seconds to 10 min, disconnected, washed and dried, and placed in CH3CN solvent, a negative potential scan shows that the surface oxide is reduced in two waves at potentials between−1.2 and−1.6 V vs. SSCE. The combined charge of the two waves amounts to 0.3–2.5 layers of oxide, depending on anodization potential and time. The more easily reduced oxide becomes non-reducible after reaction of the electrode with methyltrichlorosilane. The two oxide waves are interpreted as surface and subsurface oxide layers.

Space charge limited injection into trap-free polymers

Abstract The existence of trap-free unipolar charge transport polymers designed for application in electrophotographic receptors is exploited in a test for distinguishing ohmic from emission limited contacts. The test involves comparison of dark injection transients created by the application of a voltage step with time-of-flight transients obtained by UV photoexcitation, and comparison of steady-state dark-injection currents with the current values predicted for space charge limited currents in these materials. Contacts are characterized between electrodes such as carbon-black filled polymer, gold, indium-tin oxide and aluminum, and trap-free hole transport media, N , N ′-diphenyl- N , N ′-bis(3-methylphenyl)-[1,1′-biphenyl]-4,4′-diamine (TPD) and a condensation polymer containing TPD transport-active units (PTPB). Evidence is provided that many of these contacts are truly ohmic.

Comparison of Hole Hopping Diffusion and Migration in a Triarylamine Containing Polymer

Abstract Electron hopping charge transport rates in a triarylamine containing polymer were investigated electrochemically in the presence of a contacting electrolyte and in the solid state (absence of liquid electrolyte). Electron hopping diffusion coefficients were measured by steady state voltammetry in thin polymer films on Au microelectrode interdigitated arrays. In addition, zero-field extrapolated electron hopping mobilities and zero-field mobility activation energies are obtained from time-of-flight (TOF) measurements. Electron hopping diffusion coefficients (Dh, cm2/s) and diffusion activation energies obtained in solid state electrochemical experiments can be correlated with zero-field hole mobilities (cm2/V-s) and activation energies via the Einstein relationship.

Steady-state corona charging behavior of a corotron over a moving dielectric substrate

Charging up a dielectric surface through corona discharge from a thin wire has been a common practice in electrophotographic processes. One of the widely used corona charging devices is called a corotron and consists of a coronating wire enclosed in a shield with one constituent side being the surface to be charged. Uniform surface charge can be created on a dielectric substrate such as photoreceptor by moving the substrate at a constant velocity through a stationary corotron that steadily emits corona ions. To design efficient corotron for charging dielectric substrates, fundamental understanding of the electrostatic nature of the device is desired. In the present work, the steady-state behavior of corona charging with a corotron over a moving dielectric substrate is analyzed by computationally solving nonlinearly coupled equation system with Galerkin finite-element method and Newton iterations. The predictions based on a first-principle model are shown to agree well with experimental measurements.