Proceedings Volume 4800

Organic Light-Emitting Materials and Devices VI

Zakya H. Kafafi, Homer Antoniadis
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Proceedings Volume 4800

Organic Light-Emitting Materials and Devices VI

Zakya H. Kafafi, Homer Antoniadis
View the digital version of this volume at SPIE Digital Libarary.

Volume Details

Date Published: 3 March 2003
Contents: 11 Sessions, 30 Papers, 0 Presentations
Conference: International Symposium on Optical Science and Technology 2002
Volume Number: 4800

Table of Contents

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Table of Contents

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  • Polymers and Light-Emitting Devices
  • Organic Emissive Displays
  • OLEDs for Solid State Lighting
  • Molecular OLEDs Based on Metal Complexes and Doped Layers
  • Carrier Transporting and Luminescent Polymers
  • New Fabrication and Characterization Techniques
  • Spectroscopy and Optical Properties of Luminescent Materials and Devices
  • Carrier Transport and Interfaces in OLEDs I
  • Carrier Transport and Interfaces in OLEDs II
  • Organic TFTs and Memory Elements
  • Posters - Tuesday
Polymers and Light-Emitting Devices
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Status of red, green, and blue light-emitting polymers for passive matrix light emitting displays
Henk J. Bolink, Michael Buechel, Boudewijn Jacobs, et al.
An overview of the requirements for full color passive matrix displays and their implications for the light emitting materials will be presented. Using the performance of light emitting polymers tested in Philips devices the status of the light emitting polymers is reviewed. It will be shown that the performance of light emitting polymers is at the edge of being acceptable for practical applications. Red and green light emitting polymers can already be used for certain monochrome applications. However, for the high-resolution displays used in mobile telecom applications the efficacy for red and the lifetime for green are still somewhat low. Optimization routes for further improvement in terms of efficacies and lifetimes for red and green are identified. The peformance of blue light emitting polymers has rapidly improved over the last year, but the lifetime is still too short for full color applications. Improvement routes for the blue light emitting polymers and its device structure are outlined.
Light-emitting polymer materials for full-color displays
Hubert Spreitzer, Heinrich Becker, Esther Breuning, et al.
In the last few years, industrial research into materials fulfilling the needs of the maturing OLED display industry has intensified considerably. A first generation of polymers (phenyl-PPVs) is now being commercially exploited in first monochrome polymer LED displays. Nevertheless, due to market interest, there is a huge demand for materials for full-color OLED displays. After giving some initial results on our work in this field at last year's SPIE, we will report on the progress in the development of polymers for red, green, and blue emission. Our main focus here lies on the improvement of the properties of various polymers derived from the spiro-bifluorene core. Depending on the color, the main issues vary strongly: Whereas e.g. for BLUE materials, efficiency, color coordinates, and processibility fulfill already commercial demands, operational lifetime still needs to be improved strongly. For RED materials, in contrast, the operational lifetime is already excellent, whereas the efficiency and the driving current still need to be improved. For GREEN acquiring saturated emission, whilst maintaining the other properties (high efficiency, long operational lifetime), is still challenging. Also, we will report on advances in full-color patterning, especially techniques based on Ink-Jet Printing.
Simulation and realization of a 4-inch diagonal monochrome passive-matrix polymer QVGA display
Herbert Lifka, Aad Sempel, Piet Snijder, et al.
To explore the practical size limits of passive-matrix polymer light emitting displays, a 4-inch diagonal monochrome QVGA display was simulated, fabricated and tested. To design this display, a simulation program was developed which takes into account the multiplex rate, aperture ratio of the pixels, parasitic capacitance in the display, series resistance of the anodes and cathodes, and the decrease in efficiency at higher applied voltages. The effects of these parameters on the power consumption will be addressed. In addition, technological aspects of introducing a shunt metal in the pixels will be presented. Finally, the measurements of the fabricated display are compared to the simulation and discussed.
Organic Emissive Displays
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Recent developments in materials and processes for ink-jet printing high-resolution polymer OLED displays
Julian Carter, Anja Wehrum, Mark C. Dowling, et al.
The fabrication of high resolution light emitting polymer (LEP) OLED displays using ink jet printing to deposit the hole conducting and conjugated polymer electroluminescent components has required the development of both printing and ink technology. We review the issues associated with meeting the technology requirement and split these into the areas of ink delivery, getting the correct volume out of ink jet nozzles with well defined velocity and direction; surface energy considerations to maximize aperture ratio and display resolution; solution drying to form flat films and solution formulation to create polymer films that perform as well as films created by conventional spin coating. In addition we will describe the current status of the technology both in terms of polymer performance for both passive and active matrix applications, printing technology and polymer ink performance.
Full-color displays of passive-matrix-driven organic electroluminescence panels
Hiroshi Tada, Yoshikazu Sakaguchi, Eiichi Kitazume, et al.
2.2 inch diagonal full-color passive-matrix displays of organic electroluminescence panels were fabricated, and the possibility of mass production was examined. The panel consisted of 132 x 162 pixels, and each pixel had three sub-pixels of red, green and blue emission in line. The panel contained stripe-shaped ITO lines, multi layers of organic materials, and cathode lines orthogonal to ITO lines. The organic layers of the electroluminescence panels were fabricated by vacuum evaporation method. Full-color emission was performed by patterning directly the emitting layers of red, green and blue color through metal mask in order to take most advantage of the luminous efficiency of electroluminescence. Pixel size of 0.272 mm x 0.264 mm was performed by vacuum evaporation using metal mask. This simple-structured panel was driven with passive matrix drive method. Dual scan drive method was adopted to lower the duty ratio, which made the peak luminance lower. Total thickness of the module was only 2.5 mm. We started pilot production of the 2.2 inch diagonal full-color organic electroluminescence panels, and good manufacturability is promised.
Graded mixed-layer OLEDs
Anna B. Chwang, Raymond C. Kwong, Julie J. Brown
We describe the performance of mixed-layer, small molecule organic light-emitting devices (OLEDs) that are step-graded from a mostly hole transporting layer (HTL) to a mostly electron transporting layer (ETL) from anode-side to cathode-side, respectively. The devices are based on a green, electrofluorescent dopant and achieve luminous efficiencies of > 4.5 lm/W and 10 cd/A. These efficiencies are significantly higher than those of a uniformly mixed device, i.e., a device in which the HTL and ETL are uniformly mixed, but lower than those of a conventional heterostructure device employing the same dopant material. Operating lifetime of the graded mixed OLEDs, however, is much improved over the heterostructure device. We then compare the performance of fluorescent OLEDs at high current drive to that of phosphorescent OLEDs at high current drive in the context of passive matrix driven display suitability.
OLEDs for Solid State Lighting
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Illumination-quality OLEDs for lighting
Anil R. Duggal, Joe J. Shiang, Christian M. Heller, et al.
OLED technology has improved to the point where it is now possible to envision developing OLEDs as a low cost solid state light source. In order to realize this, significant advances have to be made in device efficiency, lifetime at high brightness, high throughput fabrication, and the generation of illumination quality white light. In this talk, a down conversion method of generating white light is demonstrated and shown to be capable of generating illumination quality white light over the full range of color temperatures required for lighting. It is also demonstrated that, due to the presence of light scattering, the down-conversion method can actually increase the overall device power efficiency.
Self-healing flexible photonic composites for light sources
Besides display application, organic light emitting diodes (OLED) have great potential for the application of thin film light sources. The new device is designed to have a self-healing mechanism against electrical shorts. The entire device can be prepared in a vacuum chamber equipped with plasma treater, monomer evaporators, radiation curing units and inorganic deposition stations. A blend of small electron-donor organic molecules and radiation polymerizable monomers is flash evaporated to provide a molecular-level vapor-phase mixture, which is then condensed and cured on a flexible ITO coated substrate. The procedure is repeated with an electron-acceptor organic substance, which is deposited over the electron-donor layer. A metallic cathode is then deposited over the electron-acceptor layer and the composite OLED product is packaged. The flash evaporation vacuum deposition technique with in-line radiation cross-linking allows the mixing of small OLED molecules with monomers or oligomers at any ratio. Using this technique, a heterogeneous blend can be flash evaporated and molecularly mixed in the vapor phase, re-condensed as a homogeneous film, and then quickly cross-linked before any phase separation occurs. This creates a unique polymer chemistry that is not possible by conventional coating techniques. The electrical characteristics and the thickness of the metallic cathode and the composition of the polymer layers are selected to produce a self-healing mechanism via gasification of elemental carbon generated by dielectric breakdowns and the oxidation of any exposed cathodic surface, thereby providing a self-healing mechanism to prevent propagation of the damage caused by electrical shorts.
Molecular OLEDs Based on Metal Complexes and Doped Layers
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Study of organic light-emitting devices with 5,6,11,12-tetraphenylnaphthacene (rubrene)-doped hole transport layer
Doping the hole transport layer (HTL) of organic light emitting devices (OLEDs) was found to increase device operational stability. To this effect, the role of 5,6,11,12-tetraphenylnaphthacene (rubrene), a widely dopant for HTLs, in increasing OLED stability has been widely investigated. However, significant disagreements between various explanations for the increased stability, ranging from rubrene being a charge injection promoter, to its being a charge trap, still exist. We conducted an in-depth study on the influence of rubrene doping of HTL on device stability. The study was carried out on OLEDs of structure: indium-tin-oxide (ITO) anode/N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB) HTL / tris(8-hydroxyquinoline) aluminum (AlQ3) electron transport layer / Mg:Ag cathode, in which different portions of the HTL were doped with rubrene. Compared to undoped devices, stability of OLEDs in which HTL doping was limited to only a thin interfacial layer at either the ITO or AlQ3 interface was essentially the same, whereas, stability of OLEDs in which a substantial portion of the HTL was doped was about an order of magnitude higher, and approached that of devices where the whole HTL was doped. In addition, for a fixed thickness of the doped portion, device stability was found to be essentially independent of the thickness of the undoped portion. The results demonstrate that increasing OLEDs stability by means of doping the HTL is associated with changes in bulk HTL hole transport properties rather than interfacial properties, and is consistent with OLED degradation mechanism based on instability of cationic AlQ3 species.
OLEDs with enhanced high-temperature operational stability
Zoran D. Popovic, George Vamvounis, Hany Aziz, et al.
Temperature dependence of electroluminescence degradation is studied in organic light emitting devices containing an emitting layer composed of a mixture of different hole transport molecules and tris(8-hydroxyquinoline)aluminum (ALQ3) electron transport and emitter molecule. The emitting layer is sandwiched between hole and electron transport layers. Devices containing the hole transport molecule N,N'-di(naphthalene-1-yl)-N,N'-diphenyl-benzidine (NPB), doped with quinacridone (DMQ) green emitter showed remarkable temperature stability. For these devices, a half-life of about 78,500 hours, 18,700 hours, and 8,600 hours can be projected for operating temperatures of 22°C, 70°C and 100°C, respectively, at an initial device luminance of 100 cd/m2. Activation energies for device degradation were determined for devices with different hole transport molecules and it was found that devices with higher activation energy show better high temperature stability. These results are consistent with the recently proposed degradation mechanism based on the unstable cationic AlQ3 species.
Organic light-emitting devices based upon divalent osmium complexes: I. Design, synthesis, and characterization of osmium complexes.
Brenden Carlson, Gregory D. Phelan, Xuezhong Jiang, et al.
In this work we present the synthesis and characterization of several novel osmium complexes of the form [Os(N-N)2L-L]2+ 2X- designed for organic light emitting device (OLED) applications. In the complex notation N-N represents a derivative of 2,2'-bipyridine or 1,10-phenanthroline and L-L represents a strong π-acid arsine or phosphine ligand. The complexes feature 3MLCT emission that ranges from 611 - 650 nm, which makes them suitable as an emission source for red OLEDs. Phosphorescent quantum yields as high as 45% and emission lifetimes as short as 400 nanoseconds have been reached.
Organic light-emitting diodes containing fluorinated asymmetrical europium cored beta-diketone complexes
Gregory D. Phelan, Brenden Carlson, Xuezhong Jiang, et al.
Novel luminescent materials based on europium-cored complexes have been synthesized and incorporated into light emitting diodes using poly (N-vinyl-carbazole) and poly (vinyl naphthalene) blends as doping hosts. The complexes consists of fluorinated β-diketone ligands chelated to europium. Excitation of the ligands and efficient transfer of energy from the excited ligands to the metal core results in the emission of optically pure red light. The ligands were designed such that they include a polycyclic aromatic compound, phenanthrene, and a second substituent to improve processibility. Phenanthrene is used to so that the ligand energy will match with the energy of the metal center. Partially fluorinated substituents were also used to help improve the efficiency and charge transfer capability of the resulting metal complex. The complex consisted of one equivalent of europium and three equivalents of the ligand. One equivalent of either 1,10-phenanthroline or 4,7-diphenyl-1,10-phenanthroline was also chelated to enhance the stability of the complex. Double and triple layer devices were synthesized with the configuration of ITO/BTPD-PFCB/Europium complex in a polymer blend/Ca/Ag for the double layer device and ITO/BTPD-PFCB/Europium complex in a polymer blend/PBD/Ca/Ag for the triple layer device. The double layer devices made with a polymer blend of PVN outperformed the devices made from PVK as the emission bands of the PVN better match the absorption bands of the ligands. A maximum brightness of 178 cd/m2 with a maximum external quantum efficiency of 0.45% was measured for the double layer device.
Carrier Transporting and Luminescent Polymers
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PAni as a hole injection layer for OLEDs and PLEDs
Bernd Werner, J. Posdorfer, Bernhard Wessling, et al.
Although polyaniline (PAni) has been proposed for use as a hole injection layer (HIL) in organic light emitting diodes (OLEDs) and polymeric light emitting diodes (PLEDs) from very early on, the material does not seem to have found widespread use on a (pre)commercial scale. Recent results will be presented showing that PAni can be efficiently used as HIL, and that it even has some advantages over the often preferred poly[ethylenedioxy-thiophene] (PEDT). Intensive investigations on the influence of conductivity, morphology and especially the work function onto device performance have led to a commercially available water-borne PAni dispersion. The stable, nanoscaled system for HILs has a particle size of about 35 nm and a lateral conductivity (when deposited and dried) of around 10-6 S/cm. Using PAni dispersions for the generation of HILs the final device performance in OLEDs and PLEDs could be significantly improved. Depending on the used light emitting polymer (LEP), luminescence data were up to 30% more efficient compared to devices made with the widespread used PEDT.
Efficient and saturated blue organic polymer light-emitting devices with an oxadiazole containing poly(fluorene) polymer emissive layer
Shu-jen Lee, Joseph R. Gallegos, Julien Klein, et al.
A new series of organic light emitting copolymers, poly(dioctyl fluorene - co - diphenyl oxadiazole)s P(DOF-DPO), are reported in this paper. We found organic polymer light emitting devices with comparable device efficiency and more saturated blue color based on this new series of copolymers with less than 10% DPO moiety when compared with poly(dioctyl fluorene) P(DOF) devices. The device structures used in this study were indium tin oxide (ITO) anode/ PEDOT-PSS layer/emissive layer/calcium (Ca)/aluminum (Al) cathode, while the emissive layer was P(DOF) or P(DOF-DPO) copolymer with different DOF and DPO ratio. The AFM data shows that the emissive layer has a smooth surface with RMS roughness about 0.5 ± 0.2 nm over a 2 μm by 2 μm area. The photo-luminescence quantum efficiency of the P(DOF-DPO) copolymer decreases with increasing DPO moiety. Both the photo-luminescence spectra and electro-luminescence spectra are slightly blue shifted when adding less than 10% DPO moiety into P(DOF). Both the emission efficiency (~0.3-0.6 cd/A at 100 cd/m2) and power efficiency (~0.1-0.2 lm/W at 100 cd/m2) of the P(DOF-DPO) based PLEDs with less than 10% DPO moiety are comparable to the P(DOF) based PLEDs.
Development of efficient electron-transporting polymers for light-emitting diodes
Michelle S. Liu, Xuezhong Jiang, Petra Herguth, et al.
Polyfluorenes are a class of very efficient conjugated polymers used in the development of LEDs that exhibit very high hole mobility. In order to balance the charge transport and enhance quantum efficiency of the LEDs, fluorene-based copolymers were synthesized based on the statistic copolymerization between fluorene and 2,5-dicyanobezene. By attaching two electron-withdrawing cyano groups onto the phenylene ring, both the electron affinity and the electron conduction of these copolymers are greatly enhanced comparing to the fluorene homopolymer. LED devices using the cyano-containing fluorene copolymers show very bright emission and low turn-on voltages. The emission color of these polymers could be also tuned by exciplex formation between the polymers and amine-containing hole transporting materials.
Poly(fluorene-co-benzothiadiazole)s: Effect of structure, molecular weight, and polydispersity on their performance in polymer light-emitting diodes
Petra Herguth, Xuezhong Jiang, Michelle S. Liu, et al.
Copolymers based on fluorene and benzothiadiazole exhibit high brightness and quantum efficiencies when incorporated into polymer light-emitting diodes (PLEDs). Their emission wavelength is strongly dependent on the benzothiadiazole-containing segment of the polymer. However, the chain structure and charge-transport and -transfer processes in these materials are not well studied. We report the synthesis of a structural-random (r-PF3B) and a structural-defined (s-PF3B) copolymer, poly-fluorene-co-benzothiadiazole whereas the ratio of the two co- monomers was chosen to be 75% and 25%, respectively. We have systematically investigated the effect of structure on their photoluminescence (PL) and electroluminescence (EL) properties. Furthermore we have also studied the effect of molecular weight and its distribution on the performance of the material in PL and EL. We have found that the absorption and emission spectra (PL and EL) of these polymers are quite independent of their structures, molecular weights, and polydispersity. However, the PL and EL efficiencies do vary with the materials studied. These materials were fabricated into a series of double-layer devices. Their external quantum efficiencies (ranging from 0.097% to 1.7%) and maximum brightness (ranging from 153 to 23300 cd/m2) are highly dependent on the structure as well as the molecular weight and polydispersity. The higher the molecular weight and the narrower the distribution are, the higher the efficiency and brightness of the devices. Likewise, the structure of the polymer also influences the efficiency. It was found that the structural-random copolymer (r-PF3B) exhibits higher efficiencies and brightness when compared with the structural-defined one (s-PF3B) in the same molecular weight range.
New Fabrication and Characterization Techniques
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Role of copper phthalocyanine in multilayer organic LEDs based on small molecules
The role of copper-phthalocyanine (CuPc) has intermediate layer between the anode and the hole-transport layer in multilayer organic light-emitting devices (OLEDs) was studied. The OLEDs consisted of CuPc, N,N'-di(naphtalene-1-yl)-N,N'-diphenyl-benzidine (NPB) as hole-transport layer and tris-(8-hydroxyquinolinato)-aluminum (Alq3) as electron-transport and emitting layer sandwiched between a high-work-function metal and a semi-transparent calcium cathode. A combinatorial approach that allows the simultaneous fabrication of 10 x 10 individual devices was used to vary the thicknesses of CuPc and NPB over a broad range from 0 to 45 nm and from 10 to 100 nm, respectively. Systematic current-voltage and impedance measurements revealed a redistribution of the internal electric field of the CuPc/NPB/Alq3 three-layer structure compared to that of the NPB/Alq3 bilayer OLED. It was demonstrated that the hole transport is mainly controlled by the internal energy barrier at the CuPc/NPB interface. The fact that CuPc strongly impedes hole injection into NPB also has a significant impact on the frequency-dependent behavior of the capacitance, especially the cutoff frequency.
Spectroscopy and Optical Properties of Luminescent Materials and Devices
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Monte Carlo modeling of organic polymer light-emitting devices on flexible plastic substrates
A Monte Carlo method for modeling the light transport phenomena in organic polymer light-emitting devices (PLEDs) is reported. In this simulation we assumed a point light source having photon emission spectrum represented by the photoluminescence (PL) spectrum of the organic polymers. This method describes the fate of photons through multiple scattering events determined by the wavelength-dependent material optical properties in a 3-D Cartesian geometry, thus considering the effects of refraction at different interfaces, back-reflection from the cathode, interference effect in the ITO thin film, and absorption within the polymer layers. We apply this method to analyze the wavelength output distribution and extraction efficiency. We found that the simulated light emission spectra of the green and red light-emitting devices are very similar to the measured PL spectra, suggesting that the light transport phenomena do not change the energy distribution significantly. We also established that the calculated extraction efficiency for the red (ηext = 19.5%) and green (ηext = 19.9%) PLEDs are approximately the same. We further investigated the light emission angular distribution of the PLEDs, and found that the simulated angular distribution shows better agreement with the experimental data than previously used models that rely on standard refraction theory at one interface.
Carrier Transport and Interfaces in OLEDs I
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Thermal detection of trapped charge carriers in organic transport materials
Norwin von Malm, Juergen Steiger, Torsten Finnberg, et al.
The effect of trap states on the transport and luminescence properties of organic light emitting diodes (OLEDs) is studied. For trap level detection energy resolved thermally stimulated current (TSC) measurements known as fractional glow are utilized to determine the density of occupied states (DOOS) in various organic semiconductors such as the small molecule systems Alq3 [aluminum tris(8-hydroxyquinoline)], 1-NaphDATA {4,4',4"-tris-[N-(1-naphtyl)-N-phenylamino]-triphenylamine} and α-NPD [N,N'-di-(1-naphthyl)-N,N'-diphenylbenzidine] and the polymeric semiconductor MDMO-PPV {poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene]}. Characteristic differences in the trap spectra are obtained and interpreted in terms of possible structural and compositional origins of the investigated materials. In order to judge the formation process of traps and their practical consequences on the charge carrier transport I-V and L-V characteristics of 1-NaphDATA doped α-NPD devices and α-NPD doped 1-NaphDATA devices were compared to respective non-doped samples. A clearly reduced current and luminescence was found only in the former case. It was possible to conclude that the detected electronic trap states either act as hole traps or as scattering centers. Furthermore, pulsed transport studies on ITO/α-NPD/Alq3/Al devices show thte critical influence of traps on the dynamical performance of the charge transport. In a two-pulse experiment the carrier injection and trap depletion can be separated.
Optimization of organic semiconductor devices by anode modification
Holger Frohne, David C. Muller, Nina Reckefuss, et al.
PEDOT [Poly(3,4-ethylenedioxythiophene)] layers were prepared by electrochemical polymerization of the respective monomer. Thereafter, these layers were electrochemically adjusted to different equilibrium potentials and they were investigated with Kelvin Probe measurements. A change in work function could be observed yielding a linear correlation with the pre-adjusted electrochemical potential. These PEDOT layers have been utilized in OLEDs in their accessible range of work functions. Internal energy conditions of the OLEDs were characterized in a photovoltaic setup yielding a linear correlation of the open circuit voltage on the pre-adjusted potential. In a second step the efficiency was determined for devices with Ca cathode (space-charge-limited electron current) as well as for devices with Al cathode (injection-limited electron current). These devices could be optimized in efficiency by adjusting the hole current to the electron current, which was determined by the work function of the cathodic metal. The optimum could be explained in zero-order approximation in terms of a balanced bimolecular reaction between holes and electrons.
Carrier Transport and Interfaces in OLEDs II
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Band mapping and frontier orbitals at the interface thin native SiO2/organics
Maria Benedetta Casu, Paolo Imperia, Sigurd K. Schrader
The properties of the interface Si<111>/SiO2/low molecular compound were studied by means of ultraviolet photoelectron spectroscopy (UPS). The investigated material, an amphiphilic derivative of 2,5-diphenyl-1,3,4-oxadiazole (NADPO), is also of interest in photonics, because it can be used as active material in opto-electronic or electro-optic devices. The UPS measurements were performed at the beam line Seya F2.2 in HASYLAB (Hamburg, Germany). The photoelectrons were collected with a Vacuum Generators ADES 400 angle resolving spectrometer system at room temperature. The photoelectron spectra were measured with an angle of incidence of 45 degrees in normal emission for various incident energies and for an incident photon energy, Einc, of 24 eV in off normal emission. We investigated the interface properties depending on the number of monolayers (2, 4, 6, 8) of well oriented Langmuir-Blodgett films, monitoring the frontier orbital position, with particular regard to the highest occupied molecular orbital energy position. The obtained results give evidence that the band structure is depending on the thickness of the organic layer at the interface Si<111>/SiO2/NADPO. We also monitored the development of the energetic position of the highest occupied molecular orbital (HOMO) from that of the monolayer towards the "true" HOMO energy position of the thin film, where the word "true" is related to the actual interface properties of the films and not to a single monolayer (ML) or a double layer. As a matter of fact the 2 ML spectra show a different behaviour than those of thicker films since the 2 ML system is strongly influenced by the substrate while the spectra of thicker films are de-coupled from the electronic structure of the substrate.
Organic TFTs and Memory Elements
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Photoemission spectroscopy study of the interface formation in organic thin film transistors
Neil J. Watkins, Li Yan, Serkan Zorba, et al.
Pentacene, perylene, and sexithiophene are all materials being used in organic thin film transistors due to their relatively large mobilities. It has been suggested that the functional behavior in these devices occurs within the first few molecular layers of the organic at the interfaces between the organic and the dielectrics used in fabrication of the thin film transistors. This makes understanding the electronic behavior of the interfaces involved in these devices critical. In order to better understand these interfaces we investigated the interface formation using photoemission spectroscopy to examine layer by layer growth of pentacene, perylene, and sexithiophene on conductors, dielectrics, and charge transfer agents and in some cases vice versa. We observed indications of dipole formation at the interfaces between the metals and organics for organic on metal deposition. There appears to be a linear relation between the interface dipole and metal workfunction with the observed dipoles ranging from a 1 eV dipole at the interface between sexithiophene and gold to a -0.46 eV dipole at the interface between pentacene and calcium. We also observed that more complex material intermixing takes place during metal on organic deposition than during organic deposition onto metal and as a result, the electronic structure of the interface differs from that of organic on metal deposition. Possible charge transfer, dipole formation and energy level bending at these interfaces will be discussed.
Posters - Tuesday
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Evolution of optical properties of tris (8-hydroxyquinoline) aluminum (alq3) with atmosphere exposure
Aleksandra B. Djurisic, Tsz Wai Lau, Chung Yin Kwong, et al.
Tris (8-hydroxyquinoline) aluminum (Alq3) represents a material of significant interest for electron transport and/or light emitting layer applications in organic light emitting diodes (OLEDs). In spite of advances in Alq3 based devices, the knowledge and understanding of the optical properties of Alq3 and its chemical and environmental stability is still limited. With the reports of decreased turn-on voltage and increased efficiency of OLEDs, the issues of lifetime and stability of those devices are attracting increasing attention. The degradation of Alq3 based OLEDs and dark spots formation and growth have been intensively studied. The studies on degradation of optical properties of Alq3 itself remain scarce. We have investigated effects of atmosphere exposure to properties of tris (8-hydroxyquinoline) aluminum (Alq3) thin films by photoluminescence (PL) and absorption measurements. Alq3 films were evaportated on glass substrates at different temperatures. The influence of annealing to the environmental stability of the films has also been investigated. It has been found that deposition at higher substrate temperature and annealing of the samples deposited at room temperature yields improvement in environmental stability of the films, i.e. less decrease of the PL intensity over time with atmosphere exposure, as well as increased PL intensity. To investigate further effects of the air exposure, films deposited at room temprature were stored for four days in air, nitrogen, and oxygen. No decrease in PL intensity has been found for storage in nitrogen, while decrease for the film stored in oxygen was smaller than that for film stored in air, indicating that both humidity and oxygen play a role in PL intensity decrease in Alq3 thin films.
Efficient blue electroluminescence from 9,10-diphenylanthracene
Gi-Wook Kang, Young-Joo Ahn, Dae Yoon Park, et al.
We report efficient blue electroluminescence (EL) devices fabricated using 9,10-diphenylanthracene (DPA) as an active emitting material. Although DPA exhibits fluorescence in the blue spectral region with high quantum efficiency, its EL efficiency has been previously reported poor. By doping DPA into a bathocuproine (BCP) layer that acts as a hole-blocking layer, we can successfully fabricate very efficient blue EL devices with the Commission Internationale de L’Eclairage (CIE) chromaticity coordinates of (0.145, 0.195). The devices show a luminous efficiency of 2.9 cd/A at 200 cd/m2 and a maximum luminance of about 10344 cd/m2 at 16.6 V.
Analysis of nanostructured blue light-emitting p-6P films on mica
Frank Balzer, Ed J. Kintzel Jr., James G. Skofronick, et al.
Blue light-emitting aggregates of para-hexaphenyl molecules ('needles') are generated via dipole-assisted self assembly on single crystalline mica substrates. By applying atomic force microscopy and fluorescence microscopy we deduce size distributions as well as height-width correlations of individual aggregates. We find growth of densely-packed, needle-like structures even in the initial growth stage at low surface temperatures. However, long (up to one millimeter) and sparsely distributed, individually addressable needles grow only at high substrate temperatures and low adsorption rates. For a given sample at constant deposition conditions the height of the needles seems to be independent of width. This opens up the possibility to control the morphology of individual nanostructured aggregates.
An integrating sphere CCD-based measurement method of the OP-LED opto-electronic characteristics
An integrating sphere CCD-based measurement system has been developed to accurately characterize the opto-electronic performances of organic polymer light-emitting devices (OP-LEDs). Comparing this method with the previously developed lens-coupled method, we have found that the integrating sphere-based measurement method provides more stable and reliable optical data in comparison with the lens-coupled measurement method. In addition, we demonstrate that an inappropriate calibration of the OP-LED measurement system can greatly exaggerate the device performances.
Enhanced light emission from self-organizing molecular semiconductor by using organic thin film transistor
Yasuo Toko, Masahiro Funahashi, Jun-ichi Hanna
Electro-optical characteristics were studied on the organic semiconductor materials exhibiting molecular alignment based on molecular self-organization in their liquid crystal phases. It was confirmed that these organic semiconductor materials exhibit very fast electron transport in the smectic mesophases as well as hole transport, both of which were independent on electric field and temperature. Above all, the ambipolar carrier transport in these materials make it possible to fabricate a simple electroluminescence cell without a layered structure required in conventional organic electroluminescence cells. In this paper, we fabricated electroluminescence cell with surface type of molybdenum or aluminum electrodes using hexyldodecylterthiophene (6-TTP-12) as a light emitting semiconducting material. The electroluminescence cell shows a green light emission when a dc bias is applied to the electrode. It was revealed that the light emission took place in the vicinity of the cathode interface, indicating that the hole is the majority carrier in the cell when the molybdenum or aluminum electrodes were applied. In addition, the light emission luminance of the cell is controllable in the external electrode which formed under insulating film as a gate electrode.
Polyurethanes with covalent attached fluorescent dyes as deep-red-emitting materials
Silvia Janietz, Hartmut Kruger, Armin Wedel, et al.
A unusual way is presented to obtain a new class of deep red emitting polymers. Polyurethanes with covalently attached fluorescent dyes were developed. The DCM-dye seems to be a favorable candidate but it has no reactive groups for linking into a polymer structure. DCM can be synthesized by the monofunctional addition of (2,6-dimethyl-4H-pyrane-4-ylidene)-malononitrile with 4-dimethylaminobenzaldehyde. We realized a bifunctional condensation of the pyrane with N-methyl-N-(2-hydroxyethyl)-4-aminobenzaldehyde to enlarge the conjugated system and to shift the emission maximum to more than 650 nm. Simultaneously we introduced two reactive hydroxy terminal groups into the dye molecule. Using this functionality we were able to synthesize several new polyurethanes with covalently attached DCM dye in the main chain. By co-condensation with non-dye molecules like N,N-bis-(2-hydroxyethyl)-aniline or butan-1,4-diole the dye content in the main chain can be varied and the influence of the absorption and emission behavior can be studied. Red emitting device structures were realized and some of the device properties will be given. It will be shown that the stability and the lifetime of the device can be increased by simple structure modification of the polyurethane, e.g. alkylation of the urethane groups or the change of the co-components.
Evidence of electron and hole transfer in metal/CuPc interfaces
Neil J. Watkins, Li Yan, Serkan Zorba, et al.
The electronic structures of interfaces between metals and Copper phthalocyanine (CuPc) organic films are investigated using the combination of ultraviolet photoemission spectroscopy (UPS) and inverse photoemission spectroscopy (IPES). The lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) can be directly observed by IPES and UPS simultaneously. We found that the Fermi level, EF, in the organic film can be modified by metals through charge transfer or doping. The FERMI level at the Cs/CuPc interface is observed to shift to less than 0.2 eV below the CuPc LUMO. The IPES observation is the first direct confirmation of Fermi level pinning near the LUMO in organic films. The pinning of the Fermi level close to the LUMO can be explained by electron transfer from Cs to CuPc, which is supported by the presence of a gap state in CuPc as observed with UPS. On the other hand, the Au/CuPc interface is characterized by electron transfer from CuPc to Au, resulting in a reduced HOMO intensity shown in the UPS spectra and a new feature below the LUMO shown in the IPES spectra. These observations shed new light onto the understanding of interface formation in organic semiconductor devices.
Efficient molecular organic light emitting diodes based on silole derivatives
We report the performance of molecular organic light-emitting diodes (MOLEDs) using silole derivatives as emissive and electron transport materials. Two siloles, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PPSPP) and 1,2-bis(1-methyl-2,3,4,5,-tetraphenylsilacyclopentadienyl)ethane (2PSP), with high PL quantum yields of 94% and 85%, respectively, were used as emissive materials. Another silole, namely 2,5-bis-(2',2"-bipyridin-6-yl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), was used as the electron transport material. MOLEDs using these two siloles and NPB as the hole transport material show a low operating voltage of approximately 4.5 V at a luminance of 100 cd/m2 and high external electroluminescence (EL) quantum efficiencies of 3.4% and 3.8%, respectively, at 100 A/m2. MOLEDs based on PPSPP exhibit a red-shifted EL spectrum which is assigned to an exciplex formed at the PPSPP:NPB interface.