• Polymer Electrochromics
  • Polymer LEDs
  • Polymer Photovoltaics
  • Polymer Composites and Liquid Crystals
  • Biophysics
  • Other (Pre-1997; highly correlated electron systems, superconductors, etc)
  • The following articles are available as Adobe Acrobat PDF (by clicking on the PDF link). Acrobat is available as a free download by clicking here.

  • Polymer Electrochromics

    1. Solid-state electrochromic devices based on poly (pheylene vinylene) polymers. Holt, A.L., Leger, J.M., Carter, S.A. Applied Physics Letters (86), 123504 (2005). (PDF)

      We present a solid state electrochromic device based on poly sphenylene vinylened light-emitting
      polymers and explore device performance as a function of salt type, salt concentration, and polymer
      layer thickness. Salts employing organic anions display improved optical contrasts. Higher salt
      concentrations and thicker devices produce higher optical contrasts at the cost of slower switching
      speeds. Devices display high reversibility, dramatic optical contrasts (>40%), and low operating
      voltages (<2 V) that are comparable to state-of-the-art conducting polymer electrochromic

    Polymer LEDs

    1. Electrochemical Doping and the Optical Properties of Light-Emitting Polymer Materials and Devices. Leger, J. PhD Dissertation, 2005. (PDF)

    2. Semiconductive Polymer Blends: Correlating Structure with Transport Properties at the Nanoscale. Ionescu-Zanetti, C., Mechler, A., Carter, S.A., Lal, R.; Advanced Materials 16(5): 385. (2004).   (PDF) (Correction)

    3. In the present study we apply multimodal AFM to map the local charge transfer properties in correlation to the molecular superstructure of the polymer blend. We identify the structural basis of the correlation between charge transfer efficiency and local blend composition. At the polymer surface, our results show that efficient charge injection occurs in regions where the lamellar edges are exposed to the probe. This correlation suggests that the efficiency of charge injection at the polymer-electrode interface can be enhanced by controlling lamellar orientation.

    4. Thickness-dependent changes in the optical properties of PPV- and PF-based polymer light emitting diodes, J.M. Leger, S.A. Carter, B. Ruhstaller, H.G. Nothofer, U. Scherf, H. Tillman, H.H. Horhold; Physical Review B. 6805(5):4209 (2003). (PDF)
    5. We explore the thickness-dependent optical properties of single layer polymer light emitting diodes for two materials, poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-ethenylene-2,5-dioctyloxy-1,4-phenylene-ethenylene] (MEH-DOO-PPV) and poly(2,7-(9,9-bis(2-ethylhexyl))fluorene)-2,7-bis(4-methylphenyl)phenylamine (PF with 2% endcap). We compare experimental electroluminescence spectra and radiance values as a function of emissive layer thickness with simulations utilizing dipole methods within a transfer-matrix formalism. The technique is then extended to explore how simulated results depend on the assumed location of emission within the polymer layer. We show that thickness-dependent optical properties of these devices are dominated by interference effects.

    6. Transient and Steady-state Behavior of Space Charges in Multilayer Organic Light-emitting Diodes, B. Ruhstaller, S.A. Carter, S. Barth, H. Riel, W. Riess, J.C. Scott; J. of Applied Physics. (PDF)

      A numerical study of space charge effects in multilayer organic light-emitting diodes (OLEDs) is presented. The method of solving the coupled Poisson and continuity equations, previously established for single-layer polymer LEDs, has been extended to treat internal organic interfaces. In addition, we consider the transient current and electroluminescence response. We discuss the accumulation of charges at internal interfaces and their signature in the transient response as well as the electric field distribution. Comparison to experimental transient data of a typical bilayer LED based on tris(8-hydroxyquinolinato)aluminum (Alq(3)) is provided and good agreement is found. Our results are consistent with commonly assumed operating principles of bilayer LEDs. In particular, the assumptions that the electric field is predominantly dropped across the Alq(3) layer and that the electroluminescence delay time is determined by electrons passing through Alq(3) to the internal interface are self-consistently supported by the results of the simulation. Moreover, the creation of emissive singlet excitons is found to be strongly confined to the Alq(3) side of the internal interface and the emission zone width is dictated by the exciton diffusion length. Design principles for trilayer LEDs with improved power efficiency are also discussed.

    7. Effects of polymer sidebranching in double- and single-layer polyfluorene light-emitting diodes, Nakazawa, YK; Carter, SA; Nothofer, HG; Scherf, U; Applied Physics Letters, V80 N20:3832-3834 (2002). (PDF)

      We study how changes in sidebranching in electroluminescent polymers affect the performance of polyfluorene-based light-emitting diodes with and without additional hole transport materials. We observe that light emission and device efficiency are determined more by the position of the exciton recombination zone than by changes in the polymer morphology induced by sidebranching. Consequently, we find that sidebranching mainly controls the relative emission between vibrational energy levels and has a minimal effect on polymer charge transport properties. Light outputs of 10 000 cd/m(2) and device efficiencies of 0.85 and 1.8 cd/A are obtained for single- and double-layer devices, respectively.

    8. Charge Transport Processes in Organic Light-emitting Devices, J.D. Scott, J.C. Brock, J.R. Salem, S. Ramos, L. Bozano, and S.A. Carter; Synthetic Metals, V111, 289-293 (2000). (PDF)
    9. The luminous efficiency of organic light-emitting diodes depends on the recombination probability of electrons injected at the cathode and holes at the anode. We have developed a numerical model to calculate the recombination profile in single- and multilayer structures, taking into account the built-in electric field, the charge injection process at each electrode, hopping transport with field-dependent mobilities, charge diffusion, trapping and Langevin recombination. By comparison of the simulation results, as well as approximate analytic solutions, with experimental data on MEH-PPV-based devices, we find that injection is thermionic with Schottky barriers for some electrode metals that are low enough to be considered Ohmic. Except at voltages close to threshold, diffusion and trapping effects are negligible. Both electrons and holes are mobile, with a field dependence that is independently confirmed both by single-carrier space-charge-limited current measurements and transient time-of-flight techniques.

    10. Bias-tuned Reduction of Self-absorption in Polymer Blend Electroluminescence, B. Ruhstaller, J.C. Scott, P.J. Brock, U. Scherf, S.A. Carter; Chem. Phys. Lett. 317, 238-44 (2000) (PDF)
    11. We present a concentration- and bias-dependent electroluminescence study on MEH-PPV aggregation in a binary polymer blend with the blue-emitting Me-LPPP as host In low-concentration blends the spectral features of MEH-PPV peak at 560 nm, identical to its photoluminescence spectrum in dilute solution, and therefore suggesting effective hindering of aggregation-induced self-absorption. At higher concentrations the electroluminescence spectra an dominated by MEH-PPV peaking at 600 nm and a dramatic shift of spectral weight to the 560 nm peak is observed with increasing bias. We attribute this novel effect to a reduction of self-absorption caused by either photo- or charge-induced bleaching.

    12. Self-assembled Nano-Composite Polymer Light Emitting Diodes with Improved Efficiency and Luminance, V. Bliznyuk, B. Ruhstaller, P. J. Brock, U. Scherf, S.A. Carter; Adv. Mat. 11 (15), 1257, (1999) (PDF)

      Nanoparticles assembled at the semiconducting polymer/ electrode interface affect charge injection into polymer light-emitting diodes (LEDs), as reported here. It is demonstrated that modification of the ITO anode using self-assembled monolayers and electrostatically assembled sie, nanoparticles can dramatically improve the electroluminescence properties of LEDs-the charged nanoparticle surface induces a dipole moment across the electrode interface, effectively increasing the local electric field anes, and on the other side of the line, voltage is constant. Analyses of decay times, the slower response time of the PDLC, show that the times peak along a line of points in temperature-composition space that is close to the transition line for increasing switching voltages. We present these results as contours an the same graphs and relate them to our understanding of the phase separation process in the PDLC mixture.

    13. Temperature- and Field-dependent electron and hole mobilities in polymer light-emitting diodes, L. Bozano, S. A. Carter, J. C. Scott, G. G. Malliaras, P. J. Brock; Appl. Phys. Lett. 74 (8), 1134-4, (1999) (PDF)
    14. We have studied the transport properties of electron- and hole-dominated MEH-PPV, poly(2-methoxy,5-(2'-ethyl-hexoxy)-p-phenylene vinylene), devices in the trap- free limit and have derived the temperature-dependent electron and hole mobilities (mu = mu(0)e(gamma root E)) from the space-charge-limited behavior at high electric fields. Both the zero-field mobility mu(0) and electric-field coefficient gamma are temperature dependent with an activation energy of the hole and electron mobility of 0.38 +/- 0.02 and 0.34 +/- 0.02 eV, respectively. At 300 K, we find a zero-field mobility mu(0) on the order of 1 +/- 0.5 x 10(-7) cm(2)/V s and an electric-field coefficient gamma of 4.8 +/- 0.3 x 10(-4) (m/V)(1/2) for holes. For electrons, we find a mu(0) an order of magnitude below that for holes but a larger gamma of 7.8 +/- 0.5 x 10(-4) (m/V)(1/2). Due to the stronger field dependence of the electron mobility, the electron and hole mobilities are comparable at working voltages in the trap- free limit, applicable to thin films of MEH-PPV.

    15. Temperature-dependent Recombination in polymer composites light-emitting diodes, L. Bozano, S. A. Tuttle, S. A. Carter, P. J. Brock; Appl. Phys. Lett. 73, 3911-3 (1998) (PDF)

      We study the temperature dependence of the current-voltage and radiance-voltage curves in double-carrier injected polymer light-emitting devices comprised of poly(2-methoxy,5-(2'-ethyl-hexoxy)-p-phenylene vinylene) (MEHPPV) and MEH-PPV/SiO2 as the active layer. The quantum efficiency increases significantly as the temperature is decreased in agreement with an increase in the recombination efficiency with decreasing temperature. Moreover, the bimolecular recombination efficiency saturates at low temperatures and high currents to a very high value for both the composite and plain MEH-PPV devices with the nanoparticles serving as charge traps only at moderately low current densities. Finally, we find that the order of magnitude improvement in radiance observed in some polymer/nanoparticle composites is due to an increase in the effective electric field across the device.

    16. Electrical and Photo-induced Degradation of Polyfluorene Based Films and LED Devices, V. Bliznyuk, S. A. Carter, J. C. Scott, G. Klärner, R. D. Miller, D. C. Miller; Macromolecules 32 (2), 361-369, (1999) (PDF)

      Degradation-induced changes in the structural and optical properties of the polyfluorene-based blue emitting films and LEDs are examined using spectroscopic (FTIR, UV-vis, photo- and electroluminescence), analytical (FTIR and ESCA), and scanning probe microscopy techniques. The materials studied are oligomers (DP ~ 10) of 9,9-di-n-hexylfluorene and its random copolymer with anthracene. In situ FTIR monitoring is used to characterize chemical changes in the active layer of operating LED devices. Two primary mechanisms of degradation are identified. In the first, photooxidation of the polymer matrix leads to the formation of an aromatic ketone, most likely fluorenone at the chain terminating monomer units, which quenches the fluorescence. The second process promotes aggregate formation, which then leads to loss of luminous intensity by exciton transfer and relaxation through excimers.

    17. Enhanced luminance in polymer composite light emitting diodes, S.A. Carter, J. C. Scott, P. J. Brock; Appl. Phys. Lett. 71 (9), 1145-7, (1997) (PDF)
    18. We demonstrate that mixing insulating oxide ranoparticles into electroluminescent polymer materials results in increased current densities, radiances, and power efficiencies in polymer light emitting diode devices. For low driving voltages, an order of magnitude increase in current density and light output is achieved with minimal loss in device lifetime. At 5 V, we achieve radiances of 10 000 cd/m with external quantum efficiencies similar to 1% for nanoparticle/MEH-PPV composite films.

    19. Bipolar charge and current distributions in organic light-emitting diodes, J. C. Scott, S. Karg, S. A. Carter; J. Appl. Phys. 82 (3), 1454-60, (1997) (PDF)
    20. The electron and hole charge distributions and the luminance profile in organic light-emitting diodes (OLEDs) depend upon the bulk properties of the emissive layer, as well as on the injection characteristics at the anode and cathode interfaces. We address the problem of separating the relative contributions of hole injection, electron injection, and recombination to the overall performance of single layer OLED devices. Using the approach of Parmenter and Ruppel [J. Appl. Phys. 30, 1548 (1959)], and including Langevin recombination, expressions are derived for the current-voltage and radiance-current dependencies in terms of electron and hole mobility, luminescence yield, and a ''current balance'' factor. When one carrier dominates the current flow, as in many practical cases, it is possible to obtain a simple asymptotic relationship which permits a test of the assumptions required to obtain the analytic solution. Experimental data from poly(2-methuxy-5(2'-ethyl)hexoxy-phenylenevinylene) diodes fabricated with various anode and cathode materials are evaluated in the context of this analytical approach. (C) 1997 American Institute of Physics.

    21. Materials and Modeling for Organic LEDs, J. Scott, S.A. Carter, S.Karg, and M. Angelopoulos, Proceedings of the SPIE 1997; Vol. 3002, 86 (1997).

    22. Polymeric anodes for improved polymer light-emitting diode performance, S. A. Carter, M. Angelopoulos, S. Karg, P. J. Brock; Appl. Phys. Lett. 70 (16), 2067-9, (1997) (PDF)
    23. We have studied polyaniline and polyethylenedioxythiophene transparent electrodes for use as hole-injecting anodes in polymer light emitting diodes. The anodes were doped with a variety of polymer and monomer-based acids and cast from either water or organic solvents to determine the effect of the dopant and solvent on the hole-injection properties. We find that the anodes with polymeric dopants have improved device quantum efficiency and brightness relative to those with small molecule dopants, independent of conductivity, solvent, or type of conducting polymer. For the most conducting polymer anodes [sigma>2(Ohm cm)(-1)], diodes could be made without an indium tin oxide underlayer. These diodes show substantially slower degradation.

    24. Polymeric Anodes for Organic Light-Emitting Diodes, J.C. Scott, S.A. Carter, S. Karg, and M. Angelopoulos; ICSM -96, Synthetic Metals, 85(1-3):1197-1200 (1997) (PDF)

      Polymer light-emitting diodes based on PPV, for example MEH-PPV, are known to be susceptible to photo-oxidative degradation. The formation of the carbonyl species in the polymer results in quenching of the luminescence. In addition the oxidation process reduces the conjugation of the polymer, leading to lower charge carrier mobilities and consequently higher operating voltages. Previous in situ FTIR studies revealed that even in a dry inert atmosphere polymer oxidation occurs, and that ITO can act as the source of oxygen. In order to explore further the nature of. the oxidation mechanism and to provide guidance for its elimination, we have studied the behavior of MEH-PPV LEDs prepared with conducting polymer anodes. When a layer of polyaniline is present between the ITO and the MEH-PPV the device characteristics improve dramatically: the injection voltage drops, the luminous efficiency increases and, most significantly, the rate of decay of the luminance decreases by up to two orders of magnitude. These data not only confirm that ITO is a source of oxygen, but also imply that the oxidation mechanism is due to direct interfacial reaction. We compare several different forms of polyaniline, with different dopants, as well as a derivative of a polythiophene.

    Polymer Photovoltaics
    1. Comparison of Blended Versus Layered Structures for Poly(p-phenylene vinylene)-based Polymer Photovoltaics, S. V. Chasteen, J. O. Härter, G. Rumbles, J. C. Scott, Y. Nakazawa, M. Jones, H.-H Hörhold and H. Tillman, S. A. Carter (PDF)
    2. We characterize and compare blended and bilayered heterojunctions of polymer photovoltaic
      devices using poly[oxa-1,4-phenylene-1,2-(1-cyano)-ethenylene-2,5-dioctyloxy-1,4-phenylene-
      1,2-(2-cyano)-ethenylene-1,4-phenylene] (CN-ether-PPV) and poly[2,5-dimethoxy-1,4-phenylene-
      1,2-ethenylene-2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylene-1,2-ethenylene] (M3EH-PPV) as
      electron- and hole-transporting polymers, respectively. We find that both blended and bilayered
      structures have substantially improved current densities (>3 mA/cm2) and power efficiencies
      (~1% under white light) over neat films. Improved exciton dissociation at multiple interfaces and
      reduced recombination due to energy and charge transfers increases the charge-carrier collection in
      both types of heterojunction devices, but low electron mobilities in the polymers lead to low fill
      factors and reduced quantum efficiency (~20%) that limit the power efficiency. Time-resolved
      photoluminescence reveals that for blended structures both the hole and electron-transporting
      polymers undergo efficient quenching with the exciton decay being dominated by the existence of
      two fast decay channels of 0.12 and 0.78 ns that are assigned to interspecies charge transfer and
      account for the increased short-circuit current observed. For layers, these components are not as
      prevalent. This result indicates that greater exciton generation at the dissociating interface and more
      efficient charge collection in the thin layers is primarily responsible for the improved short-circuit
      current, a conclusion that is further supported by numerical simulations of the exciton generation
      rate and charge collection. We also report evidence for an intermediate exciplex state in both types
      of structures with the greatest yield for blends with 50 wt % of CN-ether-PPV. Overall, the
      improved performance is due to different processes in the two structures; efficient bulk exciton
      quenching and charge transfer in blends and enhanced exciton generation and charge collection in
      layers. The optimization of each photovoltaic heterostructured device relies on this understanding of
      the mechanisms by which each material architecture achieves high power efficiencies.

    3. Numerical Simulations of Layered and Blended Organic Photovoltaic Cells, J. O. Haerter, S.V. Chasteen, S. A. Carter, J. C. Scott, Applied Physics Letters (86), 164101 (2005). (PDF)

      We present results obtained from numerical simulations of organic photovaltaic cells as the donor–
      acceptor morphology evolves from sharply defined layers, to partial blends and finally homogeneous
      blends. As the mixing percentage increases, the exciton dissociation increases and the diffusion
      counter-current decreases, resulting in substantially greater short circuit currents but reduced open
      circuit voltages. Blended structures are more sensitive to mobility than layers due to recombination
      throughout the bulk. Our model indicates that solar power efficiencies greater than 10% can be
      achieved when the zero-field charge mobilities approach 10-3 cm2 /Vs for partially blended

    4. Improving Power Efficiencies in Polymer - Polymer Blend Photovoltaics. A. J. Breeze; Z. Schlesinger; S.A. Carter; H. -H. Hoerhold; H. Tillmann, Solar Energy Mater. and Solar Cells 83, 263-271 (2004). (PDF)

    5. Efficient titanium oxide/conjugated polymer photovoltaics for solar energy conversion, A.C. Arango, L.R. Johnson, V.N. Bliznyuk, Z. Schlesinger, J. Hancock, and S. A. Carter; Advanced Materials, V12 N22:1689-1692,1642 (2000). (PDF)

      Record high efficiencies for a polymer-based solar cell are reported here for an inorganic/organic composite photovoltaic device containing a single polymer film rather than a polymer/polymer or polymer/nanoparticle blend. The high efficiencies are reported to be due to device construction (see Figure) and the electronic properties of the phenyl-amino-p-phenylenevinylene-based polymer used.

    6. Charge Transport in TiO2 / MEH-PPV Polymer Photovoltaics, A.J. Breeze, Z. Schlesinger, S.A. Carter, P.J. Brock; Phys. Rev. B 64, no.12, 125205/1-9 (2001). (PDF)

      We study the effect of polymer thickness, hole mobility, and morphology on the device properties of polymer-based photovoltaics consisting of MEH-PPV as the optically active layer, TiO2 as the exciton dissociation surface, and ITO and Au electrodes. We demonstrate that the conversion efficiency in these polymer-based photovoltaics is primarily limited by the short exciton diffusion length combined with a low carrier mobility. For MEH-PPV devices with optimal device geometry, we achieve quantum efficiencies of 6% at the maximum absorption of the polymer, open circuit voltages of 1.1 V, current densities of 0.4 mA/cm(2) and rectification ratios greater than 10(5) under 100 mW/cm(2) white light illumination. In addition, we achieve fill factors up to 42% at high light intensities and as high as 69% at low light intensities. We conclude by presenting a model that describes charge transport in solid-state polymer/TiO2-based photovoltaics and suggest methods for improving energy conversion efficiencies in polymer-based photovoltaics.

    7. Charge transfer in photovoltaics consisting of interpenetrating networks of conjugated polymer and TiO2 nanoparticles, A.C. Arango, S.A. Carter, P.J. Brock; Appl. Phys. Lett., 74 (12), 1698-700, (1999) (PDF)

      We study the effect of blended and layered titanium dioxide (TiO2) nanoparticles on charge transfer processes in conjugated polymer photovoltaics. A two order of magnitude increase in photoconductivity and sharp saturation is observed for layered versus blended structures, independent of the cathode work function. Using electrodes with similar work functions, we observe low dark currents and open circuit voltages of 0.7 V when a TiO2 nanoparticle layer is self-assembled onto the indium-tin-oxide electrode. Our results for the layered morphologies are consistent with charge collection by exciton diffusion and dissociation at the TiO2 interface.

    Polymer Composites and Liquid Crystals

    1. 1/f noise through the Metal-Nonmetal Transition in Percolating Composites, A. Breeze,  S. A. Carter, M. Heaney, G. B. Alers; Appl. Phys. Lett. 76 (5), 592-4, (2000) (PDF)
    2. We have measured the 1/f noise through the metal-nonmetal transition in carbon black/polymer composites as a function of temperature and doping. At the electronic transition, the resistivity power spectrum S-rho varies as S(rho)similar to rho(Q), with Q=2.77, in agreement with classical three-dimensional percolation. At lower temperatures, a crossover to tunneling-dominated transport occurs with S(rho)similar to ln S-rho/rho(2). Our results show that 1/f noise can be a more sensitive technique than resistivity itself for probing transport behavior near a percolation-induced electronic transition.

    3. Splitting fingers under strain: Pattern formation of a dipolar fluid in a polymer medium, S. A. Carter, J. B. Nephew, K. Amundson; Langmuir 15 (25), 8558-60, (1999) (PDF)

      We study the formation and evolution of labyrinthine patterns for dipolar fluids inside a polymer medium under mechanical strain. A new general mechanism is proposed to describe the kinetics of the pattern formation that combines orientation ordering and subsequently breaking characteristics of dipolar fluids with branching characteristics of crack-propagation This mechanism can lead to many of the patterns found in nature, including spots, stripes, labyrinthine, and starlike formations.

    4. Reaction-induced phase separation dynamics: a polymer in a liquid crystal solvent, J. Nephew, T. C. Nihei, S. A. Carter; Phys. Rev. Lett., 80 (15), 3276-9, (1998) (PDF)

      The dynamics of addition polymerization-induced phase separation in a liquid crystal solvent is examined via confocal microscopy in systems where the final morphology consists of nematic liquid crystal domains suspended in a cross-linked polymer matrix. For low polymer concentrations, we observe unusually rapid hydrodynamics and coalescence during phase separation that determine the final composite morphology. This hypercoalescence can result from polymerization-induced changes of the solubility of the polymer matrix in the liquid crystal solvent.

    5. Dependence of Morphology on the Optical and Electrical Properties of Polymer-Dispersed Liquid Crystals (PDLC), S. A. Carter, J. D. LeGrange, J. Boo, W. White, P. Wiltzius; J. of Appl. Phys. 81, 5992 (1997). (PDF)

      Using confocal microscopy, we have studied the morphology of polymer dispersed liquid crystals (PDLC) as a function of polymer/liquid crystal composition, polymer cure temperature, and ultraviolet (UV) curing power and determined how this morphology affects the electro-optical properties. The PDLC morphology consists of a spongelike texture where spherically shaped liquid crystalline domains are dispersed in a polymer matrix, These domains grow as the fraction of Liquid crystal increases and as the UV curing power decreases,We observe no significant changes in domain size with changes in the curing temperature. Instead, high-temperature cures result in coalescence and the formation of elliptical-shaped liquid crystal domains. The temperature al which this coalescence starts to be observed marks a threshold temperature T-th, above which the switching properties are strongly dependent on morphology. Below T-th the switching properties are largely independent of morphology.

    6. Dependence of the UV polymerization conditions on the Electro-optic Properties of Polymer-Dispersed Liquid Crystals, J. D. LeGrange, S.A. Carter, J. Boo, M. Fuentes, A. Freeman; J. of Appl Phys. 81, 5984 (1997). (PDF)

      We have studied the dependence of the electro-optical properties of polymer dispersed liquid crystals (PDLC) on the ultraviolet (UV) cure of the solution of monomer and liquid crystal. The kinetics of UV polymerization and its effect on the morphology of the phase separated droplets of liquid crystal determine the switching voltage, response time, and luminance of the PDLC. Using a series of statistically designed experiments, we have mapped the dependence of these responses on the weight fraction of liquid crystal, the temperature of the cell during cure, and light intensity. Temperature and composition are strongly coupled parameters that influence switching voltage, luminance, and response times. Switching voltages are minimized at 4-5 V for an 8 mu m cell gap over a large region of temperature-composition space. An abrupt transition line occurs through that space. On one side of the transition line, voltage increases linearly either as temperature increases or composition decreases, and on the other side of the line, voltage is constant. Analyses of decay times, the slower response time of the PDLC, show that the times peak along a line of points in temperature-composition space that is close to the transition line for increasing switching voltages. We present these results as contours an the same graphs and relate them to our understanding of the phase separation process in the PDLC mixture.


    1. A General Model for Amyloid Fibril Assembly Based on Morphological Studies using Atomic Force Microscopy, R. Khurana, C., Ionescu-Zanetti, M. Pope. J. Li, Marina Ramierz-Alvarado, L. Nielson, L. Regan, A. Fink, and S. A. Carter; Biophysical Journal, 85(2), 1125 (2003). (PDF)

      Based on atomic force microscopy analysis of the morphology of fibrillar species formed during fibrillation of alpha-synuclein, insulin, and the B1 domain of protein G, a previously described model for the assembly of amyloid fibrils of immunoglobulin light-chain variable domains is proposed as a general model for the assembly of protein fibrils. For all of the proteins studied, we observed two or three fibrillar species that vary in diameter. The smallest, proto. laments, have a uniform height, whereas the larger species, protofibrils and fibrils, have morphologies that are indicative of multiple proto. laments intertwining. In all cases, protofilaments intertwine to form protofibrils, and protofibrils intertwine to form fibrils. We propose that the hierarchical assembly model describes a general mechanism of assembly for all amyloid fibrils.

    2. A model for amyloid fibril formation in immunoglobulin light chains based on comparison of amyloidogenic and benign proteins and specific antibody binding, R.
    3. Khurana, P.O Souillac, A.C. Coats, L. Minert, C. Ionescu-Zanetti, S.A. Carter, A. Solomon, A.L. Fink; Amyloid. 10(2):97-109 (2003).

      In an attempt to understand the mechanism of amyloid fibril formation in light chain amyloidosis, the properties of amyloidogenic (SMA) and benign (LEN) immunoglobulin light chain variable domains (V-L) were compared The conformations of LEN and SAM were measured using secondary and tertiary structural probes over the pH range from 2 and 8. At all pH values, LEN was more stable than SMA. The CD spectra of LEN at pH 2 were comparable to those of SMA at pH 7.5, indicating that the low pH conformation of LEN closely resembles that of SMA at physiological pH. At low pH, a relatively unfolded intermediate conformation is populated for SAM and rapidly leads to amyloid fibrils. The lack of such an intermediate with LEN, is attributed to sequence differences and accounts for the lack of LEN fibrils in the absence of agitation. A kappaIV-specific monoclonal antibody that recognizes the N-terminal of SMA caused unraveling of the fibrils to the protofilaments and was observed to bind to one end of SMA protofilaments by atomic force microscopy. The antibody result indicates that each protofilament is asymmetric with different ends. A model for the formation of fibrils by SAM is proposed.

    4. Surface-catalyzed amyloid fibril formation, M. Zhu, P.O. Souillac, C. Ionescu-Zanetti, S.A. Carter, A.L. Fink; Journal of Biological Chemistry. 277(52):50914-50922, 2002 Dec 27.

      Light chain (or AL) amyloidosis is characterized by the pathd promoting carrier injection.

    5. Monitoring the assembly of Ig light-chain amyloid fibrils by atomic force microscopyC. Ionescu-Zanetti, R. Khurana, J. R. Gillespie, J. S. Petrick, A. L. Fink, S. A. Carter; Proceedings of the National Academy of Sciences of the USA, 96 (23), 13175-13179, (1999) (PDF)

      Aggregation of Ig light chains to form amyloid fibrils is a characteristic feature of light-chain amyloidosis, a light-chain deposition disease. A recombinant variable domain of the light chain SMA was used to form amyloid fibrils in vitro. Fibril formation was monitored by atomic force microscopy imaging. Single filaments 2.4 nm in diameter were predominant at early times; protofibrils 4.0 nm in diameter were predominant at intermediate times; type I and type II fibrils 8.0 nm and 6.0 nm in diameter, respectively, were predominant at the endpoints, The increase in number of fibrils correlated with increased binding of the fluorescent dye thioflavin T. The fibrils and protofibrils showed a braided structure, suggesting that their formation involves the winding of protofibrils and filaments, respectively. These observations support a model in which two filaments combine to form a protofibril, two protofibrils intertwine to form a type I fibril, and three filaments form a type II fibril.

    Other (Correlated Electron Systems, etc.)

    1. Temperature Dependence of the Hall Angle in a Correlated
      Three-dimensional Metal
      , T.F. Rosenbaum, A. Husmann, S.A. Carter, and
      J.M. Honig, Phys. Rev. B57, R13997 (1998). (PDF)

      The Hall coefficient, R-H, Of the Mott-Hubbard system vanadium sesquioxide has a strong temperature dependence in the barely delocalized metal. As in the case of the cuprate superconductors, we find that the resistivity and the Hall angle of V2-yO3 follow different power laws in temperature, implying different longitudinal and transverse scattering mechanisms. Far from half-filling, only one transport scattering rate is needed to describe the data, at which point the temperature dependence of R-H disappears.

    2. Magnetic Correlations in a Classic Mott System, W. Bao, C. Broholm,
      G. Aeppli, S.A. Carter, T.F. Rosenbaum, P. Metcalf and J.M. Honig, J.
      of Magn. Magn. Mat. Pt. 1, 177-181, 283 (1998). (PDF)

      Magnetic correlations in all four phases of pure and doped vanadium sesquioxide (V2O3) have been examined by magnetic thermal-neutron scattering. Specifically, we have studied the antiferromagnetic and paramagnetic phases of metallic V2-yO3, the antiferromagnetic insulating and paramagnetic metallic phases of stoichiometric V2O3, and the antiferromagnetic and paramagnetic phases of insulating V1.944Cr0.056O3. While the antiferromagnetic insulator can be accounted for by a localized Heisenberg spin model, the long-range order in the antiferromagnetic metal is an incommensurate spin-density wave, resulting from a Fermi surface nesting instability. Spin dynamics in the strongly correlated metal are dominated by spin fluctuations with a "single lobe'' spectrum in the Stoner electron-hole continuum. Furthermore, our results in metallic V2O3 represent an unprecedentedly complete characterization of the spin fluctuations near a metallic quantum critical point, and provide quantitative support for the self-consistent renormalization theory for itinerant antiferromagnets in the small moment limit. Dynamic magnetic correlations for (h) over bar omega<k different from in solution, but it may be more physiologically relevant because in vivo the deposits are associated with surfaces.

    3. Reaction-induced Phase Separation: a polymer in an anisotropic
      , J. B. Nephew, T. Nehei, S.A. Carter, Phys. Rev. Lett. 80,
      3276-9 (1998).
    4. Spin Waves Throughout the Brillouin Zone of (La,Pb)MnO3, T. G.
      Perring, G. Aeppli, S. A. Carter, J. P. Remeika, S-W. Cheong and S. M
      Hayden, Phys. Rev. Lett., 77, 711 (1996).
    5. Hole Doping on the CuO2 sites in Sr14 Cu24 O41, S. A. Carter, B.
      Batlogg, R.J. Cava, J. J. Krajewski, W. F. Peck, Jr., T. M. Rice,
      Phys. Rev. Lett., 77, 1378 (1996).
    6. Electronic Phase Separation and Charge Ordering in (Sr,La)2MnO4:
      Indication of Triplet Bipoloron
      , W. Bao, S.A. Carter, C. H. Chen,
      S-W. Cheong, B. Batlogg, and Z. Fisk, Solid State Commun. 98, 55
    7. A New High Sensitivity Manometer, T. F. Rosenbaum, S. A. Carter, J.
      M. Honig, Rev. Sci. Instrum. 67, 617 (1996).
    8. Crystal Structure and Elementary Physical Properties of La5 C19 P12
      and Ce5 C19 P12 2
      , R. J. Cava, T. Siegrist, S. A. Carter, J. J.
      Krajewski, and W. F. Peck, Jr., J. of Solid State Chem. 121, 319
    9. Stabilization of the 8-8-20 Strucutre-type in La8-x Cax Cu8-y Niy O20
      , G. L. Roberts, R. J. Cava, S. A. Carter, J. J. Krajewski and W. F.
      Peck, Jr., J. Solid State Chem. 121, 319 (1996).
    10. H-T Phase Diagrams of the Double Transition in Thoriated Ube13, D.S.
      Jin, S.A. Carter, T.F. Rosenbaum, J.S. Kim and G. Stewart, Phys. Rev.
      B53, 8549 (1996).
    11. Sr2RuO4-0.25 CO2 and the Synthesis and Properties of Sr3Ru2O7, R.J.
      Cava, H.W. Zandenbergen, J.J. Krajewski, W.F. Peck, Jr., B. Batlogg,
      S.A. Carter, R.M. Fleming, O. Zhou and L.W. Rupp, Jr., J. Solid State
      Chem. 116, 141 (1995).
    12. Magnetic and Nonmagnetic Ce in the Boro-carbides, S.A. Carter, B.
      Batlogg, R.J. Cava, J.J. Krajewski and W.F. Peck, Jr., Phys. Rev. B
      51, 12829, 1995.
    13. Pressure Effects and Specific Heat in the Reentrant Superconductor
      , S.A. Carter, B. Batlogg, R.J. Cava, J.J. Krajewski and W.F.
      Peck, Jr., Phys. Rev. B 51, 12644 (1995).
    14. Spin and Charge Dynamics in 2D Metal-Insulator Transition Oxides:
      (Pr,Sr)2NiO4 and Sr2(Ru,Ir)O4
      , S.A. Carter, S-W. Cheong, B. Batlogg,
      R.J. Cava, C.H. Chen, J.J. Krajewski, W.F. Peck, Jr., and L.W. Rupp,
      Jr., Physica B 206& 207, 856 (1995).
    15. Electron Correlations in the Metal-Insulator Transition Oxide:
      , S.A. Carter, B. Batlogg, R.J. Cava, J.J. Krajewski,
      W.F. Peck, Jr., and L.W. Rupp, Jr., Phys. Rev. 51, 17184 (1995).
    16. A Highly-Conducting Transparent Conductor: Zinc Indium Tin Oxide,
      J.M. Phillips
      , R.J. Cava, S.A. Carter, J. Kwo, S.Y. Hou, J.J.
      Krajewski, J. H. Marshall, W.F. Peck, Jr. and D.H. Rapkine, App.
      Phys. Lett. 67, 2246 (1995).
    17. Elementary Physical Properties and Crystal Structures of LaRh2B2C and
      R.J. Cava, T. Siegrist, B. Batlogg, H. Takagi, H. Eisaki,
      S.A. Carter, J.J. Krajewski and W.F. Peck, Jr., Phys. Rev. B 50,
      12966 (1994).
    18. New Charge-Ordered States in (La,Sr)2NiO4 for Hole Concentrations
      (nh=1/3 and 1/2)
      , S-W. Cheong, H.Y. Hwang, C.H. Chen, B. Batlogg,
      L.W. Rupp, Jr. and S.A. Carter, Phys. Rev. B 48, 7088 (1994).
    19. Magnetic and Transport Studies of Pure V2O3 Under Pressure, S.A.
      Carter, T.F. Rosenbaum, M. Lu, H.M. Jaeger, P. Metcalf, J.M. Honig
      and J. Spalek, Phys. Rev. B49, 7898 (1994).
    20. Optical Properties of a Correlated Electron System: V2O3, G.A.
      Thomas, D. Rapkine, S.A. Carter, T.F. Rosenbaum, P. Metcalf and J.M.
      Honig, J. Low Temp. Phys. 95, 33 (1994).
    21. Superconductivity at 23K in Yttrium Palladium Boride Carbide, R.J.
      Cava, H. Takagi, B. Batlogg, H.W. Zandenbergen, J.J. Krajewski, W.F.
      Peck, Jr., R.B. van Dover, R.J. Felder, T. Siegrist, K. Mizuhashi,
      J.O. Lee, H. Eisaki, S.A. Carter and S. Uchida, Nature 367, 146
    22. GaInO3: A New Transparent Conducting Oxide, R.J. Cava, J.M. Phillips,
      J. Kwo, G.A. Thomas, R.B. van Dover,
      S.A. Carter, J.J. Krajewski,
      W.F. Peck, Jr., J.H. Marshall and D.H. Rapkine, Appl. Phys. Lett. 64,
      2071 (1994).
    23. Transparent Conducting Thin Films of GaInO3, J.M. Phillips, J. Kwo,
      G.A. Thomas, S.A. Carter, R.J. Cava, S.Y. Hou, J.J. Krajewski, J.H.
      Marshall, W.F. Peck, Jr., D.H. Rapkine and R.B. van Dover, Appl.
      Phys. Lett. 65, 115 (1994).
    24. Superconductivity in LnPt2B2C, R.J. Cava, B. Batlogg, T. Siegrist,
      J.J. Krajewski, W.F. Peck, Jr., S.A. Carter, R.J. Felder, H. Takagi
      and R.B. van Dover, Phys. Rev. B 49, 12,384 (1994).
    25. Observations of the Gap and Kinetic Energy in a Correlated Insulator,
      G.A. Thomas, D.H. Rapkine, S.A. Carter, A.J. Millis, T.F. Rosenbaum,
      P. Metcalf, J.M. Honig, Phys. Rev. Lett. 73, 1529 (1994).
    26. Stabilization of Superconducting LnPt2B2C by Partial Substitution of
      Gold for Platinum
      , R.J. Cava, B. Batlogg, J.J. Krajewski, W.F. Peck,
      Jr., T. Siegrist, R.M. Fleming, S.A. Carter, H. Takagi, R.J. Felder,
      R.B. van Dover and L.W. Rupp, Jr., Physica C 226, 170 (1994).
    27. Electron Density of States in the Boro-Carbide Intermetallic
      s, S.A. Carter, B. Batlogg, R.J. Cava, J.J. Krajewski,
      W.F. Peck, Jr. and H. Takagi, Phys. Rev. B, Rapid Comm. 50, 4216
    28. Transparent Conducting Films of GaInO3 by sputtering, J. Kwo, S. A.
      Carter, R. J. Cava, S. Y. Hou, J. M. Phillips, D. H. Rapkine, G. A.
      Thomas, and R. B. Van Dover, Mat. Res. Soc. Symp. Proc. 345, 241
    29. Mass Enhancement and Magnetic Order at the Mott-Hubbard Transition,
      S.A. Carter, T.F. Rosenbaum, J. Spalek, J. M. Honig and P. Metcalf,
      Phys. Rev. B, Rapid Comm. 48, 16841 (1993).
    30. Uniaxial Stress Anisotropy of the Double Superconducting Transition
      in UPt3,
      D.S. Jin, S.A. Carter, B. Ellman, T.F. Rosenbaum, D.G.
      Hinks, Phys. Rev. Lett. 68, 1587 (1992).
    31. Incommensurate Spin Density Wave Transition in Doped Vanadium
      , W. Bao, C. Broholm, S.A. Carter, T.F. Rosenbaum, J.M.
      Honig, P. Metcalf, G. Aeppli and S. Trevino, Phys. Rev. Lett. 68,
      1597 (1992).
    32. Effect of Correlations and Disorder on Electron States in the
      Mott-Hubbard Insulator V2O3,
      S.A. Carter, J. Yang, T.F. Rosenbaum, J.
      Spalek, J.M. Honig, Phys. Rev. B43, 607 (1991).
    33. New Phase Boundary in Highly Correlated, Barely Metallic V2O3, S.A.
      Carter, T.F. Rosenbaum, J. Spalek, J.M. Honig, Phys. Rev. Lett. 67,
      3440 (1991).
    34. The Role of Disorder in Highly Correlated Metals and Insulators, T.F.
      Rosenbaum and S.A. Carter, J. Solid State Chem., 88, 94 (1990).
    35. Experiment to Demonstrate the Polychromatic Nature of X-Rays, R.
      Piccard and S.A. Carter, J. of Amer. Phys. Soc. Educ., (1989).
    36. Preferred Conformational State of the N-Terminus Section of a Bovine
      Growth Hormone Fragment (Residues 96-133) in Water is and Omega Loop
      P.R. Gooley, S.A. Carter, P.E. Fagerness and N.E. Mackenzie,
      PROTEINS: Structure, Function, and Genetics, 4, 48-55 (1988).

    Group Theses and Dissertations

    Stephanie Chasteen, PhD 2005 Exciton Dynamics in Conjugated Polymer Photovoltaics: Steady-state and Time-resolved Optical Spectroscopy. Note: PDF file is not as-published, it has been reformatted for readability. A PDF of the as-published version is available here.

    The performance of organic photovoltaics is severely limited by poor exciton dissociation and charge transport due in part to high rates of exciton recombination and low charge mobilities in polymers. This challenge can be partially overcome through the use of blended and layered heterojunctions. Such morphologies offer multiple exciton dissociation sites and separate charge pathways, thus limiting exciton recombination, and allowing for thicker, more absorbing, polymer films.
    I have performed photovoltaic device characterization and time-resolved and steady-state photoluminescence on a variety of donor-acceptor heterojunction. I have used these methods to understand excited state dynamics and how they affect device performance.

    As hole-transporters I use a derivative of poly-phenylene-vinylene (M3EH-PPV) and poly-3-hexylthiophene (P3HT). As electron-transporters I use the metal oxide titanium dioxide (TiO2), the electron-transporter CN-PPV, and a fullerene derivative (PCBM). These materials are layered and blended together to form donor-acceptor heterojunctions. All heterojunctions result in enhanced device performance, and 1:4 M3EH-PPV:PCBM resulted in the highest efficiencies.
    M3EH-PPV emission is characterized by single-chain excitations, and the decay is dominated by short components of 0.20 and 0.45 ns. CN-ether-PPV is dominated by interchain excited state species -- ie., excimers – with a decay time of 14.0 ns. The broken conjugation imposed by the ether group affect the excited state, resulting in an excited state species that is particularly vulnerable to quenching. This has important ramifications for material design.

    Hole-transporting polymers blended and layered with CN-ether-PPV have high currents (Jsc up to 3.3 mA/cm2) and good quenching relative to CN-ether-PPV (~90%) due to charge separation and generation, respectively. Hole-transporters blended with PCBM result in efficient devices (Jsc up to 14 mA/cm2) due to rapid charge transfer and the existence of charge percolation pathways caused by the presence of aggregates of PCBM. The size of the aggregates affects charge transport, and is highly dependent upon film processing and blend ratio.

    The best device performance does not necessarily correlate with the excited state lifetime, however. Morphological differences, such as charge pathways that enable efficient charge transport, often outweigh the effect of charge transfer. Suggestions for improvement of nanoscale morphology are given.

    Janelle Leger, PhD 2005.

    Electrochemical Doping and the Optical Properties of Light-Emitting Polymer Materials and Devices.


    The discovery in 1990 by Bradley et. al. that organic polymers with a conjugated backbone display semiconducting properties began an intensive research effort to understand the fundamental physics underlying these materials, to synthesize materials with certain desired properties, and to develop potential applications. Specifically, semiconducting polymers have been used in a wide range of device applications including light-emitting devices, transistors, photovoltaics, memory devices, and actuators, among others. The promising characteristics of polymer semiconductors include low-cost, easy processing, tailored synthesis, and potential for use in large-area and flexible devices.

    In this dissertation I introduce the field of semiconducting polymers and present my studies of electrochemical doping and the optical properties of lightemitting polymers in the context of improving functionality in a range of solid-state device applications. While great strides have been made toward the practical realization of a range of polymer-based optoelectronic and electronic applications, there is still much to learn about the fundamental processes affecting their optical properties. Specifically, detailed studies of the electrochemical doping of lightemitting polymers are lacking despite the emergence of a range of technologies such as polymer light-emitting electrochemical cells that depend heavily on the process of in situ electrochemical doping. In addition, the factors affecting the optical properties of light-emitting polymers and polymer-based devices have been limited in scope.

    The first three chapters of this dissertation serve as an introduction to the field of light-emitting polymers and polymer-based devices including materials, device construction, and measurement techniques. In chapter one I discuss the physical models necessary to understand semiconductivity in conjugated polymers. Chapter two reviews the device physics of several important applications. In chapter three I introduce the experimental techniques used in the following studies. Two well established light-emitting polymer devices include the polymer LED and the polymer LEC. The LEC uses electrochemical doping to achieve the charge injection necessary for light emission, while the LED injects charge directly from contact electrodes. I use a technique employing simulations of interference effects in multilayered device structures, matching experimental device spectra to simulation in order to gain insight into the location of light emission within the device. In chapter four I use this technique to explore the thickness dependence of PLEDs. In chapter five I combine simulations of interference effects in LECs with studies of planar geometry devices, thereby providing information about the fundamental operating mechanism of these devices. Several polymer-based applications include light-emitting electrochemical cells (LEC), electrochromic devices (ECD), and actuators, for which the operating mechanism depends heavily on electrochemical doping. Unfortunately, the doping of light-emitting polymers is not well understood. In chapter six I study the basic electrochemical doping reactions of one common light-emitting polymer, MEH-PPV. I explore factors affecting the fundamental doping reaction through cyclic voltammetry. Further, I investigate the optical properties of doped films in order to gain insight into the structural changes and changes in the energy band structure induced by doping.

    Finally, I explore some unique functionalities of MEH-PPV, specifically electrochromic and thermochromic effects. Chapter seven presents a MEH-PPV based electrochromic device with a layered polymer/gel electrolyte structure that displays sharp contrast and high reversibility when biased at low operating voltages. In chapter eight I study the reversible, high-contrast thermochromic behavior of MEH-PPV gel films, occurring in a practically relevant temperature range and without phase or volume changes.

    Peter Journey-Kilarney, BS, 2001 Photoaction Current Spectra of Polymer Solar Cells.
      For my thesis I have investigated the photoaction current spectra of six different polymer solar cell configurations, to determine how different polymer / electron-collector architectures affect a solar cell’s external quantum efficiency (efficiency at converting
    absorbed photon energy into electrical energy). Photoaction current spectra are taken by monitoring a solar cell’s current output while varying the color of the light impinging on the cell. This allows external quantum efficiency (EQE ) to be calculated as a function of the wavelength of the light absorbed by the solar cell. For this report I examined two different semiconducting polymer configurations, M3EH – PPV and a blend of M3EH –PPV and Cn – Ether - PPV. Each of these polymer configurations was examined with three different electron-collector configurations, a titanium oxide film (TiOx), titanium dioxide nanoparticles (TiO2), and combination of TiO2 nanoparticles spread over a TiOx film. For the M3EH solar cells it was noted that EQE performance was much better for devices with a TiO2 / M3EH interface. TiOx devices gave a peak EQE of 6.56% at
    480nm, while TiO2 devices generated a peak EQE of 16.87% at 460nm. The TiOx / TiO2 / M3EH devices gave the highest EQE for this experiment, with a peak EQE of 19.94% at 470nm. This data supports the argument that that TiO2 nanoparticles increase the contact area between the polymer and electron-collector. Blended solar cells on TiOx outperformed M3EH cells on TiOx with a peak EQE of 18.34% at 500nm for blended versus only 6.56% at 480nm for M3EH. However, unlike the M3EH cells, the blends actually performed worse when a TiO2 interface was used. Peak EQE dropped from 18.34% at 500nm to 11.23% at 480nm for the TiO2 interface and fell to 11.98% at 460nm for the TiOx / TiO2 interface. This data indicates that for TiOx the blending of the polymers enhances solar cell EQE. However, when TiO2 is used blend EQE actually drops below M3EH EQE. This indicates that the interaction between the Cn – Ether and TiO2 damages solar cell performance.