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1336402157,1362 /w EPDw UKMTEw
/w EWBALo5JGxC
Home
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About MRS®
o The Society
o MRS Mission, Vision and Values
o Contact Us
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o MRS Press Room
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Meetings & Exhibits
o MRS Spring Meetings & Exhibits
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o Technology Innovation Forum | iMatSci
o MRS Conference Services
o MRS Joint Meetings
o MRS Workshop Series
o Endorsements & Co-Sponsorships
o Additional Meeting Activities
o 8th International Workshop on Zinc Oxide and Related Materials (IWZnO 2014)
o 4th International Symposium on Graphene Devices (ISGD-4)
o 20th International Conference on Solid State Ionics (SSI-20)
o 18th International Conference on Metal Organic Vapor Phase Epitaxy
The single greatest advantage of organic molecules vs inorganic materials for electronic
applications is their capacity to self-assemble into complex and, at times, functional
architectures. Yet, practical applications of this property in organic/molecular electronics is
essentially limited to self-assembled monolayers (SAMs, where the molecules are simply grafted
on a surface with very limited lateral order) or 3D crystals (where the molecular packing is
critical for charge transport phenomenon, but little or no rational control of this packing can yet
be achieved). I will present our achievements in synthesis and surface characterization (STM and
auxiliary techniques) of 2D pi-electron functional molecular networks, highlighting two major
approaches: (i) using weak interactions to create nanotemplates on which organic
semiconducting molecules can be assembled, and which symmetry and periodicity can be
rationally and predictably controlled by the structure of a building block,[1] and (ii) exploring
surface-confined reactivity to create pi-conjugated polymers through epitaxial polymerization
reactions.[2] The latest development of the second approach opens an opportunity for creating a
new class of electronic materials- two dimensional (2D) conjugated polymers. I will describe the
principles of molecular design, the templating and catalytic role of the surface in formation of
these ordered functional materials, and provide our view on implications for the future
development of molecular electronics. [1] K.Nath et al. JACS 2006, 128, 4212; K.Nath et al.
JPC-C 2007, 111, 16996; J.MacLeod et al. Nanotechn. 2007, 18, 424031; O.Ivasenko et al.,
submitted. [2] J.Lipton-Duffin et al. submitted; (see also L.Grill et al. Nature Nanotech. 2007, 2,
687)
10:30 AM *B1.5 Conductance of Single Molecule Circuits. Latha Venkataraman, Applied Physics, Columbia
University, New York, New York.
The field of molecular electronics involves probing, manipulation, and control of single
molecules as active elements in electrical circuits. The underlying focus is to fabricate single
molecule circuits, a molecule attached to two electrodes, with varied functionality, where the
circuit structure is potentially defined with atomic precision. An experimental prerequisite to
creating functional molecular devices is to fabricate single molecule junctions reliably and
understand their physical properties. This poses fundamental challenges to nanoscience. Building
single molecule junctions with atomic precision is beyond the capability of top-down fabrication
techniques; however, variations in the device anatomy at the single bond scale can significantly
affect the junction characteristics. A bottom-up approach based on synthetic chemical control is
required to enable reproducible junction formation with well defined metal-molecule bonding
motifs. From a fundamental physics perspective, we need to understand, both experimentally and
theoretically, the electronic properties of metal-molecule-metal junctions. This will be the
primary focus of my talk. In this talk, I will review the scanning tunneling microscope break-
juncion technique we use to measure electronic transport through single molecule junctions, and
focusing on electron transport in the linear response regime, I will show that junctions formed
with gold-thiol links have large variability in conductance which can be attributed to the atomic
structure of the single molecule junctions formed with Au-S bonds. I will then discuss our
measurements using using novel metal-molecule link chemistries including amine-gold, methyl
sulfide-gold and dimethyl phosphine-gold to overcome the inherent variability of the gold-thiol
bonds. I will show that low-bias conductance of single molecules can be measured reliably and
reproducibly using these link groups [1-3]. I will then discuss the influence of the intrinsic
molecular properties, including their length, conformation, the gap between the highest occupied
molecular orbital and the lowest unoccupied molecular orbital and the alignment of this gap to
the metal Fermi level on the measured conductance. For single molecule junctions, I will show
that conductance relates to molecular conformation a biphenyl, two benzene rings linked
together by a single C-C bond [2]. For substituted benzenes, the relation between measured
conductance and the calculated ionization potential will be discussed [4]. For fused benzene
rings, I will show that conductance depends on the location of the amine groups on the molecule
[5]. [1] L. Venkataraman, et al., Nano Lett. 6, 458 (2006). [2] L. Venkataraman, et al., Nature
442, 904 (2006). [3] Y. S. Park, et al., J. Am. Chem. Soc. 129, 15768 (2007). [4] L.
Venkataraman et al., Nano Letters 7, 502 (2007). [5] J. R. Quinn et al., J. Am. Chem. Soc. 129,
6714 (2007).
11:00 AM B1.6 Charge Transport and Vibronic Effect in Nanoscale Molecular Junctions. Hyunwook Song
and Takhee Lee; Department of Materials Science and Engineering, Gwangju Institute of
Science and Technology, Gwangju, Korea, South.
Study of charge transport in molecules has become an active area of experimental and theoretical
research with promise in a variety of applications including molecular electronics [1],
optoelectronics [2], and thermoelectric energy conversion [3]. In this presentation, we interrogate
critical aspects on charge transport and vibronic effect in molecular junctions (MJs) by using
three different prototypes of device test-beds where either individual molecules or an ensemble
of molecules form a bridge between metallic electrodes in nanometer-scale junction area to
measure charge transport characteristics and electron-vibration coupling in MJs. First, we report
on a statistical method for investigating charge transport through MJs exploiting mass-produced
nanowell device structures (~7,000 devices) [4], which provides an objective criterion to
determine the most probable transport characteristics including a dominant conduction
mechanism in MJs. In the second part of the presentation, we will discuss the observation of two
tunneling pathways possible in self-assembled alkanethiol monolayers, i.e., through σ-bond and
intermolecular chain-to-chain tunneling, using conducting atomic force microscopy [5]. Lastly,
we present a method to explore the energy band lineups in MJs using a single-molecule transistor
(SMT) technique where individual molecules are linked between source and drain electrodes
with a third gate electrode. Furthermore, inelastic electron tunneling spectroscopy (IETS) is used
to observe vibrational spectra in a SMT configuration as the position of electronic resonance
level shifts by adjusting the gate voltage. The IETS provides not only in situ characterization
technique to identify the molecules associated with charge transport, but also a valuable insight
into how the interaction of charge carriers with molecular vibrational modes influences the
overall charge transport characteristics in MJs. [1] M. Galperin, M. A. Ratner, A. Nitzan, and A.
Troisi, Science 319, 1056 (2008). [2] M. S. Gudiksen, K. N. Maher, L. Ouyang, and H. Park,
Nano Lett. 5, 2257 (2005). [3] P. Reddy, S. Y. Jang, R. A, Segalman, and A. Majumdar, Science,
315, 1568 (2007). [4] H. Song, T. Lee, N. -J. Choi, and H. Lee, Appl. Phys. Lett. 91, 253116
(2007). [5] H. Song, H. Lee, and T. Lee, J. Am. Chem. Soc. 129, 3806 (2007).
11:15 AM B1.7 Electron-Phonon Interactions in Single Molecule Junctions. Joshua Hihath
1, Carols R
Arroyo2, Gabino Rubio-Bollinger
2, Nongjian Tao
1 and Nicolas Agrait
2;
1Center for
Bioelectronics an Biosensors, Arizona State University, Tempe, Arizona; 2Departamento Fisica
de la Materia Condensada CIII, Universidad Autonoma de Madrid, Madrid, Spain.
Although significant advances have been made in molecular electronics in recent years, a
complete understanding of how a molecule interacts with its environment when connected to two
electrodes is still lacking. To help alleviate this issue, spectroscopy is becoming an increasingly
important aspect molecular electronics research. And, Inelastic Electron Tunneling Spectroscopy
(IETS), which provides information about the vibration modes of the molecular junction, has
become one of the more commonly used tools for characterizing molecular systems by helping to
detail electron-phonon interactions and tunneling pathways. However, even more information
can be gained about molecular transport by applying these same principles at the single molecule
level. We have studied the electron-phonon interactions in a single molecule junction where the
molecule is covalently bound to two electrodes. In this study, the vibration modes in a single
molecule junction are measured while the strain in the junction is changed by separating the two
electrodes. This unique approach allows changes in the conductance to be compared to changes
in the configuration of a single molecule junction by recognizing changes in the vibration energy
of the junction bonds. This approach opens a new door for characterizing single molecule
junctions and better understanding of the relationship between molecular conductance, electron-
phonon interactions, and junction configuration.
11:30 AM B1.8 Reversible Conductance Switching in Molecular Devices. Auke Jisk Kronemeijer
1, Hylke B
Akkerman1, Tibor Kudernac
2, Bart J van Wees
1, Ben L Feringa
2, Paul W.M. Blom
1 and Bert de
Boer1;
1Zernike Institute for Advanced Materials, University of Groningen, Groningen,
Netherlands; 2Stratingh Institute for Chemistry, University of Groningen, Groningen,
Netherlands.
Since the theoretical study of Aviram and Ratner in 1974 predicting a single molecule
functioning as a diode, the field of Molecular Electronics has received increasing interest.
Methodologies and processing techniques have been investigated to measure the electrical
properties of (single) molecules, including breakjunction techniques and scanning probe
microscopy techniques. Our approach has been to produce molecular junctions in vertical
interconnects, predefined in an insulating photoresist matrix [1]. In the vias, a self-assembled
monolayer (SAM) is created on the underlying gold patterned substrate and the junctions are
finished with a conducting polymeric top contact. The formation of this top contact is the key
step in the processing since it has been shown that direct evaporation of metals on top of SAMs
results in penetration of the metal through the monolayer. Instead, the conducting polymer
PEDOT:PSS is spincoated on top of the monolayer. Because of this extra layer, direct
evaporation of the auxiliary top contact is possible, penetrating to some extent the polymer layer
but leaving the SAM intact. Reliable molecular junctions based on alkane(di)thiol molecules
demonstrated that the electronic properties of the molecules in the SAM are indeed measured [2].
Therefore, functionalities can now be incorporated into the molecules. Photochromic switches
are molecules which are stable in two different states and can be driven from one state into
another by applying light of specific wavelength. An interesting subgroup of switches are the
diarylethenes, because during the transition between the two states, the π-conjugation between
both ends of the molecule is formed or disrupted. Supposedly, this change in conjugation will
change the conducting properties of the molecules in molecular junctions. Molecular devices
which show (non-volatile) conductance switching are interesting for memory applications.
Therefore, diarylethenes have been assembled in a monolayer in the molecular junctions. J-V
characteristics of the molecular junctions were obtained for the two different states of the
switches, resulting in a clear difference in conductance between both forms. In situ irradiation of
the devices results in reversible switching of the conductance of the devices [3]. Reference
devices exhibit no switching. Therefore, the bidirectional conductance switching can indeed be
attributed to the monolayer of molecular switches in the devices. [1] H.B. Akkerman, D.M. de
Leeuw, P.W.M. Blom and B. de Boer, Nature 441, 69-72 (2006) [2] P.A. van Hal, E.C.P. Smits,
T.C.T. Geuns, H.B. Akkerman, B.C. de Brito, S. Perissinotto, G. Lanzani, A.J. Kronemeijer, V.
Geskin, J. Cornil, P.W.M. Blom, B. de Boer and D.M. de Leeuw, Nature Nanotechnol.
Advanced Online Publication 2008 [3] A.J.Kronemeijer, H.B. Akkerman, T. Kudernac, B.J. van
Wees, B.L. Feringa, P.W.M. Blom and B. de Boer, Adv. Mat. 20, 1467-1473 (2008)
11:45 AM B1.9 Conjugated Molecular Junctions with Strong Electronic Coupling. Richard L McCreery
1,
Adam J Bergren1, Ken Harris
1 and Sergio Jimenez
2;
1National Institute for Nanotechnology,
University of Alberta, Edmonton, Alberta, Canada; 2Cinvestav Querétaro, Libramiento
Norponiente No 2000, Mexico.
A thin layer (1-5 nm thick) of organic molecules bonded covalently to a very flat (<0.5 nm rms)
graphitic carbon substrate comprise the active region of reproducible, large area molecular
electronic junctions. UV-Vis and Raman spectroscopy indicate strong electronic coupling
between the molecule and substrate, possibly due to the nearly symmetric C-C bond between
phenyl rings. Vapour deposition of copper as a top contact completes the molecular junction,
resulting in >90% device yield, and with current-voltage characteristics that strongly depend on
the structure of the molecular component(1,2). Current-voltage curves are repeatable for millions
of cycles, and absolute current densities vary by 10-20% for several devices made with a given
molecule. In some cases, a covalent bond between the Cu top contact and a functional group in
the molecule has been detected such that both molecular layer-contact interfaces involve
covalent bonds The observed current is dependent on temperature above ~250 K, but
independent of temperature between 200 K and 5 K. The linear (i.e. “ohmic”) current-voltage
behaviour observed at 5 K indicates that the molecular layer has orbitals at the Fermi energy of
the contacts, and that transport through the molecular layer is resonant. Although transport
appears to be of the “on resonance” type, it occurs through a small subset of the available
molecules in the devices examined to date. While the current junctions behave as nonlinear
conductors, related devices show promise as photonic interfaces(3) and non-volatile
memory(4,5). References (1) Bergren, A. J.; Harris, K. D.; Deng, F.; McCreery, R.; J. Phys.
Condens. Matter 2008, 20, 374117. (2) Anariba, F.; Steach, J.; McCreery, R.; J. Phys. Chem B
2005, 109, 11163. (3) Bonifas, A. P.; McCreery, R. L.; Chem. Mater. 2008, 20, 3849. (4)
Barman, S.; Deng, F.; McCreery, R.; J. Am Chem Soc 2008, 130, 11073. (5) Wu, J.; Mobley, K.;
McCreery, R.; J. Chem. Phys. 2007, 126, 24704.
SESSION B2: Molecular Scale Electronics II
Chairs: Saw-Wai Hla and Heike Riel
Monday Afternoon, April 13, 2009
Room 2001 (Moscone West)
1:30 PM *B2.1 Charge Transport through Single Molecules - Opportunities and Challenges. Emanuel
Loertscher, Raoul Scherwitzl, Marius Trouwborst and Heike Riel; IBM Research, Rueschlikon,
Switzerland.
As device dimensions continue to shrink into the nanometer length-scale regime, conventional
semiconductor technology will be approaching fundamental physical limits. New strategies,
including the use of novel materials, innovative device architectures, and smart integration
schemes need to be explored and assessed. In that respect, molecular electronics is aimed at the
use of individual or small ensembles of molecules as functional building blocks in electronic
circuits. However, in order for molecular electronics to become a viable technology a number of
fundamental scientific issues need to be addressed. We have investigated electronic transport
through individually contacted and addressed molecules in a two-terminal break-junction
configuration under ultra-high vacuum condition at various temperatures. In this talk I will
discuss in particular our investigations of the molecule-metal coupling which we studied using
an ideal molecular system by anchoring a phenyl ring either by thiol, isocyanide or alternative
linker groups to gold electrodes. Transport measurements have been performed by repeatedly
forming and breaking the molecular junction during simultaneously recording the conductance
(under a fixed bias) and the current-voltage characteristics. The molecule-metal coupling was
derived from the molecular level broadening in resonance on the single-molecule level and from
the off-resonance conductance averaged over many thousands of junctions. The level broadening
was found to be 50% larger in case of thiol compared with isocyanide coupling, which is in good
agreement with the off-resonant conductance determined to be 25% higher for thiol coupling.
Moreover, electrical transport studies using alternative linker groups with a larger number of
chemical bonds will be presented. The conductance of the metal-molecule-metal system is
compared with thiol and isocyanide linker groups in the in-resonance as well as in the off-
resonance case.
2:00 PM B2.2 A Bistable Poly[n]rotaxane-Based Solid-State Switch. Erica Deionno
1, Wenyu Zhang
2,
William Dichtel3 and J. F Stoddart
4;
1Electronics and Photonics Laboratory, The Aerospace
Corporation, El Segundo, California; 2Department of Chemistry and Biochemistry, University of
California, Los Angeles, Los Angeles, California; 3Department of Chemistry and Chemical
Biology, Cornell University, Ithaca, New York; 4Department of Chemistry, Northwestern
University, Evanston, Illinois.
Mechanically interlocked compounds, such as bistable [2]catenanes and [2]rotaxanes, have been
shown to exhibit switching behavior in both the solution phase and the solid-state. Bistable
poly[n]rotaxanes, a new class of materials containing mechanical bonds and donor-acceptor
interactions, have been synthesized, and these polymers might be capable of similar switching
behavior in the solid-state. We report in this paper the fabrication and electrical characterization
of solid-state devices incorporating a bistable poly[n]rotaxane. Preliminary device measurements
show that the electrical properties of the devices are dependent on the electrode materials.
Devices with metal electrodes did not exhibit any switching behavior, while devices with silicon
bottom electrodes displayed a hysteretic response with applied voltage, indicating that the
devices can be switched between two conductance states, on and off. The polymer films can be
spin-cast on the bottom electrodes and the thickness of the films can be easily tuned, providing
an advantage over other molecule switches in the ease of fabrication.
2:15 PM B2.3 Collective Reactivity of Molecular Chains Self-Assembled on a Surface Peter
Maksymovych1,2
, Dan C Sorescu3, Kenneth D Jordan
2 and John T Yates, Jr.
4,2;
1Center for
Nanophase Materials Sciences, Oak Ridge National Laboratory, Oak Ridge, Tennessee; 2Department of Chemistry, University of Pittsburgh, Pittsburgh, Pennsylvania;
3U. S.
Department of Energy, National Energy Technology Laboratory, Pittsburgh, Pennsylvania; 4Department of Chemistry, University of Virginia, Charlottesville, Virginia.
Molecular self-assembly on surfaces is a route toward not only creating structures, but also
engineering new chemical reactivity afforded by the intermolecular interactions. We will
describe a new type of chemical reaction which occurs in linear chain-like molecular structures
spontaneously self-assembled from CH3SSCH3 molecules on Au(111) and Au(100) surfaces [1]:
injecting low energy electrons from a scanning tunneling microscope tip into a molecule in the
chain initiates a chain reaction that propagates through as many as 10 molecules in a row before
being quenched. Nearly every aspect of the chain reaction, as revealed by scanning tunneling
microscopy (STM) at 5K, is unprecedented. From STM-kinetic measurements, the chain reaction
can be triggered by one or two electrons, depending on the energy of the tunneling electrons. In
conjunction with the DFT-calculations of the electronic structure of the adsorbed CH3SSCH3
molecule, this identifies electron attachment to a CH3SSCH3 molecule as the triggering stage of
the chain reaction. Subsequent dissociation of the CH3SSCH3 molecule produces two hot CH3S
fragments, one or both of which then collide with neighbor CH3SSCH3 molecules, causing their
S-S bond to dissociate with the ejection of another CH3S fragment, and so forth. The propagation
of the chain reaction is thus reminiscent of the operation of the Newton‟s cradle. The majority
reaction products turn out to be CH3SSCH3 molecules that relate as mirror images to the original
CH3SSCH3 molecules. This is a unique product, since exciting an isolated CH3SSCH3 molecule
leads only to its dissociation or diffusion as a whole, but never to simultaneous rotation of both
methyl groups about the S-S bond. Self-assembly therefore provides a new reaction coordinate
for the CH3SSCH3 molecules dissociating on gold surfaces. DFT calculations of the transition
states reveal a nearly barrierless sequence of elementary steps along the new reaction coordinate
involving CH3S intermediates stabilized by neighbor CH3SSCH3 molecules. [1] P.
Maksymovych, D. C. Sorescu, K. D. Jordan, and J. T. Yates, Jr., Science (2008) in press. We
thank D. B. Dougherty for fruitful discussions. P. M. and J.T.Y: Supported by the W. M. Keck
Foundation and by the Army Research Office. K.D.J acknowledges support from the National
Science Foundation through grant CHE0518253. A grant of computer time at the Pittsburgh
Supercomputer Center is gratefully acknowledged. P.M.: Research performed in part as a
Eugene P. Wigner Fellow and staff member at the Oak Ridge National Laboratory, managed by
UT-Battelle, LLC, for the U.S. Department of Energy under Contract DE-AC05-00OR22725.
2:30 PM B2.4
Self-Assembled Monolayers of Various Conjugated Macrocycles Grafted on Silicon Oxide
for Memory Applications Virginie Gadenne1, Lionel Patrone
1, Alexandre Merlen
2, Mireille
Mossoyan-Deneux3 and Louis Porte
3;
1Institut Supérieur de l‟Electronique et du Numérique,
IM2NP, CNRS, IM2NP (UMR 6242), Maison des Technologies, Place Georges Pompidou, F-
83000 Toulon, France; 2Université du Sud Toulon-Var, IM2NP, CNRS, IM2NP (UMR 6242),
Bâtiment R, BP 20132, F-83957 La Garde cedex, France; 3Aix-Marseille Université, IM2NP,
CNRS, IM2NP (UMR 6242), Faculté des Sciences et Techniques, Campus de Saint-Jérôme,
Avenue Escadrille Normandie Niemen - Case 142, F-13397 Marseille Cedex, France.
Today self-assembled monolayers (SAM) [1,2] constitute a promising strategy to build
molecular nano-devices. Beside many studied molecules, conjugated macrocycles such as
porphyrins and phthalocyanines are good candidates for charge storage, within molecular
memory cells [3]. However their correct operation strongly depends on both molecular order and
binding to the substrate. Thus it is very important to control the deposition of these macrocycles
to form spontaneously ordered monolayers grafted on silicon for applications compatible with
microelectronics technology. In this work, we studied SAMs of three conjugated macrocycles on
silicon oxide pre-functionalized by aminopropyltrimethoxysilane (APTMS) coupling agent.
Selected macrocycles are Zn and Fe protoporphyrins bearing two carboxylic acid functions,
ZnPP and FePP respectively, and a synthesized Zn phthalocyanine surrounded by height
carboxylic acid functions, ZnPc(COOH)8. Carboxylic functional groups on peripheral cycles
allow molecules anchoring on the surface by imino link formation with APTMS. The samples
were characterized by scanning probe microscopy, ellipsometry, contact angle measurements,
Fourier transform infrared (ATR-FTIR) and UV-visible spectroscopy. Using gold surfaces and
replacing APTMS by amino-thiol coupling moieties allowed us to perform complementary
analysis of the grafting and structure of macrocycles with scanning tunneling microscopy and
surface enhanced Raman spectroscopy on appropriate nanostructured substrates. From analysis
carried out all along ZnPc(COOH)8. SAM formation, we show UV-visible spectroscopy exhibits
both a blue shift suggesting a growing face to face orientation and a splitting of the Q-band (at
698 nm). These results suggest a dimerization, due to formation of hydrogen bonds between
peripheral carboxylic acid groups [4]. They show that a great number of carboxylic acid
functions surrounding the molecule allows promoting a “π-stacking” arrangement interesting to
obtain a charge delocalization over a small memory cell. On the contrary, for ZnPP SAM, Soret
band (at 400 nm) is red-shifted during the growth, which is typical for an edge to edge
organization [5]. However no shift is observed for FePP, indicating these molecules assemble in
a different way. Furthermore FTIR spectroscopy together with ellipsometry and scanning probe
microscopy experiments indicate that ZnPc(COOH)8 and ZnPP anchor perpendicular to the
surface via imino bonds whereas FePP is linked by axial ligand position owing to iron electronic
configuration, which is less favorable for memory applications. [1] A. Ulman, Chem. Rev. 1996,
96, 1533 [2] F. Schreiber, Progress in Surf. Sci. 2000, 65, 151 [3] C.M. Carcel, J.K. Laha, R.S.
Loewe, P. Thamyongkit, K.-H. Schweikart, V. Misra, D.F. Bocian, J.S. Lindsey, J.Org. Chem.
2004, 69, 6739 [4] H. Xia, M. Nogami, Opt. Mater. 2000, 15, 93 [5] S. Boeckl, A.L. Bramblett,
California; 4Materials Science and Engineering, UC Berkeley, Berkeley, California;
5Materials
Science Division, Lawrence Berkeley Laboratory, Berkeley, California.
Single molecule circuits represent a lower limit on the scalability of electronic devices and are
hence an ultimate goal of nanotechnology. A deeper understanding of electronic transport
through single molecules is of critical importance to molecular electronics and bears added value
to the wider field of organic-inorganic heterostructured materials. While important experimental
and theoretical advances over the last decade have yielded some insight into electron transport in
metal-molecule-metal junctions, a number of fundamental questions remain unresolved. One
area of rich debate is the origin of fluctuations and stochastic switching of conductance. In
particular, (i) Compared to the energy barrier height, how large are the fluctuations in the charge
transmission function? (ii) Are thermal fluctuations more important than that of the metal-
molecule contact, or are both equally important? (iii) Do these values change with the molecule
or is there some universal behavior? By measuring the thermopower of a series of
pheneylenedithiol molecules we herein answer these questions. Junction thermopower S, an
alternative transport property to conductance, was determined by measuring the voltage
difference across molecules trapped between two gold contacts held at different temperatures (S
= V/ΔT). A modified scanning tunneling microscope (STM) was used to form a junction
between a gold STM tip and the molecule-coated gold substrate. An applied temperature bias ΔT
between the tip and substrate generated a proportional voltage V, related to S by V=S*ΔT. The
experiment was repeated 500-1000 times at each ΔT, resulting in a histogram of measured
voltages. Transport variations were quantified by ΔS/S, where ΔS is the observed spread in S.
Statistical analysis of data from repeated measurements shows that ΔS/S is dominated by
junction-to-junction differences rather than thermal fluctuations within a given junction. The
remarkably large Seebeck variation (ΔS/S~1) implies that variation in the molecular transmission
barrier is similar in magnitude to the barrier height itself. Measurements of 1,4-benzenedithiol,
4,4‟-dibenzenedithiol and 4,4”-tribenzenedithiol, show that increased degrees of freedom from
added benzene rings result in increased transport variations. These inherent sources of noise
represent a significant obstacle for single molecule electronics, but may be obscured when large
ensembles of molecules are present.
4:45 PM B2.10 Alignment of Molecular Energy Levels Between Two Biased Metallic Electrodes. Nikolai
Severin1, Daria Skuridina
1,2, Christian Seifert
1, Xie Dou
3, Ragnar Stoll
1, Stefan Hecht
1, Igor M
Sokolov1, Klaus Müllen
3 and Juergen P Rabe
1;
1Humboldt University, Berlin, Germany;
2Lomonosov University, Moscow, Russia;
3Max-Planck-Institute for Polymer Research, Mainz,
Germany.
While originally Aviram and Ratner had proposed a molecular recifyer based on tunneling
through donor and acceptor moieties linked by a spacer, it has been argued later that rectification
may be also achieved with a single physisorbed donor or acceptor molecule located
asymmetrically between two electrodes. The latter was attributed to the dependence of the
potential at the position of the molecular orbitals in the tunneling gap on their relative position
within the gap. However, it has been also claimed that the potential of a molecular adsorbate is
not dependent on the tip-sample distance but rather equal to the substrate potential. Here we
report in-situ STM and STS data on mono- and bilayers of conjugated molecules self-assembled
at the interface between an organic solution and the basal plane of graphite, in order to resolve
this issue. They confirm that the electron potential drops gradually across the molecular
adsorbate. In a first set of experiments, bias dependent STM-imaging of a molecular bi-layer
revealed a dependence of their visibility on the applied bias as predicted by the model. In a
second experiment, the relative position of a conjugated molecule within the tunneling gap was
varied by controlling the tip-surface distance. The dependence of the current rectification ratio on
the relative position of different molecules between substrate and tip is also consistent with the
model. The results indicate that resonance enhanced tunneling through physisorbed conjugated
molecules between two biased metallic electrodes depends sensitively on the gap width and the
relative position of the electronic orbitals within the gap, thereby providing a means to precisely
control current-voltage characteristics through the geometry of the gap.
SESSION B3: Molecular Scale Electronics III
Chairs: Avik Ghosh and Latha Venkataraman
Tuesday Morning, April 14, 2009
Room 2001 (Moscone West)
8:30 AM *B3.1
Imaging the Local Properties of Graphene: a New Platform for Molecular Electronics. Michael F. Crommie, Y. Zhang, V. Brar, C. Girit and A. Zettl; Physics Dept. and Materials
Graphene, a single atomic layer of carbon, provides an exciting new platform for molecular
electronics due to its chemical and structural flexibility as well as its novel electrical,
mechanical, and magnetic properties. Scanning tunneling microscopy (STM) is an ideal tool to
study the properties of graphene at lengthscales necessary to evaluate its utility for molecule-
scale device applications. We have used STM to explore backgated graphene flakes, and we
observe surprisingly strong electron-phonon coupling in electronic tunneling spectra. We also
find that the graphene charge neutral point (the Dirac point) manifests itself as a clear feature in
tunnel spectra, and it can be shifted by applied gate voltage. By spatially mapping the Dirac point
we are able to map out electron density inhomogeneities in graphene with a spatial resolution at
the nm scale. Using this new technique we have observed molecule-induced charge
inhomogeneities that coexist with energy-dependent electronic interference patterns in graphene,
giving us new insight into the microscopic mechanisms that determine graphene electron
mobility.
9:00 AM B3.2 Metal/Insulator/Metal Thin Film Electrodes for Molecular Conduction. Bing Hu, Pawan
Tyagi and Bruce Jackson Hinds; Chemical and Materials Engineering, Univ. of Kentucky,
Lexington, Kentucky.
Producing reliable electrical contacts with gaps having the dimensions of molecular lengths is a
difficult challenge for molecular electronics. As a promising alternative to break-junctions, we
use conventional film deposition and photolithography to form an exposed edge of a thin film
multilayer structure (metal/insulator/metal). Molecules can self-assemble on the exposed edge
offering an alternative conduction path through the molecules with angstrom-scale dimensional
control. Critical to this approach is to have minimal background tunnel current through the
insulator layer sandwiched between metal layers. Robust electrodes with aluminum oxide
insulator layer are found on alloys of Al/Au and Ta/Au. The readily oxidized Al/Ta reduce
surface energy for a mechanically stable interface with oxide but allow molecular contact with
metallic gold at the pattern edge. Electrodes were successfully fabricated with this strategy with
current measured through a metal coordination compound cluster composed of a cube with
cyano linked Ni or Fe at the corners. Thiolacetate ligand tethers come off of the cluster core and
bind the complex to the metal leads, allowing the molecule to span the insulator gap on the
surface of the etched pattern. Molecules that do not bridge the gap are not electrically active.
Along the 10um pattern edge approximately 6000 molecules are involved in conduction. 10nA
per molecule is seen at 10mV bias. Tunnel current through the molecules is analyzed with
Simmons model and barrier height is found to be 1.1 eV and tunnel length of 1.2nm.
9:15 AM B3.3
Dipole Effects on Electron Transport through Helical Peptides Immobilized on Gold. Shunsaku Kimura and Tomoyuki Morita; Dept. of Material Chemistry, Kyoto University, Kyoto,
Japan.
Electron injection or extraction from helical peptides to gold was studied to evaluate the dipole
effects of Au-S linkages and helical peptides. The helical lengths of the helical peptides exceeds
over 3 nm, where the electron hopping mechanism is prevailing. The molecular terminal has a
ferrocene group as a redox species, and the electron transfer from the ferrocene unit to gold was
evaluated by electrochemical methods. The electron transfer process comprises three regions; the
hopping through the helical peptide under the peptide dipole, through the linker, and through the
covalent connection between Au and the peptide under the Au-S dipole. Not only the dipoles but
also the physical properties of the helical peptide SAMs were found to influence the electron
transfer processes at the interface. Eventually, we try to extract the dipole effects on the electron
transport through the helical peptide SAMs.
9:30 AM B3.4 Creating and Characterizing Robust, Large Area Molecular Electronic Junctions. Michael
Short-circuit current, which is determined by interfacial charge separation and recombination, in
ZnO/poly 3-hexylthiophene (P3HT) solar cells is found to improve by intentional modification
of the heterojunction interface. To understand the origin of this enhancement, we investigate the
morphology of the interfacial polymer layer and decay dynamics of photoexcited species in
P3HT deposited on glass, bare ZnO, and ZnO modified with an alkanethiol monolayer. These
results are correlated with the characteristics of P3HT/ZnO and P3HT/alkanethiol-modified ZnO
bilayer photovoltaic devices. Synchrotron x-ray diffraction spectra of pristine P3HT and P3HT
on an alkanethiol-modified ZnO surface point to a more crystalline P3HT interfacial layer, while
an amorphous interfacial layer of P3HT is found on unmodified ZnO. To investigate the decay
dynamics of initial photoexcited states in the ZnO/P3HT system, the composite samples are
interrogated by pump-probe spectroscopy with sub-picosecond resolution. Transient
photoinduced absorption spectra are collected for photoexcited species in the range of 1090 nm
to 3034 nm after excitation with a 550 nm pump. Compared to P3HT/ZnO composite films, the
decay behavior for both polarons and excitons over a 500 ps time interval becomes significantly
slower with alkanethiol modification, indicating a reduction in of early-stage charge
recombination. Accompanying the decrease in recombination, we find an increase in the short-
circuit current in the alkanethiol-modified ZnO devices in spite of the electron tunneling barrier
presented by the alkanethiol monolayer. External quantum efficiency measurements in
alkanethiol-modified devices also exhibit a clear signature of crystalline P3HT within an exciton
diffusion length from the heterojunction interface. As these experiments demonstrate, charge
injection efficiency and device performance can be improved by control of the polymer
morphology at the heterojunction interface. Sandia is a multiprogram laboratory operated by
Sandia Corporation, a Lockheed Martin Company for the United States Department of Energy‟s
National Nuclear Security Administration under contract DE-AC04-94AL85000.
B5.70
Shelf Lifetime Study of Unencapsulated Organic and Hybrid Photovoltaic Devices. Matthew Thomas Lloyd
1, Dana C Olson
2, Matthew O Reese
2, Erica Fang
1, Diana L Moore
3,
James A Voigt3 and Julia W Hsu
1;
1Surface and Interface Sciences, Sandia National
Laboratories, Albuquerque, New Mexico; 2National Renewable Energy Laboratory, Golden,
Colorado; 3Sandia National Laboratories, Albuquerque, New Mexico.
As organic photovoltaics (OPVs) approach commercially viable power conversion efficiencies,
in depth investigation of degradation mechanisms and methods for their mitigation becomes
increasingly important. To address this issue, we monitored the difference in degradation rates
for polymer:fullerene bulk heterojunctions (BHJ) and polymer/ZnO hybrid devices. The device
performance was measured over the course of 6 weeks, storing the devices in the dark in ambient
atmosphere between measurements. We found that the performance of BHJ devices employing
silver electron-extracting contacts degraded rapidly (with a half-life of three days). BHJ devices
with calcium/aluminum electron-extracting electrodes also degrade, but at a slower rate. These
results are in contrast with the behavior of “inverted” BHJ and hybrid polymer/ZnO devices,
where an ITO/ZnO electrode serves as the electron-extracting contact and silver serves to extract
holes. Initially, the efficiency of these inverted devices increased; notably, the inverted BHJ
device improved by more than 50% at day nine and maintained an efficiency greater than the
initial value for the duration of the degradation study. These results indicate that the cause of
OPV shelf life degradation is not, as commonly believed, the instability of the active organic
materials. We find that the changes in the contact are a more important cause. Specifically, as
silver oxidizes in air, its work function increases and loses selectivity as an electron-accepting
electrode. A larger work function is found to advantageously increase open-circuit voltage and
short-circuit current in both inverted BHJ and hybrid devices where Ag functions as the hole-
collecting electrode. In addition, we have measured the change of many polymer/ZnO bilayer
and nanorod devices over a longer period of time and found that, over 450 days, the devices still
perform at 50% of their initial efficiency. In summary, we found that changes in the metal
contact have a larger effect than changes in the active material on the degradation of organic and
hybrid solar cells. Sandia is a multiprogram laboratory operated by Sandia Corporation, a
Lockheed Martin Company for the United States Department of Energy‟s National Nuclear
Security Administration under contract DE-AC04-94AL85000.
B5.71
Abstract Withdrawn
B5.72 Nanotube Based High Current Transistor with Good on/off Ratios. Islamshah Amlani and
Rudy Emrick; Applied Research and Technology Center, Motorola, Tempe, Arizona.
Aligned and randomly networked ensembles of single-walled carbon nanotubes (SWNT)
represent a potentially powerful platform for developing thin film semiconductor technologies
such as flexible electronics, optoelectronics, RF electronics and sensing. Electrical characteristics
of SWNT based devices that contain ensemble of nanotubes in the channel exhibit a cumulative
effect of the characteristics due to varying chiralities of the constituent nanotubes. For instance,
in an aligned SNWT device the lower limit of the off state leakage current characteristics will
depend on the fraction of metallic SWNTs in the channel. One way to improve the on/off ratio
post-fabrication is by selective joule heating of metallic and high-leakage ambipolar
semiconducting nanotubes. A typical approach is to bias the semiconducting tubes in the off-
state by applying an appropriate gate bias and apply a sufficiently high drain current to
electrically stress and break the conducting nanotubes. If not properly configured however, on-
current can also diminish substantially in this process due to undesirable burning of
semiconducting nanotubes. Here, we present an automated electrical burning process that
preferentially breaks down both metallic and high-leakage semiconducting SWNTs in a
nanotube ensemble to obtain on/off ratios higher than three orders of magnitude. Typical on-state
current of our back gated devices with interdigitated source drain geometry is in the milliampere
range due to a large number of nanotubes in the channel and the reduction in on-state current at
the end of the electrical burning process is minimal (~ 30%) with on/off ratios of 1000 or greater.
These results are among the best that have been reported. However, we were not able to obtain
electrical burning results on top gated geometry with similar efficiency. We perform two
different experiments to elucidate that nanotubes break at the center when electrically stressed,
the main reason for only marginal success with top gated devices. In the first experiment, the
channel of the back gated devices are completely passivated by PMMA to purposely block off
oxygen at the nanotube interface. Electrical burning result in this case is rather poor as lack of
oxygen availability at the nanotube interface leads to non-preferential breakdown of both
metallic and semiconducting nanotubes. In the second experiment we expose the center portion
consisting of ~30-50% of the channel by performing electron beam lithography and developing
the PMMA resist. Electrical burning results on these devices yield similar to those without any
passivation. This indirectly suggests that breakdown indeed takes place in the center of the
nanotubes and good on/off ratios can be obtained using this approach as long as the middle of the
channel is exposed to air. This has important consequences for top-gated devices where the
center portion of the channel is passivated by gate dielectric and metal.
B5.73 Control of Pentacene Growth and Its Effects on Organic TFT Characteristics Jong Sun
Choi1, Jaehoon Park
1, Jong Won Lee
1, Dong Wook Kim
1, Hyung Tak Kim
1 and Dong Myung
Shin2;
1Dept. of Electrical, Information and Control Engineering, Hongik University, Seoul,
Korea, South; 2Dept. of Chemical Engineering, Hongik University, Seoul, Korea, South.
Recently, organic semiconductor devices have been extensively investigated and steady
progresses in device performances are continuously being obtained with ever increasing range of
applications. One of the most promising organic semiconductors is pentacene, mainly due to the
high hole mobility in pentacene-based TFTs. And many researchers continue to develop high-
performance pentacene thin-film transistors (TFTs), focusing on improving the electrical
conduction in the pentacene active layer. In this study, we have fabricated blends of two different
polyimides with varying the composition ratio and investigated the growth of pentacene grains
on each blend film. It is observed that the pentacene grain size pronouncedly reduced with
increasing the content of hydrophobic polyimide in the blend film. This result can be explained
by the interaction between the adsorbed pentacene molecules and the protrusion of hydrophobic
component in the blend film. Indeed, atomic force microscopy images show that the surface
diffusion of pentacene molecules on the blend film was limited by the hydrophobic protrusions.
Accordingly, it is confirmed that the fabricated blend films can be applied to control the
pentacene grain growth. We extended these results to investigate the effects of pentacene grain
size on the TFT characteristics. Organic TFTs with larger pentacene grains exhibited improved
device performances, which might be attributed to less grain boundaries. And experimental
results show that grain boundaries act as trap sites during device operation and the trapped
charges at grain boundaries can be activated by increasing the gate-source field. However, the
threshold voltage shift upon a gate-voltage sweep direction was more pronounced for the device
with larger pentacene grains, even strongly depending on the delay time of gate-voltage step.
These results are considered to be attributed to a strong interaction between the pentacene layer
and the concomitant blend film owing to its polar feature. Of note, grain boundaries can be
identified as a limiting factor to the charge transport in organic TFTs and the interface between
the organic semiconductor and insulator layer plays a significant role in the operational stability.
Further investigations are focused on evaluating the activation energies of trapped charges at
grain boundaries as well as the interface. These results will be presented.
B5.74
Effects of Alignment Layers on Pentacene Molecular Orientation and Thin-Film Transistor
Characteristics Jae-Hoon Kim1, Jongseung Kim
1, Hyunsuck Kim
1, Jaehoon Park
2, Jong Sun
Choi2 and Dong Myoung Shin
3;
1Department of Electronics and Computer Engineering,
Hanyang University, Seoul, Korea, South; 2Dept. of Electrical, Information and Control
Engineering, Hongik University, Seoul, Korea, South; 3Dept. of Chemical Engieering, Hongik
University, Seoul, Korea, South.
Pentacene, a fused-ring polycyclic aromatic hydrocarbon, is one of the most intensively
investigated systems among various organic semiconductors due to great mobility and good
semiconducting behavior. The electrical conductivity in this material strongly depends on the
direction of applied electrical field to its long molecular axis. And also, it is known that the
vertical alignment of pentacene molecules to the gate insulator surface provides a strong π-π*
overlap and increases the electrical conductivity in the direction of perpendicular to the long-
axis. These have motivated several studies of the effects of pentacene molecular orientations on
the performance of organic thin-film transistors (OTFTs). In the present work, we use different
alignment layers to investigate their effects on pentacene molecular orientation and the
concomitant performance of organic TFTs. For the fabrication of morphological alignment layer,
polyimide films were formed by spin-coating and then rubbed in the parallel and vertical
directions to conducting channel in OTFTs. And also, liquid crystal (LC) material was used for
the fabrication of molecular alignment layer. LC molecules were aligned in the parallel and
vertical directions to channel direction. Experimental results show that the transistor
characteristics were dependent on the directions of alignment layers. The OTFTs with the
morphological alignment layer exhibited an increase in the drain current compared to the device
without rubbing treatment, independent on the rubbing directions. Dichroic ratio of the drain
current was about 1.2, which is defined as the ratio of current for the device with the parallel
alignment layer to that with the vertical alignment layer. On the other hand, the OTFTs with the
molecular alignment layer showed a significant dependence of drain current on the direction of
alignment layer: the drain current in the parallel direction increased compared with that for the
device with unaligned LC layer, but the drain current in the vertical direction even deteriorated.
In this case, dichroic ratio was about 2.1. These results indicate that the morphological effect on
the pentacene molecular orientation is intrinsically different from that of a prior molecular
orientation. We will report the detailed growth mechanism of pentacene molecule on these
alignment layers, combining with the electrical characteristics of OTFTs.
B5.75
Nanostructure-Assisted Hole Injection in Schottky Diodes and Application in Organic
TFTs Hyunsuck Kim1, Jongseung Kim
1, Jae-Hoon Kim
1, Jaehoon Park
2 and Jong Sun Choi
2;
1Department of Electronics and Computer Engineering, Hanyang University, Seoul, Korea,
South; 2Dept. of Electrical, Information and Control Enigeering, Hongik University, Seoul,
Korea, South.
Organic thin-film transistors (OTFTs) have shown great promise for a variety of electronic
applications, including flexible displays, chemical sensors, and low-cost microelectronics. The
characteristics of OTFTs have been remarkably advanced and even surpass those of TFTs with
amorphous Si. Most of efforts for OTFTs have been mainly focused on the improvements of
electrical performance. But the basic study in the interfacial characteristics related with device
performance is not so much progressed, which is also necessary for understanding device physics
and further improvements in device characteristics. The interface between an organic material
and a metal is one of critical factors for the device performance. Recently, some groups have
investigated the importance of the interfacial characteristics and attempted to enhance the
interfacial characteristics. Therefore, the interfacial properties should be investigated and must
be improved in order to make OTFTs competitive with more conventional amorphous Si and
poly-Si TFTs. In this work, we have fabricated the conic-nanostructures on the Al-bottom
electrode and investigated the growth of pentacene molecules on the rough conic-nanostructures,
combing with the barrier heights for hole injection in Schottky diodes. For the fabrication of
conic-nanostructures, the H1 solution, in which polyurethane was dissolved into acetone solvent,
was spin-coated onto the Al-bottom electrode. X-ray diffraction results show that the conic-
nanostructures can contribute to the ordered growth pentacene molecules. And the barrier height
for hole injection from the top-Au electrode was calculated by Fowler-Nordheim theory and
found to be lowered for the Schottky diode with conic-nanostructures. This injection barrier
lowering can be explained by the ordered growth of pentacene molecules under the influence of
conic-nanostructures. We also introduced these structures into the interface bewteen the
pentacene layer and gate insulator of OTFTs. It is observed that the electrical characteristics of
the OTFT with conic-nanostructures were higher for the device without nanostructures. In
particular, the field-effect mobility was significantly improved by using conic-nanostructures,
calculated to be about 2.94 cm2/Vs. Consequently, we conclude that the nanostructure-assisted
hole injection facilitated the barrier lowering, thereby contributing to achieving high
performances in OTFTs. These results will be discussed.
B5.76
Structural Phase Transition and Molecular Electron Tunneling in Self-Assembled
Monolayers Kyoungja Seo and Hyoyoung Lee; ETRI, Daejeon, Korea, South.
The electrical and chemical nature of organic molecules in self-assembled monolayers (SAMs)
on metal or semiconductors have been studied for applications of molecular electronics[1]. As a
model system, alkanethiols have been studied extensively by current-voltage (I-V) characteristics
in molecular junctions (metal-molecule-metal). Electron transport across the alkanethiol
monolayers is influenced by a tunneling pathway through the σ-bonded alkyl chain (through-
bond tunneling) and through the intermolecular charge transport (through-space tunneling)[2].
Strong electronic coupling of a sulfur atom to a gold atom by chemisorption enhances the
electron transport via through-bond tunneling across the interfaces of alkanethiol and gold. A
molecular tilt in alkanethiol SAMs enhances contribution of through-space tunneling by the
intermolecular coupling, and molecular conductance across the SAM relatively decreases[3].
Molecular orientation on alkanethiol SAMs is changed with structural phase transition by
thermal annealing in the SAMs. Molecular electron transport characteristics will be varied by
different intermolecular coupling induced in each structural phase. However, tunneling
characteristics such as an electron tunneling barrier of molecular junctions has not been reported
with different structural phases, yet. In addition, it was reported that the thermal and electrostatic
effects influence conductance across molecular junctions, dependent of the local molecular
environment induced by neighboring molecules. Thus, we expect that the structural phase
transition-induced a difference in molecular electron tunneling characteristics will depend on
different intermolecular coupling effects created in a large molecular junction and an individual
molecular junction. In this study, we demonstrate structural phase dependency of conductance
across the thiolate (e.g., alkanethiol and alkanedithiol) SAMs, dependent of a junction area in
size. The molecular electron tunneling characteristics for each structural phase are compared
with I-V curves obtained in a scanning tunneling microscope (STM)-based individual molecular
junction and a micropore-based large molecular junction. A tunneling barrier is proved as a
measure of the intermolecular coupling on different structural phases of the thiolate SAMs. It is
the first examination of the molecular electron tunneling in different structural phases of the
thiolate SAMs and dependence of the molecular electron tunneling on a junction area in size. [1]
A. Salomon, D. Cahen, S. Lindsay, J. Tomfohr, V. B. Engelkes, C. D. Frisbie, Adv. Mater. 2003,
15, 1881-1890. [2] X. D. Cui, X. Zarate, J. Tomfohr, O. F. Sankey, A. Primak, A. L. Moore, T.
A. Moore, D. Gust, G. Harris, S. M. Lindsay, Nanotechnology 2002, 13, 5. [3] H. Song, H. Lee,
T. Lee, J. Am. Chem. Soc. 2007, 129, 3806-3807.
B5.77 Semi-transparent Flexible Photo-detector using Tetracene/ZnO hybrid p-n Junction. Aaron
Park, Seongil Im, Kimoon Lee and Kwang H. Lee; physics, Yonsei university, Seoul, Korea,
South.
Over the last few decades, organic thin-film devices, such as organic light-emitting diodes
(OLED), organic thin-film transistors (OTFT), solar cells, and photodetectors have made steady
progresses in their performances. Among organic materials polyacenes like tetracene, which is
one of the important organic molecules composed of four benzene rings, have been studied for
applying to optoelectronic devices due to their good optical properties. We report on the
fabrication of p-type organic/n-type ZnO hetero-junction diode and its applications. To construct
the organic/inorganic hetero-junction, 50 nm-thick ZnO was deposited on ITO-coated flexible
PET substrate at 100 °C and then 100 nm-thick tetracene as a p-type organic semiconductor was
evaporated on the ZnO layer. Not only to make an ohmic contact with p-type organic layer but
also to investigate the photo-response properties of the p-n junction, we adopt the semi-
transparent NiOx electrode with large work-function value of ~5.1 eV and transmittance of ~30
%. Organic/inorganic hetero-junction of p-type tetracene/n-type ZnO showed rectifying behavior
with current-voltage (I-V) characteristic and exhibited quite a high current density under forward
bias. Also the tetracene/ZnO p-n diode has excellent photo-response properties under green,
blue, and weak UV illuminations. We actually fabricated pentacene/ZnO p-n diode using the
similar method, to compare with the tetracene/ZnO diode. Although the pentacene/ZnO diode
displayed a little higher forward current than that of the tetracene/ZnO, we now regard that our
p-tetracene/n-ZnO diode has clear advantages over pentacene/ZnO as an optoelectronic device
due to its excellent photodetecting potentials. More details on the comparison will be discussed
in the conference meeting.
B5.78
Characteristics of Organic Thin-Film Transistors with Anodized Aluminum Oxide as a
Gate Insulator.Jong Won Lee1, Dong Wook Kim
1, Jaehoon Park
1, Hyoung Tak Kim
1, Dong
Myoung Shin2 and Jong Sun Choi
1;
1Dept. of Electrical, Information and Control Engineering,
Hongik University, Seoul, Korea, South; 2Dept. of Chemical Engineering, Hongik University,
Seoul, Korea, South.
Organic thin-film transistors (OTFTs) have been expected by switching devices for flexible
display. Aluminum gate electrode is usually adopted to reduce the delay time because of the
property of low resistivity. And anodized aluminum oxide film is considered as a good insulator
because it is resistant to various chemical solvent and has a high dielectric constant (high-k)
contributing to reduction of the threshold voltage. The reduction of threshold voltage can lead to
lowering supply voltage and thus resulting in lowering power dissipation. In general, anodized
oxide films form two types of morphology, i.e. barrier-type and porous-type films, by altering
the condition of electrolytes. Barrie-type films in the pH range of 5 to 7 are utilized as
electrolytic capacitors due to thin, compact, and non-porous structure, while porous-type films
are applied to nano-templates due to its thick and porous structure. In this work, we fabricated
two types of anodizing aluminum oxide films with varying the pH values of electrolyte. Atomic
force microscopy images of anodized aluminum oxide films show that the surface of film
forming the barrier-type structure grown at pH 6.3 is smoother than that for the porous-like
structure grown at pH 4.1. The difference in the height of these two films was about 10-30 nm.
And the OTFT with the barrier-type aluminum oxide insulator exhibited the mobility of 0.09
cm2/Vs, the subthreshold slope of 1.1 V/decade, and the on/off ratio of 10_4, which are superior
to those for the device with the porous-type insulator. It is thought that the smooth surface of the
barrier-type film might contribute to a long range hopping of charge carriers in the conducting
channel. Indeed, we found that the activation energy for device with the porous-like anodized
insulator was much larger than that for the barrier-type structure, which demonstrates that the
characteristic improvement in the device with the barrier-type structure was attributed to low
activation energy for carrier transport in the conducting channel. These results will be discussed.
B5.79 Scanning Probe Analysis of Poly(3-Hexylthiophene) Thin Films. Rajiv Giridharagopal and
Kevin Kelly; Electrical and Computer Engineering, Rice University, Houston, Texas.
The electronic behavior of conducting polymers at the polymer-electrode interface is of great
interest, both technologically and in terms of basic materials science. We have used scanning
tunneling microscopy (STM), including spectroscopic extensions such as work function imaging
and alternating current STM (ACSTM), to probe conducting polymer thin films and monolayers
with high spatial resolution in both ambient and ultrahigh vacuum environments. In our studies
we focus on poly(3-hexylthiophene) (P3HT) films, each approximately one to three monolayers
in thickness, deposited on highly-ordered pyrolytic graphite (HOPG) and molybdenum disulfide
(MoS2) substrates. P3HT is one of a number of conducting polymers, and STM reveals that
P3HT deposited on HOPG or MoS2 self-assembles to form a highly-ordered structure with
symmetry commensurate with that of the underlying lattice. Additionally, we have used ACSTM
to analyze spatial variations in the charge carrier density in such layers, thus shedding light on
substrate-dependent charge transfer in P3HT films on different materials. Understanding such
effects is of great importance for improving P3HT devices and conducting polymer-metal
interfaces in general.
B5.80
Hybrid Nanostructures «Semiconductor/Organic Dye J-Aggregate» In Reverse Micelles. Vladimir Razumov, Lubov Nikolenko and Sergey Brichkin; Institute of Problems of Chemical
Physics RAS, Chernogolovka, Russia.
Hybrid nanostructures such as «molecular aggregate/semiconductor nanoparticle» or core/shell
structures in which core is the semiconductor nanocrystal and shell is the organic dye are very
important for the charge separation at light absorption. These structures are useful for
development of light-emitting devices, thin-film transistors, and optical memory or solar cells.
Organic/inorganic semiconductor interfaces play an important role for effective charge
separation. There are different methods for design of hybrid nanostructures. One of these
methods is a self-assembly using of reverse micelles. The main procedure includes three stages.
The first is synthesis of semiconductor nanoparticles by chemical reaction controlled by
intermicellar exchange in «water in oil» microemulsions. The second stage is molecular dye
aggregation and the third stage is selfassembly of organic/inorganic nanostructures. Self-
assembly of hybrid “nanocrystal/J-aggregate” nanostructures after mixing two reverse micelle
AOT/water/hexane solutions: one containing J-aggregates of pyridinium salt of betaine 3,3'-di(γ-
sulfopropyl)-4,5,4',5'-dibenzo-9-ethylthiacarbocyanine and the other - AgI nanocrystals was
shown. During the hybrid nanostructure formation new dye absorption bands with λmax≈673
and 695 nm appeared, these belong to J-aggregates of different structures adsorbed on
nanocrystals. It was found that the excess of iodide ions during AgI nanocrystal synthesis is the
main factor influencing the hybrid nanostructures self-assembly, so AgI crystal lattice is of great
importance in this process. Taking into account that AgI nanocrystals synthesized in iodide
excess have hexagonal crystal lattice it can be concluded that J-aggregates effective absorb on β-
AgI and do not absorb on γ-AgI. There occurs a “symbiosis” of the hybrid structure components:
nanocrystals raise the photostability of J-aggregates, and adsorbed J-aggregates efficiently
stabilize the nanocrystal size. It was shown that the stable hybrid nanostructures may be
extracted from micellar solution without further aggregation. Spectral and structure
investigations of these hybrid systems were carried out depending on average size of reverse
micelles using of light absorption and TEM techniques.
B5.81
Morphological Stabilization of Polymer Photovoltaic Cells by Using Cross-linkable
PolythiopheneShoji Miyanishi1, Keisuke Tajima
1 and Kazuhito Hashimoto
1,2;
1Enginnering,
Graduate School of The Univerity of Tokyo, Tokyo, Japan; 2JST-ERATO, Tokyo, Japan.
Polymer photovoltaic cells draw considerable attention these days for their potential of low cost
fabrication of large area devices by simple means of painting or printing from the polymer
solutions. The most commonly used structure for the polymer photovoltaic cells now is a bulk
heterojunction, which is a physical mixture of donor and acceptor materials. Recent research has
suggested that control of the mixing morphology of the donor and the acceptor in films is of high
importance to achieve highly efficient charge separation and transport. Especially in the case of
the combination of poly(3-hexylthiophene) (P3HT) and (6,6)-phenyl C61 butyric acid methyl
ester (PCBM), thermal annealing of the films has been used to control this phase separation.
During the thermal annealing, both the polymer and PCBM crystallize to form an
interpenetrating network in nanoscale, resulting in drastic enhancement of the device
performance. However, even if such phase-separated nanostructure is constructed, this
morphology is not thermally stable and may gradually undergo changes during the device
operation since the polymer and the PCBM thermodynamically prefer to segregate from each
other. In fact, a prolonged thermal treatment of the device induces the formation of large
aggregations of PCBM in the films that lowered the device performance significantly. Therefore,
morphology control that only depends on thermal annealing might be insufficient for achieving a
stable and reliable mixing morphology of the donor and the acceptor. In this work a new cross-
linkable regioregular poly(3-(5-hexenyl)thiophene) (P3HNT) was synthesized for the purpose of
stabilizing the film morphology in polymer photovoltaic cells. The vinyl group at the side chains
of P3HNT is expected to conduct cross-linking reaction by thermal treatment. After the process
of cross-linking is complete, the diffusion of PCBM into the film can be lowered to suppress the
formation of large aggregations. As a result, the thermal stability of the cells is expected to
improved in comparison to non-cross-linkable P3HT:PCBM bulk heterojunction. P3HNT was
characterized by NMR, GPC, UV-vis absorption spectra, XRD spectra and DSC. XRD and UV-
vis spectra showed that P3HNT has similar crystallinity to P3HT. As a result, the performance of
the P3HNT:PCBM bulk heterojunction devices showed comparably high efficiency over 3%
similar to P3HT:PCBM cells. Furthermore, P3HNT was certainly cross-linked during thermal
treatment, confirmed by the insolubility of the films in organic solvents. This cross-linkability
was also confirmed even in the mixture films with PCBM. As the result, the formation of large
aggregations of PCBM was prevented in P3HNT:PCBM films even after prolonged thermal
annealing. In addition, the deterioration of the photoconversion performance at a high
temperature was suppressed in the polymer solar cells compared to the control cells with
P3HT:PCBM.
B5.82 Low-Voltage Self-Supported Ion Conductive Membrane Based Transistors Nikolai J.
Kaihovirta1,3
, Carl-Johan Wikman2, Tapio Makela
1, Carl-Eric Wilen
2 and Ronald Osterbacka
1;
1Center for Functional Materials and Department of Physics, Åbo Akademi University, Turku,
Finland; 2Center for Functional Materials and Laboratory of Polymer Technology, Åbo Akademi
University, Turku, Finland; 3Graduate School of Materials Research, Turku Universities, Turku,
Finland.
Ion enhanced organic transistors are promising candidates for large-scale fabrication of low-
voltage applications [1-3]. In the ion enhanced organic transistor, an ionic insulator replaces the
traditional dielectric insulator. Two types of ion enhanced polymer transistors have been
presented in the literature: Electrochemical transistors [1, 4] and electric double layer (EDL)
gated transistors [2, 3, 5, 6]. We present a novel EDL-gated transistor using a thick (> 50 μm),
ion conducting membrane (MemFET) [7]. The membrane acts both as gate insulator and as
mechanical support. The fabrication of the membrane starts with a PVDF-film as base material.
The PVDF-film is functionalized by the roll-to-roll suitable electron beam irradiation induced
grafting technique [8]. For comparison, we have also used the commercially available, proton-
conducting Nafion®-membrane, as received. The MemFETs are fabricated by standard
laboratory fabrication techniques using soluble polymers. As semiconductor we apply the
regioregular P3HT. The highly conducting polymers PANI or PEDOT:PSS are used for the gate
electrode, while evaporated gold is chosen, for convenience, for the source and drain electrodes.
The MemFETs operate at low voltages (1 V) with a high current throughput. By using the
electron beam irradiation induced grafting technique we can fabricate membranes to conduct
different ions and/or simply tailor-make membranes to be locally ion conducting in whatever
pattern needed. MemFETs fabricated on different membranes will be presented. Furthermore, the
ion conducting membrane allows for fabrication of two (or more) devices on the same membrane
in a simple and cost-effective matter. This will be shown by driving an electrochromic display
pixel with a MemFET, both fabricated on the same membrane. [1] R. Mannerbro, M. Ranlöf, N.
Robinson, R. Forchheimer, Synth. Met. 158 (2008), 556-560. [2] D. Tobjörk, N. J. Kaihovirta, T.
Mäkelä, F. S. Pettersson, R. Österbacka, Org. Electron. 9 (2008), 931-935. [3] J. H. Cho, J. Lee,
Y. Xia, B. Kim, Y. He, M. J. Renn, T. P. Lodge, C. D. Frisbie, Nat. Mater. 7 (2008), 900-906.
[4] D. Nilsson, M. Chen, T. Kugler, T. Remonen, M. Armgarth, M. Berggren, Adv. Mater. 14
(2002), 51-54. [5] H. G. O. Sandberg, T. G. Bäcklund, R. Österbacka, H. Stubb, Adv. Mater. 19
(2004), 1112-1115. [6] M. J. Panzer, C. D. Frisbie, Adv. Funct. Mater. 16 (2006), 1051-1056. [7]
N. J. Kaihovirta, C, -J. Wikman, T. Mäkelä, C. -E. Wilén, R. Österbacka, Adv. Mater. (2008) in
press. [8] T. Lehtinen, G. Sundholm, S. Holmberg, F. Sundholm, P. Björnbom, M. Bursell,
Electrochim. Acta 43 (1998), 1881-1890.
B5.83
Understanding Aminated Silane Monolayer Formation Kinetics for Use in Organic
Electronics Justin Opatkiewicz, Melbs C LeMieux and Zhenan Bao; Chemical Engineering,
Stanford University, Stanford, California.
Aminated silane films are being used in a wide variety of fields: from biological sciences to
semiconductor research. Amines with varying degrees of substitution are common in nature and
hence can be used to modify surfaces to interface with biological systems. 3-
aminopropyltriethoxysilane (APTES) is a very common silane and has been characterized in
many studies. It is generally accepted that the amine catalyzes monolayer formation by
electrostatic attraction of the amine to a hydrophilic surface. Silanes with very similar structures,
however, have not been analyzed in as much depth. Similar molecules varying simply by methyl-
substitutions, such as N-methylaminopropyltrimethoxysilane (MAPS) and (N,N-
dimethylaminopropyl)trimethoxysilane (DMAPS), can potentially be used alongside APTES in a
variety of applications such as biological linkages and carbon nanotube separation. Here, we
compare the kinetics of MAPS and DMAPS to APTES surface reactions and determine the
influence of the methyl substitution on monolayer formation. After the kinetics of the three
silanes are understood, their abilitly to separate carbon nanotubes (CNTs) are analyzed.
B5.84 Enhanced Performance of Organic Light Emitting Diodes Using LiF Buffer Layer. Omkar
Vyavahare1 and Richard Hailstone
2;
1Materials Science and Engineering, Rochester Institute of
Technology, Rochester, New York; 2Imaging Science, Rochester Institute of Technology,
Rochester, New York.
Since the invention of organic electroluminescent devices a great deal of effort has been made to
improve their performance. Reducing the barrier and optimizing charge injection is crucial for
efficient and bright Organic Light Emitting Diodes (OLEDs). We report improvement in the
performance of OLEDs with ITO/TPD/Alq3/Al structure by inserting LiF both at electrode-
organic interface and organic-organic interface. In this paper, we elucidate the mechanism of LiF
buffer layer inserted at different interfaces. These devices show improved luminescence and
steeper IV characteristics.
B5.85
Transparent Photo-stable Complementary Inverter with Organic-inorganic Nano-hybrid
Dielectrics.Min Suk Oh1, Yong Hoon Kim
1, Sung Kyu Park
1, Jeong In Han
1, Byoung H. Lee
2,
Myung M. Sung2, Kimoon Lee
3, Kwang H Lee
3, Sung Hoon Cha
3 and Seongil Im
3;
1Flexible
Display Research Center, Korea Electronics Technology Institute, Seongnam, Korea, South; 2Department of Chemistry, Hanyang University, Seoul, Korea, South;
Matter Eng. Laboratory, The Institute of Physical and Chemical Research (RIKEN), Saitama,
Japan.
Material innovation is a key component of the research towards enhancing organic solar cells
performance. We have taken the new approach of incorporating acenes such as pentacene and
anthradithiophene into conjugated copolymers, based on their superior thin-film transistor
performance. In this work, we extend our research to report two classes of novel acene-
containing polymers. The first class consists of the regioregular 2,9- and 2,10-pentacene-
diethynylbenzene copolymers. The two polymers are observed. to differ in absorption properties
owing to foreseeably different conformations. The second class of materials consisting
anthradithiophene-cyclopentadithiophene copolymers shows excellent film absorption properties
as well as solubility in common organic solvents. High field effect mobilities of up to 10-3
cm2/Vs are measured for anthradithiophene-4,4-bis(2-ethylhexyl)cyclopentadithiophene
deposited in top-contact geometry on a silane treated silicon oxide surface. The material also
achieves a power conversion efficiency of 0.59% in preliminary solar cell devices.
B5.87
Synthesis and Optical properties of Perylene Bisimide Incorporated Low Bandgap
Polymers for Photovoltaics. Sivamurugan Vajiravelu and Valiyaveettil Suresh; Department of
Chemistry, National University of Singapore, Singapore, Singapore.
Synthesis and development of a broad range light absorbing molecules with high extinction
coefficient as suitable metarials to improve the solar cell efficiency is exciting due to the great
demand in energy for our day to day life. Perylene diimide (PDI) derivatives are most attractive
molecule owing to high charge mobility, greater electron affinity and act as good n-type
materials. In this investigation, we focused on the synthesis of alternative donor-acceptor
conjugated systems prepared using Suzuki polymerization of N,N‟-didodecyl 1,7-
dibromoperylene diimide (PDI) with diboronic acids of flurene and dithiophene respectively.
The polymers were designed to achieve molecular heterojunction through charge transfer from
donor to acceptor. The polymers were characterised using GPC, 1H, 13C NMR and elemental
analysis. TGA and DSC techniques were used to identify thermal stability and phase changes of
the polymers. The absorption spectra of polymers covered whole range of visible absorption
region from 300 to 800 nm in solution and the absorption maximum shifts to higher wavelength
in solid state.
B5.88 Multifunctional Organic Field-effect Transistor Based on Polydiacetylene. Jaehui Ahn, Doo
Ho Yang, Chunzhi Cui, Jihyun Kim and Dong June Ahn; Chemical & Biological Eng., Korea
Univ., Seoul, Korea, South.
Recently, the interest of low cost and reliable organic materials for multifunctional device is
increased. Organic field effect transistors (OFETs) are suitable for multifunctional devices
because of their potential applications. In our research, PCDA based FET was used. Among a lot
of conjugated polymer, the hole mobility of single crystal polydiacetylene is founded to be 1-10
cm2/V*s by time of flight method[1], it is good property as candidate of OFETs material.
Another unique property of PDA is electrical property to be varied as change of PDA‟s phase (so
called blue and red phase), which can be controllable by UV and external stimuli. We
demonstrated that the single device performed as both UV and gas sensor which shows that
PCDA based FET has potential application in multifunctional device. In this research, we
investigated electrical property of both red phase and blue phase PDA. First, we made thin film
of 10,12-pentacosadiynoic acid (PCDA) by the Langmuir-Blodgett (LB) deposition on SiO2/Si
substrate that has predefined source and drain metal. And then, we performed polymerization to
obtain PDA thin film, followed by thermal treatment to obtain red phase PCDA. Then, we
measured specific contact resistance by the transmission line modeling (TLM) method and the
characterization of Field Effect Transistor. We found that the red phase of PCDA can be
obtained by specific species by the surface modification to increase the selectivity. In our
experiment, the surface of PCDA was modified to have selectivity for NH3 over N2. The details
about the fabrication process and sensing mechanism will be presented.
B5.89
Abstract Withdrawn
B5.90
Doping of Perylene Diimide Derivatives n-type Semiconductor Layer and Interface
Modification for Organic Thin Film Transistor. Heng-Wen Ting and Tri-Rung Yew;
Department of Materials Science and Engineering, Nationl Tsing Hua University, Hsinchu,
Taiwan.
Doping of air-stable solution-processed tetrachloroperylene tetracarboxyldiimide based n-type
semiconductor (TC-PDI-F) layer by dipolar molecules and ionic compounds is demonstrated to
enhance the electrical properties of organic thin film transistors (OTFT). Besides, the interface
modifications between semiconductor and insulator layers by dipolar molecules with phenyl,
alcohol and fluoro-functional groups were applied to improve the molecules packing order and
the electrical performance of doped TC-PDI-F OTFTs. The electronic structures were
characterized by cyclic voltammetry (CV), UV-visible optical absorption spectroscopy (UV-Vis)
and photoluminescence spectroscopy (PL). The surface properties were also inspected by atomic
force microscope (AFM), contact angle system and X-ray diffraction spectroscopy (XRD). The
electrical properties of OTFTs were also measured. All processes were fabricated and measured
in air.
B5.91 Self-Assembled Hydrophobin Protein Membranes on Silicon Platforms Jouni Ahopelto
1,
Markku Kainlauri1, Jani Kivioja
1, Paivi Laaksonen
2, Arja Paananen
2 and Markus Linder
2;
1Micro
and Nanoelectronics, VTT, Espoo, Finland; 2Nanobiomaterials, VTT, Espoo, Finland.
We report on formation and characterisation of ordered single layer protein crystals formed by
directed self-assembly on hydrophobic substrates, such as graphite and silicon platforms. The
hydrophobin proteins form an ordered two-dimensional crystal at air-water interface with the
protein molecules all having a well defined orientation and position [1]. From the air-water
interface the crystal membranes are transferred onto surface of highly oriented pyrolytic graphite
or on patterned silicon substrates. The thickness of the membrane is about 3 nm and it has a
hexagonal-like lattice with lattice constant of about 6 nm. On silicon substrates selectivity can be
obtained between hydrophilic oxide and hydrophobic silicon areas, providing means to integrate
protein membranes with silicon microelectronics [2]. By decorating the proteins with Au
nanoparticles, nanoelectrodes for electrical measurements and, on the other hand, plasmonic
devices can be envisaged. The effect of nanoelectrodes, as measured by conducting AFM, can be
seen as largely enhanced conductivity through the protein membrane. Also, on glass slides made
hydrophobic by silanization, clear plasmon band absorption arising from the Au nanoparticles
can be seen. [1] G. Szilvay, A. Paananen, K. Laurikainen, E. Vuorimaa, H. Lemmetyinen, J.
Peltonen, M. Linder, Self-assembled hydrophobin protein films at the air-water interface:
structural analysis and molecular engineering, Biochemistry . Vol. 46 (2007) 2345 - 2354. [2] P.
Laaksonen, J. Kivioja, A. Paananen, M. Kainlauri, K. Kontturi, J. Ahopelto, M. B. Linder,
Selective nanopatterning using citrate stabilized Au nanoparticles and NCysHFBI fusion protein,
submitted 2008.
B5.92 A Self-patterned Polymer Dielectric for Low Voltage Pentacene Thin Film Transistor. Hui-
Chen Huang, Ting-Hsiang Huang and Zingway Pei; Graduate Institute of Optoelectronic
Engineering, Department of Electrical Engineering, National Chung Hsing University, Taichung,
Taiwan.
Organic thin-film transistors (OTFTs) based on pentacene has attract much attention due to low
temperature and low cost fabrication process. Although mobility of pentacene is comparable to
the a-Si:H having potential to replace a-Si:H TFT in display applications, the high driving
voltage is still a problem. Self-assembled monolayer (SAM) or ultra thin film are general
methods used to reduce the operation voltage to about 2~ 5 volts. However, SAM methods
require unique coating technique and the thickness of the ultra thin film is hard to control during
the coating process limit theses method only demonstrated by some very specific group. In this
work, a random copolymer, PS-r-PMMA, is used as dielectric material for a pentacene OTFT.
The PS-r-PMMA dielectric with 10 nm thick can be uniform coated on the surface with hydroxyl
groups in a very simple spin coating process. The PS-r-PMMA is initially coated in roughly 50
nm thick. After thermal annealing, a layer of PS-r-PMMA will connect to the OH- group
forming a 10nm thick layer no matter how thick of the initial layer. Besides the thickness control,
the PS-r-PMMA only coated on the surface having OH- groups, the self-patterning is possible to
prevent the chemical etching process during the organic circuit fabrication. The designed OTFT
is bottom gate and top contact structure. The substrate is glass and gate material is aluminum.
After the deposition of the Al, the UV/ozone process is performed to convert the surface of Al
into Al2O3 having OH- groups. After this, PS-r-PMMA is spin coated and heated sequentially.
Finally, pentacene and gold contact electrodes are deposited sequentially by thermal evaporation
to finish the OTFT. By using this self-patterning thin dielectric, the pentacene OTFT exhibits
operation voltage as low as 5V and an on/off ratio large than 105.
B5.93
Abstract Withdrawn
B5.94
Controlling the Electrical Properties of Molecule-Terminated, Non-Oxidized Silicone by
the C-C Bond Nearby the Surface Sreenivasa R Puniredd1, Ilia Platzman
1 and Hossam
Haick1,2
; 1Chemical Engineering, Technion- Israel Institute of Technology, Haifa, Israel;
2Russell Berrie Nanotechnology Institute, Technion- Israel Institute of Technology, Haifa, Israel.
The ability to exert systematic control over the electronic properties of Si is an important factor
for the realization of nanoelectronic devices. Mostly, such controllability can be achieved by
placing molecules, whose dipole can be changed systematically, at the device surface and/or
interface. In this study, we present an approach for controlling the electrical properties of Si by
inducing deliberate interaction between the energy levels of organic molecules and Si, without
the need for a dipole modification. We illustrate this approach by functionalizing 50±2% of Si
atop sites of n- and p-type Si (111) oxide-free surfaces with various organic molecules having
similar (3C) backbone but different in their C-C bond close to the Si surface (i.e., C-C vs. C=C
vs. C≡C bonds). These molecules have nearly similar dipole moment (0.9-1.6 Debye). Electrical
characterizations of molecule/Si surfaces and Hg/molecule/Si junctions showed a systematic
control over the work function and Schottky barrier of the Si, respectively, in the range from 0.2
to 0.8 eV, as compared to H-terminated Si. Our results show that the control over the electrical
properties of n- and p-type Si is dominated by systematic electron transfer from the Si surface to
the organic monolayers. The results indicate that the extent of electron transfer depends on the
difference between the energy levels of the organic molecules and the Si surface. This finding,
for which we will present a detailed explanation, has very significant implications as it suggests
the ability to control the electrical properties of semiconductors with minimal depolarization
effects.
B5.95 Ultrathin and Printable Conjugated Films for Organic Electronics. Li Tan, Engineering
Mechanics and Nebraska Center for Materials and Nanoscience, University of Nebraska-Lincoln,
Lincoln, Nebraska.
Ultrathin organic films or self-assembled monolayers (SAMs) can provide molecular or organic
electronics unmatched processing convenience and some knowledge in device operation.
Conventional concepts often come from a head-tail molecular design, where the head carries the
function of charge transport and the tail is capable of self-assembly. Amorphous nature of the
monolayers and rather significant defects along grain boundaries, however, all inhibit their
device performances at high current density and high temperature conditions. Toward this end,
we are investigating a new family of ultrathin organic thin films, i.e., self-organized nanolayers.
This type of molecules enjoys a polymeric backbone, branched with an extended pi-pi system.
Rather rigid branches, plus the chain-chain cooperation allow the polymer form long-range
ordered lamellae in solution. Printed films have a super dense, multistack morphology over tens
of microns. Unprecedented thermal stability is also realized due to an enhanced molecular
interaction between the pi-pi units, plus a strong interlayer binding between the layers. Research
efforts with scientific and engineering importance are worth of reporting, including 1) strategy of
the molecular design and synthesis; 2) film casting and characterizations; and 3) molecular
modeling to interpret the structuring mechanism of the nanolayers. Merits of well-ordered
packing, plus easiness to chemically functionalize these nanolayers, all promise great
applications in molecular or organic electronics.
B5.96
Probing Nanoscale Electric Field Fluctuations: Towards a Local Measurement of Carrier
Mobility in Organic Semiconductors. Showkat Yazdanian1, Seppe Kuehn
2, Nikolas Hoepker
1,
Roger Loring1 and John Marohn
1;
1Cornell University, Ithaca, New York;
2Rockefeller
University, New York, New York.
A rich array of dynamic physical processes can be modelled as fluctuating electric fields. These
processes include electron transfer, enzyme kinetics, ionic motion in solution and the glass
transition. We have shown that cantilever friction can be used to study dielectric fluctuations in
the vicinity of the cantilever resonance frequency. Here we show that cantilever frequency
fluctuations (“jitter”) can be used to probe low frequency dielectric fluctuations over a much
broader frequency window. We present measurements of dielectric fluctuations in polymers in
tandem with a theory predicting the shape and magnitude of the effect. We also present our
progress towards employing this technique to measure the local carrier mobility in organic
semiconductor systems.
B5.97
Porphyrins and N-confused Porphyrins as Spectral Dopants in Organic Solar Cells. Warwick J Belcher, Nathan Cooling and Paul Christopher Dastoor; Physics, University of
Newcastle, Callaghan, New South Wales, Australia.
Organic photovoltaic cells show immense promise as a new alternative for renewable energy.
However, one major problem with these devices is that the polymers typically employed in their
design absorb light in only a limited part of the solar spectrum (usually <600nm). One way to
further extend the spectral response of these devices is via the addition of complimentary
chromophores. It has been shown that the absorption of light by the Q-bands of porphyrins
incorporated into MEH-PPV/PCBM blends contributes up to 20% of the total photocurrent
generated by the device by utilising light that would not normally be absorbed [1]. Furthermore,
N-confused porphyrins could be used to extend the absorption spectra of these devices even
further, since they have Q-band absorptions extending well beyond the range of a standard
porphyrin. Despite this promise porphyrin aggregation within these ternary devices has been
observed to lead to disruption of the crucial morphology of the active layer and lowered device
efficiency. Porphyrin aggregation can be controlled by controlling the steric bulk of peripheral
substituents on the porphyrin. A series of substituted tetraphenylporphyrins and N-confused
tertraphenylporphyrins have been prepared in which the steric bulk of the peripheral substituents,
and thus the degree to which aggregation occurs, was varied. These materials have been used to
prepare a series of MEH-PPV/Porphyrin/PCBM and PPV/N-confused Porphyrin/PCBM ternary
organic photovoltaic devices. Furthermore, we have observed that the porphyrin molecules act as
“hole traps” within the devices due to the basicity of the pyrollic nitrogens, lowering device
efficiency. Reduction of this basicity has been achieved by metallation and alkylation of these
sites. The effect that these structural changes have on device performance will be presented.
References [1] P.C. Dastoor, C. R. McNeill. H. Frohne, C, J, Foster, B. Dean, C.J. Fell, W. J.
Belcher, W. M. Campbell, D.L. Officer, I. M. Blake, P. Thordarson, M.J. Crossley, N.S. Hush,
R. Jeffrey, J. Phys. Chem. 111(42) (2007), 15415-15426.
B5.98 Near Infrared Fluorescent and Phosphorescent Organic Light-Emitting Devices. Yixing
Yang1, Richard T Farley
2, Timothy T Steckler
2, Jonathan Sommer
2, Sang-Hyun Eom
1, John R
Reynolds2, Kirk S Schanze
2 and Jiangeng Xue
1;
1Department of Materials Science and
Engineering, University of Florida, Gainesville, Florida; 2Department of Chemistry, Center for
Macromolecular Science and Engineering, University of Florida, Gainesville, Florida.
There has been a growing interest in the development of near-infrared (NIR) organic light-
emitting devices (OLEDs) due to their potential applications in defense, biomedical sectors, and
telecommunications. For example, NIR OLEDs can be used as illumination and signaling
sources for night vision and friend-foe identification, and have advantages in light weight, low
thermal signature, low power consumption, and compatibility with large area and flexible
substrates. Existing NIR OLEDs have been mostly based on lanthanide-based organometallic
complexes, which generally have low external quantum efficiencies (EQE) (<0.1%). Only
recently were high efficiency NIR OLEDs reported, in which peak emission at λ≈770 nm and a
maximum EQE up to 8.5% were achieved using a phosphorescent Pt-porphyrin complex.1
Nonetheless, alternative materials and devices that exhibit longer emission wavelengths (λ>800
nm) with high efficiencies are still needed. Here we report fluorescent and phosphorescent NIR
OLEDs with peak emission wavelengths up to 892 nm and EQE up to 3.8%. First, OLEDs based
on two NIR-emitting fluorescent donor-acceptor-donor (DAD) oligomers, BEDOT-TQMe2 and
BEDOT-BBT, are demonstrated. In these molecules, the energies of the highest occupied and
lowest unoccupied molecular orbitals are controlled by the donor and the acceptor portion,
respectively. A maximum EQE of ηEQE = 1.6% and a maximum power efficiency of ηP = 7.0
mW/W are achieved in devices based on BEDOT-TQMe2, with the electroluminescence (EL)
peaked at 692 nm but extending to well above 800 nm. BEDOT-BBT based OLEDs show red-
shifted emission peaking at 815 nm (and extending to as far as 950 nm), although the maximum
efficiencies were reduced to ηEQE = 0.51% and ηP = 2.1 mW/W due to the significantly lower
fluorescent quantum yield of the NIR emitter. The efficiencies of these fluorescent OLEDs were
further increased by two to three times by incorporating a phosphorescent sensitizer in the
emissive layer to funnel the triplet excitons formed on the host molecules to the fluorescent
emitters, which are not utilized in fluorescent devices. Using this sensitized fluorescence
structure, we achieved maximum efficiencies of ηEQE = 3.1% and ηP = 12 mW/W for BEDOT-
TQMe2 based devices, and ηEQE = 1.5% and ηP = 4.0 mW/W for BEDOT-BBT based devices.
Finally, a phosphorescent NIR emitter, platinum tetraphenyltetranaphtho[2,3]porphyrin (Pt-
TPTNP), was synthesized. The more extended conjugation on this molecule compared to the Pt-
porphyrin complex used in Ref. 1 leads to a red shift in the emissive wavelength by more than
100 nm. In the Pt-TPTNP-based OLEDs, we obtain peak emission at 892 nm, and maximum
efficiencies of ηEQE = 3.8% and ηP = 19 mW/W, much higher than those of the sensitized
fluorescence OLEDs based on DAD oligomers. 1 Y. Sun et al., Appl. Phys. Lett. 90, 213503
Research Center, Northwestern University, Evanston, Illinois; 5Aspirant at the FWO Vlaanderen,
Brussel, Belgium.
Driven by potential applications of complementary logic, the field of electron-channel (n-type)
organic thin-film transistors (n-OTFT) and corresponding n-type materials has gained a lot of
attention. 5,5'''-diperfluorohexylcarbonyl-2,2':5',2'':5'',2'''-quaterthiophene (DFHCO-4T) is an
example of such a promising electron-conducting organic semiconductor. Recently, high electron
field-effect mobilities were reported for DFHCO-4T.[1,2] While most n-OTFTs need low-
workfunction metal contacts such as Mg, Ca or LiF/Al for efficient electron injection into the
semiconductor lowest unoccupied molecular orbital (LUMO), DFHCO-4T-based n-OTFTs
function properly with high-workfunction source and drain Au contacts.[1] This fact is of high
technological relevance because for use in complementary logic it is preferable to use a single
type of source and drain metal for both the p-type and the n-type OTFTs. Here, we discuss the
optimization of DFHCO-4T growth and compare the performance of DFHCO-4T transistors
with different top contact metals. Thin film growth by high vacuum evaporation of the n-type
organic semiconductor DFHCO-4T on poly-(α-methylstyrene)-coated n++
-Si/SiO2 substrates is
investigated at various deposition fluxes and substrate temperatures. Film characterization by
atomic force microscopy reveals typical Stransky-Krastanov growth. Transistors with Au source-
drain top contacts and optimized DFHCO-4T deposition conditions exhibit an apparent
saturation field-effect mobility of 4.6 cm2/Vs, whereas this parameter is 100x lower for similar
transistors fabricated with LiF/Al or Yb top contacts. We explain this reduced performance of
transistors with easily oxidizable top-contact metals such as Al by the formation of a thin
interfacial layer with poor injection properties resulting from an electron-transfer reaction
between the metal and DFHCO-4T. [1] M. Yoon, C. Kim, A. Facchetti, and T. J. Marks, J. Am.
Chem. Soc. 128, 12851 (2006). [2] M. Yoon, S. A. DiBenedetto, M. T. Russell, A. Facchetti, and
T. J. Marks, Chem. Mater. 19, 4864 (2007).
4:15 PM B7.8 Self-Assembly in Organic LEDs and FETs Simon Mathijssen
1,2, Edsger Smits
3,2, Paul van
Hal2, Ton van den Biggelaar
2, Monja Kaiser
2, Bert de Boer
3, Sergei Ponomarenko
4, Martijn
Kemerink1, Rene Janssen
1 and Dago de Leeuw
2,3;
1Applied Physics, Eindhoven University of
Technology, Eindhoven, Netherlands; 2Philips Research Laboratories, Eindhoven, Netherlands;
3University of Groningen, Groningen, Netherlands;
4Enikolopov Institute of Synthetic Polymer
Materials of Russian Academy of Sciences, Moscow, Russia.
Self assembly, the autonomous organisation of components into patterns and structures without
human intervention is the ultimate technology for mass production of large area electronics. First
we demonstrate SAMFETs, field-effect transistors where the semiconductor is a monolayer
spontaneously formed on the gate dielectric. In order to form a conducting path in between the
source and drain electrode, the molecules in the self-assembled monolayer (SAM) should be
intimately connected. Any structural imperfection as voids or grain boundaries leads to potential
barriers and, hence, to a deteriorated charge carrier mobility. In addition the nature of the
electrical contact is crucial. Formation of an effective injecting electrode to a single layer of
molecules has proven to be a historical challenge. Here we show that the SAMFET consists of a
semiconducting monolayer. The electrical connectivity between the molecules is inferred from
electrical transport measurements as well as the local surface potential as determined from
scanning Kelvin probe microscopy (SKPM) measurements. To elucidate the efficient injection of
charge carriers in the SAM, the critical region where the SAM meets the edges of the electrode
was imaged with transmission electron microscopy. Morphological studies and SKPM
measurements substantiate that the prerequisites for efficient charge transport in a field-effect
transistor are fulfilled; we observe long-range connectivity together with an intimate contact
between the semiconductor and electrodes. The small parameter variation between transistors
allowed integration into a functional 15-bit code generator. Furthermore we applied self-
assembly to produce patterned OLEDs with micro-contact printed self-assembled monolayers.
Here SAMs with opposite dipole moments change the local work function and hence the
injection, which results in a patterned light emission. The local work function is analyzed using
scanning Kelvin probe microscopy. The scanning probe measurements together with optical
micrograph images of the patterned OLEDs demonstrate a direct correlation between the local
work function and light emission. In summary, we show the incorporation of SAMs in organic
field-effect transistors and light-emitting diodes. In the first case, the monolayer is acting as the
semiconductor, in the latter as an injection modifier.
4:30 PM B7.9 An Organic-nanoparticle Transistor Behaving as a Spiking Synapse.Dominique Vuillaume
1,
Fabien Alibart1, David Guerin
1, Stephane Pleutin
1, Kamal Lmimouni
1, Christophe Novembre
2
and Christian Gamrat2;
1IEMN-CNRS, Villeneuve d'Ascq, France;
2CEA-LIST, Gif-sur-Yvette,
France.
We demonstrate that an organic transistor, made of metal nanoparticles (NP) embedded into an
organic semiconductor channel, behaves as a spiking synapse (hereafter called an organic-
nanoparticle transistor-synapse - ONTS for short). We demonstrate that this ONTS device
exhibits the main behavior of a biological synapse. For instance, the ONTS can be programmed
to work as a facilitating or depressing synapse; it exhibits short-term plasticity as well as spike
timing dependent plasticity. This behavior is obtained by virtue of the combination of two
properties of the ONTS: the transconductance gain of the transistor and the memory effect due to
charges stored in the NP. We previously demonstrated that this type of device works as a non-
volatile memory (1) but with a “leaky” behavior. This behavior is used here to implement the
synapic weight wij with a possible dynamic behavior, a mandatory condition to obtain the
training/learning of a spiking neural network (2). The gold NP are immobilized into the source-
drain channel by using surface chemistry (self-assembled monolayers) and they were
subsequently covered by a thin film of pentacene. In a biological synapse, the facilitating
behavior means that an incoming signal with a given frequency and duty cycle induces a post-
synaptic signal having an increasing trend, whereas in the case of an depressing synapse, the
post-synaptic signal tends to decrease (3). This behavior is exactly what we demonstrated for the
ONTS. Our results also compare qualitatively well with a simulation model (4). (1) C.
Novembre, D. Guérin, K. Lmimouni, C. Gamrat, D. Vuillaume, Appl. Phys. Lett. 92, 103314
(2008). (2) S. Haykin, Neural networks. A comprehensive foundation., (Macmillian, NewYork,
1994). (3) H. Markram, Y. Wang, M. Tsodyks, Proc. Natl. Acad. Sci. USA 95, 5323 (1998). (4)
M. Tsodyks, K. Pawelzik, H. Markram, Neural Computation 10, 821 (1998)
4:45 PM B7.10 Light-emitting Ambipolar Field-effect Transistors using Organic Single Crystals. Hajime
Nakantani and Chihaya Adachi; Center for Future Chemistry, Kyushu University, Fukuoka,
Japan.
An ambipolar light-emitting organic field-effect transistor (LE-OFET) based on a 1,4-Bis(4-
methylstyryl)benzene (BSP-Me) single crystal was developed. The BSP-Me single crystal has
very high photoluminescence quantum efficiency (Fai(PL)) of 89%, while Fai(PL) of the BSP-
Me vapor-deposited film is limited to a much lower value of 54%. Ambipolar operation with
successive blue electroluminescence from the FETs based on the BSP-Me single crystals was
demonstrated by realizing nearly equal electron and hole mobilities (about 0.005 cm2/Vs) with
asymmetric gold-calcium contacts. Since BSP-Me single crystals can perform light
amplification, the BSP-Me-based ambipolar LE-OFET is a promising candidate for future
electrically driven organic blue-emitting solid-state lasers. We also mention organic/inorganic
hybrid FET structures for aiming efficient carrier recombination.
SESSION B8: Charges & Transport II
Chairs: Hagen Klauk and Nobuo Ueno
Thursday Morning, April 16, 2009
Room 2001 (Moscone West)
8:30 AM *B8.1
The first principles measurement of charge mobility of organic semiconductors with UPS. Satoshi Kera
1, Hiroyuki Yamane
2, Shunsuke Hosoumi
1, Shin-ichi Nagamatsu
1 and Nobuo
Ueno1;
1Graduate School of Advanced Integration Science, Chiba University, Chiba, Japan;
2Institute for Molecular Science, Okazaki, Japan.
Most of the over 86 million registered materials are organic materials, and many of them form
solid by weak intermolecular interaction. Organic semiconductor is the representative of
electronic function brought by the weak intermolecular interaction coupled with individual
molecular characteristics, and has been increasingly studied for device application. As organic
semiconductor films show various „faces‟ depending on molecular packing structure as well as
on individual molecular structure, many mysteries exist to be elusive. A key issue of organic
FET devises is how one can improve the carrier mobility (μ) of organic thin films. As the
electrical conductivity (σ) is given by σ=nqμ, where n is the carrier concentration and q is the
charge of the carrier concerned, one must know principal mechanism which dominates μ, namely
coherent band conduction or incoherent hopping conduction. In relation to organic transistors,
many electrical measurements have been performed to investigate the charge mobility.
Unfortunately, however, most of the works have directed to obtain phenomenological μ from I-V
measurements. The coherent conduction is dominated by the band dispersion with a mean free
path of the carrier much longer than intermolecular distance, and the hopping conduction is
specified by two physical parameters, the transfer integral (t) and the charge reorganization
energy (λ). t is the measure of the intermolecular interaction, and λ is related to charge-vibration
coupling. To go into the mobility, one needs to measure t and λ experimentally. t is given by
observing the energy band dispersion or the energy level splitting in finite molecular stacks, and
λ is obtained by measuring hole- or electron-vibration coupling in organic systems at low
temperature (namely not in gas phase but in a film). UPS can in principle measure these two
targets. At this conference we will present our recent challenges on UPS of the thin films that
give (1) the energy level splitting and the band dispersion, both of which offer the value of t, and
(2) λ obtained directly from the measurement of the HOMO hole-vibration coupling in various
organic semiconductor films. From these we can obtain the ultimate hole mobility for the
coherent band conduction and the hopping conduction.
9:00 AM B8.2
Charge Transport and Microstructure Correlations using Anisotropic Polythiophene Thin
Films Fabricated via Directional Crystallization. Leslie Hendrix Jimison and Alberto H
Salleo; Materials Science and Engineering, Stanford University, Stanford, California.
In recent years much research has been devoted to the improved fabrication and understanding
macroelectronics: a category of devices that include thin film transistors, photovoltaics and light
emitting diodes. Polymeric semiconductors have great potential as the active layer for such large
scale systems, with a key attribute being their ability to be dissolved in solvents to make
printable “semiconducting inks.” While polymer semiconductors are beginning to reach
performance levels that enable them to be competitive in low-cost electronics, what is lacking is
an understanding of the fundamental charge transport processes in relation to microstructure. We
have used a means of controlling the orientation and size of crystallites in the plane of the
substrate to produce films with well-known grain-boundary types. As a result we are able to
explore the relationship between trap density within grain boundaries and charge transport.
Regioregular poly-3-hexylthiophene (P3HT) is the material under investigation. We have
fabricated anisotropic films on glass and silicon substrates via directional solidification, using
1,3,5-trichlorobenzene first as a solvent and then as a substrate for epitaxy. The technique
enables us to create films consisting of large (mm2) domains of uniform extinction under crossed
polarizers, suggesting long range orientation of the polymer chain axis. Using a film lift-off
technique, atomic force microscopy reveals the lamellar microstructure at both the polymer/air
interface and polymer/substrate interface. Film microstructure was further characterized at the
Stanford Synchrotron Radiation Laboratory (SSRL). We have collected 2D diffraction patterns,
specular diffraction patterns, and grazing incidence diffraction patterns, confirming the unique
in-plane and out-of-plane texture of the polymer films. Charge transport in the directionally
crystallized films was probed by measuring temperature-dependent mobilities using thin film
transistors with the oriented film as the active layer. Devices were made with different relative
orientations between the channel and the polymer film. Transport measurements as a function of
charge density and temperature for different orientations of our film confirm mobility anisotropy
but no activation energy anisotropy. Temperature dependent transport measurements also reveal
increased effect of bias stress between 220-280°C. Furthermore, there is marked anisotropy of
the bias stress effect between the different device orientations. Because our films are anisotropic
in the type of grain boundary present, this strongly suggests that bias stress in semicrystalline
organic thin films is dependent on the microstructure at the grain boundaries.
9:15 AM B8.3
Scanning Kelvin Probe Measurements on Pentacene based Field-Effect Transistors with
UV-modified Gate Dielectric Christopher Siol, Niels Benson, Christian Melzer and Heinz von
Seggern; Institute of Material Science, Technische Universitaet Darmstadt, Darmstadt, Germany.
The use of organic field-effect transistors (OFETs) in organic electronics is often hampered by
the fact that solely unipolar logic is implemented. In view of a performance improvement, it is
aspired to use complementary metal oxide semiconductor (CMOS) -like techniques to benefit
from the efficiency of complementary logic circuits. In recent publications we have
demonstrated a CMOS-like inverter based on pentacene n- and p-type OFETs that comprise
identical device layouts thus facilitating the production of CMOS-like elements. Solely the
treatment of the used PMMA gate dielectric with UV light prior to the pentacene deposition
allowed for an inversion of the operation mode from the usual n-type to a unipolar p-type
behaviour. Even though the stable device performance of the thus produced inverter
demonstrates the potential of the proposed UV treatment the exact mechanism allowing for the
polarity change is not known in detail. In this contribution, we try to reveal this mechanism by
discussing the results of Scanning Kelvin Probe Measurements (SKPM) performed in the
channel of UV-treated OFETs in combination with the analysis of the OFET performance. From
the current-voltage characteristics it is supposed that the UV treatment itself results in the
formation of electronic trap states, in particular in electron traps. Since the establishment of the
p-type behaviour requires, besides the UV treatment, a precedent operation of the transistor in
electron accumulation, it is speculated that the electron traps have to be negatively charged.
Indeed, SKPM disclose that the accumulation of negative charge carriers at the PMMA /
pentacene interface results in a distinct and stable trapping of electrons, which proves the
existence of efficient electron traps in UV-modified PMMA. In what way this charging helps to
convert the n-type transistor to a p-type transistor, in particular to inject holes from the employed
low workfunction metal Ca, will be discussed in detail. The stably trapped negative areal charge
density leads to a gate-field enhancement at the contacts. It is supposed that this field
enhancement is sufficient to inject holes from Ca into the channel. Under current flow, the
channel in the vicinity of the source will again partly deplete from the compensating holes
allowing anew for a field-enhanced hole injection from Ca.
9:30 AM B8.4
Trap-Dominated Charge Transport in Organic Transistors as Investigated by Field-
Induced ESR Spectroscopy. Hiroyuki Matsui1,2
and Tatsuo Hasegawa1;
1PRI, AIST, Tsukuba,
Japan; 2Department of Advanced Materials Science, University of Tokyo, Tokyo, Japan.
Recently we reported that motional narrowing effect can be observed in field-induced electron
spin resonance (ESR) spectra of high-mobility pentacene thin-film transistors (TFTs). We found
that the methods are quite useful in elucidating the carrier dynamics in organic TFTs since the
narrowed linewidth allows us to estimate the average residence time of carriers at respective
sites. In particular the analyses afford a clear evidence for the considerably long trap residence
time or the trap-dominated conduction in pentacene TFTs. Meanwhile, motional narrowing
effect is directly evidenced by temperature-dependent and gate-field-dependent single-
Lorentzian ESR spectra. However, the temperature-dependent feature deviates from the simple
motional narrowing regime in high (T > 200 K) and low (T < 50 K) temperature ranges. Here we
discuss the whole picture of field-induced ESR spectra in the temperature range of 300 - 20 K on
the basis of continuous wave saturation experiments. The method enables us to check the
homogeneity of ESR spectra as well as to estimate spin-lattice relaxation time T1. It is found
from the temperature dependence of ESR linewidth that the linewidth increases as temperature
decreases with activation energy of about 15 meV at 50 K < T < 200 K, while it deviates from
the feature at higher and lower temperature ranges. First we examined the saturation behavior of
ESR signals, which allows us to separate the contribution of spin-lattice relaxation from
motionally-narrowed inhomogeneous linewidth. The motionally-narrowed and the spin-lattice
relaxation components compete with each other at around 200 K. From this we conclude that the
small increase of linewidth at higher than 200 K with increasing tempeature should be attributed
to the spin-lattice relaxation. At the temperature lower than 50 K, on the other hand, the
linewidth tends to converge at about 0.18 mT. The ESR spectra do not show broadening under
saturation at high microwave power, demonstrating the inhomogeneity of the ESR absorption. It
means that motional narrowing is no longer effective in the low temperature range because of
long residence time at trap states. Transport and localization of field-induced carriers will be
discussed on the basis of these experimental results.
9:45 AM B8.5 Electrochemical Transistors: New Platforms to Study Interfaces in Liquids. Fabio Cicoira
1,2,
Sang Yang Yoon1, DeFranco A John
1 and George G Malliaras
1;
1MSE, Cornell UNiversity,
Ithaca, New York; 2Institute of Photonics and Nanotechnology, CNR, Trento, Italy.
The considerable research efforts in organic electronics have led to the development of a number
of devices like organic light emitting diodes, solar cells and organic thin film transistors that are
nowadays in production or prototype stage. Along with these well-established fields, exciting
emerging applications are taking advantage of the mixed ionic/electronic transport in organic
electronics devices [1]. Along this line, the application of organic semiconductor devices to
chemical and biological sensors seems to be a great fit. A promising approach towards organic-
based sensors involves the use of organic electrochemical transistors (OECTs). OECTs consist of
source and drain electrodes, and a channel containing the organic active material in ionic contact
with a gate electrode via an electrolyte solution. These devices can be operated in aqueous
environment as efficient ion-to-electron converters, thus providing an interface between the
worlds of biology and electronics and also a unique platform for the study of organic/organic and
organic/metal interfaces in liquids. Although electrochemical transistors have been known since
long time [3], they received little attention in the scientific community until the recent resurgence
due to their application in biosensors. Therefore a great deal of work is needed to understand the
fundamental processes that take place in these devices, essential for their use in sensing
applications. In this presentation we intend to address this important issue. Using
photolithography, surface engineering and micro fluidics we have developed several technique to
fabricate OECTs having different geometries. This allows us to study the basic electronic
properties and the sensing response of devices in order to understand their mechanism of
operation [4] [5]. We studied how the dimensions of the transistors (in particular on the
gate/channel area ratio) and the gate electrode material (metal or polymer) can be used to tune
the device response. The effect of the electrolyte on device response was evaluated studying
transistors in aqueous electrolytes and ionic liquids. The detection limit of OECTs based sensors
having different geometry, was analyzed for glucose and hydrogen peroxide (a species involved
in glucose sensing). [1] J. M. Leger, Adv. Mater. 2008, 20, 837. [2] M. J. Panzer, C. D. Frisbie,
Adv. Mater. 2008, 20, 3177. [3] D. Vanmaekelberg, A. J. Houtepen, J. J. Kelly, Electrochem.
Acta 2007, 53, 1140. [4] D. A. Bernards, G. G. Malliaras, Adv. Funct. Mater. 2007, 17, 3538. [5]
D.A. Bernards, G. G., Malliaras, D. J. Macaya, M. Nikolu, J. A. DeFranco, S. Takamatsu, G. G.
Malliaras, J. Mater. Chem. 2008, 18, 116.
10:30 AM *B8.6 Electronic Structure of Organic Heterointerfaces. Henning Sirringhaus, University of
Cambridge, Cambridge, United Kingdom.
Charge injection at metal-semiconductor interfaces as well as transport in the accumulation layer
of organic FETs is critically determined by the electronic structure at these interfaces. In this
presentation we will review our current understanding of the factors that determine the molecular
structure, energetic disorder and polaronic relaxation processes at interfaces and discuss recent
experiments that have yielded information about such processes with high interfacial sensitivity.
A thorough understanding of interfacial electronic structure is needed in order to identify the
factors that limit performance of state-of-the-art organic FETs.
11:00 AM B8.7
Morphology and Energy Levels in Conjugated Polymers: A Theoretical View on P3HT. Georg Heimel and Juergen P Rabe; Insitut für Physik, Humboldt-Universität zu Berlin, Berlin,
Germany.
In the field of organic electronics, π-conjugated polymers bear great promise for solution-
processible, flexible applications such as organic light-emitting devices (OLEDs), organic field-
effect transistors (OFETs), or organic photovoltaics. In addition to the bulk material properties of
the active organic part, the interfaces between organic and inorganic components are well
acknowledged to be of paramount importance for device performance and functionality. Most
importantly, the energetic position of the conducting states in the organic material crucially
impacts the energy barriers for charge injection into the device. These barriers not only limit the
overall injection rates but also give rise to often undesirably high onset voltages below which the
device remains inactive. Preparation conditions are know to crucially impact the thin-film
morphology of the prototypical π-conjugated polymer region-regular poly(3-hexylthiophene)
and, consequently, also the device characteristics of, e.g., OFETs based on this material (rr-
P3HT). In such devices, it has been shown that an edge-on configuration of highly ordered
polymer chains favorably impacts transistor performance while a face-on morphology proved
detrimental; in the former, the preferential direction for charge transport in these systems, the π-
stacking direction in co-facial polymer chains, is aligned with the direction of current flow in an
OFET device geometry. In our contribution, we present density-functional theory (DFT) based
band-structure calculations on highly ordered rr-P3HT monolayers of different morphology, i.e.,
edge-on, face-on, and intermediate regimes. Supported by electrostatic modeling, we find that
the backbone orientation importantly influences the energy-level positions in these thin films due
to intra-molecular surface dipoles. As a consequence, also the hole-injection barriers (HIB) can
be expected to depend critically on thin-film morphology. Our calculations suggest that the HIB
into rr-P3Ht films with edge-on morphology can be up to 0.5 eV lower than into films of face-on
orientation. In addition to underlining the importance of morphology control in polymer-based
organic electronic devices, understanding the impact of intra-molecular surface dipoles also
paves the way towards novel strategies for material design. To that end, we extend our
investigations towards rr-P3HT with end-fluorinated alkyl side-chains. Our calculations reveal
that, due to the strongly negative surface termination in such films, the energy levels in the
polymer could be lowered by as much as 1.5 eV in the edge-on configuration compared to face-
on. This scenario would lead to significantly reduced electron injection-barriers and, thus, favor
device operation involving negative charge-carriers. Consequently, our results imply that such
materials, together with improved control over morphology and electron traps, could potentially
serve as active organic components in n-type polymer OFETs.
11:15 AM B8.8 Influence of Contact Effects on the Switching Behavior of Organic Transistors Arne
Hoppe1, Dietmar Knipp
2, Benedikt Gburek
1, Marko Marinkovic
2 and Veit Wagner
1;
1Molecular
and Nanoelectronics Laboratory, Jacobs University Bremen, Bremen, Germany; 2Electronic
Devices and Nanophotonics Laboratory, Jacobs University Bremen, Bremen, Germany.
The influence of contact effects on the switching behavior of organic transistors was studied.
High switching frequencies can be achieved by using short channel transistors with high charge
carrier mobilities. However, most of the short channel transistors exhibit a distinct drop of the
device charge carrier mobility. The reduced device mobility is caused by the influence of contact
effects. In this study the influence of the drain/source contacts and the device geometry on the
switching frequency of high mobility dihexyl-7-thiophene (DH7T) oligothiophene thin film
transistors was investigated. The transistors were realized with channel lengths ranging from 50
nm to 50 μm. The transistors exhibit high charge charier mobilities of 0.1 cm2/Vs and high
switching frequencies between 200 kHz and 2 MHz at low operating voltages of 5 V. The
maximum of the switching frequency is limited by the specific contact resistance and the overlap
capacitance between drain/source and gate electrodes. The normalized contact resistance of the
oligothiophene transistor was determined to be 3.5 kΩcm. An upper limit of the switching
frequency for organic thin film transistors was derived, which is determined by the specific
contact resistance between the drain and source electrodes and the organic channel material. The
charge carrier mobility does not affect the upper limit of the switching frequency. Different
strategies will be discussed to maximize the cut-off frequency.
11:30 AM B8.9
Polymer-Small Molecule Semiconductor Blends for Integrated Circuits with a 712 ns
Single Stage Delay. Jeremy Smith1, Richard Hamilton
2, Donal D Bradley
1, Iain McCulloch
2,
Martin Heeney3, Dago de Leeuw
4, John E Anthony
5 and Thomas D Anthopoulos
1;
1Physics,
Imperial College London, London, United Kingdom; 2Chemistry, Imperial College London,
London, United Kingdom; 3Materials, Queen Mary University of London, London, United
Self-assembled monolayers (SAMs) provide a simple, flexible, highly ordered and convenient
system to tailor and functionalize surface and interface properties of metals, metal oxides and
semiconductors. In particular, SAMs of π-conjugated organic molecules have attracted
significant interest in the field of molecular and organic electronics because of their considerable
conductivity and there ability to change the substrate work function. We performed density-
functional theory (DFT) based slab-type band-structure calculations - including geometry
optimization in internal coordinates - to gain deeper insight into the energetic, chemical, and
physical properties of the interface between a metallic substrate and a covalently bound organic
semiconductor. In particular, we studied SAMs of anthracene-2-selenol on Au(111), which have
been characterized in detail with various experimental methods including Scanning Tunneling
Microscopy (STM), Ultraviolet Photoemission Spectroscopy (UPS), and Low Energy Electron
Diffraction (LEED) [1]. Only the combination of experiments and theoretical calculations allows
an accurate understanding of the structural ordering of the SAM on the surface. After identifying
the correct structure, the electronic properties (such as the work-function modification, the
interfacial charge rearrangements and the energy-level alignment) can be provided theoretically.
The obtained values agree very well with the experimental data which, in turn, allow a
benchmarking of the employed theoretical methods. [1] A. Bashir, D. Käfer, J. Müller, C. Wöll,
A. Terfort, G. Witte, Angew. Chem. Int. Ed. 2008, 47, 5250-5252
B10.17
Turning Gold into Lead - Reducing the Work Function by Charge Transfer Monolayers. Oliver T. Hofmann, Gerold M Rangger, Ferdinand Rissner and Egbert Zojer; Institut für
Festkörperphysik, Technical University of Graz, Graz, Austria.
Application of (sub)monolayers of organic molecules on metal electrodes in order to tune the
effective work function has become a field of significant interest. Particular electron poor or rich
molecules grow charge transfer layers, which form an infinite dipole layer across the electrode,
thus altering the vacuum potential above the surface and hence its effective work function. In this
contribution, we investigate the interaction between the electron donor viologen and an Au(111)
surface. Depending on the coverage of the monolayer, the work function of the metal can be
theoretically lowered to the value of pristine lead (ca. 4.0 eV) or even that of Mg (ca. 3.7 eV).
Using density functional theory, an in-depth analysis of the electronic structure at the interface is
presented and compared to that of the prototypical donor tetrathiafulvalene (TTF). The work
function modification in both systems is found to be determined by a subtle interplay between
adsorption induced geometric distortions and electron donation from the respective molecular
HOMO to the metal. Unlike carbon monoxide or strong electron acceptors, such as F4TCNQ, on
noble metal surfaces, no significant back donation from the metal into the molecule is observed.
Furthermore, we show that work function modification can also be tuned using spacer elements,
e.g. in the form of voluminous groups enforcing a larger distance between molecule and metal.
With increasing distance the transferred charge decreases. Interestingly, we find that the net
interface dipole increases with increasing distance as will be shown here on the example of
viologen. This work has been supported by the EC through the ICONTROL project.
B10.18
Controllable Shifts in Threshold Voltage of Top-Gate Polymer Field-Effect Transistors and
its Application to Organic Transistor Memory Kang-Jun Baeg1,3
, Yong-Young Noh3,
Henning Sirringhaus2 and Dong-Yu Kim
1;
1Dept. of Materials Science and Engineering,
Gwangju Institute of Science and Technology, Gwangju, Korea, South; 2Department of Physics,
University of Cambridge, Cambridge, United Kingdom; 3Convergence Components and
Materials Research Laboratory, Electronics and Telecommunications Research Institute,
Daejeon, Korea, South.
Organic materials are attractive for many components of electronic devices such as active
semiconductor layers, insulator layers, and electrodes, due to a lot of unique advantages over
their inorganic counterparts. Although the organic materials are not currently suitable for
applications requiring high-end performances, their low-cost and low-temperature fabrication
using solution-based processing make them ideal for large-area, flexible, transparent, and
disposable applications. Moreover, organic non-volatile memories are another emerging class of
research fields based on the advantages of organic materials. A variety of approaches have been
progressed including cross-point type organic bistable devices and organic transistor-based
memories. Here we report a solution-processed polymer FET memory device with top-gate and
bottom-contact device configuration. With incorporation of gold nanoparticles (NPs) inside
double-layered polymer gate dielectrics, the threshold voltage (VTh) of polymer FET devices
could be reversibly and systemically controlled by application of external gate fields. This
reversible shifts in VTh was originated from charge trapping in Au NPs, and might be used as
organic transistor based non-volatile memory devices.
B10.19
Ion Irradiation Effects on The Transport Properties and Degradation Mechanisms of
Organic Field-Effect Transistors. Beatrice Fraboni1, Anna Cavallini
1, Piero Cosseddu
2,
Annalisa Bonfiglio2, Yongquiang Wang
3 and Michael Nastasi
3;
1Physics, University of Bologna,
Bologna, Italy; 2Electronic Engineering, University of Cagliari, Cagliari, Italy;
3Los Alamos
National Laboratory, Los Alamos, New Mexico.
The remarkable advances recently made in the development of organic semiconductor field
effect devices (OFET) prospect challenging applications in the field of low-cost flexible,
lightweight, and conformal electronics One of the interesting features of organic active layers is
their capability to respond to the environment chemical composition, but this unfortunately
opens up the issue of long-term stability of devices based on organic materials, as oxidation is
believed to be a major reason for early device failure. The focus of our research is to investigate
the potentiality of low energy ion irradiation in the reduction and control of the degradation of
the organic material due to the exposure to atmosphere (i.e. oxygen and water). We have studied
the effects of N irradiation on pentacene and sexithiophene based OFETs. The damage induced
by the ions can induce a rearrangement of the molecular alignment by breaking covalent bonds,
crosslinking the neighboring polymer chains and forming a hydrogen-depleted three dimensional
carbon network The strong molecular structure modification affects the carrier mobility and the
threshold voltage of the device, but since the electrical transport in OFETs occurs in a few active
molecular layers at the organic/dielectric interface, we have observed that a controlled damage
depth distribution preserves the functionality of the organic active layer. We have studied the
variation of the transport parameters as a function of the irradiation energy and dose by
characterizing the optical and electrical properties of the material by means of electrical transport
analyses and photocurrent spectroscopy. We have monitored the effectiveness of the low energy
irradiation process in providing an hermetic protection to the organic active layer from the
ambient conditions.
B10.20 PN-junction Diodes made of p-type Pentacene and n-type SnO2 Nanowires. Sung Chan
Park1, Daeil Kim
1, Seongmin Yee
2, Junghwan Huh
2, Gyu Tae Kim
2 and Jeong Sook Ha
1;
1Chemical and Biological Engineering, Korea University, Seoul, Korea, South;
2School of
Electrical Engineering, Korea University, Seoul, Korea, South.
PN-junctions are of great importance in modern electronic applications for achieving the
integrated circuits and understanding the device characteristics. In our presentation, we will show
the rectifying current-voltage curves and the temperature dependent diode characteristics. Hybrid
pn-junction devices consisted of p-type organic semiconductor (pentacene) and n-type inorganic
semiconductor (SnO2 nanowires). SnO2 nanowires can be easily synthesized or grown by VLS
mechanism in CVD, showing n-type semiconducting properties with a high mobility. SnO2
nanowire networks were formed by a selective growth on the Au catalyst with various growth
conditions. Onto the SnO2 nanowire networks, pentacene was deposited by thermal evaporation
with various thicknesses under 7×10^-7 Torr. Ti (20 nm) or Cr (20 nm)/Au (300 nm) electrode
and the 300 nm thickness of Au electrode were deposited on the SnO2 channels and pentacene
channels for Ohmic contacts, respectively, which was confirmed by Ohmic current-voltage
characteristics. Microscopic and chemical analyses were investigated for characterizing each
component of the hybrid pn-junctions. As the temperature decreased, the current levels reduced
following the diode equation of I=I0(exp(ηkT)-1) with the big ideality factor reaching 420,
which indicate the big surface states at the junction parts. Considering the large ideality factor,
the soft-reverse characteristics were not so significant with a good on/off ratio of 10^3 at ±60V.
The possible application of organic/inorganic hybrid pn-diodes will be discussed from the point
of the solar-cell or photo-detectors.
B10.21
Synthesis and Characterization of High Efficiency Copolymers via Effective Energy-
Transfer for Polymer Light-Emitting Diodes (PLEDs) Qinghua Zhao1, Shuang Zhang
1, Jong-
Won Park1, Sung Ouk Jung
1, Yun-Hi Kim
2 and Soon-Ki Kwon
1;
1school of materials science and
engineering, engineering research institute (ERI),, gyeongsang national university, Jin ju, Korea,
South; 2department of chemistry, gyeongsang national university, jin ju, Korea, South.
Polymer light-emitting diodes (PLEDs) have been the subject of intense research interests
recently due to their applications in large-area flat panel displays.1 To achieve highly efficient
PLED devices, charge (holes and electrons) injection and transport from both the anode and the
cathode should be balanced at the junction of the emitting layer to yield the maximum exciton
formation.2 Although some polymer LEDs have shown high-enough efficiencies and long
lifetimes, they are mainly multilayer LEDs, which involve complicated and difficult device
fabrication processes, or single layer LEDs based on polymer blends. So, it is important to
achieve conjugated polymers which have both functions of hole/electron affinity in the single
layer LEDs. The bipolar transport characteristics of polymers are important for high EL
efficiency, because the polymers offer good recombination sites for hole and electron charge
carriers.3 In this communication, we designed and synthesized a series of polymers with hole
transporting and electron transporting ability. Their photophysical and thermal properties would
be investigated and further compared. The PLED devices would be fabricated and discussed
further. These polymers are expected to obtain high efficient from PLEDs due to effective
energy transfer from large bandgap site to narrow bandgap site. [1] J. H. Burroughes, D. D. C.
Bradley, A. R Brown, R. N. Marks, K Mackay, R. H. Friend, P. L Burn, A. B. Holmes, Nature
1990, 347, 539. [2] F. Garten, A Hilberer, F. Cacialli, E. Essenlink, Y. van Dam, B. Schlatmann,
R. H. Friend, T. M. Klapwijk, G.. Hadziioannou, Adv Mater 1997, 9, 127. [3] N. Tamoto, C.
Adachi, K. Nagai, Chem. Mater. 1997, 9, 1077.
B10.22
Abstract Withdrawn
B10.23
Para-sexiphenyl Based OLED Devices Grown on a Pre-patterned Polymeric Substrate. Gerardo Hernandez-Sosa
1, Clemens Simbrunner
1, Thomas Hoefler
2, Armin Moser
3, Oliver
Werzer3, Birgit Kunert
3, Gregor Trimmel
2, Wolfgang Kern
4, Roland Resel
3 and Helmut Sitter
1;
1Institute for Semiconductors and Solid State Physics, Johannes Kepler University Linz, Linz,
Austria; 2Institute for Chemistry and Technology of Materials, Graz University of Technology,
Graz, Austria; 3Institute of Solid State Physics, Graz University of Technology, Graz, Austria;
4Institute of Chemistry of Polymeric Materials, Montanuniversität Leoben, Leoben, Austria.
During the last years organic devices became of increasing interest in many fields of electronics.
A bright future for organic light emitting devices (OLED) is expected as organics provide a wide
spectrum of molecules emitting at various photon energies [1]. Para-sexiphenyl represents an
organic molecule which has been established as active material for blue emitting OLEDs [2, 3].
As a matter of fact, a defined control of the substrate surface properties is a good way to improve
the quality of organic films. Surface properties like polarity, hydrophilicity or hydrophobicity
can be tuned to change the way the film is growing. These changes can lead to an improvement
on the intrinsic properties of the film and consequently have an impact on the performance of the
final device. Para-sexiphenyl (PSP) has been deposited by Hot Wall Epitaxy (HWE) on
poly(diphenyl bicyclo(2.2.1)hept-5-ene-2,3-dicarboxylate) (PPNB), a photosensitive polymer,
which has been pre-treated by UV illumination leading to a change of the surface polarity [4]. A
detailed analysis of the growth morphology as a function of substrate temperature, growth time
and UV illumination of the substrate has been performed by Atomic Force Microscopy. The
surface morphology and growth kinetics of PSP were found to be different depending on which
surface it was deposited, as prepared or UV exposed. Furthermore, the crystallographic
properties has been analysed by X-ray diffraction and Grazing Incidence X-ray Diffraction
(GIXD). The crystal structure and the degree of order of the deposited PSP thin films were
determined by specular scans and reciprocal space maps. Again a clear change on the structural
properties between the films deposited on the as prepared and pre-treated surfaces was observed.
Besides the film characterization we also report on OLEDs based on Para-sexiphenyl (PSP),
which has been deposited by HWE on a UV pre-treated ITO/PEDOT:PSS/PPNB substrate. A
complete quenching of the PSP electroluminescence is observed on the UV pre- illuminated
regions while on the as-prepared one the characteristic PSP emission is observed. Consequently,
this method has proven to be a very effective pre-patterning tool which could be fully compatible
with the actual OLED production technology. [1] M. Muccini, Nature materials 5 (2006), 605 [2]
G. Kranzelbinder et. al.,Synthetic Metals 102 (1999), 1073-1074 [3] A. Niko et. al., J. Appl.
Phys. 82 (1997), 4177 [4] T. Höfler et al., Polymer 48 (2007), 1930-1939
B10.24
Chemical and Electronic Properties of Self-assembled Organic Monolayers on SiC
Surfaces. I. D. Sharp, M. Hoeb, S. J Schoell, C. Diaz Alvarez, J. Howgate, M. S Brandt and M.
Stutzmann; Walter Schottky Institut, Technische Universität München, 85748 Garching,
Germany.
Wide bandgap materials are useful systems for the study of electronic processes at the organic-
inorganic hybrid interface due to the large energetic tunability of the semiconductor Fermi level.
Here, the group IV compound semiconductor silicon carbide (SiC) is used as a model substrate
for organic monolayer self-assembly by reaction with organosilanes and alkenes on both C- and
Si-polar surfaces. Comparison of the properties of both types of films on surfaces of both
polarities reveals the important role of interfacial chemistry and the related binding dipole on the
electronic properties at the organic-inorganic interface. In the well-known case of Si, organic
functionalization is typically achieved via hydrosilylation of H-terminated surfaces or
silanization of an intermediate OH-terminated thin oxide. In contrast, SiC surfaces are OH-
terminated following HF etching and are thus ideally suited for silanization reactions. We
demonstrate that reaction of these surfaces with octadecyltrimethoxysilane (ODTMS) yields
stable, high-quality organic monolayers directly bound to the semiconductor surface without an
oxide interlayer. Furthermore, we show by x-ray photoelectron spectroscopy (XPS) and Fourier
transform infrared (FTIR) spectroscopy that reaction of OH-terminated SiC with 1-octadecene
also yields high quality monolayers but occurs over a bridging oxygen atom that is not present in
the case of hydrosilylated Si. Spectrally resolved photocurrent measurements on functionalized
surfaces reveal significant conductivity enhancements relative to oxidized surfaces, indicating a
reduction of interfacial defect concentrations. These enhancements are particularly pronounced
on C-polar SiC and are greater on alkene-reacted surfaces than on silanized surfaces. Transport
and impedance measurements with a mercury-drop-contact across monolayer-SiC structures
yield thermionic emission barriers and flat band potentials that are consistent with large dipolar
contributions to interfacial band alignment and that depend on the specific chemical binding of
the organic monolayers. Patterned arrays of covalently grafted, fluorescence-labeled proteins
onto SiC were fabricated using both functionalization routes, thus further demonstrating the
potential of the stable, biocompatible SiC surface in future biosensor and protein-integrated
electronics applications.
B10.25 Stability Issues of Organic Thin-Film Transistors. Andreas Klug
Austria; 2Christian Doppler Laboratory Advanced Functional Materials, Institute of Solid State
Physics, Graz University of Technology and Institute of Nanostructured Materials and Photonics,
Joanneum Research, Graz/Weiz, Austria; 3Macromolecular Chemistry, University of Wuppertal,
Wuppertal, Germany.
For more than two decades organic thin-film transistors (OTFTs) have been investigated, being
key devices for large-area, low-cost electronics fabricated with cheap and easy processing
techniques such as spin-coating, inkjet-printing or soft lithography. Essential properties of the
applied organic semiconductors include solution-processability, high field-effect mobility,
compatibility with adjacent layers and - not to forget - stability with respect to ambient
conditions. Here we report on the ambient, operational and shelf-life stability of OTFTs with a
polytriphenylamine-(PTPA)-based polymer active material. The results are benchmarked against
the well-established transistor polymer poly(3-hexylthiophene) (P3HT), yielding comparable
mobility values around 10-4
cm2/Vs. However, upon air exposure P3HT-based devices exhibit
switch-on voltage shifts of more than 30 V and a distinct off-current increase due to
oxygen/moisture-induced doping. Stable air operation therefore involves expensive device
encapsulation or a top-gate architecture, where P3HT is shielded by an appropriate dielectric
material. The corresponding device parameters of PTPA-based OTFTs, on the contrary, remain
rather stable and make device encapsulation obsolete. Moreover, we show that PTPA-based
devices with polyvinylalcohol as dielectric yield improved operational stability and we present
flexible OTFTs based on the two active semiconductor materials.
B10.26
Para-sexiphenyl-CdSe Nanocrystals Hybrid Light Emitting Diodes with Optimized Layer
Thickness and Interfaces. Clemens Simbrunner1, Gerardo Hernandez-Sosa
1, Eugen
Baumgartner1, Juergen Roither
1, Guenter Hesser
2, Wolfgang Heiss
1 and Helmut Sitter
1;
1Institute
of Semiconductors and Solid State Physics, Johannes Kepler University Linz, Linz, Austria; 2ZONA (Zentrum f. Oberflächen- und Nanoanalytik), Johannes Kepler University Linz, Linz,
Austria.
In the variety of molecules used for Organic Light Emitting Diode (OLED) fabrication Para-
sexiphenyl (PSP) represents a well known candidate for the fabrication of high photon energy
emitting devices [1,2]. The high energy gap (3.1 eV) of PSP and consequently its blue
electroluminescence (EL) emission recommends PSP as a component for multi-color OLED
displays and white LEDs. Recent publications promote solution-based semiconducting
nanocrystals or quantum-dots (QD) integrated within the OLED structure leading to a high
efficient electroluminescence emission of organic-inorganic hybrid devices [3-7]. This approach
requires a very good control of the layer thickness and very well defined interface properties of
the multilayer to obtain a good carrier injection and an efficient electroluminescence (EL). In this
contribution we report on the optimization of the interface and thickness between PSP and CdSe
nanocrystals as well as on the role of PSP as electron transport layer on Hybrid-LEDs. The
devices, emitting at 422nm, 549nm and 610 nm, were fabricated by spin coating the CdSe NC
from solution onto (ITO/PEDOT:PSS) anode substrates and subsequently evaporating PSP under
vacuum conditions. Aluminum contacts provide electron injection in the device structure. The
optimal layer thickness and well defined interface was determined by cross sectional scanning
electron microscopy (SEM), while the carrier injection was studied by monitoring the EL
intensity as a function of emission wavelength and applied device current. The optimized values
were found to be less than 25 nm for the PSP layer and 2 monolayers for the CdSe nanocrystals,
resulting in low onset voltages (3-4 V). Homogeneous layers are achieved across the whole
device which is verified by the resulting homogeneous light emission. The obtained spectra of
the optimized devices indicate high color purity, mainly determined by well defined emission
lines, their low FWHM and the absence of parasitic emissions. Consequently the excellent
device properties achieved by the optimization promote the demonstrated material system for a
future candidate in the fabrication of QD-PSP based LED for displays and lighting applications
[8]. [1] S. Tasch, C. Brandstätter, F. Meghdadi, G. Leising, G. Froyer, L. Althoel, Adv.Mat. 9
(1), 33 (1997) [2] G. Kranzelbinder, F. Meghdadi, S. Tasch, G. Leising, L. Fasoli, M. Sampietro,
Syn. Metals 102, 1073 (1999) [3] S. Coe, W. Woo, M. Bawendi, V. Bulovic, Nature 420, 800
(2002) [4] J. Zhao, J. Bardecker, A. Munro, M. Liu, Y. Niu, I. Ding, J. Luo, B. Chen, A. Jen, D.
Ginger, Nano Lett. 6 (3), 463 (2006) [5] Q. Sun, Y. Wang, L. Li, D. Wang, T. Zhu, J. Xu, C.
Yang, Y. Li, Nature Photonics 1, 717 (2007)
B10.27 Patterning of Organic Monolayers on GaN via UV-induced Charge Transfer. S. J Schoell, J.
Howgate, I. D. Sharp, W. Steins, M. S Brandt, M. Eickhoff and M. Stutzmann; Walter Schottky
Institut, Technische Universität München, Garching, Germany.
Understanding charge transfer processes and their mechanisms at organic-inorganic interfaces is
of fundamental importance in the emerging field of organic electronics. Wide bandgap
semiconductors are particularly useful for studying interfacial electronic processes since the bulk
Fermi level can be systematically varied over a wide energy window. Here, the affinity of GaN
to form OH-terminated surfaces by wet chemical treatments is exploited to generate functional
organosilane monolayers. In particular, wet-chemically processed layers of
octadecyltrimethoxysilane (ODTMS) and aminopropyl-triethoxysilane (APTES) on n-type as
well as on p-type Ga-face GaN were formed. For comparative purposes, identical layers were
also formed on n- and p-type 6H-SiC surfaces. The structural and chemical properties of these
layers were studied by static water contact angle measurements, thermal desorption spectroscopy
(TDS), X-ray photoelectron spectroscopy (XPS), Fourier transform IR spectroscopy (FTIR), and
X-ray reflectivity measurements. The organic layers are smooth and change their wetting
properties depending on the molecules used. Thermal desorption temperatures in the range of
500°C indicate covalent bonding of the organic molecules to the GaN surfaces. UV-induced
charge transfer between the semiconductor substrate and organic monolayers was studied after
irradiation with a low pressure Hg lamp at a wavelength of 253.7 nm. On n-type GaN this results
in a decreased water contact angle and reduced thickness of the organic layer, saturating at an
illumination time of 30 min. Furthermore, analysis via capacitance-voltage and current-voltage
measurements revealed that this effect is accompanied by the accumulation of trapped charge
within the organic layer. XPS data prove a decreased hydrocarbon signal on UV-irradiated n-
type GaN and FTIR shows a complete loss of terminating endgroups. In contrast, the wetting
behavior, as well as the XPS and FTIR results, of silanized p-type GaN, as well as n- and p-type
6H-SiC, are largely unaffected by UV irradiation, suggesting a photoinduced charge transfer
through the organic/semiconductor interface which leads to the observed degradation for n-type
GaN. This effect was exploited to achieve laterally patterned organic monolayers on n-type GaN
by selective UV illumination through a shadow mask, followed by site-sensitive covalent
grafting of proteins. We thus show that GaN is a useful platform for fundamental studies of
charge transfer processes at the organic-inorganic interface as well as a promising material for
future biosensor applications.
B10.28
Field Effect Transistor as a Platform to Study Photoinduced Processes in a Donor-acceptor
System. K. S Narayan and Manohar Rao; JNCASR, Bangalore, India.
The existence of donor-type polymer FETs exhibiting p-type characteristics and acceptor-type
molecular FETs with n-type characteristics provide an interesting possibility of a combined
active bilayer system, especially under photoexcitation. We present studies of single layer p-
channel P3HT, single layer n-channel PCBM, and bilayer PCBM/P3HT based FETs in dark and
illuminated conditions. The single layer p type mobility was estimated to be ≈ 10-2
cm2/V-s and n
type mobility in PCBM was ≈ 10-5
cm2/V-s. The bilayer structure was fabricated by introducing
the P3HT film on a PCBM device using different methods. The underlying n-type characteristic
of the PCBM device alters significantly upon illuminating the bilayer structure. Variety of
photophysical processes arising from charge generation and interfacial processes can be followed
and studied by transistor characteristics. These results are compared to the switching and
relaxation effects in single layer P3HT device under photoexcitation.1,2
1Physics, Colorado School of Mines, Golden, Colorado;
2National
Renewable Energy Laboratory, Golden, Colorado.
Excitonic hybrid organic-inorganic solar cells are gaining viability as alternatives to p-n junction
photovoltaics. Although hybrid cells typically have lower efficiencies than their inorganic
counterparts, they are more compatible with inexpensive manufacturing techniques such as spray
deposition and roll-to-roll processing, which can reduce the fabrication cost per photovoltaic
watt. Polymer devices with nanostructured ZnO as the electron-accepting layer have the potential
to improve carrier collection and power conversion efficiency in the bulk heterojunction
approach to organic solar cells. The ZnO/polymer interface, however, is not optimal and
properties such as polymer ordering and wetting at the interface need improvement.
Functionalization of the ZnO surface with molecular monolayers has the potential to resolve
these issues. In this study, we compare the performance of inverted planar ZnO/P3HT
photovoltaic cells made from sol gel-derived ZnO that was functionalized using thiol and silane
based attachment chemistries. Differences in the attachment scheme were explored using
molecules with the same end group. For example, octadecyltriethoxysilane (OTES) and
octadecylthiol (ODT) both yield surfaces with an 18 carbon alkyl chain termination. Both
showed improved polymer ordering relative to control samples. The ODT-modified devices had
higher efficiencies than OTES-treated devices, however, both treatments led to decreased short
circuit current compared to optimized control devices. Similarly, the effect of the end group was
explored using molecules that attach with the same chemistry but leave different, exposed,
terminal groups. Phenyltriethoxysilane (PTES) treated ZnO, for example, shows significantly
improved polymer wetting relative to OTES treatment and untreated surfaces. We discuss these
observations in terms of the nature of the terminal group, differences in the attachment scheme
(silicon vs. sulfur), and differences in surface coverage of the molecular layers. This work was
supported by the National Science Foundation under Grants DMR-0606054 and DMR- 0820518.
B10.41
Prediction of Dynamical Properties of Organic Field-Effect Transistors from DC
Transistor Parameters. Benedikt Gburek and Veit Wagner; School of Engineering and Science,
Jacobs University Bremen, Bremen, Germany.
Dynamical properties of Organic Field-Effect Transistors (OFETs) are of crucial importance for
almost any application. However, not direct AC data but DC measurements are usually used to
optimize transistor performance. Here we present a systematic study to which extent DC
parameters can actually be used to predict AC performance and AC limits of transistors. The
standard FET theory for long channel devices predicts a maximum device bandwidth of ωB = µ
V / L2. However, this holds only for ideal situations, e.g. without parasitic capacitances, and not
too high frequencies. Furthermore, as was recently shown [1], contact resistances can pose
severe additional high-frequency limits to the AC performance. To check the AC limit of a given
transistor directly we perform a frequency scan up to the frequency where the gate current equals
the drain current, which defines the true bandwidth of the device. The frequency-scanned AC
voltage is applied at the gate while the drain contact is kept at a constant voltage. The
dependence of the bandwidth on the drain-source voltage as well as on the DC offset of the gate-
source voltage is analyzed in detail. Among others, this analysis allows for a more accurate
determination of the threshold voltage, often difficult to be determined from standard DC
transistor characteristics. In addition, it offers better insight into carrier mobility dependencies
and distribution of energy states. A model which defines a correction factor to the ideal ωB = µ
V / L2 behavior is proposed in order to predict the measured bandwidth from DC transistor
parameters correctly. The model takes into account parasitic capacitances which occur from the
source and drain contacts. Furthermore, limitations of the bandwidth due to contact resistance are
presented and included in the model. [1] V. Wagner, P. Wöbkenberg, A. Hoppe, J. Seekamp,
Appl. Phys. Lett. 89 (2006) 243515
B10.42 Ambipolarity and Light Emission from Acene Based Transistors. Martin Schidleja, Christian
Melzer and Heinz von Seggern; Institute of Material Science, Technische Universität Darmstadt,
Darmstadt, Germany.
Polycrystalline ambipolar light-emitting organic field-effect transistors (LEOFETs) offer new
possibilities for the characterization of ambipolar devices. The main obstacles for the further
development of LEOFETs is the lack of efficiency, brightness and the limited choice of materials
allowing for high injection efficiency, charge carrier mobility and fluorescence yield. In our
contribution the realization of low injection barrier LEOFETs will be investigated using different
acenes as active layer and its relevance for organic light-emitting diodes (OLEDs) will be
discussed. Devices based on F8BT (poly(9,9-di-n-octylfluorene-alt-benzothiadiazole)) as
semiconductor and gold as source/drain metal show rather efficient light emission. Recent results
disclose a contact dominated device behavior due to high injection barriers for both charge
carrier types. In order to reduce the injection barriers Ca and Au as source and drain contacts in
combination with organic materials such as pentacene, tetracene and ditetracene with electron
affinities (EA) and ionization potentials (IP) matching the metal workfunctions are used. It will
be demonstrated that all these device structures exhibit light emission, meaning ambipolar
conduction, and how that can be utilized to investigate the injection efficiency of the individual
material combinations. Although light emission verifies the presence of electrons and holes in all
devices, the electron conduction in the channel deteriorates for decreasing EA of the organic
semiconductor. Polycrystalline pentacene allows for rather balanced electron and hole
conduction whereas the electron mobility worsens for ditetracene and tetracene. In all acene
based devices the highest intensity of the emitted light is observed in the hole dominated regime,
next to the Ca source electrode, indicating highly efficient electron injection from Ca into the
respective acene. This behavior cannot be observed in F8BT based devices due to the high
injection barriers for both charge carriers from the utilized gold electrodes. In the presented
contribution the relative impact of injection compared to transport will be highlighted and its
applicability to OLEDs will be discussed.
B10.43 Interface Engineering in Organic Thin Film Transistors. Philipp Stadler
1, Anna Track
3,
Mujeeb Ullah2, Thockchom B Singh
1, Gebhard J Matt
1,2, Helmut Sitter
2, Michael G Ramsey
3
and N. Serdar Sariciftci1;
1Institute for Organic Solarcells, Johannes Kepler University, Linz,
Austria; 2Institute for Solid State Physics, Johannes Kepler University, Linz, Austria;
3Institute
for Physics, Karl Franzens University, Graz, Austria.
Interface engineering in organic thin film transistors (oTFT) has become the key issue for
optimizing the device operation. Recent effort has been made by using passivation layers on top
of silicon dioxide (SiOx) and alumina (Al2O3) [1]. In this work we present a oTFT with C60 as
semiconductor in bottom gate and top contact structure. We compared silicon dioxide,
electrochemically grown alumina and divinyltetramethyldisiloxane-bis(benzocyclobutene)
(BCB) on alumina as gate insulator [2,3,4]. The nature of the interfaces between C60 and the
BCB and between C60 and the oxide insulators is studied in detail by X-ray and ultraviolet
photoemission spectroscopy (XPS and UPS). The interface system fullerene and oxide is
displaying large positive threshold voltages and lower drain/source currents in
accumulation/depletion regime as compared to oTFT's using organic gate insulators [5]. Here we
experimentally show that by introducing a thin polymeric buffer layer (BCB) between the oxide
and the fullerene the threshold voltage of the transistor characteristics can be shifted from
positive values in the oxide case to negative values in the polymer case [6]. The transistor is
operating within one volt and we observe an enhanced on/off ratio (4 decades) and an electron
mobility of approx. 1 cm2 V-1 s-1. We correlate this different transport performance to the
change in the work function of the semiconductor seen in the UPS specta and the shift in binding
energy of the C1s peak of the C60 in the XPS spectra respectively. [1] M. Halik, H. Klauk, U.
Zschieschang, G. Schmied, C. Dehm, M. Schütz; S. Maisch, F. Effenberger, M. Brunnbauer, F.
Stellacci, Nature 431, 963-966 (2004) [2] T. D. Anthopoulos, T. B. Singh, N. Marjanovic, N.S.
Sariciftci, A. Ramil, H. Sitter, M. Cölle, D. de Leeuw, Appl. Phys. Lett. 89 213504-1 (2006) [3]
T. B. Singh, N. S. Sariciftci, H. Yang, L. Yang, B. Plochberger, H.Sitter, Appl. Phys. Lett., 90
(2007) [4] R. Schroeder, L. A. Majewski, M. Grell, Adv. Mater. 16, 633 (2004) [5] G. Horowitz,
R. Hajlaoui, R. Bourguiga, M. Hajlaoui, Syn. Met. 101, 401-404 (1999) [6] X. H. Zhang and
Kippelen, Appl. Phys. Lett. 93, 133305 (2008)
B10.44
New Air-Stable Organic Semiconductor for p-Channel Transistors with Large Mobility
and Low-Voltage Integrated Circuits on Flexible Substrates. Ute Zschieschang1, Tatsuya
Yamamoto2, Kazuo Takimiya
2, Tsuyoshi Sekitani
3, Takao Someya
3 and Hagen Klauk
1;
1Max
Planck Institute for Solid State Research, Stuttgart, Germany; 2Hiroshima University, Higashi-
Hiroshima, Japan; 3University of Tokyo, Tokyo, Japan.
The performance of organic thin-film transistors (TFTs) often degrades upon exposure to air.
This is due to the generation of charge traps as a result of the oxidation of the conjugated
molecules. One strategy to improve the air stability of organic TFTs is thus the synthesis of
organic semiconductors with reduced susceptibility to oxidation. For p-channel TFTs this implies
a large ionization potential. However, most organic TFTs using semiconductors with large
ionization potential show rather small hole mobilities, usually below 0.5 cm2/Vs [1-3]. This has
been attributed to poor molecular ordering and hence poor overlap of the molecular orbitals.
Recently, a six-ring fused heteroarene, dinaphtho-[2,3-b:2‟,3‟-f]thieno[3,2-b]thiophene (DNTT),
was synthesized that has a large ionization potential (5.4 eV) and forms well-ordered films, with
hole mobilities as large as 2 cm2/Vs for TFTs made on doped silicon wafers (serving as a global
gate electrode) with a thermally grown SiO2 gate dielectric [4]. Here we report on the
performance and stability of DNTT transistors, inverters and ring oscillators on glass and flexible
polyethylene naphthalate (PEN) substrates. The TFTs and circuits use an inverted staggered
(bottom-gate, top-contact) device structure with patterned aluminum gates, a thin gate dielectric
based on an oxygen-plasma-grown aluminum oxide layer (3.6 nm thick) in combination with a
self-assembled monolayer (SAM) of an aliphatic phosphonic acid (1.7 nm thick), a thermally
evaporated DNTT layer and gold source/drain contacts. Gates, semiconductor, and source/drain
contacts were patterned using shadow masks. Owing to the large capacitance of the AlOx/SAM
gate dielectric (700 nF/cm2), the TFTs and circuits can be operated with low voltages of about 3
V. DNTT TFTs on glass have a mobility of 1.5 cm2/Vs, an on/off ratio of 1e7 and a subthreshold
swing of 80 mV/decade. On flexible PEN, the TFTs have a mobility of 0.6 cm2/Vs, an on/off
ratio of 1e7 and a subthreshold swing of 110 mV/decade. These mobilities are a factor of 2 larger
than those of pentacene TFTs manufactured with the same technology [5]. Because the
ionization potential of DNTT (5.4 eV) is much larger than that of pentacene (5 eV), the DNTT
TFTs have significantly better air stability, showing no degradation while stored in ambient air
for 3 months. Five-stage ring oscillators based on unipolar inverters with saturated load show
stable oscillations for supply voltages between 2.2 and 5 V, with a signal delay of 37 µsec per
stage (27 kHz) at 3 V and 18 µsec per stage (55 kHz) at 5 V. This is the highest frequency
reported for flexible organic circuits at low supply voltage. [1] H. Meng et al., J. Am. Chem.
Soc., vol. 123, p. 9214, 2001. [2] J. A. Merlo et al., J. Am. Chem. Soc., vol. 127, p. 3997, 2005.
[3] J. Locklin et al., Adv. Mater., vol. 18, p. 2989, 2006. [4] T. Yamamoto et al., J. Am. Chem.
Soc., vol. 129, p. 2224, 2007. [5] H. Klauk et al., Nature, vol. 445, p. 745, 2007.
B10.45
Optical Stability of Small-molecule Thin-films Determined by Photothermal Deflection
Spectroscopy. Marco Stella2, Monica Beatriz Della Pirriera
1, Joaquim Puigdollers
1, Cristobal
Voz1, Jordi Andreu
2, Ramon Alcubilla
1 and Joan Bertomeu
2;
1Electronic Engineering,
Universidad Politécnica de Cataluña, Barcelona, Spain; 2Fisica Aplicada y Optica, University de
Barcelona, Barcelona, Spain.
Organic semiconductors represent a new interesting class of materials for several electronic
applications. Organic solar cells performance have improved significantly in the last few years
thanks to the optimization of the solar cell structure and, specially, to the ability to process new
organic semiconductors with optimised properties. Among different deposition techniques,
thermal evaporation in high-vacuum is the more suitable process to obtain small-molecule
organic thin-films with well organize molecular structure. In this paper the optical absorption
properties of n-type (C60 and PTCDA) and p-type (CuPc) small-molecule semiconductors are
investigated by optical transmission and Photothermal Deflection Spectroscopy (PDS). Results
show the usual absorption bands related to HOMO-LUMO transitions in the high absorption
region of transmission spectra. PDS measurements also evidences exponential absorption
shoulders with different characteristic energies (47 meV for CuPc, 50 meV for PTCDA and 87
meV for C60). In addition, broad bands in the low absorption level are observed for C60 and
PTCDA thin-films. These bands have been attributed to contamination due to air exposure (1). In
order to get deeper understanding of the degradation mechanisms single, bilayer and co-
evaporated thin-films have been characterized by PDS. Dependence of the optical coefficient on
light illumination and air exposure have been studied and correlated to the structural properties
of the films (as measured by X-Ray Diffraction Spectroscopy). Results show that CuPc:PTCDA
and CuPc:C60 co-evaporated films are stable after light and air exposition. However, single
layers of C60 shows significant increase of the low level optical absorption coefficient.
B10.46
Non-Volatile Organic Memory based on Electrically Doped Organic Heterostructures. Frank Lindner, Phillip Sebastian, Bjoern Lussem and Karl Leo; Institut für Angewandte
Photophysik, TU Dresden, Dresden, Germany.
Within the last few years organic memory devices have attracted considerable attention. Several
different approaches for organic memory devices have been reported in literature that show
bistable memory behaviour with high switching speeds and high ON/OFF ratios [1]. The
drawbacks of most of these devices are a rather low stability and reproducibility and a lack of
knowledge about he precise switching mechanism. Here, we report on a novel approach based on
charge trapping in a two-well heterostructure device. Consisting of materials which are well
known from organic light emitting devices, we obtain reproducible bistable electrical switching
and memory phenomena. Depending on the number of charges stored in the wells the resistance
of the device changes, i.e. the measured current-voltage characteristic shows two states of
different conductivity at the same applied voltage. The ratio between the resistance in the OFF
state and in the ON state can be varied by the write and erase voltage. More than 2000 Write-
Read-Erase cycles are obtained without degradation. The memory state is retained for several
days before reading the devices. In conclusion, the memory concept we present shows higher
reproducibility and stability compared to other organic memories. Device performance tests
show that the heterostructure devices are a promising candidate for low-cost, electrically
phenylene-diamine) (PFB) and poly(9,9'-dioctylfluorene-co benzothiadiazole) (F8BT). Scanning
Transmission X-ray Microscopy (STXM) imaging with sub-100 nm resolution was used for
investigating the evolution of the morphology. An increase in domain size is observed with
annealing due to phase separation. Photoluminescence (PL) quantum efficiency studies of these
films indicate a direct correspondence with the domain size for blends annealed at temperatures
above 180 degC. The phase separation first evolves with the evolution of relatively pure phases
at length scales beyond the resolution of STXM. At higher temperatures (>=180degC) the
hierarchy of phase separation is lost and the length scales can be readily imaged by STXM.
Resonant Soft X-ray Scattering (RSoXS) studies show an evolution towards purer domains along
with increase in domains size with annealing. At annealing temperatures below 180oC, the
correlation function analysis of the RSoXS data shows the evolution and distribution of
subdomains that span a range of length scales (4 - 100nm).The correlation function complements
the average domain size obtained from the PL data and a Monte-Carlo model that takes the
excition diffusion into account.
B10.79 Probing the Buried Interface in Annealed Conjugated Polymer Bilayers. Sufal Swaraj
1,
Cheng Wang2, Hongping Yan
1, Chris R McNeill
3 and Harald Ade
1;
1Department of Physics,
North Carolina State University, Raleight, North Carolina; 2Advanced Light Sounce, Lawrence
Berkeley National Laboratory, Berkeley, California; 3Cavendish Laboratory, Department of
Physics,, University of Cambridge, J J Thomson Ave,, Cambridge, CB3 0HE, United Kingdom.
The properties of polymer/polymer interfaces in conducting polymeric devises critically
influences device performance, yet relatively few studies of such interfaces have been
performed. We show that Resonant Soft X-ray Reflectivity (RSoXR) as a powerful tool for the
characterization of bilayers of conducting polymers, a material class that had mostly been
investigated with neutron reflectivity. The rapid changes of optical properties near the Carbon
absorption edge provides selectivity to specific chemical moieties and high contrast for
investigated materials. We exemplify the use of this technique for conducting polymers by
characterizing the buried interface in bilayers of poly(9,9'-dioctylfluorene-co-bis-
N,N'(4,butylphenyl)-bis-N,N'-phenyl-1,4-phenylene-diamine) (PFB) and poly(9,9'-
dioctylfluorene-co benzothiadiazole). We investigate the influence of annealing on the
polymer/polymer interface and the surface and quantify the surface and interfacial widths.
RSoXR results point to an interesting strategy that will allow the interdiffusion and physical
roughness at a buried polymer/polymer interface to be determined separately by diffuse
scattering at an angle and photon energy where the top surface exhibits little scattering, yet the
polymer/polymer interfaces will exhibit total internal reflection.
B10.80 Novel Micro and Nano Patterning Techniques for Organic Electronic Systems. Alexander
Zakhidov, Jin-Kyun Lee, John DeFranco, Hon Hang Fong, Priscilla G Taylor, Christopher K
Ober and George G Malliaras; Cornell University, Ithaca, New York.
Organic electronics and optoelectronics are fast developing branches of modern science and
technology that are aiming to replace conventional inorganic materials with light, inexpensive,
flexible organic materials. One of the main issues to be solved on the route to real word
application is patterning and processing of thin layer active organic materials. In this report we
present new reliable photolithography micro and nano patterning techniques for high resolution,
high throughput patterning of solution processable organic materials [1,2]. We also demonstrate
that proposed approach is completely benign [3] to majority of organic electronic materials as
well as environmental friendly and can be easily adopted by industry. In order to demonstrate
potential application of developed patterning technique we fabricate micropatterned top-contact
organic thin film transistors and prototypes of organic light emitting displays. [1] H. S. Hwang,
Al. A. Zakhidov, J.-K. Lee, X. Andre, J. A. DeFranco, H. H. Fong, A. B. Holmes, G. G.
Malliaras, C. K. Ober, Journal of Materials Chemistry, 2008, 18, 3087. [2] J.-K. Lee, M.
Chatzichristidi, Al. A. Zakhidov, P. G. Taylor, J. A. DeFranco, H. S. Hwang, H. H. Fong, A. B.
Holmes, G. G. Malliaras, C. K. Ober, J. Am. Chem. Soc., 2008, 130, 11564. [3] Al. A. Zakhidov,
J.-K. Lee, H. H. Fong, J. A. DeFranco, M. Chatzichristidi, P. G. Taylor, C. K. Ober, G. G.
Malliaras, Advanced Materials, 2008, 20, 3481.
B10.81
Factors Determining the Efficacy of Optical Spacers in Polymer Solar Cells: The Role of
Active Layer Morphology. Anshuman Roy, Sarah Mednick, Sung Heum Park, Ji Sun Moon
and Alan J Heeger; University of California, Santa Barbara, California.
Polymer photovoltaic devices stand at the cusp of rapid commercialization today, with the
maximum power conversion efficiency reported to be over 6% [J. Y. Kim et al., Science, 317
(2007) 222-225]. The photo-active layer in these devices consists of conjugated polymers and
modified fullerenes that form a bulk heterojunction (BHJ) composite film about 100 nm in
thickness. Additionally, a layer of TiOx (Titanium Oxide) approximately 10 nm in thickness is
spin cast on top of the active layer to act as an optical spacer that maximizes the incident light
intensity in the photo-active layer [Kim et al., Adv. Mater., 18 (2006) 572-576]. Using a
combination of numerical modeling and experiments, including ellipsometry, transmission
electron microscopy and cell efficiency, we show that the efficacy of the TiOx layer as an optical
spacer is dependent not only on the thickness of the BHJ layer but also on the nano-scale
structure of the BHJ layer, which in turn depends on a host of processing conditions.
B10.82
Transport Anisotropy in Films of Organic n-type Semiconductor with Controlled In-plane
Grain Boundary Orientation. Jonathan Rivnay1, Antonio Facchetti
2,3 and Alberto Salleo
1;
1Dept. of Materials Science and Engineering, Stanford University, Stanford, California;
2Polyera
Corporation, Skokie, Illinois; 3Dept. of Chemistry, Northwestern University, Evanston, Illinois.
Solution processable small molecule organic semiconductors have gained interest due to their
potential low cost processing and field effect mobilities nearing that of their vapor deposited,
non-soluble counterparts. Unfortunately, pristine films of soluble small molecules suffer from
poor thin-film transistor device-to-device reproducibility (mobility, VT and stability) due to the
existence of grain-boundaries that are not uniformly distributed throughout the film. The effect
of grain boundaries in thin films of small molecule semiconductors is relatively large compared
to that of semicrystalline polymer devices for two reasons: the grain size is often on the order of
the channel length preventing averaging effects and boundary regions between grains are more
abrupt. Indeed, in semicrystalline polymer films, a single chain can bridge two or more adjacent
crystallites, facilitating transport through amorphous-like regions/grain boundaries. Research
should thus focus on understanding of the relationship between microstructure and charge
transport in order to design devices that do not necessarily eliminate grain boundaries, but limit
their penalty on electrical performance, while also allowing for lower device-to-device
variability. To this end, in this work we use anisotropic films of the n-type small molecule N,N‟-
bis(n-octyl)-(1,7&1,6)-dicyanoperylene-3,4:9,10-bis(dicarboximide), (PDI8-CN2) to explore the
effect of grain boundaries on field effect mobility, and understand their implication in charge
transport in thin films. To fabricate samples, we use an inclined drop casting method with a
heating stage. In-plane orientation and morphology are characterized with x-ray diffraction,
polarized light microscopy, and AFM. Thin film transistors (TFTs) were made to probe transport
as a function of charge density and temperature across two types of grain-boundaries that are
formed. These PDI8-CN2-based TFTs exhibit a mobility anisotropy of two orders of magnitude
depending on the relative orientation of the grains with the current flow. Parallel devices, with
charge transport presumably parallel to the fast growth direction of the crystallites show room
temperature mobilities above ~0.01 cm2/Vs (EA=120meV), near that of isotropic solution cast
films. Perpendicular devices, on the other hand, show low mobilities of ~10-4
cm2/Vs
(EA=340meV). The difference between the two orientations is much larger than a 2-3 fold
difference associated with crystalline anisotropy. The similar mobilities measured in parallel and
isotropic films suggest defects and grain boundaries of this device are similar to those present in
„typical‟ devices. Though the morphology and exact nature of grain boundaries in the orthogonal
direction are unknown we postulate that they are host to large energetic barriers, in agreement
with the larger EA, and may explain device-to-device non-uniformity in isotropic films, due to
the percolative nature of charge transport.
B10.83
Synthesis and Characterization of Soluble Copolymers Containing Dialkyl
Quarterthiophene. Jun Chen1, Sung J Park
1, Jae W Jang
1, Yun H Kim
2 and Soon K Kwon
1;
1School of Nano and Advanced Material Science & Engineering and ERI, Gyeongsang National
University, Jinju 660-701, Gyeonsangnam-do, Korea, South; 2Department of Chemistry and
RINS, Gyeongsang National University, Jinju 660-701, Gyeonsangnam-do, Korea, South.
Soluble conjugated 4ThFlu and 4ThNa copolymers were synthesized by the Suzuki coupling
reaction. The weight-average molecular weight of the 4ThFlu and 4ThNa copolymer were
determined to be 16848 and 13505, respectively. And the polydispersity indexes obtained by gel
permeation chromatography (GPC) using polystyrene standards for calibration in the eluent THF
is 1.55 and 1.59, respectively. The thermal, optical and electronic properties of copolymers were
investigated by thermogravimetric analysis (TGA), differential scanning calorimetry (DSC), UV-
vis absorption, photoluminescence spectroscopies, cyclic voltammetry. The absorption
maximum spectra of copolymers were observed 452, 453 nm in solution state and 563, 563 nm
in film state. The PL maximum spectra of copolymers were observed at 467, 503 nm in solution
state and 585, 624 nm in film state, respectively. The copolymers were showed highly thermal
stability of decomposition temperature over 300 oC. Especially, the copolymers were easy to be
soluble in common solvents (THF, CHCl3, Toluene etc) due to the introduction of long alky
chains.
B10.84 Blue Organic Light Emitting Diodes from Solution Processed Small Molecules. Bright
Walker, Arnold B Tamayo, Wesley Walker, Jihua Yang, Fred Wudl and Thuc-Quyen Nguyen;
Chemistry and Biochemistry, University of California at Santa Barbara, Santa Barbara,
California.
Solution-processed organic light-emitting devices provide the potential for cheap, ultrathin, light-
weight, and large-area illumination sources. In comparison with red or green light-emitting
devices, efficient blue emitting diodes are typically more difficult to obtain because the emitting
material requires a wider band gap for radiative recombination. Organic blue light-emitting
diodes were studied using the solution processed small molecules 2,7-dipyrenyl-9 ,9[|#1#|]-
dioctyl-fluorene as well as 1,3,5-tris(7,10-diphenylfluoranthene-8-yl)benzene as emissive layers
(EMLs). Devices were fabricated using the conjugated polyelectrolyte poly(9,9'-bis[6"-(N,N,N-
trimethylammonium) hexyl]fluorene-alt-co-phenylene) with tetrakis(imidazolyl)borate
counterions (PFN-BIm4) as an electron injecting layer and poly(9-vinylcarbazole) (PVK) as an
electron blocking layer. Efficient blue light emission was observed for devices with the
architecture ITO/PEDOT:PSS/PVK/EML/PFNBIm4/Al in which all organic layers were
deposited by solution processing. Using PVK and PFN-BIm4 layers results in a significant
improvement in device performance compared to devices without the layers or with common
electron injecting layers such as lithium fluoride or barium.
B10.85
Meta-stable Interfaces between Soft-contact and Closed-shell Semiconductor Surfaces. Yang Li, Wei Long and Raymond T. Tung; Physics Department, Brooklyn College, the City
University of New York, Brooklyn, New York.
The use of a self-assembled monolayer (SAM) of molecules to modify and tailor interface
electronic properties is an attractive approach for sensors and semiconductor electronic
applications, because of the molecules‟ functional variety and flexibility. In studies involving
SAM attached semiconductor surfaces, the deposition/application of metallic contacts is geared
toward minimizing disturbance to the molecular layer, i.e. for a preservation of meta-stable non-
interacting molecule-metal interfaces. However, interactions between the metal and the
molecular layer are often found to influence and even dominate the dipolar effect of the
molecular layer. Significant barrier height inhomogeneity has also been reported. In this work,
we attempt to establish a baseline for studies involving meta-stable metal-molecule interfaces. A
dual-station UHV chamber was constructed for this study. Silicon surfaces terminated with
different types of stable closed-shell configurations (Cl-, S- and H-) are used to simulate the
stable molecular termination on SAM/Si surfaces. Meta-stable metal-Si structures are fabricated
using soft-landing deposition of metal contacts (Au, Ag, and Al) with indirect deposition in an
inert gas ambient or direct deposition in vacuum, and at variable temperatures. In situ
characterization by surface spectroscopies and Kelvin probe techniques showed the varied levels
of intermixing under different deposition conditions. Electrical measurements, by variable
temperature I-V and C-V method, of Au/Si and Ag/Si diodes fabricated on n- and p-type <100>
and <111> substrates also showed a significant dependence on fabrication conditions of the
Schottky barrier. Lower deposition temperature led to more uniform contact between metal and
semiconductor, which then led to higher SBH on n-type Si, with the expected, opposite
dependence observed on p-type Si. Also, the orientation of the semiconductor surface was shown
to have a significant influence on the formation of the SBH. Electrical results and results
obtained from surface chemical analysis and microscopic techniques are presented with special
attention paid to the possible electrical inhomogeneity in the systems. These results are compared
with results involving molecular layers obtained previously in our and other groups.
B10.86
Abstract Withdrawn
B10.87
Picosecond Photoexcitation Dynamics in the Poly(2,7-Carbazole) Copolymer, PCDTBT,and
in Bulk Heterojunction Composites with PC70BM. Minghong Tong1, Nelson E. Coates
1,
Daniel Moses1, Alan J Heeger
1, Serge Beaupre
2, Mario Leclerc
2 and Russell Gaudiana
3;
1Center
for Polymers and Organic Solids, University of California, Santa Barbara, Santa Barbara,
California; 2Departement de Chimie, Université Laval, Quebec City, Quebec, Canada;
3Konarka
Technologies, Inc, Lowell, Massachusetts.
We have studied the nature of ultrafast photoexcitations and their recombination dynamics in an
alternating donor-acceptor low-bandgap Poly(2,7-Carbazole) copolymer (PCDTBT; see Blouin,
N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 - 2300) and in composites of that
polymer with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl ester (PC70BM); This
class of alternating donor-acceptor copolymers (with different acceptor units) holds promise for
photovoltaic applications because of the ability to tune the electronic energy levels by changing
acceptor units. The implied flexibility in the synthesis can lead to both a lower bandgap that
better uses the solar radiation spectrum, and a lower HOMO level that increases the open circuit
voltage of photovoltaic devices. We have used transient photinduced absorption spectroscopy,
over a wide spectral range in order to study the nature of the photoexcitations, and in particular
the carrier generation and recombination dynamics at short time scales. A very long carrier
lifetime (>>1ns) is observed in the PCDTBT-Fullerene composite.
B10.88
Steady-State and Transient Photoconductivity in the Poly(2,7-Carbazole) Copolymer
PCDTBT, and in Bulk Heterojunction Composites with PC70BM. Nelson Coates1, Minghong
Tong1, Daniel Moses
1, Alan J Heeger
1, Serge Beaupre
2, Mario Leclerc
2 and Russell Gaudiana
3;
1Physics, University of California, Santa Barbara, Goleta, California;
2Chimie, Université Laval,
Quebec City, Quebec, Canada; 3Konarka Technologies Inc., Lowell, Massachusetts.
We have studied the nature of carrier generation using steady-state and transient
photoconductivity in an alternating donor-acceptor low-bandgap Poly(2,7-Carbazole) copolymer
(PCDTBT; see Blouin, N.; Michaud, A.; Leclerc, M. Adv. Mater. 2007, 19, 2295 - 2300) and in
composites of that polymer with the fullerene derivative [6,6]-phenyl C70-butyric acid methyl
ester (PC70BM). This class of alternating donor-acceptor copolymers (with different acceptor
units, X) holds promise for photovoltaic applications because of the ability to tune the electronic
energy levels by changing X. The implied flexibility in the synthesis can lead to both a lower
bandgap that better uses the solar radiation spectrum, and a lower HOMO level that increases the
open circuit voltage of photovoltaic devices. In PCDTBT, the absorption band extends out to ~
700 nm, with two distinct but broad absorption bands that are centered at ~ 400 nm and ~ 600
nm. Higher solar cell power conversion efficiency is achieved in PCDTBT devices than in P3HT
based devices because of the improved light harvesting and larger open circuit voltage. We have
used steady-state and transient photoconductivity to investigate the carrier generation and
collection efficiency of PCDTBT and its composite with the soluble fullerene, PC70BM, after
photoexcitation at each of its distinct absorption bands. In pristine PCDTBT, higher carrier
quantum efficiency is observed with excitation at the high energy absorption band, but in the
PCDTBT-fullerene composite, this efficiency is an order of magnitude greater and relatively
wavelength independent.
B10.89 High-Mobility n-Channel Organic Thin-Film Transistors Henry Yan and Antonio Facchetti;
Polyera Corporation, Skokie, Illinois.
We report here our recent progress enabling high-performance top-gate organic thin-film
transistors (OTFT). Electron mobility of ~ 1.0 cm2/Vs was achieved for top-gate bottom-contact
devices tested in ambient using both the semiconductor and dielectric layers fabricated by spin-
coating. To the best of our knowledge, this is first report of solution-processed top-gate n-
channel TFTs with mobility higher than 1 cm2/Vs. Furthermore, we will also present updated
performance of bottom-gate bottom-contact transistors using Polyera materials. Electron
mobility of ~ 0.1 cm2/Vs was achieved in a bottom-gate bottom-contact structure with both the
semiconductor and dielectric layers fabricated by spin-coating process. In this case, the dielectric
material is an UV crosslinkable dielectric polymer having good compatibility with n-type
organic semiconductors.
B10.90 Effects of Metal and Organic Impurities on Pentacene Electronic Structures Jing Xue
1,
Geunsik Lee1 and Kyeongjae Cho
1,2;
1Physics, University of Texas at Dallas, Richardson, Texas;
2Materials Science and Engineering, University of Texas at Dallas, Richardson, Texas.
Organic semiconductors have received much attention due to their successful application in
optical and electronic areas. Pentacene is one of the most promising materials in the organic
semiconductors because of its highest hole mobility. In this work, we study the effects of metal
contact and organic impurities on pentacene‟s electronic structure by using ab initio density
functional theory (DFT) methods. Pentacene in the electronic devices directly contacts with
metal electrode, and metal atoms may diffuse into pentacene and form impurities interacting with
pentacene. Therefore, the interface between metal and pentacene can be modified by the metal
atomic diffusion. We calculate electronic structure of pentacene adsorbed with different metallic
elements (Au, Pd, Ni) and study their effects on the carrier injection barrier. We found that the
calculated energy gap for isolated pentacene molecule is 1.13 eV. The main contribution of
HOMO and LUMO come from pi orbital of C-atoms. For metal adsorption, the energy gap will
be decreased. Compared the binding energy and the bulk cohesive energy, Ni and Pd are easier
to diffuse into the pentacene than Au. Depending on the valence electron structure of the metal
atom to be adsorbed, the electronic structure of pentacene near the Fermi level has changed
differently. For Au and Ni, there will form new mid gap states and the new mid-gap states
decrease the contact barriers to improve the charge injection from the electrode. Organic
semiconductor device characteristics is also sensitive to organic impurities which may be
introduced during synthesis or by air exposure. It is worth to focus that dissociation of H2O into
H and OH may form some organic defect. We consider the defect in pentacene in forms of C-H2,
C=O and OH. All these organic impurities will give rise to gap states. For C-H2 or C=O defect,
the 6-top site is mostly favored energetically by an additional H adsorption (C22H15)or H
replacement by O (C22H13O), respectively. In both systems, the pz orbital on the perturbed C
atom no longer participates in the π bonding once the defect is introduced. For H replacement by
an OH, it seems that, there are no much differences between C22H14O and C22H14 because all
C atoms are still participates in their π bonding. However, for additional adsorption of OH,
C22H15O, the electronic structure is different from that of pentacene molecule. In addition, it is
shows that the electronic structure of pentacene with the same type of defect is affected by the
defect position. These calculations show the role of metal or organic impurities in generating the
defect states with the HOMO-LUMO gap of the pentacene molecules. These gap states may play
the critical role in limiting the mobility of the organic semiconductor channel and the efficiency
of the optical device applications. Our detailed electronic structure study provides a fundamental
insight on the effects of impurities in the pentacene device performance.
B10.91 Interface Reaction of Aluminum and 8-hydroxyquinolatolithium. Young Mi Lee
1, Yeonjin
Yi2, Jeong Won Kim
2 and Yongsup Park
1;
1Dept.of Physics, Kyung Hee University, Seoul,
Korea, South; 2Korea Research Institute of Standards and Science, Daejeon, Korea, South.
One of organic electron injection layer materials, Liq (8-hydroxyquinolatolithium) shows a few
advantages over other inorganic materials for organic light emitting device (OLEDs). As the Liq
possesses the similar structure to Alq3, the most common light emission layer material, it is
believed to provide a smooth interface and be more compatible to flexible displays. The energy
alignment and device performance including the Liq between Al and Alq3 have recently been
demonstrated. Here the interface chemical reaction at the Liq/Al interfaces was investigated by
using high resolution synchrotron radiation photoelectron spectroscopy. The different deposition
sequence gives different reactions. While strong reactions are observed throughout the Liq film
when Al is deposited on Liq layer, an interface localized reaction occurs just at the interface
upon the Liq deposition onto Al surface. Either sequence of film stacks, Liq/Al and Al/Liq
produce an interface gap state respectively at 2.1 eV and 2.8 eV below the Fermi level. Both of
the highest occupied molecular orbital (HOMO) and N 1s core level peaks are shifted to the high
binding energy side by 0.35 eV on Al/Liq whereas it is not the case on Al/Liq. Based on these
observations, the differences in electron injection barrier and interface dipole between the two
opposite deposition sequences could be drawn.
B10.92
Formation of Ohmic Carrier Injection at Anode/organic Interfaces and Carrier Transport
V/C-f measurements and atomic force microscopy are used to investigate the NH3-sensitive
materials constituting the sensor device.
10:45 AM B11.7
Organic Transistors for Sensor Applications under Gaseous and Aqueous Conditions Anatoliy N Sokolov, Mark E Roberts and Zhenan Bao; Stanford University, Stanford, California.
Organic field-effect transistors (OFETs) are ideal for inexpensive, chemical sensors owing to
their compatibility with flexible, large-area substrates, simple processing, and tunable active
layer materials. Specifically, the use of small molecule-based organic semiconductors or single-
walled carbon nanotubes allows specific recognition site incorporation to achieve selectivity.
Previously, the use of OTFTs as sensors has been limited to vapor phase systems. However,
while chemical detection for comprehensive monitoring will require the sensors to operate under
both aqueous and ambient air conditions, the OTFTs reported to-date have not been suitable for
applications in aqueous media owing to high operating voltages, degradation and delamination.
We now introduce novel low-temperature cross-linkable gate dielectric films compatible with
vapor and solution deposited semiconductors for aqueous sensors. The polymer matrix for the
gate dielectric layer in this study is poly(4-vinylphenol), selected for its dielectric characteristics
and compatibility with various organic semiconductors. These materials exhibit stability in
aqueous operation for over 10^4 electrical cycles. The work has led to the observation of a
significant drain current response for solutions with concentrations as low as parts per billion of
trinitrobenzene and methylphosphonic acid. We now describe the initial investigation of
semiconductor materials to achieve air- and water-stable sensors, as well as the introduction of
specific recognition elements to enhance sensitivity. The investigations will focus on the use of
functional groups as binding sites for the semiconductor:analyte interaction both under aqueous
and ambient air conditions, with the goal of developing environmentally stable sensors. The
mechanism for OTFT sensing is investigated by varying device parameters including
semiconductor thickness and operating conditions, such as analyte concentration and gate bias.
11:00 AM B11.8 Organic versus Inorganic Thin-Film Transistor Chemical Vapor Sensors Soumya Dutta and
Ananth Dodabalapur; Microelectronics Research Centre, University of Texas at Austin, Austin,
Texas.
This presentation will highlight the differences between responses of thin-film transistor (TFT)
based chemical vapor sensors with organic semiconductors such as pentacene and inorganic
semiconductors such as Zinc Tin Oxide (ZTO). In organic TFT chemical vapor sensors,
numerous studies have shown that trapping of charges by polar analytes at grain boundaries and
other device interfaces often results in a decrease in drain current (Id) [1-2]. In ZTO TFT
sensors, the drain current increases substantially upon exposure to such polar analytes. For
example, the drain current of a ZTO TFT sensor exhibits a substantial (~ 6-7 fold) increase of Id
upon exposure to IPA. The differences in response between the organic and inorganic TFT
sensors to the same analytes is due to the strong effects that polar molecules/materials have in
trapping charges in organic semiconductors. Increase in Id is also sometimes seen in organic
TFT sensors upon exposure to polar analytes. This happens when the molecular ordering is not
good and the mobilities are very low. Under these conditions, the extra charge density in the
channel induced by the polar analytes overwhelms the drain current reduction due to the trapping
of charges. We point out that this trapping of charges in the presence of polar molecules is also
seen when water is present in the vicinity of the transistor channel. A possible reason for the
trapping and mobility reduction in organic TFTs is the formation of bipolarons which are
stabilized by the polar analyte molecules. Since bipolarons have much lower mobilities, we have
a decrease in current. Activation energy measurements of charge carriers in organic TFT sensors
indicates that polar analyte molecules result in increased activation energies, supporting this
physical model. We further investigate the differences between OTFT sensors and inorganic TFT
sensors with the help of a novel four-terminal device configuration which allows us to extract
important information about trapping and threshold voltage shifts. In this four-terminal sensor
device, there are two coupled channels, one in an organic semiconductor and the second in an
inorganic semiconductor. These channels are electrostatically coupled, permitting both
capacitance-voltage and current voltage measurements. References: [1] L. Torsi and A.
Dodabalapur, Anal. Chem. 77, 380A (2005). [2] B. Crone, A. Dodabalapur, A. Gelperin, L.
Torsi, H. E. Katz, A. J. Lovinger, and Z. Bao, Appl. Phys. Lett. 87, 2229 (2001).
11:15 AM B11.9 The Development of Biosensors Based on Organic Thin Film Transistors Warwick J
Belcher, Xiaojing Zhou, Kathleen Sirois, Daniel Elkington and Paul Christopher Dastoor; Centre
for Organic Electronics, University of Newcastle, Callaghan, New South Wales, Australia.
Field effect transistors (FETs) are commonly used as the basis of sensor devices because of their
attractive combination of transduction and amplification properties. In particular, organic thin
film transistors (OTFTs) have received increased recent interest because they also offer
improved material compatibility with enzymes and biomolecules in general. This overcomes, in
part, the traditional issue of forming an effective recognition element - transducer interface. The
possibility of these also being “all plastic” devices means that biosensors based on OTFTs have
the potential to be easy and cheap to fabricate. A relatively new class of OTFTs are based on a
hygroscopic insulator layer. These function via a mechanism that involves ionic drift within the
insulator layer, resulting in electrochemical doping of the channel, and the formation of charged
layers at the insulator-gate and insulator-channel interfaces. The result of this dual mechanism is
a device that has remarkably low turn on voltages compared to a traditional OTFT or metal-
based FETs. This makes this class of OTFT particularly attractive for use with biological
molecules which may not be compatible with the high operating voltages required in most FETs.
We have fabricated and characterised all-polymer devices based on a variety of hygroscopic
insulators. Using glucose oxidase as a model system we have investigated interfacing enzymes
with these devices to create working biosensors. The results of this research, along with recent
developments, will be presented.
11:30 AM B11.10
Interfacial Effects Affecting Data Retention in Organic Non-volatile Memory Elements
Based on Ferroelectric Field-effect Transistors. Tse Nga Ng, Sanjiv Sambandan, Robert Street
and Ana Claudia Arias; Electronic Materials Lab, Palo Alto Research Center, Palo Alto,
California.
Non-volatile memory elements for mechanically flexible electronics are being developed using
organic materials to integrate data-sensing and storage. Ferroelectric field effect transistors
(feFETs) offer the advantages of non-destructive read-out and smaller cell size when compared
to capacitor structures. Unlike resistive switches for which the memory mechanism is still
unclear, it is well-established that ferroelectric devices switch states by dipole alignment and
allow faster switching speed than memories based on charge trapping in floating-gate transistors.
However, the retention time of ferroelectric memory is merely on the order of days, and
systematic studies of degradation mechanisms are needed to improve the retention time. In this
paper, factors affecting the current output of feFETs are examined, including changes in
semiconductor mobility, threshold voltage, and dielectric capacitance with time. Degradation in
the dielectric and in the semiconductor is separated for understanding the origins of instability.
Measurement results indicate that the semiconductor-dielectric interface is critical for
polarization retention in feFETs. The current decay of feFET hysteresis was partly caused by
charge trapping due to ferroelectric polarization. Another contribution to hysteresis loss is the
depolarization field at the channel interface; the depolarization field is not completely
compensated because of low carrier density in organic semiconductor. With better understanding
of the interfacial effects, memory cell structures are improved to retain 50% of output current
over 7 days. Due to the gradual decrease in feFET current, calibration method for extracting the
original input voltage is demonstrated, enabling feFETs to be used as analog memory. Finally,
ferroelectric transistor dimensions will be discussed with regards to improving data retention
time.
11:45 AM B11.11
Direct Tunneling into the Gate Dielectric by both Holes and Electrons in an Ambipolar