Ananth Dodabalapur, Bell Labs, Lucent Technologies
Olle Inganas, Linkoping Univ
Junji Kido, Yamagata Univ
Yang Yang, UNIAX Corp
- Basic High-Technology Laboratories
- Chemipro Kasie Kaisha, Ltd.
- dpiX, A Xerox Company
- Futaba Corporation
- Hewlett-Packard Laboratories
- IMES Co., Ltd.
- National Science Foundation
- Sanyo Vacuum Industries Co., Ltd.
- Sumitomo Electric Industries Ltd.
- Texas Instruments, Inc.
- UNIAX Corporation
- Universal Display Corporation
In sessions below "*" indicates an invited paper.
SESSION H1: LIGHT EMITTING DEVICES
Chair: Yang Yang
Monday Morning, March 31, 1997
Golden Gate B1
8:30 AM *H1.1
CONJUGATED POLYMERS AS THIN FILM SOLID-STATE LASER MATERIALS: PHOTOPUMPED LASING, GAIN-NARROWING AND WAVEGUIDING, B. J. Schwartz, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA; Maria A. Diaz-Garcia, Univ of California-S Barbara, Inst of Polymers & Organic Solids, Santa Barbara, CA; Fumitomo Hide, M. R Anderson, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA; Qibing Pei, UNIAX Corp, Santa Barbara, CA; Alan J. Heeger, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA.
We demonstrate optically pumped lasing in submicron thick films, neat and undiluted, of photoluminescent conjugated polymers. Lasing, evidenced by a dramatic collapse of the emission line width at very low pump energy thresholds (<10 J/cm), is found in over a dozen different conjugated polymers representing a variety of molecular structures, including PPV, PPP and polyfluorene derivatives. Lasing wavelengths in these materials span the visible spectrum. The short gain lengths in semiconducting polymers are attributed to the high density of chromophores, the large density of states associated with the interband () transitions in quasi-ld systems, and the Stokes shift which minimizes self-absorption and enhances stimulated emission in the absence of excited state absorption. Simple planar waveguiding structures allow the distance traveled by emitted photons to exceed the short intrinsic gain lengths. The dependence of the threshold and the gain narrowed line width on the solvent from which the film is spin cast suggests that film morphology ad chain packing can be used to control lasing in some of these materials. Prospects for producing electrically pumped solid state polymer diode lasers will be discussed.
9:00 AM *H1.2
ORGANIC ELECTROLUMINESCENT DIODES HAVING SUBMICROMETER-SIZED EMITTING AREA USING POLYMER BLENDS, Chihaya Adachi, Seiji Hibino, Toshiki Koyama, Yoshio Taniguchi, Shinshu Univ, Dept of Textile Sci & Tech, Nagano, JAPAN.
We have realized the fabrication of submicrometer-sized emitting areas using the method of polymer blends. Phase separations of host polymers and a small amount of guest hole-conductive polymers formed spherical submicrometer-sized conductive areas. As the host inactive polymers, PMMA (poly(methyl methacrylate)), PC (polycarbonate) and PS (polystyrene) were used. As the guest polymer, condensation polymers (PTPB) derived from N,N'-diphenyl-N,N'-bis(3-methoxyphenyl)-[1,1' biphenyl]-4,4'-diamine were used. Atomic force microscopy (AFM) studies confirmed the formation of spherical PTPB areas. Different weight stoichiometries of PTPB showed various diameters between 100-500nm. Using these mixed films, electroluminescent (EL) diodes, ITO/ (PTPB:PMMA)/Alq/AlLi(MgAg), were fabricated. The polymer mixture was spin coated to form a 70-100nm thick film and the Alq layer was deposited to form a 50nm thick film. These devices showed an array of intense submicrometer-sized EL emission. We expect that realization of the organic EL diodes with submicrometer-sized emitting area will contribute to a new light emitting source. Moreover, unique EL emission properties may be anticipated from controlling sphere sizes.
9:30 AM *H1.3
LIGHT EMISSIVE CONJUGATED POLYMERS: MATERIALS, PHYSICS AND DEVICE ENGINEERING, Donal D.C. Bradley, Univ of Sheffield, Dept of Physics, Sheffield, UNITED KINGDOM.
In this paper, recent progress in the program of research concerning luminescent conjugated polymers currently underway in Sheffield is reported. The talk will focus on new and improved materials, on fundamental aspects of the physics of emission under both optical pumping and within electroluminescent (EL) devices and on device optimization questions. We have been working with several polymers designed to have high photoluminescence (PL) efficiency in solid films and report here how their emissive properties are influenced by chemical structure, intermolecular interactions, and by processing procedures, including lithographic patterning. Studies focussed on EL emission are also described, with an investigation of the processes involved in exciton formation and decay. Comparison of EL device current-voltage-luminance characteristics with various theoretical models is made and the results of more sophisticated characterizations are also considered. Device optimization through he use of modified electrodes, multilayer polymer films and resonant optical microcavities is described and the performance of such optimized devices is reported. Finally, I discuss studies of laser action in both solution and thin film samples and examine the processes that compete with stimulated emission and how these may be controlled.
10:30 AM *H1.4
TEMPERATURE DEPENDENCE OF THE DEVICE CHARACTERISTICS OF POLYMER LIGHT-EMITTING DIODES, Paul W.M. Blom, M. J. M. Jong, Philips Research Laboratories, Eindhoven, NETHERLANDS.
The device characteristics of poly(dialkoxy p-phenylene vinylene) (PPV) based single carrier and two carrier devices are measured as a function of temperature. In ITO/PPV/Au hole-only devices, the hole current is found to be bulk limited. At low fields the hole current increases quadratically with voltage, as expected for space charge limited conduction effects in combination with a field-dependent mobility. The low-field hole mobility is found to be thermally activated with an activation energy of 0.5 eV. At higher fields, the current increases more than quadratically with the voltage due to a field dependence of the mobility. In Ca/PPV/Ca electron only devices, the current is strongly reduced by a large density of traps. By combining the results obtained for the hole-only and electron-only devices, the device characteristics of two-carrier polymer LEDs can be modelled. For an ITO/PPV/Ca LED it is found that the current sensitively depends on the recombination. Bimolecular recombination between free electrons and holes provides an adequate description for the experimentally observed device characteristics. The bimolecular recombination constant is strongly temperature dependent with the same activation energy as found for the charge carrier mobility. This indicates that the recombination mechanism in polymer LED is of the Langevin type. By comparing the total number of recombinations inside the polymer LED with the actual light-output, it is observed that the recombination process is highly nonradiative: only 5 of the total recombination contributes to the light output.
11:00 AM *H1.5
POLYMER LIGHT-EMITTING ELECTROCHEMICAL CELLS, Qibing Pei, Yang Yang, Alan J. Heeger, UNIAX Corp, Santa Barbara, CA.
Solid state polymer light-emitting electrochemical cells (LECs) offer a new approach to light-emitting devices fabricated from luminescent conjugated polymers. In LECs, the active polymer layer is a blend of a conjugated polymer and a solid electrolyte. The conjugated polymer is electrochemically p-doped near the anode side and n-doped near the cathode side; a light-emitting p-n junction is formed between the doped regions. The admixed electrolyte provides the necessary ionic conductivity and dopant ions. The advantages of (1) low operating voltages, (2) high quantum efficiency and luminous efficiency, and (3) long lifetime re projected for LECs, and have been demonstrated in a variety of orange, green, blue and white light emitting LECs. New materials suitable for the active polymer layer and new device architecture will be presented.
11:30 AM H1.6
ORGANIC LIGHT EMITTING MICROSTRUCTURES, Magnus Berggren, Ananth Dodabalapur, R. E. Slusher, Bell Labs, Lucent Technologies, Murray Hill, NJ.
If organic light emitting diodes (LEDs) are to be used as components in information/display technology, patterning them to control the active area will be important. In this presentation, we describe a new approach to pattern organic LEDs. This scheme allows us to fabricate organic LEDs with dimensions of order 1- 2 m. This patterning does not affect the device performance negatively. Electroluminescence and electrical data of such 2 m sized organic LEDs as well as slightly larger devices will be presented.
11:45 AM H1.7
THREE COLOR EMISSION ORGANIC LIGHT EMITTING DIODE WITH VAPOR DEPOSITED MULTI-LAYERED STRUCTURE, Yutaka Ohmori, Norio Tada, Yoshitaka Kurosaka, Satoshi Tatsuhara, Akihiko Fujii, Osaka Univ, Dept of Electronics, Osaka, JAPAN; Katsumi Yoshino, Osaka Univ, Dept of Electronic Engr, Osaka, JAPAN.
Three- or two-color tunable organic light emitting diodes have been realized utilizing vapor deposited multilayered structure. Two types of layer structure have been employed to realize three- or two-color emissions. A three-layered-structure device emits two different colors, and a five layered device emits three different colors. The former device contains 1, 2,3,4,5-pentaphenyl-1,3-cyclopentadiene (PPCP), 8-hydroxyquinoline aluminum (Alq) or N,N-bis(2,5-di-tert-butylphenyl) 3,4,9,10-perylenedicarboximide (BPPC) as blue, green or red light emitting layers, respectively, and N,N-diphenyl-N,N (3-methylphenyl)-1,1 -biphenyl-4,4-diamine (TPD) as carrier blocking layer. The emission colors are changed by applying opposite polarity of electric field in the former devices, and, in the latter devices, by applying different strength and polarity of electric field.
The mechanism of the emission color change is discussed by taking carrier injection mechanism and recombination process in the multilayer devices into consideration.
SESSION H2: OPTICAL DEVICES
Chair: Olle Inganas
Monday Afternoon, March 31, 1997
Golden Gate B1
1:30 PM H2.1
LASER ACTION IN A POLY(PARAPHENYLENE)-TYPE LADDER POLYMER, Christian Zenz, Wilhelm Graupner, Stefan Tasch, Guenther Leising, Technical Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA; J. Flieser, K. Iskra, Technical Univ Graz, Inst for Experimental Physics, Graz, AUSTRIA; T. Neger, Technical Univ, Institut fuer Experimentalphysik.
Time-resolved pump and probe experiments performed on neat films as well as liquid solutions of methylene-substituted poly(paraphenylene) type ladder polymer (mLPPP) show strong stimulated emission besides spontaneous emission. Due to its high photoluminescence yield and the lack of spectral overlap between photoluminescence and photo-induced absorption, mLPPP is a very attractive material for laser applications.
We report on the lasing properties of a transverse optically pumped cavity using mLPPP as the active material. A (1) strong spectral narrowing, (2) high degree of linear polarization, and (3) a good beam collimation of our blue green polymer laser will be demonstrated. A clear threshold for these three features is observed.
1:45 PM H2.2
DISTRIBUTED FEEDBACK LASERS MADE WITH SEMICONDUCTING CONJUGATED POLYMERS AS THE GAIN MATERIAL, Michael D. McGehee, Fumitomo Hide, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA; Maria A. Diaz-Garcia, Univ of California-S Barbara, Inst of Polymers & Organic Solids, Santa Barbara, CA; Benjamin J. Schwartz, Univ of California-S Barbara, Dept of Chemistry, Santa Barbara, CA; Daniel Moses, Alan J. Heeger, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA.
We have reported that thin films of a variety of conjugated polymers exhibit stimulated emission when optically pumped with 10 nsec long 20 µJ/cm pulses. It was shown that thin (submicron) film cast on glass substrates form simple planar waveguides which allow the distance traveled by emitted photons in the gain material to exceed the gain length. These initial experiments showed that conjugated polymers are potentially useful as materials for solid-state lasers. Here we report the fabrication of distributed feedback lasers made with the conjugated polymer poly(2-butyl-5-(2¹ -ethyl-hexyl)-1,4-phenylene vinylene) (BUEH-PPV) as the gain material. Initial results show that distributed feedback lowers the threshold for lasing by more than a factor of twenty to well below 1 µ J/cm for 10 nsec pulses.
2:00 PM H2.3
GAIN DYNAMICS IN CONJUGATED POLYMERS, Christine Spiegelberg, Axel Schulzgen, Nicolas de la Rosa-Fox, Univ of Arizona, Optical Sciences Cntr, Tucson, AZ; Pierre Marc Allemand, Donnelly Corp, Dept of Adv Tech, Tucson, AZ; Bernard Kippelen, Nasser Peyghambarian, Univ of Arizona, Optical Sciences Center, Tuscon, AZ.
Due to their potential application in optoelectronic devices, polymers have stimulated enormous research activities during the last decade. Large progress has been made in developing efficient electroluminescent devices and very recently conjugated polymers have been proposed as lasing materials.
In our contribution, we will present spectrally and time-resolved luminescence measurements for BEH-PPV, that demonstrate strong evidence for stimulated emission in this material. Gain measurements in a quasistationary regime using the second harmonic of a ns-Nd:YAG laser have revealed gain coefficients of about 100 cm over the whole spectral region of spontaneous emission. However, much higher gain can be induced in a transient regime, exciting the sample with 100 fs-laser pulses. The initial 200 nm broad luminescence spectrum evolves into a 9 nm narrow emission line at about 630 nm. This spectral change exhibits a clear threshold behavior with increasing excitation. Doubling the excitation intensity results in an increase of the integrated emission by more than two orders of magnitude. Taking into account that the thickness of the polymer film is only 100 nm, the gain coefficient has to be orders of magnitude higher than in the ns experiment. Typical for stimulated emission, the luminescence lifetime decreases at the threshold from 70 ps to less than 20 ps, which is the time resolution of our streak camera. Obviously, higher inversion can be induced by excitation pulses shorter than the depletion by nonradiative processes responsible for the fast emission decay in the low excitation regime.
2:15 PM H2.4
INTERFERENCE PHENOMENA DETERMINE THE COLOUR IN A ORGANIC LIGHT EMITTING DIODE , Thomas Granlund, Leif Pettersson, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN; Mats Andersson, Chalmers Univ of Technology, Dept of Organic Chem & Polymers, Goteborg, SWEDEN; Olle Inganas, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN.
Abstract A novel optical transition has been observed in the electroluminescence spectra from a double layer organic light emitting diode(OLED). These transitions are different from the exciplexes, which have been observed in the photoluminescence from the excited state of two dissimilar molecules. We therefore name this transition an electroplex, as it is an optical transition observed at the interface between two organic layers, but which is by definition, not seen in photoluminescence(PL). The existence of such an electroplex is concluded from a comparison between experimental results and optical modelling of the OLED structure. The two organic materials used are a polythiophene, poly(3-methyl-4-octylthiophene) (PMOT), and a oxadiazole molecule, 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,2,4-oxadiazole (PBD), where the oxadiazole molecule works as an hole blocking layer to preventing the holes from reaching the cathode(Ca/Al) before recombining with an electron and emit light. The two electrodes used are indium tin oxide (ITO) and Ca/Al. The model simulates the optical response from the layers and interfaces inside the OLED with the use of an interfacial light source, located at the interface in between the two organic materials, and taken from processed experimental EL data. Interference between light emitted directly from the interface and light reflected one time at the metal cathode can explain both the different emission spectra seen from the cathode and from the anode side of the OLED, as well as the dramatic spectral shift in the green-blue range that occurs as the thickness of the polymer and molecule layer is varied. Neither emission from the polymer nor from PBD can generate these emission spectra, so a third and novel optical transition must exist. Evidence from electrochemical spectroscopy of the PMOT and the PBD corroborate the energy of this optical transition. This novel phenomenon demonstrates how new emission characteristics may be obtained at polymer/organic junctions, well separated from quenching metal electrodes. It also demonstrates the degree of colour control possible by exploitation of optical interference inside the thin OLEDs.
3:00 PM *H2.5
NOVEL APPLICATION OF ORGANIC EL DIODE-LIGHT AMPLIFICATION DEVICE LAYERED WITH ORGANIC PHOTORESPONSIVE PIGMENT FILM AND ORGANIC EL DIODE, M. Yokoyama, M. Hiramoto, Osaka Univ, Osaka, JAPAN.
We have succeeded in designing a new light amplification device based on organic EL diode coupled with photoresponsive organic pigment film in a multilayered configuration. The 25-fold gain of light amplification has been performed owing to the large photocurrent multiplication behavior in the organic pigment film. This device has also exhibited an optical switching behavior that once the feedback of output light toward the photoresponsive pigment layer occurs, the device continues to emit keeping an input light pattern even after input light turned off. The present result may offer a new avenue for further applications of orgamic EL.
3:30 PM *H2.6
CHARGE SEPARATION, RECOMBINATION AND EXCITON DECAY IN CONJUGATED POLYMER PHOTODIODES AND LEDS, Neil C. Greenham, Cavendish Laboratory, Cambridge, UNITED KINGDOM.
In this talk I will discuss some of the issues of charge carrier separation and recombination which are important in conjugated polymer photodiodes and light-emitting diodes (LEDs). I will show how inorganic semiconductor nanocrystals can be used as efficient electron acceptors in photodiodes based on poly(2-methoxy-5-(2-ethyl)-hexyloxy- - phenylenevinylene) [MEH-PPV], giving dramatic improvements in photovoltaic efficiency. I will also describe recent work from Cambridge to control carrier recombination and exciton decay in conjugated polymer LEDs.
4:00 PM H2.7
MOLECULAR PHOTODIODE CONSISTING OF UNIDIRECTIONALLY ORIENTED A-S-D TRIAD AMPHIPHILIC MOLECULES, Masamichi Fujihira, Tokyo Inst of Technology, Dept of Biomolecular Engr, Kanagawa, JAPAN; Masaru Sakomura, Yokohama National Univ, Dept of Physical Chem, Kanagawa, JAPAN; Daisuke Aoki, Takayuki Ohno, Tokyo Inst of Technology, Dept of Biomolecular Engr, Kanagawa, JAPAN.
We constructed Langmuir-Blodgett (LB) monolayer assemblies which functions as molecular photodiodes. In this LB monolayer device, A-S-D triad amphiphilic molecules are oriented unidirectionally either towards air or towards substrate electrodes. The A-S-D triad molecule contains a viologen moiety as an electron acceptor (A), acylated perylene moiety as a sensitizer (S), and a diacylated ferrocene moiety as an electron donor (D). The triad amphiphilic molecules was deposited as a mixed monolayer with w-tricosenoic acid. Pure monolayers of -tricosenoic acid formed an alternated multilayer with the mixed monolayers. The alternated monolayer assembly was deposited on a vapor-deposited gold film on a quartz substrate plate covered with 5-layers of -tricosenoic acid. Molecular photodiode function was examined by using a He-Cd laser and a scanning surface potential microscope (SSPM). A gold coated atomic force microscope (AFM) tip of SSPM works as the other electrode of the molecular photodiode. The observed photovoltage due to photoinduced charge separation in triads in the monolayer assemblies changed sign depending upon the orientational direction of the triad. The photovoltage of a single monolayer of the triad can be easily detected by SSPM, although the photovoltage increased linearly with the number of the triad monolayers in the alternated LB films.
4:15 PM H2.8
PHOTODIODES PREPARED FROM POLYESTERS WITH OLIGOTHIOPHENES IN THE MAIN CHAIN, Yoshihito Kunungi, Augusto Canavesi, Larry L. Miller, Christopher Minor, Univ of Minnesota, Dept of Chemistry, Minneapolis, MN.
A set of polyesters with various alkoxy-oligothiophenes in the main chain have been prepared and the conductivity of oxidized films has been measured. These polymers (unoxidized) were spin coated (150 nm) onto aluminum (100 ) and coated with Au (150 ). Illumination with visible light gives photoconductivity showing that the device behaves as a Schottky diode. The action spectrum corresponds with the spectrum of the oligomer. Analysis of the current/voltage curves for a polymer with quaterthiophene shows that the polymer is p-type with an open circuit photovoltage of 0.74 V. The photoefficiency measured at low intensity is 12. Results with the other polyesters and studies of stability will be reported.
4:30 PM H2.9
HIGH-EFFICIENCY POLYMER PHOTODIODES AND THEIR APPLICATION TO IMAGE SENSORS, Gang Yu, UNIAX Corp, Santa Barbara, CA; Alan J. Heeger, Univ of California-S Barbara, Inst for Polymers & Organic Solids, Santa Barbara, CA.
High efficicncy photodiodes were developed with semiconducting conjugated polymers and polymer blends. The photosensitivity has been improved to the level of 0.1-0.3 A/W in visible spectral range [1-3]. The onset of the spectral response can be tuned by change the chemical structures of main polymer chains or its side chains. Combining with a bandpass optical filter, solar-blend UV detection was demonstrated. The possibilities of using polymer photodiodes in fabricating linear diode arrays and two-dimensional image sensors will be discussed.
4:45 PM H2.10
TRANSPORT PROPERTIES FROM TRANSIENT AND FREQUENCY BEHAVIOR OF POLYMERIC LIGHT-EMITTING DIODES, R. Osterbacka, K.-M. Kallman, Abo Akademi Univ, Dept of Physics, Turku, FINLAND; A. J. Pal, Indian Assn for the Cultivation of Science, Dept of Solid State Physics, Calcutta, INDIA; H. Stubb, Abo Akademi Univ, Dept of Physics, Turku, FINLAND.
The high fluency response and transient characteristics of polymeric light-emitting diodes (LEDs) have been studied. Langmuir-Blodgett (LB) films of poly(3-hexylthiophene) have been used as the active material. For alternating-current (ac) LED measurements, it was further sandwiched between LB films of emeraldine base polyaniline. We have shown that by reducing the thickness of the active emitting material using LB deposition technique, one can increase the high frequency limit of operation of ac LEDs. From the limiting frequency of operation, we have calculated the mobility of charge carriers in the emitting polymer layer at different applied fields. The concept of mobility has been approached also using dc transient measurements on single-layer LEDs with different thicknesses. The results obtained from both measurements are in good agreement. The field dependence of mobility has been analyzed with the Poole-Frenkel model.
SESSION H3: FIELD-EFFECT TRANSISTORS
Chair: James C. Sturm
Tuesday Morning, April 1, 1997
Golden Gate B1
8:30 AM *H3.1
SOLUTION PROCESSABILITY AND IMPROVED ORGANIC/METAL CONTACT IN OLIGOMER-BASED THIN FILM TRANSISTORS, Francis Garnier, CNRS, LMM, Thiais, FRANCE.
Conjugated oligomers, e.g., oligothiophenes and oligoacenes, have been shown to act as efficient semiconducting layers in organic-based film transisiors, the field-effect mobility of carriers being mainly controlled by structural organization of oligomor molecules in the film. Carrier mobilities determined on single crystals of conjugated oligomers suggest that mobilities of the order of 1 cmVs represent an achievable limit for highly organized films.
These results raise new research objectives for the development of organic-based thin film transistors: i) how to impose long range molecular order in thin films? In this regard, the respective advantages of using either physical means, associated to experimental conditions for film deposition, or chemical means, through molecular engineering of the oligomer, will be discussed. ii) how to increase the efficiency of the metal/organic contact at the source and drain electrodes, for allowing high current densities output? For this purpose, a local doping of the organic semiconductor will be proposed, leading to the formation of a p/p interface, which allows a significant improvement of the device characteristics. iii) are there chemical routes toward solution processable organic semiconductors? With this aim, a new promising direction will be described, which should allow full solution processability for the construction of organic-based thin film transistors.
9:00 AM *H3.2
NEW COMPLEMENTARY SEMICONDUCTORS FOR ORGAINC TFTs, Howard E. Katz, Joyce G. Laquindanum, Ananth Dodabalapur, Andrew J. Lovinger, Bell Labs, Lucent Technologies, Murray Hill, NJ.
It is well known that mobilities well above 0.02 cm/Vs are obtainable from H- or hexyl-terminated thiophene oligomers in thin film transistors (TFTs). Higher mobilities in both n- and p-channel operation and greater stability are required for applications in all-organic electronics, thus calling for a broader range of candidate compounds. 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 > 400C) and mobility as high as 0.04 cm/Vs. We have also studied pentacene as a semiconductor, and obtained evidence that its high mobility form ( cm/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 cm/Vs. Synthetic routes and device characteristics will both be described.
9:30 AM H3.3
PENTACENE THIN FILM TRANSISTORS WITH IMPROVED SUBTHRESHOLD SLOPE, Yen-Yi Lin, David J. Gundlach, Thomas N. Jackson, Pennsylvania State Univ, Dept of Electrical Engr, University Park, PA.
Using pentacene as the active material, we have fabricated organic thin film transistors (TFTs) with field effect mobility as large as 0.7 cm/V-s and on/off current ratio greater than 10; both values are comparable to those typically obtained for hydrogenated amorphous silicon (a-Si:H) devices. However, the subthreshold slope for these devices is approximately 5 volts of gate voltage per decade of drain current, similar to results obtained with other small-molecule organic materials such as -sexithienyl, but much worse than the 0.5-2 volts per decade typical for a-Si:H TFTs. A large subthreshold slope makes it difficult to take advantage of a large on/off current ratio since an undesirably large gate voltage swing is needed to turn the device from on to off. We find that the subthreshold voltage can be greatly reduced by depositing the pentacene active layer on a dielectric treated with a self assembly material. Using pentacene deposited on thermally oxidized silicon treated With octadecyltrichlorosilane we have fabricated devices with mobility greater than 0.3 cm/V-s and subthreshold slope as low as 1.64 volts per decade. This indicates that a large subthreshold voltage is not an intrinsic characteristic of these organic TFTs and increases the likelihood that organic devices will find wide application.
9:45 AM H3.4
HIGH MOBILITY THIN-FILM FIELD-EFFECT TRANSISTOR FROM SOLUBLE AND PROCESSABLE REGIOREGULAR POLY(3-ALKYLTHIOPHENE)S, Zhenan Bao, Ananth Dodabalapur, Andrew J. Lovinger, Yi Feng, V. Reddy Raju, Bell Labs, Lucent Technologies, Murray Hill, NJ.
Organic thin-film metal-insulator-semiconductor field-effect transistors (MISFETs) are potentially useful in low-cost large-area flexible displays and low-end data storage, such as smart cards. Soluble and processable organic polymeric semiconductors are essential in order to fabricate these devices at low cost. Up to now, all the soluble polymers which have been studied for field-effect transistor applications have shown very low performance, i.e., low field-effect mobilities and low on/off ratios. However, we will demonstrate in this paper that both high field-effect mobilities (ca. 0.045 cm/Vs in the accumulation-mode and 0.01 cm/Vs in the depletion-mode), and relatively high on/off current ratios (greater than 10) can be achieved for regioregular poly(3 alkylthiophene)s. We find that the film quality and field-effect mobility are strongly dependent on the choice of solvents. In addition, treating a film with ammonia or heating to 100C under N4, can increase the on/off ratio without decreasing the mobility. With these materials, it should be possible to realize organic transistors by printing technique.
10:30 AM *H3.5
FIELD-EFFECT TRANSISTORS CONSTRUCTED FROM PRECURSOR-ROUTE CONJUGATED POLYMERS, Dago M. de Leeuw, Adam R. Brown, Marco Matters, Knut Chmil, Cees M. Hart, Philips Research Laboratories, Eindhoven, NETHERLANDS.
Conjugated polymer field-effect transistors show promise for niche electronic applications. In order to realise a cheap solution-processible technology we have focussed on precursor-route conjugated polymers. Progress in the development and use of these materials in devices and simple logic circuits will be highlighted.
11:00 AM H3.6
HIGH PERFORMANCE ORGANIC FIELD-EFFECT TRANSISTOR FABRICATED BY USING SCREEN PRINTING TECHNIQUE, Yi Feng, Zhenan Bao, Ananth Dodabalapur, V. Reddy Raju, Bell Labs, Lucent Technologies, Murray Hill, NJ.
Transistors are indispensable parts of electronics. For electronic products where electronic devices and interconnections are made on a flexed plastic substrate, it is highly desirable, due to cost and environmental consideration, that all electronic materials be selectively added on the substrate only where needed. Screen printing is considered to be the standard additive process to pattern electronic materials. This talk will demonstrate that high performance transistors can be fabricated by screen printing all necessary electronic materials on a plastic film and the performance of these transistors are comparable to those made by conventional semi-conductor technologies.
11:15 AM H3.7
CHARGE TRANSPORT STUDIES OVER SMALL DOMAINS IN -SEXITHIOPHENE, Eric L. Granstrom, Michael J. Loiacono, C. Daniel Frisbie, Univ of Minnesota, Dept of CE&MS, Minneapolis, MN.
For the purpose of examining the effects of crystallographic direction and morphology on conductivity in organic semiconductors, two types of charge transport studies on small, crystalline islands of -sexithiophene (6T) are described. The 6T crystallites are grown by vacuum sublimation onto metallic or insulating substrates and range from 1 to 10 monolayers in thickness and from 50 to 500 nm in lateral width. The lattice parameters of 6T grown on SiO and gold substrates are established by scanning probe microscopy which confirms that the molecules are oriented with their long axes perpendicular to the substrate. In the first type of measurement, lateral conductivity across individual 6T grains grown on SiO is probed by contacting the grains with individually addressable gold electrodes fabricated by electron beam lithography. A gate electrode lying underneath the SiO on which the 6T islands are grown allows the lateral conductivity of the 6T to be probed in a transistor-type configuration. Transport measurements through the 6T islands are made as a function of temperature and number of monolayers in the 6T island. Measurements through two grains sharing a common grain boundary are also made. The second approach utilizes a gold-coated tip in an atomic force microscope (AFM) as a positionable, nanoscopic electrical contact. 6T crystallites grown on flat gold substrates are imaged in the AFM and a particular crystallite is selected. The gold-coated probe is positioned on top of the crystallite with a predetermined load, and the current-voltage trace through the crystallite is obtained using the substrate as the second contact. These measurements probe conduction through the thickness of the 6T island and hence down the long axes of the 6T molecules. Current-voltage characteristics as a function of island thickness reveal the conductivity per monolayer of 6T in that crystallographic direction.
11:30 AM H3.8
A FIELD EFFECT TRANSISTOR BASED ON THE MOTT TRANSITION IN A MOLECULAR LAYER , Dennis M. Newns, Ching Zhou, James A. Misewich, Pratap C. Pattnaik, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.
Here we propose and analyze the behavior of a FET-like switching device, the Mott transition field effect transistor, operating on a novel principle, the Mott metal-insulator transition. The device has FET-like characteristics with a low ``ON'' impedance and high ``OFF'' impedance. Function of the device is feasible down to nanoscale dimensions. Implementation with a class of organic charge transfer complexes is proposed.
11:45 AM H3.9
TRANSPORT IN PENTACENE FIELD-EFFECT TRANSISTORS, Ananth Dodabalapur, Joyce G. Laquindanum, Howard E. Katz, Andrew J. Lovinger, Bell Labs, Lucent Technologies, Murray Hill, NJ.
The field-effect transistor (FET) structure is a powerful means to study charge transport in the active material of the device. This technique is particularly useful when the active materials are polycrystalline organic films and conventional time-of-flight techniques to evaluate transport can be difficult to use. We have been successful in evaluating transport mechanisms using FET structures in a-sexithiophene thin films , and now apply these methods to study pentacene. Pentacene active layers, when sublimed under optimized conditions, can result in FETs with 295 K mobilities as high as 0.4-0.6 cm/V-s . The relationship between deposition conditions, morphology, and charge transport will be examined. We will specifically discuss the dependence of electron-phonon coupling (one of the factors that determines carrier mobility) on the morphology. At low temperatures (<60 K) there is strong evidence of carrier delocalization, which will be evaluated with the help of theoretical models for transport in such materials.
SESSION H4: LIGHT EMITTING DIODES
Chair: Ananth Dodabalapur
Tuesday Afternoon, April 1, 1997
Golden Gate B1
1:30 PM *H4.1
ACHIEVING COLOR AND VERY HIGH EFFICIENCY IN VACUUM-0DEPOSITED ORGANIC LIGHT- EMITTING DIODES, Stephen R. Forrest, Paul E. Burrows, Princeton Univ, Dept of Electrical Engr, Princeton, NJ; Zilan Shen, Princeton Univ, ATC/POEM, Princeton, NJ; G. Gu, Princeton Univ, Dept of Electrical Engr, Princeton, NJ; V. Bulovic, Princeton Univ, ATC/POEM, Princeton, NJ; Mark E. Thompson, Univ of Southern California, Dept of Chemistry, Los Angeles, CA.
We discuss how to achieve full color displays based on organic light- emitting diodes employing vacuum deposited thin films. The preliminary device structure consists of stacked, transparent (when switched off) (OLEDs, or ''TOLEDs''. TOLEDs, each subdevice in the stack consisting of a different color-emitting molecule. This has resulted in an integrated OLED which can be color-tuned (from red to blue) with independent control of brightness. This stacked device is ideal for extremely high resolution, full color, flat panel displays based on OLED technology. We discuss energy transfer between the organic host and light-emitting molecule to get very high output efficiencies. Finally, we discuss means to measure and obtain very high external quantum efficiency OLEDS and stacked OLEDs.
2:00 PM *H4.2
DESIGN OF WHITE-LIGHT-EMITTING ORGANIC EL DEVICES, Junji Kido, Jun Endo, Hidehiko Shionoya, Yoshinori Yamagata, Masato Kimura, Yamagata Univ, Graduate School of Engineering, Yamagata, JAPAN.
White-light-emitting organic EL devices were fabricated by using multilayer structures or dye-dispersed polymers. Multilayer devices are composed of two or three emitter layers with different emission color. One of the devices has a structure of ITO/diamine (TPD)/blue-emitting Zn complex/orange-emitting Zn complex/Mg:Ag. By adjusting the thickness of the two emitter layers, the EL spectrum was tuned to be white. The EL spectrum varied as the current density increased: The blue portion of the spectrum increased relative to the orange part. This indicates that the carrier recombination site moves toward the anode side as the current density increases. Dye-dispersed polymer systems also exhibit white emission. Hole-transporting poly(-vinylcarbazole) (PVK) was dispersed with electron-transporting 1,3,4-oxadiazole and fluorescent dyes. A device with a structure of ITO/dye-dispersed PVK/Mg:Ag, having blue-emitting 1,1,4,4-tetraphenyl-1,3-butadiene and yellow-emitting rubrene as dopants, exhibits white light. Other types of white-light emitting devices will be reported and discussed in detail.
2:30 PM H4.3
DETERMINATION OF TRAPPING PARAMETERS IN PPV LEDS WITH THERMALLY STIMULATED CURRENTS, Siegfried Karg, Wolfgang Bruetting, Martin Meier, Katja Zuleeg, Markus Schwoerer, Univ Bayreuth, Experimental Physik II, Bayreuth, GERMANY.
Employing thermally stimulated current (TSC) technique the existence of distinct trap distributions in PPV LED structures has been established. In devices with ITO bottom electrode a TSC peak in the temperature range between 100K and 130K is observed. This corresponds to a trap level with a depth of 0.15eV according to the method of Cowell and Woods. The total density of this trap species is of the order of /cm which is in good agreement with the dopant concentration obtained from capacitance measurements. The investigated peak in the TSC spectrum does not occur by using a Au electrode instead of ITO. Hence, the reaction of ITO with the elimination products (mainly HCl) during the conversion of the PPV precursor leads to the formation of these shallow traps. Further peaks in the TSC spectrum arise when the PPV LEDs are exposed to air. The evaluation gives deep traps with energies of 0.8 - 0.9eV. Their density can be varied by applying vacuum to the devices.
3:15 PM *H4.4
INTEGRATED ORGANIC LIGHT EMITTING DIODE STRUCTURES USING SINGLE-LAYER DOPED POLYMER THIN FILMS, James C. Sturm, Princeton Univ, Dept of Electrical Engr, Princeton, NJ; Chung-chih Wu, Steven D. Theiss, Minhao Lu, Sigurd Wagner, Princeton Univ, Dept of Electrical Engr, Princeton, NJ; Richard A. Register, Princeton Univ, Dept of Chemical Engr, Princeton, NJ; Mark E. Thompson, Univ of Southern California, Dept of Chemistry, Los Angeles, CA.
It is well known in organic LEDs (OLEDs) that multiple organic materials are needed to optimize the functions of electron transport, hole transport and light emission. These materials are usually prepared in a layered structure of thin films. In this talk we describe how high efficiency (>1 external quantum efficiency) and low drive voltage (<10 V) OLEDs may be achieved using single-layer doped polymer thin films with bipolar transport capabilities deposited by spin-coating. The films consist of a host material of the hole-transport polymer poly(N-vinylcarbazole) (PVK), into which various molecular electron-transport agents and red, green and blue emitting centers (e.g., nile red, coumarin 6, and coumarin 47) have been doped. Similar performance (and high efficiency) is obtained either with the typical Mg:Ag cathode or an air-stable Ag cathode. Plasma etching has been used to pattern OLEDs to integrate orange, green and blue OLEDs (from three different organic materials) onto a single substrate, all with performance similar to isolated (unetched) devices. Finally, we have demonstrated the successful integration of OLEDs and amorphous silicon thin film transistors (a-Si TFTs) to show that sufficient current may be obtained from the TFTs to drive the OLEDs. This integration has been done not only on glass substrates, but also on thin (200 m) unbreakable stainless steel foil using a top-emitting OLED structure. These finished substrates are unaffected by dropping them from a height of thirty feet onto concrete.
3:45 PM H4.5
BI-LAYER POLYMER LIGHT EMITTING DIODES WITH A SERIES OF ELECTRON-DEFICIENT POLYMERS, Weidong Chen, James L. Hedrick, Thomas Fuhrer, James Ashenhurst, Robert D. Miller, J. Campbell Scott, IBM Almaden Research Center, San Jose, CA.
A set of electron-deficient polymers based on aromatic heterocycles has been synthesised and is currently under study for applications in polymer light emitting diodes (PLEDs). These polymers, benzoxalozes, benzimidazoles, and quinoxalines, are designed to function as electron transport layer and hole blocking layer in bi-layer devices with PPV as the hole transport layer. The electron transport layer in a bi-layer PLED helps to improve electron injection. The recombination zone is moved to the heterojunction and away from the electrode interface, thereby reducing quenching. We will present the fabrication and the results of current-voltage and luminescence measurement of these devices. The electron transporting polymers are also (weakly) fluorescent and operate as single layer PLEDs. The solution and solid state photoluminescence is compared with the electroluminescence.
4:00 PM H4.6
MODELING OF POLYMER LIGHT EMITTING DIODES, Beohm R. Choi, S. K. Park, Hong H. Lee, Seoul National Univ, Dept of Chem Engr, Seoul, SOUTH KOREA.
A model is developed for the performance of single-layer polymer emitting diodes, including the effects of exciton formation, heat generation, and interface contacts. The interface effects are described in terms of surface recombination velocity. Both reactive and non-reactive recombination in the polymer are accounted for. A balance on the exciton concentration along with constitutive relationships leads to the light output and quantum efficiency.
The results provide an insight into degradation of the polymer light emitting diodes. They also shed light on the current density-voltage relationship that has been much disputed in the literature.
4:15 PM H4.7
BLUE LIGHT ELECTROLUMINESCENCE FROM A PET DERIVED POLYMER, Joel Davenas, G. Seytre, Univ Claude Bernard Lyon I, Dept of Phys des Materiaux, Villeurbanne, FRANCE.
Emission in the blue region of the spectrum, with a maximum at 420 nm, has been produced by an electric field of the order of 1 MV/cm applied to a polyethylene naphthalene 2,6 dicarboxylate (PEN) film sandwiched between 2 gold electrodes. The electroluminescence proceeds through the radiative decay of excimers. The formation of these excited states is interpreted by the short interaction of the naphthalene groups acting as chromophores. This luminescence provides a very sensitive probe for the study of the molecular order in PEN films. Modification of the PEN emission through the insertion of the film in a microcavity will be discussed.
4:30 PM H4.8
ORGANIC LIGHT-EMITTING DIODES BASED ON MOLECULAR COMPOSITES USING QUINACRIDONE DERIVATIVES AS DOPANTS, Charles D. Merritt, Hideyuki Murata, Naval Research Laboratory, Optical Science Div, Washington, DC; Darius J. Fatemi, Naval Research Laboratory, Washington, DC; Zakya Kafafi, Naval Research Laboratory, Optical Science Div, Washington, DC.
Organic light-emitting diodes were fabricated using multilayered organic nanostructures where TPD and AlQ were used as the hole and electron transport layers, respectively. The effect of doping AlQ or TPD with newly synthesized quinacridone derivatives was investigated. Photoluminescence and absorption spectra of the molecular composites were studied as a function of dopant concentration. The effect of dIfferent substituents in the quinacridone ring on the photoluminescence quantum yield was investigated. Measurements of device electroluminescence spectra, current-voltage-luminance curves, and operating lifetime as a function of dopant and dopant concentration in each region were also performed. From these experiments, luminous power and external quantum efficiencies were extracted, and were greatly enhanced for devices based on these molecular composites.
4:45 PM H4.9
ENERGY TRASFER AND CARRIER TRAPPING IN TPN:Alq AND TPN:TPD COMPOSITES, Hideyuki Murata, Charles D. Merritt, Naval Research Laboratory, Optical Science Div, Washington, DC; Darius J. Fatemi, Naval Research Laboratory, Washington, DC; Zakya Kafafi, Naval Research Laboratory, Optical Science Div, Washington, DC.
Mechanisms leading to electroluminescence (EL) enhancement in organic LEDs were investigated. 5,6,11,12-tetraphenylnaphthacene (TPN) was used as a dopant, tris-(8-hydroxyquinolinato) aluminum (III) (Alq) or N,N-diphenyl N,N-bis(3-methylphenyl)-1,1-biphenyl-4,4-diamine TPD) was the host material. Energy transfer from host to dopant molecules was demonstrated by photoluminescence (PL) spectroscopy. The PL spectra of the composite films arose from the dopant molecules, TPN, and were greatly enhanced. Similar results were observed from the EL spectra with the exception of a much greater enhancement in the EL quantum efficiency for single-layered devices. For example, in devices using TPN:Alq as the emitter layer, enhancements by factor of 4 and 30 were measured for the PL and EL quantum yields, respectively. The additional increase in EL quantum efficiency was attributed to efficient carrier trapping at the dopant molecules. Enhancement due to carrier trapping was also indicated by changing the position of the doped layer. Higher quantum efficiencies have been achieved when TPN was doped into TPD layer, where carrier trapping at TPN site seemed to be the dominant mechanism for electroluminescence.
SESSION H5: MATERIALS PREPARATION AND CHARACTERIZATION
Chair: Zhenan Bao
Tuesday Evening, April 1, 1997
Golden Gate B1
8:00 PM H5.1
THE GROWTH OF CENTIMETER-SIZED CRYSTALS OF HEXATHIOPHENE, Ch. Kloc, Univ of Konstanz, Faculty of Physics, Konstanz, GERMANY; Robert A. Laudise, Bell Labs, Lucent Technologies, Murray Hill, NJ.
Single crystals of 6 Thiophene (6T) and related oligomers are of interest for comparisons of the properties of single crystals and polycryslalline films and for the study of the anisotropy of properties of these materials which are promising as thin film transistors. We have studied and will compare melt, solution and vapor growth of 6T. We will report solubility measurements in several organic solvents contained in closed tubes at temperatures up to 250C. Some solvents give solubilities up to 18. We discuss criteria for solvent choice and preliminary crystal growth results. By far, our best crystals were obtained in growth by physical vapor deposition. We discuss the effect of inert gas pressure anal other growth parameters on volatilization, transport and deposition and report how excellent crystals can be grown in one atmosphere of inert gas, under convection limited conditions and with forced convection. Conditions for the growth of mm-sized crystals which are 15-30 m thick and for the growth of thinner cm-sized crystals are described.
8:15 PM H5.2
POLYELECTROLYTE-DYE SELF-ASSEMBLY SOLUTION ADSORPTION, Rigoberto C. Advincula, Curtiss Frank, Stanford Univ, Dept of Chemical Engr, Stanford, CA; Wolfgang Knoll, Max-Planck-Inst, Mainz, GERMANY.
The self-assembly of polyelectrolytes and dyes on a layer by layer basis is reported as investigated by a variety of spectroscopic and microscopic techniques. Using the Decher approach of polyelectrolyte self assembly, well ordered multilayers are formed involving electrostatic interaction. This is in comparison to previous deposition systems of water soluble dyes tethered to a polymeric main chain or the use of bolaform amphiphiles. The deposition process was investigated as a function of concentration, salt, pH, and time of adsorption. A key investigative tool is the use of surface plasmon spectroscopy to monitor the deposition process in-situ. The importance of this process is in the incorporation of water soluble chromophores alternating with polymers as ultrathin films. This is relevant in the area of dye incorporation in xerographic systems, display technologies, fabric modification, photoisomerizable substrates, etc. Further investigations are in progress to study the effect on dye functionalities, in particular photoismerization in the presence of electrostatic interactions.
8:30 PM H5.3
ENHANCEMENT OF ELECTRON MOBILITY BY REGULATION OF MOLECULAR ORIENTATION OF LIQUID-CRYSTALLINE OXADIAZOLE, Hiroaki Tokuhisa, Masanao Era, T. Tsutsui, Kyushu Univ, Dept of Matls Science & Technology, Kasuga-shi, JAPAN.
Glassy liquid crystal provides organic solid thin films with high degree of molecular orientation. Therefore, high charge carrier mobility is expected in the glassy liquid crystalline film, because introducing molecular orientation brings about increasing overlap between wave-function of the charge carrier transporting molecules. Recently, we demonstrated that oxadiazoles possess high potential of electron transport. Then, it is expected that in a liquid crystal with oxadiazole chromophore, high electron mobility is attained through orientation of electron transporting oxadiazole chromophores. In this study, we synthesized an oxadiazole based liquid crystal, octadecyloxybenzylidene-anminohenyloxadiazole (C18BA-OXD) and evaluated electron mobility of C18BA-OXD by using time-of-flight technique.
C18ba-OXD showed two mesophases (smectic and nematic phase) and formed a glassy liquid crystal by quenching from the mesophases. Electron mobility in the glassy liquid crystalline film was 3.6x10cmVs at an electric field of 2x10Vcm at room temperature. The value was more than ten times that at the level of 10-6 cm2V-1s-1 in isotropic glassy film of C18BA-OXD which were prepared by quenching from isotropic phase. Because oxadiazole molecules is assumed to form random orientation in the isotropic glassy phase, enhancement of electron mobility in the glassy liquid crystalline film is supposed to be caused by regulation of molecule orientation in the glassy liquid crystalline film.
8:45 PM H5.4
SYNTHESIS AND CHARACTERIZATION OF POLY(QUINOLINE VINYLENE) FOR APPLICATION AS n-LAYER IN LIGHT-EMITTING DIODES, Ramesh K. Kasim, Ronald L. Elsenbaumer, Univ of Texas-Arlington, Dept of MS&E, Arlington, TX; Satyanarayana Sripadma, Univ of Texas-Arlington, Dept of Chemistry & Biochemistry, Arlington, TX.
Conductive polymers are the subject of growing interest, originating from their novel electronic and electro-optical properties, especially light emission in the visible range upon application of voltage. Inefficient/low electron-injection via metal electrodes into the active electroluminescent polymer layer in light-emitting devices is a bottleneck for improved light emission efficiency. The need for stable n type polymers for use as intermediate electron-injection layers to improve emission efficiencies in LEDs is well recognized. A new n-type polymer, poly(2,6-quinoline vinylene) was synthesized via precursor route ant characterized by NMR, FT-IR, UV-VIS spectroscopy and thermogravimetric analysis. This polymer showed red fluorescence indicating its potential for use as a red emitter in LEDs. Along with synthesis and characterization data, results from experiments on the evaluation of polymer suitability as an electron-injection layer in combination with electroluminescent polymers such as poly(phenylene vinylene) will be presented.
9:00 PM H5.5
A STABLE, SOLUBLE PPV DERIVATIVE WITH HIGH PHOTOLUMINESCENCE EFFICIENCY, Sean E. Shaheen, M. M. Morrell, Y. Kawabe, Bernard Kippelen, Nasser Peyghambarian, Univ of Arizona, Optical Sciences Center, Tucson, AZ; M. W. Wagaman, R. H. Grubbs, California Inst of Technology, Pasadena, CA.
A new luminescent polymer has been characterized which exhibits high photoluminescence efficiency and stability. 2,3-di-t-butyl polyphenylenevinylene dicarboxylate, or bc-PPV, is more stable against oxidation due to the electron withdrawing nature of the carboxylate side chains.
The photoluminescence efficiency of bc-PPV was measured to be close to 60, which is twice that of PPV (27) or Alq (32). Stability of this luminescence was measured by exposing thin film samples of both bc-PPV and Alq to broadband UV radiation in air. After 1 hour at 100 mW/cm, the efficiency of bc-PPV had decayed to 40, whereas AIq had decayed to 1.
Electroluminescent devices were fabricated by spin casting bc-PPV, which is readily soluble in common solvents, onto ITO and subsequently depositing a Mg cathode. This simple device geometry yielded an external quantum efficiency of 0.1.
9:15 PM H5.6
ORGANIC ELECTROLUMINESCENT DEVICES FABRICATED BY IONIZED CLUSTER BEAM DEPOSITION, Ghassan Elie Jabbour, Seong Jin Cho, Bernard Kippelen, Nasser Peyghambarian, Univ of Arizona, Optical Sciences Center, Tucson, AZ.
The Ionized Cluster Beam Deposition (ICBD) technique is proposed for the fabrication of thin film organic electroluminescent devices. This technique leads to smoother and more adherent films with better electroluminescent properties than similar films deposited via the traditional thermal evaporation.
Devices fabricated had the following structure: Hole transport layers of copper phthalocyanine (40 nm) and 1, 1-bis (di-4-tolyaminophenyl) cyclohexane (60 nm) were successively deposited on an IT0-coated glass substrate using thermal deposition. The electron transporting and light emitting layer, tris(8-hydroxyquinoline)aluminum (Alq), was then deposited either thermally or by ICBD. In all devices, aluminum metal (100 nm) was used as a cathode. The full-width-at-half-maximum of the photoluminescence peak of the ICBD-Alq is 18 nm narrower than that of thermally deposited Alq, reflecting different morphological properties of the ICBD film. The ICBD-Alq also showed an extremely high adhesion: While moderate rubbing easily removed thermally deposited Alq films, rigorous rubbing with fabric caused only scratches on the ICBD films. Large leakage current at low voltage was observed in devices based on the thermally deposited Alq films. In contrast, ICBD based devices showed no leakage current, thus indicating the absence of pinholes. Since the ICBD films are 33 smoother (as measured by atomic force microscopy) than the thermal ones, they lead to a relatively smooth organic-cathode interface. This enabled the fabrication of devices exhibiting more than twice the external quantum efficiency and three times the luminance of those fabricated by the vacuum thermal deposition technique. The increase of external quantum efficiency in the ICBD Alq devices reflects the possibility of a more balanced injection of electrons and holes into the emitting layer.
9:30 PM H5.7
PREPARATION OF THIN POLY(P-PHENYLENE VINYLENE) FILMS BY CHEMICAL VAPOR DEPOSITION, Kathleen M. Vaeth, Klavs F. Jensen, MIT, Dept of Chemical Engr, Cambridge, MA.
Electroluminescent polymers such as poly(p-phenylene vinylene) (PPV) and its derivatives show promise for use as the active element in light emitting diodes and lasers. Typical routes for the fabrication of thin PPV films are solution based, and side reactions with the solvent or residual oxygen can lead to the incorporation of carbonyl defects into the polymer chain. These oxidation defects are highly undesirable, since carbonyl groups quench emission and lower the luminescent efficiency of the film. In this research, chemical vapor deposition (CVD) of PPV is explored as an alternative to solution processing methods traditionally used for fabrication of PPV films. We report the fabrication of thin PPV films from a p-dichloroxylene monomer by CVD. The UV-VIS and PL spectra of the CVD PPV are in good agreement with solution processed polymer. Film growth and thermal conversion are monitored in-situ with Fourier transform infrared reflection absorption spectroscopy (FT-IRRAS). The spectra show no carbonyl stretching at 1690 cm indicating that the as-deposited films are not oxidized. In-situ FT-IRRAS is also utilized to examine the affect of pyrolysis and sample temperature on polymerization initiation, film growth rates, and film composition. The FT-IRRAS compositional studies are complimented with ex-situ XPS. AFM and SEM characterization will be presented to illustrate the affect of processing conditions on film morphology. Implications of the CVD PPV film characteristics on device performance will also be discussed.
9:45 PM H5.8
PREPARATION AND CHARACTERIZATION OF EPITAXIALLY GROWN CdSe/CdS CORE/SHELL NANOCRYSTALS, Michael C. Schlamp, Xiaogang Peng, Andreas V. Kadavanich, A. Paul Alivisatos, Univ of California-Berkeley, Dept of Chemistry, Berkeley, CA.
The surface of a nanocrystal controls much of its structural, optical, and electronic properties simply because it is so predominant. Atoms at the surface are not fully coordinated, and act like defects unless passivated. Traditionally, homogeneous cadmium chalcogenide nanocrystals have been passivated with long chain organic surfactants. More recently, inorganic passivation has been explored, with a motivation being epitaxial growth of a surrounding material leading to a core/shell type structure with a defect-free interface. Reported here is the first demonstration of a high temperature, liquid phase preparation of an epitaxially grown, relatively uniform crystalline shell of a large bandgap material (CDs) onto a nearly monodisperse, crystalline, low bandgap core of CdSe. These novel core/shell nanocrystals exhibit excellent room temperature quantum yields (), long term stability in air and light, and excellent resistance to photo-oxidation. Samples have been prepared having core diameters ranging from 20 to 40 and shell thicknesses up to 12 - demonstrating a wide range of emission wavelengths at these high quantum yields. The core/shell nanocrystals were characterized by UV/VIS absorption, photoluminescence quantum yields, photoluminescence excitation, XPS, XRD, and TEM.
The CdSe/CDs system is ideal in many respects. The lattice mismatch of 3.9 is small enough to allow epitaxial growth but large enough to maintain a minimal degree of atomic interdiffusion. In addition, the difference in bandgaps is large enough for shell growth to increase the quantum yields and stability of the cores. The synthetic conditions chosen prevent core dissolution and CDs-only formation during epitaxial growths. The chemistry of shell growth is clean-the reaction is nearly stoichiometric. The resulting core/shell structure opens the door to three-dimensional nano-sized bandgap engineering.
Many of the properties displayed by these nanocrystals make them very attractive for incorporation into electronic devices. The fabrication of light emitting diodes (LEDs) using these nanocrystals is currently an active area of research in our lab.
10:00 PM H5.9
CORRELATION OF THE OPTICAL AND ELECTRICAL PROPERTIES OF TRIS-8-HYDROXYQUINOLINE (ALQ) FOR LED OPERATION WITH THIN FILM GROWTH CONDITIONS, Khashayar Pakbaz, Howard W. H. Lee, Lawrence Livermore National Laboratory, Photonics Group, Livermore, CA; Gildardo R. Delgado, Univ of California-Davis, Dept of Applied Science, Livermore, CA; Glenn A. Fox, Ann M. Lee, Lawrence Livermore National Laboratory, Dept of Chem & Matls Sci, Livermore, CA.
Tris-8-Hydroxyquinoline (Alq) has been the subject of much interest due to its use as the electron transport layer and the light emissive layer in organic light emitting devices (OLEDs). Thin film properties such as carrier injection and transport are of great importance in determining device performance and lifetime. It is well known that such properties are strongly dependent upon growth conditions. In this paper, we present the results of absorption, emission, electroabsorption, and photoconductivity measurements on Alq films prepared under various conditions (substrate temperature, evaporation rate, etc.). The consequences of these results are discussed in the context of OLED performance and lifetime.
SESSION H6: DEVICE RELIABILITY AND DEGRADATION
Chair: Junji Kido
Wednesday Morning, April 2, 1997
Golden Gate B1
8:30 AM *H6.1
IMPROVED STABILITY OF DOPED ORGANIC LIGHT EMITTING DIODES, Yoshiharu Sato, Shoko Ichinosawa, Hiroyuki Kanai, Mitsubishi Chemical Corp, Optical Information Media Lab, Kanagawa, JAPAN.
In the application of organic LED to flat panel displays, stability is most important from a practical viewpoint. The degradation of the diode during operation can be classified into three phenomena: decay of luminance, voltage rise in a constant-current dirve, increase of non-emissive area (dark spots). These phenomena are closely related to the physical and chemical stability of the used organic materials.
We found that the following points were effective to design a durable device:
1. interlayer between the hole transport layer and the ITO anode,
2. hole transport material with Tg higher than 80 degree C,
3. emitter layer doped with a highly efficient organic dye.
We observed that doping technique is the most effective to suppress the above degradation. Doping the emitter layer of the diode with proper dyes such as condensed aromatics greatly improved the operation performance. The morphology of organic thin layers in the diode was greatly stabilized by doping improved the half-decay time of the device. The choice of host and dopant material in the emitter layer is critical in terms of emission efficiency and durability. Alq is most stable as a host material compared with other ones.
Taking all the points into consideration, we obtained an organic LED which demonstrated a good operation performance: half-decay time of luminance is longer than 3,000 hours at an initial luminance of 500 cd/m2. Thermal and coulombic degradation process will be discussed.
9:00 AM *H6.2
STUDIES OF THE AGING MECHANISMS IN POLYMER LIGHT EMITTING DIODES, J. C. Scott, IBM Almaden Research Center, San Jose, CA; Sue A. Carter, Univ of California-Santa Cruz, Dept of Physics, Santa Cruz, CA; Marie Angelopoulos, IBM T.J. Watson Research Ctr, Yorktown Heights, NY; Weidong Chen, IBM Almaden Research Center, San Jose, CA.
If organic organic light emitting diodes are to become a commerically successful technology, their mechanisms of degradation and failure must be understood and minimized. This talk reviews the aging processes that have been identified in our own studies and by other workers, including photo-oxidation, black-spot development and short formation. Current and luminance measurements, as a function of running time at constant voltage, reveal two distinct time regimes. Long term effects are predominantly those involving ambient contamination such as oxygen and water. These can be controlled to a considerable extent by careful encapsulation. However indium-tin-oxide (ITO), frequently used as a transparent anode, has been shown to be a potential source of oxygen for photo-oxidation. Coating a thin layer of conducting polymer, such as a polyaniline or polythiothene derivative, between the ITO and the emissive polymer greatly reduces the rate of this process. The short term changes occur within the first few minutes and may account for as much as half of the brightness loss. Changes in the electrical characteristics will be discussed in the context of a model which includes effects of both surface and bulk properties.
9:30 AM H6.3
THE ITO MYSTERY IN VAPOR-DEPOSITED ORGANIC LEDS, Homer Antoniadis, Elizabeth C. Carr, M. Hueschen, J. Miller, R. Moon, D. Roitman, James R. Sheats, Hewlett Packard Co, Palo Alto, CA.
Effects of Indium-Tin-Oxide (ITO)/glass substrate characteristics on the reliability and electrical behavior of vapor-deposited organic light emitting diodes (LEDs) will be presented. Organic ITO/TPD/Alq/Mg/Ag bilayer devices were used for these studies. Five different topics will be presented: 1) relationship between the character of the ITO surface and the formation of electrical shorts, 2) ITO surface effects on the formation and growth of black spots, 3) correlation of ITO roughness and operational voltage, 4) effects of silane-modified ITO surface on the luminance current-voltage characteristics, and 5) hole-injection limitations of the ITO surface on the electrical characteristics and the quantum efficiency of the organic LED.
9:45 AM H6.4
THERMAL MODELING OF ORGANIC LED FLAT PANEL DISPLAYS, James C. Sturm, Mario Iodice, Princeton Univ, Dept of Electrical Engr, Princeton, NJ.
While most work to date on organic LEDs is performed on isolated devices, in large-area displays the power dissipation might cause excessive heating, which could lead to brightness variations and lifetime degradation. In this paper we present the thermal modeling of large-area displays based on OLEDs. The work was carried out using one-dimensional finite difference modeling including the effects of the metal contacts, substrates, air-interfaces, and any possible environmental sealing layers. Simulations were performed for several kinds of packaging, orientation, and substrates. The display size is a critical factor since it is well known that the heat transfer coefficient from a flat plate to air (h) decreases sharply as the plate size is increased.
Assuming OLEDs with 2 external quantum efficiency, 10 V drive, 100 cd/m in the green, a small size (1x1 mm), and no seating, the temperature rise in the organic is minimal (1.3C). However, assuming more realistic conditions of an environmental seal and larger size (20x20 cm), the average organic temperature rise is 22C, and some parts of the display could rise 33C. Assuming 100 cd/m in the red (where eye response is poor) would increase the average and peak temperature rise to 72C and 108C, respectively. Power inefficient color schemes, such as white light OLEDs followed by filters, can easily raise these numbers by a factor of 5. This work shows that thermal engineering will critical in reliable OLED products.
10:30 AM *H6.5
FAILURE MECHANISMS OF QUINOLINE BASED LEDs AND THE LESSONS TAUGHT TOWARDS DEVELOPMENT OF THEIR POLYMERIC ANALOGUES, Fotios Papadimitrakopoulos, D. Larry Thomsen, Univ of Connecticut, Inst of Matls Science, Storrs, CT; Keith A. Higginson, Univ of Connecticut, Dept of Chemical Engr, Storrs, CT.
Our group has reported a chemical failure mechanism for aluminum (III) 8 hydroxyquinoline (Alq)-based organic light-emitting diodes. This involves a reaction cycle that begins with the liberation of 8 hydroxyquinoline (8-Hq) from the Alq complex in the presence of trace amounts of water, followed by the oxidative coupling of the freed 8 Hq species. This regenerates water and also introduces nonemissive byproducts with lower unoccupied molecular orbitals at mid-gap positions with respect to Alq gap. Implementation of this mechanism would be discussed us a function of environmental stability of low molecular weight Alq-based LEDs. The lessons taught by this mechanism have been incorporated towards the fabrication of polymeric 8 hydroxyquinoline- based LEDs, We hereby introduce a novel self-assembly technique to grow thin films made of polymeric metal-chelate derivatives of 8-hydroxyquinoline, which are insoluble and intractable, suitable for LEDs and other semiconducting applications. The growth of a 1,200 film in about 2.7 hrs, with the potential for multilayer fabrication at high purity levels, offers an alternative to vacuum deposition and spin-coating. Single layer electroluminescent (EL) devices, with film thicknesses as thin as 400 exhibited a strong orange emission, similar to their photoluminescence (PL) response.
11:00 AM H6.6
DEGRADATION AND RADIATION NOISE IN PPV-BASED LIGHT-EMITTING DEVICES, Vadim N. Savvate'ev, Aharon Yakimov, Haiping Hong, Galina Perepelitsa, Michael Golosovsky, Alexander Galkin, Dan Davidov, Hebrew Univ, Racah Inst of Physics, Jerusalem, ISRAEL; Yair Avny, Hebrew Univ, Dept of Organic Chemistry, Jerusalem, ISRAEL; Roni Neumann, Hebrew Univ, Casali Inst of Applied Chem, Jerusalem, ISRAEL.
We have studied the degradation and photodegradation of light-emitting diodes based on thin films of poly-(phenylene-vinylene), PPV, sandwiched between ITO and Mg (or Al) electrodes. The diode degradation was studied by taking optical images of the metallic electrode and through the trasparent ITO elecrode as a fuction of time while the diode is in "off" and "on" states. We also map the resistivity of the metallic electrode using a near-field rf microscope. The microscopic observations reveal a two-stage change in the morphology of the metallic electrode. The first stage is dominated by the growth of "white bulbs" with characteristic sizes of 50-200 µ. The bulbs appear at very low light intensities. With increasing current their number increases to form complicated interconnected patterns. The second stage is marked by appearance of dark spots on the "bulbs". These spots gradually expand and form dark areas. We didn't find any light emission neither from the "bulbes" area nor from the dark regions although some strong and unstable emission appears at the borders between them. When the dark regions cover a significant part of the metallic electrode area, the electrode looses adhesion to the PPV film and disintegrates. Simultaneously, we have measured the emission intensity as function of time. At low currents the generated light is dominated by a non-Gaussian noise. At high currents the noise is superimposed on light intensity of comparable amplitude. Our results demostrate a clear correlation between the electrode damage, the light emission and the observed noise. Mechanisms for the degradation will be discussed.
11:15 AM H6.7
REAL-TIME OPTICAL CHARACTERIZATION OF DEGRADATION IN POLYMER LIGHT EMITTING DIODES, Sue A. Carter, S. Bailard, Univ of California-Santa Cruz, Dept of Physics, Santa Cruz, CA; J. C. Scott, IBM Almaden Research Center, San Jose, CA.
We present optical and electrical studies of degradation in MEH-PPV OLEDs with the goal of understanding the mechanisms that lead to device failure. We distinguish between two regimes of aging, a short-term regime which results in order-of-magnitude drops in quantum efficiency within a few hours, and long-term regimes which results in slow decay over weeks. We concentrate on real-time simultaneous current-voltage (IV) and confocal microscopy images to understand how features in the IV-curves depend on morphological changes in the luminance and electrodes. Studies are shown on several different anode and cathode materials for comparison. In addition, we measure how the decay of the radiance and device efficiencies depend on the operating temperature, driving voltage and frequency. We conclude by comparing our results to theories on charge transport and injection in OLEDs, discussing possible mechanisms for device degradation in the two regimes and the ramifications of this work on the design of long-lasting OLEDs.
11:30 AM H6.8
EFFECTS OF PLASMA TREATMENT OF THE ANODE SURFACE ON THE PERFORMANCE OF ORGANIC LIGHT EMITTING DEVICES, Chung-chih Wu, Princeton Univ, Dept of Electrical Engr, Princeton, NJ; James C. Sturm, Princeton Univ, Dept of Electrical Engr, Princeton, NJ.
Indium tin oxide (ITO) has been widely used as the bottom anode contact for organic light emitting devices. It is well known that the treatment of the ITO before the deposition of organic films is crucial to the performance of the devices. However, we found that predeposition treatments of ITO by just combinations of cleaning steps, such as sonication, boiling and rinsing in detergents, deionized water and organic solvents, is usually not satisfactory and results in devices with high drive voltage and poor reliability. In this paper, we discuss effects of plasma treatment of the anode surface on the performance of organic light emitting devices made with single-layer hole-transport polymer thin films doped with small molecule emitters and electron transporters. Both reactive or inert gases, such as oxygen and argon, were used. While an argon plasma gives only limited improvement, an oxygen plasma has dramatic effects on device performance when applied to ITO before organic deposition: the drive voltage drops substantially (from >20 V to <10 V), the external quantum efficiency (backside emission only) increases by up to 5 times (from 0.2 to 1), and a much higher driving current can be applied to achieve a much higher brightness (maximum brightness >6000 cd/m at >600 mA/cm). The lifetime of devices is also significantly enhanced by two orders of magnitude. Through the plasma treatment, the bulk properties of ITO, such as the sheet resistance, is not changed, while the ITO surface changes from hydrophobic to hydrophilic. The physical mechanisms underlying the device improvement will be discussed.
11:45 AM H6.9
DEGRADATION MECHANISM OF ORGANIC EL DEVICES, Masamichi Fujihira, Lee-Mi Do, Chimed Ganzorig, Takeshi Kato, Tokyo Inst of Technology, Dept of Biomolecular Engr, Kanagawa, JAPAN.
Degradation mechanism of layered organic molecular EL devices, i.e. ITO / TPD / Alq3 / Al, has been studied with Auger electron spectroscopy, atomic force microscopy, fluorescence spectroscopy and microscopy, and ionization potential measurements. Together with observation of degradation of the EL devices by heating, we concluded that by Joule heating a small amount of Alq3 diffused into the TPD layer which results in the drastic change in electronic properties of the hole transport layer (HTL), and thus the change in the EL characteristics. The effect of diffusion was confirmed by fabrication of organic EL cells with a mixed HTL of TPD and Alq3. We found also that the organic EL cells with the mixed HTL has a longer lifetime than the conventional layered EL cells. The increase in the lifetime is attributed to the increase in the thermal stability of the mixed organic films. Much less morphological change of the mixed HTL than the pure TPD layer by heating was observed.r thermoelectrics. 400 C. Electromigration activation energy was obtained for films deposited at the different accelerating voltages.
SESSION H7: OPTICAL/TRANSPORT PROPERTIES
Chair: Homer Antoniadis
Wednesday Afternoon, April 2, 1997
Golden Gate B1
1:30 PM H7.1
ELECTRICALLY AND OPTICALLY DETECTED MAGNETIC RESONANCE STUDIES OF SMALL ORGANIC MOLECULAR LEDs, F. Li, Jonathan Partee, Joseph Shinar, Iowa State Univ, Dept of Physics & Astronomy, Ames, IA.
The electrically and optically detected magnetic resonance (EDMR and ODMR, respectively) of tris-8-hydroxyquinoline aluminum (Alq) based LEDs is described and compared to the ODMR and EDMR of conjugated polymer-based diodes. As in the polymer-based devices multilayer indium tin oxide(ITO/(hole transporting layer(HTL} lAlq/(metal cathode) LEDs also exhibit a spin 1/2 EDMR. This resonance is compared to that of single layer ITO/HTL/(metal cathode) and ITO/Alq/(metal cathode) LEDs. The nature of this resonance, whether intrinsic or impurity-related, bulk or interface, is discussed and compared to the positive and negative spin 1/2 resonances of polymer LEDs, which are generally assigned to polaron recombination and fusion of like-charge polarons to bipolarons, respectively.
1:45 PM H7.2
ELECTRON AND HOLE CORRELATION IN AN ORGANIC MOLECULAR SEMICONDUCTOR, Chih-I Wu, Yutaka Hirose, Henning Sirringhaus, Antoine Kahn, Princeton Univ, Dept of Electrical Engr, Princeton, NJ.
Understanding basic electronic properties of organic molecular semiconductors with application in opto- and microelectronics is very important for the future development of this class of materials. We report the first investigation of electron and hole correlation in the molecular semiconductor 3,4,9,10 perylenetetracarboxylic dianhydride (PTCDA) using photoemission spectroscopy (PES) to probe the highest occupied molecular orbital (HOMO), inverse PES (IPES) to probe the lowest unoccupied molecular orbital (LUMO) and electron energy loss spectroscopy (EELS). The HOMO-LUMO gap obtained by PES-IPES represents the state of the molecule with -1 and +1 electron, respectively, with respect to its neutral charge density. The localization of electrons and holes reduces the screening of the excess charge and leads to an increase in the measured HOMO-LUMO gap relative to that of the neutral molecule obtained by EELS or optical absorption. The difference is the Hubbard on site correlation energy . In a highly correlated solid like C60, is large (1.6 eV) and the band-width small (< 0.5 eV) , indicative of weak intermolecular electronic coupling. The measured and band-width in PTCDA are 1 eV and 0.7 eV, respectively, indicative of substantial correlation in spite of the expected large intermolecular interaction along the molecular stacking direction. The effect has direct implications on spectroscopic investigations of this class of solids. We will also report ongoing measurements on other molecular semiconductors (e.g., Alq).
2:00 PM H7.3
DETERMINATION OF POTENTIAL BARRIER HEIGHTS AT INTERFACES BETWEEN CONJUGATED POLYMERS AND METALLIC CHARGE INJECTION ELECTRODES, Marius Koehler, Ivo Alexandre Hummelgen, Univ Federal do Parana, Dept of Physics, Curitiba, BRAZIL.
Potential barrier heights at interfaces between polymer and charge injection electrode materials present a preponderant role in the control of the injection current density in conjugated polymer based devices such as organic light emitting diodes. The determination of the potential barrier height constitutes a relevant step to achieve the balance between electron and hole injection currents and to maximize the radiant efficiency of a light emitting diode. It has been demonstrated that at some interfaces the charge injection into the polymer occurs via field emission, i. e., tunneling process.
In this work the tunneling current density through the triangular potential barrier at a polymer/electrode interface is calculated considering temperatures different than 0 K during the measurement of the current versus voltage characteristics of the sample. The tunneling current expression reproduces the deviations from a straight line that are normally observed in Fowler-Nordheim plots and fits very well to experimental data obtained by different authors with different polymer/electrode materials combinations. The potential barrier heights al the polymer/electrode interlaces can be directly calculated from the fit parameters of a single Fowler-Nordheim plot without the knowledge of the polymer layer thickness and without the assumption of a charge carrier effective mass value. The potential barrier height values obtained with this method agree very well with the ones determined by other ways.
2:15 PM H7.4
ELECTROLUMINESCENCE FROM ZnS NANOCRYSTALS/POLYMER COMPOSITE SINGLE LAYER STRUCTURE LIGHT-EMITTING DIODE, Yi Yang, Jilin Univ, National Integrated Optoelectronics Lab, Changchun, CHINA; Jinman Huang, Jilin Univ, Dept of Chemistry, Changchun, CHINA; Shanhua Xue, Jilin Univ, Changchun, CHINA; Shiyong Liu, Jilin Univ, Dept of Electrical Engr, Changchun, CHINA; Jiacong Shen, Jilin Univ, Key Lab for Supermolecular Structure and Spectroscopy, Changchun, CHINA.
During the last decade, the preparation and characterization of semiconductor nanocrystals have received extensive study because of their special physical and chemical properties. Their novel optical and transport properties are potentially useful for technological application. Recently, by taking advantage of developments in the preparation and characterization of direct-gap semiconductor nanocrystals, and of electroluminescent polymer, Colvin et al. and Dabbousi et al. have constructed a hybrid organic/inorganic electroluminescent device and realized electroluminescence (EL) from CdSe nanocrystals.
ZnS is a semiconductor the photoluminescence (PL) and EL of which have been studied to a large extent. Previous studies have addressed the preparation of semiconductor nanocrystals in a wide variety of matrices, but it has proved difficult to control particle size and also to obtain optical quality films. Polymers are expected to provide good mechanical and optical properties, conferring high kinetic stability on nanometer sized semiconductor particles. In this paper, ZnS nanocrystals have been synthesized in polymer matrices. Absorption spectrum and small angle x ray scattering studies showed that the size of ZnS nanocrystals was about 3.0 nm. The electron diffraction results identified the ZnS nanocrystals to be of the hexagonal structure. As the ZnS nanocrystals/polymer composite (ZnSP) possess excellent film processability, one can easily obtain good optical quality thin film of the composite by spin-coating method. ZnSP as emitter was used to fabricate a single layer structure electroluminescent device which has low turn on voltage (4 V) and the blue EL was observed at room temperature. When ZnSP was doped with an organic hole transport material, a violet-blue EL could be observed.
2:30 PM H7.5
ELECTRONICALLY EXCITED STATE PROPERTIES OF NEW ORGANIC -CONJUGATED MATERIALS, Erwin Thiel, Univ GH Siegen, Dept of Physical Chemistry, Siegen, GERMANY.
For many applications of organic -conjugated materials (e.g., electroluminescence, photovoltaic), a high photostability is required. This condition can be fulfilled by using compounds distinguished by a low probability for population of photochemically active states (long living excited triplet state). The exact knowledge of the excited state properties of organic dye molecules is important not only for photochemistry, but also for analyzing charge transfer reactions and luminescence efficiency. A new and highly sensitive method to measure transient absorption was developed. This method was employed to investigate the kinetics of electronically excited states of commercially available and new rhodamine and other structurally close related dyes. Certain compounds exhibit a quantum yield for triplet state population as low as 10. In addition, intermolecular excited state interaction processes were studied in detail. It was found that the quantum yield for triplet state population, triplet state lifetime, and quantum yield for fluorescence can be specifically influenced by the presence of different additives in the dye solution.
2:45 PM H7.6
MAGNETIC SUSCEPTIBILITY, ESR AND MAGNETOTRANSPORT STUDIES IN SOME POLYANILINE-PMMA BLENDS, Duraiswami Srinivasan, Gopala Rangarajan, T. Natarajan, Indian Inst of Technology, Dept of Physics, Madras, INDIA; Anasuya Raghunathan, Wichita State Univ, Dept of Physics, Wichita, KS; S. V. Subramanyam, S. V. Bhat, Indian Inst of Science, Dept of Physics, Bangalore, INDIA.
'Versicon' polyaniline and its blends with PMMA were studied. It is known that blending with PMMA upto 67 by weight improves not only the mechanical properties but also the electrical conductivity, even down to very low temperatures, though the mechanism of this remains yet to be understood. Magnetic susceptibility measurements down to 4.2K were performed using a SQUID magnetometer and it is observed that the unblended polyaniline exhibits almost exclusively a Pauli-like behaviour. This agrees with our ESR observation of a narrow Dysonian line with a line width of 2.4 Gauss and an asymmetry ratio 2.9. Blending with PMMA causes the Pauli like contribution to the magnetic susceptibility to decrease and this is also accompanied by corresponding changes in the ESR spectrum.
Magnetoconductivity measurements down to 1.8K in fields upto 5.5 Tesla in both blends show the following general features: 1) a decreases in magnetoconductivity with an increase in weight of polyaniline, 2) a decrease in magnetoconductivity with an increase in temperature and 3) a H variation at high fields and a H variation at low fields. These observations will be discussed interms of electron-electron interaction and localization.
3:30 PM H7.7
EVOLUTION OF THE OPTICAL PROPERTIES OF CONJUGATED POLYMERS FOLLOWING METAL DEPOSITION, E. E. Ettedgui, G. T. Davis, NIST, Polymers Div, Gaithersburg, MD; B. Hu, F. E. Karasz, Univ of Massachusetts, Dept of Polymer Science & Engr, Amherst, MA.
Because the polymer films used in LEDs are on the order of 1 000 Å thick, properties of the interfaces of conjugated polymers and the electrodes play an important role in the behavior of these devices. Previous studies have shown that metal deposition on conjugated polymers leads to the formation of an interfacial region with electronic states that may differ significantly from those found in the bulk materials. Changes in the photoluminescence (PL) and absorption properties of conjugated-nonconjugated block copolymers following the deposition of metal differ with the metal species employed. Following the deposition of 20 Å Au, the PL intensity of POPV3P (a green emitting block copolymer) decreases by 40% and a broad absorption feature centered near 630 nm arises. The PL and absorption spectrum of the polymer recover after the removal of Au. Al deposition, on the other hand, does not appear to significantly affect the PL intensity or the absorption spectrum.
3:45 PM H7.8
TIME RESOLVED PHOTOLUMINESCENCE OF LADDER-TYPE POLY(PARA-PHENYLENE), Wilhelm Graupner, Thomas Jost, Stefan Tasch, Guenther Leising, Technical Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA; M. Graupner, A. Hermetter, Technische Univ Graz, Inst fuer Biochemie und Lebensmittelchemie, Graz, AUSTRIA.
The conjugated ladder-type poly(-phenylene) (LPPP) shows blue photoluminescence and electroluminescence emission with high quantum yields. A high degree of intrachain order and low concentration of electronic defects allows to produce excellent optoelectronic devices from these materials. We discuss time-resolved photoluminescence experiments performed on neat films as well as liquid and solid solutions of LPPP. The influence of sample preparation, molecular environment, and sample treatment is quantified by combining time-resolved experiments with cw-photoluminescence and infrared spectroscopy.
4:00 PM H7.9
ELECTRONIC CONDUCTION IN ORGANICALLY LINKED TWO-DIMENSIONAL ARRAYS OF NANOMETER SIZE METAL CLUSTERS, Venkat R. Kolagunta, David B. Janes, Purdue Univ, Dept of E&CE, West Lafayette, IN; Jeffery Bielefeld, Ronald P. Andres, Purdue Univ, Dept of Chemical Engr, West Lafayette, IN; Jason I. Henderson, Clifford P. Kubiak, Purdue Univ, Dept of Chemistry, West Lafayette, IN.
An experimental study of electronic conduction through two-dimensional arrays of organically coupled, nanometer size, single crystal, metal clusters is presented. A self-assembly method for fabricating these two-dimensional superlattices using uniform metal clusters formed in a gas aggregation source and captured in a colloidal suspension has been developed. This study focuses on two-dimensional networks of 4nm gold clusters. A close packed array with inter cluster spacing of about 1nm is self-assembled from the colloidal suspension on to a TEM transparent sustrate between two lithographically defined gold contacts. The nearest neighbour clusters are then covalently linked using either a di-isonitrile or di-thiol cojugated molecule of length 1-2nm. The use of different linking molecules allows control of the inter-cluster conduction properties. This is indicated by distinct changes in low field conduction at room temperature of these cluster networks upon linking. The in-plane conduction, characterized over the temperature range of 80 - 300 K, shows an exponentially activated temperature dependence with an activation energy that can be correlated to the capacitance of the metal cluster in a two-dimensional array. The magnitude of the measured activation energy and conductance in the high temperature limit can be explained using a model incorporating single-electron charging (Coulomb) effects. Good agreement is found between the resistance and activation energy inferred from the measured data and those obtained from theoretical models.
4:15 PM H7.10
OPTICAL SPECTROSCOPY OF CHARGED EXCITATIONS AND FIELD-INDUCED LUMINESCENCE QUENCHING IN ORGANIC LIGHT EMITTING DEVICES, Thomas Jost, Technical Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA; G. Meinhardt, Technische Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA; Wilhelm Graupner, Stefan Tasch, Farideh Meghdadi, Guenther Leising, Technical Univ Graz, Inst for Solid State Physics, Graz, AUSTRIA.
Ladder-type poly(paraphenylene) (LPPP) and para-hexaphenyl (PHP) show blue photoluminescence (PL) and electroluminescence emission with high quantum yields, a high degree of interchain order, and low concentration of electronic defects. We study the physical properties of charged and neutral states in light-emitting devices made of these materials under operation by PL and charge-induced absorption spectroscopy. Pure field effects are studied in metal(M)/semiconductor(S)/insulator/metal devices, where the insulating layer prevents the current flow, whereas in M/S/M structures, we observe both field- and charge-induced effects. The dominant feature in the charge-induced absorption spectrum in the LPPP is attributed to the transition of the singly charged polaron, which is also observed in the photoinduced absorption and doping experiments. Under applied electric fields we observe quenching of the PL, which can be described within the framework of exciton-dissociation. The exciton binding-energy obtained from this experiment is compared to results from electro-modulation absorption spectroscopy.
4:30 PM H7.11
CAPACITANCE-VOLTAGE CHARACTERISTICS OF ELECTROLUMINESCENT CELL WITH 8-HYDROXYQUINOLINE ALUMINUM, Ligong Zhang, Changchun Inst of Physics, Chinese Academy of Sciences, Jilin, CHINA; Dapeng Jiang, Xieyan Lu, Xinguang Ren, Ande Lu, Jinsan Yuan, Changchun Inst of Physics, Changchun, CHINA.
Studies on organic electroluminescent (EL) cells using 8-hydroxyquinoline aluminum (Alq) as emitting material or electron transport material have been attracting wide attention in recent years. Capacitance-voltage measurement of EL cell can provide an approach to the distribution of space charge and built-in potential in cell. The EL cell in the present measurement consists of a single Alq layer as organic layer, magnesium/silver double layers as super electrode, and ITO as bottom electrode. The cell emits green-blue light at more than 17 voltage. Capacitance was measured under 1 MHz.
At forward bias (ITO on anode), the capacitance of the cell doesn't change with bias until bias exceeds 20 v, and the capacitance nonlinearly decreases with bias at more than 20 v. At reversed bias, the capacitance reduces quickly when the bias is less than 1 v, and then it slowly increases with bias. On the basis of the result of capacitance-voltage characteristics of the cell, it is thought that at lest two space charge zones exist in organic layer, and the distribution of space charges in the two zones is very different. In other words, two built-in potentials are built at the interfaces of organic layer, and they have different distribution function from each other.
4:45 PM H7.12
MOLECULAR BEAM DEPOSITED DIHEXYL-HEXATHIOPHENE (DH6T) BASED ORGANIC FIELD EFFECT TRANSISTORS (OFET) USING SiO AND VARIOUS POLYMERS AS GATE INSULATORS, Christos D. Dimitrakopoulus, Bruce Furman, Teresita G. Graham, Sampath Purushothaman, IBM T.J. Watson Research Ctr, Yorktown Heights, NY.
DH6T deposited in a UHV molecular beam deposition system has been used as a semiconducting layer in OFET devices in a configuration similar to the one used previously by Garnier et al. Polymers including various polyimides, parylene, BCB and PMMA have been used as gate insulators. Thermally grown SiO has also been used as gate insulator for comparison with the polymeric insulators. The effect of different deposition conditions of the DH6T layer on field effect mobility has been studied as well as effect of different treatments of the insulator surface. DH6T and insulator surfaces have been characterized by AFM and correlations to the transport properties have been made.
Mobility values up to 0.09 cm V sec have been obtained. Current modulation varied from 10 to more than 10.
5:00 PM H7.13
ORGANIC LED DEVICES STUDIED WITH ION SCATTERING: FILM UNIFORMITY, DEGRADATION AND METAL/ORGANIC INTERFACES, Chrysostomus H.M. Maree, Vanderbilt Univ, Dept of Physics & Astronomy, Nashville, TN; Diane Pedersen, Robert A. Weller, Vanderbilt Univ, Dept of Applied & Engr Sciences, Nashville, TN; Khashayar Pakbaz, Howard W. H. Lee, Lawrence Livermore National Laboratory, Photonics Group, Livermore, CA; Leonard C. Feldman, Vanderbilt Univ, Dept of Physics & Astronomy, Nashville, TN.
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 materials issues using 0.2-2.0 MeV ion scattering for material analysis, correlated to electrical and optical characterization of the devices Typical systems consisted of heterojunction devices of evaporated , -diphenyl , -bis(3-methylphenyl)-1,1-biphenyl 4,4 diamine (TPD) and tris-(8-hydroxyquinoline) aluminum (Alq) as well as spin coated films of poly(2-methoxy,5-(2-ethyl hexoxy)-1,4-phenylene-vinylene (MEH-PPV) sandwiched between In (ITO) and Al electrodes. In some cases a novel liquid Hg probe contact was used, providing a convenient microprobe electrode. A careful study of ion beam induced effects revealed some organic film degradations but not so severe as to inhibit meaningful measurements. The accurate ion scattering coverage (in atoms/cm) is correlated with profilometric and optical spectroscopic measurements, yielding accurate densities and absorption coefficients of the organic films. Subnanometer variations in organic film thickness can be resolved. RBS has been successfully applied for the study of the interfaces of both the ITO electrode as well as the metal electrodes. Our results indicate incorporation of aluminum into the organic layer upon failure of the devices.
SESSION H8: POSTER SESSION
Chair: Qibing Pei
Wednesday Evening, April 2, 1997
ORGANIC ELECTROLUMINESCENCE IN LIQUID PHASE, Pavel V. Jaguiro, JV "Belaya Vezha", Minsk, BYELARUS.
Theoretical and experimental investigations of organic light-emitting thin-liquid-film devices are submitted. Liquid layers (0.3-30 microns thickness) of organic dyes solutions in aprotonic solvents are used as active media. The basic peculiarities of recombination electroluminescence in a liquid phase are considered: electrochemical generation of charge carriers, hydrodynamic carry, formation of temporary associates. The characteristics of experimental devices, demonstrating an opportunity of creations high efficiency, large area and high spatial resolution displays, are brought. Liquid organic light-emitting devices are compared with solid-state ones, new opportunities and arising restrictions connected with the liquid phase are analyzed. Electrochemical degradation process for organic dye 'rubrene', connected with presence of water traces in the solution, is described. Apparently the similar mechanism presents in solid state organic devices.
PREPARATION OF AZOBENZENE DENDRITIC MACROMOLECULES AND THEIR OPTICAL PROPERTIES, Shiyoshi Yokoyama, Tatuo Nakahama, Shinro Mashiko, Communications Research Laboratory, Kobe, JAPAN.
Artificial preparation of three dimensional organizations with desired structures and physical properties has aroused much interest recently. Dendritic macromolecules, also called "dendrimers", are novel structures that have been proposed as important in macromolecular design. Dendrimers display a treelike architecture having highly branched chains and many exterior end groups. During the last decade there has been an explosive growth in studies of new types of dendrimers because of their unique structural properties and potential utility. Through judicious choice of building blocks and functional groups, one can precisely modify the physical and chemical properties of dendrimers.
In this study, we have prepared optically functional dendrimers having azobenzene branching units. Repetitive cycles of coupling reactions of multifunctional azobenzene monomers gave dendritic macromolecules called "wedges" and "dendrimers" with first to third generation. These products are designed to incorporate hyrdophobic and polar parts in order to prepare Langmuir-Blodgett (LB) films. Wedges have an unsymmetric architecture with a polar focal point and aliphatic exterior groups. While, dendrimers are prepared by coupling wedges with a polyfunctional core at a focal point, resulting in a symmetrically functionalized structure. Atomic force microscopy (AFM), polarized absorption spectroscopy, and second harmonic generation (SHG) measurements provided the structural properties of the LB films. The topological profiles in AFM images showed that growth of branching chains in terms of size increased as a function of the generation. In absorption spectroscopy and SHG measurements, results confirmed that the quantitative direction of the dipole moment in wedges was unsymmetric along the branching progression, while dendrimers had a centrocymmetric structure, where the branching progression was centered at an interior core.
POLYTHIOPHENE MEETS C : ELECTRICAL AND STRUCTURAL CHARACTERISATION OF NOVEL PHOTODIODE MATERIALS, Lucimara S. Roman, Teketel Yohannes, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN; M. R. Andersson, Chalmers Univ of Technology, Dept of Organic Chem & Pol Techn, Gottenborg, SWEDEN; Olle Inganas, Linkoping Univ, Dept of Physics, Linkoping, SWEDEN.
Fast photoinduced electron transfer has been reported in studies of conjugated polymer blends with C. This has been utilized in photovoltaic cells based on C / semiconductor polymer blends, which show a significant improvement of the carrier collection and energy conversion efficiencies. In this work we present a study made on thiophene polymers in photodiodes. These polymers may be expected to show enhanced photochemical stability, compared to polymers based on the stilbene units. Blending C at several concentrations into poly(thiophenes) of different band gaps gives us novel materials.
The photovoltaic, electrical and structural properties of poly (3-(4-octyphenyl)-2,2-bithiophene) PTOPT, poly (3-(2,5-octylbiphenyl)-thiophene) PDOPT and poly (3-cyclohexythiophene) PCHT polymers have been investigated using solid state cells in sandwich structure, with Al / active layer / ITO or poly (3,4-ethylenedioxythiophene) PEDOT. The active layer is prepared with different concentrations of C in the blend. Current versus Voltage characteristics, open-circuit voltage and short-circuit current dependence on light intensity and time have been measured. The structural properties have been studied by Scanning Force Microscopy, making it possible to compare photoelectrical and morphological properties of these blends.
BLUE LIGHT-EMITTING DIODES MADE OF MOLECULAR DOPED POLYMER AND ITS USE FOR LARGE AREA ALPHANUMERIC DISPLAYS, Baijun Chen, Haiyan An, Jilin Univ, Dept of Electronic Engr, Changchun, CHINA; Shiyong Liu, Jilin Univ, Dept of Electrical Engr, Changchun, CHINA.
In the past ten to fifteen years, mang electronic and optoelectronic devices consisting of almost all organic/polymeric materials have been demonstrated. Due to their low cost and ease of fabrication, these devices have the potential application to full color large area flat panel displays. Especially, blue emission is very important for the multicolor displays application. Compared with inorganic light-emitting diodes (LEDs) or electroluminescent devices (ELs), blue emission in particular can be obtained at high brightness levels without difficulty in organic/polymeric light-emitting diodes.
In this letter, we report that low operating voltage molecular doped polymer (MDP) LEDs were constructed by using air-stable Al as cathode, a typical conjugated polymer poly(N-vinylcarbazole) (PVK) doped with 1,1,4,4-terapheny 1-1,3-butadiene (TPB), functions as a hole transporting and emitting layer, a layer of 2-(4-biphenylyl)-5-(4-terbutypheny)-1-3,4 oxadiazole (PBD) as hole-blocking layer, and a layer of tris(8 quinlinolate)aluminum(Alq) as an electron transporting layer exhibits high luminance efficiency and significant durability.
Blue emission has been observed from the device at room temperature with the DC bias voltage of 4 v. The EL peak centers at 465 nm and blue electroluminance of over 1000 cd/m has also been observed at about 15 v. Some blue large area alphanumeric displays have also been fabricated.
OPTICAL NONLINEAR EMISSION FROM VAPOR DEPOSITED 8-HYDROXYQUINOLINE ALUMINUM/DIAMINE DERIVATIVE SUPERLATTICE STRUCTURE, Yutaka Ohmori, Norio Tada, Satoshi Tatsuhara, Yoshitaka Kurosaka, Akihiko Fujii, Osaka Univ, Dept of Electronics, Osaka, JAPAN; Katsumi Yoshino, Osaka Univ, Dept of Electronic Engr, Osaka, JAPAN.
Organic superlattice structure consisting of 8-hydroxyquinoline aluminum (Alq)/diamine derivative (TPD) exhibits emission from both Alq3 and TPD excited by pulsed THG of YAG laser light (355 nm). The emission intensity from Alq increases as decreasing the layer thickness of Alq compared with the superlattice structure which have the same total layer thickness, under the low excitation condition (below 1 kW/cm). The emission intensity from both Alq and TPD increases superlinearly as increasing the excitation intensity. However, the emission intensity ratio of Alq to TPD decreases to reach a certain value as increasing excitation power intensity. The intensity ratio of Alq to TPD reaches to the same level under high excitation conditions (above 4-5 kW/cm).
The same phenomena is observed in Alq doped poly(3 alkylthiophene) (P3AT). The intensity ratio of P3AT to Alq decreases as increasing the excitation power intensity and reaches to a certain value.
In both cases, under the weak excitation conditions, the emission from the material which has a small energy bandgap shows the strong emission compared with the wide bandgap material. The nonlinear emission from the superlattice structure and the dye dispersed polymer material has been discussed taking the energy transfer from TPD to Alq molecules or Alq to P3AT, under the high excitation conditions into consideration.
VISIBLE-INFRARED PROPERTIES OF REGIOREGULAR POLY(3-ALKYLTHIOPHENES), Pierrick Buvat, Patrick Hourquebie, CEA, Centre d'Etudes du Ripault, Monts, FRANCE.
Regiorandom and regioregular head-to-tail coupled poly(3 butylthiophenes), poly(3-octylthiophenes) and poly (3-dodecythiophenes) have been synthesized by direct oxidation of 3-n-alkylthiophenes using FeCl as oxidant and by regiospecific dehalogenation of 2-bromo-3 n-alkylthiophenes. NMR characterizations indicated 70 percent of head to-tail couplings in the polymers synthesized by FeCl route, and more than 99 percent for regioselective polymerizations. The results of both the solution and the solid state UV-Vis spectroscopic studies showed that all the regioregular poly(3-alkylthiophenes) had a lower transition energy, characteristic of a longer conjugation length. A series of visible-near IR spectra of chloloform solution of FeCl doped regioregular P3ATs is reported and shows an increase of the maximum absorption energy of doped P3ATs as a function of aklyl side chain, I.e., longer conjugation lengths are observed in the P3AT possessing the longer alkyl side chain. Moreover, the effect of solvent on electronic spectra is dramatic. The spectrum in a polar solvent, nitrobenzene, is significantly shifted toward low energy compared with spectra measured in relatively nonpolar solvent, such as dichloromethane, chloroform or toluene. Even after complete removal of the solvent, the more expended coil conformation persists in the cast film since infrared reflectance feature is more metal-like. This study allows us to generalize the concept of secondary dopant to poly(3 alkylthiophene)/FeCl/nitrobenzene systems. Finally, reflectance spectra of regiorandom and regioregular P3ATs are compared. They are the greatest ever published for P3ATs and are comparable to highly conductive polyaniline or polypyrrole reflectances.
SELECTIVE ACETYLENE POLYMERIZATION OF THIENYLACETYLENES, Ben Zhong Tang, Wan Hong Poon, Han Peng, Hong Kong Univ Sci & Tech, Dept of Chemistry, Kowloon, HONG KONG; Henry N.C. Wong, Xin-Shan Ye, Chinese Univ of Hong Kong, Dept of Chemistry, New Territories, HONG KONG; Takashi Monde, Neos Co. Ltd, Central Res Lab.
Polyacetylene and polythiophene are two prototypical conjugating polymers and their electrical conductivity, photoconductivity, photo- and electroluminescence, and nonlinear optical properties have attracted much attention. We are interested in synthesizing novel conjugating polymers containing both the acetylene and thiophene units and have studied polymerization behavior of thienylacetylenes or ethynylthiophenes. WCl6 catalyzed polymerization of 4-bromo-3-thienylacetylene in toluene at 80C for 24 h gave a mixture of soluble polymers and insoluble crosslinking products, with the soluble part having multi-model GPC peaks with a polydispersity index (PI) as high as 77. Similarly high PI was obtained when MoCl was used as a polymerization catalyst. The unusually high PIs were believed to be due to the uncontrolled simultaneous acetylene and thiophene polymerizations catalyzed by the same transition-metal halides. Because the thiophene polymerization is via an oxidation coupling mechanism, we tried to depress the thiophene polymerization by reducing the oxidation power of the transition-metal catalysts. When we added a weak reducing agent of PhSiH to the MoCl catalyst system, a completely soluble poly(thienylacetylene) with a high molecular weight (M 584000) and a narrow molecular weight distribution (PI 1.8) was obtained in quantitative yield (100). Similar results were obtained in the case of 4-trimethylsilyl-3 thienylacetylene; that is, use of the cocatalyst initiated the selective acetylene polymerization. Controlled crosslinking reactions were effected by FeCl-catalyzed further polymerizations of the thiophene units in the poly(thienylacetylenes).
STUDY ON ELECTRICAL PROPERTIES OF POLYNITROBENZOL IN POLY (N-VINYLCARBAZOLE), Jingsong Huang, Haiyan An, Jilin Univ, Dept of Electronic Engr, Jilin, CHINA; Baijun Cheng, Jilin Univ, National Integrated Optoelectronics Lab, Changchun, CHINA; Jingying Hou, Jilin Univ, Dept of Electronic Engr, Changchun, CHINA; Zhiyuan Xie, Jilin Univ, National Integrated Optoelectronics Lab, Changchun, CHINA; Shiyong Liu, Jilin Univ, Dept of Electrical Engr, Changchun, CHINA; Haipeng Zheng, Ruifeng Zhang, Jiacong Shen, Jilin Univ, Key Lab for Supermolecular Structure and Spectroscopy, Changchun, CHINA.
A number of investigations about the electronic properties of conducting polymers have been undertaken in recent twenty years. Some polymeric semiconductors have also been attracting considerable attention owing to their remarkable electrical and optical properties. And their conductivity increases by more than several orders through the charge transfer with many electron acceptors or donors to form -type or type metallic polymers, respectively. In this paper, we have used a new soluble polyphenylene (PP) derivative-polynitrobenzol (PNB) and a matrix polymer poly(N-vinylcarbazole) (PVK) to fabricate two types of devices metal/polymer/metal and metal/polymer blend/metal. PNB is one of the polynitro electron-acceptor compounds, which are easy to accept electron and transport it. Moreover, PNB is a conjugated polynitro polymer, so electron can not only hop from one molecule to another molecule, but also flow along the main chain of PNB in an external electric field. It is demonstrated that its electron mobility is larger than that of low molecular nitro compounds such as 2,4,7,trinitro-9-fluorenone (TNF). PNB has also an obvious advantage in that it is insensitive to air in contrast to other (semi)conducting polymers. We have studied the interaction of two kinds of polymers in the blends. We found that PNB could form charge-transfer complex with PVK more effectively than TNF. And this complex changes largely the original electrical properties of PVK. The PNB/PVK blend turns its semiconductor into conducting characteristics with increasing the molar-ratio of PNB/PVK. It seems that some polymer itself can be used as a dopant to improve the electrical properties of other polymers, which provides a new way to study various kinds of complex in this field.
ROOM TEMPERATURE MOLECULAR METALORGANIC SINGLE-ELECTRON TUNNELING TRANSISTOR, Eugene S. Soldatov, Moscow State Univ, Dept of Biophysics, Moscow, RUSSIA; Vladimir V. Khanin, Artem S. Trifonov, Moscow State Univ, Physics Faculty, Moscow, RUSSIA; Serge P. Gubin, Russian Academy of Sciences, Inst of General & Inorganic Chem, Moscow, RUSSIA; Serge A. Yakovenko, Gennady B. Khomutov, Moscow State Univ, Dept of Biophysics, Moscow, RUSSIA.
The molecular single-electron tunneling (SET) transistor was demonstrated for the first time and it was made at room temperature, i.e., three orders higher than for traditional thin film SET systems. Characteristics were stable and reproducible. Control characteristics (dependence of the tunnel current It on the external gate voltage V) I) have shown the clear periodicity. Charge sensitivity of the systems was about 5e/Hz, which is five orders higher than sensitivities of the best commercial electrometers currently available. The possibilities of application of such systems for design of the analog and digital electronic devices as well as various sensors are discussed. Tunnel systems was made on the base of the Langmuir-Blodgett (LB) multicomponent organic monolayers with incorporated molecules of various metalorganic clusters (carborane clusters, Pt, Fe, and other). The influence of consistence and structure of these films on the shape and parameters of tunnel characteristics of controlled SET double junction (single-molecule SET transistor) systems measured by STM as well as controlled planar multijunction SET systems are investigated in wide range of cluster concentrations and deposition parameters at the operating temperatures in the range 4,2-300 K.
TRIPLET-TRIPLET EXTINCTION COEFFICIENTS OF RHODAMINE DYES IN POLYVINYL ALCOHOL THIN FILMS, Dimitrios Peros, Univ GH Siegen, Inst fur Halbleiterelektronik, Siegen, GERMANY; Erwin Thiel, Karl-Heinz Drexhage, Univ GH Siegen, Dept of Physical Chemistry, Siegen, GERMANY.
Optical characterization of -conjugated organic compounds (e.g., dyes) includes triplet-triplet (T-T) absorption studies which are important for dye engineering. This work determines the T-T extinction coefficients () of commercially available and new rhodamine dyes which are dissolved in a rigid matrix. These dyes are highly fluorescent and have strong absorption bands in the visible region. The compounds are embedded in the rigid matrix for prolonging the triplet lifetime to fraction of seconds. The photostability of the dyes under such conditions is also considered. A cw Kr-ion laser beam is used to excite the molecules into the singlet (S) state. Due to intersystem crossing, the population in the lowest triplet state (T) increases considerably. A second beam which monitors the optical density (OD) of T is provided from a cw dye laser. The observed OD-changes are quantitatively described by solving the rate equations that govern the populations of the energy levels involved. By recording the OD-changes under steady-state conditions as a function of the excitation intensity values of can be determined.
DIELECTRIC MEASUREMENT AND ANALYSIS OF POLY(3-ALKYLTHIOPHENE) AT HIGH FREQUENCIES, Mijan Miah, Peter Foot, J. S. Ghotra, Kingston Univ, School of Applied Chemistry, Surrey, UNITED KINGDOM; I. Youngs, DRA Farnbouough, Structural Materials Centre, Hampshire, UNITED KINGDOM.
The dielectric properties of polypyrrole doped to a range of concentrations has previously been well characterized at microwave frequencies [1,2]. In this present work, the variation in the complex permittivity at room temperature is presented over a wide range of dopant concentrations to study the impact on high-frequency conductivity. In contrast to previous studies on polypyrrole, which measured either electrochemically prepared films or powder compacts, the use of poly(3-alkylthiophene) enables the investigation of thin films prepared by solution casting. The high frequency measurements were undertaken using waveguide transmission line techniques and confirm that the transmission properties are highly dependent on film conductivity.
MOLECULAR CRYSTAL STRUCTURES AND NONLINEAR OPTICAL PROPERTIES IN THE SERIES OF DICYANOVINYL AROMATIC DERIVATIVES, Mikhael Yu. Antipin, New Mexico Highlands Univ, Dept of Chemistry, Las Vegas, NM; Beatriz H. Cardelino, Spelman College, Atlanta Univ Cntr, Atlanta, GA; Ronald D. Clark, New Mexico Highlands Univ, Dept of Physical Scoience, Las Vegas, NM; Mohan Sanghadasa, Univ of Alabama, Dept of Physics, Huntsville, AL; Vladimir N. Nesterov, Inst of Organic-Element Compounds, Moscow, RUSSIA; Melvin Romero, New Mexico Highlands Univ, Dept of Chemistry, Las Vegas, NM.
Organic materials with the high optical nonlinearities are the subject of intensive studies in recent years. The usual way for design of these materials includes theoretical estimation of the optical nonlinearities, chemical synthesis, and experimental studies of the optical properties and molecular and crystal structures of the compounds under investigation. In the present work, we have studied a series of the dimethylamino and methoxy (mono-, di-, and tri-) substituted dicyanovinylbenzenes in order to make conclusions about relationship between their structure (studied by molecular mechanisms calculations and single crystal x-ray analysis) and nonlinear optical properties (estimated by quantum calculations and EFISH measurements in solution). Among the compounds studied are dicyanovinylbenzene, its dimethylamino derivative, - and -methoxy, 2,3- and 2,4-dimethoxy, and 3,4,5-trimethoxy substituted derivatives. All compounds are characterized by the relatively high values of the molecular optical nonlinearities ( and ), but only -methoxy-dicyanovinylbenzene (also known as DIVA) and dimethylamino-dicyanovinylbenzene form acentric crystals (space group P2) with the molecular dipole moment orientations with respect to the polar crystal axes close to the optimal for a manifestation of high nonlinear response. Crystal packing analysis of the x-ray structures studied together with energetic calculations revealed the factors responsible for formation of centrosymmetric/acentric crystals. High resolution x-ray diffraction data were obtained for a single crystal of DIVA and the data were used for analysis of the electron density distribution in this crystal and direct calculation from diffraction data some molecular properties (atomic charges, dipole and higher multipole moments, etc.) responsible for nonlinear characteristics.
THE EFFECTS OF SIDE-CHAIN LENGTH ON THE STRUCTURE AND PROPERTIES OF POLYALKYLTHIOPHENES, Chun-Guey Wu, Yii-Chung Lin, National Central Univ, Dept of Chemistry, Chung-Li, TAIWAN.
Polyalkylthiophene (P3AT) is one of the most interesting conducting polymers due to its characteristic properties, such as solubility for various organic solvents, fusibility at relatively low temperature, gel characteristics, solvatochromism, and thermochromism. 3 Alkylthiophenes (3AT) with side-chain carbon numbers ranging from 6 to 18 were prepared from reacting 3-bromothiophene, with corresponding alkyl granad reagent. Polymerization of 3AT was carried out in CHCl solution using FeCl as an oxidant, or electrochemically polymerized. The regioregularity of P3AT increased as side-chain length increased. Higher structure regioregularity results in smaller bandgap and therefore lower oxidation (doping) potential. The irregular change of electric conductivity vs. side-chain length may be due to the different polymer interchain distance (obtained from the XRD diffraction patterns). Furthermore, polymer films casted from different solvent showed different morphology which depends on alkyl side-chain length. The morphology of P3AT films also affect their bandgap, oxidation potential, as well as electric properties. Modification of physicochemical and charge transport properties of P3AT from various approaches will be discussed.
SYNTHESIS AND CHARACTERIZATION OF BLUE-GREEN-EMITTING 2,5-BIS[2-(4"HEXYLOXYPHENYLENE)-1-ETHENYL]3,4,-DIBUTYL THIOPHENES, Dmitry Voloschenko, Kent State Univ, Liquid Crystal Inst & Chemical Physics Program, Kent, OH; Mingquian He, Univ of Southern California, Loker Hydrocarbo
(Abstract Not Available)