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.

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.

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.

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 Lumation^TM 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.

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 E_T ~ 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,C^2,) 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.

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.

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.

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.

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).

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

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.

OLED display manufacturers are interested in white organic light emitting devices (WOLED^TMs) 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.

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.

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.

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.

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.

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.

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 g_mat ≈500 cm^-1 at an excitation density E_ex of E_ex ≈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.

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.

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.

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.

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.

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.

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.

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 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.

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.

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.

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.

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.

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.

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.

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⁺).

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.