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- Front Matter: Volume 6655
- OLED Materials I
- OLED Materials II
- Stability Issues in OLEDs
- Triplet Emitters
- OLEDs and Fabrication Processes
- Novel Light Emitting Structures and Devices
- Organic Lasers
- OLEDs and Solid State Lighting
- OLED Lighting
- Light-Emission in Organic Structures and Field-Effect Transistors
- Charge Injection and Transport in Organic Devices: Joint Session with Conference 6656
- Interfaces in Organic Devices: Joint Session with Conference 6656
- Poster Session
Front Matter: Volume 6655
Front Matter: Volume 6655
Show abstract
This PDF file contains the front matter associated with SPIE Proceedings Volume 6655, including the Title Page, Copyright information, Table of Contents, and the Conference Committee listing.
OLED Materials I
Recent progress in deep-blue phosphorescent OLEDs
Show abstract
Cyclometallated iridium N-heterocyclic carbene (NHC)-complexes have become known as efficient deep blue triplet
emitters in OLEDs. With these emitters suitable CIE color coordinates of CIE x ~ 0.15 and CIE y = 0.1...0.2 can easily
be reached. To keep the expensive and tedious synthetic and laboratory screening effort for new emitters and
complementary materials as efficient as possible a good computational pre-screening method based on quantum
chemical theory is used. In this paper, data will be presented which show a good correlation between calculated and
measured values of for example triplet energy, ionization potential and electron affinity. Only by having good control of
these parameters it is possible to design efficient and long lasting devices. Based on this, we will show our progress in
the deep-blue color region by optimizing the device setup and by employing a new, much more stable complementary
material set.
Origin of the different emission wavelengths in Alq3 analyzed by solid-state NMR
Show abstract
Tris(8-hydroxyquinoline) aluminum(III) (Alq3) is one of the most widely used materials in organic light-emitting diodes
(OLEDs), and the relationship between the structures and the luminescent wavelengths is of recent interest; yellowish-green
emissions are observed for the α- and amorphous Alq3, whereas blue emissions are found for the γ- and δ-Alq3. In
order to clarify the relationship between the structures and the emission wavelengths, we carried out solid-state nuclear
magnetic resonance (NMR) experiments on the different polymorphs of Alq3 and the amorphous state. Based on 27Al
and 13C magic angle spinning (MAS) NMR experiments, it is found that the isomeric state of the amorphous Alq3 is the
same as that of α-Alq3 and is different from those of γ- and δ-Alq3. Not only for the amorphous, but also for α-Alq3, the
local structures are found to be disordered. We also obtained clear evidence that γ-Alq3 is in the facial isomeric state. It
is suggested that δ-Alq3 is also facial. The difference between γ- and δ-Alq3 is the intermolecular packing; the effect of
intermolecular packing is found only for δ-Alq3. A further confirmation of the isomeric states of these Alq3 samples is
obtained from temperature-dependent X-ray diffraction experiments.
Tuning the properties of polythienothiophene conductive polymers for hole injection layer application
Xuezhong Jiang,
Keith Campbell,
Fabrice Amy,
et al.
Show abstract
It has been demonstrated that Air Products(R) HIL (hole injection layer) material based on the
conducting polymer polythienothiophene (PTT) and poly(perfluoroethylene-perfluoroethersulfonic
acid) (PFFSA) dramatically improves the lifetime of polymer light emitting diodes. Compared with
other conductive polymer HILs, PTT based HILs have some unique properties. The resistivity of
PTT:PFFSA films is sensitive to the annealing temperature. The resistivity dependence on annealing
temperature is not favorable for certain applications (e.g., in passive matrix display applications,
where too low a resistivity after annealing can lead to cross-talking), or from the point view of
process control. We have found that raising the pH of PTT:PFFSA dispersions can suppress the
resistivity sensitivity to annealing conditions. At the same time, raising the pH of PTT:PFFSA
dispersions also lowers the work function of PTT:PFFSA films. When LumationTM Green 1304 light
emitting polymer is used as the emitting layer, all PTT:PFFSA based devices showed lifetime that is
several times longer than that of PTT:PSSA based devices. Among the PTT:PFFSA dispersions, pH
adjusted ones show a lower leakage current, lower efficiency and shorter device lifetime compared
with the original dispersion. We have also explored the application of PTT:PFFSA in small molecule
devices. Longer device lifetime has been obtained in devices using PTT:PFFSA as HIL and
aluminum tris(8-hydroxyquinoline) (Alq3) as emitter compared with devices using copper
phthalocyanine (CuPc) as HIL. We have also found that hole injection from PTT:PFFSA into hole
transport materials commonly used in small molecule devices is very efficient.
OLED Materials II
Designing organic phosphine oxide host materials using heteroarmatic building blocks: inductive effects on electroluminescence
Show abstract
Phosphine oxide substitution of small molecules with high triplet exciton energies allows development of vacuum
sublimable, electron transporting host materials for blue OLEDs. Heteroaromatic building blocks (carbazole,
dibenzofuran and dibenzothiophene) with ET ~ 3 eV were incorporated into phosphine oxide (PO) structures. External
quantum efficiencies (EQEs) at lighting brightness (i.e., 800 cd/m2) reached as high as 9.8% at 5.2V for OLEDs using
the heteroaromatic PO hosts doped with the sky blue phosphor, iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C2,)
picolinate (FIrpic). Comparing device properties at a similar current density (i.e., J = 13 mA/cm2) showed the
dibenzothiophene-bridged PO compound exhibits the highest EQEs and lowest operating voltages at all phosphor dopant
levels. These results are explained with respect to the effects of the inductive phosphine oxide substituents on
electrochemical, photophysical and electroluminescence properties of the substituted heteroaromatic building blocks.
Synthesis and electroluminescent properties of poly(p-phenylenevinylene)s with 3’,3’-diheptyl-3,4-propylenedioxythiophene pendant group for light-emitting diode applications
Show abstract
Using the Gilch polymerization method, we synthesized a new series of green electroluminescent polymer, poly[1,4-{2-
(3,3'-diheptyl-3,4-propylenedioxythiophen-2-yl)}phenylenevinylene], poly(PDOT-PV), which is a series of fully
conjugated poly(p-phenylenevinylene) derivatives with a propylenedioxythiophene (PDOT) moiety as a side-chain. We
also synthesized copolymers, poly(PDOT-PV-co-m-SiPhPV), of poly(PDOT-PV) with poly[2-(3-dimethyldodecyl-
silyphenyl)-1,4-phenylenevinylene], poly(m-SiPhPV), segments. The resulting polymers were highly soluble in common
organic solvents and could be easily spin-coated onto an indium-tin oxide coated glass substrate to obtain high quality
optical thin films. The weight-average molecular weight (Mw) and polydispersity of poly(PDOT-PV) were 22.0 ×104
and 5.3, respectively, and those of poly(PDOT-PV-co-m-SiPhPV) were in the range of (23.2-36.7) ×104 and 5.0-5.8,
respectively. The stability of the resulting polymers is adequate for the fabrication of devices, and they provide longevity
to devices because they have high glass transition temperatures (Tg). We fabricated polymer light-emitting diodes
(PLEDs) in ITO/PEDOT/light-emitting polymer/cathode configurations using either double-layer LiF/Al or triple-layer
Alq3/LiF/Al cathode structures. For PLEDs containing poly(PDOT-PV) and poly(PDOT-PV-co-m-SiPhPV), the
performance was highest using triple-layer cathodes. The turn-on voltages of PDOT-based light-emitting polymers were
in the range of 6.0-9.0 V, and the maximum brightness and luminance efficiency were 5127 cd/m2 at 18 V and 3.75 cd/A
at 9 V.
New hole-transporting amorphous molecular materials with high glass-transition temperatures for organic light-emitting diodes
Kenji Okumoto,
Hidekaru Doi,
Hiroshi Kageyama,
et al.
Show abstract
The synthesis and properties of new hole-transporting amorphous molecular materials with high glass-transition
temperatures are described. They include 4,4',4"-tris[9,9-dimethylfluoren-2-yl(phenyl)amino]-
triphenyl-benzene (TFAPB), 4,4',4"-tris[9,9-dimethylfluoren-2-yl(4-methylphenyl)amino]triphenylbenzene
(MTFATB), and 4,4',4"-tris[bis(9,9-dimethylfluoren-2-yl)amino]triphenyl-benzene (TBFAPB).
It is shown that they function well as hole-transporting materials in thermally stable organic light-emitting
diodes.
Organic materials for blue emission OLEDs
Show abstract
In past couple of decades, organic EL materials with excellent characteristics have been searched and devices
operational stability and efficiency has been significantly improved. However, as an emerging technology for the
multibillion-dollar flat-panel-display industry, more typically with continue improvement of liquid crystal displays
(LCDs), organic light-emitting diodes (OLEDs) display technology face many challenges. In particular, organic
materials for stable and efficient blue EL emission are one of an important subject of these challenges. In this paper, we
will review our efforts in developing the organic materials for blue emission of organic electroluminescent devices after
reviewing the history of blue EL materials development in past couple decades. Our efforts in developing the organic
materials for blue emission of organic electroluminescent devices will include the following:
1. Fused aromatics fluorescent blue EL materials
2. Mixed cyano-isocyanide cyclometalated iridium complex phosphorescent blue EL materials
3. Exploration of the effects of blue emission stability and efficiency.
Efficient blue organic light-emitting diodes with 4H-cyclopenta[def]phenanthrene
Show abstract
A novel blue emitters, 2-[10-(4,4-dioctyl-4H-cyclopenta[def]phenanthrene-2-yl)-9-anthryl]-4,4-dioctyl-4Hcyclopenta[
def]phenanthrene (OCPA) and 4,4,4',4',4",4"-hexaoctyl-2,6':2',6"-ter cyclopenta[def]-phenanthrene
(TerCPP), has been synthesized and characterized. The introduction of CPP units into the structure of OCPA and
TerCPP leads to high efficiency and pure blue color property. Thermal analysis of OCPA and TerCPP reveals their high
thermal stability. Their decomposition temperatures, which correspond to a 5% weight loss upon heating during TGA,
are around 410 °C for OCPA and 425 °C for TerCPP. The high-quality amorphous films of OCPA and TerCPP with good
morphological stability can be prepared by vapor deposition. Multilayer organic EL devices constructed using OCPA
and TerCPP as an emitting layer produced bright blue emissions. The UV-visible absorption spectra of theses
compounds appear at about 290-400 nm, and their maximum PL emission spectra of OCPA and TerCPP in THF solution
appeared at about 438 nm and 390 nm, respectively. The EL spectra showed maximum peaks at about 434 nm and 440
nm, respectively. The turn-on voltage of compounds was about 6V, and the luminous efficiency is 1.0-1.2 cd/A.
Emission color of OCPA was deep blue and CIE coordinate (0.16, 0.11) is quite close to that of the National Television
System committee (NTSC) standard vlue (0.14, 0.88).
Stability Issues in OLEDs
Emission and degradation mechanism of PLED
Show abstract
Emission and degradation mechanism of polymer light emitting diode (PLED) was investigated by using trap analysis.
The device structure in this study is ITO/PEDOT-PSS/LEP/Ba/Al, where LEP (light emitting polymer) is polyfluorene
type Lumation Green 1300 series supplied from Sumation Co., Ltd. The trap behaviors of virgin and degraded devices
were investigated by using the bipolar devices, the hole only devices and the electron only devices. By analyzing the
results, we successfully clarified depth and density of each trap at the interfaces and bulks of this PLED device. PL aging
behavior and EL aging behavior were also examined for investigating mechanisms. This study shows such novel
information that carrier traps at the PEDOT-PSS/LEP interface play an important role in emission and degradation characteristics.
Triplet Emitters
Spin-orbit coupling routes and OLED performance: studies of blue-light emitting Ir(III) and Pt (II) complexes
Andreas F. Rausch,
Herbert Homeier,
Peter I. Djurovich,
et al.
Show abstract
In this study, detailed spectroscopic investigations of the blue emitting compounds Ir(4,6-dFppy)2(pic) and Pt(4,6-
dFppy)(acac) are presented. Due to spin-orbit coupling (SOC) of the emitting triplet state with higher lying singlet states
both complexes show an intense phosphorescence and are utilized as emitters in organic light emitting diodes (OLEDs).
Distinct differences with respect to important photophysical properties are found for the two compounds.
For example, the (distorted) octahedral Ir(4,6-dFppy)2(pic) complex exhibits a shorter emission decay time and shows a
larger zero-field splitting (ZFS) than the (distorted) square planar Pt(4,6-dFppy)(acac) complex (τ(Ir) = 0.4 μs and τ(Pt)
= 3.6 μs of the respective shortest-lifed triplet substate; Δ(ZFS, Ir) = 67 cm-1, ΔE(ZFS, Pt) = 8 cm-1). This behaviour is
connected with the extent of metal-to-ligand charge transfer (MLCT, dπ*) character in the emitting triplet state. High
MLCT character usually results in a high emission decay rate and indicates a good suitability as OLED emitter material.
Of crucial importance in this respect is the effectiveness of SOC. In this study it is shown that the SOC routes depend on
the coordination geometry of the emitter compound. In particular, the couplings can be more effective in (distorted)
octahedral than in (distorted) square planar compounds. Hence, the photophysical differences of Ir(4,6-dFppy)2(pic)
compared to Pt(4,6-dFppy)(acac) can be rationalized. Moreover, this investigation shows that the analysis of SOC paths
provides general guidelines for the design of efficient emitters for OLED applications.
Phosphorescent OLEDs with saturated colors
Show abstract
In this paper, two approaches are demonstrated to narrow phosphorescent OLED (PHOLED) emission lineshapes to
increase color saturation while keeping device high efficiency performance, which is critical for large area flat panel
displays. One approach uses bottom-emissive microcavity structure in green and blue devices to achieve 22 nm full
width half maximum (FWHM) emissions. The other approach is to reduce the natural width of the emission as
exemplifying in a red device. A new NTSC red with 64 nm FWHM emission is reported. In a standard device, it has a
luminous efficiency of 18.3 cd/A at 10 mA/cm2.
Harvest of triplet excitons in fluorescence emission layer based on a wide band gap host of TcTa for efficient white organic light emitting diodes
Show abstract
Selective energy transfer from triplet states of the fluorescent blue emission layer to a red phosphorescent dye in a
neighbored triplet harvesting layer has been achieved, which has provided improved efficiency with emissions from
fluorescent and phosphorescent dyes. First of all, it is crucial to find a wide band gap host for a fluorescent blue emission
layer which has larger triplet state band gap than green or red phosphorescent dye. It was found that TcTa is a good wide
band gap host for fluorescent blue dopant(BD) and a efficient blue device was obtained. A phosphorescent red dopant
(RD) was introduced into a neighboring electron transporting layer to harvest triplet states in the fluorescent blue
emission layer and by optimizing the distance between the blue emission layer and the red triplet harvesting layer, we
have succeeded in obtaining the balanced emission of the blue and the red emissions with high efficiency from the
device structure of NPB/TcTa:BD/BAlq/BAlq:RD/BAlq/LiF/Al. The device showed maximum external quantum
efficiency of 16 % at 0.1 mA/cm2 and 13 % of external quantum efficiency, (0.29, 0.23) of CIE coordinates and 920
cd/m2 at 10 mA/cm2. To realize RGB WOLED, a fluorescent green dopant was introduced into the blue emission layer.
The RGB WOLED was successfully obtained through optimization of doping concentration for green dopant and it
showed 10 % of external quantum efficiency, (0.36, 0.36) of CIE coordinates and 1400 cd/m2 at 10 mA/cm2.
Harvesting triplet excitons from fluorescent blue emitters for high-efficiency white organic light emitting diodes
Show abstract
We present a novel organic light emitting device concept for white light generation with the potential for 100%
internal quantum efficiency, which employs fluorescent blue and phosphorescent green and orange emitters. Due
to its high triplet energy, the intrinsically non-radiative triplet excitons of the fluorescent blue emitter can still
be harvested for light emission by letting them diffuse to the phosphor-containing emission layers. Thus, all
electrically generated excitons can be used for light emission without the need for phosphorescent blue emitters,
which suffer from stability problems. We demonstrate the high potential of this concept in a device achieving
57.6 lmW-1 total external power efficiency at 100 cd m-2 (20.3% external quantum efficiency) and 37.5 lmW-1
(14.4%) at an illumination relevant brightness of 1,000 cd m-2, and a high color rendering index of 86.
OLEDs and Fabrication Processes
Spectrally narrowed edge emission from leaky waveguide modes in OLEDs
Show abstract
A dramatic spectral line narrowing of the edge-emission, at room temperature, from tris(quinolinolate) Al (Alq3),
N,N'-diphenyl-N,N'-bis(1-naphthylphenyl)-1,1'-biphenyl-4,4'-diamine (NPD), 4,4'-bis(2,2'-diphenyl- vinyl)-1,1'-
biphenyl (DPVBi), and some guest-host small molecular OLEDs, fabricated on ITO-coated glass, is described. In all
but the DPVBi OLEDs, the narrowed emission band emerges above a threshold thickness of the emitting layer, and
narrows down to a full width at half maximum of only 5 - 10 nm. The results demonstrate that this narrowed emission
is due to irregular waveguide modes that leak from the ITO to the glass substrate at a grazing angle. While
measurements of variable stripe length (l) devices exhibit an apparent weak optical gain, there is no observable
threshold current or bias associated with this spectral narrowing. It is suspected that the apparent weak optical gain is
due to misalignment of the axis of the waveguided mode and the axis of the collection lens of the probe, but it is not
clear if such a misalignment can account for the for the observed evolution of the edge emission spectra with l.
Highly efficient fully transparent inverted OLEDs
Show abstract
One of the unique selling propositions of OLEDs is their potential to realize highly transparent devices over the
visible spectrum. This is because organic semiconductors provide a large Stokes-Shift and low intrinsic absorption
losses. Hence, new areas of applications for displays and ambient lighting become accessible, for instance, the
integration of OLEDs into the windshield or the ceiling of automobiles. The main challenge in the realization of
fully transparent devices is the deposition of the top electrode. ITO is commonly used as transparent bottom
anode in a conventional OLED. To obtain uniform light emission over the entire viewing angle and a low series
resistance, a TCO such as ITO is desirable as top contact as well. However, sputter deposition of ITO on top of
organic layers causes damage induced by high energetic particles and UV radiation. We have found an efficient
process to protect the organic layers against the ITO rf magnetron deposition process of ITO for an inverted
OLED (IOLED). The inverted structure allows the integration of OLEDs in more powerful n-channel transistors
used in active matrix backplanes. Employing the green electrophosphorescent material Ir(ppy)3 lead to IOLED
with a current efficiency of 50 cd/A and power efficiency of 24 lm/W at 100 cd/m2. The average transmittance
exceeds 80 % in the visible region. The on-set voltage for light emission is lower than 3 V. In addition, by vertical
stacking we achieved a very high current efficiency of more than 70 cd/A for transparent IOLED.
Fabrication of multi-layered polymer LEDs by resonant infrared pulsed-laser deposition
Show abstract
Multi-layered polymer light-emitting diodes (PLEDs) have been fabricated in a vacuum environment by resonant
infrared pulsed-laser deposition of the polymer layers. The light emitter used was poly[2-methoxy-5-(2-
ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV), and in some cases a layer of the hole-transport polymer
poly(3,4 etylenedioxythiophene:polystyrenesulfonate) (PEDOT:PSS) was also laser deposited, resulting in a device
structure of ITO/PEDOT:PSS/MEH-PPV/Al. Fourier transform infrared (FTIR) spectroscopy confirmed
that neither of the laser-deposited polymers was significantly altered by the deposition process. Laser-fabricated
devices displayed electroluminescent spectra similar to those of conventional spin-coated devices, but the differences
in electrical characteristics and device efficiency were substantial. These discrepancies can probably
be attributed to surface roughness of the deposited polymer layers. With the appropriate refinement of the
deposition protocols, however, we believe that this process can be improved to a level that is suitable for routine
fabrication of organic electronic components.
Technical issues of stainless steel foil substrates for OLED display applications
Show abstract
Key technical issues of flexible stainless steel foil substrates are addressed for OLED display backplane
applications. Surface roughness and corresponding planarization layer technology development will be the major factors
for the stainless steel foil substrates to be used for commercial applications. Promising candidates for the planarization
layer materials are reviewed and some of the properties are addressed. In addition, if the substrate is sustained to a
constant voltage for guaranteed circuit operation, capacitive coupling through the insulation and planarization dielectric
layer, from the conductive substrate to the electrode and circuit elements on it, is also carefully analyzed for panel
design and operation. Especially for large size high-resolution display applications, low k and thick planarization layer
should be used.
Novel Light Emitting Structures and Devices
Strong exciton-photon coupling in thermally evaporated polycrystalline organic materials
Show abstract
Studies of strong exciton-photon coupling in organic materials have progressed at a rapid pace since the first observation
of microcavity polaritons in tetra-(2,6-t-butyl)phenol-porphyrin zinc less than ten years ago. Current research is driven
by the potential for new optoelectronic devices based on polaritonic phenomena such as ultrafast optical amplifiers and
switches, enhanced nonlinear optical materials, and coherent light emitters, known as polariton lasers. This paper
reviews experimental advances related to strong coupling in thermally evaporated organic materials, and their potential
application in future optoelectronic devices.
Organic Lasers
Loss processes in organic double-heterostructure laser diodes
Show abstract
At high current densities, the characteristics of organic laser diode structures are strongly influenced by a variety
of loss processes such as bimolecular annihilations, field-induced exciton dissociation and induced absorptions
due to polarons and triplet excitons. Here, we investigate a TE2-mode organic double-heterostructure laser diode
by numerical simulation. The electrical properties are described using a numerical drift-difusion model and the
optical characteristics are modeled using a transfer matrix method. When annihilation processes are included,
a threshold current density of 8.5 kA/cm2 is derived for the considered device. Laser operation is not achieved
when field-induced exciton dissociation is considered. For induced absorptions, maximum relative cross sections
of 9.6 × 10-8 for polarons and 1.4 × 10-4 for triplet excitons have been calculated, which would still allow laser
operation. For higher relative absorption cross sections, laser operation is suppressed for all current densities.
Furthermore, the impact of field quenching is analyzed and the separation of singlet excitons from polarons and
triplet excitons in the time domain is studied.
Polymer lasers: recent advances
Show abstract
The development of organic thin film lasers has seen tremendous progress over the past few years. Only a few
materials are necessary to allow for continuous wavelength tunability in the spectral region from the UV to the
near IR. At the same time, the lasing thresholds of organic thin film lasers have been reduced considerably both
due to improved low-loss distributed feedback (DFB) resonator structures and highly efficient gain materials
based on guest-host energy transfer. Aside from the as yet open issue of electrical operation of organic lasers,
which we will address briefly in this paper, there are numerous applications (e.g. in biotechnology, spectroscopy)
where optically driven organic lasers may be the more cost effective and versatile solution. In this context, tunable
polymer lasers pumped by compact and inexpensive InGaN laser diodes will be shown. These lasers are based on a
modified poly(9,9'-dioctylfluorene) derivative (BN-PFO) containing 12% of -6,6'-(2,2'-octyloxy-1,1'-binaphthyl)
spacer groups doped with a few wt% of the stilbene dye 1,4-Bis(2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl-benzene
(DPAVB). With the same host polymer (BN-PFO) quasi continuous wave operation (up to 5 MHz) can be
demonstrated. Highly repetitive lasers are especially desirable for many spectroscopic applications. This regime
of operstion is found to be impeded by the photo-physics in doped organic systems where the accumulation of
absorptive species in the gain medium leads to piled-up absorption losses and consequently to termination of the
lasing process. The presence of the dopand molecules seems to strongly promote the formation and stabilization of
the species which we relate to triplet excitons. Therefore, the concentration of the dopand affects the feasibility of
quasi-cw operation of thin-film organic lasers. Strategies and results to achieve highly repetitive operation in low-threshold
guest-host systems BN-PFO:DPAVB or BN-PFO:poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene
vinylene] (MEH-PPV) will be presented.
Diode-pumped polymer lasers
Show abstract
In this paper we describe the design and performance of diode-pumped organic lasers based on the poly(paraphenylene-vinylene)
derivative MEH-PPV. To achieve the very low oscillation thresholds required for direct diode pumping, we
use a novel surface-emitting distributed Bragg reflector cavity. We describe the operating characteristics of such devices
when operating below and above threshold, and show that they can combine low threshold operation with the favourable
spectral and emission characteristics of DFB lasers. We also describe and characterize an energy transfer gain medium
using coumarin 102 laser dye as the host, which has been optimized for efficient harvesting of the diode laser excitation.
OLEDs and Solid State Lighting
Employing microcavity effects to enhance performances of white-emitting OLEDs
Show abstract
We have developed an effective approach based on wavelength-selective mirrors to implement three-peak WOLEDs that
have EL spectra matching better with transmission spectra of typical color filters and thus give much enhanced color
gamut for full-color OLED display applications. The wavelength-selective mirror used here is highly compatible with
OLED fabrication.
Progress in wet-coated organic light-emitting devices for lighting
Show abstract
Here we present recent progress in developing efficient wet-coated organic light-emitting
devices (OLEDs) for lighting applications. In particular, we describe a novel approach
for building efficient wet-coated dye-doped blue phosphorescent devices. Further, a novel
approach for achieving arbitrary emission patterning for OLEDs is discussed. This approach
utilizes a photo-induced chemical doping strategy for selectively activating charge injection
materials, thus enabling devices with arbitrary emission patterning. This approach may provide
a simple, low cost path towards specialty lighting and signage applications for OLED
technology.
OLED Lighting
Improving the light extraction efficiency of polymer LEDs using microcavities and photonic crystals
Show abstract
A significant fraction of light generated within an organic light emitting diode (OLED) is often trapped within the
structure within waveguide modes and is unable to escape usefully from the device. Addressing this issue is of
significant importance, as it potentially offers a route to improve the external efficiency of OLEDs. Here, we discuss a
number of methods to improve light extraction efficiency from conjugated-polymer LEDs. Firstly we explore the use of
low finesse optical microcavities to redistribute trapped-light into externally propagating modes. The improvements
obtained by simply adopting a microcavity structure on its own are rather small, however we then show that they can be
improved significantly by improving the reflectivity of the cathode. Finally, we show that by engineering a photonic
crystal beneath the anode of a polymer LED, a significant improvement in external efficiency (by a factor of 2) can be
achieved. Such an approach is anticipated to be readily scalable to a manufacturing environment.
Concepts for high efficient white OLEDs for lighting applications
Show abstract
Apart from usage of organic light emitting diodes for flat panel display applications OLEDs are a potential candidate for
the next solid state lighting technology. One key parameter is the development of high efficient, stable white devices. To
realize this goal there are different concepts. Especially by using highly efficient phosphorescent guest molecules doped
into a suitable host material high efficiency values can be obtained. We started our investigations with a single dopant
and extended this to a two phosphorescent emitter approach leading to a device with a high power efficiency of more
than 25 lm/W @ 1000 cd/m2. The disadvantage of full phosphorescent device setups is that esp. blue phosphorescent
emitters show an insufficient long-term stability. A possibility to overcome this problem is the usage of more stable
fluorescent blue dopants, whereas, due to the fact that only singlet excitons can decay radiatively, the efficiency is lower.
With a concept, proposed by Sun et al.1 in 2006, it is possible to manage the recombination zone and thus the
contribution from the different dopants. With this approach stable white color coordinates with sufficient current
efficiency values have been achieved.
White phosphorescent organic light emitting devices
Show abstract
OLED display manufacturers are interested in white organic light emitting devices (WOLEDTMs) because
these devices, together with color filters, eliminate the need for high resolution shadow masks, and are
scalable beyond Gen 4 substrates. Additionally, WOLEDs are well suited for general-purpose illumination,
since their power efficacies are approaching fluorescent lamps. A new structure was developed that had the
following characteristics that were measured using a 20" integrating sphere: at 100 cd/m2 normal luminance,
EQE = 35%, power efficacy is 62 lm/W, operating voltage = 4.4 V, CIE = (0.33, 0.43) and CRI = 70.
Light extraction for a doubly resonant cavity organic LED: the RC2LED
Show abstract
The RC2LED is a substrate emitting OLED which has three additional interference layers between the ITO
electrode and the glass substrate. This creates two resonant optical cavities. The RC2LED has 2 resonant
optical cavities. The first cavity is also present in regular devices and is formed by metal/organic layers/ITO.
The second cavity is formed by 3 additional layers: a high refractive index layer (Nb2O5), a low refractive index
layer (SiO2) and a high refractive index layer (Nb2O5). The additional layers introduce a strong wavelength
dependent improvement of the extraction efficiency compared to the OLED without the additional layers.
Our simulations show an improvement of the extraction efficiency of over 70% over a wavelength range of
75 nm compared to an OLED without the 3 layers. Light extraction is worse compared to the reference OLED
for wavelengths outside this wavelength range. the when compared to the OLED. This improvement has been
experimentally verified for a green OLED with an emission between 500nm and 650 nm.
A numerical study shows a relative improvement of 10% for the luminous power efficiency of a 3 color white
OLED with the additional layers. The emitted white corresponds with the light emitted by illuminant A. The
WOLED has been composed of a fluorescent blue emitter, green and red phosphorescent emitters.
Light-Emission in Organic Structures and Field-Effect Transistors
Influence of the dielectric and of the active layer doping on the FET mobility in PPV-based devices
Show abstract
We report on the influence of the dielectric/organic interface properties on the electrical characteristics of field-effect
transistors based on Poly-phenylenevinylene derivatives. We observe a direct influence of the dielectric surface on the
field-effect mobility as well as on the charge injection at the source electrode, despite the fact that we used a top contact
transistor structure.
We find that the presence of traps at the dielectric surface, decreases the hole mobility and increases the threshold
voltages. By treating the silicon dioxide dielectric surface with gas phase molecules such as octadecyltrichlorosilane
(OTS) and hexamethyldisilazane (HMDS) the hole mobility improves and the threshold voltage slightly increases.
The effects of a dielectric polymer layer spin coated onto silicon dioxide substrates before deposition of the
semiconductor polymer can be related to the density of the oxydryl groups (-OH ), which are the most efficient traps for the charges flowing in the device. We use different polymer species such as polyvinylalchol (PVA),
polymethylmetacrilate (PMMA) and a cyclotene derivative (B-staged bisbenzocyclobutene or BCB). The elimination of
the -OH groups and of other traps, produces the same effect observed with HMDS coupled to a more pronounced
enhancement of the threshold voltage, with the exception of PMMA. The electrical characteristics obtained with HMDS
and PMMA polymer dielectrics are the highest reported to date for PPV-based field-effect transistors.
We confirm that the purification of the active material is crucial to enhance the device performances and to achieve a
better device to device reproducibility.
We also investigated the effect of the dispersion of a phosphorescent dye into the active polymeric material. The
electrical characteristics of OFETs with HMDS or PMMA dielectric with and without dye doping are compared.
Charge Injection and Transport in Organic Devices: Joint Session with Conference 6656
The role of isoelectronic dopants in organic light emitting diodes
Show abstract
Power efficiency is an important parameter for all OLEDs, and is particularly critical for lighting applications. To
maximize the power efficiency one must optimize charge injection, carrier transport, and radiative quantum efficiency,
while minimizing energy losses. In this work we discuss how isoelectronic dopants can be used to address these
problems. It can be difficult to produce efficient electrical contacts, particularly to large energy gap organic materials,
and thus the contacts often limit the performance and stability of OLEDs . Recent results by several groups have
attributed improved hole injection in poly (9,9' dioctylfluorene) [PFO] based LEDs to charge trapping, but the origin of
the traps is unknown. In order to understand the role of traps in improving injection we studied poly[2-methoxy, 5-(2'-
ethyl-hexyloxy)-1,4-phenylene vinylene] (MEH-PPV) devices with C60 molecules at the anode to improve hole
injection. Isoelectronic dopants are used widely as recombination centers in organic light emitting diodes (OLEDs). In
these systems one wants to maximize quantum efficiency by effectively trapping charges on the emitting dopants, while
at the same time maximizing power efficiency by maintaining good charge transport. An understanding of the influence
of the depth of the dopant on charge capture, and charge transport will aid in optimizing doped organic LEDs. We have
looked at the OLED system consisting of the polymer PFO, and the organometallic molecule PhqIr. We show that PhqIr
acts as a shallow hole trap in PFO, and that the charge transport and luminescence properties of this system are described
by quasi-equilibrium statistics.
Interfaces in Organic Devices: Joint Session with Conference 6656
Enhanced electron injection and performance in organic light emitting devices by LiF doping
Show abstract
Optimization of charge injection in the active emitting layer and balanced transport of carriers are important in realizing
high efficiency and good reliability in organic light emitting devices (OLEDs). Electrical doping of such molecular
materials with a view to enhancing their conductivity is an attractive route for enhancing the performance and versatility
of these optoelectronic devices, in particular by enhancing carrier injection and lowering operating voltages. In the
present study, we demonstrate efficient n-type doping of tris-(8-hydroxyquinoline) aluminum (Alq3) with the inorganic
insulator lithium fluoride (LiF) by co-evaporation. The effect of dopant concentration on charge injection and carrier
transport in this system is studied. We demonstrate that optimal doping not only leads to enhanced device currents and
lower operating voltages, but also changes the charge transport from trap-limited to space-charge-limited transport.
Using this scheme, we achieve efficient electron injection without using low work function cathodes. Finally, we employ
the optimally-doped electron transport layers in OLED architectures to demonstrate devices with enhanced efficiency
and lowered operating voltages.
Self-assembled monolayer modification of PEDOT:PSS interface to improve the device performance in blue PLED
Show abstract
As a result of intensive research on polymer light-emitting diodes (PLEDs) for the last several years, the device
performances have been remarkably improved. Recently, several researchers reported on a PLEDs with an interlayer
between poly(3,4-ethylenedioxythiophene)-poly-(styrenesulfonate) (PEDOT:PSS) and an emissive polymer. It improved
the device efficiency as well as the device lifetime. The role of the interlayer is to block the electron from back diffusion
to PEDOT:PSS and/or to reduce luminescence quenching at the PEDOT:PSS interface.
We studied the improvement of the PLED by inserting an octadecyltrichlorosilane (OTS) as the interlayer between
PEDOT:PSS and the emissive layer. The OTS was treated on PEDOT:PSS through the self-assembled monolayer (SAM)
process. It improved the device efficiency of the PLED from 3.86 to 4.76 cd/A, and increased the operation lifetime from
270 to 340 minute comparing the non-OTS treated PLED with the OTS treated PLED for 10 min. In blue PLED,
inserting the OTS layer between blue polymer and PEDOT:PSS is promoted hole injection from an anode. Therefore, the
device efficiency is improved, which appears to be due to the increase of balanced recombination as a result of the
accumulated electrons near the interface between emissive layer and PEDOT:PSS.
Poster Session
Optical gain in Coumarin 545T-doped Tris(8-hydroxy-chinolinato)aluminium thin films
Show abstract
In this paper we investigate the optical gain in organic thin film waveguides using the variable stripe length
method (VSL). As active medium the guest-host system containing Tris-(8-hydroxy-chinolinato)-aluminium
(Alq3) doped by 10-(2-benzothiazolyl)-2,3,6,7-tetrahydro-1,1,7,7-tetramethyl-1H,5H,11H- (1)-benzopyropyrano-
(6,7-8-i,j)quinolizin-11-one (C545T) is studied. The doping concentration is varied over a wide range and the
gain coefficient is measured at different excitation densities to analyze the behavior of the differential gain. The
F¨orster energy transfer is responsible for the occupation of the exited state of the coumarin molecules. For
low doping concentrations with an inefficient host-guest energy transfer a low stimulated cross section can be
observed. At optimal doping concentrations (3.7-6.4 wt%) we obtain a cross section of σ =6.8x10-17 cm2 and a
high material gain of gmat ≈500 cm-1 at an excitation density Eex of Eex ≈300 μJ/cm2. A further increased
doping concentration (15 wt%) leads to a reduced cross section, due the onset of concentration quenching in
the guest-host system. Furthermore, at high excitation densities we observe a strong saturation effect of the
maximum gain which depends strongly on the doping concentration.
White light-emitting organic electroluminescent device based on a new orange organometallic iridium complexes
Show abstract
We develop the white organic light-emitting diodes (WOLEDs) with a new
orange electrophosphorescent emission, and the blue electrofluorescent or electrophosphorescent sensitizer. The new orange phosphorescent sensitizer is the thieno-pyridine framework organo-iridium complexes (PO-01). The blue phosphorsensitized
electrofluorescent is 4,4'-Bis(9-ethyl-3-carbazovinylene)-1,1'- biphenyl (DSA) doped into 4,4'-Bis(2,2-diphenyl-ethen-1-yl) diphenyl (DPVBi). Beside, the blue phosphorescent sensitizer is Bis(3,5-difluoro-2-(2-pyridyl)phenyl- (2-carboxypyridyl)iridium (FirPic). The Device Type I of WOLED based on the PO-01 and the DSA doped into DPVBi has an efficiency of 5.7 lm/W (10.6Cd/A) at 500
Cd/m2, a CIE coordinates of (0.33, 0.31), and a CRI of 71. However, the Device Type II of WOLED has an efficiency of 5.5 lm/W (10.3Cd/A) at 500 Cd/m2 and a CIE coordinates of (0.30, 0.42), while the FirPic replaces the DPVBi doped with DSA. The spectra of the Device Type II and I both response insensitive to drive current. Nevertheless, the Device Type I relatively achieves a balanced whit emission with CIE coordinates of (0.33, 0.33). They are good suitability to use in OLED lighting and full-color LCD backlights.
Optical characteristics of the OLED with microlens array film attachment
Show abstract
We investigated the luminance enhancement, spectral shift and image blur of the OLED with the microlens array
film (MAF) attachment experimentally and theoretically. Higher density, larger curvature, and smaller diameter of the
microlenses extracted more light from the substrate mode. The maximum improvements of the luminance at the normal
direction and the total power were 42.5% (80%) and 45% (85%) from our experimental (simulation) results, respectively.
The differences between the theoretical and experimental results may come from the non-Lambertian radiation of OLED
and the imperfection of the microlens array film. From observing the planar OLED, the peak wavelength is blue-shifted
and the full width at the half maximum (FWHM) decreased with respect to increasing viewing angles due to the
microcavity effect. When the MAF was attached, the spectral peak had a further blue shift (5 to 10 nm at different
viewing angles) compared to that of the planar OLED and it came from the light extraction of the MAF from the
substrate mode.
We also quantitatively investigated the "blur width" of the OLED with MAF attachment. Higher image blur was
observed as accompanied with higher extraction efficiency which showed a tradeoff between the image quality and
extraction efficiency. It means that the MAF attachment is more suitable for OLED lighting application, rather than
display application. To reduce the image blur and keep the high extraction efficiency at the same time, we re-designed
the arrangement of the microlens arrays on the film. In our optimized case, we found that the blur width can be reduced
from 79 μm to 9 μm, while the extraction efficiency is kept nearly the same. It shows a possibility to use the microlens
array film on real OLED display for improving the extraction efficiency without image quality degradation.
Oscillation method for uniform formation of solution-processed organic films and its application to organic light-emitting devices
Show abstract
We propose an oscillation method of solution-processed organic film coating. This method hindered a flow of solvent
with organic materials during drying process. We have applied this technique into fabrication of OLEDs. In this time,
oscillation is induced using piezoelectric actuator and wetted plastic roller is used for coating the organic film. By doing
this method, uniform organic film with a mean roughness of 3.8 nm was achieved. Spin-coated poly(ethylene
dioxythiophene) /poly(styrene sulfonate) (PEDOT) was used for hole buffer layer. Polyvinylcarbazole (PVCz) as a hole
transport material, (BND) as an electron transport material, and coumarin 6 (C6) as an emission material were used.
Device structure of ITO /PEDOT (40 nm)/PVCz+BND+C6 (100 nm)/LiF (1 nm)/ Al with area of 30×30 mm2 were tested. Emission was dramatically improved by changing the oscillation condition and uniform emission was observed at higher frequencies. This technique will be promising for large-area production and short tact time. And this method is also applicable for other fabrication methods, such as, spray method and ink-jet printing method.
Influence of cavity effects on light extraction efficiency in organic light emitting devices
Show abstract
In this paper, we demonstrate that the light extraction efficiency of an OLED is a strong function of the location of the
recombination zone and the ratio of the extracted mode to the substrate guided mode varies from 22% to 55%. The large
variation of the extraction efficiency in most OLEDs is the direct result of optical cavity effect present in the devices. In
addition, we show that the light intensity profile varies from a Lambertian shape to a non-Lambertian shape depending
of the device geometry.
Lasing characteristics of optically pumped edge-emitting organic semiconductor laser
Show abstract
Last year, we succeeded in reproducibly producing optically pumped edge-emitting organic semiconductor lasers using a low-temperature cleaving technique. Since the organic layer was generally soft and weak, its edge was damaged by the conventional cleaving at room temperature. This damage reduces the reflectance at the mirror edge and increases the threshold excitation energy. Stiffening the organic layer in liquid nitrogen enabled us to produce high-quality resonators with sufficient reproducibility. Slab waveguide devices consisting of Alq3:DCM film (5% DCM) were vacuum-deposited onto a polished GaAs (100) substrate coated with an 1-μm-thick layer of RF (radio-frequency) sputtered SiO2. The cleaved samples were optically pumped by a N2 gas laser (wavelength: 337 nm) resulting in a pulse width of 600 ps at repetition rate of 20 Hz. The laser oscillation was checked by measuring the full width at half maximum of the output spectrum and its polarization characteristics. The threshold density was typically 3 μJ/cm2 in a sample with a 5-mm-long resonator. We investigated the relationship between the resonator loss and the threshold density by varying the resonator length. The internal loss α and the gain coefficient β were found to be about 10.5 cm-1 and 3.2 μJ-1.cm, respectively. The threshold density was calculated as a function of the thickness of the emitting layer and compared with experimental values. We found that the optimum thickness is approximately 150 nm. Moreover, the reflectance at the mirror edge was increased by attaching a metal (aluminum) reflector to one side, resulting in a reduction in the threshold.
New conjugated polymer (PININE) with stability for LEDs
Show abstract
Fluorescent conjugated polymers have attracted much attention due to their potential applications in flat panel displays.
There are few studies on the degradation of the PPV film in air when irradiated. The photodegradation reaction is a chain
scission process involving oxygen in air to yield terminal 4-vinylbenzoic acid groups. The photodegradation of
conducting polymer may seriously effect the performance of electroluminescence devices.
In order to reduce oxidation of the vinylene group, the vinylic group was cyclized using carbon-containing 5-membered
rings. In case of PININE, it is possible to introduce four alkyl groups in the sp3 carbons in the bicycle, which will
increase the solubility of the polymer. PININE was used as the electroluminescence layer for the light-emitting diode.
PININE shows turn-on voltage of 6.5 V, and EL with maximum peak at 477 nm, maximum brightness of 2187 cd/m2 at
12 V, and efficiency of 0.34 cd/A at 162 mA/cm2. The change in luminescence following irradiation with white light on
the PININE was not observed. When irradiated with white light, the films of MEH-PPV showed significantly decreased
peaks of UV and PL. As compared to this, the films of PININE showed stable spectra when irradiated over same period of time.
Exclusive inkjet printed poly(3,4-ethylenedioxythiophene): polystyrenesulfonate as anode in polymer light-emitting diodes
Shih-Ting Lin,
Ming-Hau Chang,
Ji-Bin Horng,
et al.
Show abstract
As general practice, Poly(3,4-ethylenedioxythiophene):Polystyrene Sulfonate (PEDOT:PSS) is the most widely used
conducting polymer as electrode material in organic (polymer) devices. PEDOT: PSS film is fabricated by solution
processes such as spin-coating, dip-coating, inkjet printing (IJP), contact printing, etc. One of the most complex
operations in the fabrication process is forming conduction electrode lines or isolate devices from each other with the
pattern of polymeric film IJP, which is a non-impact printing technology in which droplets of ink are jetted directly on a
media to create a pattern. In this paper, PEDOT:PSS films, prepared by inkjet-printing and spin-coating methods, have
been studied by using atomic force microscopy (AFM), micro-Raman spectroscopy and photoelectron spectroscopy
measurements (PL). PEDOT:PSS films formed with the inkjet-printing method are appropriate for using as an anode for
simplification of the fabrication process of polymer light-emitting diodes whose performance is about 1.2 cd/A. The
performance is the same as spin-coating method. The performance was attributed to longer effective conjugation length
of PEDOT chains in inkjet-printing PEDOT:PSS films, as suggested by their micro-Raman spectroscopy.
A gas barrier film composed of SiO2/Al2O3 multilayers on flexible substrates
Show abstract
A gas barrier film composed of SiO2/Al2O3 multilayers on flexible PEN and PI substrates is studied. The multilayers are
obtained by stacking six SiO2/Al2O3 pairs utilizing the RF sputtering, and each layer is about 60 nm in thickness. The
barrier performances are further identified by the accelerated transparent Ca tests we proposed and the OLED lifetime
tests. The water vapor transmission rates (WVTR) of the gas barrier films are calculated to be 5.05×10-3 at 85°C/85%RH
and 9.5×10-6 at 20°C/60%RH, respectively. The decay rate in the Ca test at 85°C/85%RH is approximately 531 times
faster than that at 20°C/60%RH. The half lifetime of the OLED with the multilayers gas barrier as the substrate lasts as
long as the device using the glass substrate, which shows the feasibility of fabricating a thin film encapsulated OLED.
Studies of blue organic electroluminescent devices using the polymer/dopant systems as a light-emitting layer
Show abstract
In this study, blue organic light-emitting diodes (OLEDs) using polymer-dopant systems as a light-emitting layer were
fabricated. The light-emitting layer was produced via a spin-coating process. The materials used in the light-emitting
layer included: poly(9-vinylcarbazole) (PVK) as a matrix material, and blue-emission 4,4'-bis(2,2-diphenylvinyl)biphenyl (DPVBi) as a light-emitting material. The blue OLED was fabricated by blending PVK and DPVBi according to the proportion of mass as specified for a light-emitting layer. Process relevant photophysical
mechanisms and energy transfer phenomena were studied using absorption, photoluminescence (PL), photoluminescent
excitation, and electroluminescence (EL) spectra. We have learned that there was energy transfer phenomenon between
PVK and DPVBi. More specifically, the EL spectrum of PVK:DPVBi was significantly different from the PL spectra of
PVK, DPVBi, and PVK:DPVBi, shifting to a longer wavelength and showing multiple boarding emission peaks
(maximum emission at 465 and 486 nm). The electro-optical characteristics of the blue OLED and the possible origin of
the difference between the EL and PL spectra were discussed.
Carrier injection and bipolar transport in NPB for single-layer OLEDs
Show abstract
We studied the carrier injection and transporting properties of N,N'-diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-
4,4'diamine (NPB), a common hole transporter for organic light-emitting diodes (OLEDs). NPB was found to possess
significant electron mobility from time-of-flight (TOF) measurement. With bipolar transporting ability, NPB was used to
fabricate single-layer devices with a configuration of ITO/ PEDOT:PSS/ NPB/ Ca/ Ag. PEDOT:PSS was demonstrated
to form a quasi-Ohmic contact to NPB by admittance spectroscopy (AS) and dark-injection space-charge-limited current
(DISCLC) measurements. From current-voltage (JV) characteristics, single-layer NPB devices exhibited a bulk-limited
hole current in low-voltage region. Electron injection was clearly observed at a turn-on voltage of about 4V, which
coincided with the luminance-voltage measurement. In order to confine the recombination zone, dye-doped NPB layer
was inserted into single-layer devices. This intentional doping technique made a notable improvement in current
efficiency. The mechanisms of the doped devices were also addressed.
Numerical simulation of top-emitting organic light-emitting diodes with electron and hole blocking layers
Show abstract
The few reported high-contrast organic light-emitting diodes (OLEDs) all deal with bottom-emitting OLEDs and may
not be readily adapted for top-emitting OLEDs (TOLEDs), which have a few technical merits over bottom-emitting
devices for high-performance active-matrix OLED displays (AMOLEDs). The thin-film transistors on the back-plane of
an AM substrate reduce the aperture ratio of a pixel that decreases the display brightness. A TOLED, which can provide
a more flexible pixel design on an opaque AM substrate, represents a promising technique for achieving a high aperture-ratio
AMOLED. In this work, the characteristics of TOLEDs with α-NPD and LiF blocking layers are numerically
investigated with the APSYS simulation program. The α-NPD layer is used as an electron blocking layer, while the LiF
layer is used as a hole blocking layer. The TOLED structure used in this study is based on a real device fabricated in lab
by Yang et al. (Appl. Phys. Lett. 87, 143507, 2005). The simulation results indicate that when the TOLED device is with
either α-NPD or LiF blocking layer, the luminance efficiency and radiative recombination rate at the same drive voltage
can be markedly improved. The TOLED with α-NPD blocking layer has the best performance when the position of light
emission is located at the anti-node of the standing wave due to micro-cavity effect. The TOLED with LiF blocking layer
has improved performance because the LUMO of Alq3 can be lowered by band bending, which leads to better carrier
balance and thus increased radiative recombination rate.
Numerical simulation of bright white multilayer organic light-emitting diodes
Mei-Ling Chen,
Cheng-Hong Yang,
Chien-Yang Wen,
et al.
Show abstract
Transferring existing color filter technology to an organic light-emitting diode (OLED) display can greatly simplify the
fabrication of a full-color OLED since only a white light emission device is required. In this work, the optical and
electronic properties of bright white multilayer OLEDs, typically with a structure of metal/LiF/Alq3/EML/TPD/ITO
constructed by Lim et al., are numerically investigated with the APSYS simulation program. Specifically, the
emission/absorption spectra of the Alq3 (Green), Alq3:DCM (Red), and SA (Blue) light-emitting layers (EMLs) as well
as the energy band diagrams, electron-hole recombination rates, electroluminescence, current-voltage, and luminance-current
characteristics of the simulated OLED devices are investigated and compared to the experimental results. The
physical models utilized in this work are similar to those presented by Ruhstaller et al. and Hoffmann et al. The
simulated results indicate that the emission spectra of the Alq3, Alq3:DCM, and SA light-emitting layers obtained in this
study are in good agreement with those obtained experimentally by Zugang et al. We study the optical and electronic
properties of the OLEDs consisting of several dotted-line doped layers (DLDLs) and adjust all emission layers to
enhance the luminance efficiency. Finally, we insert m-MTDATA and CuPc buffer layers onto the ITO anode. Using the
double-buffer layer structure, the device performance can be greatly improved through the relative alignment of the
energy levels of the layers to enhance the holes injection and transportation. Structural optimization for the OLED
devices with better optical and electronic performance is also discussed.
Light emission optimization of europium based complex in multilayer organic light emitting diodes
G. Santos,
F. J. Fonseca,
A. M. Andrade,
et al.
Show abstract
In this work a detailed study of the electroluminescence of an Organic Light Emitting Diode based on Europium
Complex is studied as a function of the emissive layer thickness and growth rate evaporation. The device structure is
glass:ITO / TPD [N,N' - bis (3 - methylphenyl) - N,N' - diphenylbenzidine] / Eu(DBM)3phen [tris (dibenzoylmethane)
- mono (4,7-dimethylphenantroline) europium (III)] / Alq3 [aluminum - tris (8 - hydroxyquinoline)] / Aluminum. The
minimum driving voltage is about 15 Volts although the electrical current is about only 2 mA (wall plug efficiency up to
0.002 %). The electroluminescence spectra and external efficiency are clearly dependent on the Eu (DBM)3phen layer
thickness and growth rate evaporation. This results in a strong color change (CIE coordinates). With these results, a
model for the device opto-electrical behavior is presented, allowing the device final optimization.
Electro-optical measurements, stability, and physical charge behavior of rare-earth based organic light emitting diode
G. Santos,
F. J. Fonseca,
A. M. Andrade,
et al.
Show abstract
In this work we report for the first time the electroluminescence of two different kinds of rare earth complexes based
Organic Light Emitting Diodes, the Tb(ACAC)3bipy [Tris (acetylacetonate) - 2,2' - bipyridinyl - terbium(III)] and the
Eu(TTA)3bipy [tris(4,4,4 -trifluoro -1 - (2 - thienyl) -1,3 - butanediono - 2,2' - bipyridinyl - europium(III)]. In both
devices the corresponding electroluminescence spectrum is obtained (red for europium with (x,y) CIE coordinates near
(0.64, 0.34) and green for terbium near (0.28, 0.55) coordinates) at a driving voltage near 16 - 17 V with a maximum
electrical current of 1 mA. The Wall Plug Efficiency is about 10-3% in both cases.
Improved lifetime and efficiency of green organic light-emitting diodes with a fluorescent dye (C545T)-doped hole transport layer
Show abstract
We report the improvement of the electroluminescence (EL) efficiency and the device stability of green organic
light-emitting diodes (OLEDs) by doping 10-(2-benzothiazolyl)-2,3,6,7-tetramethyl-1H,5H,11H-(1)-
benzopyropyrano(6,7-8-i,j)quinolizin-11-one (C545T) in the thin interfacial region of the hole transporting layer of
N,N'-di(1-naphthyl) -N,N'-diphenylbenzidine (α-NPD) in addition to the tris(8-hydroxyquinoline) aluminum (Alq3)
emitting layer. The EL efficiency of 15.7 cd/A is obtained at 10 mA/cm2, which is about 10 % higher than the device
with C545T doped only in the Alq3 layer. In addition, the longer lifetime with very small driving voltage variation
over time is obtained under a constant current driving. This improvement in the efficiency and stability can be
attributed to the combined effect of an additional radiative recombination of electrons with holes in the C545T-doped
α-NPD layer and the reduced transport of holes into the Alq3 emitting layer, thus lowering the generation of unstable
Alq3 cationic species (Alq3+).
Tunable organic solid-state DFB laser utilizing molecular reorientation
Show abstract
Thin films of organic semiconductors have been subjected to extensive studies in the last two decades due to
applications in photonics and optoelectronics. In this paper, we investigate wavelength tuning phenomena based on
fluorene-cored oligomers (T3). We studied the optical properties of oligofluorene vacuum-deposited thin films and
found that they show high optical anisotropy but become optical isotropic after annealing at a temperature around the
glass-transition temperature. The results indicate the molecular reorientation in thin films after annealing. Using this
property, we investigated the influence of molecular orientation on stimulated emission properties of organic thin films.
Employing such properties, we have also demonstrated continuous tuning of the stimulated emission wavelength of a
slab waveguide within one sample. Finally, we employed T3 thin film for laser application. We have also demonstrated
the wavelength tuning of the organic laser with DFB structures.