When a blue polymer OLED device is driven under DC constant current the electroluminescence decays over time. Over the last two years there have been large advances in the time taken for the luminescence to fall to 50% of its original value. This is in part due to an improved recognition and understanding of the degradation mechanisms involved in the luminescence decay. Measurements made from devices during lifetest have identified a number of characteristic degradation-related changes to the device and to the light-emitting polymer. In this paper we will compare these degradation signatures together with the material properties between light-emitting polymers which have been designed to give a long DC-driven lifetime and a class of polymers which have been specifically designed to give a long lifetime under pulsed driving schemes.

In this paper, We have demonstrated a blue fluorescent organic light-emitting device (OLED) with a current efficiency of 19.2 cd/A at 100 cd/m², an estimated half-lifetime of 15611 hours at an initial luminance of 1000 cd/m², and a voltage of 4.9 V at 20 mA/cm² with a high electron mobility electron transport layer (ETL) material and high efficiency dopant material. The external quantum efficiency (EQE) in this optimized OLED is 8.32%, which is very close to the theoretical limit. Carrier balance condition is achieved due to the incorporation of the high mobility ETL, bis(10- hydroxyben-zo[h]quinolinato)beryllium (Bebq2), which can not only effectively increase the current efficiency and elongate the operation lifetime, but also reduce the driving voltage and increase the power efficiency. The EML consisted of 4,4'-bis[2-(4-(N,N-diphenylamino)phenyl)vinyl]biphenyl (DPAVBi) as the blue dopant and 9,10-bis(2'- naphthyl) anthracene (ADN) as the matrix. We found that the dopant concentration of DPAVBi did not affect the mobility value of the EML which is consistent with the J-V characteristics. Besides, although it is believed the bulk ADN is a kind of HTL materials, we found the electron mobility of ADN is one order of magnitude higher than its hole mobility in our blue OLEDs.

Poly(thieno[3,4-b]thiophene) (PTT) based transparent conductive polymers have been developed for hole injection layer (HIL) applications in polymer light-emitting devices. Using a second generation material that uses a poly(perfluoroethylene-perfluoroethersulfonic acid) as polymeric dispersant/counter-ion (PTT:PFFSA), significant improvement (up to 6 times with LUMATION Green 1304 as the light emitting layer) in PLED lifetime has been achieved compared to the lifetime of first generation PTT:PSSA (PTT:poly(styrene sulfonic acid)) based devices. Compared with the work function of 5.2 eV for PTT:PSSA film, PTT:PFFSA films have a higher work function of 5.5~5.7 eV. Interestingly, we found that the resistivity of PTT:PFFSA films is dependent on the film preparation conditions such as annealing temperature and time; while the work function of PTT:PFFSA films is independent on the film annealing conditions. We also studied the dependence of the device leakage current and lifetime on the preparation conditions of PTT:PFFSA based HIL films. Higher HIL annealing temperature results in higher device leakage current mainly due to the lower resistivity of the PTT:PFFSA HIL film prepared under such condition. However, the device lifetime is almost independent on the annealing temperature in the studied temperature range from 130 to 210°C using dispersions with PFFSA-to-TT ratios of 12:1 and 18:1.

Microdisplays are used in many applications from electronic viewfinders to rear projection television and even in non-display applications such as printers. There exist a number of microdisplay technologies based upon different materials and physical systems. We outline the various classes of microdisplay along with their suitability for certain applications. We describe an emerging microdisplay technology based on a combination of polymeric organic electroluminescent material and CMOS active-matrix backplane, making comparisons with conventional OLED display technology and other microdisplay technologies. We focus, in particular, on a novel active matrix addressing approach for the emerging technology.

A novel, fully automated, fabrication and characterization apparatus for polymer light-emitting diodes (PLEDs) was developed. This high throughput apparatus allows the fabrication of 49 devices with a controlled variation of essential parameters like material, material composition, blend concentration, layer thickness, and annealing temperature. Up to now, due to a lack of elaborate design tools, extensive experimental effort is required in order to optimize novel materials, material combinations and device structures for polymer based LEDs. Our novel apparatus provides an extensive dataset which can be used for device optimization and a profound device modeling offering a deeper theoretical understanding of underlying device physics in PLEDs.

We succeeded in the mass production of the 3.8-inch Half-VGA AM-OLED display, which is the largest one in the commercialized OLED displays. A new pixel circuit and a driving method for Low Temperature Poly-Si (LTPS) TFT were developed. The new pixel circuit and driving method can compensate not only the Vth variations but also the mobility variations of LTPS TFT. Consequently, we can achieve good pixel-to-pixel luminance uniformity.

High efficiency small molecule organic light emitting devices (OLEDs) based on light emission from an electrophosphorescent dopant dispersed in an organic host matrix are well known. Achieving blue phosphorescent OLEDs is particularly challenging because the host triplet energy should ideally be > 2.8 eV to prevent back-transfer of energy from the dopant to the host matrix resulting in loss of efficiency. A design strategy for developing new host materials with high triplet energies by using phosphine oxide (P=O) moieties as points of saturation in order to build sublimable, electron transporting host materials starting from small, wide bandgap molecular building blocks (i.e., biphenyl, phenyl, naphthalene, octafluorobiphenyl, and N-ethylcarbazole) is described. Electrophosphorescent OLEDs using the organic phosphine oxide compounds as host materials for the sky blue organometallic phosphor, iridium(III)bis(4,6-(di-fluorophenyl)-pyridinato-N,C^2,) picolinate (FIrpic) give maximum external quantum efficiencies of ~ 8% and maximum luminance power efficiencies up to 25 lm/W.

New hole-transporting pendant polymers with high glass-transition temperatures (Tgs) above 200 °C were designed and synthesized. Multilayer organic electroluminescent (EL) devices using the new polymers as the hole-transport layer and quinacridone-doped tris(8-quinolinolato)aluminum as the emitting layer exhibited high performance. One of the hole-transporting polymers functioned well as a hole injection buffer layer in organic EL devices. New green- and orange-emitting pendant polymers with high Tgs and desired ambipolar character were also designed and synthesized. Organic EL devices using these emitting polymers also exhibited good performance. One of the hole-transporting polymer showed a high hole carrier mobility of over 10^-3 cm²V^-1s^-1 at an electric field of 1.0 × 10⁵ Vcm^-1, as determined by a time-of-flight method.

Solution-processable electrophosphorescent dendrimers are an emerging class of materials for highly efficient light-emitting diodes. Here, we report time-resolved photoluminescence measurements in a fac-tris(2-phenylpyridyl)iridium(III) [Ir(ppy)₃]-cored dendrimer in neat film and blended into a 4,4'-bis(N-carbazolyl)biphenyl (CBP) host. Our results identify the existence of a photodegradation process that occurs in solution prior to processing, which significantly affects the photoluminescence kinetics of the films and leads to lower external quantum efficiencies of solution-processed phosphorescent dendrimer light-emitting displays. In parallel, we studied the triplet-triplet exciton annihilation processes in these materials from the photoluminescence decays measured at various excitation densities. From the values of the annihilation rates, we calculated the triplet exciton diffusion lengths and estimated the limiting current densities above which annihilation would dominate in phosphorescent dendrimer light-emitting devices. The results show that the triplet exciton diffusion length is small (<15 nm) in phosphorescent dendrimers and that exciton diffusion becomes still slower in the blends, which can be interpreted by the intermolecular spacing between the phosphorescent emitters being increased, thus reducing the annihilation rate.

A series of phenol-capped, oligofluorenes having 2,3,5 and 7 fluorene units and a statistical oligomer with an average of about 10 fluorene units was prepared. In a similar fashion, phenol-capped oligomers having various charge-transporting moieties incorporated into the oligomeric structures were prepared. Polymers were prepared from the oligomers by various linking reactions involving the phenol groups. Trends in the optical and electrical properties as a function of oligomer length will be reported. Device data for this family of emissive copolymers indicates that charge mobility increases with conjugation length, and can be as good as or better than that of an analogous fluorene homopolymer.

Enhanced electroluminescent efficiency using a deoxyribonucleic acid (DNA)-based biopolymer complex as an electron blocking layer has been demonstrated in both green- and blue-emitting organic light emitting diodes. The resulting bio organic light emitting diodes, or BioLEDs, achieved a maximum luminous efficiency of 8.2 and 0.8 cd/A, respectively, resulting in as much as 10× higher efficiency, 30× brighter output and 3× longer lifetime than their OLED counterparts. In this paper we describe the device fabrication and present the performance of these new structures.

Organic light-emitting devices (OLEDs) have shown great promise for general lighting applications. Over the past several years, tremendous progress has been made in improving performance attributes such as light quality, efficacy and lifetime of OLEDs. However, achieving the low cost manufacturing potential of OLEDs, another stringent requirement to enable lighting applications, has so far not been well addressed and explored. Here, we describe a vacuum-free, direct lamination process that could reduce OLED manufacturing costs substantially below what is currently possible. With this technique, OLEDs can be made by laminating an anode component to a separately engineered cathode component using a roll laminator. When coupled with a solution-based chemical n-doping strategy to enable efficient electron injection from an inert cathode into polymeric organic semiconductors, the lamination technique is able to produce high performance OLEDs with efficiency comparable to conventionally fabricated devices utilizing a vacuum-deposited, reactive metal cathode.

Organic Light Emitting Diode (OLED) is an emerging technology as one of the strong candidates for next generation solid state lighting with various advantages such as thin flat shape, no UV emission and environmental benefits. At this moment, OLED still has a lot of issues to be solved before widely used as lighting devices. Nonetheless, typical properties of OLED, such as efficiency and lifetime, have been recently made great progress. For example, a green phosphorescent OLED with over 100 lm/W and a red fluorescent OLED with an estimated half decay time of over 100,000 h at 1,000 cd/m² were reported. Large area, white OLEDs with long lifetime were also demonstrated. In this way, some of the issues are going to be steadily overcome. In this publication, we will present a phosphorescent white OLED with a high luminous efficiency of 46 lm/W and an external quantum efficiency of 20.6 percent observed at 100 cd/m². This device achieves a luminous efficiency of 62.8 lm/W with a light-outcoupling film attached on the glass substrate. This is one of the highest values so far reported for white OLEDs. And we will also show a color-tunable stacked OLED with improved emission characteristics. This device minimizes a viewing angle dependence of the emission spectra and has color tunability from white to reddish-white. These technologies will be applied to OLED lighting.

Consumer display manufacturers are increasingly interested in white organic light emitting devices (WOLEDs), because these devices offer thinner display profiles, and in combination with color filters eliminate the need for high-resolution shadow masks. Additionally, WOLEDs are well suited for general-purpose illumination, since the power efficiencies of laboratory devices have surpassed that of today's commercial incandescent bulbs. In this paper, we report on an all phosphorescent 25 cm² WOLED lighting system that achieves (31±3) lm/W at 850 cd/m² with CIE coordinates (0.37, 0.36), and an external quantum efficiency of (29±3)%.

White light emissive organic light-emitting devices (OLED) with combination with phosphorous and fluorescent materials and that phosphorous materials and transition metal complexes as emissive materials by solution process have been discussed. Firstly, we employed poly(9,9-dioctylfluorene) (PFO)-based polymer light-emitting diodes (PLEDs) doped with red emissive phosphor tris(1-phenylisoquinoline) iridium (III) [Ir(piq)₃]. In order to change of the electroluminescent (EL) spectrum, PFO-based white PLEDs have been demonstrated utilizing changing the pulse width and frequency. The maximum luminance reached 4,800 cd/m² and the Commission Internationale de L'Eclairage (CIE) coordinates obtained the white emission of (0.33,0.35) at the pulse width and frequency of 100 ns and 1 kHz, respectively. Secondly, we investigated blue phosphorescent molecule of bis[(4,6-difluorophenyl)-pyridinato-N, C2'] (picolinate) iridium(III) (FIrpic) and the red europium complex of tris(dibenzoylmethane)-mono(4,7-di-phenylphenanthroline) europium(III) [Eu(DDP)₃phen] doped in poly(N-vinylcarbazole) (PVK). We demonstrated the white OLEDs employing the energy transfer from the host to dopants, showing the CIE coordinates of (0.31, 0.38). It was found that using Eu complexes for red dopants, it is easy to control the doping ratio and obtain the white emission. The OLEDs are fabricated by solution process on a glass substrate or polymeric substrate. Color tuning and white light emission are demonstrated by driving pulsed voltage.

Organic light-emitting field-effect transistors are a new class of electrooptical devices that could provide a novel architecture to address open questions concerning fundamental optoelectronic phenomena in organic semiconductors, and can be potentially used as key components in optical communication systems, advanced display technology, solid-state lighting and organic lasers. The realisation of Organic Light-Emitting Transistors (OLETs) with high quantum efficiency and fast switching time is crucial for the development of highly integrated organic optoelectronic systems. Organic molecular materials having intrinsically ambipolar transport and high charge mobility values are restricted in number and show poor light-emission efficiency. Here, we describe the device operation principles of OLETs and report on the approach of combining p-type and n-type molecular materials in a layered structure to achieve ambipolar transport and light emission. Imaging of the individual layers and a correlation between active layer structure and device electrical performances is achieved by means of the Laser Scanning Confocal Microscopy.

We have realized an organic two-contact, light-emitting device with reduced exciton quenching and photon absorption at the metal cathode. Compared to a conventional organic light-emitting diode (OLED), the metal cathode is displaced by one to several microns from the light-emission zone. Electron transport between the cathode and the light-emission zone occurs by field-effect, and hence with an enhanced mobility compared to electron transport in a conventional OLED. The electrical characteristics and the opto-electronic performance of this light-emitting device are measured. Numerical simulations indicate that the electronic and excitonic characteristics are in good agreement with these measurements. Maximum singlet densities comparable to those in OLEDs can be achieved, while optical and excitonic losses are reduced. This might possibly result in optical gain.

By employing a combined optical/electronic model, we investigate the effect of electronic properties on the performance of three layer organic semiconductor structures, which are a potential candidate for future electrically pumped organic laser diodes. The drift-diffusion equations which describe particle transport are coupled to the spatially inhomogeneous laser rate equations to solve for the dynamics of the excited state and photon population in the laser cavity. Due to the high current densities considered, high particle densities occur, which implies that annihilation processes between the different particle species have to be considered. On the optical side, we take into account the absorption of the metal electrodes required for current injection to obtain the intensity profiles of the guided modes. Our calculations show that the inclusion of annihilation processes leads to a strong dependence of the laser threshold on the charge carrier mobilities, in contrast to the situation when exciton annihilation is neglected. We observe optimum values for the charge carrier mobilities in the emission layer regarding the threshold current and power density. On the other hand, an increase of the mobilities in the transport layers leads to a reduction of these quantities. The threshold voltage decreases for increasing mobilities, regardless of the layer in which the mobility is increased. For optimised values, we obtain a threshold current density of j_thr = 267 A/cm² with annihilation processes taken into account. The presented results can serve as guidelines in the search for material combinations and devices structures suitable for electrically pumped organic semiconductor laser diodes.

We demonstrate that attenuated luminescence and lasing in optically excited organic thin films is a sensitive probe to vapours of explosives, such as trinitrotoluene (TNT). The combined chemosensing gains from organic amplifying materials and the lasing action, promise to deliver sensors that can detect explosives with unparalleled sensitivity.

The energy level alignment at interfaces between poly(9,9'-dioctylfluorene) (F8), poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-diphenylamine) (TFB) and poly(9,9'-dioctylfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFB) and substrates with work function ranging from 4.3 eV to 5.1 eV is investigated via ultra-violet photoemission spectroscopy. Vacuum level alignment with flat bands away from the interface is found when the interface hole barrier is 0.6 eV or larger. Band bending that moves the filled states away from the Fermi level occurs when the hole barrier is smaller than 0.4 eV. This is presumably due to the accumulation of excess interface charges on the polymer side when the interfacial barrier is small. The resulting field shifts the polymer levels in a way that limits charge penetration in the bulk of the film. We also study metal-on-polymer interfaces. Different metals exhibit different growth modes. While Pt shows complete layer-by-layer type of growth, Al shows island type of growth. Current-voltage measurement shows the presence of hole traps in the Au-on top-contact device, suggesting diffusion of small Au clusters into the polymer film. Furthermore, metal-on-polymer interfaces frequently present different interface energetics than their polymer-on-metal counterpart. e.g. a 0.3 - 0.4 eV higher hole injection barrier for Pt-on-TFB than TFB on Pt.

Organic light-emitting devices (OLEDs) containing highly efficient phosphorescent emitters and highly conductive doped organic transport layers were studied. Saturated red devices with luminous efficiency of 15 cd/A operate at <4 V; hence, they have a record power efficiency of 12 lm/W at 1,000 cd/m². Additionally, two high-efficiency red OLEDs were serially connected and vertically stacked to create a stacked OLED having a luminous and power efficiency (at 1,000 cd/m²) of 28 cd/A and 12 lm/W, respectively. The electrical connection between the two OLEDs is enabled by molecular p- and n-type doped organic transport layers. The single emissive layer red OLED has a projected lifetime (time to half initial luminance) of ~150,000 hrs from an initial brightness of 500 cd/m². The stacked device shows very similar lifetime characteristics when driven at similar currents, which results in significantly prolonged lifetime of ~260,000 hrs at an initial luminance of 500 cd/m².

Organic and polymer light-emitting diodes (OLEDs/PLEDs) that emit white light are of interest and potential importance for use in active matrix displays (with color filters) and because they might eventually be used for solid-state lighting. In such applications, large-area devices and low-cost of manufacturing will be major issues. We demonstrated that high performance multilayer white emitting PLEDs can be fabricated by using a blend of luminescent semiconducting polymers and organometallic complexes as the emission layer, and water-soluble (or ethanol-soluble) polymers/small molecules (for example, PVK-SO₃Li) as the hole injection/transport layer (HIL/HTL) and water-soluble (or ethanol-soluble) polymers/small molecules (for example, t-Bu-PBD-SO₃Na) as the electron injection/transport layer (EIL/HTL). Each layer is spin-cast sequentially from solutions. Illumination quality light is obtained with stable Commission Internationale d'Eclairage coordinates, stable color temperatures, and stable high color rendering indices, all close to those of "pure" white. The multilayer white-emitting PLEDs exhibit luminous efficiency of 21 cd/A, power efficiency of 6 lm/W at a current density of 23 mA/cm² with luminance of 5.5 x 10⁴ cd/m² at 16 V. By using water-soluble (ethanol-soluble) polymers/small molecules as HIL/HTL and polymers/small molecules as EIL/ETL, the interfacial mixing problem is solved (the emissive polymer layer is soluble in organic solvents, but not in water/ ethanol). As a result, this device architecture and process technology can potentially be used for printing large-area multiplayer light sources and for other applications in "plastic" electronics. More important, the promise of producing large areas of high quality white light with low-cost manufacturing technology makes the white multilayer white-emitting PLEDs attractive for the development of solid state light sources.

We show that admittance spectroscopy (AS) can be used to determine charge carrier mobilities and transport parameters in materials relevant to organic light-emitting diodes (OLEDs). Via computer simulation, we found that a plot of the negative differential susceptance vs frequency yields a maximum at a frequency τ_r^-1. The position of the maximum τ_r^-1 is related to the average carrier transit time τ_dc by τ_dc = 0.56 τ_r. Thus knowledge of τ_r can be used to determine the carrier mobility in the material. Devices with the structure anode/phenylamines/Ag have been designed to evaluate their mobilities. The extracted hole mobility data from AS in pristine and doped material systems are in excellent agreement with those independently extracted from time-of-flight (TOF) technique. In addition, materials with different energy levels of highest occupied molecular orbital (HOMO), are further examined in order to study the effects of injection barrier on the extracted mobility by AS. In the case of an Ohmic hole contact (e.g. ITO or Au /m-MTDATA), the mobility data is good agreement with TOF results. However, for a non-Ohmic contact, the extracted mobility appears to be smaller. Thus AS can be used a means of evaluating the quality of electric contact between the injection electrode and the organic material.

Here we report results of time-of-flight (ToF) measurements on blends of different ratios of poly(9,9-dioctylfluorene-cobis- N,N'-(4-methoxylphenyl)-bis-N,N'-phenyl-1,4-phenylenediamine) (PFMO) and the structurally similar poly(9,9- dioctylfluorene-co-N-(4-methoxyphenyl)diphenylamine) (TFMO). It is shown that the hole mobility can be tuned over three orders of magnitude with a mobility minimum at 10% PFMO and 90% TFMO. We also use Raman microscopy to demonstrate that the blends do not phase separate within the one micron resolution of our experiment.

In this paper, we demonstrated that controlled charge trapping, both at the emission layer and charge transport layer with energy level alignment, is essential for charge-balanced and effective electrophosphorescent organic light-emitting device (OLED). Conditions for enhanced of efficiency and lifetime of OLED were obtained with graded doping profile at the light-emission layers (varied host-dopant concentration) and different hole (exciton) blocking materials. Conceptual device physics presented in this study can be applied at an initial design of charge-confined, balanced structure of highly efficient electrophosphorescent devices.

We develop a three-spectrum white organic light-emitting diodes (WOLED) with blue phosphor-sensitized electrofluorescent, green and new red electrophosphorescent emissions. The green phosphorescent sensitizers are bis[5-(trifluoromethyl)-2-(4-fluorophenyl)pyridine]iridium(III)acetylacetonate (Ir-2h) or Bis(2-phenylpyridine) iridium(III)3-Methyl-2,4-pentanedione (Ir(ppy)₂mac), and the new red phosphorescent sensitizer is a phenyl-qoinazoline framework organo-iridium complex. The RGB peaks of WOLED with Ir-2h are at a wavelength of 440nm, 516nm, and 633nm, respectively. However, The RGB peaks of WOLED are at a wavelength of 440nm, 550nm, and 625nm, respectively, when the green phosphorescent sensitizer is replaced by Ir(ppy)₂mac. Besides, the peak at a wavelength of 550nm is broader than the former. Therefore, The former WOLED of RGB spectra are the most distinct than the later one. The former WOLED has efficiency of 3.3 lm/W (7.15Cd/A) at 1000 Cd/m², and CIE coordinates of (0.32, 0.35). It is good suitability to use in OLED displays based on RGB color filters and full-color LCD backlights.

In this paper, we have demonstrated the time-dependent distribution of recombination-rate of a mixed-host (MH) organic light-emitting devices (OLEDs) by co-evaporating an ultra thin red-emitting doped layer (probe). With various probe position, the intensity ratio of red to green directly indicates the exciton distribution in MH layer. If the position of probe insertion is that of maximum recombination-rate, the driving voltage is also reduced which can be explained by the increase of the recombination current. From spectral and J-V analyses, the maximum recombination-rate position is 10 nm to the hole transporting layer when MH-OLED is not aged. After 48 hours of the DC aging test, the changes in the red to green intensity ratio of different devices are different. After 96 hours aging, this ratio does not change further among all devices, indicative of the achievement of steady state of recombination-rate distribution. The organic materials degrade more when it locates near the maximum of the recombination-rate.

The stability of the organic light-emitting diode (OLED) at the high temperature is important for their applications to automotive displays or various lighting applications which are more susceptible to Joule heating problems. In addition, it is known that the OLED lifetime is limited by the poor thermal stability of the hole-transport layer (HTL) material. Thus, the improvement of the thermal stability of the HTL layer is essential for enhancing both thermal stability and the operation lifetime. Here, we report that the thermal stability of OLED device can be significantly enhanced by introducing an LiF-mixed N,N'-di(1-naphthyl)-N,N'-diphenylbenzidine (α-NPD) as a HTL in the OLED having tris(8- hydroxyquinoline) aluminum (Alq₃) as a light-emitting and electron-transport layer. Compared with the reference device with the α-NPD HTL, the device having a double layer of LiF-mixed α-NPD and α-NPD as a HTL showed an increased thermal stability up to 170°C without degrading the quantum efficiency of the electroluminescence. In addition, the driving voltage variation over time (less than 3 V) was significantly suppressed while the reference device shows a variation over 6 V. The improved device stability is attributed to the enhanced thermal stability of the LiF-mixed α-NPD layer, which could be estimated from the result that the film morphology of LiF-mixed α-NPD film was nearly unchanged after heated above the glass transition temperature of α-NPD while that of α-NPD film was significantly changed.

In this paper, we demonstrate simulation results of top-emission organic light-emitting devices (TOLEDs) with red, green and blue (RGB) colors. We take the RGB spectral peaks at 460, 520, and 600 nm with full width at half maximum of 100 nm. Device structures are thick silver (Ag) anode /hole-transport layer 60 nm/ emitting layer (EML) /semi-transparent Ag cathode 20 nm/ dielectric layer. The dielectric material capped upon the cathode is zinc selenium with a refractive index of 2.6 for providing high output intensity and narrow FWHM. When monitoring the peak wavelengths of RGB device and varying the EML and dielectric thicknesses, we found the optimized value of the EML are 71, 47 and 31 nm for the red, green and blue devices, respectively. The optimized dielectric thicknesses are 80, 70 and 50 nm with periods of 117, 98 and 90 nm, respectively, for the RGB devices. Due to the limitations of the experiments, the EML thicknesses can be different and the dielectric thickness must be the same of the RGB devices. For optimizing the BGB devices simultaneously, the thickness of dielectric layer of the OLED is 667 nm. The RGB peak intensities are 0.96, 0.99 and 0.83, normalized to their optimized value. Typically, in a TOLED, green device exhibits higher efficiency than red and blue ones. That means the intensity of the green TOLED can be lower. When the dielectric layer thickness is 314 nm, the normalized RGB peak intensities are 0.99, 0.26 and 0.97.

New chromophoric molecules of 1,1-di(thiophen-2-yl)-2,3,4,5-tetraphenyl-silole (T₂TPS), 9-(diphenylmethylene)-9H-fluorene (DPMF), and tetraphenyletheneare (TPE) are designed and synthesized. When molecularly dissolved in common organic solvents, the molecules are practically nonemissive. Addition of poor solvents induces the molecules to aggregate, which turns the emission "on" and boosts their luminescence efficiencies dramatically ("aggregation-induced emission" or AIE). The photoluminescence (PL) of T₂TPS and TPE layers adsorbed on the TLC plates can be turned "off" and "on" continuously and reversibly by solvent exposure and evaporation. Transformation from amorphous phase to crystalline structure blue-shifts the PL spectrum of T₂TPS and enhances its intensity. A light-emitting devices (LEDs) device based on TPE is fabricated, which emits a blue light of 447 nm with a low turn-on voltage of 2.9 V.

While preparing parallel mirrors for edge-emitting organic semiconductor lasers (OSLs) by cleaving, the edge of an organic layer is severely damaged typically by the force of cleaving the substrate. By using an organic layer hardened at a low temperature, we were able to cleave an organic layer along the facet of the substrate and reproducibly obtain smooth and parallel mirror surfaces. Slab waveguide OSL structures consisting of 200 nm-thick Al_q3:DCM films (5% DCM) were vacuum-deposited onto polished GaAs (100) substrates coated with an 800 nm-thick layer of RF-sputtered SiO₂. The edge facets were prepared by cleaving the OSL structures in liquid nitrogen, and the facets of the organic layers were evaluated by scanning electron microscope. The samples were optically pumped using a nitrogen laser (λ=337 nm) with 600 ps pulse width at a 20 Hz repetition rate. The typical threshold power density was 68 μJ/cm² in the sample with about a 10 mm cavity length. The lasing peak wavelength was 644.5 nm. The full width at half maximum of the photoluminescence spectrum, dependence of light output power on input power, directional characteristics and polarization characteristics were measured. Our method is very useful to realize electrically pumped edge emitting OSLs.

A fluorescent star-shaped oligomer with a nitrogen atom as a core and both a hole transporting arylamine and an electron transporting 1,3,4-oxadiazole moiety, tri(4-(5-phenyl-1, 3, 4-oxadiazol-2-yl)phenyl)amine (TPOPA), has been designed and synthesized using a 5-step reaction procedure. The synthesized compound was characterized by elemental analysis, ¹H-NMR and mass spectroscopy. Thermogravimetric (TG) and differential scanning calorimetry (DSC) analysis show that TPOPA exhibits high thermal stability (T_d, 373°C) and high glass-transition temperature (T_g, 116 °C). Photoluminescence measurements indicate that the star-shaped oligomer shows intense blue emission peaked at 445 nm with a high quantum yield of 0.68 under near UV light excitation. The HOMO value of TPOPA is -5.64 eV and the LUMO is -2.58 eV based on the electro-chemical determinations. Reversible anodic oxidation results suggest that the hole-transporting is predominant for TPOPA. Two single layered devices were fabricated by vacuum evaporation with configurations of ITO / CuPC (15 nm) / TPOPA (95 nm) / Ca(30 nm) / Al(100 nm) (device 1) and ITO / CuPC(15 nm) / TPOPA (175 nm) / Ca(30 nm) / Al(100 nm) (device 2), where TPOPA was used both as emitter and carrier - transporting material, CuPC as a hole-injection and electron block material. The devices show blue wide-band emission peaked at 438 nm with a maximum luminance of 650 cd/m² and 512 cd/m² under an operating voltage of 12 V, respectively.

We examine the influence of various annihilation processes on the laser threshold current density of organic semiconductor laser diode structures. A three-layer laser diode structure is systematically investigated by means of numerical simulations. Our self consistent model treats the dynamics of electrons, holes and singlet as well as triplet excitons in the framework of a drift-diffusion model. The resulting particle distributions enter into the optical model. In our approach, we consider the actual waveguide structure and solve the resulting laser rate equation. The various annihilation processes are included as reactions between the different species in the device. We systematically vary the device dimensions and parameters of our singlet exciton annihilation model to identify the dominating quenching process in order to deduce design rules for potential organic laser diode structures. A standard material with typical material properties and annihilation rate coefficients is investigated. Singlet exciton quenching by polarons is identified as the main loss channel. The laser threshold in three layer devices is found to be very sensitive to the thickness of the emission layer.

Microdisplays are used in various optical devices such as headsets, viewfinders and helmet-mounted displays. The use of organic light emitting diodes (OLEDs) in a microdisplay on silicone substrate provides the opportunity of lower power consumption and higher optical performance compared to other near-to-eye display technologies. Highly efficient, low-voltage, top emitting OLEDs are well suitable for the integration into a CMOSprocess. By reducing the operating voltage for the OLEDs below 5V, the costs for the CMOS process can be reduced significantly, because a standard process without high-voltage option can be used. Various OLED stacks on silicone substrate are presented, suitable for full colour (RGB) applications. Red and green emitting phosphorescent OLEDs and blue emitting fluorescent OLEDs all with doped charge transport layers were prepared on a two metal layer CMOS test substrate without active transistor area. Afterwards, the different test displays were measured and compared with respect to their performance (current, luminance, voltage, luminance dependence on viewing angle, optical outcoupling etc.)

We report preliminary studies of the nature of hole injection from poly(3,4-ethylenedioxythiophene)/polystyrenesulphonic acid (PEDOT:PSS) into three commercial conjugated light emitting polymers (LEPs). Sumation's LUMATION Green 1300, LUMATION Blue, and Merck's SuperYellow LEPs are studied in combination with interlayers of poly(9,9-dioctyl-fluorene-co-N-(4-butylphenyl)-diphenylamine) (TFB), and poly[9,9-dioctylfluorene-co-(bis-N,N'-(3-carboxyphenyl)-bis-N,N'-phenylbenzidine)] (BFA). Despite the highest occupied molecular orbitals (HOMOs) of the interlayers being close to that of PEDOT:PSS and the LEP, different interlayers have different effects on hole injection and OLED device performance. We use dark injection transient current method to show that interfacial morphology changes results in modulation of hole trap densities that in turn affect hole injection. Depending on the interlayer/LEP combination partial penetration of interlayer into the LEP layer may also occur resulting in additional changes in the bulk transport properties of the LEP. Our results show that it is not the interfacial energy level alignment but the physical morphology changes at the interface which are important for varying hole injection into the device. A combination of either improved or reduced hole injection due to variations in physical contact, intermixing and trapping at the interlayer/LEP boundary dominate device performance.

One of the problems limiting the device efficiency of polymer light-emitting diodes is the imbalance of charge injection and transport between the electrons and holes. This problem is particularly serious for the case of aluminum (Al) electrode. By introducing solution-based titanium oxide (TiO_x) layer between the polymer and Al electrode, we have demonstrated that the devices exhibit an enhanced efficiency. The TiO_x layer reduces the barrier height between the polymer and Al cathode, thereby facilitating the electron injection in the devices and enhancing the device performance. Moreover, we also believe that the TiO_x layers prevent the diffusion of metal ions into the emitting polymers during the Al deposition process, reducing the degree of quenching centers in the active polymers.

We investigate current density-voltage (J-V) characteristics of copper phthalocyanine thin-film devices, with active areas ranging from S = 1,000,000 to 7.9 μm², and analyze their charge-carrier transport mechanisms under current densities between nA/cm² and kA/cm². We demonstrate injection of 128 kA/cm² in the smallest device having S = 7.9 μm². Furthermore, we find that J-V characteristics are divided into three regions between nA/cm² and kA/cm²: ohm current, shallow-trap space-charge-limited current (SCLC), and trap-free SCLC. In a shallow-trap SCLC region, we observe a large shift in J-V characteristics depending upon the active areas. From analyses of carrier traps with a thermally stimulated current (TSC) measurement, we see that TSC signal intensities of these films decrease as the active area is reduced. Hence, we conclude that a large shift in J-V characteristics is attributable to the change of carrier trap concentrations in these films.

We demonstrate that poly(3,4-ethylenedioxythiophene) doped with polystrenesulphonic acid (PEDOT:PSS) can act as an excellent hole injection material for small organic charge transporters. With PEDOT:PSS as a conducting anode, it is possible to achieve nearly Ohmic hole injection contacts to phenlyamine-based materials with HOMO values of up to 5.5 eV. In current-voltage experiment, the PEDOT:PSS anode can achieve nearly Ohmic hole injection to NPB (N,N'- diphenyl-N,N'-bis(1-naphthyl)(1,1'-biphenyl)-4,4'diamine), and TPD (N,N'-diphenyl-N,N'-bis(3-methylphenyl) (1,1'- biphenyl)-4,4'diamine). Meanwhile, dark-injection space charge limited current (DI-SCLC) transients are clearly observed and are used to evaluate the charge-carrier mobility of these phenylamine compounds. The carrier mobilities extracted by DI-SCLC are in excellent agreement with independent time-of-flight (TOF) technique. It is conceivable that PEDOT:PSS can be used as a general conducting anode for the electrical characterizations of organic materials that require Ohmic hole contacts.

In this paper, we have demonstrated a low-reflectance organic light-emitting device (OLED) by inserting a perylene diimide derivative between the emitting layer (EML) and the cathode. Such a material exhibits a good electron transport capability and good photoconductivity which absorbs light. A semi-transparent layer composed of thin aluminum (Al) and silver (Ag) was used between the EML and the n-type organic material, a perylene diimide derivative, for better electron injection and efficient destructive interference. The J-V characteristics of our low reflection and the control one are nearly identical which shows the superior conductivity of this material. In addition, the absorption peak of this ntype organic material is near 550 nm which can eliminate most of the ambient visible light. And the potocurrent is generated from self-absorption by this material. Thus, this device can also be applied as a photodetector or the applications of the self-adjustable display under different ambient illumination with suitable driving scheme.

We report a time-of-flight study of drift mobilities of hole and electron in mixed thin films of N,N'-diphenyl-N,N'- bis(1-napthyl)-1,1'-biphenyl-4,4'-diamine (NPB) and tris(8-hydroxyquinoline) aluminum (AlQ₃). Based on Poole- Frenkel model, the extracted zero-field hole mobility of pure NPB was 2.6x10^-4 cm²/Vs which is much larger than that of pure AlQ₃ (9.16x10^-10 cm²/Vs). As the AlQ₃ concentration is increased, the hole mobility decreases exponentially. In this case, AlQ₃ molecules act as blocking "hills" to the hole transport, since its HOMO energy level is 0.4 eV lower than that of NPB. In contrast, the difference in the electron mobilities of pure NPB and AlQ₃ is much smaller (5.28x10^-6 cm²/Vs vs. 1.51x10^-7 cm²/Vs) and the field-free electron mobility of the mixed films exhibits a minimum as the AlQ₃/NPB fraction ratio reaches about 75%. The LUMO energy level of AlQ₃ is 0.6 eV lower than that of NPB, making AlQ₃ become "traps" to the electron transport. When the amount of AlQ₃ reaches a certain level such that they form connected transport network, the electrons are then driven mostly in this network and the NPB molecules become blocking "hills". In summary, the HOMO and LUMO energy levels, the charge mobilities of pure compounds and the characteristics of their microscopic networks can greatly influence the resultant transport behaviors. These results may create challenges for existing transport models of disordered organic semiconductors and will be useful in designing organic light-emitting devices based on mixed-layer structures.

CuPc:C₆₀ Organic-Fullerene composites together with metals such as Au are found to form efficient hole injection structures for organic light-emitting diodes. C₆₀ concentration of 30 wt.% yielded optimum device characteristics including low driving voltage, high current efficiency, and high thermal stability. In the case of Au/CuPc:C₆₀ anode structure, extremely high current efficiency of ~ 8.7 cd/A and low operating voltage (i.e. 20 mA/cm² at ~8V) has been achieved in a simple bi-layer device using Alq as emitter. Through a study on single carrier devices, the versatility of the composite injection structure is attributed to that C₆₀ in the composite layer facilitate hole transfer from the metals to the composite whereas CuPc facilitate hole transport and transfer to a hole transport layer.

We measured electric-field-induced fluorescence quenching (EFIFQ) under various temperatures in both undoped and fluorescent dye-doped tris(8-hydroxyquinoline )aluminum (AlQ₃) layers of organic light-emitting devices (OLEDs). Results show that for a given temperature doped AlQ₃ layers demonstrate smaller EFIFQ than undoped ones. The phenomenon is attributed to the narrower energy band-gap of the guest molecule relative to that of the host material, which makes it less prone to electric-field-induced dissociation of the excited state. Results also show that for a given doping condition increasing the temperature leads to an increase in EFIFQ, indicating that the EFIFQ is a thermally assisted process.