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Polymer Composites and Liquid
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- 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
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
- Electrochemical Doping and the Optical Properties of Light-Emitting
Polymer Materials and Devices. Leger, J. PhD Dissertation,
- 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)
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.
- 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)
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
- 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.
- 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
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
- 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)
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.
- 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)
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.
- 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)
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.
- 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)
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
- 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
- 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)
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.
- 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)
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.
- 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)
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.
- 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).
- 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)
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.
- 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.
- 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.
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.
- 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
- 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)
- 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.
- 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.
- 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.
Composites and Liquid Crystals
- 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)
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.
- 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.
- 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.
- 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.
- 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
- 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
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.
- A model for amyloid fibril formation in immunoglobulin
light chains based on comparison of amyloidogenic and benign
proteins and specific antibody binding, R.
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.
- 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.
- 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,
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
(Correlated Electron Systems, etc.)
- Temperature Dependence of the Hall Angle in a Correlated
Three-dimensional Metal, T.F. Rosenbaum, A. Husmann, S.A.
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
- Magnetic Correlations in a Classic Mott System, W.
Bao, C. Broholm,
G. Aeppli, S.A. Carter, T.F. Rosenbaum, P. Metcalf and J.M.
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
- Reaction-induced Phase Separation: a polymer in an anisotropic
solvent, J. B. Nephew, T. Nehei, S.A. Carter, Phys. Rev.
- Spin Waves Throughout the Brillouin Zone of (La,Pb)MnO3,
Perring, G. Aeppli, S. A. Carter, J. P. Remeika, S-W. Cheong
and S. M
Hayden, Phys. Rev. Lett., 77, 711 (1996).
- 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.
Phys. Rev. Lett., 77, 1378 (1996).
- Electronic Phase Separation and Charge Ordering in (Sr,La)2MnO4:
Indication of Triplet Bipoloron, W. Bao, S.A. Carter, C.
S-W. Cheong, B. Batlogg, and Z. Fisk, Solid State Commun. 98,
- A New High Sensitivity Manometer, T. F. Rosenbaum,
S. A. Carter, J.
M. Honig, Rev. Sci. Instrum. 67, 617 (1996).
- Crystal Structure and Elementary Physical Properties of
La5 C19 P12
and Ce5 C19 P12 2, R. J. Cava, T. Siegrist, S. A. Carter,
Krajewski, and W. F. Peck, Jr., J. of Solid State Chem. 121,
- 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
Peck, Jr., J. Solid State Chem. 121, 319 (1996).
- H-T Phase Diagrams of the Double Transition in Thoriated
Jin, S.A. Carter, T.F. Rosenbaum, J.S. Kim and G. Stewart, Phys.
B53, 8549 (1996).
- Sr2RuO4-0.25 CO2 and the Synthesis and Properties of Sr3Ru2O7,
Cava, H.W. Zandenbergen, J.J. Krajewski, W.F. Peck, Jr., B.
S.A. Carter, R.M. Fleming, O. Zhou and L.W. Rupp, Jr., J. Solid
Chem. 116, 141 (1995).
- Magnetic and Nonmagnetic Ce in the Boro-carbides, S.A.
Batlogg, R.J. Cava, J.J. Krajewski and W.F. Peck, Jr., Phys.
51, 12829, 1995.
- Pressure Effects and Specific Heat in the Reentrant Superconductor
HoNi2B2C, S.A. Carter, B. Batlogg, R.J. Cava, J.J. Krajewski
Peck, Jr., Phys. Rev. B 51, 12644 (1995).
- Spin and Charge Dynamics in 2D Metal-Insulator Transition
(Pr,Sr)2NiO4 and Sr2(Ru,Ir)O4, S.A. Carter, S-W. Cheong,
R.J. Cava, C.H. Chen, J.J. Krajewski, W.F. Peck, Jr., and L.W.
Jr., Physica B 206& 207, 856 (1995).
- Electron Correlations in the Metal-Insulator Transition
Sr2(Ir,Ru)O4, 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).
- A Highly-Conducting Transparent Conductor: Zinc Indium
J.M. Phillips, R.J. Cava, S.A. Carter, J. Kwo, S.Y. Hou,
Krajewski, J. H. Marshall, W.F. Peck, Jr. and D.H. Rapkine,
Phys. Lett. 67, 2246 (1995).
- Elementary Physical Properties and Crystal Structures of
LaIr2B2C, R.J. Cava, T. Siegrist, B. Batlogg, H. Takagi,
S.A. Carter, J.J. Krajewski and W.F. Peck, Jr., Phys. Rev. B
- 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.
L.W. Rupp, Jr. and S.A. Carter, Phys. Rev. B 48, 7088 (1994).
- Magnetic and Transport Studies of Pure V2O3 Under Pressure,
Carter, T.F. Rosenbaum, M. Lu, H.M. Jaeger, P. Metcalf, J.M.
and J. Spalek, Phys. Rev. B49, 7898 (1994).
- Optical Properties of a Correlated Electron System: V2O3,
Thomas, D. Rapkine, S.A. Carter, T.F. Rosenbaum, P. Metcalf
Honig, J. Low Temp. Phys. 95, 33 (1994).
- Superconductivity at 23K in Yttrium Palladium Boride Carbide,
Cava, H. Takagi, B. Batlogg, H.W. Zandenbergen, J.J. Krajewski,
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,
- GaInO3: A New Transparent Conducting Oxide, R.J. Cava,
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.
- Transparent Conducting Thin Films of GaInO3, J.M. Phillips,
G.A. Thomas, S.A. Carter, R.J. Cava, S.Y. Hou, J.J. Krajewski,
Marshall, W.F. Peck, Jr., D.H. Rapkine and R.B. van Dover, Appl.
Phys. Lett. 65, 115 (1994).
- Superconductivity in LnPt2B2C, R.J. Cava, B. Batlogg,
J.J. Krajewski, W.F. Peck, Jr., S.A. Carter, R.J. Felder, H.
and R.B. van Dover, Phys. Rev. B 49, 12,384 (1994).
- Observations of the Gap and Kinetic Energy in a Correlated
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).
- Stabilization of Superconducting LnPt2B2C by Partial Substitution
Gold for Platinum, R.J. Cava, B. Batlogg, J.J. Krajewski,
Jr., T. Siegrist, R.M. Fleming, S.A. Carter, H. Takagi, R.J.
R.B. van Dover and L.W. Rupp, Jr., Physica C 226, 170 (1994).
- Electron Density of States in the Boro-Carbide Intermetallic
Superconductors, S.A. Carter, B. Batlogg, R.J. Cava, J.J.
W.F. Peck, Jr. and H. Takagi, Phys. Rev. B, Rapid Comm. 50,
- Transparent Conducting Films of GaInO3 by sputtering,
J. Kwo, S. A.
Carter, R. J. Cava, S. Y. Hou, J. M. Phillips, D. H. Rapkine,
Thomas, and R. B. Van Dover, Mat. Res. Soc. Symp. Proc. 345,
- Mass Enhancement and Magnetic Order at the Mott-Hubbard
S.A. Carter, T.F. Rosenbaum, J. Spalek, J. M. Honig and P. Metcalf,
Phys. Rev. B, Rapid Comm. 48, 16841 (1993).
- Uniaxial Stress Anisotropy of the Double Superconducting
in UPt3, D.S. Jin, S.A. Carter, B. Ellman, T.F. Rosenbaum,
Hinks, Phys. Rev. Lett. 68, 1587 (1992).
- Incommensurate Spin Density Wave Transition in Doped Vanadium
Sesquioxide, W. Bao, C. Broholm, S.A. Carter, T.F. Rosenbaum,
Honig, P. Metcalf, G. Aeppli and S. Trevino, Phys. Rev. Lett.
- Effect of Correlations and Disorder on Electron States
Mott-Hubbard Insulator V2O3, S.A. Carter, J. Yang, T.F.
Spalek, J.M. Honig, Phys. Rev. B43, 607 (1991).
- New Phase Boundary in Highly Correlated, Barely Metallic
Carter, T.F. Rosenbaum, J. Spalek, J.M. Honig, Phys. Rev. Lett.
- The Role of Disorder in Highly Correlated Metals and Insulators,
Rosenbaum and S.A. Carter, J. Solid State Chem., 88, 94 (1990).
- Experiment to Demonstrate the Polychromatic Nature of X-Rays,
Piccard and S.A. Carter, J. of Amer. Phys. Soc. Educ., (1989).
- Preferred Conformational State of the N-Terminus Section
of a Bovine
Growth Hormone Fragment (Residues 96-133) in Water is and Omega
P.R. Gooley, S.A. Carter, P.E. Fagerness and N.E. Mackenzie,
PROTEINS: Structure, Function, and Genetics, 4, 48-55 (1988).
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
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
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
||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.