Papers

AVS1997 Session EM+FP-WeM: Organic Materials for Microelectronics, Optoelectronics and Flat Panel Displays: Fundamentals

Wednesday, October 22, 1997 8:20 AM in Room C1/2

Wednesday Morning

Start	Invited?	Item
8:20 AM		EM+FP-WeM-1 Electronic and Chemical Structure of Conjugated Polymer Surfaces and Interfaces: Applications in Polymer-based Light Emitting Devices W.R. Salaneck, M. Lögdlund, Th. Kugler (Linköping University, Sweden) Since conjugated polymers are viable electronic materials for molecular based opto-electronic devices, it is of critical importance to understand the nature of the electronic structure of the polymer surface and the interface with other polymers, semiconductors and metals. Many details of the early stages of interface formation with metals deposited on clean surfaces of conjugated polymers and model molecular solids in ultra-high vacuum have been studied over the past several years¹^,². There is at least a reasonable degree of understanding of how metallic contacts should be made to polymer light emitting devices in order to help optimize device performance³. Relatively little detailed spectroscopic work on the chemical interactions at the polymer-ITO interface (the 'other electrode') has been carried out. The surfaces of various types of ITO have been studied employing X-ray and ultra-violet photoelectron spectroscopy, XPS and UPS⁴, in connection with Atomic Force Microscopy (AFM). In addition, the interfacial interactions are probed by studying ultra-thin overlayers (ideally, mono-layers) of one of conjugated polymers, or model molecules there of, employing the same techniques. First, a mini-review of the reported studies on the behavior of metal atoms on conjugated polymer surfaces will be presented, in order to illustrate the issues which occur and which can be studies using the above experimental approach. Some consequences for device performance, derived from the results of these studies, will be pointed out. In addition, turning to 'the other electrode', we will present some of our latest results on the chemical and electronic structure of the interface between ITO and adsorbed conjugated polymer materials. ¹W. R. Salaneck and J. L. Brédas, Adv. Mat., 8, (1996) p. 48. ²W. R. Salaneck, S. Stafström, and J. L. Brédas, 'Conjugated Polymer Surfaces and Interfaces' (Cambridge University Press, Cambridge, 1996). ³P. Bröms, J. Birgersson, N. Johnsson, M. Lögdlund, and W. R. Salaneck, Synth. Met., 74, (1995) p. 179. ⁴M. Lögdlund, P. Dannetun, and W. R. Salaneck, 'Electronic and chemical structure of polymer surfaces and interfaces as studied by photoelectron spectroscopy' in 'Handbook of Conducting Polymers-II', edited by T. Skotheim, J. R. Reynolds, and R. L. Elsenbaumer (Marcel Dekker, New York, 1997).
9:00 AM		EM+FP-WeM-3 Organic LED Devices Studied with Ion Scattering: Conductivity, Efficiencies and Degradation at Metal/Organic Interfaces C.H. Marée, Y. Wu, R.A. Weller (Vanderbilt University); K. Pakbaz, H.W.H. Lee (Lawrence Livermore National Laboratory); L.C. Feldman (Vanderbilt University) There is a considerable interest in using organic electroluminescent (EL) devices for display applications because of the low processing costs, the high efficiencies and the wide range of emission wavelengths achieved in organic semiconductor films. Currently, reliability and short lifetimes of the devices are limiting factors precluding successful application. These instabilities have been attributed to changes in the organic films, in the electrodes and to interfacial reactions between organics and the electrodes. We have investigated some of these issues using 0.2-2.0 MeV ion scattering for materials analysis, correlated to electrical and optical characterization of the devices. Typical systems consisted of heterojunction devices of evaporated N,N'-diphenyl-N,N'-bis(3-methylphenyl)-1,1'-biphenyl-4,4'-diamine (TPD) and tris-(8-hydroxyquinoline) aluminum (Alq) sandwiched between In₂O₃(ITO) and Al electrodes. A careful study of ion beam induced effects revealed some organic film degradation, but not so severe as to inhibit meaningful measurements. The accurate ion scattering coverage (in atoms/cm²) is correlated with profilometric and optical spectroscopic measurements, yielding accurate densities and absorption coefficients of the organic films. Using the sub-nanometer depth resolution and sub-millimeter spatial resolution the thickness dependence of IV curves, efficiencies and degradation has been studied and compared with some current models for charge transport, like injection limited or space charge limited currents. Furthermore, RBS has been successfully applied for the study of reactions and diffusion at the interfaces of both the ITO as well as the metal electrodes.
9:20 AM		EM+FP-WeM-4 Degradation Processes in Polymeric Electroluminescent Devices J. Sheats, D. Roitman, Y.-L. Chang (Hewlett Packard Laboratories) We have studied a variety of electroluminescent polymer devices using the primary technique of time-of-flight secondary ion mass spectrometry (TOF-SIMS). Three general types of information can be obtained from these experiments: 1) pinhole densities; 2) extents of metal ion migration; and 3) identification of very small amounts (ppm) of organic contaminants and degradation products. We find, as have other workers, that In migration through the polymer film toward the cathode is an important process. The extent to which pinholes can be detected in films spun under ordinary "clean" conditions will be examined. Electrical stress-induced chemical changes detectable only by TOF-SIMS will be described.
9:40 AM		EM+FP-WeM-5 Photoluminesence Quenching of Alq3 by Metal Deposition: A Surface Analytical Investigation* V.-E. Choong, Y. Park, Y. Gao (University of Rochester); M.G. Mason, C.W. Tang (Eastman Kodak Company) The importance of the interfacial properties in organic light emitting devices (OLED) is well recognized. We will describe our recent efforts to understand interfaces in OLEDs using surface/interface analytical techniques in a well controlled ultra high vacuum (UHV) environment. We observed severe photoluminescence (PL) quenching of organic thin films comprising of a model OLED material, namely tris-(8-hydroxyquinoline) aluminum (Alq3), upon sub-monolayer deposition of a number of metals. Such quenching may severely affect the EL device efficiency. We have investigated the interfaces using x-ray photoemission spectroscopy (XPS) and ultraviolet photoemission spectroscopy (UPS), and studied the intriguing process of the interface formaiton at an atomic/molecular level. We will show that microscopic surface and interface properties are intimately related to the device characteristics and performance. * Supported in part by DARPA DAAL 0196K0086.
10:00 AM		EM+FP-WeM-6 Energetics of Multilayer Organic Light Emitting Diodes by UV-Photoelectron Spectroscopy and Electrochemical Measurements N.R. Armstrong, J.D. Anderson, P.A. Lee (University of Arizona) The frontier energy levels of the components of a number of new and existing OLED multilayer technologies have been estimated by a combination of UV-photoelectron spectroscopy (for thin films deposited on atomically clean metals) and by electrochemical measurements of first oxidation and first reduction potentials in solution. The operation of these devices may be explainable in simple molecular terms, based upon established principles of electrochemically generated chemiluminescence (ECL) during annihilation reactions of radical cation and radical anion states. For a two-layer OLED such as ITO/HTA/Alq₃/Mg,Ag we find that the hole, trapped at the HTA/Alq₃ interface, at low applied fields, may be sufficiently energetic so as to react directly with the radical anion state of Alq₃, generating the emissive state of Alq₃, without the need for injection of holes into the lumophore layer. For Alq₃ OLEDs "sensitized" with quinacridones (QAD) UPS and electrochemical measurements suggest that this material acts as a charge trap, with excellent stability for both radical cation and anion states, from which emission eventually occurs. The output emission of several new exciplex-based OLEDs is also predicted well through the combined use of electrochemistry and UPS. The emission from the exciplex is predicted within 50 mV by the difference between the first reduction and first oxidation potential of the acceptor and donor molecules respectively which comprise these two-layer devices.
10:20 AM		EM+FP-WeM-7 Energy Offsets of Molecular Levels at Organic Heterojunctions A. Rajagopal, A. Kahn (Princeton University) The development of microelectronics and optoelectronics based on small molecule organic semiconductors requires a detailed understanding and control of the interface properties of these materials. In particular, the electrical behavior and chemical stability of metal contacts and organic-organic heterojunctions are of great importance for the efficiency and life-time of increasingly complex, multi-layer devices. We present here a direct determination of energy offsets between molecular levels at interfaces between three small-molecule organics, i.e. the hole-transporter 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) and N,N'-diphenyl-N,N'-bis-(1-nappthyl)-1,1'biphenyl-4,4'diamine (α-NPD), and the electron-transporter and light emitter tris(8-hydroxy-quinoline)aluminum (Alq₃). The offsets are measured with ultra-violet photoemission spectroscopy on layers incrementally deposited by thermal evaporation in ultra-high vacuum. We investigate the PTCDA-Alq₃ and Alq₃-α-NPD pairs. In each case, we measure two heterojunctions obtained by inverting the deposition sequence (A/B and B/A) and find no difference in the interface offset within experimental resolution. For PTCDA-Alq₃, we find the highest occupied molecular orbital (HOMO) of Alq₃ 0.35 eV above that of PTCDA. For α-NPD-Alq₃, the Alq₃ HOMO is approximately 0.1 eV below that of α-NPD. Using the optical gap of each materials as a lower bound for the transport gap (2.2 eV, 2.7 eV and 3.1 eV for PTCDA, Alq₃ and α-NPD, respectively), these measurements indicate that the Alq₃ lowest occupied molecular orbital (LUMO) is above that of PTCDA by about 0.8 eV whereas it is lower than that of α-NPD. Thus electrons (holes) injected on the Alq₃ (PTCDA) side of the heterojunction can cross into the PTCDA (Alq₃ ). On the other hand, electrons should be contained in Alq₃ at the α-NPD interface, in good agreement with the much higher efficiency of Alq₃ electroluminescence in devices using the Alq₃ -α-NPD pair.
10:40 AM		EM+FP-WeM-8 Saturated Orange and Red Organic Light Emitting Diodes M.E. Thompson (University of Southern California); S.R. Forrest (Princeton University); Y. You, A. Shoustikov (University of Southern California); P.E. Burrows (Princeton University) A range of vacuum-deposited, single-heterojunction organic LEDs have been prepared and studied. The electron transporting layer consists of aluminum tris(8-hydroxyquinoline) (Alq₃) or aluminum tris(5-hydroxyquinoxaline) (Alx₃) (host) doped with a red fluorescent dye or C₆₀ (guest) at 0.1 - 3 % guest by weight. A device prepared with Alq₃ undoped gives green emission, centered at 510 nm, while an similar device prepared with Alx₃ gives emission red shifted by roughly 80 nm to 600 nm. Current-voltage characteristics, UV-visible, photoluminescence (PL) and electroluminescence (EL) spectra have been measured and will be discussed. Devices prepared with dye dopes into Alq₃ or Alx₃ emit orange or red light upon applying moderately low voltage (< 10 V). In the many of the devices the emission comes solely from the dopant in both EL and PL spectra, even though the host material itself is quite emissive. Correlations observed in the energy transfer efficiency between the host and guest, and the degree of overlap between the emission spectrum of the host and the absorption spectrum of the guest, suggest Forster energy transfer is the dominant pathway for exciton transfer to the guest molecules. If the energy match between the host material and the guest is poor, some luminescence from the host is observed in the PL spectra. The EL spectra for devices made with these mixtures, however, show very little host luminescence, suggesting that in addition to Forster energy transfer alternate pathways for host -> guest energy transfer must be operative, such as carrier trapping at the guest molecules. We will discuss the optimal mixture of materials to obtain saturated red emission.
11:00 AM		EM+FP-WeM-9 Electronic Structure Changes in α-sexithiophene upon Exposure to Oxidizing and Reducing Gases C. Kendrick, S. Semancik (National Institute of Standards & Technology) The organic molecular semiconductor α-sexithiophene (α6T) is attracting considerable attention for application to organic transistors and optoelectronic devices due to its high carrier mobility (0.01-1 cm²/Vs), which rivals that of amorphous silicon. α6T has been studied as a model compound for thiophene-based polymers, which may be functionalized to improve solubility and therefore processibility. In this work we present the changes in electronic properties that result when α6T films interact with the gases O₂, NO₂, CO, CH₄, H₂S, NH₃, and water vapor in order to explore the material's potential for use in chemical sensors. Vacuum sublimed thin films of α6T are deposited on gold-coated silicon substrates and initially characterized by x-ray and ultra-violet photoemission spectroscopy (XPS/UPS), inverse photoemission spectroscopy, and atomic force microscopy. These films are then exposed to the various gases while being heated to elevated temperatures. We discuss the changes in chemical and electronic structure observed in XPS and UPS, respectively, brought about by the interaction of gases with the film, as well as contact potential difference measurements which yield changes in the ionization potential. Finally, we report on the reversibility of α6T films to gas absorption/adsorption by acquiring temperature programmed desorption spectra through the temperature at which α6T sublimes. Subsequent photoemission spectra show a thin residual film which cannot be desorbed, presumably containing the constituents of α6T which have partially reacted with the gold layer.
11:20 AM		EM+FP-WeM-10 Controlled Formation of Organic Monolayers on Si(001) H.L. Liu, J.S. Hovis, R.J. Hamers (University of Wisconsin, Madison) We have previously reported that the interaction between Si=Si dimers on the Si(001)-2x1 surface and the C=C double bond of unsaturated organic molecules, such as cyclopentene, can lead to the formation of well ordered organic monolayers on Si(001) surfaces. To explore the potential for growing multilayered organic structures on silicon surfaces, we have investigated a two-step process in which the heterocyclic molecule 3-pyrroline is first bonded to the surface and is subsequently reacted with the molecule acetyl chloride. The deposition of 3-pyrroline molecules on the Si(001) surface at room temperature results in the formation of a monolayer with ordered and disordered regions on the surface. Our X-ray photoelectron spectroscopy (XPS) study shows that the 3-pyrroline molecule can bond with the Si(001) surface in two configurations. One configuration involves bonding through the C-Si bond, and the other through the N-Si bond. Nevertheless, we have found that the formation of a 3-pyrroline monolayer on the surface can be controlled by changing the dosing conditions, and we have succeeded in forming a 3-pyrroline monolayer on the Si(001) surface on which molecules bond essentially only through the C-Si bond, leaving the secondary amine group open for the subsequent growth of an additional organic layer. The experiments were conducted in a UHV system utilizing X-ray photoelectron spectroscopy (XPS), scanning tunneling microscopy (STM) and Fourier transform infrared (FTIR) spectroscopy.
11:40 AM		EM+FP-WeM-11 Ultrathin Films of Perylenedianhydride Dyes on (001) Alkali Halide Surfaces: Characterization by Luminescence, FT-IR, Fluoresence Anisotropy and AFM A. Back, N.R. Armstrong, B. Schilling, P.A. Lee (University of Arizona); D. Schlettwein (University of Bremen, Germany) Ordered thin films of perylenetetracarboxylicdianhydride (PTCDA) and its related bisimides (C4-PTCDI and C5-PTCDI) have been created on several different (001) faces of single crystal alkali halides, using protocols which provide for their characterization through electron diffraction, scanning probe microscopies, in situ luminescence at submonolayer coverages, ex situ FT-IR and total internal reflection fluorescence anisotropy. Extrapolation of these protocols to the characterization of other electroluminescent dye thin films is also discussed. We find that submonolayer coverages of these perylene dyes leads to luminescence emission strongly reminiscent of monomer emission from dilute solutions, corresponding to mobile and weakly interacting discrete molecular species on the substrate. Decay of this emission and the corresponding rise of a longer wavelength excimer emission can be used to estimate surface diffusion rates of the initially deposited material. Widely different morphologies of these dyes are observed by AFM analysis, with layer-by-layer growth confirmed on certain substrates and growth temperatures, and island-plus-layer growth on others. A combination of FT-IR and TIRF is essential to determine the average orientation of these lumophores, and departures from stable bulk structures are indicated for the bisimides at low coverages. Interaction with atmospheric oxygen, which would normally be predicted to quench fluorescence in these materials, actually enhances the luminescence, consistent with the notion that the high dark conductivities seen previously for these thin films deposited in vacuo is due to impurity doping. Removal of these dopants through interaction with oxygen lowers the dark conductivity and removes centers responsible for exciton dissociation in the as-deposited material.