Papers

AVS2001 Session OF+EL+TF-ThA: Electronic Properties of Organic Thin Films

Thursday, November 1, 2001 2:00 PM in Room 131

Thursday Afternoon

Start	Invited?	Item
2:00 PM		OF+EL+TF-ThA-1 Interface Formation in Organic Thin Film Transistors: A Photoemission Spectroscopy Study N.J. Watkins, L. Yan, Y. Gao (University of Rochester) Pentacene, perylene, and sexithiophene are all materials being used in organic thin film transistors due to their relatively large mobilities. It has been suggested that the functional behavior in organic thin film transistors occurs within the first few molecular layers of the device at the interfaces between the organic and the metals and dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation of pentacene, perylene, and sexithiophene on conductors and dielectrics using photoemission spectroscopy to examine layer by layer organic growth onto these materials. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition, ranging from a 1eV dipole at the interface between sexithiophene and gold to a -0.46eV dipole at the interface between pentacene and calcium. There appears to be a linear relation between the interface dipole and metal workfunction. On the other hand, for metal on organic deposition, more complex material intermixing takes place and as a result, the electronic structure of the interface differs from that of organic on metal deposition. Possible charge transfer, dipole formation and energy level bending at these interfaces will be discussed.
2:20 PM		OF+EL+TF-ThA-2 Controlling Metallic Contacts to Self-Assembled Monolayers and Molecular Electronic Devices A.V. Walker, B.C. Haynie, T. Tighe, D.L. Allara, N. Winograd (Pennsylvania State University) An understanding of the nature of the metal â€“ atom organic monolayer interaction is vital in the development and design of molecular electronic devices. To fully characterize these interactions, we employ a multi technique approach using time-of-flight secondary ion mass spectrometry (ToF SIMS), infrared spectroscopy (IRS), x-ray photoelectron spectroscopy (XPS) and atomic force microscopy (AFM). Using a methoxy terminated alkanethiol monolayer on Au{111}, we demonstrate that the metal organic monolayer contact can be varied from complete destruction of the monolayer to contact formation at the terminal group to complete penetration through the layer. For metals of intermediate reactivity, e.g. Cu, we observe that the metal atoms interact with the terminal group and penetrate through the layer to the monolayer / Au{111} interface. We have also studied the interaction between promising molecular wire and electronic and device candidates and metal atoms. By using a variety of metals, the formation of the metal molecule contact can also be controlled. This leads us to suggest new metallic contact materials for molecular electronic devices.
2:40 PM		OF+EL+TF-ThA-3 Dependence of Electrical Properties of Metal/Organic/Metal Systems on Interface Electronic Structure, Morphology and Chemistry C. Shen, A. Kahn (Princeton University) Interactions between metals and organic molecules are varied, and depend sensitively on the nature of the metal, of the organic molecule, and on the mode of the formation of the interface. In general, an interface formed by deposition of a metal on an organic film is broader than the interface formed by reverse deposition sequence. Metal atoms diffuse and/or react in the organic layer, acting as electrically active dopant and altering the electronic properties and the structure of the molecular film. These interactions have a profound impact on the injection of charge carriers across these organic interfaces. We present here a comprehensive investigation of these effects performed on a series of metal/organic/metal structures. All experiments are performed in ultra-high vacuum to eliminate extrinsic effects. The organic materials are Alq₃ (tris-(8-hydroxy quinoline) aluminum), amorphous emissive material extensively used in molecular OLEDs; and F₁₆-CuPc (hexadecafluoro copper phthalocyanine), a good electron-transport material that forms molecular stacks. The metals are Al and Au. Combinations of structures including permutations of Al and Au as top and bottom contacts, and even ultra-thin metal layers inserted into the bulk of the organic film, are used to investigate the effects. Key results are: 1. The predominant role of chemistry-induced electronic gap states in leading to identical metal/organic barriers for systems like Al-on-Alq₃ and Alq₃-on-Al; 2. the "p-like" doping induced by Au atoms deposited on, and diffused deep into, organic films like Alq₃ and F₁₆-CuPc; and the structural disruption in the F₁₆-CuPc stacks resulting from Al-F₁₆-CuPc chemical reaction, leading to considerable decrease in electron mobility through the material. General conclusions concerning metal/organic contacts will be discussed. Work supported by the MRSEC program of the National Science Foundation (DMR-9809483) and the NJCOE.
3:20 PM		OF+EL+TF-ThA-5 Organic Modified Schottky Contacts: Barrier Height Engineering and Chemical Stability D.R.T. Zahn, T.U. Kampen, S. Park (TU Chemnitz, Germany); A. Bushell (University of Wales Aberystwyth); M. Rus (TU Chemnitz, Germany) The electronic properties of metal-GaAs(100)-contacts have been modified using 3,4,9,10-perylene-tetracarboxylic dianhydride (PTCDA) and dimethyl-3,4,9,10-perylen-tetracarboxylicdianhydride (DiMe-PTCDI). Silver is used as a top electrode. The influence of the organic films on the electronic transport properties was investigated using in situ current-voltage (IV) and capacitance-voltage (CV) measurements. The IV curves show a strong dependence on the modification of the Schottky contacts with PTCDA. The effective barrier height varies between 0.81 ~ 0.64 eV by changing the thicknesses of the PTCDA interlayer between 0 ~ 60 nm. For layer thickness above 60 nm space-charge limited currents in the organic layer determine the carrier transport in the diodes. The CV characteristics do not vary upon introducing an organic interlayer, indicating that the overall capacitance is dominated by the depletion layer within the GaAs substrates and that the width of the depletion region is hardly affected by the PTCDA modification. Therefore, the change in the effective barrier height can be explained by an increasing image-force lowering in the presence of the organic interlayer. The decrease in barrier height as a function of the organic layer thickness is not observed after an exposure of the samples to air. Here, barrier heights are independent of the organic layer thickness and have a value similar to the one of a bare Ag/GaAs(100) Schottky contact. This sensitivity to air is attributed to a reaction of the anhydride groups of the PTCDA with oxygen and/or water leading to carrier type conversion in the organic layer. Similar experiments are currently being performed using DiMe-PTCDI. First results also reveal a comparable decrease in barrier height. In addition, the DiMe-PTCDI modified Schottky contacts are found to be less sensitive to exposure to air because the imide groups are chemically more stable compared to the anhydride groups of the PTCDA.
3:40 PM		OF+EL+TF-ThA-6 Characterization of Organic/organic' Heterojunctions using UV and X-ray Photoemission Spectroscopies and Luminescence Quenching D. Alloway (University of Arizona); D. Schlettwein (University of Bremen and University of Oldenburg, Germany); N.R. Armstrong (University of Arizona) Heterojunctions based on ultrathin films of perylenedicarboxylicbisimide (PTCDI) dyes, and various phthalocyanine (Pc) overlayers, have been explored using combinations of UPS and XPS, to define the band-edge offset of these systems, and quenching of the luminescence response of the PTCDI layers as a function of coverage of the Pc layer. Depending upon initial PTCDI coverage, the quenching of its luminescence response is extremely efficient using less than 1-2 monolayer coverages of Pc, and can be seen to be due to a combination of energy transfer, and exciton dissociation events. UPS measurements suggest small interface dipoles are formed for some of the PTCDI/Pc heterojunctions, especially those which are based on perfluorinated Pcs, which have high electron affinities. These interface dipoles have the potential to accelerate exciton dissociation, and may be of importance in the application of these systems to organic photovoltaic applications.
4:00 PM		OF+EL+TF-ThA-7 The Influence of Substrate Interactions on the Metallicity of Polyaniline Films* B. Xu (University of Nebraska, Lincoln); J. Choi (Louisiana State University); P.A. Dowben (University of Nebraska-Lincoln) Both Na doped and undoped short chain vapor deposited polyaniline (PANI) thin films were studied using angular-resolved photoemission spectrascopy (ARPES) and X-ray photoemission spectrascopy (XPS). Films, grown on two different substrates, Si and Au, using vapor deposition method show distinctly different properties. Some indication of preferential molecular orientation was observed for ultrathin film deposited on Au substrate, but not on the Si substrates. Clear indication of metallic character was observed for the undoped polyaniline films consistent with recent theory suggesting that for alignment of chain parallel with a metal substrate enhances metallicity. The polyaniline films became more insulating with sodium doping, suggesting that Na doping leads to an increase in insulating behavior that may due to the completion of band filling. *Supported by the Office of Naval Research and the Nebraska Research Initiative.
4:20 PM		OF+EL+TF-ThA-8 The Electronic Structure Studies of Fluoride Layer Insertion between Al and Organic EL Material¹ Y. Park, J. Lee (Korea Research Institute of Standards and Science); D.Y. Kim (Hallym University, Korea); S.K. Lee (Chonnam University, Korea) We have investigated the electronic structures of interfaces between Al and tris-(8-hydroxyquinoline)aluminum (Alq₃), which is a prototypical organic electroluminescent (EL) material. It has been well known that the insertion of alkali metal fluoride, such as LiF, greatly enhances the EL performance and the origin of such behaviors have been controversial. We used photoelectron spectroscopy techniques including X-ray and UV photoelectron spectroscopy (XPS and UPS) to probe the electronic structures of Al/fluorides/Alq₃ interfaces. While the presence of LiF layer exhibited enhanced gap states and concomitant formation of shoulder peak in N 1s core level peaks, insertion of other fluorides showed distinctively different behaviors. For example, the presence of MgF₂ does not form N 1s shoulder peak although weak gap states were observed. Other fluorides including CsF and CaF₂ are also studied. In addition, the mixture of Al, LiF, and AlF₃ sequentially deposited on Alq₃ were investigated. The various core level peaks in this system revealed that this combination does not distinguish LiF and AlF₃, rather it is more like a homogeneous mixture of LiF, AlF₃ and Al, which is in contrast to the view that the deposition of Al on LiF/Alq₃ forms AlF₃ and liberated Li. We discuss the implications of these results in the electrical properties of the interface and eventually the organic ELD performance that employs this type of electrodes. ¹ This work was supported in part by MOST of Korea through NRL Program and ASSRC.
4:40 PM		OF+EL+TF-ThA-9 Direct Observation of Fermi Level Pinning at LUMO in Alkali Metal Doped Organic Films L. Yan, N.J. Watkins (University of Rochester); C.W. Tang (Eastman Kodak Company); Y. Gao (University of Rochester) The electronic structures of pristine and alkali metal doped organic films are investigated using the combination of photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) can be directly observed by IPES and UPS simultaneously. We found that the Fermi level position in the organic film can be modified by alkali metal doping. For example, in CuPc films, the observed LUMO of the CuPc film is shifted by the Cs doping to less than 0.2eV above the Fermi level. This pinning may be explained by the charge transfer process from Cs to CuPc. Our observation is the direct confirmation of widely used assumption that the LUMO can be inferred from HOMO position in organic films when a valance band shift is observed. The result indicates that energy alignment and charge injection properties of the organic materials can be modified by a simple doping process. Our findings could lead to various interface-engineering methods useful in organic optoelectronic devices. Possible interface dipoles, energy level shifts, charge transfer, metal diffusion and chemical reactions at the interface will be discussed.