A preview of the papers appearing in the May 2005
issue of the 
Journal of the SID, available on-line at www.SID.org.

Edited by Aris Silzars

Highly transparent and conductive CdO thin films as anodes for organic light-emitting diodes: Film microstructure and morphology effects on performance

Yu Yang
Qinglan Huang
Andrew W. Metz
Shu Jin
Jun Ni
Lian Wang
Tobin J. Marks

Northwestern University

Abstract — Highly conductive and transparent CdO thin films have been grown on glass and on single-crystal MgO(100) by MOCVD at 400°C and were used as transparent anodes for fabricating small-molecule organic-light emitting diodes (OLEDs). Device response and applications potential have been investigated and compared with those of control devices based on commercial ITO anodes. It is demonstrated that highly conductive CdO thin films of proper morphology can efficiently inject holes into such devices, rendering them promising anode materials for OLEDs. Importantly, this work also suggests the feasibility of employing other CdO-based TCOs as anodes for high-performance OLEDs.

CdO was one of the first transparent conducting oxide materials discovered and has been extensively employed for transparent electrodes in photovoltaic devices due to the nearly metallic conductivity. However, there have been no reported attempts to fabricate OLEDs using CdO-based TCOs as the electrodes. Although the band gap of pure bulk CdO is only 2.3 eV, leading to relatively poor optical trans-parency in the short-wavelength range, significant widening of the band gap can be achieved via a Burstein–Moss (B–M) shift using fluorine or aliovalent metal doping, due to the small effective CdO carrier mass. For example, In doping widens the band gap from 2.6 eV in pure CdO to 3.2 eV at 5% In doping, which is comparable to reported band-gap values for commercial ITO (3.0–3.7eV).


FIGURE 2 — Optical transparency of as-deposited CdO thin films on MgO(100). Inset: derivation of the apparent optical band gap.


Passive-matrix displays based on light-emitting polymers

Homer Antoniadis
M. W. Lui

OSRAM Opto Semiconductors

Abstract — The design architecture, product specification, and reliability of the first high-resolution commercially available passive-matrix displays based on light-emitting polymers is described. Also, the applications and benefits of this new low-cost display technology are discussed.

The display module is comprised of a polymer-OLED glass, a circular polarizer (CP) laminated on the glass, typically used to enhance contrast and eliminate the unwanted reflections of the cathode electrode, a high-density thin-film flex (attached on glass) which carries the driver IC [chip on flex – (COF)], and several surface-mount (SMT) components. The driver IC is mounted on the COF flex using ACF processing (anisotropic conductive film). Such an assembly of components is achieved by seamlessly integrating the OLED glass with the driver IC aiming to meet application specific requirements.



FIGURE 1 — A standard polymer-OLED passive-matrix display (Pictiva 80 x 48) having 80 x 48 pixels showing all critical components.


High-efficiency p–i–n organic light-emitting diodes with long lifetime

Philipp Wellmann
Michael Hofmann
Olaf Zeika
Ansgar Werner
Jan Birnstock
Rico Meerheim
Gufeng He
Karsten Walzer
Martin Pfeiffer
Karl Leo

Novaled GmbH

Abstract — High-performance organic light-emitting diodes (OLEDs) are promoting future applications of solid-state lighting and flat-panel displays. We demonstrate here that the performance demands for OLEDs are met by the PIN (p-doped hole-transport layer/intrinsically conductive emission layer/n-doped electron-transport layer) approach. This approach enables high current efficiency, low driving voltage, as well as long OLED lifetimes. Data on very-high-efficiency diodes (power efficiencies exceeding 70 lm/W) incorporating a double-emis-sion layer, comprised of two bipolar layers doped with tris(phenylpyridine)iridium [Ir(ppy)3], into the PIN architecture are shown. Lifetimes of more than 220,000 hours at a brightness of 150 cd/m2 are reported for a red PIN diode. The PIN approach further allows the integration of highly efficient top-emitting diodes on a wide range of substrates. This is an important factor, especially for display applications where the compatibility of PIN OLEDs with various kinds of substrates is a key advantage. The PIN concept is very compatible with different backplanes, including passive-matrix substrates as well as active-matrix substrates on low-temperature polysilicon (LTPS) or, in particular, amorphous silicon (a-Si).

All OLEDs presented in this paper were prepared by thermal evaporation of different organic layers in ultrahigh vacuum with a base pressure of 10–8 mbar, applying the PIN structure onto an indium tin oxide (ITO) coated, pre-structured glass substrate without breaking the vacuum. For the layer growth, two different kinds of evaporation tools were used: a cluster system where p- and n-doped layers, emission layers, and interlayer are deposited in separated chambers and a multisource one-chamber system. Comparable diode structures from both systems show equal performance.



FIGURE 1 — Organic molecules typically used for the doped transport layers. Left: F4-TCNQ is a well-known acceptor (p-dopant); middle: MeO-TPD can be used as a host for F4-TCNQ (p matrix); right: BPhen is used as host for Cs (n matrix).


Development of a novel emitting vinyl polymer and its application in organic electroluminescent devices

Daisuke Nagamatsu
Masanori Maeda
Keisuke Hashimoto
Kenji Okumoto
Hiroshi Kageyama
Yasuhiko Shirota

Osaka University

Abstract — A novel emitting vinyl polymer, poly[4-(7-{4-[N,N-bis(9,9-dimethylfluoren-2-yl) amino]phenyl}-2,1,3-benzothiadiazol-4-yl)phenylethylene] (PVFABT), was designed and synthesized. The new vinyl polymer was found to form smooth amorphous films with a high glass-transition temperature of 199°C. The polymer possesses bipolar character with both electron donating and accepting properties. It undergoes reversible anodic oxidation and cathodic reduction to give stable cation and anion radicals. It exhibits intense orange fluorescence in solution and as film. A multilayer organic electroluminescent device using PVFABT as an emitting material emitted orange light, exhibiting high performance.

The emitting layer in organic EL devices functions as the recombination center for holes and electrons injected from the anode and cathode, respectively. Therefore, the materials for use in the emitting layer should have bipolar character, accepting both holes and electrons. In addition, emitting materials should also have high luminescence quantum efficiencies. Based on this concept, we have designed and synthesized a new vinyl polymer, PVFABT. The introduction of a 4-[bis(9,9-dimethylfluoren-2-yl) amino]phenyl group is intended to provide electron-donating properties and to facilitate the formation of an amorphous glass because of its nonplanar molecular structure. The introduction of a 2,1,3-benzothiadiazole group is intended to provide electron-accepting properties.



FIGURE 3 — Cyclic voltammogram of PVFABT in THF (1.0 x 10–3 M). Supporting electrolyte: tetra-n-butylammonium perchlorate (10–1 M). Scan rate: 100 mV-sec–1.


Metal (IV) tetras (8-hydroxyquinoline) (M = Zr, Hf) used as electroluminescent material and electron-transport layer in OLEDs

Simona Garon
Eric K. C. Lau
Siew-Ling Chew
S. T. Lee
Mark E. Thompson

University of Southern California

Abstract — In this paper, we report on the utilization of zirconium (IV) tetras (8-hydroxyquinoline), Zrq4, and hafnium (IV) tetras (8-hydroxyquinoline), Hfq4, as an electroluminescent material in fluorescent organic light-emitting diodes (OLED) and as an electron-transport layer (ETL) for high-efficiency electrophosphorescent organic light-emitting diodes (PHOLEDs).

The electronic properties of the metal tetra-quinolates are very similar to Alq3. These Alq3 and Mq4 complexes are yellow solids, with nearly identical absorption spectra. Both types of quinolate complex emit from predominantly quinolate based states, with the only difference being a small red shift for the Mq4 materials. Alq3 emits with a λmax of 518 nm, while both Zrq4 and Hfq4 have λmaxvalues of 538 nm. The substitution of the heavier metal-ion zirconium or hafnium is expected to give a red-shift compared to the aluminum chelate.



FIGURE 3 — Device performance data for phosphorescence-based devices (structure: ITO/NPD/CBP-Ir(ppy)3/BCP/Mqn/LiF/Al, Mqn= Alq3, Zrq4,Hfq4).


Highly efficient green polymer light-emitting diodes through interface engineering

Qianfei Xu
Jinsong Huang
Yang Yang

University of California

Abstract — By interface engineering, we have improved the quantum efficiency for green polyfluorene polymer light-emitting diodes by three fold: the efficiency improved from 9 to 28 cd/A. This interface engineering was achieved by inserting a thin layer of calcium (2) acetylacetonate, denoted as Ca(acac)2, at the polymer/metal cathode interface. The Ca(acac)2 layer behaves in a multifunctional way. It assists the electron injection by lowering the electron-injection barrier. In the meanwhile, hole injection is enhanced by the accumulated electrons in the polymer layer. This effect is believed to lower the barrier height for hole-injection through the Schottky effect. Finally, the Ca(acac)2 layer works as a hole block layer to block the holes. As a result of the charge balance and charge confinement, the device quantum efficiency increases dramatically.

The improvement in device efficiency reported in this paper is believed to have resulted due to several effects, such as the enhancement in electron injection due to the Ca(acac)2 layer, the enhancement in hole injection due to the accumulated electron density (electron-induced hole injection), and finally, hole blocking (or confinement) due to the Ca(acac)2 layer. The PLEDs were fabricated on pre-cleaned glass substrates coated with high-work-function indium tin oxide (ITO) as the anode. A buffer layer of 50 nm poly(ethylene dioxy thio-phene)/polystyrene sulfonate (PEDOT:PSS) was used as a hole-injection layer at the anode interface, between the ITO and the emission polymer. The emitting polymer layer was fabricated by spin-coating inside a pure nitrogen-filled glove box.



FIGURE 2 —The I–V–L curve of the device with Ca(acac)2 as the interfacial modification layer. The inset is the plot of luminance efficiency vs. current.


Charge-recombination processes in oligomer- and polymer-based light-emitting diodes: A molecular picture

David Beljonne
Zhigang Shuai
AiJun Ye
Jean-Luc Brédas

Georgia Institute of Technology

Abstract — An overview of our recent work on the mechanisms of singlet and triplet exciton formation in electroluminescent π-conjugated materials is presented. According to simple spin statistics, only one-fourth of the excitons are formed as singlets. However, deviations from that statistics can occur if the initially formed triplet charge-transfer (CT) excited states are amenable to intersystem crossing or dissociation.

The efficiency of organic light-emitting diodes (LEDs) depends to a large extent on the nature of the excited species that are formed upon recombination of injected positive and negative charges. These excitations are known to be a function of both electron-vibration and electron-electron interactions; they are generally believed to be excitons with a binding energy in excess of kT. Of importance is that singlet and triplet excitons possess different energies; the singlet–triplet energy difference, that is the exchange energy, is estimated to be larger than 0.5 eV for the lowest excitation in a range of conjugated polymers. They also display different geometry relaxations; due to the possibility of exchange between like spins, triplet wavefunctions usually display a more spatially confined character, a feature that is especially pronounced for low-lying excitations.



FIGURE 5 — Ratio between the singlet and triplet charge recombination rates, r = kS/kT, as a function of ΔS and λS, in a cofacial arrangement of an OPV2 chain.


Clamped-inverter circuit architecture for luminescent-period-control driving of active-matrix OLED displays

Hajime Akimoto, Hiroshi Kageyama,
Mitsuhide Miyamoto,
Yoshiteru Shimizu, Naruhiko Kasai,
Hiroki Awakura, Akira Shingai,
Naoki Tokuda, Kenta Kajiyama,
Shigeyuki Nishitani, Toshihiro Sato

Hitachi Central Research Laboratory

Abstract — An innovative pixel-driving technology for high-performance active-matrix OLED flat-panel displays is described. Called "clamped-inverter circuit architecture," it uses luminescent-period-control driving to reduce the inter-pixel non-uniformity caused by the device-to-device variability of low-temperature poly-Si TFTs. A prototype full-color display shows a luminous deviation of less than 1.6%, which corresponds to only the LSB-error in 6-bit gray-scale.

The clamped-inverter circuit architecture for active-matrix OLED displays that is described here, unlike the conventional luminescent-level-control and digital-control architectures, controls the luminescent period of the pixels. The inter-pixel non-uniformity caused by pixel-to-pixel variation of poly-Si TFT performance can be effectively suppressed by the ON–OFF drive scheme of the new architecture. Gamma correction can be implemented by modulating an analog input signal voltage. A full-color display that performs only the LSB error in 6-bit brightness control and gamma correction was demonstrated using a conventional low-temperature poly-Si CMOS TFT process technology. The prototype panel provided a wide viewing angle: of +170 to –170° and its luminous dynamic range was more than 2000.



FIGURE 11 — Luminance deviation of 20 panel pixels.


LUT-based compensation model for OLED degradation

David Antonio-Torres
Paul F. Lister
Paul Newbury

University of Sussex

Abstract — A compensation model for blue OLED devices, which is based on an OLED degradation model derived from luminance measurements of OLED devices, has been developed; it reduces degradation effects by monitoring the activity of a set of subpixels and adjusting the driving conditions of the blue subpixels of the display accordingly. To evaluate its performance, the compensation model has been embedded in an OLED display controller model, where its implementation is based on a lookup table. Simulations show that even with a reduced number of monitored subpixels, degradation is effectively reduced.

By keeping track of the levels of brightness that each subpixel has resolved, the corresponding operating time can be computed. This mechanism needs to receive the updated information each row of the display is refreshed via the column driver and at the same time be informed of the refreshing period via the row driver. The compensation model can be programmed in a lookup table (LUT) with the operating time as an index. The benefit of using a LUT to hold the compensation model is that it can be programmed to match the degradation model of the OLED devices. Figure 5 shows how the LUT and the DMU are placed around the display drivers in an OLED display controller. The compensation scheme's functionality can be decomposed into three major phases, as shown in Fig. 5.



FIGURE 5 — Functional view of the compensation scheme (Ref. 6).


The role of LiF buffer layer in tris-(8-hydroxyquinoline) aluminum-based organic light-emitting devices with Mg:Ag cathode

B. J. Chen
S. C. Tan
X. W. Sun

Nanyang Technical University

Abstract — The device characteristics of organic light-emitting devices based on tris-(8-hydroxyqunoline) aluminum with a thin layer of LiF inserted at the ITO and organic interface or organic and Mg:Ag cathode interface were investigated. By inserting both a 1.0-nm LiF layer at side of the ITO anode and a 0.5-nm LiF layer under the Mg:Ag cathode, the device, at a current injection of 10 mA/cm2, exhibited the highest current efficiency of 8.2 cd/A and power efficiency of 1.93 lm/W for all the types of devices investigated in this study. Both the current efficiency and power efficiency of the device were improved by 1.2 times at a current injection of 10 mA/cm2, compared to the standard device without any LiF buffer layer.

A thin layer of LiF had been used to modify the low-work-function Mg:Ag alloy cathode and the ITO anode in Alq3-based OLEDs. A thin layer of LiF can enhance electron injection when inserted only between the organic electron transporting layer and the Mg:Ag alloy cathode, but can block hole injection when it was inserted only between the ITO anode and organic hole-transport layer. By both inserting a 1.0-nm LiF layer at the side of the ITO anode and a 0.5-nm LiF layer under the Mg:Ag cathode, the device demonstrated the highest performance among all the types of devices investigated in this study. This is due to increased electron injection and simultaneously decreased hole injection, which results in a more balanced electron and hole injection, thus eliminating non-productive hole current and hence a higher value of cd/A.



FIGURE 2 — Luminance versus voltage characteristics of the four types of devices with or without a 1.0-nm LiF-buffered layer inserted on one or both sides of the OLED stack, between the electrode and organic layer.


A new driving method introducing a display period for AMOLEDs

H. Kageyama
H. Akimoto
Y. Shimizu
T. Ouchi
N. Kasai
H. Awakura
N. Tokuda
T. Sato

Hitachi Central Research Laboratories

Abstract — We have proposed an Advanced-Clamped-Inverter driving method for the fabrication of AMOLEDs, in which one frame period is divided into an addressing period and a display period. This driving method enables the AMOLEDs to produce excellent moving images without motion blur and false pixels, and has peak-luminance characteristics because of its unique light-emission scheme of the OLED elements. Good inter-pixel uniformity was also achieved in a previous clamped-inverter driving method. We fabricated AMOLEDs and experimentally confirmed their characteristics.

A pixel circuit for this driving method was designed and fabricated for a 3.5-in. AMOLED panel with QVGA resolution driven by the LCD driver IC. The amplitude of its output voltage (5 V) is sufficient in order to achieve a contrast ratio of over 1000. In the experiment, the gray-scale characteristics showed good inter-pixel uniformity of 1.2% at a VDATA of 5 V, and a peak-luminance characteristic was obtained by having a voltage drop independent of the gray-scale control. We succeeded in displaying moving images with no apparent motion blur and no false pixels.



FIGURE 4 — Simulation results for the light-emitting scheme of the OLED element by Advanced-CI driving method. The light-emitting period varies symmetrically around the central point in the displaying period.


Applications of OLEDs that use VUV-CVD films

Kiyohiko Toshikawa
Junichi Miyano
Atsushi Yokotani
Kou Kurosawa
Yoshiie Matsumoto

Miyazaki Oki Electric Co.

Abstract — In photo-CVD (chemical vapor deposition) in which vacuum-ultraviolet (VUV) excimer lamps (VUV-CVD) are used, thin films were deposited at room temperature because VUV photons have the energy to decompose material gases. For the use of OMCTS (octa-methylcyclotetrasiloxane), an organic siloxane, we can deposit a self-flatness film for high-pressure conditions. The reactants generated by VUV photons have excellent migration characteristics for this condition. Also, the VUV-CVD film demonstrates low stress, comparatively hard hardness, good electrical properties, and good thermal resistance. The VUV-CVD film is optimum for planarizing film in the over-coating deposition step in the production of OLEDs, which requires a low-temperature process.

Recently, a novel photo-CVD method using vacuum-ultraviolet ex-cimer lamps (VUV-CVD) has gained attention. Because VUV-CVD uses only photon energy which decomposes material gases, SiO2 films can be deposited at room temperature using TEOS (tetraethoxysilane) as a material gas. The characteristic of VUV-CVD film depends on the structure of the material gas. In electrical properties, the VUV-CVD film using an annular organic siloxane demon-strates low leakage current. Also, the fast deposition rate will be useful in mass production. In this study, we focus on the characteristics of VUV-CVD film using OMCTS (octamethylcyclotetrasiloxane) of an annular organic siloxane. This film has self-flatness coverage during the high-pressure condition. Also, it has features of low damage, low stress, good electrical properties and good thermal resistance. The VUV-CVD film is very suitable for thin-film preparations of OLEDs which require low-temperature processes.



FIGURE 8 — Applications to cathode separator using VUV-CVD film.