Papers

AVS1997 Session EM+FP-WeA: Organic Materials for Microelectronics, Optoelectronics and Flat Panel Displays - State-of-the-Art Devices

Wednesday, October 22, 1997 2:00 PM in Room C1/2

Wednesday Afternoon

Start	Invited?	Item
2:00 PM		EM+FP-WeA-1 Light Emitters Based on Organic Thin Films Deposited in Vacuum S.R. Forrest, P.E. Burrows (Princeton University); M.E. Thompson (University of Southern California) During the past five years, enormous strides have been made towards realizing an entirely new generation of displays based on vacuum deposited, small molecular weight organic thin film light emitting devices (OLEDs). Indeed, numerous companies worldwide are moving rapidly towards introducing the first display products based on vacuum deposited OLEDs due to their demonstrated performance characteristics including: high brightness (>10^5 cd/m^2), high quantum and power efficiencies (1%-3% and 5-10 lm/W, respectively), and long operational lifetimes (>10^4 hr.). In our laboratory, we have taken advantage of the unique properties of vacuum deposited thin film OLEDs to demonstrate several new device concepts. Chief among these devices is the transparent OLED, or TOLED, which is 70%-80% transparent when switched off; and a compound, full color pixel in which the red, green and blue primary color sub pixels are placed in a vertical stack, or SOLED. Each of the separately contacted sub-pixels in the stack afford independent control of both color and brightness. In addition, we have demonstrated a flexible, ultra-lightweight OLED, or FOLED, also based on vacuum deposited thin organic films (in contrast to polymers). All of these devices provide an entirely new family of light emitting devices which are useful in flat panel display applications. In other developments, we have recent demonstrated the first solid state, organic thin film laser based on vacuum-deposited materials. These remarkable devices have ~50W peak output power, emitting in the 600nm to 650nm wavelength range when optically pumped in the UV. Due to the very low threshold energies of these efficient devices (~1J/cm2), the prospects for realizing electrically pumped organic lasers deposited on lightweight, flexible plastic substrates appear excellent. These potentially low cost lasers may eventually find use in such applications as laser printing, memories, scanners, etc. In this talk, we will describe recent developments made in our laboratory and worldwide in the emerging field of organic light emitters. This work was supported by AFOSR, DARPA, NSF(MRSEC), and Universal Display Corp.
2:40 PM		EM+FP-WeA-3 Enhancing the Brightness, Efficiency and Color Purity in Organic Electroluminescent Devices with Al Cathode. G.E. Jabbour, S.E. Shaheen, Y. Kawabe, M.M. Morrell, J.F. Wang, M.-F. Nabor, B. Kippelen, N. Peyghambarian (University of Arizona) We report the fabrication of ultra-bright and highly efficient organic light emitting devices (OLEDs) based on an electron transporting and light emitting layer of quinacridone (QAD) doped Alq₃, and a hole transport N-N'-diphenyl-N-N'bis(3-methylphenyl)-[1-1'-biphenyl]-4-4'-diamine (TPD) layer. Maximum luminance exceeding 20,000 cd/m², and about 3 % external quantum efficiency can be seen for devices with the structure Al/LiF/Alq₃:QAD/TPD/ITO. At 15 V, the luminance level of the Al/LiF devices is 70 times higher, and the external quantum efficiency is 100% greater than those of devices without the LiF layer. The presence of the LiF layer also results in a shift of the operating voltage to lower values than those of devices without LiF. We also fabricated blue light OLEDs using TPD as a hole transporting layer and distyrylarylene (DPVBi) as light emitting layer. Devices made with the structure Al/LiF/DPVBi/TPD/ITO showed lower operating voltage, are much brighter and almost six times as efficient as Al/Alq₃/DPVBi/TPD/ITO. Also, OLEDs with the insulating layer showed a unique color purity and are more stable under continuous bias operation than those with the structure Al/ Alq₃ /DPVBi/TPD/ITO . We will discuss the possible mechanismfoot1-5 behind the performance enhancement in our devices. Initial results on the extension of this technique to fabricate pure red color OLEDs will also be presented. We will elaborate on the effects of the position of the insulating layer (or layers), with respect to the organic layers, on device performance. In conclusion, an optimum thickness of LiF inserted as a buffer layer between the Al cathode and the organic layer greatly enhances the brightness and the efficiency of the OLED. It also eliminates the need for the Alq₃ layer in the blue devices thus allowing for purer colors and more stable devices to be fabricated. This work has been funded by ONR through the MURI Center CAMP, NSF, and AASERT student support through BMDO/AFOSR. ¹A. G. Fischer and H. J. Moss, J. Appl. Phys., 34, 2112 (1963). ²Y-E. Kim, H. Park, and J-J. Kim, Appl. Phys. Lett. 69, 599 (1996). ³F. Li, H. Tang, J. Anderegg, and J. Shinar, Appl. Phys. Lett. 70, 1233 (1997). ⁴L. S. Hung, C. W. Tang, and M. G. Mason, Appl. Phys. Lett. 70, 152 (1997). ⁵G. E. Jabbour, Y. Kawabe, S. E. Shaheen, J. F. Wang, M. M. Morrell, B. Kippelen and N. Peyghambarian, CLEO, postdeadline paper # CPD 19 (1997).
3:00 PM		EM+FP-WeA-4 New Developments in the Photonic Applications of Conjugated Polymers M.A. Díaz-García, F. Hide, A.J. Heeger (University of California, Santa Barbara) High performance photonic devices fabricated from conjugated polymers have been demonstrated, including light emitting diodes, photovoltaic cells, photodiodes, optocouplers, and thin film transistors. In some cases, performance parameters have been improved to levels comparable to or better than their inorganic counterparts. Notably absent from this list of semiconducting polymer devices is the polymer laser diode. As the first important step in exploring the feasibility of such laser diodes, optically pumped stimulated emission, gain, and lasing have recently been observed in over a dozen different semiconducting polymers representing a variety of molecular structures with emission wavelengths spanning the visible spectrum. We give a brief review with emphasis on these laser-related phenomena and suggest two routes for constructing 'plastic' laser diodes. We discuss resonant structures for providing the feedback requiered for lasing or gain narrowing in submicron thick films, neat and undiluted, of photoluminescent thin solid films of conjugated polymers. These include planar waveguides, microcavities, distributed feedback (DFB) structures, and high-Q microresonators. Lasing and gain narrowing have been compared using two of these structures: waveguides and microcavities. In both cases, the gain narrowing or lasing threshold is at 0.05 -0.1 µJ per 10 ns pulse focused to ~1.5 mm spot size diameter. Single-mode microcavity lasers are obtained when a microcavity resonance occurs at the wavelength where the gain of the polymer is maximum.
3:20 PM		EM+FP-WeA-5 Recent Progress of Organic Electroluminescent Materials and Devices Y. Taga, S Tokito, H Fujikawa (TOYOTA Central Research & Development Labs, Inc., Japan) In this review paper, we first demonstrate the high temperature operation of organic electroluminescent (OEL) devices using a novel hole transporting material of a tetramer of triphenylamine (TPTE) with a high glass transition temperature (Tg) in combination with a emitting material of tris (8-quinolinolato) aluminum (Alq). The OEL thus formed showed a maximum temperature of continuous operation up to 140C° without breakdown and also durability at 100C° for more than 100hrs. The results can be explained in terms of thermal stability of TPTE. In addition, the energy structure of TPTE together with a dimer of triphenylamine (TPD) was evaluated to have high hole injection structure by measuring the ionization potential and work function. On the other hand, several experiments revealed the spectral narrowing and intensity enhancement using a Fabry-Perot microcavity. We demonstrate how sharply a single-mode light emission pattern can be obtained by a sophisticated microcavity designing technique. The microcavity structure was designed to have a resonance wavelength of 490nm which is located on the shorter wavelength side of the non-cavity OEL structure. As a result, the microcavity OEL device shows only one resonance mode, i.e., a single mode emission, located at 500nm with the full width at half maximum of 12nm. Furthermore, the emission intensity in the normal direction is higher than those of nonmicrocavity devices. In conclusion, future challenges of research and development of OEL materials and devices and the forthcoming fields in industrial applications are presented.
3:40 PM		EM+FP-WeA-6 Conjugated Oligothiophenes, a New Class of Organic Semiconductors and Laser Crystals D. Fichou (CNRS: Centre National de la Recherche Scientifique, France) Since our pioneering works in the late 80's, conjugated oligothiophenes became a new class of organic semiconductors with potential applications in devices such as thin film transistors (TFTs). These low molecular solids can be vacuum-deposited as oriented thin films with low concentrations of structural defects thus leading to excellent transport properties. In this talk, we first describe the use of oligothiophenes as TFTs materials and investigate the influence of chain length, degree of purity and molecular ordering on carrier mobility. In particular, we present recent results on high purity octithiophene (8T), the longest non-substituted oligothiophene so far, and a dialkylated 8T derivative. Field-effect mobilities in the range µ=2-3x10^-2 cm²/V.s are obtained, showing that m tends to saturate for chains longer than 6T. Beside, we demonstrate optically pumped laser emission in ultra thin (3.5 µm) single crystals of 8T. Low threshold gain narrowing and dual energy dependence of two emission lines constitute clear evidences of stimulated emission. Lasing results from the net alignment of molecular dipoles in a two-dimensional crystal (crystallographic ratio a/c=10!) acting as a natural optical resonator. Since 8T is an organic semiconductor, it bridges the gap between conventional laser dyes and electroluminescent polymers in the search of a polymer laser diode. To our knowledge this is the first evidence of laser action in a monolithic (undoped) organic single crystal thus opening a new way toward organic solid state microlasers.
4:20 PM		EM+FP-WeA-8 High-Mobiilty and Liquid-Phase-Processable Organic TFT Semiconductors H.E. Katz, J. Laquindanum, A. Dodabalapur, Z. Bao, A.J. Lovinger (Bell Laboratories, Lucent Technologies) Hole mobilities ranging from 0.02-1 cm²/Vs are obtainable from H- or alkyl-terminated thiophene oligomers and fused carbocyclic and carbosulfocyclic aromatic ring compounds in thin film transistors (TFTs). Comparable mobilities from n-channel operation and liquid phase deposition processes for both n- and p-channel compounds are required for all-organic electronics to be technologically and economically worthwhile. We have investigated new compounds with varied conjugated frameworks, such as a newly synthesized benzodithiophene dimer that packs in a strictly planar ribbon geometry perpendicular to the device surface. The dimer exhibited excellent thermal stability (mp > 400 C) and mobility as high as 0.04 cm²/Vs. We have also studied pentacene as a semiconductor, and obtained evidence that its high mobility form (mobility = 0.1-1 cm²/Vs) possesses a particularly effective single-crystal morphology. In addition to the above p-type compounds, we have investigated complementary n-type materials. Surprisingly, the relatively small naphthalene framework appears advantageous, with naphthalene-based derivatives showing electron mobilities >10^-3cm²/Vs. Synthetic routes to liquid-processable materials and characteristics of devices and higher-order circuits will both be described.
4:40 PM		EM+FP-WeA-9 Analysis of a Nanoscopic Field Effect Transistor based on Metallization of a Molecular Layer C. Zhou, D.M. Newns, J.R. Misewich, W.E. Donath, P.C. Pattnaik (IBM T.J. Watson Research Center) A manufacturable switch manifesting adequate engineering properties will be a key component in computer circuits operating at nanoscopic scales. One solution to the switch problem is a device with FET-like geometry fabricated from a channel material which in its 'ON' state has a significantly smaller surface area per carrier, and a shorter screening length, than the channel dimensions. These materials specifications require a non-semiconductor solution. We present a detailed engineering analysis of a device satisfying these criteria based on the metallization of an organic layer through the Mott-Hubbard metal-insulator transition ¹. We consider channel materials from which the device can be fabricated, and give a fully quantum-mechanical analysis of the static and timing aspects of the device. ¹'A Field Effect Transistor based on the Mott Transition in a Molecular Layer', Applied Physics Letters, 70, 598 1997.
5:00 PM		EM+FP-WeA-10 Photoemission Spectroscopy Analysis and Organic Light Emitting Material* Y. Gao, V.-E. Choong, Y. Park (University of Rochester); B.R. Hsieh (Xerox Wilson Center for Research & Technology) We found that a short period of exposure of a light- emitting conjugated oligomer, namely 1,4-bis[4- (3,5-di-tert-butylstyryl) styryl]benzene (4PV), to the radiation sources used in UV or x-ray photoelectron spectroscopies (UPS or XPS) resulted in total photoluminescence (PL) quenching. The quenching occurred while there was no detectable change in the XPS C 1s core level, but UPS valence spectra indicated the disappearance of the vinylene peak. The optical absorption and PL spectra suggested a cleavage of 4PV to smaller segments such as 2PV by the UV radiation exposure typical for surface analysis. This suggests the destruction of the vinylene groups, and the possible creation of radical species, which may serve as PL quenching sites. We also found that the modification in electronic structure by Ca deposition dominated that by radiation, although both quenches the PL effectively. * Supported in part by NSF DMR-9612370.