Fig.1: Effect of UV-illumination on a PHP-sample. Order of measurements: A-F
Conference Contributions Without Proceedings
Willi Graupner, Section
Head, Process Integration at
concerto! - silicon foundry
at austriamicrosystems AG.
This page lists the
conference contributions which did NOT result in proceedings, therefore
the full abstracts are listed. This page is organized the following way:
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Methods and Applications of Fluorescence Spectroscopy, Graz/Austria,
10.-11.10.1989 :
Annual Meeting of the German Physical Society, Berlin - 20.--24.3.1995
MRS
Fall 1996 Meeting, Boston, December 2-6, 1998 (this
page)
MRS
Spring 1997 Meeting, San Francisco, March 31 - April 4, 1997 (this
page)
March
Meeting of The American Physical Society March 16-20, 1998 Los Angeles,
CA (this page)
48. ÖPG-Jahrestagung 1998 (this page)
Electrooptical Properties of Polymers and Related Phenomena, Villa
Monastero, Varenna, 13 - 17 September, 1998, Italy (this
page)
MRS
Fall 1998 Meeting, Boston, November 30 - December 4, 1998 (this
page)
American
Physical Society Centennial Meeting March 20-26, 1999 Atlanta, GA (this
page)
III Convegno
Nazionale Materiali Molecolari per Fotonica ed Elettronica, Tanka Village
Villasimius 15-17 Settembre 1999 (this page)
9th Sede Boker Symposium on Solar Energy Production, July 12th
to 13th,
Ben Gurion National Solar Energy Center, Sede Boker, Israel (this
page)
Materials Research Society - Spring 2000 Meeting, April 24-28, 2000, San Francisco, California (this page)
Organic Optoelectronic
Devices Virginia Academy of Sciences - May 25, 2000 (this
page):
(transparencies)
220th ACS National Meeting August 20-24, 2000 Washington, D.C. (this page):
Efficient
charge generation in conjugated molecules
MRS Fall meeting
* Phototvoltaics Paper
March Meeting of the American Physical Society 2001, March 12 - 16,
2001, Washington State Convention Center, Seattle, Washington (this
page)
* A high pressure study of phonons in oligophenyls using Raman scattering - (this page).
* Raman Spectroscopy of Endohedral Metallofullerene C80 - (this page).
* Optical Spectroscopic Studies of Soluble Fluorene-based Conjugated Polymers - (this page).
* Pressure effects on the Davydov splitting of T6 single crystal - (this page).
Fall Meeting of the Materials Research Society 2001, November
,2001, Boston (this page)
MRS Spring Meeting in San Francisco (this page)
MRS
Fall 1996 Meeting, Boston, December 2-6, 1996
2:00 PM D9.2
PHOTOPHYSICS OF PARA-HEXAPHENYL,
Wilhelm Graupner, Technical Univ, Inst fur Festkoerperphysik, Graz,
AUSTRIA;
Andrea Mura, Univ of Cagliari, Dept of Physical Sciences, Cagliari,
ITALY;
Sandro De Silvestri, Mauro Nisoli, Politecnico di Milano, CEQSE-CNR,
Milano, ITALY;
Alessandra Piaggi, Guglielmo Lanzani, Univ of Sassari, Dept of Mat
& Physics, Sassari, ITALY;
Guenther Leising, Farideh Meghdadi, Technical Univ, Inst for Solid
State Physics, Graz, AUSTRIA.
D13.1
NATURE OF THE LOW-LYING TRIPLET STATE IN p-CONJUGATED
POLYMERS,
Jonathan Partee, Joseph Shinar, Iowa State Univ, Dept of Physics &
Astronomy , Ames, IA;
S. W. Jessen, Arthur J. Epstein, Ohio State Univ, Dept of Physics,
Columbus, OH;
Wilhelm Graupner, Guenther Leising, Technical Univ, Inst for Solid
State Physics, Graz, AUSTRIA.
D13.2
DYNAMICS OF LONG-LIVED POLARONS IN POLY(para-PHENYLENE)
(LPPP)-TYPE LADDER POLYMERS,
Jonathan Partee, Joseph Shinar, Iowa State Univ, Dept of Physics &
Astronomy , Ames, IA;
Wilhelm Graupner,Guenther Leising, Technical Univ, Inst for Solid State
Physics, Graz, AUSTRIA;
U. Scherf, Max-Planck-Inst, For Polymerforschung, Mainz, GERMANY.
MRS
Spring 1997 Meeting, San Francisco, March 31 - April 4, 1997
1:30 PM H2.1
LASER ACTION IN A POLY(PARAPHENYLENE)-TYPE LADDER
POLYMER,
Christian Zenz, Wilhelm Graupner, Stefan Tasch, Guenther Leising,
Technical Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA;
J. Flieser, K. Iskra, T. Neger,
Technical Univ, Institut fuer Experimentalphysik, Graz, AUSTRIA;
Time-resolved pump and probe experiments performed on neat films as
well as liquid solutions of methylene-substituted poly(paraphenylene) type
ladder polymer
(mLPPP) show strong stimulated emission besides spontaneous emission.
Due to its high photoluminescence yield and the lack of spectral overlap
between
photoluminescence and photo-induced absorption, mLPPP is a very attractive
material for laser applications.
We report on the lasing properties of a transverse optically pumped
cavity using mLPPP as the active material. A (1) strong spectral narrowing,
(2) high degree of
linear polarization, and (3) a good beam collimation of our blue green
polymer laser will be demonstrated. A clear threshold for these three features
is observed.
3:45 PM H7.8
TIME RESOLVED PHOTOLUMINESCENCE OF LADDER-TYPE POLY(PARA-PHENYLENE),
Wilhelm Graupner, Thomas Jost, Stefan Tasch, Guenther Leising,
Technical Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA;
Marion Graupner, Albin. Hermetter,
Technische Univ Graz, Inst fuer Biochemie und Lebensmittelchemie, Graz,
AUSTRIA.
The conjugated ladder-type poly(-phenylene) (LPPP) shows blue photoluminescence
and electroluminescence emission with high quantum yields. A high
degree of intrachain order and low concentration of electronic defects
allows to produce excellent optoelectronic devices from these materials.
We discuss
time-resolved photoluminescence experiments performed on neat films
as well as liquid and solid solutions of LPPP. The influence of sample
preparation, molecular
environment, and sample treatment is quantified by combining time-resolved
experiments with cw-photoluminescence and infrared spectroscopy.
4:15 PM H7.10
OPTICAL SPECTROSCOPY OF CHARGED EXCITATIONS AND FIELD-INDUCED
LUMINESCENCE QUENCHING IN ORGANIC LIGHT EMITTING DEVICES,
Thomas Jost, G. Meinhardt, Wilhelm Graupner, Stefan Tasch, Farideh
Meghdadi, Guenther Leising,
Technical Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA.
Ladder-type poly(paraphenylene) (LPPP) and para-hexaphenyl (PHP) show
blue photoluminescence (PL) and electroluminescence emission with high
quantum
yields, a high degree of interchain order, and low concentration of
electronic defects. We study the physical properties of charged and neutral
states in light-emitting
devices made of these materials under operation by PL and charge-induced
absorption spectroscopy. Pure field effects are studied in
metal(M)/semiconductor(S)/insulator/metal devices, where the insulating
layer prevents the current flow, whereas in M/S/M structures, we observe
both field- and
charge-induced effects. The dominant feature in the charge-induced
absorption spectrum in the LPPP is attributed to the transition of the
singly charged polaron,
which is also observed in the photoinduced absorption and doping experiments.
Under applied electric fields we observe quenching of the PL, which can
be
described within the framework of exciton-dissociation. The exciton
binding-energy obtained from this experiment is compared to results from
electro-modulation
absorption spectroscopy.
March
Meeting of The American Physical Society
March 16-20, 1998
Los Angeles, CA
Session E8 - Electric and Optical Properties.
MIXED session, Monday afternoon, March 16
402A, Los Angeles Convention Center
Photophysics
Studies of a White Light Emitting Organic Guest-Host System
M. Wohlgenannt, W. Graupner, G. Leising (Technische Universitat
Graz, Dept. of Solid State Physics), Z.V. Vardeny (University of Utah,
Dept. of Physics)
We report the optical properties of highly fluorescent guest host systems of two conjugated polymers. For the guest-host mixture the blue emitter methyl-substituted laddertype poly(para-phenylene) (mLPPP) was blended with a red emitter poly(perylene-co-diethynylbenzene) (PPDB) as a guest at sub-percent and percent level concentrations. We have used transient and steady state photoluminescence as well as near steady state photoinduced absorption (PA) to show that an efficient excitation energy transfer of Forster type occurs between the blue emitting host and the red emitting guest. This energy transfer can be exploited to produce white light emitting organic LEDs. In addition, modulation-frequency dependent PA has been employed to probe the energy transfer dynamics. The spectral signatures of emissive and absorptive photogenerated species in both polymers are presented. PA detected magnetic resonance spectroscopy (PADMR) of the system was also studied. PADMR allows to measur! e the spins of emissive and absorptive photogenerated species and gives spectral information about generation and recombination processes of these species occurring at magnetic resonance condition. In mLPPP we found a narrow spin 1/2 resonant band at g close to 2, superimposed on a much broader feature. The PADMR spin 1/2 spectrum shows a band at 1.35eV, close to a PA band which was previously identified as due to triplet photoexcitations.
Session M24 - Conducting Polymers III: Properties
of Doped Polymers (DMP Focused Session).
MIXED session, Wednesday morning, March 18
501C, Los Angeles Convention Center
We present a systematic Raman study of planar and nonplanar oligomers of poly(para-phenylene) under hydrostatic pressure. In hexaphenyl, the relative intensity of the inter-ring C-C stretch Raman mode at 1280 cm^-1 to the C-H bending mode at 1220 cm^-1, which is indicative of the conjugation length shows the influence of planarization under high pressure. At 1 bar, the ratio of the intensities is 0.8 and above 20 kbar the ratio is 0.3. These results are interpreted within the framework of SQMOFF calculations.\footnote L. Cuff, M. Kertesz, Macromolecules \bf27, 762 (1994). Related X-ray studies show that increasing the oligomer length decreases the lattice constants, which results in a closer crystallographic packing. In the hexaphenyl molecules this close packing tends to planarize the molecule.
Session O24 - Conducting Polymers
IV: Optical Properties (DMP Focused Session).
MIXED session, Wednesday morning, March 18
501C, Los Angeles Convention Center
We present photoluminescence(PL), absorption and photoinduced absorption studies of planar and non-planar oligomers of poly(para-phenylene) under hydrostatic pressure. These materials show strong luminescence with high quantum yield of 30 and 100 percent in films and solutions, respectively. Both PL and absorption show a red shift under hydrostatic pressure. The PL vibronic peaks shift by -30cm^-1/kbar and display changes in intensity with pressure. From our work, we gain insight into the influence of intermolecular interactions on the electronic and optical properties.
Session O24 - Conducting Polymers IV: Optical
Properties (DMP Focused Session).
MIXED session, Wednesday morning, March 18
501C, Los Angeles Convention Center
Ladder-type poly(p-phenylenes) (LPPPs) are attractive as blue emitters in polymer LEDs. When small amounts of guest red-emitting poly(perylene-co-diethynylbenzene) (PPDB) are added to an LPPP host, the emission shifts from blue to red with increasing guest concentration due to efficient excitation energy transfer. The results of photoluminescence-, electroluminescence-, and current-detected magnetic resonance measurements on LEDs with 0.05 - 2w% PPDB in LPPP are presented and discussed.
MRS
Spring 1998 Meeting, San Francisco, California, April 13-17, 1998 (this
page)
G8.5
ULTRAFAST DIRECT OPTICAL DETECTION OF CHARGE GENERATION
IN BLUE ORGANIC LIGHT EMITTING DIODES.
W. Graupner, E. List, G. Leising , Institut fuer Festkoerperphysik,
TU Graz, AUSTRIA,
G. Lanzani, Istituto di Matematica e Fisica, Universita' di Sassari,
ITALY;
G. Cerullo, M. Nisoli, S. De Silvestri, Dipartimento di Fisica, Politecnico
di Milano, ITALY.
Conjugated oligo- and polyphenylenes are used as active layers in blue, blue-green and white light emitting diodes (LEDs) as well as the active component in red-green-blue display elements due to their high photoluminescence quantum yields and good processibility. We present results of transient pump and probe spectroscopy on these materials. In particular we investigated LEDs under applied electric field. This allows us to optically probe the charge generation mechanism with a time resolution of a few 100 fs. Since we can directly monitor the generated polaron as well as the reduced singlet exciton population in the LEDs we can describe the process of charge generation on a sub picosecond timescale quantitatively.
We acknowledge the support of the bilateral Italo/Austrian 1997 Research
Program.
G8.11
INFLUENCE OF THE FILM MORPHOLOGY ON THE ELECTRONIC
PROPERTIES OF HEXAPHENYL.
W. Graupner, R. Resel, F. Meghdadi, G. Leising, Institut fuer Festkoerperphysik,
TU Graz, AUSTRIA;
A. Piaggi, G. Lanzani, Istituto di Matematica e Fisica, Università
di Sassari, ITALY;
G. Bongiovanni, Dipartimento di Scienze Fisiche, Università
di Cagliari, ITALY.
The blue emitting hexaphenyl is used for active layers in blue light
emitting diodes (LEDs) as well as the active component in red-green-blue
display elements due to its high photoluminescence quantum yield and good
processibility. We present results of X-ray structural investigations on
hexaphenyl films prepared under defined different conditions. These experiments
are combined with atomic force microscope experiments to characterize the
film morpholgy and the following optical
experiments: cw photoinduced absorption and transient photoluminescnece
to probe the long living absorptive states and the decay kinetics of the
emissive species. Based on these experiments we can directly determine
the influence of the degree
of order on the electronic properties.
We acknowledge the support of the bilateral Italo/Austrian 1997 Research Program.
Photovoltaic effect in organic compounds made of mLPPP,
PHP, DOT, and PPV
G. Meinhardt, W. Graupner, G. Feistritzer, R. Schröder, R. Kiebooms,
G. Leising,
Electrooptical Properties of Polymers and Related Phenomena, Villa Monastero, Varenna, Italy, 13 - 17 September, 1998
Photovoltaic Properties of Al/PHP/ITO
Sandwich Devices
G. Feistritzer, W. Graupner, E. Zojer, G. Meinhardt, J. Brandt, G.
Leising,
Institut für Festkörperphysik, TU Graz, Petersgasse 16,
A-8010 Graz, Austria
Para-hexaphenyl (PHP) devices are of particular interest since PHP films
can be fabricated by vapour deposition. Adjusting the evaporation rate
provides a good control of the thickness of the films and the orientation
of the chains within the layer. The characteristic optical absorbance of
these layers which is below 415 nm makes them suitable for several applications,
like a solar-window, which transmits light in the visible region whereas
the UV-component is used for photovoltaic purposes.
In our work photovoltaic properties of Al/PHP/ITO sandwich devices
on glass substrates were investigated by measuring the photocurrent action
spectra resulting from illumination through the ITO electrode. I/U-characteristics
for the devices were collected showing an onset of the forward current
at about 10 V.
Photocurrent action spectra were performed either in the dark or under
UV-illumination. The absolute values of the photocurrent decrease by a
factor of 2, when the dark spectrum is compared to the spectrum under illumination.
We observe a gradual decrease of the responsivity from run to run which
is shown in Fig. 1.
Fig.1: Effect of UV-illumination on a PHP-sample. Order of measurements:
A-F
However, when the device was left in the dark for 15 hrs the photocurrent recovered to values close to that of measurement C in fig. 1. In addition to this behaviour, the spectra changed their shape under UV-illumination. At higher wavelengths a shift to higher energies is observed whereas at lower wavelengths the spectra are shifted to lower energies. This results in a narrowing of the whole spectrum.
Upon tilting the specimen with respect to the direction of the incident
light we observed a shift of the photocurrent action spectrum in the range
below 380 nm to higher wavelengths. All the above outlined experiments
were carried out using the lock-in technique. So the photocurrent was recorded
versus chopper frequency in the range 7 Hz to 200 Hz. While measuring at
three different wavelengths we observed deviations in the photocurrent
time constants which indicate distinct origins of charge carriers for each
spectral region.
Ultrafast dynamics of field-induced
charge generation in conducting polymers
M. Nisoli, G. Lanzani, G. Cerullo, S. Stagira, C. R. Zenz, S. De Silvestri,
W. Graupner, E. J. W. List, G. Leising – Milano, Italy
Three-Dimensional Interactions in Conjugated Molecules Studied by High Pressure Spectroscopy
W. Graupner1, S. Guha2,S. Yang3, M. Chandrasekhar3, H.R. Chandrasekhar3, R. Glaser4, R. Resel1, G. Leising1
1Institute for Solid State Physics, Petersgasse 16, A
8010 Graz, Austria
2Department of Physics, Marquette Univeristy, Milwaukee
3Department of Physics and Astronomy, University of Missouri-Columbia
4Department of Chemistry, University of Missouri-Columbia
Conjugated molecules are low dimensional electronic systems which are attractive for many electronic and optoelectronic applications. However, the electronic properties of bulk samples of these materials do substantially depend on three-dimensional interactions [1,2]. One of the most appropriate ways to experimentally determine the relationship between three-dimensional interactions and electronic properties is to (1) probe the latter by optical spectroscopy while (2) varying the former by the application of hydrostatic pressure [3-7].
In this contribution the results of Raman, photoluminescence, and absorption spectroscopy on para-phenylene backbone systems will be discussed. Hydrostatic pressures of up to 70 kbars were applied to samples in a temperature range of 4 to 300 K. The spectral positions of all spectral features show a continuous shift upon application of pressure, however there are also changes in the peak shapes and peak distribution. In the case of Raman spectroscopy the pressure induced shifts are the expression of a change in the force constants of the molecules - a stiffening of the backbone as well as the C-H bonds. The intensity distribution of C-H modes can be used as a tool to investigate structural changes on a single chain, e.g. as an indicator of planarity in para-hexaphenyl. Changes in the planarity of molecules via the application of pressure originate in the stronger intermolecular interactions which also result in a distinct behaviour of the volume compressibility. In contrast to Raman spectroscopy which probes structural changes photoluminescence and absorption spectroscopy are susceptible to the corresponding changes in the electronic properties. The observed results for these spectra are very well in agreement with theoretical predictions for interacting conjugated molecules [1,2]
The work in Graz was supported by Österreichische Nationalbank, project 6608.
[1] J. Cornil, A. J. Heeger, J. L. Bredas, Chem. Phys. Lett.272,
463 (1997).
[2] J. Cornil, D. A. dos Santos, X. Crispin, R. Silbey, J. L. Bredas,
J. Am. Chem. Soc. 120, 1289 (1998).
[3] W. Graupner, S. Eder, K. Petritsch, G. Leising, U. Scherf, Synth.
Met. 84, 507 (1997).
[4] W. Graupner, S. Guha, S. Yang, M. Chandrasekhar, H. R. Chandrasekhar,
G. Leising, U. Scherf, K. Müllen, Mat. Res. Soc. Symp. Proc.
488, 873 (1998).
[5] S. Guha, W. Graupner, R. Resel, M. Chandrasekhar, H. R. Chandrasekhar,
G. Leising, Mat. Res. Soc. Symp. Proc. 488, 867 (1998).
[6] L. Rossi, W. Graupner, R. Resel, F. Meghdadi, G. Leising, F. Sannicolo,
G. Lanzani, Tubino, Mat. Res. Soc. Symp. Proc. 488, 897 (1998).
[7] S. Guha, W. Graupner, M. Chandrasekhar, H. R. Chandrasekhar, R.
Resel, G. Leising, R. Glaser, submitted to J. Chem. Phys.
MRS Fall 1998 Meeting, Boston, November 30 - December 4, 1998
4:45 PM R2.9
LASING OF A PURE CONJUGATED HOMO-POLYMER FILM IN AN
ADJUSTABLE MICROCAVITY.
Christian Zenz , INFM, Universite di Sassari, ITALY;
Salvatore Stagira, Margaritha Zavelani-Rossi, Mauro Nisoli, Sandro
De Silvestri, INFM, Politecnico di Milano, ITALY;
Wilhelm Graupner, Stefan Tasch, Günther Leising, Inst. f. Festkörperphysik,
TU-Graz, AUSTRIA.
Optically pumped lasing in a high Q-value cavity with adjustable cavity
length is demonstrated using methyl substituted laddertype poly-(parahexaphenyl)
(m-LPPP) as active material. Due to its high cross section for stimulated
emisson , the high photoluminescence quantum yield, and the low subgap
absorption m-LPPP is a promising candidate for organic solid state devices
which has been proved by spectral narrowing in simple waveguides at low
pump thresholds. The cavity is
formed by a flat dielectric mirror with a reflectivity of >99,99 in
the spectral range of the m-LPPP emission and an output coupler with a
curvature radius of 8 m and a reflectivity of about 97. A film of m-LPPP
with a thickness of approximately 200 nm is prepared on the flat mirror.
The cavity length can be adjusted using a translation stage to obtain a
stable cavity where the cavity mode is matched to the pump mode. For optical
excitation with 150fs pulses at 3.18eV we
observe a photoluminescence emission spectrum that is modulated by
the modes of the cavity. Pumped above a threshold value of 150 J/cm2, single
mode operation with a small output beam divergence is observed. The temporal
behaviour of the cavity is studied using up-conversion technique to measure
the output pulse duration. The results are compared with theoretical simulations.
R3.21
STRUCTURAL PROPERTIES OF LIGHT EMITTING DEVICES BASED
ON PARA-HEXAPHENYL.
A.Pogantsch 1, R.Blyth2, G.Feistritzer1,
W.Graupner1, N.Koch1, M.Ramsey2, F.Netzer2,
R.Resel1, S.Tasch1 and G.Leising1
1 Institut fuer Festkoerperphysik, Technische Universitaet
Graz
2 Institut fuer Experimentalphysik, Universitaet Graz.
We investigate different device parameters of a monolayer sandwich structure
based on the blue electroluminescent oligomer p-Hexaphenyl: p-Hexaphenyl
molecules show a high anisotropy of their electronic properties and their
molecular ordering in the film can be controlled by varying the vacuum
deposition conditions. We perform I/V and luminance characterization as
well as photocurrent measurements to investigate the effects of the ordering,
characterized by X-ray diffraction and infrared spectroscopy, on the probed
macroscopic properties of the devices. We study the structure of a device
in an ITO/p-Hexaphenyl/Al configuration using an elemental depth profiling
technique based on Auger electron spectroscopy. Devices operated for a
given period of time as well as freshly prepared devices have been investigated
by this technique. The influence of the organo-metallic interfaces on the
onset voltage and the efficiency of electroluminescence and also on the
photovoltaic properties of the device is thereby highlighted. Depending
on the aluminum deposition conditions we observe drastic changes in the
device performance: A significant drop of the onset voltage and an increase
in quantum efficiency can be achieved. We do not find evidence of the formation
of an aluminum oxide layer.
The financial support of the Spezialforschungsbereich Elektroaktive
Stoffe is gratefully acknowledged.
9:45 AM R6.4
ON THE WORKING PRINCIPLE OF LIGHT-EMITTING ELECTROCHEMICAL
CELLS.
L. Holzer , G. Feistrizer, W. Graupner, S. Tasch, F.P. Wenzl, G. Leising,
Institut fuer Festkoerperphysik, Technische Universitaet Graz, Graz, AUSTRIA;
R. Blyth, M.G. Ramsey, F.P. Netzer, Institut fuer Experimentalphysik,
Universitaet Graz, Graz, AUSTRIA.
We present experimental results to the working principle of light-emitting
electrochemical cells (LECs) based on poly(phenylene vinylene) and poly(paraphenylene)
type polymers. In order to monitor the distribution and the binding state
of the ions in the active layer, we performed secondary ion mass spectroscopy
(SIMS) in combination with Auger spectroscopy. These results are used to
classify the nature of the interfacial layer between the electrode and
the polymer. The binding state
of the ions as a function of the applied voltage was studied via infrared
spectroscopy on the device under operation. The motion of the polarons
and dopants in the visible and near infrared spectral range was investigated
with charge induced absorption spectroscopy. These results clearly show
that doping occurs within the layer during operation.
2:45 PM R7.4
ULTRAFAST TRIPLET STATE PHOTOGENERATION MECHANISMs
IN ORGANIC SEMICONDUCTORS.
G. Lanzani , Christian Zenz, University of Sassari, Dept of Mathematics
and Physics, ITALY;
G. Cerullo, S. De Silvestri, Dept of Physics, Politecnico di Milano,
ITALY;
D. Comoretto, G. Dellepiane, University of Genova, Dept. of Chimica
and Chimica Industriale, ITALY;
W. Graupner, G. Leising, Graz University of Technology, Institute for
Solid State Physics, AUSTRIA.
Triplet excited states (TS) play a fundamental role in the photophysics
and photochemistry of organic molecules and polymers. TS formation is an
effective non radiative deactivation channel for the singlet excited state
and occurs in Organic Light Emitting Devices (OLED), following charge carrier
recombination. The general knowledge is that TS cannot be directly photogenerated,
but result due to inter system crossing (ISC) from the singlet manifold
in the ns time scale. ISC is not however the only available mechanism for
TS formation following photoexcitation: in this work we demonstrate ultrafast
TS generation in p-conjugated semiconductors
by applying the femtosecond pump-probe scheme. Singlet, triplet and charged
states that are involved in the process are identified by characteristic
bands in the photoinduced absorption spectrum and monitored with 200 fs
time resolution. Electric field assisted pump-probe experiments in exaphenyl
LEDs
suggest that TS formation occurs in the ultrashort time domain by polaron
recombination. The field-assisted experiments allow to single out the charged
excited state contribution, and to demonstrate the bimolecular character
of the recombination process. In addition we found that upon high density
photexcitation a second mechanism is active in PHP films, assigned to singlet
fusion and fission into triplet pairs. A third TS generation mechanism
is direct singlet fission (< 200 fs) into triplet pairs, as demonstrated
in matrix-isolated chains of polycarbazolyldiacetylene. This is a non-luminescent
material wherein fission occurs probably from the lowest lying even parity
state. Triplet pairs are transient species that decay in the ps time domain
because of geminate recombination after one-dimensional diffusion. Trapping
at the chain ends hampers the latter process, and give rise to localized
long lived triplets.
R8.25
INTERNAL COLOUR-CONVERSION IN SINGLE LAYER POLYMER
LIGHT EMITTING DIODES REALISED BY EXCITATION ENERGY TRANSFER.
E.J.W. List , G. Dicker, W. Graupner, G. Leising, Institut für
Festkörperphysik,
Technische Universität Graz, AUSTRIA; A.D. Schlueter Institut
für Organische Chemie, FU Berlin, GERMANY.
We present internal colour conversion within the active layer of polymer
light emitting diodes (PLED) consisting of a guest host system of a polymer
and a dye dopant. This technique is very promising to realise single layer
PLEDs emitting different colours depending on the dopant concentration.
Colour tuning in single layer PLEDs is very important due to the ease of
the fabrication for future applications in flat panel displays. Since the
emission spectrum of the blue-emitting
conjugated ladder-type poly(paraphenylene) (LPPP) and the absorption
spectrum of RS19 - a red-emitting macromolecule - show a strong overlap
we observe excitation energy transfer of F–rster type from the host m-LPPP
to the guest RS19 so that dependent on the concentration one can control
the emission color of a single layer PLED between the CIE coordinates 0.23,
0.25 and 0.51 , 0.45. Using this technique we realised highly efficient,
stable single layer PLEDs emitting different colours including white light
emission. We report on electroluminescence and photoluminescence properties
as well as I/V characteristics. Furthermore we investigated the internal
field distribution in the devices via electromodulation spectroscopy. Charge-
and photoinduced states are probed in order to monitor the energy and charge
transfer processes in the devices.
R8.26
INFLUENCE OF THREE-DIMENSIONAL INTERACTIONS ON THE
ELECTRONIC PROPERTIES OF POLY(PARA-PHENYLENE).
S. Yang1, W. Graupner 2, S. Guha1,
M. Chandrasekhar1, H.R. Chandrasekhar1, G. Leising2;
1 Dept. of Physics and Astronomy, Univ. of Missouri, Columbia,
MO;
2 Institut für Festkörperphysik, TU Graz, AUSTRIA.
The anisotropic electronic properties of conjugated molecules can be
utilized to build devices which produce linearly polarized light either
by anisotropic absorption or emission. The anisotropy has its origin in
the quasi one-dimensional nature of the isolated linear conjugated molecule.
Three-dimensional interactions however are very important in bulk samples
of conjugated materials. In this contribution we investigate the electronic
properties of a class of efficient, blue emitting oligo- and polyphenyls.
The experimental tools comprise (1) absorption, (2) photoluminescence emission,
(3) photoinduced absorption and (4) bleaching of absorption bands. The
spectral dependence of (1)-(4) depends on the configuration of the neutral
singlet ground and excited states as well as on the neutral triplet and
charged polaronic excitations. The temperature range of our investigations
spans 20-300 K. In order to investigate the influence of three-dimensional
interactions on the electronic properties we have performed the above experiments
in a range from ambient pressure to 70 kbar. The observed spectra show
two effects upon increasing pressure: (a) a broadening and (b) a continuous
redshift of the involved bands. Both effects (a) and (b) are distinctly
different for different involved species - hence we can identify the influence
of a varying degree of three-dimensional interactions on the involved species.
2:15 PM R10.3
PHOTOVOLTAIC PROPERTIES OF MULTILAYER AND MOLECULARLY
DOPED ORGANIC DEVICES.
G. Feistritzer 1, G. Meinhardt1, R. Schroeder1, E. Zojer1, E. List1,
A. Pogantsch1, G. Dicker1, W. Graupner1, A.D. Schlueter2,
G. Winter3, M. Hanack3, G. Leising1
1Institut fuer Festkoerperphysik, TU Graz, Graz, AUSTRAIA;
2Institut fuer Organische Chemie, FU Berlin, Berlin,
GERMANY;
3Institut fuer Organische Chemie, Uni Tuebingen, Tuebingen,
GERMANY.
For optoelectronic devices organic materials exhibit the advantages of low price, low processing costs, the ability of easy tuning of the optical properties and the use in large area application. In our work we investigated the photovoltaic properties of three groups of devices:
(A) single layer devices made of para-hexaphenyl
(PHP), pure and photo-oxidised methylated-laddertype poly-para-phenylene
(mLPPP), poly-phenylene-vinylene (PPV) sandwiched between ITO
and Al electrodes
(B) mLPPP blended with TiOphthalocyanine (TiO-Pc),
poly-phenyl-co-diethynyl benzene (PPDB) and RS19 (1); for the mLPPP/TiO-Pc
the efficiency increased by one order of magnitude with respect to the
pure material.
(C) multilayer devices consisting of PHP/mLPPP/PHP
and mLPPP/PHP/mLPPP with different layer thicknesses.
For all experiments a modulated probe beam in combination with lock intechnique was used. The influence of the location of the photoactive region on shape and magnitude of the action spectra was investigated with illumination through the ITO and semitransparent Al-electrodes. Bias-dependent measurements were used to investigate charge separation mechanisms and their spatial location. In order to obtain further inside in the charge generation process we performed photovoltaic experiments under optical bias and at different excitation conditions (light intensity and chopper frequency). We obtained the best photovoltaic yield for a triple layer device made of PHP/mLPPP/PHP (1.2mA/W) which is two orders of magnitude higher than an mLPPP/PHP/mLPPP-structure and three orders of magnitude above the single layer mLPPP. A variation of the thickness of the organic layers shows that this values can be further improved.
(1) B. Schlicke et al., Synthetic Metals 83 (1996) 173.
Midwest Solid State Meeting Ames/Iowa - October 1998
Electronic Properties of Organic
Molecules under High Pressure -
a comparative study for a polycrystalline and an amorphous
conjugated molecule
S. Guha 1* , S. Yang 1 , W. Graupner 2 , H. R. Chandrasekhar 1 , M. Chandrasekhar 1 , G. Leising 2
1 Department of Physics and Astronomy, University of Missouri, Columbia,
MO 65211, U.S.A.
2 Institut für Festköperphysik, Technische Universität
Graz, A-8010 Graz, Austria.
* Department of Physics, Marquette University, Milwaukee, WI 53201
We present optical studies using photoluminescence and absorption of
conjugated polymers from two families, a polycrystalline “soft” non-planar
polymer, para hexaphenyl (PHP) and an amorphous “rigid” planar material,
methyl substituted ladder poly para phenylene (m-LPPP) with long side chains,
under hydrostatic pressure. These studies provide insight into the fundamental
properties of the ground and excited states of these materials. Both materials
show a red-shift in their
transition energies with increasing pressures, characteristic of a
B-B* transition. It is observed that the photoluminescence vibronic transitions
broaden linearly as a function of pressure in m-LPPP though there is no
significant broadening in PHP. From our measurements under pressure and
using a simple model, we extract atmospheric pressure values for the shift
in the configuration coordinate between ground and excited states and the
ratio of force constants describing the potential wells that define these
states.
European
Conference on Organic Solar Cells Cadarache,
France, December 3-5,
1998
Photovoltaic Properties of Poly-Para-Phenylene
- Vinylene - Polyaniline Blends and
Influence of Encapsulation on the Photooxidation
Raoul Schröder (1), G. Freistritzer (1), W. Graupner (1), Peter Preishuber-Pflügl (2), D. Faiman (2), F. Stelzer (3), B. Weßling (4), G. Leising (1)
(1) Solid State Physics, Technical University
of Graz, Austria
(2) Inst. for Chemic. Technol. of Inorg. Compounds,
Technical University of Graz, Austria
(3) National Solar Energy Center, Ben-Gurion
University, Sede Boquer, Israel
(4) Ormecon Chemie, Ammersbek, Germany
Due to the simple chemical tuning of the electronic properties of organic molecular semiconductors these materials are investigated for a wide variety of optoelectronic devices, covering the field from laser diodes to solar cells. In all these applications (1) a proper choice and (2) stability of the active organic layer is crucial.
We report studies of solar cells based on poly(para phenylene vinylene)
(PPV) and doped polyaniline (PAni). The high photocurrent response of the
PPV in the green and blue spectral region is combined with the charge transport
capability of the PAni. Solution/Dispersion processing of both materials
secures a simple production of the devices by using different ways of blending
PPV and PANI. The different preparation conditions affect the spectral
dependence of the measured photocurrent. In order to characterize the internal
fields acting on the molecules in the device we performed electro-modulation
spectroscopy in the devices.
Further we show how encapsulating polymer materials can prevent photooxidation
and therefore stabilize them. The spectral response and electrical characteristics
of the stabilized cells were tested under laboratory conditions and also
under AM1.5 conditions in the desert.
Conference
on Lasers and Electro-Optics and
International
Quantum Electronics Conference (CLEO/ IQEC '98), May 1998,
QWI4 - Direct observation of ultrafast field-induced charge photogeneration in conducting polymers,
G. Cerullo, M. Nisoli, S. Stagira, S. De Silvestri, Politecnico Istituto
Nazionale Fisica della Materia, Italy;
G. Lanzani, Univ. Sassari, Italy,
W. Graupner, E.J.W. List, G. Leising, Technische Univ. Graz, Austria.
Field-induced charge photogeneration was directly observed in a poly-(para-phenylene) ladder-type polymer by applying the pump-probe technique. Polarons, identified by an absorption band at 1.9 eV, are formed by exciton dissociation in the picosecond time domain.
QThG19 - Amplified spontaneous emission in blue emitting poly(para-phenylene) ladder-type polymer,
S. Stagira, G. Cerullo, M. Nisoli, S. De Silvestri, Politecnico Istituto
Nazionale Fisica della Materia, Italy;
G. Lanzani, Univ. Sassari, Italy;
W. Graupner, G. Leising, Technische Univ. Graz, Austria.
A strong narrowing is observed in the photoluminescence band of poly(para-phenylene)
ladder-type polymer in the blue spectral region under high excitation fluence.
Experimental evidence of the presence of an efficient amplified spontaneous
emission process is reported.
American
Physical Society Centennial Meeting March 20-26, 1999 Atlanta, GA
Session WC01 - Organic Electronic Materials
IX: Optical and Electronic Characterization I.
ORAL session, Thursday afternoon, March 25
Room 360W, GWCC
We introduce and apply a new version of the photomodulation technique, measured far from the steady state, for obtaining the action spectra of the photogeneration quantum efficiency of neutral and charged excitations in films of a ladder-type poly(para-phenylene). Whereas the photogeneration action spectrum for singlet excitons shows a step function increase at a photon energy, E, close to the optical gap, both triplet and polaron photogeneration action spectra show, in addition, a monotonous rise at higher E. The rise for triplets is explained by singlet fission into triplet pairs, whereas for polarons it is modeled by an electron interchain tunneling process.
Session XC01 - Organic Electronic Materials
X: Optical and Electronic Characterization II.
ORAL session, Friday morning, March 26
Room 360W, GWCC
We present photoluminescence and absorption studies of conjugated molecules from two distinct families, a polycrystalline "soft" non-planar, para hexaphenyl (PHP) and an amorphous "rigid" planar, methyl substituted ladder poly para phenylene (m-LPPP), under hydrostatic pressure. These studies provide insight into the fundamental properties of the ground and excited states of these materials. Both materials show a red-shift in their transition energies with increasing pressures. The photoluminescence vibronic transitions broaden linearly as a function of pressure in m-LPPP though there is no significant broadening in PHP. >From our pressure measurements and using a simple model, we extract atmospheric pressure values for the shift in the configuration coordinate between ground and excited states and the ratio of force constants describing the potential wells that define these states. These results are also interpreted in terms of the Huang-Rhys factor. This was supported by Univ. of Missouri research board.
Session VC01 - Organic Electronic Materials
VIII: Fluorescent Properties.
FOCUS session, Thursday morning, March 25
Room 360W, GWCC
Films and both liquid and solid solutions of para-phenylene type molecules are highly emissive media in the UV-blue-green range. The quasiparticles governing the emission processes in light emitting diodes (LEDs) made of these molecules are emissive singlet excitons as well as absorptive triplet excitons and polarons. We present the results of a detailed and comprehensive study of The photoexcited states in para-phenylene type polymers and oligomers The photoinduced absorption (PIA) representing the photogenerated polaron and triplet exciton density, photoluminescence (PL) representing the photogenerated singlet exciton density, and optically and electrically detected magnetic resonance (ODMR/ELDMR) representing the interaction between the polarons/triplets and singlets is described. We show how a model relying on the PL-quenching effect of the nonemissive quasiparticles and a model relying on the fusion of triplets can both account for the observed phenomena
Session VC01 - Organic Electronic Materials
VIII: Fluorescent Properties.
FOCUS session, Thursday morning, March 25
Room 360W, GWCC
Films and both liquid and solid state solutions of para-phenylene type molecules are highly emissive media in the ultraviolet-blue-green spectral range. They are used in light emitting diodes (LEDs) and photopumped lasers. Photoexcited para-phenylene type molecules give rise to (1) strong emission from singlet excitons, (2) bleaching of the singlet exciton absorption, (3) induced absorption from triplet excitons and (4) induced absorption from polarons. Since the processes (3) and (4) can absorb the emission of singlet excitons, the presence of polarons and triplet excitons might be a fundamental problem for laser diodes made from para-phenylene type molecules.
In our experiments we modify the molecular geometry by the application of hydrostatic pressures up to 70 kbar. To our knowledge this is the first thorough study of the photobleaching and photoinduced absorption signals under these conditions. In particular we show how triplet and polaron states, which are created at a ratio of 3:1 in LEDs under operation, react to the induced geometric changes.
Conference
on Lasers and Electro-Optics (CLEO) Quantum Electronics and Laser Science
Conference, Baltimore, Maryland May 23 - 28, 1999
Photogeneration of Triplet States by Polaron Recombination in para-Hexaphenyl
C. Zenz(1,3), G. Cerullo(2), G. Lanzani(1), W. Graupner(3), F. Meghdadi
(3),
G. Leising(3), S. De Silvestri(2)
(1) Istituto Nazionale per la Fisica della Materia, Istituto di Matematica
e Fisica, Università di Sassari, Via Vienna 2, I-07100, Sassari,
Italy
(2) Istituto Nazionale per la Fisica della Materia,CEQSE-CNR, Dipartimento
di Fisica, Politecnico di Milano, Pza. L. d. Vinci 32, I-20133 Milano,
Italy
(3) Institut für Festkörperphysik, Technische Universität
Graz,
correspondence: C. Zenz, Petersgasse16, A-8010 Graz, Austria, Fax: ++43-316-873-8478, email: f513zenz@mbox.tu-graz.ac.at
Abstract
Triplet generation in para-hexaphenyl by bi-molecular non-geminate recombination of photo-generated polarons is observed. Electric field-assisted femtosecond pump-probe measurements can establish this process.
Dissociation of photo-generated excitons into charge carriers (polarons) is the working principle of organic photovoltaic cells, whereas recombination of field-injected charge carriers into luminescent singlet excitons and non-radiative triplet excitons governs the efficiency of organic light emitting devices (OLED). In this paper, we address these processes in a single layer OLED based on para-hexaphenyl (PHP) by conventional and electric field-assisted femtosecond pump/probe measurements. Due to its well defined conjugation length and its chemical structure PHP exhibits well resolved photo-induced absorption (PA) features, that can be assigned to singlet, triplet and polaron absorption. This allows to monitor the dynamics of field-induced singlet dissociation and polaron recombination in a real single layer OLED structure consisting of aluminum/ PHP/ Indium Tin Oxide (ITO) with 200 fs time resolution.
Figure 1 (a) shows conventional differential transmission spectra for 0 ps and 200 ps pump-probe delays (t D). We observe three PA-bands peaking at 1.59 eV (PA1), 1.8 eV (PA2), and 2.09 eV (PA3), and a strong stimulated emission (SE) band for probe energies higher than 2.5 eV. The field-induced changes of the bands under the same experimental conditions for an applied reverse bias of 16 V and a pump- probe delay of 20 ps are depicted in Figure 1 (b). From this spectrum, a field-induced quenching of SE and PA1 is evident, whereas both PA2 and PA3 are increased. PA1 is assigned to singlet exciton absorption due to the correspondence with SE-dynamics and the field-induced quenching by exciton dissociation. PA3 at 2.09 eV can be assigned to polaron absorption as indicated by theoretical calculations and comparison with doping induced measurements. This assignment is also supported by the observed field-induced increase of PA3.
The absorption band at 1.8 eV (PA2) has been assigned to
triplet absorption being the only persistent spectral feature for pump-probe
delays longer than 200ps (see Fig. 1a) and a comparison with cw-PA measurements.
The differential transmittance spectrum at 0 ps delay, that already contains
this feature, suggests a triplet exciton formation within the pump pulse
( t P<200 fs ). Due to the observed
dynamics and the energetic position of the pump, we exclude intersystem
crossing (ISC) and singlet fission to be the dominant triplet generation
mechanism under these experimental conditions. More insight into the triplet
formation can be gained by studying the dynamics of the field-induced changes
of PA2. In this case we cause exciton breaking into polarons
and test the effect on the triplet band. We observe an increase of triplet
absorption with a finite rise time, that is not correlated to the observed
exciton dissociation dynamics. These findings suggest that triplet formation
occurs by a two step process: (i) field-induced dissociation of
singlet excitons into polarons followed by (ii) non-geminate recombination
into triplets and excitons following the spin statistics. The bi-molecular
character of this process can be demonstrated by Figure 2, where the significant
decrease of the field-induced triplet formation time upon increasing the
excitation density. We conjecture, that the same triplet formation mechanism
is also active without external electric field. The ultrafast dissociation
of a fraction of the photo-generated singlet exciton, possibly assisted
by disorder or singlet exciton fusion, gives rise to an initially high
density of polarons, which recombine to triplet excitons within the experimental
resolution of our set-up.
Figure
1. (a) Conventional transient pump/probe spectra of the polycrystalline
PHP film for different pump probe delays (tD),
measured in an OLED-structure. (b) Field-induced (D
T/T)MF-spectrum of PHP films for pump-probe delay of 20 ps and
an applied reverse bias of 16 V. Data points are obtained using a monochromator
or interference filters. The solid line is a guide to the eye.
Figure
2. Normalized field induced build-up of the triplet absorption
(PAtrip) at 1.8 eV for increasing excitation densities (ED).
Traces are vertically displaced for clarity.
III
Convegno Nazionale Materiali Molecolari per Fotonica ed Elettronica,
Tanka
Village Villasimius 15-17 Settembre 1999
Migration and decay behaviour of singlet excitons in ladder-type poly- and oligophenyls
Christoph Gadermaier (a), M.A. Loi (b), E.J.W. List (a), W. Graupner (c), U. Scherf (d), G. Bongiovanni (b), A. Mura (b), G. Leising (a)
(a) Institut für Festkörperphysik, TU Graz, Petersgasse 16,
A-8010 (Austria)
(b) Dipartimento di Fisica and INFM, Università di Cagliari,
I-09042 Monserrato (Italy)
(c) Department of Physics, Virginia Tech, VA-24060 Blacksburg (USA)
(d) Max-Planck-Institut für Polymerforschung, D-55128 Mainz (Germany)
The class of methyl substituted ladder-type poly(paraphenylenes) (m-LPPP) is highly attractive for applications in organic light emitting diodes (OLEDs) and potentially laser diodes due to its high photoluminescence (PL) quantum yield (QY) of 30% in thin films and 100% in solution. The suppression of the inter-ring torsional twists in PPP results in a planarisation of the polymer backbone, which improves the conjugation. Very silmilar emission properties are found in planarised oligophenyls (xLOPPs, with x denoting the number of phenyl rings). The effective conjugation length of the oligomers is lower than that of the polymer (which has an effective conjugation length of approximately 10 – 15), so their energy gap is higher, resulting in a PL spectrum which is blue-shifted with respect to the polymer. We present comprehensive PL studies (time-resolved, site-selective excitation and QY) performed on 5LOPP and 7LOPP in thin films, in an inert matrix (PMMA) and in solution and compare the attained results to the m-LPPP data. The oligomers have short solubility side groups, which enables a large variety of inter-molecular conformations. Inter-molecular interactions affect the energy gap, so the conformational disorder leads to a broad energy distribution, which enables inter-molecular exciton migration. We found broad PL spectra of both oligomers in bulk film as well as a significant aggregate emission, which is not observed in either solution or in matrix, where inter-chain interaction is suppressed. The polymer has longer side chains, which increases inter-molecular distances and therefore prevents aggregate formation. The PL of both films exhibits a temporal behaviour that can be modelled with a double exponential decay, with both decay times increasing and the weight on the fast component decreasing when viewing only the lower energy section of the emission spectrum. In 5LOPP dispersed in a low amount of PMMA, an analogous decay behaviour is observed, although the PL spectrum does not show any aggregate emission. For higher PMMA concentrations and solutions of both types of oligomers a monoexponential PL decay with generally longer lifetimes compared to bulk films is found. We explain the different temporal behaviour with fast non-radiative decay channels present only in pristine films. This assumption is supported by the considerably lower QY in bulk films compared to matrix films and solutions. Site-selective excitation measurements provide additional information to explain the migration process and will also be presented at the conference.
Optical characterisation of Poly-2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene
M. A. Loi (a), E. J. W. List (b), C. Gadermaier (b), W. Graupner (c), G. Leising (b), G. Bongiovanni (a), A. Mura (a), J.-J. Pireaux (d) and K. Kaeriyama (e)
(a) Dipartimento di Fisica and INFM, Università di Cagliari,
I-09042 Monserrato (CA), Italy
(b) Institut für Festköperphysik, Technische Universität
Graz, Petersgasse 16, A-8010 Graz, Austria
(c) Department of Physics, Virginia Tech, VA-24060 Blacksburg, Virginia
(d) Laboratoire LISE, Facultés Universitaires Notre-Dame de
la Paix, B-5000 Namur, Belgium
(e) Department of Materials Science, Kyoto Institute of Technology,
Kyoto 606, Japan
We have investigated the electronic properties and the photoluminescence decay of the poly-2,5-diheptyl-1,4-phenylene-alt-2,5-thienylene copolymer made up by the regular alternation of high and low band-gap units. Experimental data show that the large twisting of the neighbouring constituents of the polymer chain leads to a considerable shift of the optical transitions towards high energies. This blue shift is much more pronounced than the expected band-gap lowering caused by the low-gap thiophene units. Transient photoluminescence experiments show that inter-chain migration of excitons towards centers where excitons decay non-radiatively reduces the photoluminescence quantum yield.
9th
Sede Boker Symposium on Solar Energy Production, July 12th to 13th,
Ben Gurion National Solar
Energy Center, Sede Boker, Israel
MRS Fall 1999 Meeting, November 29 - December 3, 1999, Boston, Massachusetts
PHOTOVOLTAIC RESPONSES IN POLYMER-FULLERENE
IONICALLY SELF-ASSEMBLED NANOSTRUCTURES.
P.J. Neyman , W. Graupner, J.R. Heflin, Virginia Tech, Dept of Physics,
Blacksburg, VA;
D. Marciu, M. Miller, A. Drake, Luna Innovations, Inc., Blacksburg,
VA;
H. Wang, H.W. Gibson, H.C. Dorn, Virginia Tech, Dept of Chemistry,
Blacksburg, VA;
R.M. Davis, Virginia Tech, Dept of Chemical Engineering, Blacksburg,
VA.
The ultrafast photoinduced electron transfer from conjugated polymers
to fullerenes has allowed the production of effcient organic photovoltaic
devices. Since the exciton diffusion distance is on the order of 10 nm,
however, the charge transfer cannot occur unless the fullerene acceptor
is within this distance of any given optically-excited electron-hole pair.
We have recently fabricated polymeric photovoltaic devices from ionically
self-assembled monolayers (ISAMs) and variations thereon. The ISAM nanostructure
fabrication method simply involves the alternate dipping of a charged substrate
into aqueous cationic and anionic solutions at room temperature. Each monolayer
is fully formed within a couple minutes of immersion, and the bilayer thickness
can be controlled from 0.3 to greater than 5.0 nm by variation of the solution
parameters. Large-area, conformal, flexible, optoelectronic thin lms can
thus be fabricated with detailde structural and thickness control at the
sub-nanometer level combined with ease of fabrication and low cost. We
have employed several approaches to combine the tetrahydrothiophenium precursor
of
poly(para-pheneylene vinylene) (PPV) with fullerenes. As two examples,
hydroxylated C forms direct ionic bonds with the cationic PPV precursor
while pristine C forms covalent bonds with the amine groups of a second
polycation incorporated into the ISAM films. Comparison of the short-circuit
currents, open-circuit voltages, and energy conversion efficiencies are
presented for photovoltaic devices made from these and other novel polymer-fullerene
self-assembled nanostructures.
Materials
Research Society - Spring 2000 Meeting,
April 24-28, 2000, San
Francisco, California
PHOTO-MODULATION SPECTROSCOPY
ON NANOSTRUCTURED POLY(PARA PHENYLENEVINYLENE)-SYSTEMS
E.J.W. List(1,2), J. Partee(2), J. Shinar(2), W. Graupner(3), C. Gadermaier(1),
R. Smith(4), D. Gin(4) and G. Leising(1)
(1)Institut für Festkörperphysik, Technische Universität
Graz, 8010 Graz, Austria
(2)Ames Laboratory-USDOE* and Department of Physics and Astronomy,
Iowa State University, Ames, Iowa 50011, USA
(3)Department of Physics, Virginia Tech, Blacksburg, 24060 Virginia,
USA
(4)Department of Chemistry, University of California,94720 Berkeley,
California, USA
The photophysical properties, such as spectral line-shape and site selectivity
of photoluminescence (PL), PL quantum yields, cw photoinduced absorption
spectra (PIA) and the X-band PL-detected magnetic resonance (PLDMR) responses
of bulk poly(para-phenylenevinylene) (PPV) films and films of isolated
PPV chains ordered on the nanometer scale (nano-PPV) are presented and
compared. The ordering is a result of the lyotropic liquid crystalline
character of the matrix material, which by self-assembly leads to the formation
of a regular hexagonal array of hollow cylinders (channels) with an outer
diameter of about 40Å containing the conjugated polymer chains.
The optical spectra and the spectral dependence of the PLDMR show that
the incorporation of the polymer chain into the self-assembled matrix largely
eliminates the interchain charge transfer excitons and only allows for
intrachain migration of singlet excitons (SE´s). This isolation alters
the emission properties of nano-PPV as well as the transition energies
of the intergap triplet and polaron states as found from PIA. In particular,
from the observed sublinear dependence of the PL intensity on the excitation
density in nano-PPV, which is in strong contrast to the perfectly linear
dependence in bulk PPV, we find that SE´s which are subject to intrachain
migration are strongly nonradiatively quenched at trapped polarons. This
quenching process is believed to be responsible for the polaron PLDMR.
From a comparison of the spectral dependence of the PLDMR in nano and bulk
PPV, we find that in nano-PPV the chains are not only isolated in the channels,
but moreover are composed of distinct shorter isolated conjugated segments
and connected longer conjugated segments.
*Ames Laboratory is operated by Iowa State University for the USDOE
under Contract W-7405-Eng-82.
APS
March Meeting 2000, March 20-24, Minneapolis Convention Center, Minneapolis
MN.
We report X-ray powder diffraction studies on oligophenyls for pressures up to 2 kbar. Geometries of biphenyl were optimized using 3D band structure calculations based on the obtained unit cell dimensions. The optical absorption under increasing pressure has been computed for the relaxed structures. It displays a red shift in agreement with experimental findings. We probed polycrystalline oligophenyl powders under pressures up to 80 kbar and temperatures of 10-300 K with Raman scattering. The results are compared to Hartree Fock simulated Raman spectra of isolated molecules and band structure computed phonon frequencies of the crystalline polymer. We find temperature and pressure induced non-planar to planar phase transitions in these materials.
Organic Electronic Materials and Devices (DMP)
Financial support:Univ. of Missouri Research Board, OeNB project 6608.
HIGH
PRESSURE STUDIES ON THE STRUCTURAL INFLUENCE ON THE ELECTRONIC
PROPERTIES
OF SEXITHIOPHENE SINGLE CRYSTALS
M. A. Loi, G. Bongiovanni, A. Mura (Dipt. di Fisica and INFM, Univ.
di Cagliari, Monserrato, Italy),
Q. Cai, H. R. Chandrasekhar, M. Chandrasekhar (Dept. of Phys. and
Astronomy, Univ. of Missouri, Columbia, MO),
W. Graupner (Dept. of Phys., Virginia Tech, Blacksburg VA),
C. Botta (Ist. di Chimica delle Macromolecole, CNR, Milano, Italy),
F. Garnier (Lab. des Materiaux Moléculaires (CNRS), Thiais,
France)
Sexithiophene shows electronic properties strongly dependent on the
order in the films. The focus of this study are structural properties of
sexithiophene single
crystals. We perform Raman spectroscopy at 10 K and under pressure
up to 80 kbar to probe structural changes, phase transitions and interchain
interactions.
Upon changing pressure we detect changes in frequency, position and
intensity of the Raman modes. The observed stiffening with increasing pressure,
is indicative
for increasing force constants. Finally we discuss the effect of both
the variations in the intermolecular interactions (Davydov splitting) and
in the conformational
properties of the oligomer, induced by the applied pressure.
15.9.3 High Pressure Physics: Primarily Experimental
Financial Support: University of Missouri Research Board, ASPIRES 1999/Virginia
Tech.
In this contribution, we present the study of the cw and time-resolved photoluminescence (PL) of T6 single crystal under high hydrostatic pressure at low and room temperature. The PL spectrum of T6 single crystal at 300 K has been related to the excitonic emission. Under pressure, from room to 80 Kbar, we observe for the cw PL spectra: i) a red shift of the PL spectra, ii) and a (reversible) reduction of the quantum efficiency of roughly 10 times. Conversely, the decay dynamics is weakly affected by pressure, indicating that the non-radiative decay rate is almost unchanged. We discuss the strong quenching of the PL in terms of mixing between singlet and charge transfer excitons as predicted by the theory when the interchain distances reduces.
Financial Support: University of Missouri Research Board, ASPIRES 1999
program of Virginia Tech.
A new combined photoluminescence detected magnetic resonance (PLDMR)
and thermally stimulated current (TSC) study of defects in wide bandgap
para-phenylene type semiconductors is described. As TSC probes the density
of mobile charge carriers after detrapping and PLDMR reveals the influence
of trapped charges on the PL, their combination yields the concentration
of traps, their energetic position, and their contribution to PL quenching.
The reported trap densities which are 2x10^16 for the polymer and 10^14
cm^-3 for the oligomer are the lowest reported for para-phenylene type
materials.
The photophysics of bulk poly(para-phenylenevinylene) (PPV) films are
compared to those of isolated PPV chains incorporated into a self-assembled
lyotropic liquid-crystalline matrix ordered at the nanometer scale (nano-PPV).
As expected, the optical spectra and the spectral dependence of the photoluminescence
(PL)-detected magnetic resonance (PLDMR) show that the nano-PPV structure
largely eliminates the interchain charge transfer excitons, allowing only
intrachain migration of singlet excitons (SE´s). The observed unusual
sublinear dependence of the PL on the excitation density in nano-PPV indicates
that these SE´s are strongly nonradiatively quenched at trapped polarons.
A rate-equation model based on this quenching mechanism provides strong
evidence that this quenching process is responsible for the polaron PLDMR.
*Operated by ISU for the USDOE under Contract W-7405-Eng-82.
The XVIIthInternational Conference On Raman Spectroscopy (ICORS 2000), 20-25 August 2000, Peking University, Beijing, China
UNDERSTANDING THE STRUCTURE OF CONJUGATED MOLECULES UNDER PRESSURE
H.R. Chandrasekhar(1), M. Chandrasekhar (1), S.Guha (2) and W. Graupner (3).
(1) Department of Physics and
Astronomy, University of Missouri, Columbia, MO 65211, U.S.A.
(2) Physics Department, Marquette
University, Milwaukee, WI 53201-1881, U.S.A
(3) Department of Physics, Virginia
Tech, Blacksburg, VA 24061-0435, U.S. A.
This study involves organic conjugated molecules (polymers) which exhibit interesting electronic and optical properties such as doping induced conductivity, high quantum efficiency of photoluminescence and non-linear optical response. The delocalized and highly polarizable character of the p -orbitals make these materials attractive for photonic and opto-electronic applications. The optical and electronic properties of poly para-phenylene (PPP) type molecules depend strongly on the molecular arrangement of the phenyl rings and intermolecular interactions. A controlled and reversible means of changing the intermolecular distances without altering the chemical nature of the material is via the application of hydrostatic pressure. The usual method of tuning the intermolecular interaction via chemical substitution has the unwanted side effect of introducing torsion in the polymeric backbone; the hydrostatic pressure technique is free of such problems [1].
The choice of materials includes compounds of two generic families with distinct inter-chain interactions and intra-chain order, namely an oligo- and a polyphenyl. The oligophenyl, para hexaphenyl (PHP), is known to form monoclinic crystallites and is characterized by a torsional degree of freedom between the neighboring phenyl rings. In the crystalline state the molecules are arranged in layers forming a herringbone type of arrangement. The polyphenyl ladder type poly para phenylene is called m-LPPP due to the methyl (substituted) group in the Y position. It does not form crystallites due to the bulky side groups and shows no torsional degree of freedom between the neighboring phenyl rings owing to the methin bridge between the rings. m-LPPP exhibits a reduction of defects coupled with high intra-chain order due to planarization of phenyl rings, and excellent solubility owing to the large side groups. It has proved to be the best material within the LPPP family from the point of view of stability and PL quantum yields.
The Raman spectrum of the oligophenyls is mainly characterized by four intense modes of Ag symmetry. A theoretical approach by Rumi and Zerbi [2] shows that Raman intensities are a test bench for probing the change in the potential well between neighboring phenyl rings when oligo-p-phenylenes change from a non-planar to a planar geometry. In particular, the ratio of intensity of the C-C stretch mode at 1280 cm-1 to the C-H in-plane bending mode at 1220 cm-1 (I1280/I1220) provides a test for the torsional angle between two neighboring phenyl rings, since simulations show that a higher number of conjugated phenyl rings results in a lower torsional angle between the phenyl rings. For a biphenyl molecule where d»± 50° , I1280/I1220 ~25. Beyond six phenyls, d»± 20° and I1280/I1220 ~1.
The temperature dependence of PHP at atmospheric pressure shows that I1280/I1220 decreases with increasing temperature (10 to 300 K). This is indicative of an average planar conformation at room temperatures changing to a "frozen" non-planar conformation at lower temperatures. While temperature affects the population of the higher-lying (more planar) states, pressure increases the proximity of the molecules. The most striking feature of the Raman spectrum of PHP is the decrease in the intensity of the 1280 cm-1 peak relative to the 1220 cm-1 peak, (I1280/I1220) at high pressures. At 1 bar the ratio of the intensities is 0.84 and at higher pressures beyond 15 kbar, the average value of I1280/I1220 ~0.3. This relative decrease in intensity of the inter-ring C-C stretch mode at higher pressures is due to the planarization of the molecule. Indeed, at 20 kbar, no change is observed in the intensity ratio between 10 and 300 K since the system does not leave the planar conformation.
In order to obtain a quantitative estimate of the changes in the Raman spectrum due to planarization we have used a restricted Hartree-Fock method to calculate the Raman spectrum of biphenyl both in planar form (D2h symmetry) and with the phenyl rings tilted at 52° (D2 symmetry), which is the minimum energy condition for the molecule. The calculation for the non-planar biphenyl yields I1280/I1220=14.5 and for the planar geometry I1280/I1220 decreases to 3.2. In comparing these theoretical results of the biphenyl molecule to the experimental results on PHP, one has to compare the ratio of I1280/I1220 for the non-planar to the planar geometry. Experimentally the non-planar geometry corresponds to the 1 bar case and the planar geometry corresponds to higher pressures. The experimentally obtained ratio of the intensities of the non-planar geometry to the planar geometry is 2.8 (=0.84/0.3) for PHP which is in close agreement to the calculated value of 4.5 (=14.5/3.2) for a biphenyl molecule.
We present a detailed account of the hydrostatic pressure effects on the Raman spectra of polymeric systems. Measurements at 10K show abrupt changes in the pressure derivatives for the 1220 and 1280 cm-1 modes indicative of a transition from non-planar to planar configuration. Pressure-induced changes in the geometry of the molecules and their effects on the opto-electronic properties will be discussed.
1. S. Guha, W. Graupner, R.Resel, M. Chandrasekhar, H.R. Chandrasekhar
and G. Leising, Phys. Rev. Lett. 82, 3625 (1999).
2. M. Rumi; and G. Zerbi, Chem. Phys. , 242, 123 (1999).
Workshop
at Virginia Tech - April 21st, 2000, Inorganic/Organic Optoelectronic Devices
Prospects for Hybrid Organic-Inorganic Technologies - part1,part2.
Willi Graupner
Department of Physics, Virginia
Tech, Blacksburg, VA 24061-0435, U.S. A.
Workshop
at Virginia Tech - May 15, 2000, organized by the NSF
Science and Technology Center for High Performance Polymeric Adhesives
and Composites
Organic Optoelectronic Devices
Raoul Schroeder, Willi Graupner
Department of Physics, Virginia
Tech, Blacksburg, VA 24061-0435, U.S. A.
This presentation highlights two distinct aspects of organic optoelectronic devices: (1) the use of Förster Energy Transfer to color tune the emission of an organic Light Emitting Diode (LED) [1],[2], (2) the use of organic LEDs in ultrafast time-resolved spectroscopy in order to probe the charge generation process [3],[4].
In the first part of the presentation an alternative method for producing efficient white light-emitting polymer diodes based on a blend of two polymers is reported [1]. The white light emission is composed of a broad blue emission of laddertype(polyparaphenylene) (m-LPPP) and a red-orange emission of a new polymer, poly(perylene-co-diethynylbenzene) (PPDB). The red-orange electroluminescence emission is promoted by an excitation energy and charge transfer from m-LPPP to the PPDB. A concentration of 0.05% PPDB in the polymer blend is required in order to obtain white light emission. By inserting an insulating material in the blend, so that a maximum external quantum efficiency of 1.2% is obtained. The excitation energy transfer also has been investigated by ultrafast spectroscopy [2]. The excitation-intensity dependent emission color of the organic films and the concentration dependent transfer times were two of the main results of this study.
In the second part of the presentation the results of combined electrical/optical experiments on an organic LED are reported. Electric field-induced charge photogeneration in ladder-type poly(para-phenylene) is investigated by field-assisted femtosecond pump-probe experiments carried out on light emitting diodes. The characteristic photoinduced absorption band at 1.9 eV allows one to directly monitor the polaron population. We find that polarons are formed by exciton fission without intermediate states on a time scale of 10 ps. The buildup kinetics of the polaron population suggests a dissociation driven by exciton diffusion during interchain thermalization. The experimental results, which showed for the first time directly the generation process of polarons on a sub-ps timescale are in excellent agreement with a simultaneously published theoretical study [5].
[1] S. Tasch, J. W. E. List, O. Ekström, W. Graupner, G. Leising,
P. Schlichting, U. Rohr, Y. Geerts, U. Scherf, K. Müllen, Applied
Physics Letters 71, 2883-2885 (1997).
[2] G. Cerullo, M. Nisoli, S. Stagira, S. De Silvestri, G. Lanzani,
W. Graupner, E. J. W. List, G. Leising, Chemical Physics Letters
288, 561-566 (1998).
[3] W. Graupner, G. Cerullo, M. Nisoli, G. Lanzani, E. J. W. List,
G. Leising, S. De Silvestri, Physical Review Letters 81,
3259-3262 (1998).
[4] C. Zenz, G. Cerullo, W. Graupner, G. Lanzani, F. Meghdadi, G. Leising,
S. De Silvestri, Physical Review B 99, 14336-14341 (1999).
[5] V. I. Arkhipov, E. V. Emelianova, and H. Bässler, Physical
Review Letters 82, 1321-1324 (1999).
Virginia
Academy of Sciences - Year 2000 meeting (program)
May 25:
(1) Department of Physics, Virginia Tech, Blacksburg, VA 24061-0435,
Fax: 540-231-7511, graupner@vt.edu,
(2) Luna Innovations, Inc,
(3) Department of Chemistry, Virginia Tech,
(4) Institut fuer Festkörperphysik, Technische Universität
Graz,
(5) Department of Chemical Engineering, Virginia Tech
Conjugated molecules are built by a sequence of carbon atoms with an
alternating series of single and double bonds connecting them. The energetic
distance between the highest occupied molecular orbital (HOMO) and the
lowest unoccupied molecular orbital (LUMO) is determined by two influences:
(1) the number of interacting double bond, denoted as the effective conjugation
length and (2) the strength of interaction between these double bonds which
is mainly determined by their relative positions with respect to each other.
Therefore the electronic properties of conjugated molecules can easily
be tuned by varying the chemical composition. The tuning of HOMO and LUMO
and various ways of influencing the nanomorphology of the molecular films
are discussed with the focus of efficient charge generation. The talk will
concentrate on particular optoelectronic devices and techniques to probe
efficient charge generation in organic devices with the particular goal
of photovoltaic devices in mind.
We present a study of Raman phonons in three oligophenyls, namely, ter-, quarter- and hexaphenyl under hydrostatic pressure at low temperatures. The strongest modes observed are those due to the C-H bending mode at 1220 cm-1, the inter-ring C-C stretch Raman mode at 1280 cm-1, and the ring C-C stretch mode at 1600 cm-1. The intensity ratio of the first two modes is indicative of the degree of torsional motion between neighboring phenyl rings. Pressure brings the molecules closer to one another, thereby planarizing the molecule. In addition to the abrupt change in the intensity ratio caused by planarization, the frequencies of these modes increase discontinuously, and the linewidths change as well. The effect is clearly seen at pressures of 7.6, 7.8 and 13.6 kbar for the three materials, respectively.
Metal-containing endohedral fullerenes have attracted special attention as a new class of spherical molecules with novel properties that are different from those of empty fullerene cages. Endohedral metallofullerenes hold great promise for applications in opto-electronic devices, since varying the encapsulated metal cluster can alter the optical and electronic properties without changing the structural features of the outer carbon shell. A new family of stable endohedral metallofullerenes, A_3-nB_nN@C_80 (n=0-3, A,B=metal), has recently been reported [1]. These structures are stabilized by transfer of up to six electrons from the trimetallic nitride metal cluster to the C_80 cage. The Raman spectra of Sc_3N@C_80 are reported for the first time. The observed spectra are consistent with a C_80 cage of icosahedral symmetry. Our experimental results are further supported by first-principles calculations of the vibrational modes of a charged icosahedral C_80.
[1] S. Stevenson et al. Nature, \bf401, 55 (1999).
*Supported by Research Corporation Award CC4841
Soluble fluorene-based conjugated polymers are known for high chemical
purity, high photoluminescence quantum yield (PLQY) and small inhomogeneous
broadening in the optical spectra. We present Raman and optical absorption/reflection
spectroscopic studies from two polymers with distinct differences in their
backbone conformation, namely, poly(9,9-bis-(2-ethylhexy)fluorene-2,7-diyl)
(PF) and ladder type poly(para-phenylene) (m-LPPP). PF is an intense blue
emitter with a substantially higher PLQY compared to m-LPPP and is also
known to show phase transitions with changing temperatures. Spectroscopic
properties of conjugated polymers have varying degrees of sensitivity to
backbone conformation. The key to understanding the fundamental differences
between the two materials is the ribbon-type backbone conformation in m-LPPP
as opposed to the degree of freedom introduced by only one single bond
between two neighboring fluorene units in PF. The consequences of this
difference in terms of the vibronic and electronic excitation will be discussed.
Highly crystalline organic semiconductors have recently been used to
demonstrate the first laser diodes based on organic emitters. The optical
and electronic properties of such crystals are strongly influenced by intermolecular
interactions. When n molecules are present in the elementary cell of a
molecular solid, each excited state of the molecules gives rise to n excitonic
bands. The degeneracy of these states is lifted by intermolecular interactions.
The size of this splitting (Davydov splitting) is proportional to the interaction
strength. We present transmission spectra of a sexithiophene single crystal
under high hydrostatic pressure at low temperature (10K). We observe well
resolved spectra even at the highest pressure, which allows us to study
the behavior of the Davydov splitting with increasing of the intermolecular
interaction. We observe an unexpected reduction (~3060 kbar. Possible explanations
for this reduction will be proposed.
Fall Meeting of the Materials
Research Society 2001,
November ,2001, Boston
DIRECT EVIDENCE FOR ANNIHILATION OF SINGLET EXCITONS
BY TRIPLET EXCITONS IN PL-CONJUGATED POLYMERS.
E.J.W. List , U. Scherf , K. Mullen , W. Graupner, C.-H. Kim and J.
Shinar.
Institut f ur Festk orperphysik, Technische Universit at Graz, Graz,
AUSTRIA;
Ames Laboratory - USDOE, Iowa State University, Ames, IA;
Institut fur Physikalische und Theoretische Chemie, Universit at
Potsdam, Golm, GERMANY;
Max-Planck-Institut f ur Polymerforschung, Mainz, GERMANY;
Department of Physics, Virginia Tech, Blacksburg, VA;
Department of Physics and Astronomy, Iowa State University, Ames, IA.
*Present address: eMagin Corporation, Hopewell Junction, NY.
Ames
Laboratory is operated by Iowa State University for the USDOE
under Contract W-7405-Eng-82.
We provide the fi rst direct evidence for nonradiative quenching of
singlet excitons (SE's) by triplet excitons (TE's) in conjugated polymers
and an estimate of the strength of this interaction. The elucidation of
this issue is highly desirable since in polymer light emitting devices
injected carriers form TE's with a yield of up to 75% Moreover, as a result
of the dipole forbidden transition of TE's to the ground state, the lifetime
of the TE is extremely long relative to that of the SE's. This results
in a high steady state TE density and hence a high probability for interacting
with other species, such as quenching SE's. For this study a blend of two
conjugated polymers which consists of the blue-emitting methyl-substituted
poly(para-phenylene)-type LPPP host and the range-emitting poly(perylene-co-diethynylbenzene)
guest was studied by photoinduced absorption (PA) and hotoluminescence
(PL)-detected magnetic resonance. The guest was chosen because both its
PL and its TE half- eld-for-resonance at g 4 are distinct from those of
the host. Hence we could alter the population of the TE spin sublevels
of each of the two polymers separately and simultaneously monitor its e
ect on the decay of SE's in each of the polymers. From the latter we nd
that a reduction of TE's on either of the two polymers enhances the SE
decay on both polymers nearly identically. The PA results rule out other
possible mechanisms to yield this signal, except one: a long range SE-TE
annihilation due to a dipole-dipole transfer mechanism. Based on this nding,
we calculate the transfer radius and quantify the annihilation process
for the presented system and for other conjugated polymers such as poly(p-phenylene
vinylene) and polyfluorene.
ENHANCED PHOTOVOLTAIC RESPONSE IN IONICALLY SELF-ASSEMBLED
MONOLAYER THIN-FILM DEVICES.
D. Marciu, M.B. Miller, C. Kozikowski, Luna Innovations, Inc.,Blacksburg,
VA;
J.R. He in, S. Cho, B. Reid, K. Kuroda, W. Graupner, Department of
Physics, Virginia Tech, Blacksburg, VA;
H.Wang, H.W. Gibson, Department of Chemistry, Virginia Tech Blacksburg,
VA;
R.M. Davis, Department of Chemical Engineering, Virginia Tech, Blacksburg,
VA.
We describe detailed studies of ionically self-assembled monolayer (ISAM)
photovoltaic devices incorporating various electron acceptor materials:
fullerenes, phthalocyanine and carbon nanotubes. The conducting polymer
material generates electron/hole pairs when exposed to photons, and the
electron acceptors aid in collecting these electrons before they can recombine
with the holes, thus improving the eciency of the PV process in the polymer.
The ISAM technique allows the deposition of conducting polymer and fullerene
materials in alternating layers of subnanometer thickness. This ensures
that every photoexcited electron-hole pair is in proximity to a fullerene
electron acceptor, thus minimizing electron-hole recombination and increasing
the photocurrent. The individual thickness of each monolayer and the
interpenetration of adjacent layers can be precisely controlled through
the parameters of the electrolyte solutions. Using the ISAM technique,
we demonstrated that it is possible to create ultrathin lms (100 nm) of
PV material that have performance on the order of that of silicon based
photovoltaic devices.
OPTICAL SPECTROSCOPIC STUDIES OF SOLUBLE FLUORENE-BASED
CONJUGATED POLYMERS: A
HYDROSTATIC PRESSURE AND TEMPERATURE STUDY.
C.M. Martin , S. Guha , J.D. Rice , M. Chandrasekhar , H.R. Chandrasekhar
, W. Graupner , and U. Scherf ,
Dept. of Physics & Astronomy, University of Missouri-Columbia,
MO;
Dept. of Physics, Astronomy & Materials Science, Southwest Missouri
State University, Spring eld, MO;
Dept. of Physics & Astronomy, Virginia Tech, Blacksburg, VA; present
address: eMagin Corporation,
Hopewell Junction NY;
Physical and Theoretical Chemistry, Universitaet Potsdam, GERMANY.
Soluble fluorene-based conjugated polymers show the highest photoluminescence
quantum yield (PLQY) of all conjugated polymers in the solid state.[1]
Optical studies are presented as a function of temperature and hydrostatic
pressure, using Raman scattering and photoluminescence from two polymers
with distinct di erences in their backbone conformation, namely, poly(9,9-bis-(2-ethylhexy)
uorene-2,7-diyl) (PF) and ladder type poly(para-phenylene) (m-LPPP). PF
has a substantially higher PLQY compared to m-LPPP and is also known to
show phase transitions with changing temperatures.Spectroscopic properties
of conjugated polymers have varying degrees of sensitivity to backbone
conformation. The key to understanding the
fundamental di erences between the two materials is the conformation
of the backbone, namely the ribbon-type backbone in
m-LPPP as opposed to single bond between two neighboring fluorene units
in PF, which introduces a torsional degree of freedom. The consequences
of this di erence will be discussed in terms of the Huang Rhys factor as
well as the mode Grueneisen parameters.
[1] A.W. Grice et al. Appl. Phys. Lett. 7 3, 629 (1998).
NANOSCALE CONTROL OF THE POLYMER-FULLERENE INTERFACE
IN PHOTOVOLTAIC DEVICES BY THERMALLY-INDUCED INTERDIFFUSION.
M. Drees, J.R. He in, W. Graupner, Virginia Tech, Department of Physics,
Blacksburg, VA;
D. Topasna, M.B. Miller, Luna Innovations, Blacksburg, VA;
R.M. Davis, Virginia Tech, Department of Chemical Engineering, Blacksburg,
VA.
Ultrafast photoinduced charge transfer from conjugated polymers to fullerenes
has led to extensive studies of these systems as photovoltaic devices.
The charge transfer process prevents radiative electron-hole recombination,
resulting in free, mobile charges. A limiting factor is the exciton di
usion distance, which is of the order of 10 nm. If the fullerene is not
within this distance of the optical excitation site, no charge separation
will take place. The simplest system for such devices is a bilayer system
in which a lm of C60 is evaporated onto a spin-cast lm of MEH-PPV, for
example. If each lm has a thickness of 100 nm, the active region for charge
transfer is only a small volume at the interface. We describe studies in
which the polymer is heated above the glass transition temperature in an
inert environment, inducing an interdi usion of the polymer and the fullerene
layers. Cooling the polymer lm below the glass transition temperature terminates
the interdi usion. With this process, a controlled, bulk, gradient heterojunction
is created. Monitoring of the photo-luminescence and the short circuit
currents of the devices show a dramatic decrease in photoluminescence and
concomitant increase in short circuit currents, demonstrating the improved
interface. Initial studies with MEH-PPV and C60 have shown an increase
in photocurrent of more than a factor of 6 compared to the unheated bilayer
system. We will present systematic studies of the time/temperature pro
le of the interdi usion process as correlated with photoluminescence quenching,
increase of photocurrent, and the resultant nanostructured polymer-fullerene
interface.
SINGLET EXCITON (SE) QUENCHING MECHANISMS IN $\pi$-CONJUGATED MATERIALS
AND ORGANIC LIGHT-EMITTING DEVICES (OLED’S).
J. Shinar, Ames Laboratory – USDOE and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011; E. J. W. List, Institut fuer Festkoerperphysik, Technische Universitaet Graz, A-8010 Graz, Austria; W. Graupner, Department of Physics, Virginia Tech, Blacksburg, Virginia 24060; C.-H. Kim, A. Naik, and G. Li, Ames Laboratory – USDOE and Department of Physics and Astronomy, Iowa State University, Ames, IA 50011; D.-W. Lee and J.-I. Jin, Center for Electro and Photo-responsive Molecules and Department of Chemistry, Korea University, Seoul 136-701, Korea; G. Jabbour, Optical Sciences Center & Department of Materials Science and Engineering, Univesity of Arizona, Tucson, AZ 85721.
Several significant SE quenching mechanisms are reviewed:
(i) Quenching by polarons. Recent analysis of photoluminescence (PL)-detected
magnetic resonance (PLDMR) measurements lead to the conclusion that the
rate of this bulk mechanism becomes comparable to the SE radiative decay
rate when the polaron density approaches 10^17cm^(-3), a situation commonly
reached in OLED’s.
(ii) Quenching by triplet excitons. Other ODMR measurements, conducted
on guest-host polymer blends, suggest that at high excitation densities
this bulk mechanism is also significant.
(iii) Quenching charges, including negative bipolarons, at the organic/cathode
interface. Electroluminescence (EL) and electrically-detected magnetic
resonance (ELDMR and EDMR, respectively) measurements on small molecular
OLED’s yield negative (EL- and current-quenching) spin ½ quenching
resonances which are attributed to the spin-dependent formation of negative
bipolarons at the organic/cathode interface. The results indicate that
the charge density at this interface depends strongly on its nature, but
is generally very high, and may account for the differences in the behavior
of OLED’s with different interfaces.
(iv) Quenching by negative bipolarons in bulk polymer films, the formation
of which is stabilized by holes which are trapped at, e.g., electron donating
sidegroups.
Triplet excitons (TE’s), which are more localized, have a higher
binding energy, and are less mobile than SE’s, are expected to be largely
impervious to these quenching mechanisms. However, recent ELDMR measurements
on 2,3,7,8,12,13,17,18-octaethylporphine Pt (PtOEP)-based electrophosphorescent
OLED’s yield negative spin ½ quenching resonances similar to those
exhibited by fluorescence-based OLED’s. It is therefore suspected that
quenching of SE’s by charges at the organic/cathode interface may compete
significantly with the transfer of the SE energy to TE’s.
Ames Laboratory is operated by Iowa State University for the USDOE under
Contract W-7405-Eng-82.
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