Meetings & Events

Publishing Alliance

2007 MRS Fall Meeting & Exhibit

November 26 - 30, 2007 | Boston
Meeting Chairs: Duane Dimos, Mary Galvin, David Mooney, Konrad Samwer

Symposium F : Interfaces in Organic and Molecular Electronics III

2007-11-26 Show All Abstracts

Symposium Organizers

Julia W. P. Hsu Sandia National Laboratories

Leeor Kronik Weizmann Institute of Science

George G. Malliaras Cornell University

Nobuo Ueno Chiba University

F1: Charge Transport and Injection

Session Chairs

C. Lee

John Marohn

Monday PM, November 26, 2007

Back Bay C (Sheraton)

9:30 AM - **F1.1

Polaron Formation and Charge Carrier Trapping in Organic Semiconductor Thin Films.

Xiaoyang Zhu ¹
¹Department of Chemistry, University of Minnesota, Minneapolis, Minnesota, United States

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10:00 AM - F1.2

Interfacial Hole Traps in Conjugated Polymers Thin Films.

Leonid Fradkin ¹, Kwang Jik Lee ¹, Paul Barbara ¹
¹, The University of Texas at Austin, Austin, Texas, United States

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10:15 AM - F1.3

Charge Transport Measurements on Anisotropic Polythiophene Thin Films Fabricated via Directional Crystallization.

Leslie Jimison ¹, Alberto Salleo ¹
¹Materials Science and Engineering, Stanford University, Stanford , California, United States

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Recently, there has been considerable interest in the development of semiconducting polymers for use in printable electronic devices, such as transistors for display backplanes. These materials offer a cost effective alternative to conventional semiconductors, with key advantages of organic materials being their low processing temperature and possibility of solution processing in ambient, leading to roll-to-roll printing. Performance over the past few years has improved, but charge transport in these materials is not yet fully understood. These conjugated polymers generally form a semicrystalline microstructure, consisting of lamellar crystalline regions separated by amorphous grain boundaries. A mobility edge model has been proposed that suggests charge delocalization in the crystallite regions where mobile states exist, with the effective mobility limited by the localized states within the disordered grain boundaries and at defects. We have used a means of controlling the orientation and size of crystallites in the plane of the substrate to explore the relationship between trap density within grain boundaries and charge transport. Regioregular poly-3-hexylthiopene (P3HT) is the material under investigation. We have fabricated anisotropic films on glass and silicon substrates by taking advantage of the needle-like growth of 1,3,5-tricholorobenzene (TCB). We use TCB first as a solvent at an high temperature and then as a nucleating agent and substrate for epitaxy once cooled. When TCB is removed from the P3HT/TCB film, an oriented single-component polymer film is left behind, consisting of large (mm2) domains where the film extinguishes uniformly under crossed polarizers, suggesting long range orientation of the polymer chain axis. Characterization with an AFM reveals crystalline lamellae stacked along the fiber direction. Films were further characterized at the Stanford Synchrotron Radiation facility. We have collected 2D diffraction patterns, specular diffraction patterns, and grazing incidence diffraction patterns. Charge transport in the directionally crystallized films was probed by measuring in plane 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 were used to explore the effect of microstructure on trap distribution.

10:30 AM - F1.4

Anisotropic Transition in the Transport Mechanism in a Thin Film Phase Standing Pentacene Monolayer Studied by Angle-resolved Photoelectron Spectroscopy.

Toshihiro Shimada ¹
¹Department of Chemistry, University of Tokyo, Tokyo Japan

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10:45 AM - F1.5

Hopping Mobility of Perfluoropentacene Films Measured with High-resolution UPS: Impacts of Intermolecular Interaction on the Hole-vibration Coupling.

Satoshi Kera ¹, Shunsuke Hosoumi ¹, Shin-ichi Nagamatsu ¹, Nobuo Ueno ¹
¹Graduate School of Advanced Integration Science, Chiba Univ, Chiba Japan

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Pentacene (PEN) is currently the most potential conjugated organic molecule as active material in novel electronic devices, such as organic thin-film field effect transistors (OFFT). Therefore, extensive research efforts are being undertaken to investigate its electronic and electrical properties from both of fundamental and applied aspects. A newly synthesized molecule of perfluoropentacene (PFP) was reported to act as n-channel OFET and ambipolar transistors with pentacene was fabricated [1]. When one discusses the hopping charge mobility, both the intramolecular charge reorganization energy (λ), which is related to the electron/hole-phonon coupling, and the transfer integral (t), which is related to the intermolecular electronic interaction, are the key parameters. That is, in principle, larger t and smaller λ are required in order to increase the charge mobility. However, very few detailed discussion on λ and t has been performed so far due to lack of high-resolution ultraviolet photoelectron spectra (UPS) of organic films. Recently, we have succeeded to assess λ and t directly from the fine features in high-resolution UPS of a well-ordered organic monolayer deposited on graphite [2,3]. In this paper, we will report determination of both λ and t with high-resolution UPS of a PFP bilayer on graphite. Furthermore, the electronic structures of PEN(monolayer)/graphite, PFP(monolayer)/graphite and PFP(bilayer)/graphite are compared. The UPS band derived from the highest occupied state (HOMO) for the two monolayer systems (PEN and PFP) clearly shows fine structures, and the band shape at 50 K is significantly different between the two systems due to changes in the vibration energy and the wave function spread of HOMO. The results of the monolayers show that λ of PFP is 2.4 times larger than PEN. Moreover, we observed a considerable difference in the HOMO band shape between the monolayer and bilayer of PFP. The HOMO of the bilayer splits into two prominent peaks with vibration satellites. The splitting could be explained by the bonding and antibonding states due to the strong HOMO-HOMO interorbital interaction [3]. The vibration-satellite intensities in the bilayer are reduced to ca. 80 % of those in the PFP monolayer due to the interorbital interaction. We could estimate the hopping mobility (μ) of a PFP film to be about 10 cm²/Vs at 300 K using the energy separation (2t) of 0.43 eV and λ of 0.184 eV at 300K.[1] Y. Inoue et al, Jpn, J. Appl. Phys. 44, 3663 (2005) and Y. Sakamoto et al, J. Am. Chem. Soc. 126, 8138 (2004).[2] S. Kera et al, Chem. Phys. Lett. 364, 93 (2002) and H. Yamane et al, Phys. Rev. B 72, 153412 (2005). [3] S. Kera et al, Phys. Rev. B 75, 121305R (20

11:00 AM - *

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11:30 AM - **F1.6

Experimental Manifestation of Excess Electron Delocalization in Oligoacene Molecular Aggregates.

Atsushi Nakajima ^{1 2}, Masaaki Mitsui ¹
¹Department of Chemistry, Keio University, Yokohama Japan, ², JST-CREST, Yokohama Japan

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Organic molecular clusters, consisting of 2-1000 molecules, are finite aggregates, providing a microscopic model to investigate the chemical and physical properties of molecular assemblies in organic nanomaterials or at interface. A unique nature of organic molecular clusters is originated from weak intermolecular van der Waals interactions, and thus organic molecules in a cluster fairly preserve their molecular identity. This feature results in narrow electronic bandwidths, and electron (or hole) charge carriers in organic molecular aggregates are often strongly localized on individual molecules, where molecular ions are formed. Such a molecular ion is instantaneously stabilized by the electronic polarization of its neutral neighbors, which significantly affects charge-transport energy levels in organic molecular aggregates. An intriguing subject of research on organic molecular clusters is to explore the evolution of the structural, electronic, thermodynamic, and chemical properties from molecular to condensed matter systems in a step-by-step manner. Since the number of constituent molecules can be tuned accurately in combination with mass spectrometry, size-selective investigations on a broad size range of organic molecular clusters can provide a deep understanding on the evolution of the electronic and geometric properties. In particular, oligoacenes such as tetracene and pentacene are classical examples for the study of charge carrier localization and transport. We have recently realized an efficient formation of large oligoacene molecular nanoclusters up to more than 200 constituent molecules; naphthalene (Nph), anthracene (Ac), tetracene (Tc), and so on. By adding a single excess electron, the corresponding nanocluster anions were produced in the gas-phase, and the size-selective properties could be revealed by photoelectron spectroscopy. In particular, two types of anion states are shown to coexist in nanometer-scale cluster anions of oligoacene of Nph, Ac, and Tc. The photoelectron spectra of size-selected cluster anions containing 2 to 100 molecules revealed that rigid “crystal-like” cluster anions emerge, greater than ~2 nanometers in size, and coexist with the “disordered” cluster anion in which the surrounding neutral molecules are reorganizing around the charge core. These two anion states appear to be correlated to negative polaronic states formed in the corresponding crystals.[1] M. Mitsui, N. Ando, A. Nakajima, K. Kaya, et al., J. Chem. Phys. 121, 7553 (2004).[2] M. Mitsui, A. Nakajima, K. Kaya, U. Even, J. Chem. Phys. 115, 5707 (2001).[3] N. Ando, S. Kokubo, M. Mitsui, A. Nakajima, Chem. Phys. Lett. 389, 279 (2004).[4] M. Mitsui, A. Nakajima, Bull. Chem. Soc. Jpn. (2007) in press[5] M. Mitsui, N. Ando, A. Nakajima, submitted.

12:00 PM - **F1.7

Field Effect Transistors using Charge-transfer-complex Layer.

Kazuhiro Kudo ¹, Masatoshi Sakai ¹, Masakazu Nakamura ¹
¹Electrical and Electronics Eng., Chiba University, Chiba Japan

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12:30 PM - F1.8

Modeling of Charge Transport in an Hybrid Metal / Organic / Inorganic Device.

Henry Mendez ¹, Ilja Thurzo ², Dietrich Zahn ²
¹Physics, Pontificia Universidad Javeriana, Bogota, Cundinamarca, Colombia, ²Physics, Chemnitz University of Technology, Chemnitz, Sachsen, Germany

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12:45 PM - F1.9

Interfacial Structure Modifying Interlayers: Impact of LiF Deposition on Highly Ordered Semiconducting Organic Thin Films.

Ayse Turak ¹, E. Barrena ¹, H. Dosch ¹
¹, Max Planck Institute for Metals Research, Stuttgart, Baden-Wuerttemberg, Germany

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F2: Organic Thin Film Transistors

Session Chairs

Kazuhiro Kudo

H. Sirringhaus

Monday PM, November 26, 2007

Back Bay C (Sheraton)

2:30 PM - **F2.1

Organic Electronics: Towards the Realization of an Ideal Device.

Christof Woell ¹
¹Phys. Chemistry I, Univ. Bochum, Bochum Germany

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After light emitting diodes made form organic materials (OLEDs) have already reached the market also organic field effect transistors (OFETs) using organic molecules as active semiconductors are reaching the stage where products come into sight, e.g. in “smart” identification tags. For transistors based on oligomers (rather than polymers) organic molecular beam deposition (OMBD) is a key requisite for the defined manufacturing of working devices. Since correlations of organic thin film structural quality with electrical properties have revealed that defects in the organic material severely limit the mobility of the charge carriers, a key parameter for the switching-speed of an OFET, the fabrication of high-performance devices will require the ability to grow highly ordered, epitaxial thin films on appropriate substrates. True epitaxy yielding structural qualities comparable to that of single crystals, however, has been realized only in a few cases [1]. The reasons for these severe difficulties, including the effect of molecular conformations [2], will be discussed in this talk. Recent work has demonstrated that one of the most severe limitations results from pronounced dewetting [3,4].The last point to be presented in the talk is the fabrication of an “ideal” organic electronic device, where true intrinsic properties governing carrier injection and transport can be studied in the absence of any structural defects. The current-voltage characteristics of this device reveal a pronounced asymmetry. For negative polarity, the current characteristics is almost independent of the layerthickness. For positive polarity, the current onset is shifted significantly to larger voltages withincreasing layer thickness. Numerical simulations for a two-dimensional model system allow toidentify as reasons for this pronounced asymmetry the injection properties of the STM tip. Fornegative substrate bias the creation of holes in the valence band occurs by tunneling of electrons to the tip whereas in the opposite case holes have to be transported through the OSC-layer from the substrate. Thus, for low positive voltage the hole current limits the device current. Once the resulting voltage drop between layer and tip becomes larger than the barrier for electron injection, direct tunneling of electrons into the pentacene conduction band becomes possible and n-conduction begins to dominate [5]. Reasons for the presence of n-conduction in pentacene, an organic semiconductor which normally only shows p-conductivity, will be discussed in the talk.References[1] G. Witte and Ch. Wöll, J. Mater. Res., 19, 1889, (2004)[2] D. Käfer, L.Ruppel, G. Witte, Ch. Wöll, Phys. Rev. Lett., 95, 166602, (2005)[3] G. Beernink, T. Strunskus, G. Witte, Ch. Wöll, Appl. Phys. Lett., 85, 398, (2004)[4] S. Lukas, S. Söhnchen, G. Witte, Ch. Wöll, Chem. Phys. Chem, 5, 266 (2004)[5] L. Ruppel, A. Birkner, G. Witte, C. Busse, T. Lindner, G. Paasch, Ch. Wöll, submitted for publication

3:00 PM - F2.2

Surface Potential Imaging of Grain Boundary Effects in Solution Processable Acene-Based Thin Film Transistors.

Lucile Teague ¹, Behrang Hamadani ², Sankar Subramanian ³, John Anthony ³, David Gundlach ², James Kushmerick ²
¹, Savannah River National Laboratory, Aiken, South Dakota, United States, ², National Institute of Standards and Technology, Gaithersburg, Maryland, United States, ³Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States

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3:15 PM - F2.3

Looking into the Interface Between Organic Semiconductor/organic Dielectric Interface in Pentacene Thin-film Transistors: Trapping Mechanisms Characterization, Modeling and Correlation with Transistor Performances.

Emanuele Orgiu ^{1 2}, Mohammad Taki ¹, Simone Locci ^{1 2}, Beatrice Fraboni ³, Annalisa Bonfiglio ^{1 2}
¹DIEE, department of Electrical and Electronic engineering, University of Cagliari, Cagliari Italy, ²Centre S3 nanoStructures and bioSystems at Surfaces, INFM, Modena Italy, ³Dipartimento di Fisica, University of Bologna, Bologna Italy

Show Abstract

The interface between an organic semiconductor/dielectric pair plays a decisive role in the functional performance of the Organic Thin-Film Transistors (OTFTs). Since it is well-known that the charge carriers flow through the channel in the first few nanometers of the channel, namely at the interface between the semiconducting and the insulating layer, a better understanding of which phenomena are involved in the conduction mechanism is due. Designed interface that promotes synergistic interactions between the semiconductor and dielectric is essential in achieving optimum FET performances.Surface properties, i.e. surface energy and surface roughness, of a dielectric layer represent distinctive factors which determine potential improvements in electric characteristics of OTFTs because they strongly affect the pentacene growth. In fact the carrier transport efficiency is increased through the presence of well-ordered crystalline structure and/or the large grain size. On the other hand comparative studies of pentacene grown both on polyvinyl alcohol (PVA) and polyvinylphenol (PVP) have shown that although the well-structured morphology of the pentacene film on PVP should result in better OTFTs performances in terms of mobility of the carriers the performances of OTFTs with a pentacene film grown on PVA were better than the former. In this work we aim at giving a clearer picture of the charge trapping, charge transport and the defects at the interface by means of either Capacitance-Voltage measurements with varying the frequency on Metal-Insulator-Semiconductor (MIS) structures and Ids-Vds/Ids-Vgs characterization on OTFTs devices. Different organic dielectrics have been tested as an insulating layer for the above-mentioned structures, namely PVA, PVP, polydimethylsiloxane (PDMS) and polymethylsilsesquioxane (pMSSQ). The semiconductor employed in all these structures was pentacene that exhibits a pretty higher mobility compared to many organic semiconductors. Variation of the OTFTs performances in terms of mobility, threshold voltage and contact resistance will be shown with only varying the dielectric. Pentacene films have been grown simoultaneously both on MIS and OTFTs structures and in the same environmental conditions (temperature, evaporation rate, light). By doing this we are sure that the only parameter ranging in the experiments is the interface between organic semiconductor and dielectric.Correlation between dielectric and field effect mobility will be provided together with modeling of the classic equation of the transistor that is often misused since it does take into account only the thickness and the permittivity constant of the dielectric. Dielectric/organic semiconductor interface properties rather affect both the mobility of the carriers that flow through the channel and the contact resistance as it can be shown by our investigations. The role of the dielectric in the physics of OTFTs saturation regime will be covered as well.

3:30 PM - F2.4

Current Injection in Top Contact Pentacene Thin Film Transistors with Copper Electrodes.

Sui-Dong Wang ^{1 2}, Kazuhito Tsukagoshi ^{1 2}, Takeo Minari ^{1 2}, Tetsuhiko Miyadera ^{1 2}, Yoshinobu Aoyagi ^{1 2}
¹, The Institute of Physical and Chemical Research (RIKEN), Wako, Saitama Japan, ², CREST, JST, Honcho, Kawaguchi, Saitama Japan

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The performance of organic thin film transistors (OTFTs) have been improved in the last decade, however, large contact resistance (R_C) of OTFTs is one of the main problems for practical applications. Gold (Au) is now commonly used as source and drain electrodes in OTFTs due to its large work function. Other metals with lower cost, for example copper (Cu), are highly desirable in manufacturing. We found that copper (Cu) electrodes can produce lower R_C in top contact pentacene TFTs than Au, though Cu is known to have smaller work function. The transfer line method (TLM) [1] was employed to derive channel mobility and R_C in the pentacene TFTs. We define the local charge mobility in the contact region as contact mobility (μ_C), which can be simply calculated from R_C. From the temperature dependence of μ_C, μ_C for both Cu and Au appear thermally activated with activation energies (E_A) around several tens meV, and follow the Meyer-Neldel relation [2]. Furthermore, the Meyer-Neldel energy (E_MN) in the Cu contacts is much lower than that in the Au contacts, implying a narrower trap states distribution in the case of Cu. On the other hand, Cu contacts show relatively weak gate-bias (V_GS) dependence of R_C than Au, and the V_GS dependence decrease with increasing temperature. These features can be well explained by the low E_MN for the Cu contacts. We argue that R_C in OTFTs is not dominated by the interfacial Schottky barrier, but by the access resistance to the channel. The reasons are that (1) If R_C is dominated by Schottky barrier, the current should be limited by injection barrier and flow by either tunneling or thermal excitation processes across the barrier. The tunneling process is temperature independent, the thermal activation behavior of μ_C rule out this possibility. (2) The E_A of μ_C are much smaller than the reported Schottky barrier height for pentacene/Au interface, and the Cu contacts show higher μ_C in spite of an expected higher hole injection barrier. Therefore, the thermal excitation process across injection barrier cannot interpret the results. (3) Both the tunneling and thermal excitation processes predict strong electric field dependence of the injection current, however, the derived electric field at the contacts are almost unchanged and decreased with the drain current for Cu and Au, respectively. This confirms our argument. The experimental results suggest that the conduction through the contact region in OTFTs is limited by exponentially distributed trap states, the effects of the trap distribution will be discussed in the presentation.[1] T. Minari et al, Appl. Phys. Lett. 88 (2006) 083514.[2] T. Minari et al, Appl. Phys. Lett. accepted.

3:45 PM - F2.5

Frequency Response Analysis of Pentacene Thin Film Transistors and the Effect of Contact.

Tetsuhiko Miyadera ^{1 2}, Takeo Mnari ^{1 2}, Sui-Dong Wang ^{1 2}, Kazuhito Tsukagoshi ^{1 2}, Hiromi Ito ¹, Yoshinobu Aoyagi ^{1 2 3}
¹, RIKEN, Wako, Saitama, Japan, ², JST-CREST, Kawaguchi, Saitama, Japan, ³, Tokyo Inst. of Tec., Nagatsuda, Kanagawa, Japan

Show Abstract

Frequency response analysis is one of the most significant issues of organic thin film transistors (OTFTs). However, only a few papers have been reported on this subject. In this report, we present frequency response analysis of pentacene OTFTs and metal - insulator - semiconductor (MIS) capacitors fabricated on Si/SiO₂ substrates. Based on a simple CR equivalent circuit, the cutoff frequency (f_c) can be expressed as f_c ~ V_gμL^-2. Based on the model, channel length reduction and the increase of mobility improve the f_c. However, the parasitic impedance distributed at various interfaces in an OTFT limits the response speed of the practical OTFT. In this work, the contact interface was found to limit the ac-characteristics of the OTFT. MIS analysis revealed the existence of parasitic impedance at the contact. The model for current transport at the contact was proposed. The parasitic impedance was analyzed quantitatively by means of Cole-Cole plot. The equivalent circuit model consists of charge injection barrier (C_i, R_i) and charge transport obstacle underneath the contact (C_b, R_b) well reproduced the measurement results. The parasitic impedance was effectively suppressed by local doping of F₄TCNQ inserted at the interface between electrode and pentacene thin film. The ac-characteristics of OTFT were measured with different contact condition. The doped contact OTFT with short channel length (L = 100 um) reached a higher f_c, up to 98 kHz, compared with the non-doped contact OTFT. The F₄TCNQ doping effectively improves the ac-characteristics. The equivalent circuit model of an OTFT with parasitic impedance was proposed (T. Miyadera et. al., Appl. Phys. Lett., in press). The model well agrees with the frequency response results and the effect of the parasitic impedance at the contact was analyzed quantitatively. From the ac-response measurement and model analysis, the suppression of parasitic impedance at the contact by F₄TCNQ doping was found to improve the f_c. These results indicated that the F₄TCNQ doping for short channel length OTFTs is a significant approach to achieve high-speed operation of OTFTs.

4:00 PM - *

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4:30 PM - F2.6

Using Self-assembled Monolayer to Improve the Charge Injection in Field Effect Transistors.

Saghar Khodabakhsh ^{1 2}, Ruth Rawcliffe ³, Steve Francis ⁴, Neville Richardson ⁴, Alasdair Campbell ³, Tim Jones ²
¹Chemistry, University of Cambridge, Cambridge United Kingdom, ²Chemistry, Imperial College London, Londn United Kingdom, ³Physics, Imperial College London, Londn United Kingdom, ⁴Chemistry, University of St. Andrews, St. Andrews United Kingdom

Show Abstract

Self-assembled molecules containing aromatic molecules are of great interest because of their potential application in optoelectronic devices. Self-assembly of small aromatic thiols on gold substrates is studied using scanning tunnelling microscopy (STM), polarisation modulation reflection absorption infra-red spectroscopy (PM-IRRAS) and Goniometry is studied in this work. Aromatic thiols are of greater interest because of their conjugated nature as opposed to conventional aliphatic thiols which exhibit insulating properties due to them comprising long carbon chains. However, two aliphatic thiols were also studied in order to compare the insulating properties. The aromatic molecules are all thiophenol derivatives, with different Para substitutions, chlorine (CTP), fluorine (FTP), and methoxy (MOTP). 2,2,2-trifluoro ethne thiol (TFET) and 1h1h2h2h-perfluoro-1-decanethiol (MAX) were also studied as short and long aliphatic thiols, respectively. Direct evidence for chemisorption of thiols on Au substrate was obtained using PM-IRRAS. It was found that assembling a monolayer of aromatic thiols onto gold, results in formation of gold islands, which agrees well with literature. The formation of Au islands is attributed to the increased mobility of gold atoms as result of pi-pi interactions within the monolayer. Assembling a monolayer of molecules with large dipole moments result in creation of an electric field just outside the Au substrate which ultimately result in dramatic increase in the work function of Au. Au substrates coated with SAMs were then used as an electrode in polymer field effect transistors. We also demonstrated that the field-effect mobility is limited by injection and contact resistance originating from a work function mismatch between the metal and organic layer. This can be reduced or increased by modifying the metallic electrode using thiols to induce a dipole at the interface. The magnitude and direction of this dipole directly affects values derived for the contact resistance and mobility. Aromatic thiols performed best, as the short, conjugated spacer groups did not significantly inhibit injection, as compared to alkyl spacer groups. This is certainly a very useful means to combat the problem of non-ohmic injection in transistor structures, which is becoming more commonplace as air stable polymers tend to have ionisation potentials higher than gold.

4:45 PM - F2.7

Investigation of Carrier Modulation at the Semiconductor Channel in Organic Ferroelectric Field Effect Transistor.

Takuya Honjo ¹, Kei Noda ¹, Shuichiro Kuwajima ¹, Kenji Ishida ², Hirofumi Yamada ¹, Kazumi Matsushige ¹
¹Electric Science & Engineering, Kyoto University, Kyoto City Japan, ²Chemical Science & Engineering, Kobe University, Kobe City Japan

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5:00 PM - F2.8

Controlling the Characteristics of Organic Thin Film Transistors through Interface Modifications.

Peter Pacher ¹, Barbara Stadlober ², Alexandra Lex ³, Veronika Proschek ¹, Ursula Haas ², Herbert Goled ², Anja Haase ², Norbert Koch ⁴, Stephan Rentenberger ¹, Gregor Trimmel ³, Christian Slugovc ³, Egbert Zojer ¹
¹Institute of Solid State Physics, Graz University of Technology, Graz Austria, ²Institute of Nanostructured Materials and Photonics, Joanneum Research, Weiz Austria, ³Institute for Chemistry and Technology of Organic Materials, Graz University of Technology, Graz Austria, ⁴Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany

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In this contribution we discuss two conceptionally different approaches to controlling the characteristics of organic thin film transistors (OTFTs) by modifying the properties of the interface between the active layer and either the dielectric or the metal electrodes.In the first approach, a covalently bond reactive layer containing sulfonyl chloride and sulfonic acid groups is prepared on the surface of the SiO2 gate dielectric prior to the deposition of the active layer (here, poly(3-hexylthiophene). This results in a large positive turn-on voltage (VT) of around 50 V. When exposing the device to a variety of amines (in particular to ammonia vapour), VT shifts to negative voltages by approximately 65 V. This huge effect is caused by the interaction of the interfacial layer with the active material and is attributed to an efficient, localized chemical doping and dedoping mechanism, acting in close spatial proximity of the conducting channel. This process allows a switching of the device from depletion to accumulation mode and holds high promise for application in chemical sensing.In the second set of experiments, we optimize the contact resistance in short-channel bottom-contact OTFTs (in this case based on pentacene) by a UV/ozone treatment of the Au electrodes. This procedure is highly effective in (i) decreasing the hole-injection barrier between Au and pentacene (as measured by photoelectron spectroscopy) and (ii) in improving the morphology of pentacene on top of the Au contacts. In this way, the contact resistance in our devices is reduced by an order of magnitude reaching values as low as 80 Ohm.cm for gate voltages well above the threshold and a channel-length independent carrier mobility in the range of 0.3cm2/Vs<µ<0.4cm2/Vs is observed.In both cases, an in-depth analysis of the interface properties turns out to be crucial for understanding the processes in the devices. Therefore, the interfaces were studies by numerous surface sensitive techniques including X-ray photoelectron spectroscopy, UV photoelectron spectroscopy, surface infrared absorption, and atomic force microscopy.Financial support by the ISOTEC cluster of the Austrian Nano-initiative is gratefully acknowledged.

5:15 PM - **F2.9

Polymeric Diodes for Non-volatile Memories.

Kamal Assadi ¹, Dago de Leeuw ^{1 2}, Bert de Boer ¹, Paul Blom ¹
¹Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, ², Philips Research Laboratories, Eindhoven Netherlands

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5:45 PM - F2.10

Charging Effects in Hybrid Structures Based on Polyoxometalate Layers for Molecular Memory Applications.

Eleni Makarona ¹, Antonios Douvas ¹, Eleftherios Kapetanakis ¹, Dimitris Velessiotis ¹, Panagiotis Argitis ¹, Pascal Normand ¹, Nikos Glezos ¹, Jerzy Mielczarski ², Ela Mielczarski ², Teodor Gotszalk ³, Woszczyna Miroslav ³
¹Inst. of Microelectronics, NCSR "Demokritos", Athens Greece, ², LEM, INPL/CNRS, Vandoeuvre lès Nancy France, ³, Faculty of Microsystem Electronics and Photonics ul. Janiszewskiego , Wroclaw Poland

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We present a vertical device of hybrid organic/inorganic self-assembled monolayers (SAMs) fabricated with the combination of the layer-by-layer (LBL) method with a CMOS-compatible process. The monolayers were based on tungsten polyoxometalates (POM) which recently have attracted considerable research interest as candidates for molecular electronics applications due to a unique combination of properties. POMs, especially the tungsten and molybdenum ones, have well-defined and stable structure consisted of coordination polyhedra MOn that have a metal ion in their center and connect each other through common edges and apices. In POMs electron exchange reactions take place without significant alterations of the molecular structure (mainly Keggin structure), whereas electron delocalization to many metal centers is encountered, to some extent depending on the specific structure and composition, even at room temperature. The POM monolayers were organized on wet oxidized p- and n-type Si substrates on top of Amino-Propyl-Tetraethoxy Silane (APTES) SAMs. The fabrication of capacitor-like MIS structures was then completed by coverage with an Al top electrode deposited by e-beam evaporation. POM monolayers were fabricated in order to deconvolute the role of different substrates, while samples with only the wet oxide layer were used as reference. All types of devices exhibited remarkable stability with no signs of degradation even after 2 months of storage under ambient conditions. Capacitance versus voltage (C-V) and current versus voltage (I-V) characteristics were obtained and analyzed. STM measurements were also performed in order to complement our study of the SAM transport properties on the microscopic level. The charging and transport properties of the SAM capacitors are important for the the future development of hybrid memory elements. It was proved that the POM molecules can be selectively charged from either side depending on the applied voltage and the selection of the type of substrate.Fabrication of well-organized POM/1,12-diaminododecane multilayers following a layer-by-layer method has been also demonstrated. The electron transport through these structures with Scanning Tunneling Microscopy, revealing molecular charging effects through hysterisis loops, will be discussed.

F3: Poster Session: Interface I

Session Chairs

George Malliaras

Nobuo Ueno

Tuesday AM, November 27, 2007

Exhibition Hall D (Hynes)

9:00 PM - F3.1

Effect of Hydroxyl Group on the Characteristics of Solution Processed n- and p-type OFETs using Poly(p-silsesquioxane) Derivatives as a Gate Insulator.

Yutaka Ohmori ¹, Hirotake Kajii ¹, Shohei Fukuda ¹, Toshiyuki Ogata ², Motoki Takahashi ²
¹Ctr for Adv Sci & Innov (CASI), Osaka University, Suita, Osaka, Japan, ², Tokyo Ohka Kogyo Co. Ltd, Koza-Gun, Kanagawa, Japan

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Organic field effect transistors (OFETs) fabricated on the polymer substrate including conductive organic materials and polymeric gate insulator is one of promising devices for the flexible electronic devices. For OFETs with a conductive organic layer formed on the gate insulating materials, the performance of devices strongly depends on the molecular structure of the gate insulators. In this study, we investigated the effect of hydroxyl group of gate insulating materials on the characteristics of dihexylsexithiophene (DH6T) OFETs for p-type, and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) for n-type conducting layer, using poly(p-silsesquioxane) (PSQ) derivatives as a polymeric gate insulator, which contained various ratios of phenol-group with a hydroxyl group bonded to a phenyl ring in the side chain of their molecular structures. Four kinds of PSQ derivatives have been prepared in the experiments. PSQ (all-OH) has a hydroxyl group in all the side chain. The ratio of the hydroxyl contained in the polymer insulator decreases in the order of PSQ (all-OH), PSQ (part-OH), PSQ (part-CH3) and PSQ (all-CH3). PSQ (all-CH3) does not have a hydroxyl group in all the side chain. For DH-6T OFETs, the threshold voltage is shifted from + side to – side in the order of PSQ (all-OH), PSQ (part-OH), PSQ (part-CH3) and PSQ (all-CH3). From the results for the DH6T OFETs, the threshold voltage is dependent on the ration of hydroxyl group in the polymer gate insulator. The current on/off ratio of OFET increases with decreasing the ration of hydroxyl group, when the gate voltage is scanned from 0 to –30 V. The OFET with PSQ (part-OH) has a maximum carrier mobility of 0.14cm2/Vs. N-type conducting PCBM is one of the candidate materials to realize n-type transistors with solution process. First, thin film of CsF layer were examined as a carrier injection layer into the conducting layer of OFETs with Ag metal as source and drain electrodes. The electron mobility of the OFETs decreased with increasing CsF layer thickness. On the other hand, the threshold voltage of the OFETs drastically decreased at the CsF layer thickness of 0.5-2 nm, and then increased above 3nm. This results shows that the threshold voltage of the OFETs has a minimum value at around 2-nm-thick CsF layer inserted between metal electrode and the organic layer. Since the CsF layer is an insulating layer, optimum thickness of CsF layer realizes the carrier injection from the electrodes into organic layer through CsF layer.We investigated the effect of hydroxyl group of polymer gate insulators on the characteristics of PCBM OFETs using PSQ derivatives. The electron mobility increases with decreasing the ratio of hydroxyl group in side chains of the polymer gate insulator, and the OFET with PSQ (part-OH) has a maximum carrier mobility of 0.024 cm2/Vs.

9:00 PM - F3.10

Properties of Blue Light Emitting Diodes Coated on Surface Treated ITO/Glass Substrates.

Sang Baie Shin ¹, Su Cheol Gong ¹, Ji Keun Chang ¹, Ho Jung Chang ¹, Young Chul Chang ², Yong Bin Sun ³
¹Electronics Engineering, Dankook University, Cheonan, Chungnam, Korea (the Republic of), ²Mechatronics Engineering, Korea University of Technology and Education, Cheonan, Chungnam, Korea (the Republic of), ³The graduate school of industrial technology and information, Kyonggi University, Suwon, Kyoung gi, Korea (the Republic of)

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Generally, organic light emitting diodes (LEDs) are classified into two groups such as low molecule LED and polymer LED (PLED) depending on the applied emitting materials in the devices. PLEDs have attracted much attention for the applications of large and flexible displays because of the simple structure and soluble processes by using spin coating, ink-jet printing and deep coating method. Until now, many researchers have been studied on the PLED devices. However, there are few papers on the effect of ITO plasma treatment and the optimization of film structure. In this study, we fabricated the blue lighting PLED with ITO/PEDOT:PSS/(PVK)/PFO-poss/LiF/Al structure. All organic film layers were prepared by the spin coating method on the plasma and heat treated ITO(indium tin oxide)/glass substrates. PFO-poss [polyhedral oligomeric silsesquioxane-terminated poly(9,9-dioctyl fluorine)] and PVK [N-vinycabozole] were used as the light emitting and hole transport(electron blocking) materials, respectively. The heat treatment of ITO/Glass substrate was carried out at 180°C for 2 hrs in a vacuum oven. The condition of O2 plasma treatment of ITO/glass substrate was 100 watt for 30 sec in RF power under 40 mtorr pressure. The electrical and optical properties of PLEDs were investigated by using HP4145B semiconductor measurement system and CS-1000 spectro radiometer. The surface morphology of ITO electrode films was analyzed by the atomic force microscope(AFM).The dependences of the plasma and heat treatment of ITO anode films on the optical and electrical properties of the PLEDs were investigated. From the AFM measurement, the RMS value of the ITO film without plasma treatment was 4.8 nm, and decreased to 2.7 nm when the ITO film was treated by the plasma and heat treatment. As a result, the surface roughness of the ITO film was improved by the plasma and heat treatment. In order to study the effect of PVK layer, we prepared the PLED samples with PVK and without PVK layer. From the emission spectrum analysis, the intensity of the secondry(excimer) peaks of PFO-poss decreased for the PLED device with PVK film compared with the one without PVK layer. The maximum current density was 428 mA/cm2. The maximum luminance and the light efficiency of the PLEDs were found to be about 486 cd/m2 at 12V and 0.25 lm/watt at 7V, respectively. The color coordinators (CIE chart) of blue PLEDs were ranged x=0.17~0.20, y=0.13~0.15, and the peak emission spectrum was about 430 nm showing a relatively pure blue color.

9:00 PM - F3.11

Characterization and Electrical Properties of Silicon / Porphyrin / Metal Devices.

Franklin Anariba ¹, James Diers ¹, David Bocian ¹
¹Chemistry, University of California, Riverside, California, United States

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9:00 PM - F3.12

Conformation-Dependent Conductance of a Series of Metal-Molecule-Metal Junctions.

Le-Jia Wang ¹, Lin Tan ¹, Hao-Li Zhang ¹
¹State Key Lab of Applied Organic Chemistry, Lanzhou University, Lanzhou China

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9:00 PM - F3.13

The Role of Order on the Diode Mobility in Semiconducting Polymers.

Brian Hardin ¹, Alex Mayer ¹, Michael McGehee ¹
¹Material Science and Engineering , Stanford University , Stanford, California, United States

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9:00 PM - F3.14

Transport Properties through Organic Molecules by First Principles Calculations and Green's Function Approach.

Hiroshi Mizuseki ¹, Rodion Belosludov ¹, Sang Uck Lee ¹, Amir Farajian ^{1 5}, Olga Pupysheva ^{1 5}, Chiranjib Majumder ², Jian-Tao Wang ³, Hao Chen ⁴, Yoshiyuki Kawazoe ¹
¹Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan, ⁵, Center for Nanoscale Science and Technology, Department of Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States, ², Novel Materials and Structural Chemistry Division, Bhabha Atomic Research Center, Mumbai India, ³, Institute of Physics, Chinese Academy of Sciences, Beijing China, ⁴, Physics Department, Fudan University, Shanghai China

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Molecular devices are potential candidates for this next step, and they would make it possible to realize the most advantageous devices. However, a major predicament and source of expenditure is necessary that such a large number of organic molecules can be obtained by synthetic chemistry, so any means of exploring their properties and behavior in order to predict the relevant properties of a molecule in advance of its synthesis would be extremely useful. One established approach is to use the computational methods developed for the prediction of a stable molecular structure and conductance properties. Our group has covered a wide range of molecular system which have potential application in molecular electronics using first-principles calculations [1]; supramolecular enamel wires (covered wires) [2], connection between organic molecules and metal electrodes [3], self-assembled nanowires on silicon surface [4, 5]. Moreover we examine electronic transport properties through small molecules for a building block, such as fused thiophene rings [6, 7], bent carbon nanotube, DNA, porphyrin and ferrocene[8] molecules and so on. In this presentation, we will present recent investigations related to organic devices, using molecular orbital analysis.[1] http://www-lab.imr.edu/~mizuseki/nanowire.html[2] R. V. Belosludov, A. A. Farajian, H. Mizuseki, K. Ichinoseki, and Y. Kawazoe, Jpn. J. Appl. Phys., 43, 2061 (2004).[3] C. Majumder, H. Mizuseki, and Y. Kawazoe, J. Chem. Phys., 118, 9809 (2003).[4] J.-T. Wang, C. Chen, E. G. Wang, D.-S. Wang, H. Mizuseki, and Y. Kawazoe, Phys. Rev. Lett., 97, 046103 (2006).[5] R. V. Belosludov, A. A. Farajian, H. Mizuseki, K. Miki, and Y. Kawazoe, Phys. Rev. B, 75, 113411 (2007).[6] Y. X. Zhou, F. Jiang, H. Chen, R. Note, H. Mizuseki, and Y. Kawazoe, Phys. Rev. B, 75, 245407 (2007).[7] F. Jiang, Y. X. Zhou, H. Chen, R. Note, H. Mizuseki, and Y. Kawazoe, J. Chem. Phys., 125, 084710 (2006).[8] T. Uehara, R. V. Belosludov, A. A. Farajian, H. Mizuseki, and Y. Kawazoe, Jpn. J. Appl. Phys., 45, 3768 (2006).

9:00 PM - F3.16

Statistical Analysis of Electronic Transport Properties of Metal-Molecule-Metal Junctions: Criteria for Intrinsic Molecular Devices.

Takhee Lee ¹, Tae-Wook Kim ¹, Hyunwook Song ¹, Gunuk Wang ¹
¹Materials Science and Engineering, Gwangju Institute of Science and Technology, Gwangju Korea (the Republic of)

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9:00 PM - F3.17

Organic Logic Devices Based on Pentacene and Zinc Oxide Materials for Flexible Displays.

Yasuyuki Watanabe ¹, Hiroyuki Iechi ^{2 4}, Hiroshi Yamauchi ³, Kazuhiro Kudo ^{3 4}
¹Center for Frontier Science, Chiba University , Chiba Japan, ²Advanced Technology R&D Center, Research and Development Group, Ricoh Co. Ltd, Yokohama Japan, ⁴Graduate school of Engineering, Chiba University, Chiba Japan, ³Faculty of Engineering, Chiba University, Chiba Japan

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Recently, flexible electronic devices have attracted much attention to the potential application for the flexible displays, radio frequency identification cards (RFIDs) and photovoltaic cells etc. It has been reported in the articles that flexible pentacene field effect transistors (FETs) [1] and flexible amorphous InGaZnO FETs [2] show the stable characteristics under bending condition. For the more advancing flexible electronics, it is important to fabricate the electronic devices based on p-type and n-type semiconductors. For example, logic circuits are very important devices for realizing flexible sheet displays. To realize the complementary circuits and logic circuits, both the p-channel and n-channel transistors are needed. It has been known that most of organic semiconductors with high carrier mobility, such as pentacene, show p-type characteristics and ZnO thin film show n-type characteristics. We reported the basic characteristics of various electronic devices based on organic semiconductors and/or ZnO thin film such as photovoltaic cells, organic light-emitting transistors (OLETs) [3] and inverter circuits [4]. It should be noted that pentacene SITs with vertical structure on flexible substrate show stable electronic characteristics under bending condition [5]. On the other hand, we have also investigated the electrical characteristics of static induction transistors (SITs) based on pentacene [6] and ZnO [7] for the realization of low voltage operation. In particular, we propose that the new type OLETs using pentacene SITs and ZnO thin film. In addition, we also report that the basic characteristics of the logic devices with new device structures consist of p-channel pentacene transistors and n-channel ZnO transistors. The obtained results demonstrate that there are two key points for improvement in the characteristics of electronic devices using pentacene SITs and/or ZnO materials. In pentacene SITs, it is important to control the electronic interface state at the interface of pentacene/ITO for fabrication of the organic logic devices and the OLEDs. Furthermore, it is indispensable to control the film orientation associate with the electronic characteristics of ZnO thin films used in FETs and OLETs.References[1] T. Sekitani, Y. Kato, S. Iba, H. Shinaoka, T. Someya, T. Sakurai, and S.Takagi, Appl. Phys. Lett. 86, 073511 (2005).[2] K. Nomura, H. Ohta, A. Takagi, T. Kamiya, M. Hirano, and H. Hosono, Nature (London) 432, 488 (2004).[3] H. Yamauchi, M. Iizuka, and K. Kudo, Jpn. J. Appl. Phys. 46, 4B, 2678 (2007). [4] H. Iechi, Y. Watanabe and K. Kudo, Jpn. J. Appl. Phys. 46,4B, 2645 (2007).[5] Y. Watanabe, H. Iechi and K. Kudo, Appl. Phys. Lett. 89, 233509 (2006). [6] Y. Watanabe, H. Iechi and K. Kudo, Jpn. J. Appl. Phys. 46, 4B, 2717 (2007).[7] H. Iechi, M.Sakai, K. Nakamura, M. Iizuka. M. Nakamura and K. Kudo, Synthetic Metals 154, 149 (2005).

9:00 PM - F3.18

Low Ionization Energy Molecules for Efficient N-doping of Organic Films.

Calvin Chan ¹, Stephen Barlow ², Seth Marder ², Antoine Kahn ¹
¹Dept. of Electrical Engineering, Princeton University, Princeton, New Jersey, United States, ²Dept. of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States

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Chemical or electrical doping of molecular organic films is recognized as a viable means of improving charge injection and transport in organic devices, and for creating semiconductor device junctions. P-doping with the electronegative molecule tetrafluoro-tetracyano-quinodimethane (F₄-TCNQ) has been used on a number of hole-transport materials. Yet, molecular n-type doping has been hindered by the energetic requirements for electron transfer from the dopant to host. The low dopant ionization energy (IE) required for such a charge transfer makes donor molecules prone to oxidation. Until recently, n-doping has been mainly the domain of fast diffusing and inefficient alkali metals (i.e., Li or Cs) and their compounds.

We have recently demonstrated n-doping of the electron transport material tris{2,5-bis(3,5-bis-trifluoromethyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene (THAP), which has an electron affinity (EA) of 4.59 eV as measured by inverse photoemission spectroscopy (IPES). The donor molecule was cobaltocene (CoCp, IE = 4.07 eV) [Chan et al., Chem. Phys. Lett. 431, 67-71 (2006)]. In this current work, we introduce an even stronger molecular n-dopant and demonstrate n-doping of copper phthalocyanine (CuPc, EA = 3.25 eV) by decamethylcobaltocene (CoCp^*₂). CoCp^*₂ has a remarkably low IE of 3.30 eV, as measured by ultra-violet photoemission spectroscopy (UPS). N-doping is observed by a 1.4 eV upward swing of the Fermi-level in the gap of CuPc (single particle gap, Eg = 1.9 eV) between the p- and n-doped films. N-doping is also confirmed by current-voltage (I-V) measurements on undoped, interface-doped, and uniformly doped Au/CuPc/Au devices. A 10⁴- to 10⁷-fold increase in current density of the interface-doped device as compared to the undoped CuPc device is due to enhanced injection. An additional 10³-fold increase in current density is observed for the uniformly doped device and is attributed to enhanced conductivity of the bulk film.

The successful application of p- and n-doping to make organic semiconductor junction devices (e.g., p-i-n and i-n) devices is also demonstrated. For example, p-i-n CuPc homojunction devices show good rectification behavior with an appreciable built in voltage of 1.47 eV, which is promising for organic photovoltaic devices.

9:00 PM - F3.19

Solution-processible Organic Thin Film Transistors with Work-function-tunable Electrodes by Self Assembled Monolayers of Molecules.

Seonghoon Lee ¹, Jungpyo Hong ¹
¹Chemistry, Seoul National University, Seoul Korea (the Republic of)

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9:00 PM - F3.2

Estimation of Field Distrbution in Organic Photovoltaic Device by Kelvin Probe Measurement.

Eiji Itoh ¹, Keiichi Miyairi ¹
¹Department of Electrical and Electronic Engineering, Shinshu University, Nagano Japan

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9:00 PM - F3.20

Electroluminescence Degradation and Its Recovery at an Organic/Organic Interface in OLEDs.

Liang-Sheng Liao ¹
¹Display Science and Technology Center, Eastman Kodak Company, Rochester, New York, United States

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9:00 PM - F3.21

The Influence of Metal-Molecule Contacts on Decay Coefficients and Specific Contact Resistances in Molecular Junctions Studied by Multi-Barrier Tunneling Model

Gunuk Wang ¹, Tae-Wook Kim ¹, Hyoyoung Lee ², Takhee Lee ¹
¹, Department of Materials Science and Engineering,Gwangju Institute of Science and Technology , Gwangju Korea (the Republic of), ², National Creative Research Institute, Center for Smart Molecular Memory, IT Convergence Components Laboratory, Electronics and Telecommunication Research, Daejeon Korea (the Republic of)

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The idea of utilizing functional molecules as the electronic components in future ultrahigh-density electronic devices has generated tremendous attention. Before such device applications, understanding the charge transport mechanisms through molecular systems is important. Over the past years, it has been clear that metal-molecule contacts play an essential role in the charge transport of metal-molecule-metal (M-M-M) junctions [1]. The conductance of molecular junctions strongly depends on the contacts, for example, whether the molecules form chemisorbed or physisorbed contact to the metal electrode. In M-M-M junctions, alkanedithiol (HS-[CH2]n-SH) junctions having chemisorbed contacts ([S-Au]) at both sides showed higher current density (smaller resistance) than that of alkanemonothiol (HS-[CH2]n-1-CH3) junctions having only one chemisorbed contact and the other physisorbed contact ([CH3-Au]) [2]. In this study, we performed current-voltage characterization on different-length alkyl (alkanemonothiol and alkanedithiol) self-assembled monolayers (SAMs) contained in microscale via-hole structures in M-M-M junctions. We fabricated and characterized ~ 30,000 M-M-M molecular devices using alkyl SAMs and obtained large enough device yield from this mass-fabricated molecular devices. Based on the statistical analysis of a large number of molecular devices, we studied charge transport mechanisms, especially the metal-molecule contact effects using a multi-barrier tunneling model where M-M-M junction can be divided into three parts conceptually: the molecular body and two metal-molecule contacts on either side of the molecule. We investigated and compared the current densities and contact resistances of alkanemonothiol versus alkanedithiol M-M-M junctions. Our results revealed that alkanemonothiol and alkanedithiol junctions have almost identical molecular-body electron decay coefficient beta_(Body), which was found to be ~ 0.92Å-1. In the contrast, decay coefficient beta_(o) that can be determined from Simmons tunneling fitting model [3] were different for alkanemonothiol versus alkanedithiol M-M-M junctions. Furthermore, the contact resistances and specific contact resistances were determined and compared for alkanemonothiol versus alkanedithiol M-M-M junctions in terms of different molecular length and different natures of metal-molecule contacts.

9:00 PM - F3.22

STM Study of Mixed Ru Complex/1-Dodecanethiol Self-assembled Monolayers on Au(111).

Alexander Konchenko ¹, Kyoungja Seo ¹, Junhghyun Lee ¹, Hyoyoung Lee ¹
¹Center for Smart Molecular Memory, Electronics and Telecommunications Research Institute (ETRI), Daejeon Korea (the Republic of)

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9:00 PM - F3.23

Dielectric Polymer Materials as Gate Insulator for OTFT.

Zisu Moon ¹, Sunglan Choi ¹, Hongdoo Kim ¹
¹Chemistry, Kyunghee Univ., Yongin Korea (the Republic of)

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9:00 PM - F3.24

Negative Differential Resistance Behaviors from Redox-switchable Metalloproteins on the Carbon Nanotube Nanogap Planar Device.

Qun Tang ¹, Hee cheul Choi ¹
¹chemistry, nanscale materials research lab, Pohang Korea (the Republic of)

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9:00 PM - F3.27

Effect of Self-assembled Monolayer Modified Electrodes on Bottom-contact OFETs.

Jong Won Lee ¹, Ki Pyo Hong ¹, Se Hyun Kim ¹, Sang Yoon Yang ¹, Chan Eon Park ¹
¹Chemical Engineering , POSTECH, Pohang Korea (the Republic of)

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To produce commercial OFET devices, fine lithography is needed and top-contact OFETs are difficult for it due to damage of active layer. Therefore, bottom-contact OFETs are more desirable for a manufacture processes. However, it has been demonstrated that the bottom-contact configuration gives inferior performance to the top-contact one because of relative high contact resistance which were originated from the formation of injection barrier at the metal-organic semiconductor (OSC) interface. It has been shown previously that, by self-assembled molecular layer with intrinsic electric dipole moment, the work function of metal electrodes can be lowered or raised, affecting the size of the injection barrier at the metal-OSC interface. However, limited testing has been made to use this approach to engineer injection barriers in OFET field. In this study, gold surfaces modified with 1-decanethiol (DT), heptadecafluoro-1-decanethiol(HDFDT), tridecafluoro-1-octanethiol(TDFOT) were characterized by He(I) ultra-violet photoelectron spectroscopy (UPS). We have tuned the Au work functions by chemically modifying the Au surface through the formation of chemisorbed self-assembled monolayers (SAMs) having electron donating group and electron withdrawing group. The ordering in the SAMs creates an effective, molecular dipole at the metal/SAM interface, which increased the work function of Au (bare Au~4.9 eV) to 5.5~5.7 eV for electron withdrawing group. On the other hand electron donating group shifted work function of Au to 4.1 eV. The hole injection barriers at the pentacene/Au, pentacene/DT/Au, pentacene/HDFDT/Au and pentacene/TDFOT/Au were also determined using in-situ thin film deposition in combination with X-ray and ultraviolet photoelectron spectroscopy at PAL-4B1 beam line. By using these SAMs to engineer the effective Au work function, the charge injection process was markedly affected. In addition, we have fabricated bottom-contact pentacene-based organic field effect transistors (OFETs) by using these SAMs to investigate the effect of hole injection barriers at interface between the metal electrode and the vacuum deposited pentacene films on the performances of OFETs. We have demonstrated a correlation between hole injection barriers at the metal/OSC interface and device parameters such as mobility, threshold voltage and sub-threshold swing. As tuning OFET electrodes with partially fluorinated alkanethiol SAM, bottom-contact OFET device performance was dramatically improved. We have also found that the different pentacene film morphology between bare Au electrodes and SAM-treated Au electrodes on the side slope and the upper surface of the electrodes was another important factor on the device performance of OFETs. Furthermore, in order to avoid the small grain size region near the Au-electrode, photo-lithography was used to have the almost perpendicular profile of Au-electrode.

9:00 PM - F3.28

Vapor Deposited Aluminum on Polymer Films: Important Aspects for Organic Semiconductor Devices.

Korhan Demirkan ¹, Anoop Mathew ¹, Conan Weiland ¹, Micheal Reid ², Robert Opila ¹
¹Department of Materials Science and Engineering, University of Delaware, Newark, Delaware, United States, ²Centre for Telecommunications Valve-Chain Research, Stokes Institute, University of Limerick, Limerick Ireland

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The clustering and diffusion of aluminum and its chemical interactions, with the polymer surface complicates the electrical properties of the metal/organic semiconductor interface. Microscopic (Atomic Force Microscopy, Scanning Electron Microscopy, Focused Ion Beam) and chemical (X-ray and synchrotron source Ultraviolet Photoelectron Spectroscopy) analyses were performed on aluminum metallized poly(2-methoxy-5,2’- ethyl-hexyloxy-phenylene vinylene) (MEH-PPV), polystyrene and ozone treated polystyrene surfaces in order to investigate the morphology and the interface chemistry. SEM analysis showed that during the initial stages of the metallization, significant clustering of aluminum takes place on all the polymer surfaces. Focused Ion Beam (FIB) cross-sectional analysis showed that the final aluminum layer is notably porous. Photoelectron spectroscopy gives some insight about the degree of chemical interaction for each polymer; namely, for MEH-PPV, the chemical interactions were mainly through the C-O present in the side chain of the polymer structure. The chemical interaction of aluminum with polystyrene was less significant, but it showed an abrupt increase with the ozone treatment of the polystyrene surface (due to the formation of exposed oxygen sites). Results showed a strong relationship between the surface reactivity and the condensation / sticking of the aluminum atoms on the surface where evaporated aluminum is found to have a higher sticking coefficient on oxygen containing organic surfaces. Correspondingly, aluminum metallization on non-oxygen containing organics surface leads to a very slow growth and poor surface wetting (Volmer-Weber type growth). Both cases leads to complicated and undesirable electrical properties at the interface: either due to the presence of an insulating aluminum oxide layer, or due to a reduced contact area (non-uniform/non-continuous interface) between the polymer and the aluminum. The oxygen diffusion through the metal/polymer interface, interface delamination and metal migration are the predominant routes for failure of organic semiconductor devices. Therefore, engineering the right morphology at the interface is crucial for enhanced device lifetime.

9:00 PM - F3.29

Degradation of Hole Injection at the Interface of a Conducting Polymer and a Fluorene Copolymer.

Alexios Papadimitratos ¹, Aharon Yakimov ², Anil Duggal ², Hon Hang Fong ¹, George Malliaras ¹
¹, Cornell University, Ithaca, New York, United States, ², GE Global Research, Niskayuna, New York, United States

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9:00 PM - F3.3

Estimation of Barrier Height at Conducting Polymer / Cathode Interface by Internal Photoemission Measurement Using Three Layered Structure.

Eiji Itoh ¹, Keiichi Miyairi ¹
¹Department of Electrical and Electronic Engineering, Shinshu University, Nagano Japan

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9:00 PM - F3.30

Degradation in iTMC OLEDs: Potential Role of Neutral Complexes.

Leonard Soltzberg ², Velda Goldberg ¹, Michael Kaplan ^{1 2}, Heather Bankowski ^{1 2}, Shannon Browne ^{1 2}, Heather Concannon ^{1 2}, Megan Damour ^{1 2}, Samantha Green ^{1 2}, Elthea Hendrickson ^{1 2}, HengLian Huang ^{1 2}, Virginia Liu ^{1 2}, Lindsey Piirainen ^{2 1}, Suwathna Reel ^{1 2}, George Malliaras ³, Jason Slinker ³, Stefan Bernhard ⁴
²Chemistry, Simmons College, Boston, Massachusetts, United States, ¹Physics, Simmons College, Boston, Massachusetts, United States, ³Materials Science & Engineering, Cornell University, Ithaca, New York, United States, ⁴Chemistry, Princeton University, Princeton, New Jersey, United States

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9:00 PM - F3.31

Solution-Processed Organic Field-Effect Transistors and Unipolar Inverters from Self-Assembled Dipole Gate Dielectrics

Cheng Huang ¹, Howard Katz ¹, James West ²
¹Materials Science and Engineering Department, Johns Hopkins University, Baltimore, Maryland, United States, ²Electrical and Computer Engineering Department, Johns Hopkins University, Baltimore, Maryland, United States

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9:00 PM - F3.32

Correlations Between SFG Spectra and Electrical Properties of Organic Field Effect Transistors.

Hongke Ye ¹, Jia Huang ², Jung-Rae Park ¹, Howard Katz ², David Gracias ¹
¹Chemical and Biomolecular Engineering, Johns Hopkins University, Baltimore, Maryland, United States, ²Materials Science and Engineering, Johns Hopkins University, Baltimore, Maryland, United States

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Transistors fabricated with organic semiconductors allow for the possibility of solution based processing of electronic devices. The ability to monitor the channel region (semiconductor-dielectric interface) within these devices is critical to understanding structural and electronic changes occurring in the layer during gated conduction. Results from such studies could shed light on strategies to develop organic semiconductor devices that overcome some present day limitations such as large operational voltage, irreproducibility and limited reliability. Monitoring the channel region during gated conduction in an operational OFET, however, is very challenging because of the fact that the semiconductor-dielectric interface is buried. Most surface probes rely on vacuum operation, or place considerable restrictions on the types of substrates that can be analyzed. Hence, while there have been many elegant spectroscopic studies of organic semiconductors, many were conducted ex-situ or using restrictive substrates. Additionally since the critical channel in the OFET, the semiconductor-dielectric interface is buried, it is extremely challenging to probe and previous surface spectroscopic studies have focused on characterizing the semiconductor-air interface or the bulk, which may have limited relevance to the operational characteristics of the OFETs. Second order non-linear optical spectroscopies however are inherently surface sensitive and allow nondestructive probing of buried interfaces.We have developed instrumentation that combines second order sum frequency generation (SFG) vibrational spectroscopy and electrical probe measurements. Simultaneous SFG spectra and electrical measurements to obtained on organic field effect transistors (OFETs) fabricated with the semiconductors: 5, 5’-bis(4-hexylphenyl)-2,2’-bithiophene (6pttp6), 5,5’-bis(4-ethylphenyl)-2,2’-bithiophene (2pttp2) and pentacene. In-situ measurements during gating of the OFETs showed strong correlations between vibrational spectra and electronic properties. One correlation involved structural changes in the hexyl and ethyl groups, of 6pttp6 and 2pttp2 respectively, and saturation source-drain current; the correlation was observed only at negative gate voltages (when carrier injection was possible) and was more pronounced for 6pttp6, with the introduction of gauche defects in the longer hexyl chains. A second correlation between the dependence of SF non-resonant background on gate voltage and electronic mobility was observed on OFETs of all three semiconductors, at both positive and negative gate voltages. This correlation suggests that a common molecular structural packing element may determine the magnitude of both the electronic mobility and higher order non-linear optical susceptibilities in oligomeric thin films. These results also demonstrate the utility of SFG in probing molecular structural and electrical field effects at the buried semicronductor-dielectric interface of OFETs.

9:00 PM - F3.33

Atomic-Scale Analysis of Self-Assembled Monolayers on Silicon.

Jui-Ching Lin ¹, Josh Kellar ¹, Jun-hyun Kim ², Nathan Yoder ¹, SonBinh Nguyen ², Mark Hersam ¹, Michael Bedzyk ¹
¹Materials Science and Engineering, Northwestern University, Evanston, Illinois, United States, ²Chemistry, Northwestern University, Evanston, Illinois, United States

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9:00 PM - F3.34

Modification on the Unoccupied Electronic Structure of Organic Semiconductor by Alkali Metal.

Huanjun Ding ¹, Yongli Gao ¹
¹, University of Rochester, Rochester, New York, United States

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9:00 PM - F3.35

Metal Nanoparticle Modification of Transparent Conducting Oxides for Organic Solar Cells and Organic Light Emitting Diodes.

Diogenes Placencia ¹, Neal Armstrong ¹, P. Veneman ¹, Michael Brumbach ¹
¹Chemistry, University of Arizona, Tucson, Arizona, United States

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9:00 PM - F3.36

Coverage Effects in Thiol-bound Self-assembled Monolayers on Gold.

Lorenz Romaner ³, Georg Heimel ^{1 2}, Egbert Zojer ³
³Institute of Solid State Physics, Graz University of Technology, Graz Austria, ¹Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, ²School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, United States

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Self-assembled monolayers (SAMs) are widely used to modify physical surface properties. Most importantly, single-molecule electronic devices largely rely on organic molecules self-assembled on some noble metal electrode structure. Here, the energetic alignment of the electrode Fermi level and the frontier molecular orbitals within the SAM crucially impacts the overall device characteristics. Moreover, SAMs have been employed to tune the work function of electrodes in conventional organic electronic devices which allows controlling the barrier for charge-carrier injection into the active organic layer.While a number of theoretical studies have addressed the interface energetics in closely packed SAMs, the situation of the highest possible packing density does not necessarily reflect the reality encountered in experiments. Depending on the preparation conditions and local substrate/electrode morphology, different packing densities are achieved. Especially for highly dipolar molecules, desirable to induce an important change in the substrate work function, the maximum coverage that can be achieved seems to be limited by dipole-dipole repulsion.In the present theoretical work, we investigate the impact of coverage on the two most relevant parameters for the applications of SAMs in (single) molecular electronics: the modification of the substrate work function upon SAM formation and the energetic alignment of the frontier molecular orbitals in the SAM with the metal Fermi level. Relying on slab-type band-structure calculations within the framework of density-functional theory (DFT), we systematically vary the coverage in SAMs of two highly dipolar conjugated molecules, 4’-X-4-mercaptobiphenyl (where X are amino- and cyano-groups), on Au(111). In this way, we can clearly differentiate between effects arising from intermolecular interaction and molecule-metal interactions. For the former, we find that the effective dipole moment per molecule in the SAM is largely reduced at higher coverage. The observed depolarization effects are approximated by electrostatic models and ultimately rationalized in terms of charge re-arrangements on the atomic length scale. Also the metal-molecule interactions are significantly affected by the packing density, as adsorption-induced charge transfer in the SAM tends to be suppressed at high coverage. This has important implications for the induced work-function changes as well as the level alignment, leading to a highly nonlinear coverage-dependence of both.We conclude that the modification of the work function of an electrode by self-assembly of dipolar molecules can only be understood upon accounting for the microscopic electronic structure of the molecules. For single-molecule electronics, we can conclude that the packing density should significantly impact the device characteristics.

9:00 PM - F3.38

Fluorination Effects on Molecular Orientation and Electronic Structure of Phthalocyanine by Soft X-ray Spectroscopy.

Koji Okudaira ¹, Eiichi Kobayashi ², Kazuhiko Mase ³, Nobuo Ueno ¹
¹Graduate School of Advanced Integration Science, Chiba University, Chiba Japan, ², Saga light Source , Saga Japan, ³, Institute of Materials Structure Science, Tsukuba Japan

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Fluorinated organic material such as hexadecafluoro fluorinated zinc phthalocyanine (FZnPc) and perfluoro pentacene is a very interesting material, since it is a promising electron-transport material for organic devices. To develop a highly efficient organic light-emitting diodes (OLED), it is important to clarify molecular orientation as well as electronic structure such as the characteristics of unoccupied states of the electron-transport layer in the OLED. Near edge x-ray absorption fine structure (NEXAFS) spectra of FZnPc and metal-free phthalocyanine (H2Pc) films with various thickness on hydrogen-terminated Si(111) (H-Si(111)) and pristine (native oxide-covered) Si(100) (SiO2/Si(111)) substrates were observed in order to investigate the fluorination effect on the molecular orientation and electronic structure. For the thick FZnPc films (thickness of 5 nm) on H-Si(111) and SiO2/Si(111) the lowest peak (π*) intensities in the carbon K-edge NEXAFS spectra show the same incidence angle dependence. Analysis of the incidence angle dependence of C K-edge NEXAFS spectra shows that FZnPc molecules in the thick films incline with a large tilt angle on the H-Si(111) and SiO2/Si(111) substrate. The simulation of NEXAFS spectra was carried out by using DFT theory including the core-hole state. The calculated NEXAFS spectra are in good agreement with observed ones. This simulation provided that the lowest peak in fluorine 1s NEXAFS spectra of FZnPc can be assigned to the transition from F1s to σ*(C-F), not π*. For the thin FZnPc films (thickness of 0.5 nm) on SiO2/Si(111) C K-edge NEXAFS spectra does not show incidence angle dependence clearly. It indicates that the FZnPc molecules orient randomly in the thin film on SiO2/Si(100). For the H2Pc films on H-Si(111) the lowest peak (π*) intensities in the carbon K-edge NEXAFS spectra show the opposite incidence angle dependence to those for the same thickness films on SiO2/Si(111). Analysis of the incidence angle dependence of carbon K-edge NEXAFS spectra shows that molecules in the thick films nearly lie flat on the H-Si(111) substrate, on the contrary on the SiO2/Si(111) substrate they incline with a large tilt angle. The fluorination of phthalocyanine affects on not only the electronic structure of the molecule but also the molecular orientation.

9:00 PM - F3.39

Organic Thin Film Deposition from a Supersonic Cluster Beam Source Yields Novel Growth Behavior.

Aram Amassian ¹, Sukwon Hong ², Sugandha Bhargava ², Arthur Woll ³, Detlef Smilgies ³, Todd Schroeder ², Aravind Killampalli ², George Malliaras ¹, James Engstrom ²
¹Materials Science and Engineering, Cornell , Ithaca, New York, United States, ²School of Chemical and Biomedical Engineering, Cornell, Ithaca, New York, United States, ³Cornell High Energy Synchrotron Source, Cornell, Ithaca, New York, United States

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9:00 PM - F3.4

Mechanisms of High Efficient Organic Light-emitting Devise with MoO3 Layers.

Chang Lin ¹, Guan Lee ¹, Chih Wu ¹, Ting Cho ², Chung Wu ², Tun Pi ³
¹, Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China, Taipei Taiwan, ², Graduate Institute of Electronics Engineering, National Taiwan University, Taipei, Taiwan 10617, Republic of China, Taipei Taiwan, ³, National Synchrotron Radiation Research Center, Hsinchu, Taiwan 300, Republic of China, Hsinchu Taiwan

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9:00 PM - F3.40

Observation of Conductivity Fluctuation at Pentacene/SiO₂ Interface in Working Thin-Film Transistors Using Atomic-Force-Microscope Potentiometry.

Masakazu Nakamura ¹, Noboru Ohashi ¹, Hiroshi Tomii ¹, Ryousuke Matsubara ¹, Kazuhiro Kudo ¹
¹Department of Electrical and Electronic Engineering, Chiba University, Chiba Japan

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9:00 PM - F3.41

Improving the Electrical and Optical Properties of Organic Light Emitting Diodes using Na doped tris (8-hydroxyquinoline) Aluminum Layer.

Kisoo Kim ¹, Kihyon Hong ¹, Soo Young Kim ¹, Jong-Lam Lee ¹
¹, POSTECH, Pohang Korea (the Republic of)

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9:00 PM - F3.5

The Electron Transport Properties and Interfacial Chemical Reactions of Tris-(8-Hydroxyquinoline)-Aluminum Doped With Cesium-Derivatives In Organic Light Emitting Devices.

Mei-Hsin Chen ¹, Yin-Jui Lu ², Chung-Chih Wu ², Chih-I Wu ¹
¹, Department of Electrical Engineering and Graduate Institute of Electro-optical Engineering, National Taiwan University, Taipei Taiwan, ², Department of Electrical Engineering, National Taiwan University; Graduate Institute of Electro-Optical Engineering, National Taiwan University; and Graduate Institute of Electronics Engineering, Nati, Taipei Taiwan

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9:00 PM - F3.6

Linker-free Grafting of Conducting Polymer Films on Various Organic and Inorganic Substrates via Oxidative Chemical Vapor Deposition.

Sung Gap Im ^{1 2}, Pil Yoo ^{1 2}, Paula Hammond ^{1 2}, Karen Gleason ^{1 2}
¹Chemical Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, ²Institute for Soldier Nanotechnologies, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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9:00 PM - F3.7

The Functional Hydrophobic Buffer Layer for Enhanced Performance of Organic Thin Film Transistors.

June-Yong Song ¹, Jae-Il Jung ¹, Yoonseuk Choi ², Hak-Rin Kim ³, Jae-Hoon Kim ^{1 2}
¹Department of Electronics and Computer Engineering, Hanyang University, Seoul Korea (the Republic of), ²Research Institute of Information Display, Hanyang University, Seoul Korea (the Republic of), ³School of Electrical Engineering and Computer Science, Kyungpook National University, Daegu Korea (the Republic of)

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9:00 PM - F3.8

Influence of Self-Assembled Organic Thin Film Monolayer on Ambient Copper Surfaces Oxidation.

Ilia Platzman ¹, Hossam Haick ¹, Rina Tannenbaum ^{1 2}
¹Chemical Engineering, Technion – Israel Institute of Technology, Haifa, Israel, Haifa Israel, ²School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, Georgia, United States

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9:00 PM - F3.9

C60 Fullerenes on the Functionalized Si(111) Surface as a Model for the Interfaces between Biomolecules and Semiconductor Substrates.

Xiaochun Zhang ¹, Andrew Teplyakov ¹
¹Chemistry and Biochemistry, University of Delaware, Newark, Delaware, United States

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Buckminster fullerenes C60 were used as a model to understand the attachment chemistry of large molecules with amine-terminated self-assembled monolayers (SAM) on semiconductor substrates. This type of interface serves as a prototype for the future devices in such fields as solar energy conversion, biosensing, catalysis and molecular electronics. Fullerenes C60 were attached to 11-amino-1-undecene self-assembled monolayers on Si(111) surface. The chemical state and topography of the C60-modified surface was characterized by surface analytical spectroscopic and microscopic methods and by computational investigation. X-ray photoelectron spectroscopy (XPS) revealed that secondary amine group is formed between the C60 and the amino-undecene SAM on the surface. Infrared spectroscopic (IR) studies verified several characteristic features of C60 skeleton vibration and amino-undecene vibrational signature. The C-H stretching region confirmed that the SAMs produced were well-ordered. The atomic force microscopy (AFM) investigation suggested that the fullerene molecules form surface features with apparent height of ~2 nm and average apparent width of ~20 nm. Parallel study was performed on Au(111) surface for comparison with the results from the silicon substrate. The reaction between fullerene molecules and amino-undecene diluted in decene (~1% v/v) SAM on Si(111) surface yielded accordingly diluted and uniformly distributed C60 molecular features, which indicated that the amine groups were the reactive sites. Preliminary characterizations have revealed the structural and compositional information of the interface between DNA molecules and Si substrate. Microscopic tools such as AFM and contact angle measurement were used to study the change of the surface topography and the change of the surface hydrophilicity or hydrophobicity among each step of the DNA attachment chemistry on Si surfaces accordingly. From the contact angle measurement, it was observed that the hydrophilicity of the surface decreased after attaching the cross linker molecule (SSMCC) onto the amine terminated Si(111) surface and increased after attaching the DNA molecules to the SSMCC modified surface. The AFM images presented the topography change before and after binding of the DNA to the Si substrate, and the topography change after treating the DNA attached surface with DNA digest enzyme. XPS and Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) were employed to learn the chemical composition of these interfaces. The results combined from XPS and TOF-SIMS evidenced the present of the cross linker and the DNA molecules covalently bonded on the surfaces. The attachment chemistry of the model system is very robust as confirmed by investigating alternative silicon-based nanostructured substrates, and could be useful for the future applications.

9:00 PM - F3: posters I

F3.37 Transferred to F9.41

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2007-11-27 Show All Abstracts

Symposium Organizers

Julia W. P. Hsu Sandia National Laboratories

Leeor Kronik Weizmann Institute of Science

George G. Malliaras Cornell University

Nobuo Ueno Chiba University

F4: Interfacial Electronic Properties

Session Chairs

Jean-Luc Bredas

David Cahen

Tuesday AM, November 27, 2007

Back Bay C (Sheraton)

9:30 AM - **F4.1

Effect of Gases on the Electronic Structure of Organic Films and Interfaces Studied by Photoelectron Yield Spectroscopy (PYS) , UPS, and Kelvin Probe.

Kazuhiko Seki ¹, Masato Honda ¹, Yuta Isogai ¹, Toshiro Yamamoto ¹, Yukio Ouchi ¹, Kaname Kanai ²
¹Department of Chemistry, Nagoya University, Nagoya Japan, ²Res. Center for Mat. Science, Nagoya University, Nagoya Japan

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It has long been known that electrical properties of organic semiconductors are significantly affected by atmospheric gases. For clarifying such effect, the knowledge of the change of the electronic structure of organic films and interfaces are indispensable. Some information can be obtained by UPS by exposing the specimen to gases and measuring the spectra after re-evacuation, as reported here for the case of C60 exposed to O2 [1]. Still the specimen in such measurements is no longer in the gas, and development of a method for measuring the electronic structure both in vacuum and under gas for the same specimen has been highly desired. Recently we have been developing photoelectron yield spectroscopy (PYS), where the photoemission yield (number of photoelectrons/incident photon) as a function of the photon energy is measured using a femtoammeter. The onset gives the ionization threshold energy Ith. By applying a voltage between the sample and the collecting electrode, this method can be used for samples not only in vacuum but also under various gases. Our initial study for titanyl phthalocyanine (TiOPc) and other materials showed that the value of Ith is changed by about 0.2 eV at exposure-evacuation cycle [2, 3]. Here we report the results on other materials, and also the development of further analysis. Since Ith is the energy separation between the vacuum level Evac and the highest occupied molecular orbital (HOMO), another method is required for separating the effect of the gas to these levels. We performed such separation by introducing a Kelvin probe to the PYS chamber. The studies of the effect of H2O on TiOPc showed that most effect comes from the change of Evac. This probably corresponds to the adsorption/desorption of the gas molecules to the surface, leading to the change of the dipole layer at the surface. Other related results will be also presented.[1] Y. Tanaka et al., Chem. Phys. Lett., 441, 63 (2007).[2] M. Honda et al., Mol. Cryst Liq. Cryst., 455, 219 (2006).[3] M. Honda et al., submitted to J Appl. Phys.

10:00 AM - F4.2

Electronic Structures of Dyes and Phthalocyanines Estimated with ``Photo-electron Spectroscopy in the Air (PESA)."

Y. Nakajima ¹, D. Yamashita ¹, A. Ishizaki ¹, B. Pellissier ², M. Uda ³
¹, Riken keiki Co.,Ltd, Tokyo Japan, ², RKI Instruments Ink., Union City, California, United States, ³, Waseda University, Tokyo Japan

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The electronic structures of organic materials used for semiconductor devices can be analyzed with the aid of “Photo-electron Spectroscopy in the Air (PESA)” by estimating the work function (WF), the ionization potential (IP) and the density of states (DOS)[1]-[3]. For this purpose the “open counter” must be employed as a detector, because it can detect and count small numbers of low energy photoelectrons, one by one, in the air under an atmospheric pressure. A PESA measurement was carried out as follows. UV photons emitted from a deuterium lamp were monochromatized by a grating spectrometer and focused on a sample. Photoelectrons emitted from the sample were counted by the open counter. Here, the energies of monochromatized UV photons were shifted at 0.05 eV intervals up to 6.20 eV, if necessary up to 7.00eV. When photons with higher energies than 6.20eV are used, the air in the monochromator must be replaced by N2 gas because the photons with higher energies than 6.20eV are absorbed in the air. PESA has, if compared with such photoelectron spectroscopies as XPS and UPS, the advantage of performance in a non-vacuum measurement, a high energy-resolution and low photo-excitation energies. A non-vacuum measurement is very much useful for ones engaged in investigations on organic semiconductors in powdered and liquid states. Photoelectron excitation efficiency is high when an electron, localized on the highest occupied molecular orbital (HOMO) or nearby of a condensed matter, is ionized with low energy photons. In addition, radiation damage can be ignored when organic materials are lightly irradiated with photons with low energies. IP of a dye, i.e. cis-di(thiocyanato)-bis(2,2'-bipyridyl-4-carboxylate-4'-carboxylic acid)-ruthenium(II) (N719) was derived from an observed photoemission threshold energy to be 5.30 eV. Here the dye has been used as a raw material for a Dye-Sensitized Solar Cell. DOSs of Fe-, Ni-, Cu- and H2-phthalocyanines were deduced from differentiation of observed photoemission yields with incident photon energies. The DOSs thus obtained were well compared with calculated ones estimated with use of the DV-Xα molecular orbital calculation method.[1] M. Uda and H. Kirihata, Japanese Patent S55-179922 (1980), 1447157 (1988)[2] H. Kirihata, and M. Uda, Rev. Sci. Instr. 52, 68 (1981).[3] M. Uda, Jpn. J. Appl. Phys. 24, 284 (1985)

10:15 AM - F4.3

Energy Level Alignment at the Rubrene/metal Interface.

Huanjun Ding ¹, Yongli Gao ¹
¹, University of Rochester, Rochester, New York, United States

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10:30 AM - F4.4

Changing Band Offsets in Copper-Phthalocyanine to Copolymer Poly(Vinylidene Fluoride with Trifluoroethylene) Heterojunctions.

Jie Xiao ¹, Andrei Sokolov ¹, Peter Dowben ¹
¹Physics and Astronomy, University of Nebraska-Lincoln, Lincoln, Nebraska, United States

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10:45 AM - F4.5

The Electronic Structure of Pentacene/Fullerene Layered and Co-deposited Thin Films on poly(ethylenedioxythiophene) :poly(styrenesulfonate) (PEDOT:PSS).

Ingo Salzmann ¹, Steffen Duhm ¹, Ricarda Opitz ¹, Jian Zhang ¹, Juergen Rabe ¹, Norbert Koch ¹
¹Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany

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11:00 AM - *

break

11:30 AM - **F4.6

Energetics of Polyfluorene Interfaces.

Jaehyung Hwang ¹, Eung-Gun Kim ², Jean-Luc Bredas ², Antoine Kahn ¹
¹Electrical Engineering, Princeton University, Princeton, New Jersey, United States, ²School of Chemistry & Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States

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We present a comprehensive investigation of the electronic structure of three polymers, poly(9,9’-dioctylfluorene) (F8 or PFO) and two fluorene-arylamine copolymers, poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl) diphenyl-amine) (TFB) and poly(9,9’-dioctylfluorene-co-bis-N,N’-(4-butylphenyl)-bis-N,N’-phenyl-1,4-phenylenediamine) (PFB) and their interfaces with metals and conducting polymers and oxides. Ionization energy (IE), electron affinity (EA) and molecular levels relevant for charge carrier injection and transport are measured using ultra-violet photoemission and inverse photoemission spectroscopies (UPS, IPES). The polymer films are spun under controlled atmosphere (N2) and introduced in ultra-high vacuum (UHV) without ambient exposure. Occupied and unoccupied states measured for F8 and TFB are compared with theoretical calculations performed at the density functional theory (DFT) level. The very good agreement between measured and calculated densities of states provides an accurate determination of the energy of single electron and hole transport level and energy gap [1]. Films of F8 and TFB are prepared on various metal, oxide and polymer substrates exhibiting a range of work functions (WF). Electron and hole barriers vs. substrate WF closely follows the Schottky-Mott rule for both polymers. In particular, interface-enhanced measurements on ultra-thin (3 nm) films demonstrate interface vacuum level alignment. Flat bands are observed near the interface when the barrier is 0.6 eV or larger. However, band bending away from the interface is observed when the interface barrier is equal to or smaller than 0.4 eV, suggesting that the Fermi level cannot approach closer than about 0.6 eV from the band edge (HOMO in this case) in the bulk of the material. We suggest that the charge induced in the gap tail of the broad frontier molecular level (the HOMO in the present case) is responsible for this quasi Fermi level pinning [1]. Metal-on-polymer interfaces are prepared by UHV evaporation of a range of metals with WF between 2.7 and 5.5.eV. The dependence of the interface barrier on the metal WF is smaller than in the polymer-on-metal case. The interface S-parameter is 0.5-0.6, vs. S=1 in the Schottky-Mott case. Interface chemistry plays a greater role at metal-on-top interfaces. However, we suggest that the difference between the two types of interfaces can also be rationalized in terms of the model previously used to explain the energetics of interfaces formed between small molecule films and clean vs. contaminated metal surfaces [2]. [1] J. Hwang, E.-G. Kim, J. Liu, J.-L. Brédas, A. Duggal and A. Kahn, J. Phys. Chem. C 111, 1378 (2007)[2] A. Wan, J. Hwang, F. Amy and A. Kahn, Organic Electronics 6, 47 (2005)

12:00 PM - **F4.7

Interfaces of Organic Semiconductors and Their Implications to Device Performance.

C. Lee ¹, J. Tang ¹, Y. Zhou ¹, M. Fung ¹, S. Lee ¹
¹Phys. & Mater. Sci, City University of Hong Kong, Hong Kong, _, China

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12:30 PM - F4.8

Electrode - Molecular Semiconductor Contacts: Work-function-dependent Charge Injection Barriers versus Fermi-level Pinning.

Norbert Koch ¹, Antje Vollmer ²
¹Institut f. Physik, Humboldt-University Berlin, Berlin Germany, ², BESSY GmbH, Berlin Germany

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12:45 PM - F4.9

Electronic Structure at Polyethylene/Metal Interfaces Studied by Ultraviolet Photoemission Spectroscopy.

Kaname Kanai ¹, Kenji Koizumi ², Yukio Ouchi ², Kazuhiko Seki ²
¹, Research Center for Materials Science, Nagoya University, NAGOYA Japan, ², Graduate School of Science, Nagoya University, NAGOYA Japan

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Recently there have been extensive studies of interfaces between organic thin film and metal substrate (organic/metal) in relation to the role of those interfaces in organic electronic devices. Ishii et al., reported that the energy level alignment at the organic/metal interface is strongly affected by the formation of the interfacial double layer (IDL).[1] The IDL brings about the abrupt shift in the vacuum level of the film which reaches to about several hundred meV. Despite several experimental and theoretical efforts the origin of the IDL has not yet been fully understood.[1,2] One of the reasons for its difficulty in revealing the origin is that many effects contribute to the IDL formation, such as chemical interaction, alignment of permanent dipoles of the adsorbed molecules on the metal surface, induced mirror force, charge transfer interaction and push-back of the electron cloud spilled out from the metal surface.[1] To reveal the origin of formation of the IDL it is needed to clarify each contribution. We report the electronic structure at the interface between polyethylene (n-C44H90) thin film and the single crystalline metal surfaces studied by ultraviolet photoemission spectroscopy (UPS) and X-ray photoemission spectroscopy (XPS). Based on common sense, there is little chance that chemical interaction and charge transfer interaction between n-C44H90 molecule and the metal occurs due to inertness of the molecule. It is expected to give an insight into the origin of the IDL formation limiting in the case of physisorption system. Yoshimura et al., reported the IDL formed at the n-C44H90/Cu(100) interface decreases the vacuum level of the film by about 0.3 eV.[3] This drop in the vacuum level clearly indicates even the physisorption of such inert molecule reconstructs the charge distribution at the organic/metal interface. We observed large drop in the vacuum level of about 0.7 eV at the interface between n-C44H90 thin film and Au(111) surface. The C 1s peak in XPS spectra of the thin film showed a clear chemical shift compared with that of the thick film, indicating there exists the interaction between the molecule and Au(111) surface. These observed IDL formation on several metal surfaces will be discussed by comparison with the reported theoretical simulation.[2] [1] H. Ishii et al., Adv. Mat., 11(8) 605 (1999),[2] Y. Morikawa et al., Phys. Rev. B, 69 041403 (2004),[3] D. Yoshimura et al., J. Electron Spectros. Rela. Phenom., 144-147, 319 (2005).

F5: Organic Light Emitting Diodes

Session Chairs

Paul Blom

James Durrant

Tuesday PM, November 27, 2007

Back Bay C (Sheraton)

2:30 PM - **F5.1

Ambipolar Transport at Organic-organic Heterointerfaces.

Henning Sirringhaus ¹
¹, Cambridge University, Cambridge United Kingdom

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3:00 PM - F5.2

Switching the Hole Injection Barrier – Integration of Photochromism into OLEDs.

Philipp Zacharias ¹, Malte Gather ¹, Anne Koehnen ¹, Nina Rehmann ¹, Klaus Meerholz ¹
¹Physical Chemistry, University of Cologne, Cologne Germany

Show Abstract

Dithienylethenes (DTE) are known as thermally irreversible photochromic compounds with good fatigue resistance and have therefore been applied in optical memories and switches.[1] The uncolored open-DTE transforms by UV illumination via a conrotatory 6π6-electrocyclisation into the deeply colored closed-DTE which switches back to the open form by visible light.We have synthesized the photochromic 1,2-bis[5’-(3-ethyl-oxetane-3-methylenoxy-hexyloxy- phenyl)-2’-methylthien-3’-yl]perfluorocyclopentene (XDTE) that is a hole-conductor, and at the same time film-forming and crosslinkable. The two oxetane units at the end of the flexible spacer allow for a cationic ring opening polymerization (CROP) which we have successfully used for the subsequent immobilization of each layer during the fabrication of multilayer-OLEDs from solution.[2,3]Here we demonstrate for the first time a photochromically switchable OLED. The device consists of a layer of the XDTE (35nm thick) sandwiched between a layer of a blue-emitting polymer (60nm) on the cathode side and several layers of different crosslinked TPD-based hole-conductors (XTPDs) (15nm) on the anode side. Current density and electroluminescence, respectively, of the OLED in the ON state (closed-XDTE after illumination with 312 nm light) and in the OFF state (open-XDTE after 595 nm illumination) differ by a factor >1000. The switching is based on the change of the HOMO energy of XDTE – from -5.33 eV for closed-XDTE to 5.79 eV for open-XDTE. Combined with the right choice of adjacent layers, this leads to a minimized hole-injection barrier in the ON state and a maximized barrier in the OFF state. To optimize these two barriers seven different XTPDs with HOMO energies ranging from -5.15 to -5.50 eV have been used.[4] The electroluminescent polymer was chosen such that the emission (431 and 452 nm) did not overlap with the absorption bands of the photochromic system (590 nm ON, 300 nm OFF) to prevent self-induced optical switching.The switching kinetics for external optical switching have been investigated. Furthermore, we found that the stability of the two states towards thermal, electrical and internal optical switching is extremely good. The efficiency of the photochromic OLED reaches 0.9 Cd/A, the device emits deep blue light (CIE coordinates x/y = 0.16/0.09), and achieves a brightness of 100 Cd/m² at 9 V. [1] M. Irie Chem. Rev. 2000, 100, 1685.[2] C. D. Müller, T. Braig, H. G. Nothofer, M. Arnoldi, M. Gross, U. Scherf, O. Nuyken, K. Meerholz Chemphyschem 2000, 1, 207.[3] Meerholz, K.; Müller, C. D.; Nuyken, O. In Organic light emitting devices; Mullen, K., Scherf, U., Eds.; Wiley-VCH: Weinheim, 2005.[4] P. Zacharias, M. C. Gather, M. Rojahn, O. Nuyken, K. Meerholz Angew. Chem. Int. Ed. 2007, 46, 4388.

3:15 PM - F5.3

Enhanced Performance of Organic Light-emitting Diodes using Carbon Nanotube Hole-injection Layers.

Li Wei Tan ¹, Ross Hatton ¹, Anthony Miller ¹, Silva Ravi ¹
¹School of Electronic and Physical Science, Advanced Technology Institute, Guildford, Surrey, United Kingdom

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3:30 PM - F5.4

Interface Engineering through Versatile Phosphonic Acid Self-assembled Monolayers for OLEDs and OFETs.

Hong Ma ¹, Julie Bardecker ¹, Orb Acton ¹, Guy Ting ¹, Hin-Lap Yip ¹, Yen-Ju Cheng ¹, Fei Huang ¹, Michelle Liu ¹, Alex Jen ¹
¹Materials Science & Engineering, University of Washington, Seattle, Washington, United States

Show Abstract

There are several advantages to use organophosphonic acids to replace the conventional silane-based molecules for forming self-assembled monolayers (SAMs). These include: 1) better stability to moisture; 2) less tendency to form homocondensation between the phosphonic acids; and 3) the reaction between organophosphonic acid and the metal oxide substrate is not limited by the content of surface hydroxyl groups. These features enable organophosphonic acids to form dense, robust, and structurally well-defined functional phosphonate monolayers on metal oxide (MO) surface and provide a versatile approach of interface engineering for high-performance organic/polymeric light-emitting diodes (LEDs) and field-effect transistors (FETs).We have demonstrated improved performance of polymer LEDs using SAMs of hole-transporting triarylamine-based molecules with phosphonic acid binding groups to modify the surface of indium tin oxide (ITO) anode. Compared to bare oxygen plasma-treated ITO, the devices using SAM-modified anodes decrease in turn-on voltage of up to 3 V, increase in current density of up to 18-fold and in brightness of up to 17-fold at 10 V. The molecular structures of SAMs play a crucial role in charge injection at the interface of anode/hole-transporting layer. Although hole-injection from the ITO anode can be significantly improved through the addition of a hole-transporting SAM at the interface, we have found that the existence of different interfacial factors such as dipole moment and charge blocking can interact with the hole-transporting effect, which can be either beneficial or detrimental to charge-injection.By using aryl-terminated long alkyl (pi-sigma) phosphonic acid SAMs on native silicon oxide (SiO2) or aluminum oxide (Al2O3) as gate dielectrics and templates, we have realized OFETs with high charge carrier mobilities, large on/off current ratios, low leakage currents, small threshold/operating voltages and good stability at ambient conditions. This is achieved by: a) modification of hydroxy-containing MO dielectric surface with SAM to decrease charge carrier traps; b) tailoring of the surface energy of MO dielectric and aryl-terminated SAM-mediated ordering to control the molecular orientation and morphology of subsequently deposited semiconductor layer; c) well-packed and dense SAM (2-3 nm) as ultrathin dielectrics.

3:45 PM - F5.5

Interfacial Electronic Structure of N, N′-bis(1-naphthyl)-N, N′-diphenyl-1, 1′-biphenyl-4, 4′-diamine/copper phthalocyanine:C60 Composite/Au Studied by Uultraviolet Photoemission Spectroscopy.

Sang Wan Cho ¹, Yeonjin Yi ², Myungkeun Noh ³, Man ho Cho ¹, Kyung-Hwa Yoo ¹, Kwangho Jeong ¹, Chung-Nam Whang ¹
¹Institute of Physics and Applied Physics, Yonsei University , Seoul Korea (the Republic of), ²Division of Advanced Technology, Korea Research Institute of Standards and Science, Deajon Korea (the Republic of), ³, LOT vacuum co., Ltd, Gyeonggi-do Korea (the Republic of)

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4:00 PM - *

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4:30 PM - F5.6

Direct Measurement of the Electric Field Distribution in an Organic Light-Emitting Electrochemical Cell.

Jason Slinker ¹, John DeFranco ¹, Michael Jaquith ², Hector Abruna ², John Marohn ², George Malliaras ¹
¹Materials Science and Engineering, Cornell University, Ithaca, New York, United States, ²Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States

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4:45 PM - F5.7

Stacking of Organic Light Emitting Diodes: Field-induced Charge Separation at Doped Organic/organic Interfaces.

Michael Kroeger ¹, Jens Meyer ¹, Sami Hamwi ¹, Thomas Dobbertin ², Thomas Riedl ¹, Hans-Hermann Johannes ¹, Wolfgang Kowalsky ¹
¹Institut fuer Hochfrequenztechnik, TU Braunschweig, Braunschweig Germany, ², Osram Opto Semiconductors GmbH, Regensburg Germany

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We examine field-induced charge-carrier separation at doped organic/organic interfaces consisting of tetrafluorotetracyanoquinodimethane doped 4,4’,4”-tris(N-1-naphtyl-N-phenylamino)-triphenylamine as hole-transporting layer and Li-doped 1,3,5-tri(phenyl-2-benzimidazole)-benzene as electron-transporting layer. Layer structures comparable to the given one, were employed as interconnecting units in highly efficient stacked organic light emitting diodes [1], which showed drastically improved quantum efficiencies. This increase can only be attributed to extra charge carrier pairs, which dissociate under the influence of the electric field. The phenomenon has been reported several times, but so far the involved physical processes have only been described for charge separation at doped organic/inorganic interfaces [2]. Low-temperature I-V characteristics, , thickness-dependent I-V characteristics, and Kelvin probe measurements are used to derive the energy-level alignment at the interface. We found, that the separation process is independent of temperature. Thickness-dependent I-V- characteristics and Kelvin Probe measurements give evidence for a 5-nm-thin depletion layer adjacent to the interface. Further, an interfacial dipole and band bending within the first 5 nm next to the interface shift the lowest unoccupied molecular orbit (LUMO) of the electron transporting layer, so that the difference between the highest occupied molecular orbit (HOMO) of the hole transporting layer lies only a few hundred meV below the LUMO level of the electron transporting layer. Consistent with our experimental results, we propose a model of electrons tunneling through the depletion zone from the HOMO of the hole-transporting material to the LUMO of the electron-transporting material. This generates an electron-hole pair, which dissociates under the intense electric field close to the interface. [1] L. S. Liao, K. P. Klubek, and C. W. Tang, Appl. Phys. Lett. 84, 167 (2004).[2] M. Terai and K. Fujita, and T. Tsutsui, Jpn. J. Appl. Phys., Part 2 44, L1059 (2005).

5:00 PM - F5.8

Defect-free Polyfluorenes for Polymer LEDs.

Andrew Holmes ¹, Sung Yong Cho ^{2 1}, Scott Watkins ¹, David Jones ¹, Andrew Grimsdale ¹
¹Bio21 Institute, University of Melbourne, Parkville , Victoria, Australia, ²Melville Laboratory, University of Cambridge, Cambridge, Cambs, United Kingdom

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5:15 PM - F5.9

A Novel Structure Studying Triplet Exciton Diffusion in OLED Devices.

Chao Wu ¹, Eugen Polikarpov ², Peter Djurovich ², Mark Thompson ²
¹Department of Materials Science, University of Southern California, Los Angeles, California, United States, ²Department of Chemistry, University of Southern California, Los Angeles, California, United States

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5:30 PM - F5.10

Efficient Förster Energy Transfer from Phosphorescent Organic Molecules to J-aggregate Thin Films.

Yasuhiro Shirasaki ¹, Polina Anikeeva ¹, Jonathan Tischler ¹, M. Bradley ¹, Vladimir Bulovic ¹
¹, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States

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5:45 PM - F5.11

Alternative Transparent Conducting Oxides for use as Anodes in Organic Light Emitting Diodes.

Joseph Berry ¹, Matthew Reese ¹, Paul Burrows ², David Ginley ¹
¹, National Renewable Energy Laboratory, Golden, Colorado, United States, ², Pacific Northwest National Laboratory, Richland, Washington, United States

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2007-11-28 Show All Abstracts

Symposium Organizers

Julia W. P. Hsu Sandia National Laboratories

Leeor Kronik Weizmann Institute of Science

George G. Malliaras Cornell University

Nobuo Ueno Chiba University

F6: Electrode Modification

Session Chairs

Toshiaki Munakata

K. Seki

Wednesday AM, November 28, 2007

Back Bay C (Sheraton)

9:30 AM - **F6.1

Structure and Electronic Properties of Molecular SAMs on Au: Insights from First Principles Simulations.

Annabella Selloni ¹
¹Chemistry, Princeton University, Princeton, New Jersey, United States

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10:00 AM - **F6.2

Electronic Structure of Metal/Organic Interfaces: Self-Assembled Monolayers on Noble Metals and Conducting Oxides.

Jean-Luc Bredas ¹
¹of Chemistry and Biochemistry, Georgia Tech, Atlanta, Georgia, United States

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10:30 AM - F6.3

The Effect of Thin Film Structure of Organic Semiconductor on Hole-injection Barrier at 1-hexadecanethiol Modified Gold/organic Semiconductor Interface.

Chan Eon Park ¹, Kipyo Hong ¹, Jong Lee ¹
¹, Pohang University of Science and Technology, Pohang Korea (the Republic of)

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We studied the effect of crystalline and amorphous structure of organic semiconductor on hole-injection barrier between 1-hexadecanethiol-modified gold (HDT-Au) and organic semiconductors (OSC) by in-situ ultraviolet photoemission spectroscopy (UPS) study. Since the work function of HDT-Au is about 1.08 eV lower than that of bare Au, higher hole-injection barrier is generally expected between HDT-Au and OSCs. At the interface between HDT-Au/amorphous OSC, such as N,N’-diphenyl-N,N’bis(1-naphthyl-1,1’-biphenyl-4,4’-diamine (α-NPD) and Tris(8-hydroxyquinolinato)aluminum (Alq3), we found that estimated hole-injection barrier was coincident with that general expectation. 0.19 eV (and 0.29 eV) higher hole-injection barrier was measured between α-NPD (and Alq3) and HDT-modified gold than that between α-NPD (and Alq3) and bare gold. However, smaller hole-injection barrier between HDT-Au and crystalline OSC such as pentacene and α-Sexithiophene was observed in this study. Hole-injection barrier at HDT-Au/pentacene (and α-sexithiophene) interface was reduced by 0.11 eV (and 0.12 eV) as compared with that at bare Au/pentacene (and α-sexithiophene) interface. All the OSCs deposited on HDT-Au showed a very small vacuum level shift due to a reduction of induced density of interface states (IDIS), while a large vacuum level shift of OSCs occurred on bare gold. However, the variation of hole injection barrier depending on OSC thin film structure can not be explained by the reduction of IDIS because it was observed regardless of all OSCs. The more important factor we should consider in the system is the ionization energy of OSCs. The ionization energies of amorphous OSCs are constant regardless of underlying substrates, which results in the higher hole-injection barrier at HDT-Au/OSC interface as compared with bare Au/OSC interface. However, the ionization energy of crystalline OSC on HDT-Au is smaller than that on bare Au. The smaller ionization energy of crystalline organic semiconductor comes from polarization energy determined by crystallinity of crystalline organic semiconductor, which was established by X-ray diffraction analysis and atomic force microscopy. Therefore, the smaller injection barrier between crystalline organic semiconductor and HDT-modified gold comes from the polarization energy originated from the crystalline structure of organic semiconductor.

10:45 AM - F6.4

On the Interaction of the Strong Organic Acceptor F4TCNQ with Coinage Metals.

Gerold Rangger ¹, Lorenz Romaner ¹, Georg Heimel ², Steffen Duhm ³, Norbert Koch ³, Alexander Gerlach ⁴, Frank Schreiber ⁴, Egbert Zojer ¹
¹Solid State Physics, University of Technology Graz, Graz Austria, ²Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, ³Institut für Physik, Humboldt-Universität zu Berlin, Berlin Germany, ⁴Institut für Angewandte Physik, Universität Tübingen, Tübingen Germany

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The physical and chemical properties of organic/metal interfaces are of high interest for the application of molecular (sub) monolayers to modify surface properties. They are applied for, e.g., the tuning of injection barrier in organic electronic devices, chemical sensing, or molecular electronics.In this contribution we present a joint theoretical and experimental study on the strong acceptor F4TCNQ adsorbed on the Cu (111) surface. The electronic and structural properties were determined by ultraviolet photoelectron spectroscopy (UPS) and X-ray standing wave (XSW) measurements. F4TCNQ is found to adsorb in a heavily distorted conformation accompanied by pronounced changes in the UPS spectrum and an increase of the surface work function. To gain a deeper insight into the nature of the bonding between metal and molecule, the interface was modelled by means of density-functional theory based band-structure calculations. This allows a precise analysis of the interfacial charge re-arrangement upon monolayer formation.By projecting the orbitals of the isolated molecule onto the states of the metal/monolayer system and by analysing the crystal-orbital overlap population (COOP), we find forward-donation from the lone pairs of the molecule into the metallic states and back-donation from the metal into the LUMO of the molecule. Furthermore, a dipole layer is seen to form upon adsorption. Together with the intrinsic dipole moment of the heavily bent molecule, the total work function of the surface is obtained. The density of states in the molecular region, the work-function modification, and the geometric distortion of the molecule upon adsorption can be directly compared to the experimental UPS and XSW data. Additional calculated observables like X-ray photoemission (XPS) and infrared (IR) spectra as well as scanning tunnelling microscopy (STM) images also compare favourably to experiment and thus permit to establish a complete picture of the complex interface energetics in the F4TCNQ/Cu(111) system. In order to gain a broader understanding, the results obtained for the Cu(111) surface are then compared to calculations on F4TCNQ on Au(111) and Ag(111) substrates.

11:00 AM - *

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11:30 AM - F6.5

Odd-even Effects in Self-assembled Monolayers of Alkane-biphenylthiols.

Georg Heimel ^{2 1}, Lorenz Romaner ³, Jean-Luc Bredas ², Egbert Zojer ³
²School of Chemistry and Biochemistry and Center for Organic Photonics and Electronics, Georgia Institute of Technology, Atlanta, Georgia, United States, ¹Department of Material Science and Engineering, Massachusetts Institute of Technology, Cambridge, Massachusetts, United States, ³Institute of Solid State Physics, Graz University of Technology, Graz Austria

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Self-assembled monolayers (SAMs) are widely used to modify physical surface properties. Notably, SAMs have been employed in organic electronic devices to either control the barrier for charge-carrier injection into the active organic layer or as gate dieletrics in organic field-effect transistors. Moreover, the nascent field of single-molecule electronics largely relies on organic molecules self-assembled on some noble metal electrode structure. Here, the energetic alignment of the electrode Fermi level and the frontier molecular orbitals within the SAM crucially impacts the overall device characteristics.Recently, ω-(biphenyl-4-yl)alkanethiols have been shown to form SAMs with a high degree of order and uniformity over large areas. Intriguing odd-even effects with the number of methylene units between the conjugated biphenyl core and the thiol docking group have been observed in a series of experimentally accessible observables, including infrared spectra (IR), core-level shifts in X-ray photoelectron spectroscopy (XPS), and images from scanning tunneling microscopy.In this contribution, we use quantum-mechanical methods to study the two most relevant parameters for the applications of this class of SAMs in (single) molecular electronics, i.e., the modification of the substrate work function upon SAM formation and the energetic alignment of the frontier molecular orbitals with the Fermi level of the electrode. We focus on SAMs of ω-(biphenyl-4-yl)alkanethiols containing (n=) 0 to 6 methylene spacer units between the biphenyl core and the thiol docking group on Au(111). On the basis of density-functional theory calculations we address the local bonding geometry at the sulfur-gold interface which we identify as the microscopic origin of the observed odd-even effects. IR and XPS spectra are calculated and compared to available experimental data. We then discuss the modification of the substrate work function and the level alignment as a function of n. Pronounced odd-even effects are found for the latter while no clear trend is seen in the former.

11:45 AM - F6.6

Electrostatics of Ideal and Non-ideal Polar Organic Monolayers.

Amir Natan ¹, Leeor Kronik ¹, Hossam Haick ², Raymond Tung ³
¹Materials and Interfaces, Weizmann Institute of Science, Rehovoth Israel, ²Chemical Engineering, Technion - Israel Institute of Technology, Haifa Israel, ³Physics, City University of New York, Brooklyn, New York, United States

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12:00 PM - F6.7

Substrate Dependence of the Electronic Structure of an Organic-organic Heterojunction.

Wei Zhao ¹, Qing Zhang ², Stephen Barlow ², Seth Marder ², Antoine Kahn ¹
¹Electrical Engineering, Princeton University, Princeton, New Jersey, United States, ²Dept. of Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States

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Vacuum level offset (ΔE_vac) and molecular level offsets at an organic-organic (OO) heterojunction are generally independent of the work function (WF) of the substrate on which the OO bi-layer is formed. Unless the organic film is doped, the Fermi level (E_F) can move over a large fraction of the energy gap upon changes in interface conditions, e.g. varying substrate WF, without generating significant charge density in the electron or the hole state in the bulk or at the heterojunction. E_F adjusts to equilibrium on both sides of the junction, and ΔE_vac and the molecular level offsets at the OO interface result from intrinsic properties of the pair of organic materials.¹ However, it was recently reported that the substrate can have substantial effect on ΔE_vac when one of the heterojunction constituents has a small band gap.^2,3 We study here the OO heterojunction formed between a tris(thieno)-hexaazatriphenylene derivative (THAP) and copper phthalocyanine (CuPc), two organic materials with similarly small energy gaps (1.75 and 1.9eV, respectively) but significantly different ionization energies (6.4 and 5.1 eV, respectively). We use ultraviolet photoemission spectroscopy (UPS) and Kelvin probe contact potential difference (KP-CPD) measurements to study the electronic structure of the heterojunction as a function of substrate: air-exposed contaminated Au, ultra-high vacuum sputtered Au, ozone-treated Au and PEDOT:PSS. The WF of the bottom THAP film (~100Å thick) varies from 4.94eV on contaminated gold to 5.94eV on PEDOT:PSS. As E_F moves down in the THAP gap, and as equilibrium is maintained throughout the substrate/THAP/CuPc system, the CuPc highest occupied molecular orbital must remain below E_F. Therefore, ΔE_vac at the CuPc/THAP heterojunction increases from 0.32eV to 1.37eV. A significant charge transfer occurs, indicated by the appearance of new filled states in the THAP gap, consistent with the fact that the starting WF of THAP is larger than the ionization energy of CuPc. Significant downward band bending at the surface of the THAP film is also observed when the substrate WF increases and EF moves down in the gap. After correction for band bending, a linear relation is obtained between the WF of the underlying THAP and the OO interface dipole. KP-CPD confirms the ΔE_vac results obtained via UPS on the thin-film heterojunction CuPc(100Å)/THAP(100Å)/PEDOT:PSS, and allows an extension of the electronic structure measurements to CuPc overlayer thicknesses that are too large for UPS measurements. As the CuPc thickness increases, Evac gradually shifts down and the WF increases by 0.8eV, saturating at thickness of ~2000Å with E_F approximately at mid-gap. The electronic structure of the OO heterojunction, however, remains clearly dependent on the substrate. 1 H. Vazquez et al., Phys. Rev B Rapid Comm. Phys. Rev. B 71, 041306 (2005); 2. J. X. Tang et al. Appl. Phys. Lett. 88, 232103(2006); 3 J. X. Tang et al., J. Appl. Phys. 101, 064504 (2007)

12:15 PM - F6.8

Contact Properties in Molecular Electronics: The Effect of Molecular Orientation on Potential at Organic- Metal Interfaces.

Maxim Nikiforov ¹, Ulrich Zerweck ², Christian Loppacher ², Tae-Hong Park ³, Michael Therien ³, Lukas Eng ², Dawn Bonnell ¹
¹Materials Science and Engineering, University of Pennsylvania, Philadelphia, Pennsylvania, United States, ², Dresden University of Technology, Dresden Germany, ³Department of Chemistry, University of Pennsylvania, Philadelphia, Pennsylvania, United States

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12:30 PM - **F6.9

Why is the Process of Injecting Charge from a Metal into an Organic Semiconductor is So Hard to Understand? Help from Temperature Dependent Electric Force Microscopy.

John Marohn ¹, Tse Ng ^{1 2}, Showkat Yazdanian ¹, Michael Jaquith ¹
¹Chemistry and Chemical Biology, Cornell University, Ithaca, New York, United States, ², Palo Alto Research Center, Palo Alto, California, United States

Show Abstract

It is widely regarded that charge injection from a metal into a film of pi-conjugated molecules may be understood as a field-assisted diffusional escape of holes (or electrons) from the attractive potential of their image charges in the metal. While a number of diffusion-limited thermionic emission theories have been developed to describe metal-organic charge injection, none of these theories properly accounts for the energetic disorder known to be present in films of pi-conjugated molecules. It is not clear whether recently revisited diffusion-limited thermionic injection theories can be applied at all to organic semiconductors without extensive modification.Rigorously testing theories of metal-organic charge injection has proven extremely challenging. Studying devices by current-voltage measurements requires a careful disentangling of injection and bulk transport steps; electric force microscopy has become a big help here. The main remaining challenge to testing charge injection theories is that, according to diffusion-limited thermionic emission theory, injection current is proportional to the charge mobility in the organic film. Because of energetic disorder, the hole and electron mobility in films of amorphous pi-conjugated molecules has the same dependence on temperature and electric field as injection processes.In this talk we present measurements of temperature dependent charge transport and local electric fields, measured using vacuum electric force microscopy, in a molecularly doped polymer film. From these data we can infer, for the first time in a single experiment, the temperature dependence of the main factors governing the injection current: the electric-field induced lowering of the image-potential barrier, the interfacial charge density, and the mobility. In the triarylamine studied, we find that the Schottky effect is anomolously large and the interfacial charge density is larger than expected and strikingly non-Arhennius – both, we will argue, a logical but somewhat unexpected consequence of the energetic disorder present in the organic film. We will also discuss ongoing work using measurements of the friction and frequency fluctuations experienced by ultrathin silicon cantilevers near an organic semiconductor surface as a tool for measuring the local mobility.

F7/H11: Joint Session: Interfacial Issues in Organic Photovoltaics

Session Chairs

David Ginley

Wednesday PM, November 28, 2007

Back Bay C (Sheraton)

2:30 PM - **F7.1/H11.1

Charge Photogeneration at Nanostructured Organic and Organic/inorganic Interfaces.

James Durrant ¹
¹, Imperial College London, London United Kingdom

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3:00 PM - F7.2/H11.2

Effect of Surface Modification on the Polymer Disorder at the P3HT/ZnO Interface.

Dana Olson ¹, Erik Spoerke ¹, Yun-Ju Lee ¹, Matthew Lloyd ¹, Todd Alam ¹, Nolanne Chang ¹, David Wheeler ¹, Cary Allen ², Darick Baker ², Thomas Furtak ², Rueben Collins ², James Voigt ¹, Julia Hsu ¹
¹, Sandia National Laboratories, Albuquerque, New Mexico, United States, ²Physics, Colorado School of Mines, Golden, Colorado, United States

Show Abstract

Hybrid conjugated polymer/metal oxide photovoltaic devices offer a low-cost alternative to current inorganic PV technologies through the use of solution processed organic and inorganic composite materials. In nanostructured poly(3-hexylthiophene) (P3HT)/ zinc oxide (ZnO) solar cells, an electron is transferred from P3HT to ZnO upon photoexcitation. We found that the P3HT layer at the ZnO interface is disordered, as evident by a blue shift (up to 55 nm) in the absorption spectra. A blue shift indicates a disruption in the conjugation length in the polymer chains, which leads to an increased band gap and degrades the transport properties of P3HT. This effect depends on the substrate on which the P3HT was deposited, the solvent of the P3HT solution, the solution concentration, and subsequent annealing and cooling processes. We determined that P3HT films less than 10 nm thick are disordered when deposited on solution-based ZnO films or nanorods. Annealing the composite P3HT/ZnO films further increases the disorder. The blue shift has not been observed on single crystal ZnO or a variety of other oxide substrates, and it is believed to be the result of defects at the surface of the solution deposited ZnO.To understand the origin of this disorder, the P3HT films are characterized by optical absorption, photoluminescence, and solid-state nuclear magnetic resonance. Additionally, modification of the P3HT/ZnO interface using molecular monolayers is observed to reduce or eliminate the blue shift of the polymer. Finally, the effects of such modifiers on the optical and morphological properties of the P3HT and the hybrid device performance have been studied.The authors would like to thank the IC Post Doctoral Fellowship for partial funding this research. A portion of this material is based in part upon work supported by the National Science Foundation under Grant No. DMR 0606054. 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.

3:15 PM - F7.3/H11.3

The Role of Bottom Contact Electrical Uniformity at the Nanometer Scale in the Performance of Organic Solar Cells.

Peter Veneman ¹, Neal Armstrong ¹, Michael Brumbach ¹
¹Chemistry, University of Arizona, Tucson, Arizona, United States

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3:30 PM - F7.4/H11.4

Electronic Structure at the Interface Between Bathocuproine and Metal Electrode.

Susumu Toyoshima ^{1 2}, Takeaki Sakurai ¹, Taima Tetsuya ², Hiroo Kato ³, Kazuhiro Saito ², Katsuhiro Akimoto ¹
¹Institute of Applied Physics, University of Tsukuba, Tsukuba Japan, ²Recerch Center for Photovoltaics, National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba Japan, ³Faculty of Science and Technology , Hirosaki University, Aomori Japan

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The electrical properties of the organic/metal interface are sensitive to the performance of organic devices, so it is important to understand and control their properties. In organic photovoltaic cells composed of Phthalocyanine and C60, bathocuproine (BCP) is used as the buffer layer between C60 and metal electrode to improve the cell efficiency[1]. The highest occupied molecular orbital (HOMO) level of the BCP is situated at 6.5 eV from the vacuum level and the energy difference between HOMO level and the lowest unoccupied molecular orbital (LUMO) level is ~3.5 eV. The role of BCP, however, on the performance of photovoltaic cells has not been clearly understood yet. In this work, we studied the electronic structure at the interface between BCP and various kinds of metal (K, Ca, Mg, Al, Ag, Cu, Au) by ultraviolet photoemission spectroscopy (UPS). The various kinds of metal were deposited on Au/Si substrates and then BCP was deposited in a vacuum chamber. The specimens were transferred to the UPS measurements chamber without exposing air. The UPS measurements were carried out with the photon energy of 21.2 eV. It was found that the energy difference between the HOMO of BCP and the Fermi level of metal was almost constant with 3.7 eV for the case of K, Ca, Mg, Al, Ag, whose work function were relatively low. Considering the HOMO-LUMO energy difference in BCP, the energy position of the LUMO level almost accords with the Fermi level of these metals. Further, we observed new peaks at around 1 eV below the Fermi level which may be due to the formation of the interface states. These results suggest that electrons can through from LUMO of BCP to metal via interface state, that is, the barrier height for electrons is lowered by the interface state. For the specimens of BCP deposited on Cu, Au, whose work function are relatively high, the position of the HOMO level from the Fermi level was varied depending on the work function of the metals, suggesting the increase of the barrier height at the interface. No new peak was observed for the case of BCP on Cu and Au. We fabricated solar cells, whose structure is ITO/PEDOT/ZnPc/ZnPc:C60/C60/BCP/metal, with varying metal electrode. The performance of the cells strongly depended on the Fermi level of the electrode metal and higher efficiency was obtained using metal electrode with lower work function. From these results, it is considered that the interaction between BCP and metal depends on the work function of metal, and the close position of the LUMO level to the Fermi level is important to make an interaction between BCP and metal. The interface states induced by the interaction may play an important role in the electrical properties at the interface. [1] P. Peumans and S. R. Forrest, Appl. Phys. Lett. 79, 126 (2001).

3:45 PM - F7.5/H11.5

Interelectrode Morphology of Bulk Heterojunction Photovoltaic Devices as Revealed in-situ by Grazing-incidence X-ray Scattering.

Brian Pate ^{1 2}, Michael Durstock ¹
¹Materials and Manufacturing Directorate, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States, ²Engineering Division, Universal Technology Corporation, Dayton, Ohio, United States

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4:00 PM - *

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F8: Interfacial Issues in Nanocomposites

Session Chairs

Neil Armstrong

Wednesday PM, November 28, 2007

Back Bay C (Sheraton)

4:30 PM - **F8.1

Polymer – Nanoparticle Interface: Impact of Structure on the Responsive Characteristics of Polymer NanoComposites.

Richard Vaia ¹, Loon-Seng Tan ¹, David Wang ¹, Michael Arlen ¹, David Jacobs ¹
¹, Air Force Research Laboratory, Wright-Patterson AFB, Ohio, United States

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5:00 PM - F8.2

Valence Band and Core Level Characterization of Monolayers of Tethered Semiconductor Nanoparticles Using Photoelectron Spectroscopy.

Amy Graham ¹, R. Shallcross ¹, Neal Armstrong ¹
¹Chemistry, University of Arizona, Tucson, Arizona, United States

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5:15 PM - F8.3

Conducting Polymer / Oxide Semiconductor Schottky Junctions.

Masaki Nakano ¹, Tomoteru Fukumura ¹, Kazunori Ueno ¹, Atsushi Tsukazaki ¹, Ryosuke Gunji ¹, Yoshinori Yamada ¹, Akira Ohtomo ¹, Masashi Kawasaki ^{1 2}
¹, Institute for Materials Research, Tohoku University, Sendai Japan, ², CREST, Japan Science and Technology Agency, Tokyo Japan

Show Abstract

Poly(3,4-ethylenedioxythiophene) poly(styrenesulfonate) (PEDOT:PSS) is one of p-type conducting polymers, widely used for organic electronic devices such as OLED and/or OFET. Due to its low resistivity (~10^-3 Ωcm) and large work function (~5.1 eV), heavily doped PEDOT:PSS can be used as a metal electrode of the metal / semiconductor Schottky junction with various n-type semiconductors. Recently, unique electronic properties related to the two-dimensional electron gas formed at heterointerface in oxide semiconductors have been of great interests [1-2]. Schottky junction is a useful tool for evaluating the electronic state at such heterointerfaces. In addition, high-quality Schottky junction can be used for various applications such as field-effect transistors, photodetectors, and tunneling junctions. However, a good quality Schottky contact on wide-gap n-type oxide semiconductors has not yet been established, partly because the use of conventional physical vapor deposition processes may cause degradation of the interfaces, resulting in poor device performance. On the other hand, one can expect soft wet fabrication process with PEDOT:PSS may reduce interface reaction, resulting in high device performance and good reproducibility. Here, we report on the high quality Schottky junction with PEDOT:PSS as the metal electrode and a number of n-type oxide semiconductors; Nb-doped SrTiO₃, Nb-doped TiO₂, and ZnO as semiconductor layers. PEDOT:PSS was spin-coated on the top of Nb-doped SrTiO₃, Nb-doped TiO₂, and ZnO single crystals. Au capping layer was deposited onto PEDOT:PSS by thermal evaporation. Mesa structures with a diameter of 350 μm were formed by Ar⁺ milling and photolithography technique. Junction properties were examined by current-voltage and capacitance-voltage measurements at ambient condition. All devices showed good rectifying behavior and ideality factor was estimated to be near unity based on thermionic emission model, indicating the present junctions are high quality. In particular, the junction with ZnO represented good reproducibility with a negligible small variation of the junction parameters among different samples. A typical behavior was observed in the temperature dependence of the device characteristics as conventional inorganic metal / semiconductor Schottky junctions. M.N. is supported by Research Fellowships of the Japan Society for the Promotion of Science for Young Scientists. [1] A. Ohtomo et al., Nature 427, 423 (2004) [2] A. Tsukazaki et al., Science 315, 1388 (2007)

5:30 PM - F8.4

Interfacial Effects in Organic – Inorganic Laminar Composite Dielectric Structures.

Pratyush Tewari ^{1 2}, Eugene Furman ², Ram Rajagopalan ², Michael Lanagan ^{1 2}
¹Engineering Sciences and Mechanics, Pennsylvania State University, University Park, Pennsylvania, United States, ²Materials Research Laboratory, Pennsylvania State University, University Park, Pennsylvania, United States

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5:45 PM - F8.5

Computational Study of Polymer Nanocomposite Dielectrics with Inorganic Oxide Fillers: The Role of Interfaces.

Ning Shi ¹, Steven A. Boggs ¹, Rampi Ramprasad ¹
¹Deparment of Chemical, Materials & Biomolecular Engineering, University of Connecticut, Storrs, Connecticut, United States

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The development of polymerics dielectrics with improved high voltage endurance is a major enabling technology for a wide range of high voltage applications from capacitor to transmission class cable. Nanocomposites based on a polymer matrix filled with inorganic dielectric nanoparticles have shown promise for such applications, motivated by recent observations that the incorporation of SiO2 nanoparticles into polyethylene can increase the breakdown strength, while incorporation of micron sized SiO2 particles does not. It thus appears that the interface between SiO2 and polymer plays an important role in improving the dielectric strength. Such interface states could act as potential electron traps, thereby scavenging “hot”electrons, and strong coupling between these interface states and phonons in SiO2 could provide a mechanism for cooling of hot electrons, thereby increasing dielectric strength. A systematic investigation of various types of inorganic oxide-polymer interfaces focusing on interface states and electron-phonon coupling will help identify trends related to dielectric breakdown strengths, and can aid in the rational design of polymeric systems with superior dielectric properties. We present density functional theory (DFT) based simulations of the interfaces between SiO2 and Al2O3 and polymers, including the effect of coupling agents. The extant to which interface effects modify the electronic and dielectric properties in these systems was explored in detail. New computational methodologies have been developed within the framework of DFT to study (i) the dielectric constant variation across interfaces, (ii) band edge variation across interfaces, and (iii) electron-phonon coupling in multi-component systems. Position dependent dielectric constant profiles have been computed based on the theory of local permittivity [N. Shi and R. Ramprasad, Phys. Rev. B 74, 045318 (2006)]. Band edge variations across polymer-oxide interfaces have been quantified in terms of the layer-decomposed density of states to identify atomic-level origins, and the tendency to trap the itinerant conduction electrons. The increase in dielectric strength increase has been correlated with increased coupling between phonons in SiO2 and Al2O3 and defect states at the oxide-polymer interface. Our method for the computation of the electron-phonon coupling solves some difficulties in the standard derivation of this interaction. This study identifies some of the fundamental factors responsible for the high breakdown strength displayed by SiO2 based polymeric nanocomposites, and contrasts this with Al2O3-based systems.

F9: Poster Session: Interface II

Session Chairs

Leeor Kronik

Nobuo Ueno

Thursday AM, November 29, 2007

Exhibition Hall D (Hynes)

9:00 PM - F9.1

Molecular Resonant Tunnelling Diodes on Silicon: Synthesis and Grafting of Active Molecules on Hydrogenated Silicon.

Fabrice Moggia ¹, Bruno Jousselme ¹, Pascale Jegou ¹, Vincent Derycke ², Gael Robert ², Jean-Philippe Bourgoin ², Serge Palacin ¹
¹Chemistry of Surfaces and Interfaces, CEA, Gif sur Yvette France, ²Molecular Electronics Laboratory, CEA, Gif sur Yvette France

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As CMOS goes towards 65nm technology node, the integration of traditional memory cells such as DRAMs and non-volatile Flash gets limited. The solutions proposed for their replacement (FeRAM, MRAM, PCM) show a large energy consumption. Therefore there is still a need for a high-density and ultra low-power memory devices suited to respond to the growing market of wireless Systems On a Chip (SOC). Within this scope, the MEMO project targets the study of molecular memory cells which are technologically compatible with CMOS logic. Our goal is to study the physics involved in such devices, to evaluate their electrical performances as well as to status on the possibility to co-integrate – or “hybridize”- these molecular memories on CMOS circuits. For this purpose, the concept of Negative Differential Resistance memory (NDR) based on Resonant Tunneling molecular Diode (RTD) will be studied. Our target devices rely on a silicon/molecule/Carbon nanotube junction in order to (i) involve a very few number of molecules in the junction; (ii) prevent any damage resulting from evaporation of a top-metal electrode. Silicon was selected as injection electrode to take benefit of pinned-density of states, as already demonstrated by Hersam et al (1).Two molecules, one donor (based on the naphthalene carboxidiimide core) and one acceptor (based on the terthiophene core) with respect to silicon orbitals, were synthesized with proper substituents to allow direct chemical grafting on hydrogenated silicon. Electrochemical and photophysical studies in solution give the corresponding figures for both molecules: redox potentials -0.9V and +0.5 V (vs Ag+/Ag) and optical gaps 3.2 and 2.9 eV, respectively. Each molecule will thus probe a particular injection pathway from the silicon electrode.The molecules are chemically grafted on Si-H and the resulting monolayers studied by IRRAS and XPS to probe the density of grafting and the amount of silicon reoxidation.The monolayers will then be prepared in a designated host geometry to allow localized deposition of CNTs by solution processing (2).(1) Guisinger, N. P.; Greene, M. E.; Basu, R.; Baluch, A. S.; Hersam, M. C. Nano Letters 2004, 4, 55-59.(2) Auvray, S., V. Derycke, M. Goffman, A. Filoramo, O. Jost, Bourgoin J.-P.. Nano Letters, 2005, 5, 451

9:00 PM - F9.10

Electronic Structure of Ionic Liquids Studied by Photoemission, Inverse Photoemission and Soft X-ray Emission Spectroscopies.

Toshio Nishi ¹, Takashi Iwahashi ¹, Yoshihisa Harada ³, Yukio Ouchi ¹, Shik Shin ^{3 4}, Kazuhiko Seki ¹, Kaname Kanai ²
¹Department of Chemistry, Nagoya University, Nagoya-shi Japan, ³, RIKEN/SPring-8, Hyogo Japan, ⁴Institute for Solid State Physics, University of Tokyo, Kashiwa-shi Japan, ²Research Center for Materials Science, Nagoya University, Nagoya-shi Japan

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Ionic liquids are salts which are in the liquid phase at room temperature. They attract much interest because of their unique properties such as excellent chemical stability, low vapor pressure, wide electrochemical window, high ionic conductivity, and high solubility with most inorganic and organic materials. Therefore they are regarded as the environment-friendly “Green” solvents and new promising electrolytes for electrochemical applications such as dye sensitized solar cell. For understanding their properties and refining the performance of these materials, the knowledge about the electronic structure is indispensable. A basic question is how the electronic structures of the cation and the anion are combined to form the electronic structure of the ionic liquid under the strong effects of the electrostatic interaction among the ions. In particular, the character of the top of the valence states and the bottom of the conduction states is important. We investigated the electronic structures of the ionic liquids formed by 1-alkyl-3-methylimidazolium cation [Cnmim]+ (n = 4, 8, 10, where n is the carbon number in the alkyl chain) with various inorganic and organic anions by ultraviolet photoelectron spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). Although UPS and IPES are powerful methods for studying the electronic structure of materials, it is usually difficult to apply these methods to liquid samples with large vapor pressure, since the experiments must be performed in vacuum. Fortunately, the low vapor pressure of ionic liquids due to the strong inter-ion coulombic force enables the use of these methods even under ultra-high vacuum (UHV) condition. On the other hand, it is difficult to assign the observed spectral features of UPS spectra because the ionic liquids are composed of both cations and anions [1-4]. In order to study the electronic structure in more detail, we also performed soft X-ray emission spectroscopy (SXES) measurements of ionic liquids. In SXES measurements, the core hole created by an X-ray absorption process is filled by the decay of a valence electron. Non-resonant SXES spectrum measured at the excitation energy far above the ionization threshold gives direct information of the elemental partial densities of the occupied states. In the case of [C4mim]+PF6- and [C4mim]+BF4-, we found that the top of valence states is derived from the cation, based on the comparison between non-resonant SXES and UPS spectra. This is in sharp contrast to the case of usual salts such as alkali halides. On the other hand, for the case of [C4mim]+(CF3SO3)- and [C4mim]+(CF3SO2)2N-, the MO of the anions contributes to the topmost valence states.[1] D. Yoshimura et al., J. Electron. Spectrosc. Relat. Phenom. 144-147 (2005) 319.[2] O. Hoefft et al., Langmuir 22 (2006) 7120.[3] S. Krischok et al., Z. Phys. Chem. 220 (2006) 1407.[4] S. Krischok et al., J. Phys. Chem. B 111 (2007) 4801.

9:00 PM - F9.11

Probing the Origin of Green Emission Impurities in Poly-fluorene Based OLEDS with Single-molecule Spectroscopy.

Michael Odoi ¹
¹Chemistry, University of Massachusetts, Amherst, Massachusetts, United States

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9:00 PM - F9.12

Improving Stability of Pentacene Field-Effect Transistors with Post-Annealing.

Shun-Wei Liu ^{1 2}, Guo-Jun Huang ³, Chin-Chien Lee ³, Chin-Ti Chen ¹, Juen-Kai Wang ^{4 5}
¹Institution of Chemistry, Academia Sinica, Taipei Taiwan, ²Department of Electrical Engineering and Graduate Institute of Electro-Optical Engineering, National Taiwan University, Taipei Taiwan, ³Department of Electronic Engineering, National Taiwan University of Science and Technology, Taipei Taiwan, ⁴Center for Condensed Matter Sciences, National Taiwan University, Taipei Taiwan, ⁵Institute of Atomic and Molecular Sciences, Academia Sinica, Taipei Taiwan

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9:00 PM - F9.13

Impact of Contact/Semiconductor Thickness Ratio on OTFT Device Performance: Carrier Mobility and Contact Resistance.

Srinivas Gowrisanker ¹, Manuel Quevedo-Lopez ¹, Huiping Jia ¹, Eric Vogel ¹, Bruce Gnade ¹
¹Material Science and Engineering, University of Texas at Dallas, Richardson, Texas, United States

Show Abstract

In recent years pentacene based thin film transistors (TFT) have seen significant improvements in device performance. Performance is now comparable to hydrogenated amorphous silicon (a:Si:H) thin film transistors. Pentacene-based TFTs must show reproducible transistor parameters such as mobility, threshold voltage and drive current for the technology to become pervasive. A limiting factor to achieve these requirements on plastic substrates is that the TFTs must be processed at temperatures below 120°C. Similar to Si-based CMOS, device structure and gate insulator selection are critical to achieve the desired device performance. A good source/drain contact with pentacene ensures good charge injection and low contact series resistance because most carrier injection occurs from the sidewalls of the contacts. Therefore, it is important to optimize the pentacene and Au contact thicknesses to reduce contact resistance and improve pentacene coverage on the Au bottom contacts. In addition, a good insulator interface with pentacene also ensures low interface trap density. Understanding these two parameters is necessary to improve TFT device performance. Additionally, in order to achieve realistic applications for organic TFTs, such as RFID devices and flexible displays, pentacene TFTs must also be photo-lithographically integrated with other circuit components . In this work, we fabricated and tested pentacene-based TFTs using a photo-lithography based bottom contact (BC) transistor structure with parylene as the gate insulator. Device optimization was carried out by varying the ratio of pentacene to Au contact thickness. We identified a critical pentacene/Au film thickness ratio to maximize effective carrier mobility. We also report contact resistance dependence with the pentacene/Au ratio. Contact resistance was extracted using Transfer-Line-Method (TLM) [1,2]. FTIR, AFM, XRD and SEM analysis were performed in the systems evaluated to correlate electrical properties with materials properties of the different organic thin film transistor constituents. Film morphology and micro-structure as a function of pentacene thickness are reported, along with transistor parameters such as threshold voltage, drain current and sub-threshold slope.References[1]. Dieter K. Schroder, “Semiconductor Material and Device Characterization”, 3rd edition, pp 138-149[2]. Necliudov PV, Shur MS, Gundlach DJ, Jackson TN, Silid-State Electronics 47 (2003) 259-262,

9:00 PM - F9.14

Conduction Mechanisms in Tri-layer Organic Memory Devices from C₆₀ Molecules in Insulating Polymers.

Alokik Kanwal ¹, Manish Chhowalla ¹
¹Materials Science and Engineering, Rutgers University, Piscataway, New Jersey, United States

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9:00 PM - F9.15

Charge Injection and Mobility Study of OLED Materials by Dark Injection Method.

Takayuki Chiba ¹, Nobuhiro Ide ², Tatsuo Nishina ¹, Yong-Jin Pu ¹, Ken-ichi Nakayama ¹, Masaaki Yokoyama ¹, Junji Kido ^{1 2}
¹Organic Device Engineering, Yamagata University, Yonezawa, Yamagata, Japan, ², Optoelectronic Industry and Technology Development Association, Bunkyo-ku, Tokyo, Japan

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9:00 PM - F9.16

Optimization of High Frequency Organic Diodes Fabricated Using Photolithography-based Processes.

Yuming Ai ¹, Srinivas Gowrisanker ¹, Huiping Jia ¹, Manuel Quevedo-Lopez ¹, Isaac Trachtenberg ¹, Eric Vogel ¹, Robert Wallace ¹, Bruce Gnade ¹, Raymond Barnett ¹, Havey Stiegler ¹, Hal Edwards ¹
¹Materials Science and Engineering, University of Texas, Dallas, Richardson, Texas, United States

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9:00 PM - F9.17

Effect of Crystal Packing on Charge Transport in Cofacial π-Stacking Molecular Organic Semiconductors.

Oana Jurchescu ^{1 2}, Behrang Hamadani ¹, Brandon Vogel ¹, Sankar Subramanian ³, Sean Parkin ³, John Anthony ³, Thomas Jackson ², David Gundlach ¹
¹, National Institute of Standards and Technology, Gaithersburg, Maryland, United States, ²Department of Electrical Engineering, Penn State University, University Park , Pennsylvania, United States, ³Department of Chemistry, University of Kentucky, Lexington, Kentucky, United States

Show Abstract

We report on a structural phase transition around room temperature in fluorinated 5,11-bis(triethylsilylethynyl) anthradithiophene (diF-TESADT). We combine differential scanning calorimetry, X-ray diffraction and field-effect transistor mobility measurements to characterize the phase transition. The transition is manifested as thermal expansion along a and b crystallographic axis and contraction along the c axis, leading to a 0.15 A decrease in the interplanar distance and rearrangement of the molecules. Single crystal X-ray diffraction data points to a small rotation around the alkyne - Si bond, which accounts for the decreased d-spacing at higher temperature. The existence of two different molecular packing for the same organic system provides a unique opportunity to understand the influence of the crystal structure on the electronic properties, since these systems only differ with the orientation of the constituent building blocks with respect to each other, and effects like contact injection issues or grain boundaries are identical. We present evidence that the electronic transport is sensitive to orientation of the molecules. diF-TESADT TFTs are particularly well suited for these studies given the high degree of order of spin cast diF-TESADT films induced by chemical treatment of the interface between the contacts and the organic semiconductor. This treatment gives rise to large (10’s of micrometers) grains growing off the contacts and into the channel region of the device. We measure the evolution of mobility with temperature using thin film transistor devices in vacuum, and observe a “step-like” variation in its value when passing through the phase transition temperature. This effect occurs for different device geometries, suggesting that this is an intrinsic material property. The electronic coupling between molecules is modulated by the molecular arrangement, and this is reflected in a 20 % increase in mobility in the high temperature phase (more compact crystal structure) due to different molecular orientation. Our results are particularly important for chemists that use crystal engineering techniques to make new materials, as they demonstrate how subtle changes in orientation of the molecules induce considerably changes in electronic properties.

9:00 PM - F9.18

Morphology and Structure of Thin Film Transistor from Solution-deposited Triisopropylsilylethynyl Pentacene.

Songtao Wo ¹, Frederic Sansoz ², Randall Headrick ¹, John Anthony ³
¹Department of Physics, University of Vermont, Burlington, Vermont, United States, ²Mechanical Engineering, University of Vermont, Burlington, Vermont, United States, ³Department of Chemistry, University of Vermont, Burlington, Vermont, United States

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9:00 PM - F9.19

Enhance the Field-effective Mobility of OTFTs by Surface Modification of Dielectric.

Lin Jiang ¹, Jie Zhang ², Dan Gamota ², Christos Takoudis ^{1 3}
¹Chemical Engineering, University of Illinois at Chicago, Chicago, Illinois, United States, ², Motorola company, Chicago, Illinois, United States, ³Bioengineering, University of Illinois at Chicago, Chicago, Illinois, United States

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This work studies the effects of dielectric materials, which are organic so that they can be applied on flexible substrates, to semiconductor properties. We focus on the interface study between dielectric and semiconductor layers. The semiconductor layer is made by conjugated polymers which derive their semiconducting properties by having delocalized p-electron bonding along the polymer chain. As it is known, the ability of conjugated polymers to transport charge due to the π-orbital overlap of neighboring molecules provides their semiconducting and conducting properties. The self-assembling or ordering of these polymers enhances this π-orbital overlap and is the key to improvements in carrier mobility. Since the semiconductor materials are deposited on dielectric layer, the functional groups of dielectric materials on surface do affect the alignment and the crystal formation of semiconductor. Therefore, to do the research on the interface between semiconductor and dielectric layers plays an important role in enhancing the mobility of OTFTs.For industrial application requiring large area coverage, structural flexibility, and low cost, such as printed electronics, each layer is printed on the flexible plastic substrate, so the dielectric materials have to be printable, but the materials now used in this area are either printable or able to give good performance. The solution is to do the surface modification of printable materials with silane derivatives according to the previous research being done on silicon wafer as substrate to enhance the field-effective mobility. We functionalized the dielectric materials, poly(4-vinyl phenol-co-methyl methacrylate) (PVP-PMMA), polyamide, polyurethane etc., with silane derivatives which have different functional groups, such as –OH, -NH2 which make the hydrophobic surface turn to hydrophilic. Those functional groups help the assembling of semiconductor molecular on dielectric surface. X-ray photoelectron spectroscopy (XPS) and Fourier transform infrared (FTIR) spectroscopy are used to characterize the chemical composition of dielectric surface. Atomic force microscopy (AFM) characterizes the crystal formation of the semiconductor. As the crystal size twice bigger, it can enhance the field-effective mobility by an order of 10 to 100.

9:00 PM - F9.2

Evaluation Organic Device Interfaces By Current Noise Measurements.

Lin Ke ¹, Soo Jin Chua ¹
¹OESC, Institute of Materials Research & engineering, Singapore Singapore

Show Abstract

9:00 PM - F9.20

Fully Flexible And Transarent All-Organic Ambipolar FETS With Organic Bulk Heterojunctions.

Piero Cosseddu ^{1 2}, Annalisa Bonfiglio ^{1 2}, Ingo Salzmann ³, Juergen P. Rabe ³, Norbert Koch ³
¹Dept. of Electrical and Electronic Engineering, University of Cagliari, Cagliari Italy, ²INFM-S3, nanoStructures and bioSystems at Surfaces, Modena Italy, ³Department of Physics, Humboldt-University zu Berlin, Berlin Germany

Show Abstract

9:00 PM - F9.21

Influence of Electrode Material in Light Emitting Polymer Electrochemical Cells.

Donna Hohertz ¹, Jun Gao ¹
¹Physics , Queens University, Kingston, Ontario, Canada

Show Abstract

Polymer light emitting electrochemical cells (PLECs) are light emitting devices consisting of an active layer blend (luminescent polymer, an ionic species and an ion transport polymer) sandwiched between two electrodes. Upon application of a voltage bias n-and p-type doping is introduced at each electrode via electrochemical redox reactions. N-and p-doped regions propagate through the film meeting to form a p-n junction. The doped polymer is highly conductive allowing for efficient injection of charge from each electrode. It is at this junction charges recombine and emit light. The high conductivity of the doped luminescent polymer means the contact between the film and electrode is ohmic. This has lead to the assumption that PLEC performance is insensitive to the electrode material. Indeed PLECs have been shown to work under forward and reverse bias. However until the junction is formed, contact is not ohmic. Therefore during the initial turn on while the junction is being formed the difference between the electrode work function and polymer LUMO (lowest unoccupied molecular orbital) and HOMO (highest occupied molecular orbital) will affect the injection of charge and the performance of the resulting device. A series of large (1mm) planar LEC devices, with different electrode materials, were prepared, tested and subjected to time lapse fluorescence imaging under identical conditions. Electrode combinations included symmetric devices of Au-Au, Ag-Ag, Al-Al, Ca-Ca, and asymmetrical Ca-Au both forward (Au positive) and reverse bias (Au negative). The active region consisted of 5.1/1/5.1 by weight % of MEH-PPV (poly[5-(2-ethylhexyloxy)-2-methoxy-1,4-phenylene vinylene] )/Lithium Triflate(LiCF3SO3) /PEO (poly-ethylene oxide). The current, luminance, doping profile, doping speed and turn on time were all affected by the choice of electrode material. Of the symmetric devices silver had the highest luminance while gold had the smoothest doping profile. Asymmetrical devices demonstrated remarkable differences in performance between forward and reverse bias. Under forward bias the doping profile was remarkably smooth; the time to junction formation and light emission was short and the current and luminance was high. Under reverse bias the doping profile was jagged and a discontinuous shifting emission zone was formed. The time needed to first light emission tripled while the current and luminance fell by a factor of 4. Full results and future work/implications of this research will be discussed in the presentation.

9:00 PM - F9.22

Monolayer of Au Nanoparticles at the Pentacene/dielectric Interface: Improved OFET and Memory Device.

Dominique Vuillaume ¹, Christophe Novembre ², David Guerin ¹, Kamal Lmimouni ¹, Christian Gamrat ²
¹, IEMN-CNRS, Villeneuve DAscq France, ², CEA-LIST, Saclay France

Show Abstract

Organic memory has recently attracted an increasing interest. Among many physical principals, some of them use metal NP included in an organic matrix sandwiched between two metal electrodes in a two-terminal vertical configuration [1]. NP is used to store charges and modify the conductivity of the organic matrix. On the other hand, organic memory FET has been suggested, exploiting the memory effect of a ferroelectric gate dielectric (both inorganic and organic) [2]. The three-terminal FET configuration has several advantages considering a possible use as memory cross-bar arrays and as addressing devices for OLED displays. Here we report on a new organic memory FET featuring a NP/organic semiconducteur heterojunction as the active channel material. Starting from a bottom gate, bottom source-drain configuration, the SiO2 gate dielectric and the Au source and drain electrodes are functionalized by an amine-terminated alkyl self-assembled monolayer (SAM). The SAM is used to attach Au-NP (20 nm in diameter). Finally, a 50 nm thick pentacene layer is vacuum sublimated to form the organic memory FET. The gate voltage is used to charge/discharge the Au-NP layer, the drain-source current Ids is used to read the state ("on" or "off") of the memory. After a negative pulse on the gate, Ids is strongly reduced, turning the memory in the "off" state. A positive pulse erases the data. Electrical characterizations reveal the following memory performances: the "on/off" ratio is about 3x10^4 and the retention time (estimated as the half-life time of the current variation) of 1500-3000 s. Since dynamic effects also exist in pentacene OFET, we compare with a reference device made at the same time but without the Au-NP. Only a weak effect is observed ("on/off" of about 2-3). Thus the memory effect can be attributed to the charge stored in the Au-NP. The basic physical mechanisms involved in this organic memory FET will be discussed. We also show that, in the "on" state – when charges have not been trapped in the NP, the FET mobility is increased by almost a factor 10 compared to the standard pentacene OFET without the NP. [1]. L. Ma, J. Liu, S. Pyo, and Y. Yang, Appl. Phys. Lett. 80, 362-364 (2002); L. Ma, J. Liu, and Y. Yang, Appl. Phys. Lett. 80, 2997-2999 (2002); D. Tondelier, K. Lmimouni, D. Vuillaume, C. Fery & G. Haas. Appl. Phys. Lett. 85(23), 5763-5765 (2004).[2]. G. Velu, C. Legrand, O. Tharaud, A. Chapoton, D. Remiens & G. Horowitz. Appl. Phys. Lett. 79(5), 659 (2001); E. Katz et al., J. Appl. Phys. (2002)

9:00 PM - F9.23

Controlled Growth and Patterning of Inorganic Dielectrics and Metals through Stencils on Organic Single-crystals by Pulsed Laser Deposition.

Peter de Veen ¹, Guus Rijnders ¹, Dave Blank ¹
¹MESA+ Institute for Nanotechnology, University of Twente, Enschede Netherlands

Show Abstract

Focusing on the fabrication of devices based on organic molecular single-crystals (e.g. pentacene, rubrene) will be the best approach to study the intrinsic electronic properties and to explore the physical limits of organic semi-conducting materials.[1] In general, two approaches are used in the preparation of devices (field-effect transistors, FET) based on organic single-crystals.[2] Mostly encountered is the “flip-crystal” method, where a single-crystal is simply placed on top of a pre-fabricated source/drain/gate structure and then ‘bonds’ electrostatically to this structure. However, the nature and quality of the interfaces remain unknown when this method is used. Less encountered is the other approach, where the contacts and gate insulator are fabricated directly on top of the crystal surface. With most techniques, the deposition damages the crystal surface, thus destroying the interface.Objective in our research is to study the electronic transport in the field-effect geometry, i.e. metallic contacts and a gate electrode will be placed on the organic single-crystal, electrically isolated by an inorganic dielectric. To reach high break down field strength, high quality dielectrics need to be deposited. However, it is a challenge to deposit the various layers on the surface of the organic crystal without destroying the interface. The mobility of the charge carriers in a FET is largely affected by the quality of this inorganic dielectric – organic crystal interface. In our research, we make use Pulsed Laser Deposition (PLD) through stencils with enables deposition of nano-structures. We applied this direct patterning technique to deposit Au dots on a self-assembled monolayer (SAM) without destroying the SAM.[3] Our intention is to use this method in this research, enabling controlled deposition of structures directly on the fragile crystals.First results show that we can deposit and pattern 5-100 micrometer features with a height up to 100 nm of gold and various inorganic dielectrics (e.g. CeO2) on the surface of pentacene single-crystals with this method very controllably. The fact that the typical pentacene substrate terrace steps (inter-monolayer distance d(001) of 1.41 nm) are still clearly visible on top of the islands indicate a high-quality growth.In this contribution we will focus on our results on the controlled growth and patterning through stencils of inorganic dielectrics and metallic contacts on the pentacene single-crystal surface by PLD. Besides that, the electrical characterization of the fabricated devices will be presented. [1] C. Reese, Z.N. Bao, Journal of Materials Chemistry, 16, 4, 329-333 (2006)[2] R.W.I. de Boer, M.E. Gershenson, A.F. Morpurgo, V. Podzorov, Physica Status Solidi A, 201, 6, 1302-1331, (2004)[3] E.A. Speets, B.J. Ravoo, F.J.G. Roesthuis, F. Vroegindeweij, D.H.A. Blank, D.N. Reinhoudt, Nano Letters, 4, 5, 841-844, (2004)

9:00 PM - F9.24

Singlet and Triplet Bimolecular Annihilation Dynamics in Chiral Supramolecular Nanostructures of a Sexithiophene Derivative.

Carlos Silva ¹, Jean-Francois Glowe ¹, Paul-Ludovic Karsenti ¹, Mathieu Perrin ¹
¹Department of Physics, Université de Montréal, Montreal, Quebec, Canada

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9:00 PM - F9.25

BEEM Study of Silicon Molecular Diodes.

Wenjie Li ¹, Karen Kavanagh ¹, Marcus Kuikka ¹, Hogan Yu ¹
¹, Simon Fraser Univesity, Burnaby, British Columbia, Canada

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9:00 PM - F9.26

Metal Suppression of Pentacene Grain Growth.

Hsi-wen Lo ¹, Yu-Chong Tai ¹
¹Electrical Engineering, California Institute of Technology, Pasadena, California, United States

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9:00 PM - F9.27

Oxidation Characteristics of Single Molecule MEH-PPV with a High Energy Hole Transport Layer.

Kwang-Jik Lee ^{1 2}, Josh Bolinger ², Paul Barbara ²
¹Material Science & Engineering, The University of Texas at Austin, Austin, Texas, United States, ²Center for Nano and Molecular Science and Technology, The University of Texas at Austin, Austin, Texas, United States

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9:00 PM - F9.28

C60-terminated Self-Assembled Monolayers for Optimized Photoinduced Charge Transfer in Organic Field Effect Transistors.

Byoungnam Park ¹, Peerasak Paoprasert ¹, Insik In ², Jodi Zwickey ¹, Padma Gopalan ², Paul Evans ²
¹Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States, ²Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States

Show Abstract

Creating and characterizing functional interfaces provides opportunities in incorporating new effects in electronic devices. Photoinduced charge transfer effects, when have been applied to the photovoltaic devices, can be observed in organic field effect transistors (OFETs) that incorporate a functional semiconductor/gate insulator interface. The high sensitivity of the accumulation layer to this interface in OFETs makes it possible to measure the charge carriers resulting from photoinduced charge transfer. We measured the threshold voltages of OFETs formed from pentacene films deposited on the functionalized interface consisting of an amine-terminated self-assembled monolayer (SAM) and the C60 molecules attached to it. The amine-terminated SAM on the SiO2 surface attached C60 molecules to it were characterized with X-ray photoelectron spectroscopy and atomic force microscopy. We calculated the charge density induced in the pentacene film from the shift in threshold voltage. The charge density induced by the charge transfer effect depends on the light intensity, the gate electric field and the concentration of C60 molecules per unit area. To understand the roles of these parameters we varied the duration, intensity of the illumination and the magnitude of the gate electric field. The threshold voltages were measured in the dark after the illumination was finished. Increasing either the light intensity or gate electric fields increases the threshold voltage shift. The threshold voltage shift saturated at high light intensity, suggesting that the charge transfer process was saturated due to the limited number of C60 molecules. The maximum value of the threshold voltage shift was 84 V which corresponds to 9×1012 cm-2 trapped charges. This is far less than the concentration of C60 molecules in a closed packed monolayer. The highly controlled nanometer-scale interface allowed us to quantify and control the photoinduced charge transfer. OFETs can be used as test structures to evaluate donor-acceptor interfaces where the photoinduced charge transfer occurs.

9:00 PM - F9.29

Charge Injection and Transport in Conjugated Polymers.

H. H. Fong ¹, George Malliaras ¹
¹Materials Science and Engineering, Cornell University, Ithaca, New York, United States

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9:00 PM - F9.3

Characterization of Interface using Bias Dependent Photoluminescence of the Schottky Cell.

Vipul Singh ¹
¹Biological Functions and Engineering, Kyushu Institute of Technology, Kitakyushu-shi, Fukuoka, Japan

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Polymer based organic devices are catching up fast. The design concepts of these devices are in many ways similar to the amorphous Silicon based Field Effect Transistors (FETs). These materials are preferred for their potential in the futuristic development of cheap and flexible electronic devices. Besides the ease of their processibility is what makes them lucrative materials for various potential applications. Their wide range of applications includes the Polymeric FETs (PFETs), Polymeric Solar Cells (PSCs) and Polymeric Light Emitting Diodes (PLEDs). It is envisioned that while PFETs will be driving the next generation RFID tags on one hand and on the other PLEDs will be driving the flexible displays of the future. Although PLEDs based displays are already into the market. The development of PFETs is still in its nascent stage. Presently there are many key factors governing the functionality of these devices. One such parameter is the nature of the interface. The interface is important for both the planar and the stacked configuration devices. While in the PFETs the interface between the insulator and semiconductor plays a key role in the performance of the device. Even a major limiting factor in the performance of the PFETs the contact resistance is believed to originate from the metal polymer interface. In the case of PLEDs also the interface is very significant from the point of view of charge injection and its recombination. The role of interface is becoming even more popular because of the growing interest in the injection based stacked type transistors. In this article, we study and characterize the nature of interface between LiF, Al, Al:LiF with Poly (3-hexylthiophene-2,5-diyl) (P3HT). A fundamental understanding of the nature of interface is crucial from the point of development of PFETs and PSCs.. The photoluminescence (PL) investigations can be used to characterize a variety of material parameters. Intensity of PL signal provides information on nature of surfaces and interfaces. We have investigated bias dependent PL spectra of these interfaces.In this article we report that a quenching in PL was obtained in both forward and reverse bias regime. Further it was observed that a higher level of quenching resulted due to positive bias application. Moreover, it was also observed that the quenching patterns in the PL were different for the ITO/P3HT/Al and ITO/P3HT/LiF:Al type of schottky cell devices. These effects combined with their capacitance-voltage characterization of these devices have been explained on the basis of band bending model and the trap induced dead layer formation at the cathode. The higher level of quenching observed in the forward bias regime has been explained on the basis of temporary deformation created by injected charge carriers which act as trapping centers for excitons and leading to non radiative decay of excitons. Hence, resulting in the observed PL quenching in the forward bias regime.

9:00 PM - F9.30

Charge Injection and Transport in Polymeric Light-emitting Device and Transistors.

H. H. Fong ¹, George Malliaras ¹
¹Materials Science and Engineering, Cornell University, Ithaca, New York, United States

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9:00 PM - F9.31

Self-Assembled Monolayers of Dipolar Chromophores for Photoinduced Charge Transfer in Organic Field Effect Transistors.

Padma Gopalan ¹, Peerasak Paoprasert ¹, Byoungnam Park ¹, Paula Colavita ², Hamers Robert ², Paul Evans ¹
¹Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States, ²Department of Chemistry, University of Wisconsin-Madison, Madison, Wisconsin, United States

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9:00 PM - F9.32

Investigation of the Electronic Structure at the Copper-phthalocyanine/ITO Interface.

Timo Hofmann ¹, Roberto Felix ¹, Lothar Weinhardt ¹, Marcus Baer ¹, Clemens Heske ¹
¹Chemistry, UNLV - University of Nevada Las Vegas, Las Vegas, Nevada, United States

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The power conversion efficiencies of thin-film organic solar cell devices (η < 5%) have not yet reached those of their inorganic counterparts, but the perspective of low-cost production using common industrial coating processes drives the development of organic photovoltaic cells [1]. As for all electronic devices, the transport of charges through interfaces is crucial for organic solar cells. However, little is known about the chemical (e.g., intermixing) and electronic (e.g., electronic level alignment) structure at metal/organic and organic/organic interfaces. Yet they are vital to be understood in detail to model, control, and tailor the properties of interfaces [2,3]. In this contribution we will present an in-depth investigation of the chemical and electronic interface structure between the dye molecule copper-phthalocyanine (CuPc) and indium tin oxide (ITO). These materials are widely used as p-type organic semiconductor (CuPc) and transparent front contact (ITO), respectively. We have investigated the electronic level alignment at the CuPc/ITO interface both for occupied as well as for unoccupied states using a combination of X-ray photoelectron spectroscopy (XPS), UV photoelectron spectroscopy (UPS) and inverse photoelectron spectroscopy (IPES). We have studied the interface formation by gradually evaporating CuPc onto ITO substrates cleaned by Ar+ ion treatments. Particular focus was on determining the dipole at the interface as well as the changes in work function when changing the thickness of the CuPc film. The results will also be discussed in view of the morphology of the films as determined by Atomic Force Microscopy. By combining UPS and IPES, we determine the electronic surface HOMO-LUMO gap of organic films (as opposed to the optical band gap), being of central importance for the charge carrier transport in solar cell devices.

[1]H. Hoppe and N.S. Sariciftci, J. Mater. Res. 19, 1924 (2004).
[2]H. Ishii, K. Sugiyama, E. Ito, and K. Seki, Adv. Mater. 8, 605 (1999).
[3]D. Cahen, A. Kahn, and E. Umbach, Materials Today July/August, 32 (2005).

9:00 PM - F9.33

Organic oxide/Al Complex Cathode for Efficient Organic/polymer Light-emitting Diodes and Bulk-heterojunction Solar Cells.

Tzung-Fang Guo ^{1 2}, Fuh-Shun Yang ¹, Zen-Jay Tsai ¹, Ten-Chin Wen ³, Chia-Tin Chung ⁴
¹Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan Taiwan, ²Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan Taiwan, ³Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan, ⁴, Chi Mei Optoelectronics Corporation, Tainan Taiwan

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9:00 PM - F9.34

Influence of the Interfacial Properties at Polymer Gate Dielectrics for n-type Pentacene-based Organic Field-effect Transistors.

Tzung-Fang Guo ^{1 2}, Zen-Jay Tsai ¹, Shi-Yu Chen ¹, Jer-Wei Chang ^{1 2}, Ten-Chin Wen ³
¹Institute of Electro-Optical Science and Engineering, National Cheng Kung University, Tainan Taiwan, ²Advanced Optoelectronic Technology Center, National Cheng Kung University, Tainan Taiwan, ³Department of Chemical Engineering, National Cheng Kung University, Tainan Taiwan

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9:00 PM - F9.36

Surface Plasmon-mediated Luminescence in Organic Field-effect Transistors.

Akira Watanabe ¹, Hirokazu Tada ^{1 2}
¹Materials Physics, Osaka University, Toyonaka Japan, ², JST-CREST, Kawaguchi Japan

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9:00 PM - F9.37

Low Voltage and High Performance Pentacene and Polyfluorene-based Transistors using Hybrid High-k/ultra Low-k Dielectrics.

Minseong Yun ¹, Korampally Venumadhav ¹, Mohammad Arif ², Suchismita Guha ², Shubhra Gangopadhyay ¹
¹Electrical and Computer Engineering, University of Missouri, Columbia, Missouri, United States, ²Physics and Astronomy, University of Missouri, Columbia, Missouri, United States

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Organic field-effect transistors (FETs) have been widely investigated due to their potential applications in low cost, large area, and flexible electronics. Despite the rapid progress in organic FETs there are still obstacles- high density of defect in organic semiconductor and poor interface between dielectric and organic semiconductor, which leads to relatively high operating voltage. The nature of the gate dielectric in organic FETs still remains controversial. On the one hand, high-k inorganic dielectrics reduce the operating voltage in organic FETs but increase the gate leakage current. On the other hand, low-k dielectrics increase the field-effect mobility although operating voltages are enhanced. In this work we present a study of an improved device performance of pentacene and polyfluorene-based FETs using hybrid high-k and ultra low-k dielectrics. Our focus has been on reactive electron-beam evaporated thin Al2O3 (H2 annealed) and spin-coated nanoporous organosilicate films. Ultra low-k films were prepared from solutions containing nanoparticle dispersions of Polymethyl Silsequioxane (PMSSQ). The insulating properties of the PMSSQ dielectric were characterized by capacitance-voltage and current-electric field (J-E) measurements. PMSSQ films with dielectric constant as low as 1.4 have been obtained using Al/PMSSQ/p+-Si structures. J-E measurement with different thickness of PMSSQ show that gate leakage current can be significantly reduced with ultra low-k material compared to Al2O3. Optical characterization of these films has been carried out for refractive index measurements. Scanning electron microscope images of the film’s cross section indicates that the film is comprised of nanoparticles in the size range of 2-5 nm.Thin films of pentacene were deposited on hybrid high-k and ultra low-k/heavily doped Si substrates to fabricate organic FETs. A notable enhancement in the mobility is found compared to single layered control samples. Output and transfer characteristics of pentacene FETs with hybrid gate dielectrics show a low operating voltage (≤ -2V), exhibit a mobility of 1.06 cm^2/V s, an on/off ratio of 10^5, and a small subthreshold slope when devices are operated at -3 drain-source voltage.

9:00 PM - F9.38

Vacancies in Pentacene Films on Chemically Modified Surfaces.

Soonjoo Seo ¹, Paul Evans ²
¹Materials Science Program, University of Wisconsin-Madison, Madison, Wisconsin, United States, ²Department of Materials Science and Engineering, University of Wisconsin-Madison, Madison, Wisconsin, United States

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9:00 PM - F9.39

Optical Evidence of Filament Formation in Cross Bar Molecular Electronic Devices.

Nazanin Davani ¹, Ken Shimizu ², Michael Preiner ³, Nicholas Melosh ²
¹Chemical Engineering, Stanford University, Stanford, California, United States, ²Material Science and Engineering, Stanford University, Stanford, California, United States, ³Applied physics, Stanford University, Stanford, California, United States

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9:00 PM - F9.4

Improved Mobilities in Organic Thin Film Transistors with Embedded Electrodes.

Hyeyeon Ryu ¹, Pilsoo Kang ¹, Gyutae Kim ¹, JeongSook Ha ²
¹, Korea university Electrical Engineering, Seoul Korea (the Republic of), ², Korea university Chemical and Biological Engineering, Seoul Korea (the Republic of)

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9:00 PM - F9.41

Transport through a Gated Single-molecule Transistor: Switching and Negative Differential Resistance.

Amir Farajian ⁴, Rodion Belosludov ², Hiroshi Mizuseki ², Yoshiyuki Kawazoe ², Tomihiro Hashizume ³, Boris Yakobson ¹
⁴Mechanical and Materials Engineering, Wright State University, Dayton, Ohio, United States, ²Institute for Materials Research, Tohoku University, Sendai, Miyagi, Japan, ³Advanced Research Laboratory, Hitachi Ltd, Hatoyama, Saitama, Japan, ¹Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States

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9:00 PM - F9.5

Instability of Pentacene Ultrathin Films and its Influence on Field Effect Mobility.

Yuki Tsuruma ¹, Genki Yoshikawa ¹, Susumu Ikeda ¹, Koichiro Saiki ¹
¹Department of Complexity Science and Engineering, The University of Tokyo, Kashiwa Japan

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Organic field effect transistors (OFETs) offer the fascinating possibility of fabricating a next generation devices, such as flexible displays, plastic smart cards, sensors, organic light-emitting diodes and lasers. Among a number of organic materials being used as an active layer in such devices, pentacene (C₂₂H₁₄) is regarded as one of the most promising candidates for OFETs. It is known that in organic-based FETs the carriers induced by the gate bias are located within few monolayers (MLs) just above the dielectric layers. In order to improve the FET performance, they are commonly modified by self-assembled monolayers (SAMs), such as hexamethyldisilazane (HMDS) and octadecyltrichlorosilane (OTS) for SiO₂. Owing to these benefits, the performance of the pentacene FET has exceeded that of amorphous Si FET.However, we have found that pentacene molecules deposited on HMDS-treated SiO₂ substrates aggregate with time even under high-vacuum and ambient temperature conditions. We constructed an in situ atomic force microscopy (AFM)-FET measurement system and found that the FET mobility significantly decreased with the aggregation. The 1.25 ML pentacene film grown on HMDS/SiO₂ showed striking morphological change. Apparently good pentacene ultrathin films grown on HMDS easily break owing to spontaneous aggregation of molecules even under high vacuum and ambient temperature conditions. Electrical measurement of the same film indicates that the as-deposited film shows a typical FET characteristic even though the film thickness is only 1.25 ML. The flat wetting layer works as a conduction channel, which is consistent with the accumulation layer thickness (1-2 molecular layer) estimated from our past study (PRB 71, 35332). With degradation of film continuity, the FET mobility decreases and it finally disappears 3 hours later, which completely corresponds to separation of aggregated islands. A close inspection of the change teaches us that the first degradation starts at the periphery of the second layer, resulting in formation of grooves around the periphery. Although thicker pentacene films are more stable, this kind of change appears at the thinnest region of the film. These results provide a direct visualization of the instability inherent in pentacene films. This aggregation should be one of the major origins of the instability and irreproducibility of pentacene-based devices. We are now investigating detailed aggregation mechanism by putting the ultrathin pentacene film under different circumstances (exposure to various gases, exploration of SAM materials, etc.) or changing the growth condition to find the way of stabilizing the pentacene film.

9:00 PM - F9.6

Novel Water-Soluble Polyfluorenes as an Electron Injection Layer in PLED for Extremely High Quantum Efficiency.

Seung-Hwan Oh ¹, Doojin Vak ^{1 3}, Seok-In Na ¹, Tae-Woo Lee ², Dong-Yu Kim ¹
¹Dept. of Materials Science and Engineering, Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of), ³Heeger Center for Advanced Materials (HCAM), Gwangju Institute of Science and Technology (GIST), Gwangju Korea (the Republic of), ², Samsung Advanced Institute of Technology, Yongin-Si Korea (the Republic of)

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9:00 PM - F9.7

In-plane Structures of Ultrathin Organic Films by Grazing Incidence X-ray Diffractmetry.

Noriyuki Yoshimoto ¹, Wan-Yan Li ², Keijyu Aosawa ¹, Mutsuo Ito ¹, Toshinori Tanisawa ¹
¹Dep. Materials Science & Engineering, Iwate University, Morioka Japan, ², JST Iwate, Morioka Japan

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9:00 PM - F9.9

Current-voltage Characteristics of Lutetium Endohedral Metallofullerenes using Gold Nanogap Electrodes Fabricated by Surface Catalyzed Electroless Gold Plating Technique.

Yuhsuke Yasutake ¹, Keijiro Kono ¹, Hisashi Umemoto ², Yasuhiro Ito ², Haruya Okimoto ², Charles Smith ³, Hisanori Shinohara ^{2 4}, Yutaka Majima ¹
¹Dept. of Physical Electronics, Tokyo Institute of Technology, Tokyo Japan, ²Dept. of Chemistry, Nagoya University, Nagoya Japan, ³Dept. of Physics, Cavendish Laboratory, University of Cambridge, Cambridge United Kingdom, ⁴Institute fur Advanced Reseach, Nagoya University, Nagoya Japan

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Single molecular switching devices composed of a single functional molecule have been extensively studied as basic components of the molecular nanodevices. We have demonstrated the switching phenomena by utilizing orientation of endohedral metallofullerene. The current-voltage characteristics of a single Tb@C₈₂ molecule on an octanethiol SAM at 13 K exhibited hysteresis and negative differential conductance repeatedly; this was interpreted in terms of switching of the Tb@C₈₂ molecular orientation caused by the orientation change in its electric dipole moments derived from the trivalent Tb³⁺C₈₂^3– electronic states due to the application of an external electric field.[1] For the practical use of single molecular devices such as single molecular orientation switching devices, it is important to develop the fabrication procedure for integrated nanogap electrodes with high yield. Here, we report the fabrication procedure of integrated nanogap by using surface catalyzed electroless gold plating technique[2] and measurements of the current-voltage characteristics of lutetium endohedral metallofullerenes (Lu@C₈₂) by using these gold nanogap. We used safety electroless gold solution which consists of iodine tincture as gold solvent and L(+)-ascorbic acid as reducing agent. [3] By adjusting the electroless gold solution density and plating time, we succeeded in fabricating the gold nanogap below 5 nm distance with process yield of 41 %. Then, Lu@C₈₂ molecules were evaporated onto gold nanogap with measuring the nanogap conductance during the vacuum sublimation of Lu@C₈₂. After confirming Lu@C₈₂ molecules were trapped between the gold electrode by changing in nanogap conductance, we measured the current–voltage characteristics of Lu@C₈₂ molecules and observed the electrical transport properties through Lu@C₈₂ molecules.[1] Y. Yasutake, et al., Nano Lett., 5, 1057 (2005). [2] C. S. Ah et al., Appl. Phys. Lett., 88, 133116 (2006). [3] A. Umeno, et al., Appl. Phys. Lett., 86, 143103 (2005).

2007-11-29 Show All Abstracts

Symposium Organizers

Julia W. P. Hsu Sandia National Laboratories

Leeor Kronik Weizmann Institute of Science

George G. Malliaras Cornell University

Nobuo Ueno Chiba University

F10: Thin Film Growth

Session Chairs

Karen Kavanagh

Xiaoyang Zhu

Thursday AM, November 29, 2007

Back Bay C (Sheraton)

9:30 AM - F10.1

Interfacial Electronic Structure of Pentacene on Cu(110) and Cu(100) Surfaces: Energy Level Splitting and Energy Band Dispersion.

Hiroyuki Yamane ¹, Kyuho Lee ², Kaname Kanai ³, Yukio Ouchi ¹, Yoshitada Morikawa ⁴, Kazuhiko Seki ^{1 3}
¹Department of Chemistry, Graduate School of Science, Nagoya University, Nagoya Japan, ², National Renewable Energy Laboratory, Golden, Colorado, United States, ³Research Center for Materials Science, Nagoya University, Nagoya Japan, ⁴The Institute of Scientific and Industrial Research, Osaka University, Osaka Japan

Show Abstract

The electronic structure at interfaces formed between an organic semiconductor film and a metal surface plays a crucial role in the performance of organic devices such as light-emitting diodes, field-effect transistors, and solar cells. In particular, the energy positions of the highest-occupied and lowest-unoccupied molecular orbital (HOMO and LUMO) levels with respect to the Fermi level of metal electrodes and the formation of the interfacial dipole layer are of fundamental importance in discussing the barrier heights for the charge injection and separation at organic/metal interfaces. Although many research groups have been studying the energy level alignment at organic-related interfaces, the clarification of the origin of the interfacial electronic structure including the dipole layer is still an important issue in the recent studies of organic-related interfaces.In this work, in order to study the electronic structure at organic/metal interfaces in detail, we have performed angle-resolved UV photoemission spectroscopy experiments on the ordered pentacene (Pn) monolayers on Cu(110) and Cu(100) surfaces. It was reported that (i) Pn molecules on Cu(110) form a highly ordered monolayer with planar adsorption geometry, where the molecular long axis is parallel to the [1-10] substrate direction [Söhnchen et al., J. Chem. Phys. 121, 525 (2004)], and that (ii) Pn molecules on Cu(100) form a multi-domain monolayer with planar adsorption geometry [Baldacchini et al., Surf. Sci. 566-568, 613 (2004)].We observed the distinctive electronic structures at the Pn/Cu(110) and the Pn/Cu(100) interfaces, which are completely different from those of the gas- and bulk-phase pentacene. For both interfaces, we observed the evidence of the HOMO-level splitting of about 0.6 eV. The resultant split levels at the Pn/Cu(110) interface showed the periodic shifts with the photoelectron take-off angle, which can be ascribed to the energy band dispersion. On the other hand, there are no periodic shifts in the split levels at the Pn/Cu(100) interface. The lattice constant deduced from the observed dispersion for the Pn/Cu(110) interface is consistent with the reported one based on the low-energy electron diffraction experiments. Thus the observed dispersion can be ascribed to the intermolecular energy band dispersion in the Pn monolayer on Cu(110). These results indicate that (i) the splitting behavior can be ascribed to the hybridization of the MO(s) and the substrate wavefunction, and (ii) the dispersive behavior observed at the Pn/Cu(110) originates from the lateral ordering of the film and the hybridization between the MO(s) and the substrate wavefunction.We will report the above experimental results in detail, in comparison with the theoretical calculations based on density-functional theory within pseudopotential formulation and generalized-gradient approximation. Also we will discuss the possible origin of the dipole layer at the Pn/Cu interface.

9:45 AM - F10.2

Spontaneous Charge Transfer Across Hexyl Layers to Establish Thermodynamic Equilibrium.

Steffen Duhm ¹, Hendrik Glowatzki ¹, Robert Johnson ², Juergen Rabe ¹, Norbert Koch ¹
¹Department of Physics, Humboldt University Berlin, Berlin Germany, ²Department of Experimental Physics, Hamburg University, Hamburg Germany

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10:00 AM - **F10.3

Bonding, Structure and Function of Highly Ordered Organic Semiconductor Layers on Metal Surfaces.

Stefan Tautz ¹, Ruslan Temirov ¹, Serguei Soubatch ¹
¹School of Engineering and Science, Jacobs University Bremen (Formerly: International University Bremen), Bremen Germany

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During recent years, molecular adsorbate layers on solid surfaces have moved into the focus, because organic semiconductors and their interfaces offer application potential in organic and molecular electronics. Although some applications are close to being marketed commercially (e.g. displays on the basis of organic light emitting diodes or RFID tags on the basis of organic field effect transistors), many fundamental properties of molecular materials and their interfaces are still the subject of active research.For oligomeric semiconducting π-conjugated molecules, the technique of organic molecular beam deposition (OMBD) has been developed to such an extent that the controlled and reproducible preparation of highly ordered ultra-thin molecular layers on a range of inorganic substrates has become possible, thereby making these films accessible to the sensitive spectroscopies and microscopies of UHV-based surface science. Apart from the adsorption process itself, the intrinsic properties of organic semiconductors as well as their modification by the presence of the interface can thus be studied with precision. In the present contribution, we will focus on the aspects of bonding, structure and function of small-molecule semiconductors on metal surfaces. In detail, the following issues will be discussed: (1) The bonding of π-conjugated molecules often involves a chemical as well as a physical contribution. Using a model molecule on noble metal surfaces as an example, the particular properties of such bonds are discussed. (2) In addition to the molecule-substrate bonding, the intermolecular interaction has an important influence on the structural evolution at the interface. Depending on the molecule, these interaction can be attractive or repulsive, yielding very different phase behaviour. This will be illustrated by a comparison between two different types of molecules on the same surface. (3) In organic and molecular electronics, the organic-metal contact is of prime importance for charge injection and charge transport – the prime function of organic semiconductors. In the case of highly ordered organic monolayers on a metallic substrate, charge transport across the interface and through the molecule can be investigated under structurally well-defined conditions. We will illustrate this with two examples, (a) the inelastic tunneling transport through an adsorbed molecule and (b) the charge transport through a single-molecular wire.

10:30 AM - F10.4

Structural and Electronic Aspects of Organic Donor-acceptor Interfaces:C₆₀ and Pentacene.

D. Dougherty ¹, W. Jin ², W. Cullen ³, G. Dutton ², J. Reutt-Robey ², S. Robey ¹
¹, NIST, Gaithersburg, Maryland, United States, ²Department of Chemistry and Biochemistry and Materials Research Science and Engineering Center, University of Maryland , College Park, Maryland, United States, ³Department of Physics and Materials Research Science and Engineering Center, University of Maryland , College Park, Maryland, United States

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Successful utilization of organic donor-acceptor systems for photovoltaic and light emitting diode applications requires an understanding of factors controlling molecular structure in mixtures and at interfaces, along with the development of correlations between structure and electronic features such as band alignment. This will allow the optimization of structural and electronic characteristics. We have studied several organic systems using STM, STS, and photoemission to examine real-space and electronic structure and provide clues to the relative importance of competing intermolecular interactions and their impact on band alignment. Initial studies have focussed on the technologically relevant donor-acceptor system consisting of C₆₀ and pentacene, with Ag(111) and Au(111) as substrates. Organic interfaces were formed starting with the initial growth of C₆₀ or pentacene mono-, bi- and multilayers, followed by deposition of the second organic componnent. STM measurements show that C₆₀ deposited on a pentacene bilayer on Ag(111) produces two unique structures in the first layer. The pentacene bilayer forms a well-ordered structure on Ag(111) with the long molecular axis nearly parallel to the surface. At low coverage, C₆₀ always adsorbs in-between two adjacent pentacene rows and isolated C₆₀ molecules are easily observed at room temperature. This indicates that the mobility of C₆₀ on pentacene is significantly smaller than on most metal surfaces where island formation is rapid. In some cases, images of C₆₀ reveal structure that suggests C₆₀ is oriented with the 6=6 bond between hexagons down and in the gap between two penatcene molecules, similar to the orientation proposed for C₆₀-porphyrin systems. With increasing coverage, C₆₀ forms linear chains, still templated by underlying rows of pentacene. Further increases in coverage finaly result in domains of C₆₀ with no long range order. We propose that these changes in structure result from competing C₆₀- C₆₀ and C₆₀-pentacene interactions.We have developed simple ideas based on electrostatic interactions to help explain these structures, as well as the proposed orientation between C₆₀ and the pentacene bilayer structure, and are testing these ideas using DFT calculations for simple model systems built around clusters consisting of C₆₀ plus one- or more pentacene molecules.Electronic aspects of these donor-acceptor systems were also examined. Information on local transport gaps and band alignment was provided by constant-current distance-voltage spectroscopy. A gap of 4.5±0.2 eV is found over the linear C₆₀ chains compared with a gap of 3.6±0.2 eV for the surounding pentacene bilayer. These values are compared and contrasted with analogous measurements of occupied state alignment based on photoemission and with ideas from theroetical examinations of the influence of local polarization on the band gap.

10:45 AM - F10.5

Multilayer Growth Investigation of a Tris(thieno)hexaazatriphenylene Derivative on Au(111) by Scanning Tunneling Microscopy.

Sieu Ha ¹, Qing Zhang ², Stephen Barlow ², Seth Marder ², Antoine Kahn ¹
¹Electrical Engineering, Princeton University, Princeton, New Jersey, United States, ²Chemistry and Biochemistry, Georgia Institute of Technology, Atlanta, Georgia, United States

Show Abstract

Layer-by-layer growth of the electron transport material tris{2,5-bis(3,5-bis-trifluoromethyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene (THAP) is probed by scanning tunneling microscopy (STM). A relative of discotic liquid crystals, THAP is demonstrated to grow in well ordered planes from the first to fourth monolayers when deposited on Au(111). Within each monolayer, the molecular planes align approximately parallel to the substrate. The first monolayer orders in a square unit cell with two molecules per cell. The molecules appear significantly smaller in size as compared with subsequent monolayers, and they are likely heavily influenced by close proximity to the metal substrate. The second monolayer, at fractional coverage, initially grows commensurate to the first layer, suggesting that interlayer forces control adlayer order. At full coverage, however, intralayer interactions prevail and the second layer adopts a markedly different order, characterized by a hexagonal close-packed lattice with a herringbone pattern of alternating rows of rotational orientation. As a result, the unit cell is rectangular with two molecules per cell, and the STM images are more representative of the chemical structure of THAP than in the first monolayer, in size and in structure. The third and fourth monolayers both show an ordered structure concordant with the second layer, and all three have nearly equivalent unit cells. This suggests some registry between layers such that one monolayer influences the order and lattice vectors of the subsequent layer. Monolayers beyond the fourth are also expected to order in the same manner. In addition to structural similarities, occupied state STM images from the second to fourth monolayers reflect a decrease in the influence of the substrate, as expected. The metal surface causes delocalization of the density of states throughout neighboring molecules, but as thicker layers are probed, this effect should diminish. In STM, molecules in the second monolayer appear uniform in contrast, but in the third and fourth layers the molecules have more distinct features. Indeed, the latter molecules more closely resemble the highest occupied molecular orbital as calculated by density functional theory, which accounts for a single molecule isolated from external effects.

11:00 AM - *

break

11:30 AM - **F10.6

Photoemission Microspectroscopy and Scanning Microscopy for Organic Films.

Toshiaki Munakata ¹
¹Graduate School of Science, Osaka University, Toyonaka, Osaka, Japan

Show Abstract

A critical difficulty of the electronic structures at the interfaces between organic molecules and inorganic substrates arises from the spatial inhomogeneity of the organic films. The growth of organic films is not spatially homogeneous, because it is affected by substrate-molecule interaction and intermolecular interaction. The geometrical structure causes inhomegeneity of electronic structures. The impact of spatial inhomegeneity on the interface electronic structure remains elusive to date. In order to resolve the complicated electronic structures, we have developed micro-spot photoemission spectroscopy (micro-UPS) and micro-spot two-photon photoemission (micro-2PPE) spectroscopy. Micro-UPS achieves a lateral resolution of 0.3 micrometer and an energy resolution of 30 meV by using the 6th harmonics (wavelength of 140 nm) of a Ti:Sa laser of 100 fs pulse duration as a light source. By simply changing the light source to tunable UV fs-radiation, unoccupied electronic states can be observed by micro-2PPE. The lateral resolution for micro-2PPE was 0.4 micrometer. Here, we report micro-UPS and 2PPE results for films of several phthalocyanine (Pc) derivatives grown on a highly oriented pyrolitic graphite (HOPG) substrate. The HOMO band of CuPc appeared as two split peaks when the thickness was less than 1 monolayer (ML). The two peaks converged to the binding energy of 1.2 eV as the thickness increased to 1 ML. The lower and higher energy peaks arise from isolated and aggregated molecules, respectively. The HOMO band of CuPc film of 1.5 ML thickness appeared again as two peaks at 1.2 and 1.4 eV; the former is due to the 1st layer, and the later, to the 2nd layer. The HOMO band of OTiPc appeared as a single peak at areas of monolayer films, and as split peaks, at islands of metastable bilayer films. Microscopic images revealed islands of the bilayer film. The islands dissolved slowly by migration and reorientation of molecules. From micro-2PPE for mono- and bilayer films of PbPc, we have successfully identified HOMO-1, HOMO, LUMO(+1), LUMO+2 and the image potential state. The occupied states are in good agreement with micro-UPS results. Resonant excitations between the states confirmed the assignments. Lateral distributions of unoccupied states were recorded as surface images. The results demonstrate the capability of the photoemission microspectroscopy to resolve the complicated electronic structure of realistic organic films.

12:00 PM - F10.7

Image Potential States of Pentacene on Ag(111): Comparison between Two-Photon Photoemission and Tunneling Spectroscopy.

Gregory Dutton ¹, Dan Dougherty ², Janice Reutt-Robey ¹, Steven Robey ²
¹Chemistry & Biochemistry, University of Maryland, College Park, Maryland, United States, ²Surface and Microanalysis Science Division, NIST, Gaithersburg, Maryland, United States

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12:15 PM - F10.8

Influence of Interface Engineering on the Growth Behavior and Performance of Organic Thin Film Crystals.

Aram Amassian ¹, Sukwon Hong ², Sugandha Bhargava ², Aravind Killampalli ², Tushar Desai ², Vladimir Pozdin ¹, Arthur Woll ³, Detlef Smilgies ³, George Malliaras ¹, James Engstrom ²
¹Materials Science and Engineering, Cornell , Ithaca, New York, United States, ²School of Chemical and Biomedical Engineering, Cornell, Ithaca, New York, United States, ³Cornell High Energy Synchrotron Source, Cornell, Ithaca, New York, United States

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12:30 PM - F10.9

Highly Oriented Growth of CuPc/PTCDA Heterojunctions via Organic Vapor-Phase Deposition.

Richard Lunt ^{1 2 3}, Jay Benziger ¹, Stephen Forrest ^{2 3}
¹Chemical Engineering, Princeton University, Princeton, New Jersey, United States, ²Electrical Engineering and Computer Science, University of Michigan, Ann Arbor, Michigan, United States, ³Physics, University of Michigan, Ann Arbor, Michigan, United States

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12:45 PM - F10.10

Selective Nucleation of Organic Single Crystals from Nanoscopically Rough Surfaces.

Stefan Mannsfeld ¹, Alejandro Briseno ^{2 1}, Shuhong Liu ¹, Mark Roberts ¹, Colin Reese ¹, Zhenan Bao ¹
¹Chemical Engineering, Stanford University, Palo Alto, California, United States, ²Department of Materials Science & Engineering and Chemical Engineering, University of Washington, Seattle, Washington, United States

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F11: Molecular Junctions

Session Chairs

Stefan Tautz

Christof Woell

Thursday PM, November 29, 2007

Back Bay C (Sheraton)

2:30 PM - F11.1

Scanning Tunneling Microscopy Studies of Organic Molecules Adsorbed on Silicon NanoWires.

Eric Salomon ¹, Antoine Kahn ¹
¹Dept. of Electrical Engineering, Princeton University, Princeton, New Jersey, United States

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One way to grow one dimensional (1D) structures of organic semiconductors is to adsorb molecules on a template surface presenting a 1D pattern. With that purpose, we investigated the ability to form organic NanoWires (NWs) when adsorbing specific molecules on silicon NWs.In the present work, all experiments were done in interconnected ultra-high vacuum chambers (base pressure p~5x10-11 Torr) equipped with sputtering and annealing for Ag surface preparation, a Si evaporator for SiNW formation, organic molecule sublimation stations, and a room-temperature Omicron STM. The template surface was made of SiNWs oriented along the [-110] direction of the Ag. Those NWs were obtained upon adsorption of Si atoms on a Ag(110) surface and were characterized mainly by Scanning Tunneling Microscopy (STM) and Spectroscopy (STS). As expected from previous photoemission measurements and density functional theory calculations , the I(V) spectra recorded on several individual SiNWs exhibit a metallic behavior and the local density of states (LDOS) confirmed the existence of discrete electronic states close to the Fermi level.Then, in order to check selective adsorption of molecules on the SiNWs according to their chemical structure and functionalization, we used different organic compounds, with different functions, on the template surface and probed their physical properties using STM. We describe here the adsorption of tris{2,5-bis(3,5-bis-trifluoromethyl-phenyhl)-thieno}[3,4-b,h,n]-1,4,5,8,9,12-hexaazatriphenylene (THAP) and 9,10-phenan-threnequinone (PQ). At sub-monolayer coverage, STM data demonstrate that THAP molecules adsorb randomly all over the surface without any preferential order (Fig.1a). The electronic properties of the molecules are probed by STS and compared with those of a sub-monolayer of THAP adsorb directly on a clean Ag(110) surface. Both spectra behave a similar shape confirming that molecules such as THAP adsorb on the surface regardless to the SiNWs. In the case of the PQ molecules (fig.1b), the STM images exhibit bright structures oriented along the [-110] direction and exclusively adsorbed on the SiNWs. Such observation establishes that those molecules react preferentially with the SiNWs. In that special case, the interaction might occur between the Si atoms of the NWs and the dicarbonyl group of the molecule through a cycloaddition such as [4+2] or Diels-Alder reaction, as demonstrated when PQ adsorb on Si(100) .These two examples show, not surprisingly, that the reactivity towards the SiNWs depends on the functionalization of the molecule. Studying the detail of the adsorption of quinones molecules alike PQ might be interesting for the following reasons. Such reaction involves strong, well known and reproducible molecule - Si bonds. Furthermore, since many PQ derivatives are known, this technique may provide a rather general method for fabrication of new types of 1D functional organic layers.

2:45 PM - F11.2

Interfacial Effects in Inelastic Electron Tunneling Spectra of Molecular Cross-Wire Junctions.

Lam Yu ¹, James Kushmerick ¹
¹Surface and Microanalysis Science Division, NIST, Gaithersburg, Maryland, United States

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3:00 PM - **F11.3

Studies of Molecular Contacts using Ballistic Electron Emission Microscopy.

Karen Kavanagh ¹
¹Physics, Simon Fraser University, Burnaby , British Columbia, Canada

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3:30 PM - F11.4

True Molecular Resistance of Alkanes in Molecular Junctions.

Hylke B. Akkerman ¹, Auke Jisk Kronemeijer ¹, Paul A. van Hal ², Tom C. T. Geuns ², Dago M. de Leeuw ^{1 2}, Paul W. M. Blom ¹, Bert de Boer ¹
¹Zernike Institute for Advanced Materials, University of Groningen, Groningen Netherlands, ², Philips Research Laboratories, Eindhoven Netherlands

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We have demonstrated a technology to fabricate reliable molecular metal-self-assembled monolayer (SAM)-metal junctions with unprecedented device diameters up to 100 µm with a yield close to unity. Stability investigations have shown a shelf life of months and no deterioration upon cycling. Key ingredient is the use of a highly conducting polymer layer sandwiched between the SAM and the top electrode in order to prevent electrical shorts [1]. Furthermore, we have shown that for alkanes a rectangular tunneling barrier with the image potential included consistently describes the current-voltage characteristics of the molecular junctions up to 1V bias for different molecule lengths [2]. For extraction of the intrinsic molecular conduction both the formation of the monolayer as well as the nature of its contact to the electrode have to characterized. We demonstrate that depending on the concentration of long alkanedithiols in ethanol for the self-assembly process, the formation of the SAM and the molecular orientation are varied between either a SAM consisting of molecules in a looped phase, i.e. with both thiols attached to the Au electrode, or with molecules in a standing-up phase where only one thiol is attached. We show that C14-dithiol assembled from 0.3 mM concentration results in an almost complete looped phase monolayer and the current is increased by a factor 50 compared to the standing-up phase, due to a smaller tunneling distance [3]. We found no effect on the conduction properties of devices based on alkane(di)thiols when the conduction of the PEDOT:PSS is modified by 3 orders of magnitude. This clearly indicates that the resistance of the monolayer is the dominant resistance in the device. However, when the nature of the interface between the monolayer and the PEDOT:PSS is altered by modifying the endgroup of the molecules, i.e., by comparing alkanemonothiols with alkanedithiols, the absolute conductance values change. A similar exponential decrease of the current with increasing molecule length is found, which shows that the transport through the molecular junctions can be factorized in a resistance of the molecules and a resistance arising from the interface. This factorization enables us to derive the interface resistance and the true molecular resistance of both alkanemono- and dithiols [4]. [1] H. B. Akkerman, P. W. M. Blom, D. M. de Leeuw, B. de Boer, Nature 441, 69 (2006).[2] H. B. Akkerman, R. C. G. Naber, B. Jongbloed, Paul. A. van Hal, P. W. M. Blom, D. M. de Leeuw, B. de Boer, Proc. Natl. Acad. Sci. USA, in press.[3] H. B. Akkerman, A. J. Kronemeijer, P. A. van Hal, D. M. de Leeuw, P. W. M. Blom, B. de Boer, submitted for publication. [4] H. B. Akkerman, A. J. Kronemeijer, S. J. G. Tamminga, M. J. Smid, D. M. de Leeuw, P. W. M. Blom, B. de Boer, submitted for publication.

3:45 PM - F11.5

Photoelectric Junctions between Metals, Semiconductors and Photosynthetic Reaction Center Proteins.

Ludmila Frolov ², Elad Koren ¹, Yossi Rosenwaks ¹, Shachar Richter ³, Chanoch Carmeli ², Itai Carmeli ³
²Biochemistry, Tel Aviv University, Tel Aviv Israel, ¹Physical Electronics, Tel Aviv University, Tel Aviv Israel, ³Chemistry, Tel Aviv University, Tel Aviv Israel

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4:00 PM - *

break

4:30 PM - **F11.6

Electron Transport Across Molecules: From Well-defined Monolayers of Simple Alkyls to Proteins.

David Cahen ¹
¹Materials and Interfaces, Weizmann Institute of Science, Rehovoth Israel

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We measure electronic transport across high quality semiconductor/alkyl chain monolayers/metal structures, where the molecules are chemically bound to the semiconductor (Si, GaAs) and/or the metal (Hg, Au, Pd). By combining the results with those from photoemission measurements (Kahn, Ueno) and electronic structure calculations (Kronik) we are led to question some “conventional wisdoms”: To what extent should we use the simple HOMO/LUMO concept for these systems; How relevant is the concept of a transport barrier with one well-defined energy barrier and spatial width; How important is molecule-electrode chemical bonding for transport and, more trivial(?), do we actually know what are the contact areas in molecular electronics? Notwithstanding these uncertainties, our work with the simple systems, showed a route to work with the much more complicated biological ones. I will illustrate the latter with our results on bacteriorhodopsin. While much more work is needed, already the results hold some surprises.----------------------------------------------------------------------------Work done with A. Salomon, O. Seitz, H. Shpaisman, F. Thieblemont, R. Har-Lavan, G. Nesher, A. Vilan, I. Ron, Y.-D. Jin, and with the groups of ., L. Kronik and M. Sheves, Weizmann Inst, with T. Boecking & J. Gooding, UNSW, Sydney, Australia and with the groups of Antoine Kahn, Princeton Un., Eberhard Umbach, Würzburg Un., Germany, and Nobuo Ueno, Chiba Un., Japan.----------------------------------------------------------------------------References----------------------------------------------------------------------------D. Cahen et al., Mater. Today, 8 (2005) 32; A. Salomon et al., Phys. Rev. Lett. 95 (2005) 266807, Nano. Lett. 2006, 6, 2873; Adv. Mater. 19 (2007) 445-450; O. Seitz et al. Langmuir, 22 (2006) 6915; JACS, in press,; G. Nesher et al., J. Phys. Chem. B, 110 (2006) 14363; JACS 129 (2007) 734 L. Segev et al.; Phys. Rev. B 74 (2006) 165323,; F. Amy et al., J. Phys. Chem. B., 110 (2006) 21826; Y. Jin et al., PNAS 103 (2006) 8601.

5:00 PM - F11.7

Molecular Modulation over Electronic Structures of Si/Molecule Interface.

Tao He ^{1 2 3}, Huanjun Ding ⁴, Naama Peor ⁵, David Nackashi ⁶, Meng Lu ^{1 2 3}, Yuval Ofier ⁵, Yongli Gao ⁴, Shlomo Yitzchaik ⁵, Paul Franzon ⁶, James Tour ^{1 2 3}
¹Department of Chemistry , Rice University, Houston, Texas, United States, ²Computer Science, Mechanical Engineering and Materials Science, Rice University, Houston, Texas, United States, ³Smalley Institute for Nanoscale Science and Technology, Rice University, Houston, Texas, United States, ⁴Department of Physics and Astronomy, University of Rochester, Rochester, New York, United States, ⁵Department of Inorganic and Analytical Chemistry, The Hebrew University of Jerusalem, Jerusalem Israel, ⁶Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina, United States

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The performance of silicon-based devices can be influenced greatly by their surface states, which can be modulated via surface engineering. The drain current and threshold voltage in pseudo metal-oxide-semiconductor field-effect transistors (MOSFETs) were shown to be controllably tuned by grafting a monolayer of a structurally varied series of molecules (-C6H4-X, with X = NMe2, NH2, NO2, and Mo6 cluster) atop oxide-free H-passivated silicon surfaces of the channel region. The resulting systematic modulation of the device conductance is attributed to charge transfer between the device layer and the molecules. To further elucidate this, the silicon/molecule heterojunctions were fabricated by direct covalent grafting of the same molecules onto the surface of four types of silicon substrates (both n- and p-type with different dopant densities). The electronic structures of the resultant interfaces were studied by x-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS), inverse photoemission spectroscopy (IPES), and Kelvin probe technique. The results indicated the electronic structures were systematically tuned in accordance with the electron-donating ability, redox capability, and/or dipole moment of the grafted molecules. The contribution from the hydrogen adatoms was also considered. The surface band bending, dependent on charge transfer between the silicon substrates and the grafted molecules, exhibits the same trend as the charge transfer barrier, which agrees well with those observed in the pseudo-MOSFETs. The electron affinity, dependent on the dipole moment of the grafted molecules, has the same trend as the surface potential step and work function. The results reinforce the potential utilization of surface grafting by organic molecules as a means to tune the electronic properties of semiconductors and, consequently, to achieve controllable modulation of electronic characteristics in small semiconductor devices at future technology nodes where consistent impurity doping is difficult to achieve.

5:15 PM - F11.8

1/f Tunnel Current Noise Through Si-Bound Alkyk Monolayers.

Dominique Vuillaume ¹, Nicolas Clement ¹, Stephane Pleutin ¹, Stephane Lenfant ¹, Oliver Seitz ², David Cahen ²
¹, IEMN-CNRS, Villeneuve DAscq France, ², Weizmann Institute, Rehovot Israel

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5:30 PM - F11.9

Can Molecules on Semiconductors be Described as Heterojunctions?

Fabrizio Cleri ^{1 2}, Christophe Delerue ¹
¹, Institute of Electronics, Microelectronics and Nanotechnology (UMR CNRS 8520), Lille France, ², University of Science and Technology of Lille , Lille France

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Compared to the substantial bulk of experimental work concerning single molecules and molecular monolayers contacted to metal surfaces, considerably less work has been done for molecules contacted to semiconductor substrates. Even less studied are the complex poly-interfaces of a semiconductor/molecule/metal contact. When considering the semiconductor-molecule system, it may be tempting to assume a heterojunction model, together with all its related concepts of band lineup, band offset, interface dipole, and so on. However, to what extent such concepts, originally formulated for bulk semiconductor interfaces, can be transferred to a thin molecular monolayer of relatively modest planar density ? And, when a metal comes into contact with such a system (from the side of the monolayer), do the two halves form some kind of Schottky barrier ? Is there a critical planar density of the molecular layer beyond which the above concepts can, or cannot, be applied ? Are the free-molecule properties of any relevance in predicting the characteristics of the heterojunction ? What is the influence the interfacial bonding of the molecule and of the attending charge transfer on, e.g., the overall energy spectrum, the valence-band alignment, the vacuum-level alignment, and so on ?Stimulated by recent experimental work in our group (Lenfant et al., J. Phys. Chem B 110 (2006) 13497), and in the attempt to propose possible answers at least to some of the above questions, we performed first-principles calculations of a model multilayer, formed by polyalkylsilane chains covalently bonded to Si:H surfaces, and contacted at the other extremity by a metal (Al or Au). The molecules were terminated with different π-conjugated head groups. We used density-functional theory with GGA and standard norm-conserving pseudopotentials to describe the ground-state charge density in a two-dimensional periodic, dense planar stacking of Si/molecular layer/metal, the whole sandwich being isolated by enough void space in the perpendicular direction. We generally observe a strong effect on the ordering and alignment of the molecular energy levels with respect to the Fermi level of Si, consequent to intermolecular screening within the monolayer and of the appearance of surface localized states, as a function of the different interfacial bonding arrangements. However, the results for the complete Si/molecule/metal junction are not yet conclusive, showing quite a different behavior as a function of the molecular termination, and of the type of bonding to the metal contact. (Preliminary results in : Cleri et al., J. Phys. Chem. B 110 (2006) 11496.)

5:45 PM - F11.10

Sensitive Sub-Bandgap Measurements of Polythiophene-based Thin Films Using Surface Plasmon Resonance Spectroscopy (SPRS).

Jonathan Rivnay ¹, Michael Preiner ¹, Nicholas Melosh ¹, Alberto Salleo ¹
¹Materials Science and Engineering, Stanford University, Stanford, California, United States

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Although the physics of charge transport in organic semiconductors is not fully understood, it is agreed upon that transport and charge injection is limited by charge trapping. The trap states responsible for lowering carrier mobility must be located within the band gap of the organic semiconductor. Thus these trap states lead to weak sub-gap absorption features in the optical absorption spectrum of the material. Measurement of such low-level absorption in thin films is difficult due to the inadequate sensitivity of common spectroscopic techniques. While photothermal deflection spectroscopy can measure weak absorption, it is limited to thick films. Many electronic devices of interest (e.g. transistors, LEDs) however are made with thin (<100 nm) layers of semiconductor material. Thin and thick films often have significantly different microstructures; therefore, there is need for a technique that measures weak, sub-gap absorption in polymeric thin films. Recently, surface plasmon resonance spectroscopy (SPRS) has been employed to probe the optical properties of monolayer thick molecular electronic devices with high sensitivity[1]. SPRS relies on the high electric fields created by surface plasmon polaritons at the metal/organic interface to allow for sensitive measurement of optical properties at the buried interface. This approach is able to simultaneously extract both the real (n) and imaginary (k) components of the index of refraction based on the efficiency of coupling incident light into surface plasmon polaritons. We use SPRS to measure sub-bandgap absorption in thin (ca. 20nm) polymeric films of regioregular poly-3-hexylthiopene (P3HT) and poly(2,5-bis-alkylthiophen-2-yl)thieno[3,2-b]thiophene (PBTTT) on Au. Absorption spectra of P3HT and PBTTT obtained with SPRS were in good agreement with conventional UV-vis spectra, and showed additional features below the absorption edge. In order to establish the sensitivity of the technique, we measured the optical absorption of a 20-nm film of poly(methyl methacrylate) (PMMA) in the visible. The P3HT films show sub-bandgap absorption features approximately one order of magnitude higher than the PMMA baseline. SPRS promises to be a powerful tool in investigating charge transport in organic thin film semiconductors of varying order, processing conditions, and degradation. It will allow for a further understanding of the relation between microstructure and electrical properties for thin films, and will help to determine the optical properties of charge injected in field-effect devices.1. K.T. Shimizu, et al. Nano Letters, 6, 2797 (2006).

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