Dept of Elec & Computer Engr
Blacksburg, VA 24061-0356
Optical Sciences Center
Univ of Arizona
Tucson, AZ 85721
Palo Alto, CA 94303
*Lightwave Microsystems Corporation
*Office of Naval Research
Proceedings published as Volume 561
of the Materials Research Society
Symposium Proceedings Series.
* Invited paper
SESSION F1: NONLINEAR OPTICAL MATERIALS
9:00 AM *F1.1
DESIGN AND APPLICATIONS OF ORGANIC MOLECULES WITH LARGE
TWO-PHOTON ABSORPTION CROSS SECTIONS. Seth R. Marder , Department of Chemistry,
The University of Arizona, Tucson, AZ.
Chair: Bernard Kippelen
Tuesday Morning, April 6, 1999
Franciscan II (A)
Recently we found that conjugated organic molecules with
symmetrical charge transfer from the ends of the molecule to the middle
have large two-photon cross sections. Based on this notion we designed
and synthesized a variety of molecules with unprecedented two-photon cross
sections. Here we will discuss the design strategy in detail as well as
applications of these compounds in the field of optical limiting, two-photon
microscopy, and two-photon induced polymerization.
9:30 AM F1.2
COMPARISON OF FEMTOSECOND AND NANOSECOND TWO-PHOTON CROSS
SECTIONS MESUREMENTS OF AFX CHROMOPHORES IN VARIOUS SOLVENT ENVIRONMENTS.
Max D. Alexander, Jr. , Lisa R. Denny, Jeffrey W. Baur, Richard A. Vaia,
Paul A. Fleitz, Bruce A. Reinhardt, Materials and Manufacturing Directorate,
Air Force Research Laboratory, Wright-Patterson AFB, OH; Sean M. Kirkpatrick,
SAIC, Dayton, OH; Eric Pooler, SOCHE, Dayton, OH.
Accurate and reproducible determination of the intrinsic
two-photon cross section remains one of the most difficult tasks of organic
two-photon active materials. The measured response may represent a superposition
of the fundamental two-photon and thermal perturbations as well as excited
state absorption, small linear absorption contributions, and other multi-photon
processes. The goal of this work is to separate the intrinsic two-photon
cross section from these other contributions. The AFX family of chromophores,
and specifically AF50, developed in the Air Force Research Laboratory have
exhibited extremely large effective cross section values when measured
with nanosecond pulses. These previously reported cross sections are seen
to vary greatly with their local molecular environment and solvent system.
Here we present intrinsic cross section data measured by nonlinear fluorescence
and z-scan using femtosecond pulses. The effects local molecular environment
has been examined in a variety of solvent systems to elucidate the intrinsic
two-photon cross section. A Ti-sapphire pump-probe experiment has also
been employed to examine the excited state contribution to the apparent
two-photon cross section . The use of this technique allows for determination
of the solvent relaxation kinetics and any excited state absorption, which
can be separated as a function of the local molecular environment.
9:45 AM F1.3
MULTIFUNCTIONAL FULLERENE FILMS FOR PHOTONICS. Yanjing
Liu 1, You-Xiong Wang2, Hongxia Lu2 and
Richard O. Claus2, 1NanoSonic Inc., 2Virginia
Multilayer fullerene thin films up to more than one micron
in thickness have been synthesized using a novel electrostatic self-assembly
process, and their nonlinear optical properties characterized. The fullerenes,
a C60 and C70 mixture (fullerite, 9:1), were made water soluble using a
procedure detailed in prior publications. Consecutive monolayers of these
molecules and water soluble polymers were then adsorbed from solutions
by dipping. The regular linear buildup of the the thickness of the stable
fullerene films was characterized by UV-vis spectroscopy and ellipsometry.
Atomic force microscopy reveals ultrauniform structure of the outermost
layers of the films, and uniform particle size. Electro-optic and pulsed
laser measurements indicate both second order optical nonlinearity and
optical limiting behavior. The relationship between nonlinear behavior
and variations in the way the fullerene molecules are incorporated into
the films is discussed.
10:00 AM F1.4
RESIN PHOTOCURE VIA TWO-PHOTON UP-CONVERTED FLUORESCENCE.
Lisa R. Denny , Bruce A. Reinhardt, Max D. Alexander, Jr., Jeffery W. Baur,
Paul A. Fleitz, Richard A. Vaia, Materials Directorate, Air Force Research
Laboratory, Wright-Patterson AFB, OH; Sean A. Kirkpatrick, SAIC, Dayton,
OH; Stephen J. Clarson, University of Cincinnati, College of Engineering,
The use of organic chromophores to produce two-photon
up-converted emission as an initiation source for photocuring of resins
represents a promising new processing technique for the fabrication of
bulk structures. Resin photocure is a well-established polymerization technique
which has been used commercially for decades. Traditional single-photon
photoinitiated cures use light sources that, due to strong linear absorption
of the material, are unable to penetrate effectively. Thus, limiting fabrication
to thin configurations such as films and coatings. In contrast, use of
internally generated uv-vis radiation from an infrared two-photon initiated
fluorescence presents a tremendous advantage in terms of the ability of
that light to penetrate deeper into the resin system. As a result thicker
polymeric structures can be fabricated. The two-photon fluorescence photocure
process involves an organic chromophore exhibiting two-photon up-converted
fluorescence and a photoinitiator blended into the resin system. Once the
chromophore fluoresces the photoinitiator is activated and polymerization
can occur. The polymeric structure is fabricated in the portion of the
resin where sufficient two-photon fluorescence is produced to activate
the photoinitiator. Chromophores developed in the Air Force Research Laboratory
exhibit large effective two-photon cross-section values and thus provide
the efficient fluorescing behavior required to facilitate the photocure.
In general, the most critical factors associated with this process are
large two-photon cross-sections, efficiency of conversion, wavelenght of
emission, power threshold, as well as resultant polymer initiator chemistry.
Discussions from initial studies will include the observation of a strong
correlation between the molecular environment surrounding the chromophore
and the range in which fluorescence occurs. In these studies, using a femtosecond
pulsed laser light source, the cure of optical epoxy and acrylic polymeric
structures of greater than one centimeter in thickness have been demonstrated.
10:45 AM F1.5
LANGMUIR-BLODGETT FILMS OF SQUARYLIUM DYE J-AGGREGATES
EXHIBITING FEMTOSECOND OPTICAL RESPONSES. Makoto Furuki , The Femtosecond
Research Association, Tsukuba, JAPAN; Hitoshi Kawashima, Toshiro Tani,
Electrotechnical Laboratory, Tsukuba, JAPAN; Lyong S. Pu, Corporate Research
Lab., Fuji Xerox Co. Ltd., Kanagawa, JAPAN.
J-aggregates of squarylium dyes formed in their Langmuir-films
have been found to exhibit efficient and ultra-fast non-linear optical
properties, even in the form of a mono-molecular layer at an air-water
interface. This time, we established a method to make a Langmuir-Blodgett
(LB) film of J-aggregates with a single, intense (O. D. = 0.2 / monolayer)
absorption band at 784 nm, almost that in its Langmuir-film. Deposition
of the LB-film was carried out by withdrawing a glass substrate from an
area slightly less than monolayer occupation during compression at a constant
rate of 40 excess to the deposited area.
Over-coating the surface of the LB-film with a glassy poly-perfluorocarbon
film after deposition, enhanced and stabilized the formation of J-aggregates
on the glass substrate. Characterizations by a near-field scanning optical
microscope clarified a highly ordered structure of the LB-film, which filled
with 2-dimensional domains of J-aggregates of sub-m
diameter and mono-molecular thickness, in which the dye molecules orient
in the same directions. We also observed femtosecond nonlinear-optical
responses from this LB-film. An ultra-fast decay of absorption change (200
fs) with a quite low saturation energy (3.4 mJ/cm2pulse)
was observed in femtosecond pump-probe measurements. These results suggest
a high delocalization of the excited states in the J-aggregates of the
LB-film within ordered molecular arrangements, which maintaining 2-dimensional
mono-molecular layer structures, initially formed in the Langmuir film
at the air-water interface. This work was performed under the management
of a technological research association: the Femtosecond Technology Research
Association (FESTA), supported by New Energy and Industrial Technology
Development Organization (NEDO).
11:00 AM *F1.6 RECENT ADVANCES IR POLYMERS FOR
INTEGRATED OPTICS. William H. Steier , S.S. Lee, S. Garner, A. Chen, H.
Zhang, Univ. Southern Calif., Dept. Electrical Eng., CA; Larry R. Dalton,
Univ. Southern Calif., Dept. Chemistry, CA; Harold R. Fetterman, A. Udupa,
D. Bhattaachaya, UCLA, Dept. Electrical Eng., CA.
The recent advances in passive and active polymer materials
and their application to integrated optics will be reviewed. The latest
results on high speed traveling wave electro-optic modulators, rf phase
shifters, and recent advances in achieving three dimensional integrated
optics using polymers will be reviewed.
SESSION F2: ELECTRO-OPTICS
1:30 PM *F2.1
ENHANCEMENT OF NONLINEAR OPTICAL PROPERTIES THROUGH SUPRAMOLECULAR
CHIRALITY. Thierry Verbiest, Sven Van Elshocht, Martti Kauranen, Louis
Hellemans, Johan Snauwaert, André Persoons , University of Leuven,
BELGIUM; Colin Nuckolls, Thomas Katz, Columbia University, New York, NY.
Tuesday Afternoon, April 6, 1999
Franciscan II (A)
Nonlinear optical (NLO) effects like second-harmonic generation
and the electro-optic effect require materials that are non-centrosymmetric
on the molecular and macroscopic level. They are usually designed by incorporating
noncentrosymmetric chromophores into a noncentrosymmetric macroscopic structure
such as poled polymer films, Langmuir-Blodgett films, self-assembled films
or crystals. Improvements to the nonlinearity can be made by optimization
of the molecular nonlinear response or by improving the alignment of the
chromophores in the macroscopic structure. Here we report the nonlinear
optical properties of Langmuir-Blodgett (LB) films of chiral helicene derivatives
that self-organize in LB-films to form supramolecular structures with extremely
strong chiral properties. We show that the second-harmonic response of
nonracemic (chiral) LB-films of the helicene is several orders of magnitude
higher than the response of racemic (achiral) LB-films of the same material.
The magnitude of the NLO response of the nonracemic films was estimated
to be a respectable 50 pm/V, although the helicene structure lacks features
that are commonly associated with high nonlinearity. We present evidence
that it is the supramolecular organization of the helicene molecules that
enhances the molecular NLO response, while the presence of chirality in
the nonracemic films effectively breaks the macroscopic symmetry. Therefore,
the NLO response of the nonracemic films is much higher.
2:00 PM *F2.2
IMPORTANCE OF MATERIAL AND OPERATIONAL PARAMETERS LIMITING
THE PHOTOSTABILITY OF ELECTRO-OPTICS CHROMOPHORE DOPED POLYMERS. Adriana
Galvan-Gonzales1, Michael Canva 1,2, George Stegeman1,
Robert Twieg3, Seth Marder4; 1CREOL, School
of Optics, University of Central Florida, Orlando, FL; 2IOTA,
Laboratoire Charles Fabry du CNRS, Université d'Orsay, FRANCE; 3Liquid
Crystal Institute, Kent State University; 4JPL and Beckmann
Institute, CalTech, Pasadena, CA.
Electro-optics chromophore doped polymer materials have
been studied for many years. They have been constantly improved and many
efficient potential applications have now been demonstrated. Compared to
other materials, they usually present the advantages of better efficiency,
ease of integrability and low cost. Their Achille heel now remains in their
chemical stability. The orientational stability issue, necessary to retain
a chi(2) nonlinearity, has been addressed by many means, including covalently
grafting the chromophores to the matrix, increasing the Tg of this latter
and in parallel increasing the thermo-stability of the chromophore in order
to resist the high temperature inherent to the electric poling process.
Photostability of the chromophores, i.e. degradation reaction induced by
absorption of a photon, has remained far less studied. However in an optical
device which aim is to operate with photons, this issue will ultimately
limit the component lifetime. We have investigated the photodegradation
behavior of a number of chromophore doped systems, guest/host and side-chains,
as a function of excitation wavelength. Systematic behaviours have been
unravelled and allow us now to forecast material properties as a function
of photon energy using only very few experimental data. We have also quantified
the impact of operational parameter such as temperature, atmospheric composition,Ö
Currently, we are interested in assessing the impact of chromophore structure
and chromophore interactions with its host. Photostability figures of merit
have been defined that easily allow to calculate device lifetimes in any
operational configuration. Necessary material improvements are thus quantified.
2:30 PM F2.3
ENHANCED ELECTRO-OPTIC COEFFICIENT (R33) OF DOPED POLYMER
FILMS THROUGH OPTIMIZATION OF CHROMOPHORE ENVIRONMENT. Richard A. Vaia
, Stephen Caracci, Bruce Reinhardt, Materials and Manufacturing Directorate,
Air Force Research Laboratory, WPAFB, Dayton OH; Michael Banach, Department
of Materials Science and Engineering, University of Cincinnati, Cincinnati,
The influence of chromophore-matrix and chromophore-chromophore
interactions on processing and final electro-optical (EO) properties of
guest-host polymer systems are discussed. Molecular interactions between
chromophore and polymer define the local chromophore environment and dictate
the alignment (poling) efficiency and subsequent thermal stability of the
non-linear polymer. In addition, specific intermolecular interactions will
retard cooperative relaxations (enhancing stability) and promote molecular
level miscibility (increasing effective chromophore loading and EO coefficient
(r33)). The molecular environment of Disperse Red 1 (DR1) and 4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostryl)-4H-pyran
(DCM) doped in poly(styrene) (PS), poly(2-vinyl pyridine) (P2VP), poly(methyl
methacyrlate) (PMMA), and styrene-methyl methacyrlate copolymers was examined
with UV-VIS absorption, in-situ thermal ellipsometry and FTIR spectroscopy.
These observations are correlated to processing parameters (poling field,
temperature) and EO coefficient. The final EO coefficient varied as much
an order of magnitude depending on the host polymer. Dispersion and local
field effects due to the dielectric constant of the polymer do not account
for these large variations. Specific chromophore-host interactions such
as H-bonding increase chromophore solubility by preventing aggregate formation
through dipole-dipole interactions. Additionally, a field-responsive host
facilitates chromophore alignment. Combining these effects, such as in
DR1-P2VP system, leads to an enhancement of r33 by greater than 40% with
respect to conventional DR1-PMMA systems.
2:45 PM F2.4
BRIDGED POLYSILSESQUIOXANES WITH IMPROVED SECOND ORDER
NONLINEAR OPTICAL PROPERTIES AND STABILITY. Stephen T. Hobson , US Army
Inst of Chem Defense, Adv. Drug Assessment, Aberdeen Proving Ground, MD;
Kenneth J. Shea, Univ of California-Irvine, Dept of Chemistry, Irvine,
CA; Jarek Zieba, Paras N. Prasad, Dept of Chemistry, SUNY University at
Buffalo, Buffalo, NY.
We report the synthesis and sol-gel polymerization of
4-nitro-N,N-bis[(3-triethoxysilyl)propyl]aniline. An efficient synthesis
of the monomer was developed by the hydrosilylation of N, N-diallyl-4-nitroaniline.
Optical quality thin films were synthesized by spin coating a n-butanol
solution of 4-nitro-N,N-bis[(3-triethoxysilyl) propyl]aniline using formic
acid both as catalyst and source of water. We improved the temporal stability
of the NLO signal from films prepared from 4-nitro-N,N-bis[(3-triethoxysilyl)propyl]aniline
by increasing the intensity of the poling field and extending the heating
period during the poling/curing stage. By Maker fringe analysis, a (2)
value of 9 10-8
esu was measured for these polysilsesquioxanes. If one assumes that the
major component of the NLO effect is along the z axis, the (2)
value corresponds to a d33 coefficient of 18.9 pm/V and a r33
value of 4.7 pm/V.
3:30 PM *F2.5
HIGH SPEED LAYER-BY-LAYER SELF-ASSEMBLY OF POLARIZED
CHROMOPHORIC SUPERLATTICES. Yanjing Liu , NanoSonic, Inc., Richard O. Claus,
Dept of Electrical and Computer Engineering, Dept of Material Science and
Engineering, Virginia Tech, VA; James R. Heflin, Daniela Marciu, and Charley
Figura, Dept of Physics, Virginia Tech, VA.
A new method for high speed build-up of the macroscopic
non-centrosymmetric NLO films at room temperature has been developed, using
layer-by-Layer electrostatic self-assembly (ESAM) technique. It has been
demonstrated that this molecular-assembly technique can provide not only
highly homogeneous, micron-thick films, but also relatively high (2)
values. The ESAM films have exhibited no measurable decay of (2)
at room temperature over a period of more than eighteen months in contrast
to electric field poling films. It will also be demonstrated that (2)
values can be increased significantly by applying a dc electric field during
the self-assembly process.
4:00 PM F2.6
SECOND ORDER NONLINEAR OPTICAL FILMS BY ALTERNATING POLYELECTROLYTE
DEPOSITION ON HYDROPHOBIC SUBSTRATES. M. Joseph Roberts , Geoff A. Lindsay,
Naval Air Warfare Center, Chemistry and Materials Division, China Lake,
CA; Warren N. Herman, Naval Air Warfare Center Aircraft Division, Electro-Optic
Sensors Branch, Patuxent River, MD.
Alternating polyelectrolyte aqueous solution deposition
technique (APD) may be used to process nonlinear optical polymers (NLOP)
into noncentrosymmetric ordered films near room temperature. In this study,
second-order NLOP APD films of a stilbazolium-substituted polyepichlorohydrin
and poly(sodium 4-styrenesulfonate) were prepared by alternately dipping
a substrate into aqueous solutions of each polymer. Evidence for uniform
layer to layer deposition includes a linear increase of UV-Visible absorbance
and quadratic increase of second harmonic generated light intensity as
a function of film thickness. Films have been uniformly deposited up to
116 bilayers. The Maker fringes exhibit additional minima due to interference
of the second harmonic generated at the two sides of the substrate that
supports films on both sides. The fact that these minima go to zero indicates
high quality films. Azimuthal scans indicate that the polar order is isotropic
in the plane of the film. Thermal stability experiments show that these
films can maintain 90 of the polar order
up to 150C during a slow temperature
ramp of 1C/min. Films have been
further characterized by contact angle measurements, profilometry, and
polarized light microscopy. Work is in progress to deposit thicker films
of the same quality and to quantify NLO figures of merit.
4:15 PM F2.7
STRUCTURE-PROPERTY RELATIONSHIPS IN ORGANIC NONLINEAR
OPTICAL MATERIALS. Eric M. Breitung and Robert J. McMahon, University of
Wisconsin, Department of Chemistry and Materials Science Program, Madison,
Tuning the degree of bond-length alternation in organic
nonlinear optical materials is a powerful paradigm for the design of organic
materials with large molecular hyperpolarizabilities ().
Several research groups employed this paradigm in the design and synthesis
of NLO materials incorporating donor-acceptor polyenes. Increased bond-length
alternation in polyenes leads to decreased barriers to rotation about C=C
bonds and, hence, increased conformational flexibility. Since the degree
of bondlength alternation is solvent dependent, so is the degree of conformational
flexibility. In an effort to probe the influence of conformational flexibility
on NLO response, we synthesized a series of simple donor-acceptor polyenes
that are either conformationally flexible (eg. dimethylaminopentadienal)
or rigid (eg. 7-dimethylamino-4,4a,5,6-tetrahydro-2(3H)-naphthalenone).
For each pair of molecules ZINDO sum-over-states calculations predict a
larger value of ,
for the conformationally flexible isomer, but EFISH measurements (CHCl3)
reveal a larger value of,
for the conformationally rigid system. Various explanations for this behavior
will be considered.
4:30 PM F2.8
CARBAZOLE CYCLIC OLIGOMERS AND CHROMOGENIC CALIXARENES
FOR NONLINEAR OPTICS. Tatsuo Wada 1,2, Atsushi Gunji1,2,
Yadong Zhang1, Sumio Maruyama2, Stephan Houbrechts2,
Hiroyuki Sasabe1,2, 1Core Research for Evolutional
Science and Technology (CREST), JST, Frontier Research Program, The Institute
of Physical and Chemical Research (RIKEN), Wako, JAPAN; Yuji Kubo, Saitama
University, Dept. of Applied Chemistry, Urawa, JAPAN.
We have developed cyclic oligomers as a new class of multifunctional
molecules. We investigated second-order nonlinear optical responses on
two kinds of molecular systems such as carbazole cyclic oligomers and chromogenic
calixarenes. By Knoevenagel condensation between bisformyl carbazole
and bis(cyanoacetate), we can control the ring size of carbazole cyclic
oligomers with a high yield. Efficient energy transfer was observed from
carbazole moieties to acceptor-substituted ones in the tetramer. This carbazole
cyclic oligomer can be used as an electron-transport and luminescent materials
in electroluminescent devices. Acceptor-substituted carbazole and indoaniline
derivatives with a different number of moieties were introduced into the
calixarenes platform. The topological shapes of indoaniline-derived
calixarenes were studied by hyper-Rayleigh scattering. The two indoaniline
moieties in the chromogenic calixarenes were pre-aligned and net molecular
hyperpolarizability was enhanced.
4:45 PM F2.9
POLING AND RELAXATION DYNAMICS OF 2ND ORDER NLO-ACTIVE
NEMATIC LIQUID CRYSTALLINE MAIN CHAIN POLYMERS. A.T.H. Koch , M. Warner,
J.J.M. Halls, S.V. Fridrikh, Cavendish Laboratory, Cambridge, UNITED KINGDOM;
E. Toussaere, Ecole Normale Supíerieure de Cachan, FRANCE; C.E.
Schwarzwîalder, S.C. Moratti, Melville Lab for Polymer Synthesis,
Cambridge, UNITED KINGDOM; R.H. Friend, Cavendish Laboratory, UNITED KINGDOM.
We report the characterisation of the second order nonlinear
optical properties of a semiflexible main chain random copolymer with a
head to tail structure synthesized from 4-carboxy-4-[N-(11-hydroxyundecyl)-N-methylamino]-azobenzene
and hydroxybenzoic acid. This polymer exhibits a nematic liquid crystal
mesophase with a nematic-isotropic transition temperature of
C. We have carried out measurements of the temperature and field dependent
poling behaviour by means of a time-resolved ellipsometric set-up after
Teng and Man. We measure values of
of up to 90 pm/V. As a function of temperature, the plateau values of the
poling response to an applied electric dc field vary stepwise with increased
mobility of the various segments of the polymer chain. These changes in
mobility correspond to conformational changes of the polymer which we have
studied by differential scanning calorimetry (DSC), thermally stimulated
discharge current (TSDC) and powder diffractometry x-ray analysis. In the
time domain, we find that above the glass transition temperature Tg
of C up to a temperature
of C a fast and a slow
poling response can be distinguished. In that temperature range is not
possible to anneal out the nematic defects even with applied field strengths
up to V/m. Between
C and the nematic-isotropic transition only the fast response can be observed.
We show that the contribution
to the measured NLO-response is less than 5% for the particular material
studied even at the highest applied fields. We model the dependence of
the poling on the applied field strength with a modified self-consistent
Maier-Saupe approach. We have used electroabsorption to measure the imaginary
part of over a wavelength
range from 350 to 1000 nm and to determine the degree of polar order in
the polymer films. Furthermore, we find sub-band gap features in the electroabsorption
spectra which we assign to charge transfer states resulting from charge
injection into the polymer films.
SESSION F3: POSTER SESSION
APPLICATION OF THE FÖRSTER MODEL TO A STUDY OF REVERSE
SATURABLE ABSORPTION IN FULLERENE-CONTAINING POLYIMIDE. N.V. Kamanina ,
L.N. Kaporskii, Vavilov State Optical Inst, St. Petersburg, RUSSIA; V.I.
Berendyaev, B.V. Kotov, Karpov Research Physical-Chemical Inst, Moscow,
Tuesday Evening, April 6, 1999
Metropolitan Ballroom (A)
Effect of reverse saturable absorption (RSA) is the characteristics
property of polymers doped with fullerenes. In the present work RSA has
been investigated in the structures based on polyimide. The structures
were a 1m thick film of the aromatic
polyimide solution in 1,1,2,2-tetrachloroethane doped with the fullerene
mixture (C60:C70=87:13) and/or the malachite green
dye. The RSA investigations were carried out under an irradiation of a
pulsed Nd-YAG laser at wavelength of 532 nm.
RSA was detected in both types of the structures with
a level of optical limiting depended on the concentrations of the fullerene
mixture and of the dye. The most optical limiting was observed in the polyimide
structure doped with the fullerene mixture simultaneously with the dye.
Since the absorption spectrum of the fullerene-polyimide
system is overlapped with the fluorescence spectrum of the malachite green
dye at the wavelength of the excitation radiation (532 nm), resonance conditions
are fulfilled in the polyimide-dye-fullerene structure. This fact allows
the RSA peculiarities of that structure to be explained in the framework
of the Förster model . In this case overlapping the electron shells
of the dye and fullerene molecules provides the favourable conditions for
the formation of the charge transfer complexes as the result of the free
electron exchange between donor (dye) and acceptor (fullerene).
 T. Förster, Disc. Farad. Soc., 27 (1959) 7.
RAMAN SCATTERING BY COMPOSITIONS OF TRANS-NANOPOLYACETYLENE.
Valerii Kobryanskii , Institute of Chemical Physics RAS, Moscow, RUSSIA.
The polymerization of acetylene on a catalytic system,
composed of binuclear rhenium complexes and reducing agent, in solution
of saturated polymers leads to soluble compositions containing nanoparticles
of polyacetylene with low content of conformational and chemical defects.
Studying the structure of polyacetylene obtained in different polymeric
matrices indicates that the compositions of polyacetylene with poly(vinyl
butiral) are of greatest interest for theory and practice. In this report,
fundamental regularities of Raman scattering by the trans-nanopolyacetylene
(trans-NPA) compositions are reported. The results make it possible to
formulate four basic features that distinguish the Raman scattering by
the trans-NPA compositions from the standard trans-PA modifications. (1)
The weak frequency dispersion of polyene C=C and C-C stretching bands,
which is evidence for the narrow length distribution of conjugation chains
and the narrow energy distribution of the lowest-lying excited states 1Bu
and 2Ag in trans-NPA. (2) The occurrence of long sequences of overtone
and combination bands in room-temperature resonance Raman spectra and the
occurrence of at least two overtones and combination band in room-temperature
off-resonance Raman spectra (785- and 1064 nm excitations) of trans-NPA
compositions. (3) High intensities of the anti-Stokes lines of the fundumentals
C=C and C-C stretching vibrations of the polyene chain. (4) Extremely high
intensities of the fundamentals C=C and C-C stretching vibrations of the
polyene chain. Compositions of trans-NPA could be used for elaboration
of new types of Raman lasers and security of objects and documents against
PUSH-PULL MOLECULES: SOLVENT AND VIBRATIONAL AMPLIFICATION
OF NLO RESPONSES. Anna Painelli , Parma Univ, Dept Chimica GIAF, Parma,
NLO properties of push-pull molecules are usually modeled
in terms of the DA dimer model, a simple two-state model. This model proved
useful in understanding the physical behavior of these molecules, as well
as in providing guidelines for the synthesis of molecules with large NLO
responses. Recently I have shown that this model can be easily extended
to account for the coupling of electrons with molecular vibrations: In
push-pull molecules vibrations originate a large amplification of static
NLO responses, and strongly affect the responses even at optical frequencies.
In the same spirit, I show that the coupling of the molecular dipole moment
to the polarization of the surrounding solvent can be modeled in terms
of an effective DA dimer. In fact, the fast component of the solvent polarization
simply renormalizes the parameters of the DA model. Moreover, in polar
solvents, the slow orientational polarization adds to vibrations in amplifying
the static NLO responses. Solvent effects on spectroscopic properties of
push-pull molecules are highly non-trivial and cannot be caught, even at
a qualitative level, without properly accounting for the non-linearity
of the solvent-solute interaction. In particular, the implications of this
non-linearity are discussed in connection with solvatochromic effects (for
both absorption and emission bands) and electroabsorption measurements
in solution. The ground and excited state dipole moments of push-pull molecules
are usually extracted from these experiments using standard theories accounting
for linearized solvent-solute interactions: the corresponding estimates
are proved to be unreliable for molecules characterized by large non-linear
responses and alternative interpretative schemes, based on truly non-linear
interactions, are presented.
A STUDY OF GROUP VELOCITY DISPERSION OF SINGLE-MODE OPTICAL
WAVEGUIDE UTILIZING SILOXANE POLYMER. Yuriko Ueno , Katsuhiro Kaneko and
Shigeo Tanahashi, Kyocera Corporation, R&D Center Keihanna, JAPAN.
In this paper, we describe result of measurement on a
group velocity dispersion for an optical wave guide utilizing siloxane
polymer on ceramic substrate. The single-mode optical waveguide utilizing
siloxane polymer on ceramic substrate has been developed for opto-electronic
multichip modules to operate femto seconds light wave pluses and THz electrical
signals. Propagation loss of the single-mode optical waveguide formed on
ceramic substrate is 0.14 dB/cm at 1.3 micron meter wavelength. However,
siloxane polymer has an absorption spectrum band at about 1.53 micron meter
which is assumed by combination band of C-H vibration modes. Then propagation
loss of the waveguide is 2.0 dB/cm at 1.55 micron meter wavelength. The
group velocity dispersion of the optical waveguide is measured by a following
method. At first, an optical signal was modulated by a tunable laser source
and passed through the optical waveguide. Second, that optical signal passed
through the optical waveguide was detected by an optical receiver and compared
against its modulation electrical signal as reference. Third, the difference
of phase between the two signals was measured as a group delay. A group
velocity dispersion is calculated from the value. The result showed that
the group velocity dispersion of the optical waveguide was significantly
related to the absorption spectrum of siloxane polymer. The group velocity
dispersion of the optical 50mm long waveguide is about 2 fsec/nm at 1.55
micron meter wavelength. For example, when a 500 femto seconds optical
pulse which has 16 nm for spectrum width passed through the optical wave
guide, an expansion of the spectrum width is estimated as about 32 nm.
Then, this expansion of spectrum is well to distinguish femto seconds optical
signals. Thus, the optical waveguide utilizing siloxane polymer is well
capable for femto seconds optical pluses transmission.
TWO PHOTON SPECTROSCOPY OF DITHIENYL POLYENES. Thomas
M. Cooper , Paul A. Fleitz, Laura Sowards, Air Force Research Lab, Wright-Patterson
Air Force Base, OH; Lalgudi V. Natarajan, Sean M. Kirkpatrick, SAIC, Dayton,
OH; Charles W. Spangler, Dept. of Chemistry, Montana State University,
To develop novel nonlinear absorbers for optical limiting
applications, we have been investigating the properties of dithienyl polyenes.
Spectroscopic measurements include UV/Vis, fluorescence, fluorescence lifetime
and two photon absorption and fluorescence as a function of the polyene's
conjugation length. Structure-property trends will be used to explore the
feasibility of using these compounds for optical limiter applications.
COMBINED MAIN- AND SIDE-CHAIN AZOBENZENE POLYESTERS:
A POTENTIAL FOR PHOTOINDUCED NONLINEAR WAVEGUIDES. Frederik Sahlén,
Tommy Geisler, Svren Hvilsted, Condensed Matter Physics and Chemistry Department,
Ris National Laboratory, Roskilde, DENMARK; N.C.R. Holme, P.S. Ramanujam,
Optics and Fluid Dynamics Department, Ris National Laboratory, Roskilde,
DENMARK; Jan C. Petersen , Danish Institute of Fundamental Metrology, Lyngby,
New combined main- and side-chain azobenzene polyesters,
which exhibit an intensity dependent refractive index, have been prepared
in order to optically fabricate nonlinear waveguides. The general design
idea of our polymers comprises a (3)-active
chromophore in the main-chain, and a chromophore, which can be oriented
by polarized light, in the side-chain. The photoinduced orientation of
the side-chains results in anisotropy or birefringence, which provides
the basis for the waveguide properties. Furthermore large surface-relief
gratings have been found to arise in azobenzene polymers. This grating
formation could facilitate the coupling of light into and out of the waveguides.
Novel sulfonyl azobenzenes, a diester, (4-[[5-(ethoxycarbonyl)pentyl]sulfonyl]-4í-
[[5-(ethoxycarbonyl)pentyl]methylamino] azobenzene, and a diol, 4-[[(8-hydroxy-7-methylhydroxy)-octyl]sulfonyl]-
azobenzene, have been used to prepare new polyesters by transesterification
in the molten state. The polyesters have been characterized by 1H
NMR, 13C NMR and UV-visible spectroscopy, differential scanning
calorimetry (DSC), size exclusion chromatography (SEC), third harmonic
generation (THG) and optical anisotropy measurements. The molar masses
of the polyesters were in the range of 5000-19000 g mol-1, which
was sufficient in order to spin coat thin films. Optical anisotropy measurements
have been carried out in thin films of these polymers. Polarization holography
was employed to write both an anisotropic and surface relief grating. A
diffraction efficiency of approximately 12
was obtained from preliminary measurements. From atomic force microscopic
investigations, it was found that a stable surface relief grating with
a typical depth of 700 nm was also formed. The peaks of the surface relief
grating were found to coincide with linear polarization perpendicular to
the grating vector. From THG measurements the polymers are shown to possess
an off-resonance electronic (3)
of the order 10-12 esu corresponding to a non-linear refractive
index of 2.310-14 cm2/W.
DESIGN AND SYNTHESIS OF DENDRIMERS WITH ENHANCED TWO-PHOTON
ABSORPTION. El Hadj Elandaloussi , Charles Spangler, Montana State Univ,
Dept of Chemistry and Biochemistry, Optical Technology Center, Bozeman,
MT; Carl Dirk, Dept of Chemistry, Univ Texas, El Paso, TX; Martin Casstevens,
Deepak Kumar, John Weibel, Ryszard Burzynski, Laser Photonics Technology,
Inc, Amherst, NY.
There has been considerable recent interest in the design
of new organic chromophores, oligomers and polymers with potentially large
two-photon cross-sections. One particularly attractive system is based
on poly[p-phenylene vinylene] (PPV) oligomers containing electron-donating
substituents. We have recently designed and synthesized several PPV dimers,
with bis-(diphenylamino) donor groups attached to the terminal phenyl rings,
which have demonstrated large TPA cross-sections. We have now extended
this design concept to dendrimer structures based on bis-(diphenylamino)stilbene
repeat units. The efficacy of this approach will be discussed, as well
as projected future design paradigms for even greater TPA enhancement.
PHOTONICS APPLICATIONS OF BIPOLARON FORMATION IN BIS-(DIPHENYLAMINO)DIPHENYLPOLYENES.
Kimba Ashworth , Benjamin Reeves, Amy Frost, Charles Spangler, Montana
State Univ, Dept of Chemistry and Biochemistry, Optical Technology Center,
Bis-(dialkylamino) and bis-(diphenylamino) substituted
diphenylpolyenes have recently been shown to have exceptionally large two-photon
cross-sections, and have also been used in optical power limiting (OPL)
applications. Another potential mechanism for OPL in these materials involves
the photogeneration of highly absorbing bipolaronic dications which can
then function as reverse saturable absorbers (RSAs). The synthesis of two
series of these unique polyenes has been carried out, as well as studies
of bipolaron formation in solution. These new chromophores have also been
incorporated into pendant polymers, copolymers and dendrimer formulations,
and the efficiency of photonic processes based on polymer type will be
THE SYNTHESIS OF DIPHENYLPOLYENES AND PPV-OLIGOMERS INCORPORATING
DIPHENYLPHOSPHINO DONOR GROUPS. Luis Madrigal , Charles Spangler, Dept
of Chemistry and Biochemistry, Optical Technology Center, Montana State
Univ, Bozeman, MT; Carl Dirk, Dept of Chemistry, Univ Texas, El Paso, TX.
Previous studies of the second and third order nonlinear
optical properties of organic molecules have shown enhancement of the nonlinear
optical response when second row elements replace first row elements in
the structure (e.g. S for O in donor groups). Various substituted amino
groups have been used extensively in the design of molecules for both second
and third order NLO applications as strong donor groups. However, to date
there have been no reported systematic studies of the effect of replacing
P for N in various chromophore functionalities. In this presentation we
will discuss the syntheses of several organic polyenes with diphenylphosphino
substituents, and compare structure-property relationships to the equivalent
SYNTHESIS AND OPTICAL LIMITING CHARACTERIZATION OF A
PORPHYRIN-BUCKMINSTERFULLERENE DYAD. Kenneth J. McEwan and Keith L. Lewis,
Defence Evaluation and Research Agency, Malvern, UNITED KINGDOM; How-Ghee
Ang, Zhi-Heng Loh , Leng-Leng Chng and Yiew-Wang Lee, DSO National Laboratories,
Buckminsterfullerene (C60) and porphyrins are
well-known optical limiters in the visible region. Furthermore, the enhanced
nonlinear absorption by the fullerene anion (C60-)
relative to pristine C60 is expected, due to the increased electronic
transition dipole moment of the unpaired electron in the excited state.
We synthesized a porphyrin-C60 dyad in which the spherical C60
moiety is positioned above the porphyrin plane. Molecular modeling yielded
a structure with a through-space inter-chromophore distance of 3.28 .
It is hoped that the close proximity of the two moieties would facilitate
inter-chromophore electron transfer to yield the charge-separated species,
which in turn, is expected to exhibit enhanced optical limiting performance
relative to its components. However, electronic spectroscopy did not show
any through-space interaction. This was attributed to the unfavorable disposition
of electron-rich bonds in each chromophore towards one another. Optical
limiting measurements using 3-ns laser pulses at 532 nm were performed
on solutions of the porphyrin-C60 dyad, the reference porphyrin,
and C60. The linear transmittances of the solutions was 40.
Optical limiting was observed for all the samples. However, the transmitted
saturation level exhibited by the porphyrin-C60 dyad was about
4 times higher than that of C60 and 1.3 times higher than that
of the reference porphyrin. This could be due to the absence of through-space
interaction as predicted from molecular modeling, as well as a shift in
the excited state absorption spectra of the dyad relative to its individual
components. The latter awaits further confirmation from time-resolved laser
spectroscopy experiments. Nevertheless, the vast structural variation possible
in such dyads, as presented by their ease of chemical modification, should
enable the rational design of dyads with enhanced optical limiting performances
relative to their respective components.
ELETRICAL FIELD EFFECTS ON CRYSTALLINE PERFECTION OF
MBA-NP CRYSTALS BY MAPPING OF BRAGG-SURFACE DIFFRACTION. S.L. Morelhao
, Sao Paulo Univ., Dept of Applied Physics, Sao Paulo, BRAZIL; L.H. Avanci,
M.A. Hayashi, L.P. Cardoso, Univ. of Campinas, Inst. of Physics, Campinas,
High quality non-centrosymmetric organic single crystals,
such as 2-(alfa-ethylbenzylamino) - 5-nitropyridine (MBA-NP), have exceptionally
large second order polarisabilities. The possibilities of exploiting this
fundamental property are perceived to be extensive because of the almost
endless variations in chemical structure that can be produced through modern
organic synthesis. In addition, to exceptional nonlinearities the space
groups are all of the type that also produces piezoelectricity. In terms
of controlling material non-linear properties, characterization of the
piezoelectric tensor has been important. X-ray diffraction techniques are
often used to determine small changes in the crystal lattice under an applied
electric field. Recently [Phys. Rev. Lett., 1998 (in-press)], a method
based on the X-ray multiple beam diffraction phenomenon were developed
for determining more than one piezoelectric coefficients from a single
scan. A strong limitation for applying such method is the non-constant
mosaicity of the MBA-NP crystals. In this article, we have essentially
investigated effects of weak electric fields to the spatial misorientation
of the large perfect diffracting regions (mosaic blocks) of the crystals.
The misorientation are monitored by mapping of Bragg-surface diffraction
(BSD) [Appl. Phys. Lett. 71(18), 2614 (1997)], which is the unique diffraction
technique to allow the spatial view of the misorientation. The BSD is a
special three-beam diffraction that involve one Bragg reflection and one
surface reflection (diffracted beam in the surface parallel direction).
Reciprocal space mapping are also used for comparison. The misorientation
effects are correlated to the direction of molecular electrical dipole.
TWO-PHOTON ASSISTED NONLINEAR ABSORPTION OF DIFLUORENYLIDENETHENE.
Anders Eriksson , Linkoping University, Dept of Physics and Measurement
Technology, Linkoping, SWEDEN; Per Andersson, Bertil Eliasson, Umea University,
Dept of Organic Chemistry, Umea, SWEDEN; Mikael Lindgren, Svren Svensson,
Defence Research Establishment, Linkoping, SWEDEN.
A cumulene named difluorenylidenethene (DFE) has been
examined with regard to its third order optical nonlinearities. The material
was found to be stable with a strong absorption peak at 479 nm but with
very weak absorption above 500 nm. The z-scan method and nonlinear absorption
measurements were used to specify the origin of the optical nonlinearity.
A Sunlite OPO laser allowing for wavelength tuning was used to characterize
the wavelength dependence of the response of the material. Luminescence
spectra were collected for a number of different exciting wavelengths and
energies. From a preliminary analysis the maximum effective nonlinear absorption
coefficient (beta) was found to be ca 44 cm/GW at 532 nm for 3.4 ns pulse-length,
however, with a large spreading in experimental data depending on pulse
energy. The nonlinear behaviour can possibly be explained in terms of two-photon
assisted reverse saturable absorption.
SYNTHESIS AND CHARACTERIZATION OF SOME NEW METAL-CONTAINING
POLYMERS FOR THIRD ORDER NONLINEAR OPTICAL APPLICATIONS. Mamoun M. Bader
, Phuong-Truc T. Pham, Justin Moser, Leslie Albright, Kevin Myeres and
Angela Molli, Department of Chemistry, Pennsylvania State University, Hazleton,
Continuing with our earlier theoretical investigations
of 8-hydroxyquinoline and its derivatives, we report here on the synthesis
and characterization of a new family of metal-containing polymers incorporating
8-hydroxyquinoline, for third order nonlinear optical applications. These
polymers have the following structural features: a metal center, a chromophore,
and a flexible spacer group with variable lengths. These structural features
allow us to have an appreciable degree of control over the various physical
properties of these materials. We report on three different types of such
polymers: polyethers, polyesters, and polyamides. Finally the chromophores
in these polymers are either incorporated in the backbone and/or as a side
OPTICAL NONLINEAR PROPERTIES OF SELF-ASSEMBLED J-AGGREGATES
CYANINE MOLECULES DOPED IN SPIN-CASTED SILICATE FILMS. Itaru Honma , *T.
Watanabe, H.S. Zhou, *K. Ishigure and *K. Asai, Energy Division, Electrotechnical
Laboratory, AIST, Umezono, Tsukuba, Ibaraki, JAPAN; *Department of Quantum
System Engineering, Faculty of Engineering, University of Tokyo, Hongo,
Bunkyo-ku, Tokyo, JAPAN.
The functional opto-electrical materials with molecular-to-nano
size structure has attracted much attention for devices with excellent
properties. We present, in this paper, a new synthetic route of self-organization
process to produce self-assembled J-aggregates cyanine molecules doped
in sol-gel derived silicate films. The J-aggregates of cyanine molecules
(1-methyl-1octadecylquino cyanine perchlorate) with sufficient optical
density are spontaneously doped in the sol-gel derived silicate thin films
through spin casting technique. The cyanine molecules are solvated in the
solution of HCl/H2O mixture, ethanol and TEOS. The mixed solution
was spin casted on the silica substrate at the rotational speed of 2000
rpm. The as-prepared film was red-colored where only isolated molecular
absorption can be observed, however, as the hydrolysis and condensation
of silica proceeds, the doped molecules start forming J-aggregates in the
glassy matrix and optical absorption changes to that of the aggregates.
The process provide simple and efficient method to prepare optical grade
J-aggregates-doped silica thin films while the optical density is much
larger than the that formed by LB technique. The third order nonlinear
optical properties of the film was measured by Z-scan method in a vacuum
at liquid nitrogen temperature. The measuring wavelength of 577 nm and
light intensity of 20-80 MW/cm2 was used. The large nonlinear
susceptibility of 5 x 10-7 esu was obtained for doped J-aggregated
in the transparent silica film.
ULTRAFAST OPTICAL RESPONSES OF THE SQUARYLIUM DYE J-AGGREGATES
FILMS. Satoshi Tatsuura , Min-quan Tian, Yasuhiro Sato, Lyong Sun Pu, Fuji
Xerox Co., Ltd., Corporate Research Labs., Kanagawa, JAPAN; Makoto Furuki,
FESTA Laboratories, Ibaraki, JAPAN.
Some organic molecules are attractive candidates for the
ultrafast optical devices because of their fast recovery of the bleached
and induced absorption. Recently they have been observed that the J-aggregates
of pseudoisocyanine bromide have a fast decay component of 1.3 ps at 77K
and that the monolayer of squarylium(SQ) dye J-aggregates on water surface
has 280 fs decay time at room temperature(RT). But in consideration for
utilizing them as optical devices, operation at RT and easiness of handling
should be very important. We tried to fabricate the films consist of SQ
J-aggregates on solid substrates and evaluate their decay times of the
bleached absorption. We fabricated the SQ films on glass substrates by
spincoating method. The SQ derivatives differed from each other markedly
in their spectra and behavior under heating according to their alkyl side
chain length. Progress of aggregation was observed for some SQ derivatives
under heating and/or the existence of acid vapor. Especially we found it
for the first time that the SQ derivatives with dipropylamino bases(SQ33)
and dibutylamino bases(SQ44) showed a sharp absorption peak of J-aggregates
in the spincoated films. Time-resolved difference absorption spectra were
measured about the SQ33 and SQ44 spincoated films by femtosecond pump-probe
spectroscopy. The fast decay time of SQ44 was determined to 220 fs at 50
MW/cm2 pump power and the max rate of an absorbance change was
about 25 % at 220 MW/cm2 pump power. All measurements were performed
in air at RT. We suppose that the SQ dye has great advantages both in ability
and cost in the field of the ultrafast optical applications. This work
was supported by New Energy and Industrial Technology Development Organization
(NEDO) within the framework of the Femtosecond Technology Project.
INFLUENCES ON THE SPECTROSCOPIC PROPERTIES OF AFX SERIES
TWO-PHOTON ABSORBING HETEROCYCLIC CHROMOPHORES. Jeffery W. Baur , Max D.
Alexander, Jr., Lisa R. Denny, Bruce A. Reinhardt and Richard A. Vaia,
Materials and Manufacturing Directorate, Air Force Research Laboratory,
Wright-Patterson AFB, OH; Ramamurthi Kannan, Systran Corporation, Dayton,
OH; Sean A. Kirkpatrick, SAIC, Dayton, OH.
Due to the recent development of organic chromophores
with relatively large effective two-photon cross-section values, there
has been intense interest in utilizing and optimizing nonlinear absorption
processes in a variety of applications including optical limiters, nondestructive
inspection of paint with underlying corrosion, and low energy photocuring.
In addition to being thermally stable and processible, asymmetric chromophores
consisting of -electron donating
and -electron accepting heterocyclic
moieties separated by a conjugated aromatic core have been shown to give
some of the largest nonlinear responses. However, these nonlinear responses
have also been shown to vary with local molecular environment and with
the pulse width of the incident radiation. Here we consider the influence
of molecular environment on the linear and nonlinear spectroscopic properties
of a series of structurally similar chromophores. Because the two-photon
fluorescence was nearly identical to that of the single-photon fluorescence
for a given molecular environment, information about the fluorescing state
obtained from a linearly excited molecule could be applied to that of a
nonlinearly excited molecule. For the series of chromophores examined,
the Stokeís shift was observed to increase linearly and substantially with
solvent polarity. Such behavior was adequately described by existing solvent
relaxation theory and indicates that the fluorescing species is highly
sensitive to a local molecular environment which can be described in terms
of the properties of the bulk solvent. An excited state absorption from
this solvent sensitive state has been suggested as a possible cause for
the solvent sensitivity of the nanosecond effective two-photon cross-section
values and for the large discrepancy between values recorded with femtosecond
and nanosecond laser pulses. Detail studies of intermolecular energy transfer
interactions between chromophore substituents will further the understanding
of this fluorescing excited state.
ELECTROSTATIC SELF-ASSEMBLY OF MULTILAYER NONCENTROSYMMETRIC
THIN FILMS AND DEVICES. Kristie M. Lenahan , You-Xiong Wang, Wei Zhao,
Richard O. Claus, Virginia Polytechnic Institute & State University,
Fiber & Electro-Optics Research Center, Blacksburg, VA; Yanjing Liu,
NanoSonic, Inc., Blacksburg, VA.
The electrostatic self-assembly of multilayer thin films
by alternate adsorption from polyelectrolyte solutions spontaneously leads
to the formation of noncentrosymmetric structures if the molecules themselves
have net dipole moments. The observation of significant second-order nonlinear
optical susceptibility in such films has been observed, using both commercially
available chromophores and molecules specifically designed to yield an
enhanced net dipole moment. The nature of the deposition process results
in an alignment of the chromophores that is stable over time and to temperatures
up to 150C, in contrast with poled
polymers. ESA films offer the additional major advantages of excellent
homogeneity and low optical loss, high thermal and chemical stability,
and low cost.
SESSION F4: PHOTOREFRACTIVE POLYMERS
8:30 AM *F4.1
DESIGN AND OPTIMIZATION OF CHROMOPHORES FOR LIQUID CRYSTAL
AND PHOTOREFRACTIVE APPLICATIONS. R.J. Twieg , M. He, L. Sukhomlinova,
F. You, Department of Chemistry, Kent State University, Kent, OH; K.D.
Singer, V. Ostroverkhov, Department of Physics, Case Western University,
Cleveland, OH; W.E. Moerner, M.A. Diaz-Garcia, D. Wright, J.D. Casperson,
B. Smith, E. Glazer, Department of Chemistry, UC San Diego, La Jolla, CA;
R. Wortmann, C. Glania, P. Kramer, K. Lukaszuk, R. Matschiner, Institute
of Physical Chemistry, University of Mainz, Mainz, GERMANY.
Wednesday Morning, April 7, 1999
Franciscan II (A)
Organic chromophores are employed in a wide range of discrete
optical or electronic functions as well as combined opto-electronic functions.
Well-known and established applications include photosensitizers and charge
transport agents for electrophotography and liquid crystals and dichroic
dyes for displays. New and emerging applications for organic chromophores
include those for electro-optic, electroluminescent and photorefractive
devices. Fortunately, many common structure features can be shared amongst
these chromophores and then synthetic, design and function properties are
portable amongst them. Our own current efforts on organic chromophores
reflect this diversity of structure and function but also attempt to utilize
common structure concepts as often as possible. For both photorefractive
and liquid crystal applications we are working with chromophores that contain
the diphenylacetylene and methylenedihydropyridine functionalities. We
are also attempting to extend these pyridine based systems to the analogous
methylenetetrahydro- and methylenehexahydro- systems as well. Photorefractive
materials are generally fabricated and their diffraction efficiency and
speed are evaluated as composite systems in a charge transport material
such as PVK. Chirality is another general feature of current interest as
it applies to NLO materials with helical pi-systems for Kleinman-disallowed
hyperpolarizability. Here the pseudotensor contributions to the nonlinearity
are evaluated by hyper-Rayleigh light scattering experiments. Presently
the important design features that will give rise to large nonlinearities
via these contributions is not well understood. The chiral molecules we
are presently evaluating almost invariably are derived from natural sources
such as camphor or the steroid estrone. Chirality, of course, has already
been of great relevance to liquid crystals including cholesteric and ferroelectric
liquid crystals. The potential liquid crystal behavior of our chromophores
is evaluated by thermal analysis and polarized optical microscopy. Many
of the relevant electronic and optical properties of these photorefractive
and liquid crystal systems are also conveniently obtained by electro-optical
absorption measurements (EOAM). In all cases we are also attempting to
design these molecules mindful of ultimate requirements for durabilty as
9:00 AM *F4.2
NEW INSIGHTS INTO TRAPPING AND COMPENSATION IN PHOTOREFRACTIVE
POLYMERS. W.E. Moerner , Stanford Univ, Dept of Chemistry, Stanford, CA;
D. Wright, M. Diaz-Garcia, A. Goonesekera, J. Casperson, B. Smith, M.S.
DeClue, E. Glazer and J.S. Siegel, Univ of California San Diego, Dept of
Chemistry, La Jolla, CA; R.J. Twieg, Kent State Univ, Dept of Chemistry,
In the past few years, several photorefractive polymer
materials have attained impressive performance, with two-beam coupling
gain coefficients of 100's per centimeter. We have utilized these large
gain coefficients coupled with multiple layer geometries and/or grating
translation to achieve beam fanning, self-pumped phase conjugation and
single-pass gains of a factor of 500. In spite of these large strides,
the understanding of trapping is still in its infancy. We consider a class
of photorefractive polymers based on the photoconducting host polymer poly(N-vinyl
carbazole), doped with various nonlinear optical chromophores based on
dicyanostyrenes, and with a liquid plasticizer and the sensitizer fullerene
C60. Detailed measurement of the trapping dynamics as a function
of time, intensity, and electric field allows us to propose a new trapping
model. Using a combination of infrared spectroscopy, extensive photorefractive
performance measurements, and electrochemical measurements, we show that
the fullerene anion acts as the primary hole trap with compensation provided
by the nonlinear optical chromophore. New insights into the optimal design
of photorefractive polymers will be presented.
9:30 AM *F4.3
PHOTOREFRACTIVE POLYMERS WITH HIGH SPEED. N. Peyghambarian
, K.B. Ferrio, J. Herlocker, E. Hendrickx, B.D. Guenther, Univ. of Arizona,
Tucson, AZ; S. Mery, GMO/IPCMS, Stasbourg, FRANCE; Y. Zhang, B. Kippelen,
Univ. of Arizona, Tucson, AZ.
We report on a photorefractive polymer with a 4-ms response
time constant in transient four-wave mixing experiments at 0.5 W/cm2
writing irradiance, 95 V/m applied
electric field, and a grating period of 3.1 m.
Complementary transient ellipsometry, however, reveals orientational birefringence
response two orders of magnitude faster, with a time constant of 30 s.
Orientation does not limit the dynamic formation of photorefractive gratings
in this polymer, which suggests that even faster photorefractive responses
are possible for polymer composites with improved charge generation and
10:00 AM F4.4 PHOTOREFRACTIVE EFFECT IN A NEW COMPOSITE
BASED ON BI-FUNCTIONAL HOST POLYMER. Yi-Wang Chen , Yuan-Kang He, Hui-Ying
Chen, Institute of Polymer Science, College of Chemistry, Peking University,
Beijing, P.R.CHINA; Feng Wang, Zhi-Jian Chen, Qi-Huang Gong, Department
of Physics, Peking University, Beijing, P.R. CHINA.
The guest-host approach for photorefraction always leads
to a polymeric system with low stability. Instead, bi-functional side chain
polymeric materials, which have both the photoconductivity and the electro-optic
effect intrinsically, exhibit higher stability of the EO effect. We have
synthesized a new bi-functional side chain polymeric material obtained
from poly(carbazolylpropyl methacrylate) (PCPMA) as starting material.
A portion of the carbazole groups were reacted with p-nitroaniline to form
3-(p-nitrophenyl)azo carbazole (NPAC) groups having second-order NLO properties.
The photorefractive effect was observed in composites based on such bi-functional
side chain copolymer of N-methacryloxypropyl-3-(p-nitrophenyl)azo carbazole
with N-methacryloxypropyl carbazole (PCPMA-NPAC). A two-beam coupling gain
coefficient of 9.4 /cm and an electro-optic coefficient of 9.3 pm/V were
measured at the applied electric field of 92.4V/mm by the typical two-beam
coupling experiment and the compensation electro-optic measurement technique.
The erase-write properties of the gratings were discussed as well.
SESSION F5: ELECTRONIC AND OPTICAL PROCESSES
1:30 PM *F5.1
ORGANIC AND POLYMER TRANSISTORS: DEVICE PHYSICS AND APPLICATIONS.
A. Dodabalapur , Z. Bao, R. Sarpeshkar, Y.Y. Lin, H.E. Katz, W.J. Li, A.J.
Lovinger and V.R. Raju, Bell Laboratories, Lucent Technologies, Murray
Wednesday Afternoon, April 7, 1999
Franciscan II (A)
This presentation will begin with a description of the
basic physics of typical organic and polymer transistors and the factors
which determine and influence the apparent mobility. The transient characteristics
and modeling of organic transistors will be described. The integration
of organic light emitting diodes and transistors is promising for emissive
displays. We have developed designs for `smart' pixels in which an analog
circuit consisting of 6-7 transistors drives each LED. The simulated and
experimental characteristics of such pixels will be presented. The development
of air-stable n-channel organic transistors led to our demonstrating the
first organic complementary circuits. The design considerations and characteristics
of organic complementary circuits with > 100 transistors will be described.
Such circuits will be compared with circuits based upon only p-channel
2:00 PM F5.2
SYNTHESIS, MATERIAL PROPERTIES, AND TRANSISTOR PERFORMANCE
OF OLIGOTHIOPHENES AND THEIR THIAZOLE ANALOGUES. Wenjie Li , Howard E.
Katz, Andrew J. Lovinger, Karl R. Amundson, Bell Laboratories, Lucent Technologies,
Murray Hill, NJ.
Beginning with the original reports of Garnier on the
utility of vacuum-sublimed thiophene oligomers in field-effect transistors
(FETs), this compound class has been an important vehicle for exploring
the performance limits and applications of these devices. We have previously
reported the synthesis and semiconducting properties of oligothiophenes
with solubilizing end-substituents, especially dihexylsexithiophene (DH6T)
and dihexylquinquethiophene (DH5T).
In this paper, the liquid phase fabrication of large area FETs based on
these compounds is investigated. Thin films cast from solutions of these
compounds onto different dielectric surfaces showed high field-effect mobilities
(> ca. 0.03 cm2Vs) and good film continuity over large areas
(1-2 cm2). In addition, several new p-type semiconducting materials
with lower electron-donating ability than the parent oligothiophenes were
synthesized and their thermal, morphological, and FET properties were studied.
The incorporation of thiazole rings into oligothiophenes was designed to
make them less susceptible to p-doping. The thiazole-containing compounds
generally have lower melting points and better solubility than the corresponding
oligothiophenes. The off currents are also lower due to the reduced susceptibility.
Relatively high on/off current ratios (> 103) can be routinely
obtained from devices operating in air, eliminating the need for strict
exclusion of oxygen. However, the trade-off is their lower field-effect
mobility which is possibly caused by the larger charge injection barrier
and poorer ordering in the film.
2:15 PM *F5.3
NANOSCALE LIGHT AMPLIFICATION AND CONVERSION IN ORGANIC
CRYSTALS. Denis Fichou , Laboratoire des Materiaux Moleculaires, C.N.R.S.,
Thiais, FRANCE; Fabrice Charra, Alexandre Gusev, DRECAM, CEA Saclay, Gif
sur Yvette, FRANCE; Jean-Michel Nunzi, Vincent Dumarcher, LETI, CEA Saclay,
Gif sur Yvette, FRANCE.
We recently observed amplified stimulated emission (ASE)
in plane-parallel single crystals of quaterthiophene (4T), sexithiophene
(6T) and octithiophene (8T) under one-photon transverse pumping. Following
a single-pass amplification, SE light is waveguided throughout the organic
crystals on several millimeters and comes out essentially from the edges
in a total internal reflection process. These lamellar crystals also generate
two-photon fluorescence when they are pumped in a longitudinal geometry
by a near-infrared light beam, i.e. when the beam of a Ti:sapphire laser
is directed right in between the two lower and upper faces of the crystal.
Importantly, these two phenomena are highly dependent on the structural
morphology of the crystal samples and it becomes necessary to investigate
the origin of light emission at the microscopic and molecular levels in
order to determine its primary mechanism. In this context, we used the
tip of a scanning tunneling microscope (STM) to image the morphology of
the front face of 4T, 6T and 8T crystals. Although these bulk materials
(typically a few microns thick) are resistive, their good photoconductivity
allows to flow enough charges when they are deposited on top of a transparent
conducting ITO substrate and irradiated by a continuous He-Cd laser light.
The weak photocurrent which is produced then allows to realize the structural
mapping of the crystal surface. But more importantly, laser-induced STM
also provides a powerful tool to generate photovoltaic conversion of light
into electricity at the microscopic level. The STM tip, the air gap and
the crystal then form a metal-insulator-semiconductor (MIS) junction aimed
at injecting charges into the photoconductive material under laser irradiation.
Correlation between the structural and the photovoltaic images of the 4T,
6T and 8T crystals unveils some unexpected features with a spatial resolution
of a few nanometers.
2:45 PM F5.4
TIME RESOLVED TRANSPORT OF ELECTRONS AND HOLES IN CONJUGATED
POLYMERS. D.J. Pinner , N. Tessler, R.H. Friend, Cavendish Laboratory,
University of Cambridge, UNITED KINGDOM.
We present a wide range of time resolved properties of
various polymers in light emitting diode configurations. Using a combination
of time resolved current, voltage, charge induced absorption, light emission,
and light emission spectra we are able to provide direct evidence for the
motion of both types of carriers. Using the charge induced absorption,
the unknown properties of the contact interface are decoupled from the
charge transport. We report reliable data of both electron and hole mobility
in efficient LEDs and address the issue of pulsed versus CW properties.
Knowing the above properties, efficient light emitting diodes are constructed
which allow us to demonstrate light intensity in excess of 20 million cd/m2.
We discuss the implications of this work for future electrically-pumped
3:30 PM F5.5
ORGANIC LASERS BASED ON ONE- AND TWO-DIMENSIONAL PHOTONIC
BANDGAP STRUCTURES. Martin Meier1 , Attila Mekis2,
Ananth Dodabalapur1, Zhenan Bao1, John A. Rogers1,
John D. Joannopoulos2, Richard E. Slusher1, 1Bell
Laboratories, Lucent Technologies, Murray Hill, NJ; 2MIT, Cambridge,
Organic gain media have been very useful in investigating
he properties of several classes of laser resonators such as distributed
feedback (DFB), distributed Bragg reflectors (DBR), planar microcavities
and whispering-gallery mode. We describe the characteristics of two-dimensional
distributed feedback lasers with organic thin-film gain media (Alq /DCM)
deposited on lithographically patterned Si/SiO2 photonic band
gap structures. Bragg reflections caused by the gratings diminsh the group
velocity of photons along some directions of crystallographic symmetry
to zero, and the resulting feedback gives rise to laser oscillations. Dispersion
relations for photons were calculated analytically and are used to interpret
the laser emission spectra. We will describe techniques which make possible
the realization of a complete two-dimensional photonic bandgap in the visible.
A promising and quite ease method to fabricate one- and two dimensional
distributed feedback resonators for organic lasers are soft lithographic
techniques. We demonstrate the successful application of microcontact printing
and replica molding techniques to the fabrication of DFB, DBR and two-dimensional
photonic bandgap structures with a feature size of 300nm. Using Alq:DCM
as organic gain media either the precursor polymer PPV or a acrylate-based
pre-polymer (NOA 72) can be used to form the resonator structures. Thus
photoexcited plastic lasers have narrow emission profiles. The good performance
of the lasers demonstrates the suitability of these techniques for fabrication
certain types of functional optoelectronic devices that require submicron
3:45 PM F5.6
MORPHOLOGY AND CHARGE TRANSFER IN INTERPENETRATING NANOPARTICLE/CONJUGATED
POLYMER NETWORK PHOTOVOLTAICS. Alexi Arango , Sue Carter, UCSC, Dept of
Physics, Santa Cruz, CA; Thomas Daubler, Dieter Neher, Max-Planck-Institut
fur Polymerforschung, Mainz, GERMANY; Phil Brock, IBM Almaden Research
Center, Almaden, CA.
We study the effect of nanoparticle morphology and dye-sensitization
on quantum efficiency and photovoltage in conjugated polymer / nanoparticle
blends and layered photovoltaic cells. Additionally, we examine the consequence
of exposure to ambient atmosphere and the effect of a self-assembled nanoparticle
layer at the ITO electrode. In ambient atmosphere, blended sandwich cell
devices with ITO and Au electrodes, reveal large photocurrents of over
100 electrons per incident photon at low bias voltages of
1 Volt, yet produce no significant photovoltage or short circuit current.
Layered devices have lower photocurrents under bias, yet generate open
circuit voltages of 0.7 Volts and short circuit current quantum efficiencies
approaching 5%. We utilized DC photocurrent action spectra and current-voltage
characteristics, from which conclusions about photo-induced charge transfer
at the nanoparticle interface, charge trapping and hole injection may be
SESSION F6/B13/E8: JOINT SESSION:
CONCURRENT WITH B11, B12 AND F5
ORGANIC LUMINESCENT MATERIALS
Wednesday Afternoon, April 7, 1999
Franciscan I (A)
1:30 PM *F6.1/B13.1/E8.1
EXCIMER FORMATION IN COPOLYMERS OF 9,9-DIALKYL FLUORENES
AND ANTHRACENE(S). R.D. Miller , G. Klaerner, F.G. Klaerner, M. Kamieth,
M.H. Davey, J.C. Scott, J.P. Chen, IBM Almaden Research Center, San Jose,
Poly(fluorene) derivatives comprise an interesting class
of electroactive materials with potential application in polymeric light
emitting diodes. The formation of excimers upon thermal annealing or the
passage of current is a ubiquitous problem for these materials. The interaction
of polymer chains with themselves and/or their environment depends on the
chemical structure and shape of the chain and the nature of the functional
groups. We have prepared a variety of novel linear, U- and S- shaped rigid
rod polymers by the copolymerization of 2,7-dibromo-9,9-dihexylfluorene
with 2,6-, 1,8, 1,5-, and 9,10-dihaloanthracenes. Polymer chain conformations
have been studied by molecular modeling and predictions regarding excimer
formation have been verified by spectroscopic studies.
2:00 PM F6.2/B13.2/E8.2
PHOTOLUMINESCENT POLYMER FILMS FOR DISPLAY APPLICATIONS.
Anja R.A. Palmans , Andrea Montali, Christoph Weder, Paul Smith, ETH Zurich,
Dept. of Materials, Zurich, SWITZERLAND.
In order to increase the efficiency of liquid crystal
displays, our research has focused on the introduction of photoluminescent
(PL) polarizers into traditional displays. These PL polarizers typically
comprise rigid poly-p-phenyleneethynylene (PPE) polymers, embedded and
uniaxially oriented in a polyethylene (PE) matrix. Since only a limited
fraction of incident light can directly be used by these PL polarizers
- because their absorption is also anisotropic - we recently proposed a
new approach in which coumarin dyes have been isotropically mixed into
such oriented PPE/PE films to act as sensitizer. In these ternary blends,
the overall efficiency of transferring the unpolarized incident light to
polarized colored light has been dramatically improved through a polarizing
energy transfer from the sensitizer to the PPE. To explore the scope and
limitations of this approach, a variety of sensitizers was mixed into the
PPE/PE blends. However, we observed that upon mixing certain sensitizers
into the PE/PPE polymer blends, problems related to phase separation and
crystallization arise. As a result, we have introduced the concept of covalently
attaching sensitizers to the conjugated polymer backbone. This approach
allows us to introduce a well-defined distance between sensitizer and PPE,
which ameliorates the control over the required proximity between the sensitizer
and PPE. As such, energy transfer efficiency may be further improved. In
addition, problems related to phase separation are circumvented. Finally,
the polarizing nature of the energy transfer can be enhanced.
2:15 PM F6.3/B13.3/E8.3
LASING IN ORGANIC THIN FILMS USING CASCADE FORSTER ENERGY
TRANSFER. Martin Meier , John A. Rogers, Ananth Dodabalapur, Zhenan Bao,
Richard E. Slusher, Bell Laboratories, Lucent Technologies, Murray Hill,
NJ; Attila Mekis, MIT, Dept. of Physics, Cambridge, MA.
Optically pumped lasing in thin films of an absorbing
host 8-hydroxyquinolinato aluminum (Alq) or PBD doped with different laser
dyes (C490, DCM II, LDS 821) is investigated. In our materials, an initial
molecular excited state is generated in the host compound by absorption
of light. This state is then resonantly and non-radiatively transferred
through one or more steps down in energy. Suitably matched dye molecules
dispersed in the host ensure that the absorption losses at the final emission
wavelength are very small. This approach can lower the threshold power
density required for light amplification and provide broad tunability of
the emission wavelength. Furthermore the optical emission properties are
decoupled from the transport properties which is important for the design
of future electrically driven device structures. In order to provide the
necessary feedback, we employed distributed feedback (DFB), distributed
bragg reflectors (DBR) and two-dimensional photonic bandgap structures
based on thermally oxidized Si wafer as resonators. The carefully chosen
gain media together with the resonator structures which act as wavelength-selective
mirrors allow us to fabricate optically pumped lasers in the whole visible
spectral range (390nm -800nm). We also fabricated rib-waveguide based DFB
and DBR lasers in which the gratings are defined by soft-lithographic procedures.
2:30 PM F6.4/B13.4/E8.4
ELECTRO-LUMINESCENCE AND PHOTOLUMINESCENCE OF POLYFLUORENE-PHK
COPOLYMER LED. O. Stephan, J.C. Vial , F. Muller, Lab. de spectrometrie
Physique, Univ. J. Fourier and CNRS, St. Martin d'Heres cedex, FRANCE.
Thin layers of polyfluorene-polyhexylcarbazole copolymer
sandwiched between ITO and aluminum electrodes show stable photoluminescence
and electro-luminescence at room temperature and ambient atmosphere. I(V)
and Electro-luminescence versus voltage curves present a threshold at 7
Volts for layer thicknesses of the order of 0.1 m.
The emission degrades slowly when the layer is electrically polarized or
UV exposed (The lifetime of the device is typically 12 hours for 25 Volts
- 150 A polarization conditions).
Photoluminescence and electro-luminescence spectra show the same features
and are similar to those obtained for low dilution of the polymer in chloroform.
Time resolved photoluminescence spectra present a fast blue band (in the
sub nanosecond range) centered at 450 nm and a slower yellow one (in the
range of 5 ns) centered at 550 nm. The yellow band is absent for fresh
samples but become dominant (specially for the electro-luminescence) for
aged samples. As already proposed, the spectral behavior suggests that
this band results from the re-crystallization of the polymer, here, we
add other observations in favor of this explanation. In addition, We explain
the slower decays as taking their origins in indirect type electron-hole
recombination, enhanced in the crystalline phase.
2:45 PM F6.5/B13.5/E8.5
FLUORESCENT, HEXACATENAR LIQUID CRYSTALS. Ben Hoag, Douglas
L. Gin , Univ of California, Dept of Chemistry, Berkeley, CA; Udo Theissl,
Emil J.W. List, Egbert Zojer, Guenther Leising, Technische Univ Graz, Inst
für Festköperphysik, Graz, AUSTRIA.
A novel family of phasmidic liquid crystals incorporating
an oligo(-phenylenevinylene) core
was produced. These compounds exhibit blue luminescence in both solution
and the solid state. The emission profiles of these compounds are influenced
by the degree of organization in the solid state. The synthesis and liquid-crystalline
behavior of these compounds will be discussed. Their photophysical properties,
as studied by c.w. photoluminescence excitation, emission spectroscopy,
and site selective spectroscopy, as well as their optoelectronic properties
will be presented.
3:15 PM *F6.6/B13.6/E8.6
MULTICOMPONENT DESIGNS FOR POLYMER LEDs. Frank E. Karasz
and Bin Hu, Department of Polymer Science & Engineering, University
of Massachusetts, Amherst, MA.
Multilayer architectures provide substantially enhanced
efficiencies in polymer LEDs principally by virtue of their ability to
provide a more balanced charge injection and good excitonic confinement
within the chromophore layer. Further improvement may be gained by using
the same charge transport materials in a multi-component design which relies
on the immiscibility of the respective polymeric constituents to provide
micro-domains (``quantum wells'') of the chromophore in a matrix of the
other components. Using alternating hard-soft block copolymers in which
the oligomeric hard block forms the chromophore while the soft block provides
solubility and additional phase separation, we have designed systems which
emit blue, green or red, as required, from a single layer device. Further
advantages of the multicomponent design include more versatility in optimizing
charge injection by adjusting the ratio of the electron and hole transporting
materials, and reduced concentration quenching in the chromophore because
of its dispersed state. By using a chromophores blend, one may also take
advantage of energy transfer from the higher to the lower band gap material.
Fabrication of a single layer device is simplified in comparison to a multi-layer
Furthermore the multi-component design has been developed
to produce a white light emitting polymer LED. The chromophore domain is
a blend of the blue, green and red emitters in appropriate ratio, while
the charge transport components optimized by the usual criterion of balanced
injection from the matrix. Because of the differing band gaps and ionization
potentials of the chromophore components the emission spectrum is field
dependent; a CIE chromaticity diagram analysis showed a shift in the spectrum
as the bias voltage was increased, progressing from orange-red to white
as the green and blue chromophores became activated.
3:45 PM F6.7/B13.7/E8.7
OPTICAL PROPERTIES AND CHARACTERIZATION OF Eu3+ ACTIVATED
PMMA:PAAc COPOLYMER SAMPLES: Eu CONCENTRATION EFFECTS. Rebeca Sosa , Mario
Flores, Antonio Muñoz and Rogelio Rodríguez, Dept. of Physics,
Universidad Autónoma Metropolitana-Iztapalapa, Distrito Federal,
Samples of Eu3+-activated PMMA:PAAc copolymers were prepared
in order to investigate their spectroscopic properties. The densities,
refractive indices. UV-Visible absorption and emission spectra and fluorescence
decay times were obtained experimentally. These data were used in conjunction
with Judd-Ofelt theory to calculate the spontaneous emission probabilities
and branching ratios, as a function of the europium content. The range
of the concentrations was 0.1 to 2.5
mol. Our results are combined with previous information on the optical
properties of rare-earth activated polymers in order to get a better understanding
of the role played by the Eu3+ ions on the polymerization process in our
4:00 PM F6.8/B13.8/E8.8
PHOTOLUMINESCENCE OF POLY(P-PHENYLENE VINYLENE) ENCAPSULATED
IN MESOPOROUS SILICA. Andras G. Pattantyus-Abraham, Michael O. Wolf , Dept
of Chemistry, University of British Columbia, Vancouver, BC, CANADA.
We have prepared poly(p-phenylene vinylene) (PPV) encapsulated
in a mesoporous silica, MCM-41. In situ polymerization of xylylene bis(tetrahydrothiophenium
chloride) yields a highly luminescent, yellow powder with a fluorescence
spectrum matching that of unencapsulated PPV. Nitrogen absorption isotherms
show that the pore size of the PPV-loaded MCM-41 is significantly smaller
than that of empty MCM-41 and a polymer loading of ca. 8% by weight is
obtained by thermogravimetric analysis. The hybrid material has also been
characterized by IR and solid-state NMR. Detailed photoluminescence data
for the encapsulated PPV will be presented, and the use of these nanostructured
materials in device applications will be discussed.
4:15 PM F6.9/B13.9/E8.9
FLUORESCENCE EMISSION FROM SINGLE STRETCH-ORIENTED CONJUGATED
POLYMER MOLECULES. Paul J. Carson , Jessie A. DeAro, Steven K. Buratto,
Department of Chemistry, University of California, Santa Barbara, CA; Cuiying
Yang, IPOS, University of California, Santa Barbara, CA.
Fluorescence from blends of derivatized poly(p-phenylenevinylene)
(PPV) in stretch-oriented polyethylene (PE) has been observed with single-molecule
spatial isolation using confocal microscopy. The photoluminescence and
electroluminescence, as well as mechanical properties and ease of processing,
have made conjugated polymers desirable as potential active materials in
device applications. Single-molecule fluorescence has the advantage over
ensemble spectroscopies in that it allows the direct observation of the
heterogeneity of spectral line shapes and fluorescence intermittency among
individual emitters. These photophysical properties, as well as others,
reveal the distribution of nanoenvironments and optically relevant conformational
distributions of the fluorescent species in the active material. Results
on single-molecule fluorescence intermittency and spectral line shapes
will be presented for the derivatized-PPV/PE system. Polarization anisotropy
in both stretch-oriented and unoriented polymer blends will be discussed.
New techniques for characterizing the underlying rates of molecular state
fluctuation will be presented.
4:30 PM F6.10/B13.10/E8.10
PROBING OF TRIPLET STATES IN POLYMER LEDS. V. Cleave
, N. Tessler, R.H. Friend, University of Cambridge, Physics Department,
Cambridge, UNITED KINGDOM; G. Yahioglu, Imperial College, Chemistry Department,
London, UNITED KINGDOM; P.Le Barny, Thomson-CSF, Orsay, FRANCE.
We report a study of the triplet state in doped polymer
light-emitting diodes (LEDs). Using a system consisting of a luminescent
polymer doped with triplet-emitting molecules, we employ time resolved
electroluminescence (EL) and photoluminescence (PL) as well as sub-gap
(triplet) absorption, to give direct evidence for the transfer of triplet
as well as singlet excitons from the polymer host to the molecular dopant.
These transfer processes offer normally non-radiative triplet excitons,
as well as singlet ones, access to a radiative decay channel, lifting the
25% efficiency limit imposed by the ratio of 3:1 triplet:singlet excitons
created in conventional polymer LEDs. Device efficiency can be maximized
by optimising the trapping of carriers, singlet excitons (Förster
transfer), and triplet excitons (Dexter transfer) by a single guest molecule.
The incorporation of triplet emitting molecules also enables us to probe
the position of the triplet level in the polymer host via knowledge of
the guest molecule, and build a more complete picture of the polymer energy
levels. We show that the established procedure can be applied to a wide
range of luminescent polymers to shed light on their basic photophysics.
4:45 PM F6.11/B13.11/E8.11
FROM A REACTION MECHANISM TO NEW PPV'S FOR HIGH PERFORMANCE
POLYMER-LEDS. H. Spreitzer , H. Becker, W. Kreuder, E. Kluge, H. Schenk,
Aventis Research & Technologies, Frankfurt, GERMANY.
In the recent years, organic light emitting materials
made dramatic progress in comparison to classical semiconductor light-emitting
diodes. Especially light emitting polymers provide an ideal combination
of processability and performance to allow for an innovative low cost technology
for LCD-backlighting, illumination, and display applications. Recent advances
in materials and device preparation lead to high-performance polymer LEDs
(PLEDs) with power efficiencies exceeding 10 lm/W.
Soluble Poly-para-phenylenes vinylenes (PPVs)
have been proven to be highly efficient light emitting materials for different
parts of the visible spectrum. Those materials can be prepared by dehydrohalogenation
polymerization of 1,4-bischloromethyl benzenes. The mechanism of this polymerization
is assumed to incorporate a p-quinodimethane species as a key intermediate.
We could show, that this mechanism may lead to polymerization defects in
the polymer which then will cause interruptions in conjugation. Due to
the fact, that high charge carrier mobilities are required for a stable
polymeric emitter, they have been closely investigated by 13C
NMR measurements of 13C isotope labeled polymers. It has now
been shown, that the expected breaks in the conjugation chain could indeed
be identified. The amount of defects differs regarding the substitution
pattern of the PPV. The detrimental effect of conjugation breaks can clearly
be observed in lifetime tests of PLEDs.
By modifying the substitution pattern of the PPV, the
occurrence of polymerization defects was reduced drastically. Those new
materials now allow the preparation of very stable and highly efficient
PLEDs. Efficiencies of up to 16 lm/W (@ 100 cd/m2) and lifetimes
exceeding 25000 h (as determined by accelerated aging) are now routinely
achieved. The new class of high performance polymers allows easy access
to various colors.
SESSION F7: POSTER SESSION
OBSERVATION OF LARGE BAND BENDING AT ORGANIC SEMICONDUCTOR
HETEROINTERFACES. P.G. Schroeder 1, M.W. Nelson1,
P.A. Lee2, K.W. Nebesny2, N.R. Armstrong2,
R. Schlaf1 and B.A. Parkinson1, 1Colorado
State University, Department of Chemistry, Fort Collins, CO; 2University
of Arizona, Department of Chemistry, Tucson, AZ.
Wednesday Evening, April 7, 1999
Metropolitan Ballroom (A)
Band bending of up to 0.8eV has been observed at heterointerfaces
involving organic semiconductor materials with different Fermi level energies.
Our results demonstrate that this effect is very significant for many of
the active materials being investigated for luminescent displays (OLEDís)
and organic thin film transistors (OTFTís). The interfaces investigated
were grown by vapor deposition in a multi step growth procedure. After
each growth step the samples were characterized in situ using a combination
of x-ray and ultraviolet photoemission spectroscopy (XPS, UPS). In these
measurements UPS was used to measure the highest occupied molecular orbital
(HOMO) positions and interface dipoles while XPS was used to independently
determine the band bending at the interface. Our experiments show that,
due to the large magnitudes of band bending observed, it is crucial to
incorporate band bending into the determination of the energy level alignment.
This procedure leads to a more precise determination of the electronic
structure at heterocontacts in order to gain better understanding of the
charge injection processes and rectification behavior of such interfaces.
TOWARDS THE SYNTHESIS OF MONO- AND BI-FUNCTIONAL AMPHIPHILIC
OLIGOTHIOPHENES. Rigoberto C. Advincula , Seiji Inaoka and Dan Phillips,
Department of Chemistry, University of Alabama at Birmingham, AL; Mi-Kyoung
Park, Dong-Myung Shin, Department of Chemical Engineering, Hong-Ik University,
It is our objective to investigate the mono- and bi- functionalization
of oligothiophene derivatives to achieve amphiphilicity and telechelic
functionality. The telechelic functionality is attractive for further polymerization
reactions and utility in copolymer architectures. In this report, we describe
our initial synthesis and characterization of 5,5'''''-di((6-bromohexyl)-2,2':5',
2-terthiophene) 2,2':5',2:5,2''': 5''',2:5,2''''' - a sexithiophene derivative.
The synthesis was done primarily by the symmetrical coupling of terthiophene
derivatives. We also synthesized a thiophene pentamer, bromoalkyl-oligothiophene
derivate 5-(6-bromohexyl)- 2,2';5',2;5,2';5',2- quinquethiophene and characterized
its initial photoluminescence properties. N,N'-dimethyl-N,N'-bis(6-(a-quinquethienyl)hexyl)-
1,6-hexanediamine was formed from the reaction with N,N-dimethyl-1,6-hexanediamine.
These materials can eventually be functionalized as an amphiphile or attached
to an ionene polymer backbone as side groups.
DETERMINATION OF CRYSTALLITE SIZE AND LATTICE STRAIN
IN HEXAPHENYL THIN FILMS. Hans-Jürgen Brandt a,
Roland Resela, Jozef Keckesb, Brigitte
Koppelhuber-Bitschnauc, Norbert Kocha
and Günther Leisinga, aInstitute
of Solid State Physics, Technical University Graz, Graz, AUSTRIA; bDepartment
of Theoretical Chemistry, Slovak Academy of Sciences, Bratislava, SLOVAKIA;
cInstitute of Physical and Theoretical Chemistry, Technical
University Graz, Graz, AUSTRIA.
Hexaphenyl (6P) thin films are used as an organic material
for efficient blue light emitting diodes. Well known structural properties
are of special interest for achieving high efficiency electroluminescence.
6P thin films are prepared by thermal vacuum deposition on glass substrates,
one substrate held at room-temperature (RT) and the other one heated to
170°C (HT). The thin films were studied by x-ray diffraction line profile
analysis and by a atomic force microscope. We investigated the crystallite
size and lattice strain of the RT-sample, using the HT-sample and highly
pure silicon as standard. We determine the crystallite size and lattice
strain on 3 different types of preferred grown crystallites. Multiple line
analysis was performed on (00l)-reflections (l = 1 .. 13) and by single
line analysis of the (11-1), (11-2) and the (20-3) reflections. We observe,
that the size of the 6P crystallites in (00l)-direction are in the same
order of magnitude as the film thickness. The line broadening is determined
by the small size of the crystallites.
THE VALENCE ELECTRONIC STRUCTURE OF N-DOPED P-SEXIPHENYL.
Udo Theissl , Emil J.W. List, Norbert Koch, Guenther Leising, Technical
Univ Graz, Inst of Solid State Physics, Graz, AUSTRIA; Antje Vollmer, Free
Univ Berlin, Inst of Physical Chemistry, Berlin, GERMANY; Sigurd Schrader,
Univ Potsdam, Inst of Solid State Physics, Potsdam, GERMANY; Jean-Jacques
Pireaux, Univ Namur, LISE, Namur, BELGIUM.
Thin films of p-sexiphenyl (6P) were doped with increasing
amounts of potassium in situ, and the change in the valence electronic
structure of 6P upon the alkali metal deposition was followed with ultraviolet
photoelectron spectroscopy. We observe the evolution of new intra-gap emissions,
which are attributed to the formation of bipolarons, even for very low
doping concentrations. The low binding energy intra-gap emission exhibits
a pronounced asymmetric lineshape, in contrast to the findings when cesium
is used as dopant. In order to investigate whether this lineshape is due
to different emissive electronic species in the bulk and on the surface
of the 6P film, the take-off angle for the photoelectrons was varied. As
no change in the lineshape is found when going from normal to near-grazing
emission we can exclude that charged 6P molecules in the bulk and on the
surface yield different valence electronic spectra. Therefore, the characteristic
lineshape of the low binding energy emission is proposed to be related
to the interaction of the doped organic molecule with the different counterions.
EL BEHAVIORS OF STYLYL COMPOUNDS WITH BENZOXAZOLE AND
BENZOTHIAZOLE FOR ORGANIC LIGHT-EMITTING-DIODE. Koichi Yamashita , Tatsuo
Mori, Teruyoshi Mizutani, Nagoya Univ, Dept. of Electrical Engineering,
The use of distylylbenzene derivatives in blue organic
light-emitting-diodes (OLEDs) was first discovered in Idemitsu Kousan by
Hosokawa et al. In our previous work, we have studied on the stylyl derivatives
having triphenylamine and benzoxazole at side group as emitting layer.
These devices emitted a blue-green light. However, these devices did not
have an excellent EL efficiency (0.1 lm/W). For the current density versus
applied voltage characteristics, the current densities of devices using
these stylyl compounds as an emitting material were higher than that of
device using Alq3 as an emitting materials at same applied voltage. From
these results, we think that these stylyl compounds having a triphenylamine
group can flow out holes from emitting layer to cathode metal. In addition,
the poly-crystallized rates of these materials were faster than that of
TPD at room temperature in air. From these reasons, we designed new stylyl
compounds. In this presentation, we study on two series of new stylyl compounds
as emitting and hole transport materials for the characteristics of OLEDs.
We synthesized two series of new stylyl compounds. One have distylyl structure
eliminated triphenylamine group which have benzoxazole or benzothiazole
groups at both sides. Another have the structure of ditriphenylamine derivative
(TPD) having benzoxazole or benzothiazole groups as side group. We studied
emitting and hole transport characteristics using these new stylyl compounds
ENHANCEMENT OF THE MORPHOLOGICAL STABILITY OF ALUMINUM(III)
8-HYDROXYQUINOLINE FILMS BY INCREASING ENTROPIC DISORDER. K.A. Higginson
, B. Yang and F. Papadimitrakopoulos, Department of Chemistry, Polymer
Science Program, Nanomaterials Optoelectronics Laboratory, Institute of
Materials Science, University of Connecticut, Storrs, CT.
Aluminum(III) 8-hydroxyquinoline (Alq3), a premier material
for the development of organic light-emitting diode (OLED) technology,
has been shown to be a semicrystalline material which undergoes structural
relaxations at temperatures well below its glass transition. This crystallization
and/or aging affected the fluorescence and conductivity of Alq3 films.
In an effort to develop OLED materials with greater stability, materials
which form thermally stable amorphous glasses have been pursued. Aluminum(III)
5-methyl-8-hydroxyquinoline (AlMeq3) was used to retard crystallization
in emitting films, both as a pure material and in coevaporated blends with
Alq3. Crystallization and aging kinetics of Alq3, AlMeq3, and their blends
were studied with x-ray diffraction and luminescence techniques. The blends
of Alq3 and AlMeq3 were found to be resistant to crystallization in OLED
CURRENT-VOLTAGE CHARACTERISTICS OF SINGLE LAYER POLYMER
LIGHT EMITTING DIODES UNDER HIGH ELECTRICAL EXCITATION. B. Ruhstaller ,
S.A. Carter, Univ. of California, Dept. of Physics, Santa Cruz, CA; P.J.
Brock, IBM Almaden Research Center, San Jose, CA; U. Scherf, Max Planck
Inst. fur Polymerforschung, Mainz, GERMANY.
We study the current-voltage characteristics of both double
and single carrier polymer light emitting devices in the high intensity
electrical excitation regime. Single layer devices are being investigated
with either the orange emitting MEH-PPV or the blue emitting ladder polymer
Me-LPPP as the active polymer. Performing very low duty (0.001)
pulsed electroluminescence measurements up to 80 volts we observe a crossover
from a space charge limited to an ohmic regime with a weakly field dependent
mobility and a saturating external quantum efficiency of 1%. We report
current densities of 100 A/cm2 and brightnesses of 5105
cd/m2. The results are compared to DC measurements and the spectral
features are monitored below the onset of degradation.
CONJUGATED POLY-P-PHENYLENE (PPP) FROM POLY(1,3-CYCLOHEXADIENE)
(PCHD) HOMO- AND BLOCK COPOLYMERS: CONTROLLED PROCESSABILITY AND PROPERTIES.
Kunlun Hong, Jimmy Mays, Yuming Wang, Rigoberto Advincula , University
of Alabama at Birmingham, Department of Chemistry, Birmingham, AL.
Poly(p-phenylene) (PPP) and its derivatives are an interesting
class of polymers, which exhibit conductivity upon doping. It also exhibits
photo-or electro-luminescence upon excitation with light or application
of a bias voltage in a diode configuration. Pure PPP is an insoluble and
infusible material, stable at high temperatures. Its thermal stability
is an attractive property, considering the requirements for future opto-electronic
applications. In this report, we indicate our initial results on the synthesis
of PCHD by living anionic polymerization, block copolymer configuration,
and subsequent conversion to PPP derivatives. We have used a variety of
initiators, solvent, and temperature conditions to determine the right
parameters for obtaining narrow MWD polymers. We then proceeded with conversion
to PPP derivatives using a dehydrogenation reaction with chloranil. Our
results indicate a systematic conversion to a conjugated polymer with increased
solubility and photoluminescence properties. The microstructure, MW and
block copolymer composition affecting the processability and conversion
HOLOGRAPHIC NANOPATTERNING OF POLY(P-PHENYLENE VINYLENE)
AND THE INFLUENCE ON PL EFFICIENCY AND RADIATIVE DYNAMICS. G. Gigli, M.
Lomascolo, R. Rinaldi, C. Turco, P. Visconti and R. Cingolani, Unita INFM,
Dipartimento Scienza dei Materiali, Universite di Lecce Lecce, ITALY; F.
Cacialli , Cavendish Laboratory, University of Cambridge, Cambridge, UNITED
We report a flexible method for the patterning of organic
semiconductors in the submicrometer range, which we have successfully applied
to thin films of poly(p-phenylene vinylene) (PPV) prepared on a variety
of substrates, such as quartz, indium-tin oxide coated glass, or inorganic
dielectric mirrors. The method is based on the holographic lithography
of a standard photo-resist layer, followed by Ar-ion etching of the PPV.
The holography was carried out with the help of an Ar-ion laser beam (363.8
nm) which we used in combination with a corner cube interferometer of our
own design and construction, in order to fabricate 2-dimensional periodic
structures. The use of multiple exposures of the photo-resist allows effective
control of the geometry of the repeat element which we exemplify for stripes
and pillars of virtually arbitrary shape (squares, circles, diamonds, ellipses),
with feature size ranging between 250 nm and 1 µm. We characterised
the morphology of the structures by contact mode atomic force microscopy,
and the impact of the technology on the material properties by monitoring
the photoluminescence quantum yield at different stages of the process.
We also report and discuss the temporal decay of the radiative photoexcited
species with a view to the damage introduced by the patterning procedure.
SESSION F8/B15: JOINT SESSION:
8:30 AM *F8.1/B15.1
APPROACHES TO COLOR GENERATION IN ORGANIC LIGHT EMITTING
DEVICES. S.R. Forrest , M.A. Baldo, V. Bulovic, R. Desphande, D. O'Brien,
Princeton University, Department of Electrical Engineering, Princeton,
NJ; A. Shoustikov and M.E. Thompson, University of Southern California,
Department of Chemistry, Los Angeles, CA.
Thursday Morning, April 8, 1999
Metropolitan I (A)
We discuss the use of both phosphors and polarizable fluorescent
dyes as means for generating highly efficient, saturated color emission
from organic light emitting devices (OLEDs). We have recently discovered
that electrophosphorescence coupled with Dexter energy transfer, can lead
to red light emission in vacuum deposited OLEDs with external efficiencies
approaching 6%. Using electrophosphorescence, we have accurately measured
the spin statistics of exciton formation in such hosts as Alq to result
in a singlet fraction of only 16+/-2%. This surprising result is of both
fundamental and technological importance. While phosphorescence can be
extremely efficient at low intensities, due to triplet-triplet annihilation
and color center saturation, the efficiency tends to fall off at higher
luminance levels. In other work, we have demonstrated that some fluorescent
dyes are highly polarizable, leading to large shifting of the emission
color as a function of dopant density. This effect, which we term polarization
color shifting, has numerous uses in tuning the emission of OLEDs. In this
talk, we will discuss energy transfer, electrophosphorescence, polarization
color shifting and related effects observed in OLEDs based on vacuum deposited
molecular organic materials.
9:00 AM F8.2/B15.2
HIGHLY TRANSPARENT POLYMER LIGHT-EMITTING DEVICE. Shun-Chi
Chang, Yang Yang , Dept. of Materials Science and Engineering, UCLA, Los
Transparent light-emitting devices are desired for many
unique applications, such as head-on displays. Usually, transparent organic
LEDs requires the sputter deposition of the transparent ITO electrode on
top of the EL organic materails, and this process is rather time consuming.
We have developed a novel technique for the fabrication of highly transparent
polymer LEDs without the sputter deposition of the top transparent ITO
electrode. The transparency of the device is higher than 90% in the visible
spectrum, when a blue polymer poly [9,9-bis(3,6-dioxaheptyl)-fluorene-2,7-diyl]
(BDOH-PF) is used as the active material. When unbiased, the transparent
LED is as transparent as glass. On biasing, the brightness of the LED can
reach 100cd/m2 under10 volts. This device represents a new approach for
the fabrication of a large area transparent display panel.
9:15 AM F8.3/B15.3
SYSTEMATIC STUDY OF INSULATING BUFFER LAYER THICKNESS
EFFECTS ON THE ELECTROLUMINESCENT CHARACTERISTICS OF ORGANIC LIGHT-EMITTING
DEVICES. G.E. Jabbour , S.E. Shaheen, B. Kippelen, N. Peyghambarian, Univ.
of Arizona, Tucson, AZ.
The fabrication of efficient and bright organic light-emitting
devices (OLEDs) based on more environmentally stable cathodes, such as
aluminum (Al), has already been made possible by incorporating an inorganic
buffer layer such as lithium fluoride (LiF), cesium fluoride (CsF) etc.,
between the cathode and the organic layer. A systematic study of the effects
of the buffer layer thickness on device performance shows that OLEDs with
sub-monolayer thickness of LiF, for example, have the same external quantum
efficiency and light output as devices with LiF thickness of 0.8 nm. While
the mechanism behind the enhanced performance in not yet completely understood,
the above experimental results eliminate few of several arguments currently
used to explain the enhancement in OLEDs performance, mainly, tunneling-through-an-insulator
and elimination of gap-states. Short time operational stability data of
devices with LiF thickness ranging from 0 nm to 1.5 nm will be presented.
9:30 AM F8.4/B15.4
INVESTIGATION OF THE ELECTRODE/ORGANIC INTERFACE IN MOLECULAR
ORGANIC LIGHT EMITTING DIODES USING COMBINED XPS AND UPS MEASUREMENTS.
R. Schlaf , 1L.A. Crisafulli, 2P.G. Schroeder, 1M.W.
Nelson, 1H. Murata, 2C.D. Merritt, 2B.A.
Parkinson, 1Z.H. Kafafi, 21Colorado State University,
Dept of Chemistry, Fort Collins, CO; 2US Naval Research Laboratory,
Optical Sciences Division, Washington, DC.
The electronic properties of the electrode/organic interfaces
in molecular organic light emitting diodes (MOLEDs) have considerable influence
on the device performance. Device performance strongly depends on the charge
injection barrier at both the anode/hole transport layer and the cathode/electron
transport layer interfaces. We have determined the frontier orbital alignment
between a number of electron transporters (ex. tris-(8-hydroxyquinolinato)
gallium or indium (Gaq3, Inq3)) and commonly used
cathode materials (ex. Ag, Mg, Al), and hole transporters (ex. 4,4í -bis(1-naphtylphenylamino)
biphenyl (NPB)) and commonly used anode materials (ex. indium tin oxide
(ITO)). The experiments were carried out in ultra high vacuum by growing
well-defined organic thin films in multiple steps on metal or ITO coated
glass substrates. After each growth step, the surface was characterized
by combined XPS and UPS measurements. This method uses UPS to measure the
highest occupied molecular orbital (HOMO) position and the work function
while XPS is employed to detect the band bending developing across the
interface during film growth. This procedure allows the distinction between
interface dipole and band bending related spectral phenomena with high
accuracy. Our results demonstrate that the metal/organic interfaces show
strong interface dipoles and band bending.
9:45 AM F8.5/B15.5
POLYMER PHOTONIC DEVICES WITH FROZEN P-I-N JUNCTIONS.
Jun Gao , Alan J. Heeger, Institute for Polymers and Organic Solids, University
of California, Santa Barbara, CA; Gang Yu, Uniax Corp., Santa Barbara,
Polymer light-emitting electrochemical cells (LECs) and
polymer photovoltaic cells have been made from polymer blends.The operation
of these solid-state devices involves both electronic and electrochemical
processes, as a result of the introduction of polymer electrolyte in the
polymer blend film. When a large enough bias is applied across the device,
a p-i-n junction is induced due to electochemical doping on both sides
of the film. The p-i-n junction which is created in situ is dynamic. When
the external bias is removed, the junction will disappear by relaxation.
We report the stabilization of the p-i-n junction in these devices by lowering
the temperature. We found that at temperature below 200K, the ions were
essentially immobile a nd the p-i-n junction was ``frozen''. Detailed study
shows that polymer ``frozen-junction'' devices exhibit significant advantages
over ``dynamic-junction'' devices.
10:30 AM *F8.6/B15.6
DOPED POLYMER LIGHT-EMITTING DEVICES. Alan G. MacDiarmid
and Feng Huang, University of Pennsylvania, Dept of Chemistry, Philadelphia,
Even unsuspected trace amounts of residual doping either
in the light-emitting polymer layer (MEH-PPV) or in a polymer layer (polyaniline)
adjacent to an ITO electrode in a light-emitting device, can greatly reduce
electron and/or hole injection energies and mask the conventional work
functions of the electrode materials. This effect can improve certain important
parameters of a device including light emission in a reverse bias mode
in addition to the conventional forward bias mode. Poly(ethylene oxide)
can enhance this effect. The effects are consistent with the presence of
(polaron/bipolaron)+ A- p-doped ionic species (A-:
dopant anion) initially present in the polymer, the number of which does
not change during operation. Potential-induced diffusion of A-
ions controls the properties of the systems. Application of a pulsating
DC or AC potential promotes operating lifetime of the devices. General
conclusion : in the past, electronic polymers in the highly conducting
(fully doped) metallic regime have attracted the most attention.
In the future, it seems likely that the effect of unsuspected traces of
impurities on certain properties of organic semiconductors will attract
considerably more attention as the semiconducting regime becomes
technologically more important.
11:00 AM *F8.7/B15.7
CONJUGATED POLYMERS AS MULTI-COLOR ELECTROCHROMIC AND
ELECTROLUMINESCENT MATERIALS. John R. Reynolds , David J. Irvin, Michael
B. Ramey, Christopher A. Thomas, Dean M. Welsh, Department of Chemistry,
Center for Macromolecular Science and Engineering, University of Florida,
Gainesville, FL; Wing-Keung Woo, Michael F. Rubner, Department of Materials
Science and Engineering, MIT, Cambridge, MA.
A series of variable gap conjugated polymers have been
synthesized as multi-color electrochromic and electroluminescent materials.
Low bandgap (ca. 1.0 to 1.5 eV), cathodically coloring and high visible
light contrast, electrochromic polymers are designed utilizing a broad
selection of electron rich (e.g. 3,4-alkylenedioxythiophene) units, along
with electron poor (e.g. pyridinyl and cyanovinylene) units. Electrochromic
polymers with three distinct color states are prepared using polymers containing
chromophores of discrete lengths. Using carbazole and biarylamine linked
chromophores, yellow neutral, green cation-radical, and blue dication states
are attained. Electropolymerization methodologies are shown to produce
electrochromic materials with the most rapid and long-lived switching characteristics
as compared to solution processed films. Relatively high bandgap (2.4 to
3.0 eV), luminescent, conjugated polyelectrolytes based on poly(p-phenylene),
poly(2,5-thienylene-alt-p-phenylenes), and poly(p-phenylene ethynylenes)
have been utilized in layer-by-layer electrostatic depositions to yield
luminescent films with emission colors ranging from yellow to green to
11:30 AM F8.8/B15.8
EFFECTS OF IONIZATION POTENTIAL OF THE HOLE TRANSPORT
LAYER ON THE ELECTROLUMINESCENT CHARACTERISTICS OF ORGANIC LIGHT-EMITTING
DEVICES. B. Kippelen , G.E. Jabbour, S.E. Shaheen, M.M. Morrell, J.D. Anderson,
P. Lee, Univ. of Arizona, Tucson, AZ; E. Bellmann, S. Thayumanavan, S.
Barlow, R.H. Grubbs, CalTech, Pasadena, CA; S.R. Marder, N.R. Armstrong,
N. Peyghambarian, Univ. of Arizona, Tucson, AZ.
We report on organic electroluminescent devices with Al
cathode with luminous efficiency of 20 lm/W and external quantum efficiency
of 4.6 . When pulsed in air at
room temperature and without any encapsulation, remarkably high peak brightness
of 4.4 x 106 cd/m2 and high efficiency of 4.4 cd/A are obtained.
In contrast to current belief, we demonstrate that device quantum efficiency
can be increased by tuning the ionization potential of the hole transport
moieties to reduce energy barrier at the hole transport/light-emitting
interface. We also show that high efficiency and long stability can be
combined by cascading hole transport layers with increasing ionization
11:45 AM F8.9/B15.9
TEMPERATURE AND FIELD DEPENDENT MOBILITIES IN POLYMER
LED'S. L.D. Bozano , S.A. Carter, Univ. of California, Dept. of Physics,
Santa Cruz, CA; J.C. Scott, G.G. Malliaras, P.J. Brock, IBM Almaden Research
Center, San Jose, CA.
Transport properties of electron and hole-dominated MEH-PPV
devices are reported. The IV characteristics were measured in the range
of temperature from 200 K to 300 K. Different anode/cathode combinations
were used to obtain single and double polarity devices. The temperature
dependent mobilities are derived from the space charge limited behavior
at high electric fields. At 300 K the zero field mobility
and the electric coefficient
for electrons and holes are compared. For holes we found
= 1. x 10-7 cm2/Vs and
= 4.3 x 10-4 (m/V)1/2. For the electrons, we found
a an order of magnitude lower
than holes but a larger of
7.0 x 10-4 (m/V)1/2. Due to the stronger electric
field dependence of the electron mobility, electron and hole mobilities
are comparable at working voltages in the trap-free limit.
SESSION F9/B16: JOINT SESSION:
1:30 PM *F9.1/B16.1
MOLECULAR SELF-ASSEMBLY APPROACHES TO ORGANIC PHOTONIC
ARRAYS. T.J. Marks , W. Li, Q. Wang, J. Cui, J. Malinsky, Chem Dept and
the Materials Research Center, Northwestern U, Evanston IL; G. Jabbour,
J. Anderson, B. Kippelen, N. Armstrong, N. Peyghambarian, Optical Sciences
Center, U. of Arizona, Tucson, AZ.
Thursday Afternoon, April 8, 1999
Metropolitan I (A)
Molecular self-assembly as embodied in siloxane-based
self-limiting chemisorptive processes offers the prospect of constructing
optically-functional organic superlattices composed of tailored chromophoric
building blocks deposited with conformal, self-sealing/healing coverage
and Angstrom-level precision. Reported here is a new approach to organic
LED assemblies in which hole transport, emissive, and electron transport
layers are assebled into robust, covalently interlocked thin films using
siloxane condensation/ coupling processes. The resulting structures have
been characterized by a battery of microstructural, electrochemical, spectroscopic,
and photonic techniques. Properties of the resulting assemblies include
high structural regularity, absence of pinholes, the possibility of ultra-thin
layers, hight quantum efficiencies, and low driving voltages.
2:00 PM F9.2/B16.2
ORGANIC QUANTUM-CONFINED STRUCTURES VIA MOLECULAR LAYER
EPITAXY. Vladimir Burtman and Shlomo Yitzchaik, The Hebrew University of
Jerusalem, Department of Analytical and Inorganic Chemistry, Jerusalem,
A new molecular assembly approach - molecular layer epitaxy
(MLE) is introduced for the vapor-phase assembly of organic multilayers
integrated in molecular electronic devices. The MLE approach uses carrier
gas-assisted epitaxial deposition, covalent bonding and horizontal p-stacking,
in a pulsed mode for layer-by-layer growth. The substrates in this process
are firstly self-assembled with a template layer, n-propylamine, susceptible
for chemisorption. The subsequent steps rely on self-limiting reactions
that forms monolayers from the vapor phase and self-cleaning processes
where the excess precursor is sublimed away. Repeating such a reactions
cycles with 1,4,5,8-naphtalenetetracarboxylic-dianhydridde (NTCDA) and
1,6-Diamino-n-hexane (DAH) or/and 4,4- Diamino-diphenylmethan (DAPM) or/and
1,4-Phenylenediamine (PDA) building blocks leads to an imide linked layer-by-layer
growth. Resulting organic superlattices were studied by a spectroscopic
ellipsometry, absorption and emission spectroscopy and current voltage
measurements. MLE-derived structures are highly regular, exhibiting exciton
confinement effects and are thermo-chemically robust. An ultra-thin (ca.
4 nm) organic light-emitting devices (OLED) with low turn on voltage (0.5
V) were obtained via MLE where the emission is also size and voltage dependent.
Structural manipulations in the frame of MLE method on the molecular level
are highly flexible and therefore are very suitable for obtaining tunable
quantum-confined layered structures.
2:15 PM F9.3/B16.3
CRYSTALLINE SELF-ASSEMBLIES FROM ZINC-BISQUINOLINE; STRUCTURE,
MORPHOLOGY TOWARDS EPITAXIALLY GROWN OLEDS. D. Laurence Thomsen III and
Fotios Papadimitrakopoulos , Department of Chemistry, Polymer Science Program,
Nanomaterials Optoelectronics Laboratory, Institute of Materials Science,
University of Connecticut, Storrs, CT.
The reactive self-assembly of 8,8'-dihydroxy-5,5'-biquinoline
(bisquinoline) with diethyl zinc has shown to form a linear coordination
polymer. The packing and film uniformity of these films has been instrumental
for the fabrication of pinhole free films capable of electroluminescence.
Spectroscopic ellipsometry indicates that these assemblies possess the
highest refractive index among prior reported multilayer assemblies (n
= 1.69 at 633 nm). The suspected long-range crystalline order is currently
elucidated as a result of a detailed morphological and spectroscopic characterization.
Optical microscopy and selective area electron diffraction demonstrates
the presence of randomly-oriented poly(zinc-bisquinoline) single-crystals
dispersed throughout the self-assembled film. Crystal structure characterization
indicates a preferred growth orientation with the molecular axis perpendicular
to the growth substrate, along with a strong lateral bisquinoline self-organization.
The potential of this method to produce epitaxially grown organic semiconductors
for light emitting diodes is presently discussed.
2:30 PM F9.4/B16.4
SYNTHESIS AND SELF-ASSEMBLING PROPERTIES OF POLY (PHENYLENE
VINYLENE) WITH DENDRITIC SIDE-CHAINS. X. Linda Chen , Zhenan Bao, Karl
R. Amundson and Andrew J. Lovinger, Bell Laboratories, Lucent Technologies,
Murray Hill, NJ.
Novel types of poly(phenylene vinylene) with dendritic
side-chains have been synthesized to investigate their self-assembling
and device properties. These conjugated polymers, which have weight-average
molecular weights 20 - 60 kDalton, were synthesized by Heck reactions.
The introduction of the bulky side-chains improved the main-chain PPV separation
and enhanced photoluminescence was observed. In solid state, the polymers
self-organized into thermotropic liquid crystals with nematic textures,
suggesting potential application as polarized light-emitting media. In
addition, results of poly(phenylene vinylene)s with various structure modifications,
their associated self-assembling properties as well as device applications
will be presented.
2:45 PM F9.5/B16.5
SYNTHESIS AND CHARACTERIZATION OF NOVEL METAL COMPLEXES
FOR ELECTROLUMINESCENCE APPLICATIONS. Xu-Tang Tao ,Tatsuo Wada, Hiroyuki
Sasabe, Bio-Polymer Physics Laboratory, The Institute of Physical and Chemical
Research (RIKEN), JAPAN; Hironori Suzuki, Toshiyuki Watanabe, Seizo Miyata,
Graduate School of Bio-Applications and Systems, Tokyo University of Agriculture
and Technology (TUAT), Tokyo, JAPAN.
We report the synthesis and characterization of a series
of new metal complexs for electrolminescence applications.The complexes
are prepared by the reaction of 2-[3-(2H-benzotriazol-2-yl)-4- hydroxyphenyl]ethyl
methacrylate with various metal salts. These complexes can be polymerized
and show strong photoluminescence under a UV-lamp illumination. High optical
quality films were obtained by vacuum evaporation or by spin-coating methods.
The electroluminescence of single and double layer devices have been characterized.
The results indicated that these metal complexes could act as both electron
transport and emissive layers for EL devices.
3:30 PM *F9.6/B16.6
POLYQUINOLINES: MULTFUNCTIONAL POLYMERS FOR ELECTRO-OPTIC
AND LIGHT-EMITTING APPLICATION. Alex K.Y. Jen , Hong Ma, Xiaoming Wu, Jianyao
Wu, Yunqi Liu, Michelle Liu, Sen Liu, Craig Urian, Department of Chemistry,
Northeastern University, Boston, MA.
Polyquinolines (PQs) represent a class of high performance
polymers with outstanding thermal stability (> 450C),
low dielectric constants (2.5-3.0), low moisture absorption (0.1-0.15),
very high Tg (250-400C),
and compatibility with plasma or reactive ion etching. Our studies of their
electro-optic (E-O) properties have shown very promising results for side-chain
polyquinoline systems. They possess large E-O coefficients after poling
(r33 as high as 40 pm/V at 1.3 m),
and good temporal alignment stability at 100C.
Recently, polyquinolines have also been explored as good electron transporting
materials for light-emitting diodes (LEDs). The results from electrical
and optical characterization of these polymers showed that they possessed
very good LED performance (low turn-on voltage, bright light emission and
high rectification ratio). In addition, they possessed high thermal stability,
good electrochemical reversibility and excellent film-forming property.
4:00 PM F9.7/B16.7
1.5 M LUMINESCENCE
FROM ERQ BASED OLED'S. Richard Curry , William Gillin, Dept. of Physics,
Queen Mary and Westfield College, University of London, UNITED KINGDOM.
A 1.5 m emission
source which is compatible with silicon technology would have huge potential
in optoelectronic communications. To this end there has been a considerable
research effort over the last 10 years to study the incorporation of erbium
into silicon. Erbium is of interest because it has a sharp intra-atomic
transition at 1.5 m. We have produced
the chelate erbium (III) tris(8-hydroxyquinoline) (ErQ) and demonstrated
bright, sharp luminescence lines at 1.5 m
which are due to the erbium intra-atomic transitions. We present photoluminescence
data as a function of excitation energy and absorption data. These show
that the erbium is being excited via absorption into the molecule rather
than by direct absorption into the atom. Organic light emitting diodes
(OLED's) have been fabricated on indiun tin oxide (ITO) using N, N'-bis(3-methylphenyl)-N,
N'-diphenyl-[1, 1'-biphenyl]-4, 4'-diamine (TPD) as a hole transporting
layer and ErQ as an electron transporting/emitting layer. As AlQ based
OLED's have been demonstrated to operated using silicon as one electrode
we propose that ErQ may be a potential candidate for a silicon compatible
1.5 m emission source.
4:15 PM F9.8/B16.8
NARROW SPECTRUM LIGHT-EMISSIVE DEVICES MADE FROM BLENDS
WHICH EXHIBIT ENERGY TRANSFER FROM CONJUGATED POLYMERS TO RARE EARTH COMPLEXES.
Michael D. McGehee , Troy Bergstedt, Andrew P. Saab, Chi Zhang, Marie O'Regan,
Alan J. Heeger, Guillermo Bazan and Vojislav Srdanov, Univ of California,
Santa Barbara, CA.
Conjugated polymers have proven to be very useful as the
emissive component in light-emitting diodes and more recently in lasers
and photonic band gap materials. One of the limitations for some of these
applications is the very large width of the emission spectrum (50-100 nm).
To address this problem we have blended -diketone
chelated europium complexes into blue emitting polymers such as CN-PPP
and PVK. In these blends energy can be transferred in a multistep process
from the polymer to the -diketone
ligands and then to the europium atoms. The fluorescence spectrum from
the europium is centered at 613 nm and has a linewidth of less than 10
nm. To achieve efficient energy transfer we have synthesized a whole family
of europium complexes with different -diketone
ligands to tune the energy levels involved in the energy transfer process.
We will present spectroscopic data which show the mechanisms of energy
transfer and that energy transfer can be efficient if the energy levels
of the europium chelates are chosen properly . We will also present quantum
efficiency measurements and temperature dependent fluorescence lifetime
measurements which show that highly efficient fluorescence can be achieved
from the europium if it is shielded in an environment which prevents quenching.
Furthermore, we will show the characteristics of light-emitting diodes
made from the blends and discuss research which is presently being done
to make lasers and photonic band-gap devices.
4:30 PM F9.9/B16.9
NEW METHOD FOR DETERMINING MOBILITY AND EXCITON BINDING
ENERGY IN ORGANIC SEMICONDUCTING MATERIALS. Wim Geens , Suresh C. Jain,
Jef Poortmans, Johan Nijs and Robert Mertens, IMEC vzw, Leuven, BELGIUM.
We have fabricated Schottky diodes using PPV-oligomer:C60
blends and measured their dark and illuminated I-V characteristics.
We found that the current of the illuminated devices under reverse bias
increases strongly with the applied voltage. This result is similar to
that observed with Schottky diodes fabricated using other conducting polymers
. Both LED and solar cell structures show this behaviour. This behaviour
is strikingly different from that of a Si Schottky diode where the current
is independent of the reverse bias voltage. We suggest that this behaviour
arises on two accounts, (1) the excitons dissociate under the action of
the applied field  increasing the number of charge carriers and hence
the current, and (2) the mobility of the carriers increases as the applied
field increases . We have modified the chemical rate equations to take
into account the effect of the electric field on the rate of dissociation
and derived an expression for the equilibrium fraction of dissociated excitons.
Using this expression for the number of carriers and the expression for
mobility given in , we have derived an expression for the current as
a function of applied field. Exciton binding energy and field enhanced
mobility occur as fitting parameters in this equation. We have derived
the value of the binding energy and field enhanced mobility by fitting
this equation with the experimental results for several different cases.
We find that even for the same organic semiconductor, the binding energy
comes out to be different for samples prepared in different laboratories.
 X. Wei et al., Phys. Rev. B 49, 17480 (1994); J. Gao et al., Synth.
Metals 84, 979 (1997).  R. Kersting et al., Phys. Rev. Lett. 73, 1440
(1994); V. I. Arkhipov et al., Phys. Rev. B 52, 4932 (1995).  S. Karg
et al., Synth. Metals 67, 165 (1994).
4:45 PM F9.10/B16.10
DELAYED FLUORESCENCE AND TRIPLET-TRIPLET ANNIHILATION
IN PI-CONJUGATED POLYMERS. J. Partee, E.L. Frankevich, B. Uhlhorn, J. Shinar
, Ames Laboratory - USDOE* and Physics Department, Iowa State University
(ISU), Ames, IA; Y. Ding and T.J. Barton, Ames Laboratory - USDOE and Chemistry
Department, ISU, IA.
The delayed fluorescence (DF) of poly(para-phenylene vinylene)
(PPV) and poly(para-phenylene ethynylene) (PPE) derivative solids and frozen
solutions at 20 K was measured by modulating the photoexcitation power
at a frequency f and monitoring the response of the photoluminescence at
2f. The behavior of the response vs f provides strong evidence for triplet-triplet
annihilation to singlets excitons accounting for up to 3%
of the total emission in PPV films and 1.5%
in PPE powder. It also yields triplet lifetimes of 70 and 110 microsec
in PPV films and frozen solutions, and 200
and 500 microsec in PPE powder
and frozen solutions, respectively. However, while the response of the
PPV films and solutions agreed with a single monomolecular triplet lifetime,
the behavior of the PPE powder with such a single lifetime was only rough,
and that of the PPE solution was poor. In the films, the DF was slightly
red-shifted from the prompt fluorescence, but in the frozen solutions the
two coincided. This behavior is consistent with the high energy of the
lowest-energy singlet excitons in the shortest conjugated segments in the
films, which appears to be more than twice that of the triplet excitons.
*Ames Lab is operated by ISU for USDOE under Contract W-7405-Eng-82.
SESSION F10/B17: JOINT SESSION: ORGANIC EL 1
8:30 AM *F10.1/B17.1
DARPA HIGH DEFINITION SYSTEMS PROGRAM - RECENT DEVELOPMENTS.
Bruce E. Gnade , DARPA, Arlington, VA.
Friday Morning, April 9, 1999
Metropolitan I (A)
Recent developments from the DARPA High Definition Systems
program will be reviewed. A brief review will be presented showing how
new display technologies are being incorporated into military systems,
as well as some of the new technologies which are being supported by the
DARPA HDS program.
9:00 AM *F10.2/B17.2
MULTICOLOR POLYMER ELECTROLUMINESCENT DEVICES. Junji
Kido , Takayuki Suzuki, Satoshi Shirai, Graduate School of Engineering,
Yamagata University, Yonezawa, Yamagata, JAPAN.
Fabrication of micropatterned multicolor polymer electroluminescent
(EL) devices by the newly developed photobleaching method is described.
In this approach, we take advantage of the instability of organic fluorescent
molecules upon photooxidation; that is, organic molecules can be oxidized
or bleached by exposure to light in the presence of oxygen. For color tuning
and patterning, a polymer emitter layer containing fluorescent dyes is
exposed to ultraviolet (UV) light after formed by spin coating so that
a specific dye is photooxidized and becomes non-emissive. Due to this photobleaching
process, the UV exposed area have different emission colors from the non-exposed
area, and, using a fine-patterned photomask, micropatterned multicolor
EL devices are fabricated. This photobleaching method is the simplest method
for the fabrication of multicolor polymer EL devices.
9:30 AM F10.3/B17.3
R-G-B MULTI-COLOR ORGANIC LIGHT-EMITTING PIXELS PROCESSED
BY HYBRID INKJET PRINTING. Shun-Chi Chang, Yang Yang , Dept. of Materials
Science and Engineering, UCLA, Los Angeles, CA; Jun Onohara and Junji Kido,
Graduate School of Engineering, Yamagata University, Yamagata, JAPAN.
Organic light-emitting diodes represent a novel approach
for fabricating future high quality multicolor emissive displays. Traditionally,
the deposition of organic semiconductors is carried out by the thermal
sublimation process in an ultra-high vacuum environment. This is a rather
time consuming process and the patterning of fine multicolor pixels is
also quite complicated. In this presentation, we report the successful
demonstration of controllable patterning of red-green-blue (R-G-B) multi-color
organic light-emitting pixels using a hybrid inkjet printing technique.
For this demonstration, the blue polymer, which serves as the buffer layer,
was partially soluble in the solvent using for the red and green dopants.
We observed an efficient charge transfer when small amount of dopants,
DCM and Almq3, were introduced into the buffer layer. This demonstration
represents a new approach to solving an old problem of patterning the multi-color
organic LEDs for the future display application.
9:45 AM F10.4/B17.4
SURFACE-MODIFIED TRANSPARENT ELECTRODES FOR ORGANIC LIGHT-EMITTING
DIODES. Peter K.H. Ho , Univ of Cambridge, Cavendish Laboratory, Cambridge,
UNITED KINGDOM; Jeremy H. Burroughes, Cambridge Display Technology Ltd,
Cambridge, UNITED KINGDOM; Richard H. Friend, Univ of Cambridge, Cavendish
Laboratory, Cambridge, UNITED KINGDOM and Cambridge Display Technology
Ltd, Cambridge, UNITED KINGDOM.
Super-efficient poly(p-phenylenevinylene) derivative-based
single-layer polymer light-emitting diodes (LEDs) with external forward
quantum efficiencies near to one-quarter of the internal photoluminescence
efficiency of the emitter layers have been obtained on modified indium-tin
oxide (ITO) anodes. The ITO surface is modified with an ultrathin functional
polymer interlayer of the order of 100-
thick in a two-step process comprising first a surface silylation and derivatisation
reaction to anchor ionic charges on the oxide surface, followed by self-assembly
(alternate polyelectrolyte deposition) of the desired polymer interlayer.
Switching of the polymers during assembly allows composition-graded interlayers
to be fabricated with charge-transport and other properties tunable with
nanometer precision in the assembly direction. These interlayers can be
designed to give a virtual anode interface with the emitter polymer layer
that is conducive to hole injection but which presents a blocking barrier
to electrons transiting in the opposite direction. The resultant devices
exhibit a sharp turn-on soon after attaining the thermodynamic flat-band
condition. This is a signature of facile bipolar carrier injection at the
respective interfaces, together with efficient carrier confinement, capture
and radiative recombination in the emitter polymer layer. For a polyfluorene
derivative-based single-layer LED emitting in the green, where the eye
phototopic response peaks, we have recorded a forward luminous efficiency
of 25lm/W peaking at a drive voltage of 3.0V; and a luminance level of
1000cd/m2 at 5.6V.
10:30 AM *F10.5/B17.5
THE PHYSICS OF ORGANIC LIGHT-EMITTING DEVICES. J. Campbell
Scott , IBM Research Division, Almaden Res. Ctr., San Jose, CA; George
G. Malliaras, Dept. of Materials Science and Engineering, Cornell Univ.,
Ithaca, NY; Luisa Bozano and Sue A. Carter, Dept. of Physics, Univ. of
California, Santa Cruz, CA.
Qualitatively, the basic principles behind the operation
of OLEDs are well established. In order to optimize device parameters such
as power efficiency and operating voltage, to explore the limits of performance
and to understand changes in the electrical properties as diodes age, it
is necessary to develop a quantitative understanding of each of the relevant
processes: injection of electrons and holes at the cathode and anode, charge
transport, recombination and exciton formation, and emission. In this talk,
I will summarize our experimental, theoretical and numerical studies to
address these issues.
11:00 AM *F10.6/B17.6
HIGHLY EFFICIENT POLYMER LEDS FOR FULL COLOUR DISPLAY
APPLICATIONS. Jeremy Burroughes, David J. Lacey , Ian Millard, Craig Murphy,
Carl Towns, Cambridge Display Technology Ltd, Cambridge, UNITED KINGDOM.
We report the results of our programme of work on a range
of poly(fluorene)-based Polymer Light Emitting Diodes (PLEDs). These have
been developed with a common device architecture for all emission colours,
which achieves facile injection of both electrons and holes, and features
polymers with common core chemistry. It is a primary concern to ensure
that the emission colour of our PLEDs makes them suitable for use in full
colour display applications. By variation of the chemistry of the emissive
layer we have been able to tune the emission colour whilst retaining the
general device architecture and with little compromise of the electrical
performance. This approach has enabled rapid development of the different
PLED colours and enabled demonstration of efficient, stable EL emission
at various wavelengths. Synthesis of high molecular weight polymers based
on the fluorene unit has been achieved using a Suzuki polymerisation route.
Careful control of both the polymerisation reaction and the work-up has
yielded polymers of high purity and with high photoluminescence (PL) efficiency.
Variation of the chemistry of the monomers allows control of PL emission
colour, variation of the HOMO and LUMO levels and independent variation
of the chemical compatibility of the resultant polymer. We are now able
to demonstrate significant device efficiencies for red, green and blue
PLEDs using bilayer structures on Indium-Tin Oxide substrates with evaporated
metal cathodes. For example, green light emitting structures with efficiencies
in excess of 20 lm/W have been produced regularly. Similar green PLEDs
driven with constant current show very low drive voltage drift over thousands
of hours (less than 0.4 mV per Hour). The most recent efficiency and lifetime
data on our Red and Blue structures will also be presented.
11:30 AM F10.7/B17.7
LONG-TERM DEGRADATION MECHANISM OF ORGANIC LIGHT EMITTING
DEVICES BASED ON SMALL MOLECULES. Hany Aziz , McMaster University, Department
of Materials Science and Engineering, Hamilton, Ontario, CANADA; Zoran
D. Popovic, Nan-Xing Hu, Ah-Mee Hor, Xerox Research Centre of Canada, Mississauga,
Ontario, CANADA; Gu Xu, McMaster University, Department of Materials Science
and Engineering, Hamilton, Ontario, CANADA.
The intrinsic degradation of (8-hydroxyquinoline) aluminum-based
organic light emitting devices, that leads to the long-term drop in the
electroluminescence efficiency of the devices operated under constant current
conditions, has been studied. The role of stabilizing agents, such as introducing
a copper pthalocyannine buffer layer at the hole-injecting contact and
using mixed layers of hole and emitting electron transport molecules, has
been investigated. Based on the results, the major cause of the long-term
degradation in the devices has been elucidated.
11:45 AM F10.8/B17.8
THREE-COLOR DOPING OF POLYMER OLEDS BY MASKED DYE DIFFUSION.
F. Pschenitzka , J. C. Sturm, Princeton University, Dept. of Electrical
Engineering, Princeton, NJ.
Achieving three-color OLEDs is problematic since the local
deposition and/or local patterning of three different organic materials
is difficult especially for polymer based devices. In this work, we demonstrate
masked dye diffusion technique to locally change the emission spectrum
of an OLED, and apply this method for three-color integration. The key
novel feature of the work is the use of a large-area dopant source plate,
as opposed to a conventional evaporation point source. At elevated temperatures,
the dye will transfer from the source and diffuse into the organic layer
of the OLED which is in close proximity. The pattern of the transferred
dopant may be controlled by either a conventional shadow mask or a printed
diffusion barrier. We can create the source plates and transfer the dopant
without vacuum processing. This method allows us to integrate RGB-OLED
into a single polymer film with resolution. The device was made with a
PVK and PBD as the matrix and bimane (blue), C6 (green) and Nile Red (red)
as emissive dyes. The method can be easily applied to other polymers and
small molecule devices.