Meetings & Events

spring 1998 logo1998 MRS Spring Meeting & Exhibit

April 13 - 17, 1998 | San Francisco
Meeting Chairs: John A. Emerson, Ronald Gibala, Caroline A. Ross, Leo J. Schowalter









Symposium G—Science and Technology of Organic Electroluminescent Devices

Chairs

Homer Antoniadis 
Hewlett Packard Co 
Palo Alto, CA 94304 
415-857-2386

Donal Bradley
Dept of Physics
Univ of Sheffield
Centre for Molecular Matls
Sheffield, S3 7RH UNITED KINGDOM
44-114-2824575

Ian Campbell 
Los Alamos National Laboratory 
MS D429 
Los Alamos, NM 87545 
505-667-5757

Junji Kido
Graduate School of Engineering
Yamagata Univ
Yamagata, 992 JAPAN
81-238-26-3052

Symposium Support 
*Chemipro Kasei Kaisha, Ltd. 
*Futaba Corporation 
*Hewlett Packard Laboratories 
*IMES Co., Ltd. 
*Los Alamos National Laboratory 
*Office of Naval Research 
*Sanyo Vacuum Industries Co., Ltd. 
*Sumitomo Electric Industries, Ltd. 
*UNIAX Corporation 

* Invited paper

TUTORIAL 

STG: ORGANIC LIGHT-EMITTING DIODES 
Sunday, April 12, 1:00-5:00 p.m. 
Pacific H 
Organic light emitting diode (OLED) devices promise a new and exciting form of emissive display technology. Unlike many of the existing display technologies, OLED's offer a path to low cost, large area emissive displays by virtue of their simple fabrication techniques. This tutorial will be an introductory review to the field of OLED's with emphasis on the organic materials used in this technology. Both types of materials systems, small molecules and polymers, will be covered. The first session of the tutorial will provide a historical overview of OLED's followed by the development of small-molecule OLED's and the second session will be dedicated to polymer LED's. 

Instructors: 
Tetsuo Tsutsui
, Kyushu University 
Bing R. Hsieh, Xerox Corporation 


SESSION G1: NEW MATERIALS FOR OLEDs I 
Chair: Homer Antoniadis 
Monday Morning, April 13, 1998 
Nob Hill D
8:30 AM *G1.1 
POLYMER LIGHT EMITTING DIODES: STATUS AT PHILIPS RESEARCH. Herman F.M. Schoo, Rob C.J.E. Demandt, Jeroen J.M. Vleggaar, Coen T.H. Liedenbaum, Peter van de Weijer and Yvo Croonen, Polymer & Organic Chemistry Department, Philips Research Laboratories, Eindhoven, NETHERLANDS. 

The successful application of polymers as the active layer(s) in organic LEDs depends on several technologies. The performance of a polymer LED is the end product of many steps, and some of these will be addressed in this paper. First of all, the polymer materials have to satisfy a number of criteria, regarding bandgap, photoluminescence efficiency, charge mobility, defect density, processibility, etc.. Every one of these properties can be optimised by manipulating the molecular structure of the polymer, however this has to be done in conjunction with the other properties. Furthermore, modelling of the device characteristics, and the investigation of failure mechanisms can help to optimise materials and device structure. The current status of polymer LED devices at Philips Research will be discussed. 

9:00 AM *G1.2 
HIGHLY EFFICIENT LED BASED ON FLUORENE HOMOPOLYMERS AND COPOLYMERS. Mark Bernius, Michael Inbasekaran, Edmund Woo, Weishi Wu, and Lisa Wujkowski, Central Research & Development, The Dow Chemical Company, Midland, MI. 

Fully conjugated polymers have been prepared from 2,7-disubstituted fluorenes in consistently high molecular weight and purity. The same synthetic methodology has been applied for the preparation of a large number of random, alternating and block copolymers. All polymers are excellent film formers and highly fluorescent. Physical and electronic properties of the polymers can be manipulated by the nature of substituents and monomer structures. This talk describes the properties of some of these polymers and the highly efficient light emitting diodes therefrom. 

9:30 AM G1.3 
POLYMER LIGHT EMITTING DIODES: NEW MATERIALS AND DEVICES. Zhenan Bao, Zhonghua Peng, Ananth Dodabalapur, Mary E. Galvin, Bell Laboratories, Lucent Technologies, Murray Hill, NJ. 

Single layer polymer LEDs are ideal candidates for practical applications due to their easy processing conditions. However, low quantum efficiency of light generation is often obtained due to imbalanced charge injection and transport of holes and electrons. In this paper we report new conjugated polymers with electron deficient oxadiazole side-chains. These polymers have shown an order of magnitude increase in electroluminescence efficiencies and better charge injection properties compared to their corresponding conjugated backbone polymers. Better LED performance is obtained with polymers having higher concentration of electron-transporting molecules. In addition, other new polymers with improved quantum efficiencies will be presented. Finally, issues regarding integration of polymer transistors with organic LEDs will be discussed. 

9:45 AM G1.4 
STRUCTURE OF MOLECULAR SELF-ASSEMBLED ZINC-BISQUINOLINE ELECTRLUMINESCENT FILMS. D. Laurence Thomsen III, Keith A. Higginson, Thomas Phely-Bobin, Fotios Papadimitrakopoulos*, University of Connecticut, Institute of Materials Science, Polymer Science Program, Chemistry Department, Storrs, CT. 

The recent discovery of electroluminescence in molecular self-assemblies of zinc-bisquinoline has initiated furher structural investigation in the structure and order of such assemblies for advancing the understanding of these semiconductive films. At early film growth the coverage and density of zinc bisquinoline self-assemblies was monitored by spectroscopic ellipsometry and quartz crystal microbalances (QCM). Non-linear effects for both density and coverage were observed at early stages of film growth near the substrate surfaces. AFM was performed on zinc bisquinoline self-assemblies to probe the coverage at early growth, also. Linearity in film growth after 20 nm was observed by both ellipsometry and QCM with the pace of self-assembly being approximately 2 repeat units per dipping sequence, attributed to partial reactance, diffusion, and swelling of the grown layers. The density of zinc bisquinoline films was estimated to be 1.4-1.5 g/cm3 by QCM measurements, and a refractive index of 1.7 was determined from an ellipsometric trajectory plot of films ranging in thicknesses from 0 - 128 nm. Optimization of electron and hole injection in these devices have further enhanced the device behavior and longetivity. 

10:30 AM *G1.5 
ORGANIC ELECTROLUMINESCENT DEVICES USING NOVEL FAMILIES OF COLOR-TUNABLE EMITTING MATERIALS. Yasuhiko Shirota, Department of Applied Chemistry, Faculty of Engineering, Osaka University, Osaka , JAPAN. novel family of amorphous molecular materials containing an oligothiophene moiety has been designed and synthesized. They readily form stable amorphous glasses with high glass-transition temperatures, when the melt samples are cooled. They form uniform amorphous films by vacuum deposition. Both the single-layer and the double-layer devices consisting of the emitting layer and the electron-transport layer of tris(8-quinolinolato)aluminum were fabricated and their performances examined. Turning of the emitting color from light blue to orange was achieved simply by varying the conjugation length of the oligothiophene moiety. The color of light emitted by organic electroluminescent devices can also be tuned by using the emission from the exciplex formed at the interface between the electron-transport layer and the hole-transport layer. Tuning of the emitting color was achieved in the double-layer organic electroluminescent device using 1,3,5-tris(4-tert-butylphenyl-1,3,4-oxadiazolyl)benzene as the electron-transport material and a series of novel hole-transport materials with varying ionization potentials. 

11:00 AM *G1.6 
NOVEL BLUE EMITTING PHENYLANTHRACENE-DIMERS FOR ORGANIC LIGHT-EMITTING DIODES. Tetsushi Inoue, Kenryo Namba, Kenji Nakaya, R&D Center, TDK Corporation, Chiba, JAPAN. 

We report on newly synthesized two type of phenylanthracene-dimers(type 1; 10,10-diaryl-9,9-bianthryl and type2; 9,9,10,10-tetraaryl-2,2-bianthryl) and their application to organic LED. Type 1 was prepared by the two-step reaction of 9,9-bianthrone and aryllithium, and type 2 was prepared by the coupling reaction of 2-chloro-9,10-diarylanthracene using Ni(0) complex. Both of the dimers indicated strong blue fluorescence and high glass transition temperature. Four-layer devices was fabricated by vacuum deposition. Blue to blue-green emission with luminance of over 20,000cdm2 was observed. The characteristics of blue emitting devices will be described. 

11:30 AM G1.7 
BLUE ORGANIC LIGHT EMITTING DIODES: INFLUENCE OF DOPING ON EFFICIENCY AND STABILITY. Horst Vestweber, Horst Vestweber, Heike Riel, Walter Riess, IBM Research Division, Zurich Research Laboratory, Rueschlikon, SWITZERLAND. 

Efficient and stable blue organic light emitting diodes (OLEDs) are considered as corner stones for achieving full-color organic electroluminescence displays. In this paper we will give a detailed overview of our studies on blue organic electroluminescence. In our devices we typically use indium tin oxide as the transparent hole injecting electrode, CuPc as the hole injection and buffer layer, aromatic diamines as the hole transport layers, BAlq or DSAs as the emitting layers and Alq as the electron injection and transport layer. In order to enhance the efficiency, the emitting layer was doped with different types of dopants. We investigated the influence of the doping concentration, the individual layer thicknesses and the cathode materials on the luminous efficiency and stability of these devices. Optimization of these parameters leads to highly efficient and stable blue OLEDs. 

11:45 AM G1.8 
DOPED ORGANIC LIGHT EMITTING DIODES (DOLED) HAVING A 650nm-THICK HOLE TRANSPORT LAYER. Chihaya Adachi, Asuka Yamamori, Toshiki Koyama and Yoshio Taniguchi, Shinshu Univ, Dept. of Functional Polymer Science, Ueda, Nagano, JAPAN. 

We have succeeded in fabricating aÅ@novel thick-film organic light emitting diodes having a doped hole transport layer (DHTL). The basic cell structure is anode / DHTL / emitter layer / cathode. The DHTL is composed of a hole transporting polycarbonate polymer, PC-TPD-DEG, and tris(4-bromophenyl)aminium hexachloro antimonate, TBAHA, as a dopant. As an emitter , we used tris(8-hydroxyquinoline) aluminum, Alq. With a 650nm-thick DHTL, the device showed considerable reduction of resistance compared with an anode / non-doped HTL / Alq / cathode device with same HTL thickness. Although the EL quantum efficiency was rather low in the doped device, we could surely increase the efficiency by interposing a thin tetraphenylbendidine, TPB, layer between the DHTL and the Alq emitter layer, keeping low driving voltage. The anode /DHTL(650nm) / TPD(50nm) / Alq(50nm) / cathode showed the luminance of above 4000cd/m2 at 8.5V and 220mA/cm2.

SESSION G2: NEW MATERIALS FOR OLEDs II 
Chair: Junji Kido 
Monday Afternoon, April 13, 1998 
Nob Hill D
1:30 PM *G2.1 
A BLUE ORGANIC LIGHT EMITTING DIODE. Y. Kijima, N. Asai, and S. Tamura, Research Center, Sony Corporation, Yokohama, JAPAN. 

A blue OLED which has a structure like an SH-B type diode has been developed. The blue OLED consists of a hole-injection layer (m-MTDATA), a hole-transporting emissive layer, a hole-blocking layer and an electron-injection layer (Alq3) formed on an ITO anode by vacuum vapor deposition.  -NPD was used for the hole-transporting emissive layer, which has an emission peak at around 460 nm. For the hole-blocking layer, we found that 2, 9-Dimethyl-4, 7-diphenyl-1, 10-phenanthroline (Bathocuproin) is a very effective material. As Bathocuproin has a deep HOMO level, the recombination area is in the -NPD layer at the boundary between -NPD and Bathocuproin. The electroluminescence peak from the new blue OLED is at around 460 nm. The color coordinate in CIE chromaticity is (0.15, 0.16). The blue OLED has a potential of over 10,000 cd/m2 at 9.5 V under DC operation with an AlLi cathode. When this OLED is driven under a 1/100 duty ratio, the peak luminance is over 50,000 cd/m2. The luminous efficiency was 1.1 lm/W at 150 cd/m2. The blue OLED device is also a good green device without the Bathocuproin layer. The green OLED shows electroluminescence from the Alq3layer with luminance of 40,000 cd/m2 at 10 V under DC operation, and the color coordinate in CIE chromaticity is (0.33, 0.55). 

2:00 PM G2.2 
NOVEL DIAMINES AS THERMALLY STABLE HOLE TRANSPORTERS IN ORGANIC LIGHT EMITTING DEVICES. Bryan E. Koene, Douglas E. Loy, Mark E. Thompson, Dept of Chemistry, University of Southern California, Los Angeles, CA; Paul E. Burrows, Stephen R. Forrest, Department of Electrical Engineering, Princeton University, Princeton, NJ. 

One of the major causes of device degradation in organic thin film devices is the thermal instability of the hole transporting layer. In our efforts to design thermally stable hole transporters, as well as to understand how chemical structure relates to Tg, we have synthesized a variety of symmetric and asymmetric biphenyl diamines. The symmetric diamines are typically derivatives of phenyl-benzidines, e.g. NPD = napthyl-phenyl-benzidine. Both a and b versions of NPD (1- and 2- substituted napthyl groups), have been investigated and found have Tg values of 95C. The phenanthrenyl derivative (PPD) has also been prepared and found it to have a Tg of 150C. In addition to these symmetric materials, we also examined a variety of asymmetric hole transporting materials. These amines consist of two different triaryl amines attached at the 4 and 4 positions of a biphenyl group, i.e. Ar1Ar2N-C6H4C6H4-NAr3Ar4. Seven different amines were used to create a family of asymmetric biphenyl diamines, whose Tg values vary from 55C to 125C. The syntheses, thermal properties, and performance of both the symmetric (NPD and PPD) and asymmetric diamines as HTL materials in OLEDs will be discussed.

2:15 PM G2.3 
EFFICIENT BLUE ELECTROLUMINESCENT DEVICES BASED ON DOPED HOLE OR ELECTRON TRANSPORTERS. Charles D. Merritt, Hideyuki Murata and Zakya H. Kafafi, U. S. Naval Research Laboratory, Washington DC. 

Efficient blue electroluminescent devices were fabricated based on multilayered organic nanostructures where the emitter layer consists of highly fluorescent molecules such as perylene or substituted anthracenes dispersed in hole (ex. substituted biphenyl-4,4í -diamines) or electron (ex. oxadiazole derivatives) transporters. Photoluminescence spectra of the molecular composites indicate very efficient energy transfer from host to guest molecules. In addition, it was found that in some cases doping was quite effective in preventing exciplex formation. Hamada et. al previously suggested[1] exciplex formation between a triphenylamine dimer used as a hole transporter and some substituted oxadiazoles employed as electron transporters at the organic-organic interface in electroluminescent devices. The paper discusses the mechanism(s) giving rise to efficient electroluminescence when the host is a hole or electron transporter. 
Reference: 

2:30 PM G2.4 
SATURATED RED ELECTROLUMINESCENCE FROM PHOSPHORESCENT PORPHINE DOPED OLEDS. Mark E. Thompson, Andrei Shoustikov, Yujian You, Department of Chemistry, University of Souterhn California, Los Angeles, CA; Scott Sibley, Goucher College, Baltimore, MD; Marc Baldo, Valdimir Koslov, Paul E. Burrows, Stephen R. Forrest, Department of Electrical Engineering, Princeton University, Princeton, NJ. 

We have previously reported red OLED prepared by doping tetraphenyl porphine (TPP) into Alq3 based OLEDs. We have extended this doping study to other porphine complexes, including phosphorescent metallo porphines, e.g. platinum octaethylporphine, PtOEP. The platinum ion in PtOEP efficiently promotes intersystem crossing to the triplet state and efficient phosphorescence from this complex. The doping levels required fro efficient energy transfer from Alq3 to TPP are low (0.5 - 1.5%). In contrast, when PtOEP doped into Alq3 at 0.5 - 1.5 %, the energy transfer is not efficient and substantial amounts of Alq3 are observed at moderate to high current levels. At higher doping levels the Alq3 is completely quenched and the device gives saturated red emission at all current levels up to device breakdown. The reason that higher doping levels are required for efficient energy transfer is related to the significantly longer lifetime for emission from the phosphorescent transition in PtOEP compared to the fluorescent transition for TPP. We will discuss the color, efficiencies and electrical properties of porphine doped OLEDs, esp. PtOEP doped devices. The emissive lifetimes of these doped materials and how that lifetime effects energy transfer in OLEDs doped with phosphorescent and fluorescent dopants will also be discussed.

SESSION G3: CHARACTERIZATION AND 
FAILURE ANALYSIS 
Chair: James R. Sheats 
Monday Afternoon, April 13, 1998 
Nob Hill D
3:15 PM *G3.1 
ELECTRONIC STATE ALIGNMENT OF ORGANIC MATERIALS DETERMINED BY STM AND STM-EXCITED LUMINESCENCE. S.F. Alvarado, L. Libioulle, and P.F. Seidler, IBM Research Division, Zurich Research Laboratory, Rueschlikon, SWITZERLAND; D.G. Lidzey and D.D.C. Bradley, Dept. of Physics, University of Sheffield, Sheffield, UK. 

The relative alignment of energy levels at organic/electrode interfaces is a determining factor in charge injection and, hence, overall performance of organic light-emitting devices (OLEDs). By using scanning tunneling microscopy and STM-excited luminescence, we have been able to probe the position of the highest occupied and lowest unoccupied molecular orbital (HOMO and LUMO) of OLED materials on metallic and semiconductor surfaces. Specifically, we make use of two threshold phenomena: (1) the tunneling energy for the onset of charge carrier transport within the organic materials and (2) the tunneling energy for the onset of luminescence induced by injecting charge carriers from the tip. In the first case we measure the relative tip height as a function of the tunneling voltage VT at constant current. For VT such that the Fermi level of the tip lies within the HOMO-LUMO gap, tunneling into states of the organic material is not possible, and the tip penetrates the organic thin film until charge can tunnel into the substrate. The threshold bias voltage for penetration marks the onset of charge carrier transport across the organic material and is also approximately the point at which electroluminescence begins. In this manner one can determine the alignment of the molecular levels with repect to the Fermi energy of the substrate. Furthermore the measurements allow us to determine the charge transfer energy gap of the organic materials. Preliminary results show that for evaporable species Alq3 and NPB, the charge transport energy gap is approximately equal to the optical absorption edge, while 1,12-alcoxy-substituted PPV and the polyfluorene PFO appear to have conductance gaps greater than the absorption edge. 

3:45 PM G3.2 
THE EFFECT OF REVERSE BIAS ON DEGRADATION PROCESSES IN ORGANIC EL DEVICES. Masayuki Yahiro, Tetsuo Tsutsui, Department of Materials Science and Technology, Graduate School of Engineering Sciences, Kyushu University, Kasuga, Fukuoka, JAPAN; Dechun Zou, Core Research for Evolutional Science and Technology, Japan Science and Technology Corporation, Kasuga, Fukuoka, JAPAN. 

The mechanism of recoverable degradation in organic EL devices was investigated. It was found that spontaneous recovery occurred after the cessation of continuous driving of organic EL devices. We have found the recovery was enhanced when reverse bias was applied. The recovery associated to electric-field induced recovery is considerably larger than that arising from spontaneous recovery. Both the recovery range and speed depend on the magnitude of the reverse bias as well as the time interval during which it is applied. A large reverse bias gives rise to a fast recovery with a high recovery range. The electric field induced recovery phenomena were observed in both single layer and double layers devices. In addition, all the devices prepared by dry and wet processes showed similar recovery. We propose an internal-field formation model to explain the mechanisms of the degradation and the recovery. Generally, EL devices are expected to contain some kind of ionic impurities. When a forward bias is applied to a device, ionic impurities diffuse towards electrodes. Consequently, an internal electric field in opposite direction to the external field is formed. This internal field reduces the effective electric field for carrier injection. The internal electric field may also arise from permanent dipoles under a strong external electric field. Theses are likely to be one of the causes for degradation of organic EL devices. When the forward bias is released the internal electric field decreases with time due to the depolarization of ions and dipoles. Re-setting the forward bias to its initial value, a higher injection current and luminance can be obtained. If a reverse bias is applied to the device, an internal field in forward direction can be formed. When a forward external bias is applied to the treated device, the effective electric field is expected to be comparable to or stronger than the applied external field. Consequently, a larger current density and luminance can be obtained, and an enhancement of EL intensity is observable. This internal electric-field model for the degradation mechanisms suggests that employing dry processing techniques and the use of extra-pure materials are important. Moreover a pulsed bias with a carefully tuned reverse bias is expected to improve the durability of EL devices. 

4:00 PM G3.3 
TIME-OF-FLIGHT SECONDARY ION MASS SPECTROMETRY ANALYSIS OF THE EFFECTS OF ELECTRICAL STRESS ON POLYMER LEDS. James R. Sheats, Ying-Lan Chang, and Daniel Roitman, Hewlett Packard Laboratories, Palo Alto, CA. 

Electroluminescent polymer devices can now be made with room temperature lifetimes of several thousand hours. Degradation is still an important focus of research, however, since still longer lifetimes are desired and not all colors can be obtained with the same longevity. We have used TOF-SIMS as a primary tool for investigating the chemical consequences of electrical stress in polymers such as PPV and polyfluorenes. The results show, as was surmised, that only very small amounts of chemical change are associated with substantial levels of luminance decay, and give valuable insight into the source of the changes 

4:15 PM G3.4 
INTERFACIAL ELECTRONIC STRUCTURES IN ORGANIC LIGHT EMITTING DIODES. Y.M. Wang, K.W. Wong, S.T. Lee and X.Y. Hou, Department of Physics and Materials Science, City University of Hong Kong, HONG KONG; M.G. Mason and C.W. Tang, Eastman Kodak Company, Rochester, NY; P.S. Chung, Department of Electronic Engineering, City University of Hong Kong, HONG KONG. 

The electronic structures of some organic materials commonly used in organic light emitting diode device and that of the indium tin oxide (ITO) were studied using ultraviolet photoemission spectroscopy (UPS) and Kelvin probe (KP). The organic materials chosen were tris(quinoline-8-hydroxylate)aluminum (Alq3), copper phthalocyanine (CuPc), N,N-dimethylquinacridone (DMQA) and N,N'-bis-(1-naphhyl)-N,N'-diphenyl-1,1'-biphenyl-4,4''-diamine (NPB). Energy diagram of various interfaces involved has been determined. The largest band offset occurs at the conduction band or unoccupied state levels. The band energy alignment at the interfaces can be explained using a vacuum level alignment due to the small difference in the vacuum levels for the organic materials. The effect of surface treatments on the electronic structure of ITO was also investigated. An oxygen plasma treatment followed by a brief annealing was found to provide an efficient way for modifying the surface of ITO. The stability of organic interfaces and the possible interface mixing will be discussed. 

4:30 PM G3.5 
INTER-DIFFUSION AT THE Mg/Alq INTERFACE: AN IN SITU XPS STUDY. Ye Tao, Armin Kundig, Chengzhi Cai, Iris Gamboni, Bert Muller and Peter Gunter, Swiss Federal Institute of Technology, Institute of Quantum Electronics, Nonlinear Optics Laboratory, Zurich, SWITZERLAND. 

Electroluminescent devices based on organic thin films are promising candidates for flat-panel displays. Tri (8-hydroxychinoline) (Alq) is one of the most efficient and stable emitters. While a great deal of efforts has been devoted to improve the bulk properties of Alq-based emitting materials, it is crucial to control and engineer a well defined and stable interface between Alq and metallic contact for efficient and long-lasting devices. Magnesium has been widely used as a cathode material for organic LEDs due to its relatively low work function which reduces the electron injection barrier. It was believed that reactive metals such as Mg would strongly interact with the organic surface and form a sharp interface with negligible diffusion. By using in situ photoelectron emission spectroscopy to follow the formation of Mg/Alq interfaces under ultra-high vacuum, we have found that the first 0.51.0 nm Mg reacts with the Alq surface. With further deposition, Mg diffuses into Alq and reacts with it. A metallic Mg layer is formed after deposition of more than 16 nm Mg on Alq. This result demonstrates, for the first time, that strong inter-diffusion occurs at Mg/Alq interface during cathode fabrication. Furthermore, we have found that the inter-diffusion process is significantly reduced by pre-depositing an ultra-thin (1.0 nm) Ag layer on Alq, and thus a sharp interface between the Mg and Ag/Alq can be fabricated. The influence of reduced Mg diffusion on EL characteristics and device performance will be discussed. 

4:45 PM G3.6 
CHARACTERIZATION OF THE INTERFACE BETWEEN Alq3 AND TPD IN A SERIES OF BILAYER OLEDS. G. Jabbour+, T. Schuerlein#, E. Principe#, R. Schlaf*, P.A. Lee*, N. Peyghambarian+, B. Kippelen+, N.R. Armstrong*, Jeff Anderson* - + = Optical Sciences Center, and * =Department of Chemistry University of Arizona, Tucson, AZ; # =Charles Evans Associates, Redwood City, CA. 

The interface between the hole transport layer (typically a TPD derivative) and the luminescent layer (typically Alq3 or another vacuum compatible lumophore), has been implicated as important in controlling the output efficiency of OLEDs based upon bilayers of these materials. Radical annihilation reactions between the cation radical form of TPD, and the anion radical form of Alq3, have been shown to be sufficiently energetic to produce the singlet emissive state of Alq3, which is responsible for light emission in these devices. Characterization of the interface between TPD and Alq3 layers has been undertaken using AFM, TOF-SIMS and angle-resolved XPS, as a function of deposition conditions for the TPD and Alq3 layers. Control of the apparent roughness of this interfacial region is undertaken by changing substrate temperature and deposition rate, and correlating this roughness with device performance. The combination of AFM, TOF-SIMS and ARXPS allows for the investigation of not only the surface roughness, but also the extent of conformal growth between the two organic layers. Controlling the growth mode of the organic layers allows for an overall reduction in pin-hole defects and leads to device efficiency and stability. 
SESSION G4: ORGANIC LED DISPLAYS 
Chair: Yang Yang 
Tuesday Morning, April 14, 1998 
Nob Hill D
8:30 AM *G4.1 
POLYMER LIGHT-EMITTING DIODE DISPLAYS - DEVICE PERFORMANCE AND APPLICATIONS. I.D. Parker, J. Allen, E. Baggao, P. Bailey, Y. Cao, S. Draeger, B. Gusner, A.J. Heeger, J. Kaminski, C. Knudson, J. Long, B. Nilsson, J. Peltola, R. Pflanzer, M. Raffetto, T. Ronnfeldt, B. Webber, UNIAX Corporation, Santa Barbara, CA. 

Polymer LEDs have now reached a level of performance that is attractive for commercial applications with operating lifetimes in excess of 10,000 hrs.