This PDF file contains the front matter associated with SPIE Proceedings Volume 7051, including the Title Page, Copyright information, Table of Contents, Introduction, and the Conference Committee listing.

The geometric and electronic structures of organic materials in OLEDs are the underlying basis of the devices. However, the materials are in the amorphous states and the detailed analysis has been difficult so far. In this study, we attempt the detailed analysis of a hole-transport material, N,N'-diphenyl-N,N'-di(m-tolyl)benzidine (TPD), by solid-state NMR measurements. The chemical shifts are found to depend significantly on the molecular structures. The chemical shifts also reflect the electronic states of TPD. With the help of DFT calculations, the geometric and electronic structures of TPD are analyzed. The DFT calculations are also carried out for TPD and the related materials, replacing N atoms of N,N,N',N'-tetraphenylbenzidine to P or B. Not only the calculations of reorganization energies for single molecules but also those of intermolecular interactions between adjacent two molecules provide us information on the performances of carrier transports for respective materials.

Organic electronic devices generally have a layered structure with organic materials sandwiched between an anode and a cathode, such organic electronic devices of organic light-emitting diode (OLED), organic photovoltaic (OPV), organic thin-film transistor (OTFT). There are many advantages of these organic electronic devices as compared to silicon-based devices. However, one of key challenge for an organic electronic device is to minimize the charge injection barrier from electrodes to organic materials and improve the charge transport mobility. In order to overcome these circumstances, there are many approaches including, designing organic materials with minimum energy barriers and improving charge transport mobility. Ideally organic materials or complex with Ohmic contact will be the most desired.

Organic photochromic materials have been studied as active materials in optical devices since they show a reversible change of color in the visible region and appreciable reversible changes of the refractive index (Δn) in the NIR. The latter peculiar property can be suitably exploited for the realization of holographic optical elements (HOEs) and in particular volume phase holographic gratings (VPHGs). Photochromic polyester based on diarylethene alcohol was synthesized and characterized both in solution and in thin film. The optical properties of the films were good enough for making optical devices and the modulation of the refractive index (measured by spectral ellipsometry) was large at 1.5 micron and it became even larger at shorter wavelength where the material is still transparent. Such photochromic material is a good candidate for making rewritable efficient HOEs.

Our investigations refer to highly efficient emitting materials used in organic light-emitting diodes (OLED). We are especially interested in the possibility of shifting the emission wavelength in phosphorescent iridium(III) complexes. Depending on the mesomeric and inductive behavior of different substituents, the emission spectrum can be varied by introducing those substituents at various positions of the chromophoric ligand. Therefore, we synthesized Ir(ppy)₃- analogue complexes with nitrile, trifluoromethyl and methoxy groups at different positions of the ligand's phenyl ring to determine the influence of the position and of each substituent on the emission spectrum. To further study the adjustability we prepared several heteroleptic complexes and changed the ancillary ligand therein. In addition, we developed a new and as yet unknown ligand system based on hetero five membered rings, cyclometalated to iridium to generate homo- and heteroleptic complexes. Devices obtained with these emitting materials have shown high luminescence efficiencies of up to 30 lm/W @ 500 cd/m².

We investigated the effect of thermal stress, caused by the deposition of aluminum on top of an organic light emitting diode (OLED), on the device performance and proved our simulated results by experimental tests. Concerning the temperature of the substrate, we found a much larger influence of thermal radiation, caused by the evaporation source and the environmental setting compared to the kinetic and thermal energy of the deposited material itself. Due to these results, we developed a new system for metal deposition, using the flash-evaporation technique. Using it, we were able to minimize the influence of thermal radiation and geometry on the evaporation. Therefore the substrate heating was reduced by more than 90 % and the photometric efficiencies of test-devices were improved slightly. Additionally the time of deposition and retention was lowered by 90 %, with an increased material yield of more than 55 % at the same time. The resistance of the conducting layer decreases by two orders of magnitude, caused by emerging micro crystals. Surprisingly, the roughness of the surface actually decreased slightly.

We report on the performance of green phosphorescent organic light-emitting diodes (OLEDs) based on the well-known device structure of a hole-transport layer, an emissive layer with host 4,4'-di(carbazol-9-yl)-biphenyl [CBP] and the green phosphor emitter fac tris(2-phenylpyridinato-N,C^2,) iridium [Ir(ppy)₃], a hole-blocking layer of 2,9-dimethyl-4,7- diphenyl-1,10-phenanthroline [BCP] and and tris-(8-hydroxyquinolinato-N,O) aluminum [Alq₃] as an electron-transport layer. Using spin-coated hole-injection/transport layers with increasing ionization potentials and decreasing hole mobilities, external quantum efficiencies of up to 18.1% at 100 cd/m² were measured in such devices. Furthermore, by removing the electron-transport layer of Alq₃ and increasing the thickness of BCP, devices with efficiencies of 21.2% and 72 cd/A at 100 cd/m² were obtained. Achieving such high efficiencies with a simplified hybrid structure in which one layer is processed from solution and only two other organic layers are deposited from the vapor phase is desirable for the fabrication of low-cost OLEDs.

Phosphorescent organic light emitting device (PHOLED^TM) technology has demonstrated record high efficiencies and long operational stability. Here we report on the introduction of an additional charge transporting dopant into the device emissive layer to further improve the luminous efficiency and device lifetime. The performance enhancement is attributed to the separation of polarons and excitons in the device emissive layer, which results in reduced triplet-triplet and triplet-polaron interactions as well as minimizing self quenching and reabsorption. Specifically we report a 50% improvement in the luminous efficiency of a red PHOLED and a 3 fold improvement of the device lifetime due to the use of dual doping. A dual doped sRGB red device with 28 cd/A and the lifetime over 300,000h at 1,000 nits is demonstrated.

Some recent photoluminescence (PL)- and electroluminescence (EL)-detected magnetic resonance (PLDMR and ELDMR, respectively) studies of the negative (PL- and EL-quenching) spin 1/2 resonance are reviewed. These include the resonances in poly[2-(N-carbazolyl)-5-(2'-ethyl)-hexoxy-1,4-phenylenevinylene] (CzEh-PPV) films, rubrene films, tris(quinolinolate) Al (Alq₃) OLEDs, rubrene-doped Alq₃ OLEDs, and fac tris(2-phenylpyridine) iridium [Ir(ppy₃)]-doped poly(N-vinyl carbazole) (PVK) polymer LEDs (PLEDs). The resonances are all assigned to quenching of SEs by trions, which are bipolarons stabilized by a counterpolaron or counterion. As bipolarons are spinless, their formation from two like-charged polarons is spin-dependent, and hence enhanced at resonance. This enhanced formation, and the resulting enhanced quenching of SEs, yields the negative spin 1/2 PLDMR and ELDMR. As previously shown, since trion formation also reduces the mobility of the trapped carriers, this process also results in a negative spin 1/2 electrical current-detected magnetic resonance (EDMR). Importantly, since the counterpolaron is usually trapped, e.g., at organic/cathode or organic/organic interfaces, or at impurity sites such the oxygen center in rubrene, it is suspected that the trions might be responsible for the long term degradation of OLEDs and PLEDs associated with abrupt junctions or impurities.

Despite active scientific research and substantial business interest, the nature of the operational degradation of OLEDs remains only partly understood. From the perspective of device physics, there are numerous studies indicating that operational degradation, i.e., monotonic loss of luminance efficiency and increase of drive voltage, is caused primarily by the accumulation of traps-nonradiative recombination centers and luminescence quenchers in the vicinity of the recombination zone. However, chemistries underlying the formation of those species remain elusive. In this work, we focused on two representative hole transport materials, NPB and TAPC, to study their chemical transformations in operating OLED devices. We found that the presence of these hole transport materials in several representative fluorescent and phosphorescent devices may result in the formation of new chemical materials, corresponding to homolytic dissociation of weak C-N bonds of NPB, and C-N/C-C bonds of TAPC. Qualitatively similar to luminance efficiency loss in operating devices, the accumulation of the degradation products is monotonic and nonlinear. Using bilayer hole transport layers, we established that the chemistries of NPB and TAPC are strongly confined to the immediate vicinity of the main recombination interfaces and are likely to be initiated by singlet excited states of these arylamines. It is concluded that the differences in singlet excited state energies and bond dissociation energies are responsible for dramatic differences in the degradation rates of OLED devices using NPB- and TAPC-based hole transport layers.

A comprehensive electronic-optical simulation tool for the design of complex organic multi-layer device structures is presented. The physical models comprise the key optical and electronic processes governing organic light-emitting (OLEDs) and light-harvesting devices. The simulation of such devices is demonstrated for electronic-only or optics-only models as well as for electronic-optical and optical-electronic coupled device models. Validation examples with experimental data and applications for device simulations are also discussed. It is shown that both light-emitting and light-harvesting devices require careful optical as well as electronic multilayer design and characterization.

A giant surface potential (GSP) has been observed on a tris-(8-hydroxyquinolate) aluminum (Alq₃) film deposited on a glass or a metal substrate under dark conditions. However, the effects of a GSP on the device properties of Alq₃-based organic light-emitting diodes (OLEDs) has not been considered. In this paper, we report on the effects of ambient light during the fabrication of an Alq₃-based OLED on the device properties by displacement current measurement. We found that the light irradiation significantly reduces the density of charge existing at the 4,4'-bis[N-(1-naphthyl)-N-phenylamino]-biphenyl/Alq₃ interface and results in the formation of charge traps in the Alq₃ layer. Considering the similarities between the GSP and the interfacial charge, they can be attributed to the same origin; the orientaion polarization of Alq₃ film.

The charge conduction properties of the organic phosphorescent emission layer doped with iridium-based green and red phosphorescent emitters, fac-tris(2-phenylpyridine) iridium(III) (Ir(ppy)₃) and bis(2-(2'-benzo [4,5-a]thienyl)pyridinato-N,C3')iridium(acetyl-acetonate) (btp₂Ir(acac)), were studied and compared to those of the reference host of 4,'-N,N'-dicarbazole-biphenyl (CBP). In the CBP host layer, both dopants act as hole traps but they affect the electron transport differently. Compared with the pristine CBP film, the electron mobility is similar for Ir(ppy)₃-doped CBP but it is more than two orders of magnitude lower for btp2Ir(acac)-doped CBP. Because of such difference in the electrical conduction properties between the Ir(ppy)₃- and btp2Ir(acac)-doped CBP, the main recombination zone position and the electron-hole balance changes. Based on these findings, we optimized white organic light emitting diodes (OLEDs) with multi-emitting layer (EML) structures in which CBP layers doped with Ir(ppy)₃ and btp2Ir(acac) and fluorescent dopant of 4,4'-bis[2-{4-(N,N-diphenylamino)phenyl}vinyl]biphenyl (DPAVBi) were used as green (G), red (R), and blue (B) EMLs, respectively. The white OLEDs with the R/G/B EML sequence show improved electron and hole balance, resulting higher efficiency, better color stability and longer lifetime compared to the G/R/B EML sequence. A high luminous current efficiency of 13.5 cd/A at 100 cd/m² was achieved with the R/G/B EML sequence.

Long life-time molecular-based organic electronics, such as organic light-emitting diodes (OLEDs), organic solar cells, or organic transistors etc, inevitably demand their constituent molecules to be highly thermal-stable. Coupling with special needs in molecular design, the resultant increasing molecular weight (MW) will eventually make the molecules difficult to deposit if via dry-process, while using wet-process would frequently result in undesired relatively poorer efficiency. Surprisingly, two high-molecule composing OLEDs with relatively high-efficiency were obtained by using solution-process. A blue OLED with a blue dye doped in a novel high-MW, wide band-gap host, 3,5-di(9H-carbazol-9-yl) tetraphenylsilane (SimCP2), yielded 24 lm/W (38 cd/A) at 100 nits, and a green OLED using a novel high-MW green dye, bis[5-methyl-7-trifluoromethyl-5H-benzo (c)(1,5) naphthyridin-6-one] iridium (picolinate) (CF3BNO), yielded 70 lm/W (89 cd/A), while their dry-processed blue and green counterparts yield 1.7 and 21 lm/W, respectively. Importantly, although the comparatively high MW has made the resulting molecules extremely difficult to vacuum-evaporate and has resulted in poor device performance, the wet-process has been proven effective in fabricating two high molecule-containing OLEDs with relatively high efficiency. The successful demonstration suggests that the same approach may as well be extended to other organic devices that compose or should compose high molecules.

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. 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 spot meter: at 100 cd/m² normal luminance, EQE = 20%, power efficacy is 34 lm/W, operating voltage = 3.6 V, CIE = (0.44, 0.44) and CRI = 75.

We present highly efficient red and green phosphorescent devices comprising only two organic layers. Novel host materials having good electron transporting and new narrow band-gap fluorescent characteristics lead to the fabrication of simplified high efficiency devices. The driving voltage value of 3.3 V to reach luminance of 1000 cd/m² is reported in simple bi-layer red and green phosphorescent OLEDs, respectively. The maximum current- and power-efficiency values of 38.30 cd/A and 46.60 lm/W are demonstrated in this green device. The current and power efficiencies of bi-layered red PHOLED are 9.38 cd/A and 11.72 lm/W. Results reveal a practical way to fabricate highly efficient truly bi-layer organic devices for trouble-free manufacturing processes.

The dynamics of triplet recombination in fluorene trimers have been studied using steady state photoinduced absorption (PA) spectroscopy. We investigated two type of oligomeric films, deposited by different techniques: thermal evaporation and spincoating. The different molecular arrangement in both films is manifested in a red-shift of the absorption, PL and T1-Tn triplet PA spectra of the sublimated film relative to the spincoated one. Triplet recombination dynamics follow a dispersive bimolecular recombination model away from the trap filling regime. Moreover we report on the characteristics of a host-guest lasing system obtained by co-evaporation of the most promising oligofluorene derivative (T3) with the red-emitter 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dye (DCM). The blend satisfies the necessary condition for an efficient Förster energy transfer to take place from T3 matrix to DCM molecules.

Organic Light-Emitting Device (OLED) has been attracted much interests to realize a flat panel display owing to its promising characteristics. Active matrix OLED can make it possible to make big displays with the good image qualities and create new values and concepts, such as a flexible display, transparent display, and so on. However, in spite of the big potential of the AMOLED, it is still needed to improve lifetime, efficiency and productivity for large size displays. In this paper, we present current development status and issues of the AMOLED toward an ultimate champion among flat panel displays.

We report on three-dimensional numerical optical simulations of the emission extraction efficiency in light emitting devices with field effect carrier transport. The finite difference time domain (FDTD) method is applied for organic thin film structures on silicon substrates with metal and metal oxide electrodes. Simulations are performed for Au, Ag and indium tin oxide electrodes in bottom gate, bottom contact geometries. Special attention is paid on the dependence on electrode thickness and contact shape. It is demonstrated that in unipolar driven devices with Si gate, silicon dioxide insulator and 40 nm-thick organic films the maximum out-coupling efficiency is below 10 %. This value can be doubled by an implementation of metal reflecting layers on the Si substrate. Furthermore, the emission efficiency in the ambipolar regime is investigated. The results present the dependence of light extraction on the distance between light source and electrode. Additionally, the influence of the contact edge shape is investigated for two different designs with rectangular and wedge electrodes. Interference effects cause an oscillation in the distance dependence.

Fullerene (C₆₀) has been found to form a universal hole injection interface in the Metal/C₆₀ anode structure. This bilayer structure opens up possibilities to select various highly conductive metals as anodes to replace indium tin oxide (ITO). Organic light emitting diodes (OLEDs) utilizing Au/C₆₀ and Mg/C₆₀ bilayer anodes were fabricated. Electroluminescence (EL) efficiencies of devices with Au/C₆₀ anodes surpassed the ITO baseline device by ~ 25%. It was found that the hole injection current for the Au/C₆₀ anode can be tuned via the C₆₀ interlayer thickness in the range of 1 - 5 nm. Single carrier hole-only (HO) devices with different metal and metal/C₆₀ bilayer anodes were studied. With the insertion of a 3 nm thick C₆₀ buffer layer between the anode metal and hole transport layer (HTL), an increase in hole injection current of more than two orders was realized. Based on device modeling we extracted C₆₀-induced dipoles on the metal surfaces. These dipole values agree well with values obtained by Kelvin probe and photoemission measurements. What is more, the dipole values effectively pin the work function of all metals to a common value of ~ 4.6 eV, creating a universal hole injection barrier regardless of the pristine anode metal work function.

We investigated the electronic structure of organic thin films doped with alkali metal using photoemission and inverse photoemission spectroscopy (UPS, XPS and IPES). We found that doping induces energy level shift, which can be seen as in two different stages. The first stage is predominantly due to the Fermi level moving in the energy gap as a result of the doping of electrons from the alkaline metal to the organic, and the second stage is characterized by the significant modification of organic energy levels such as the introduction of a new gap state, new core level components, and change of binding energies with respect to the frontier orbital. In addition, we observed that the energy level shift in the first stage depended approximately in a semi-logarithmic fashion on the doping concentration, whose slope could not be explained by the conventional model used in inorganic semiconductors. The lowest unoccupied molecular orbital (LUMO) is observed to diminish as doping progresses. Furthermore, we observed that the doping induced modification can be compensated by depositing Au or O₂ on alkali metal doped organic films. The modification of the electronic structure by other inorganic or organic dopants will also be discussed.

Current density-voltage (J-V) characteristics of hole-only devices using indium tin oxide (ITO) anode and N,N'- diphenyl-N,N'-bis(1-naphthyl)-1,1'-biphenyl-4,4'-diamine (α-NPD) layer were measured with various thicknesses of a molybdenum oxide (MoO₃) buffer layer inserted between ITO and α-NPD. The device with a 0.75-nm-thick MoO₃ layer forms Ohmic hole injection at the ITO/MoO₃/α-NPD interfaces and J-V characteristics of this device are controlled by a space-charge-limited current. Results of X-ray photoelectron and ultraviolet/visible/near-infrared absorption studies revealed that this Ohmic hole injection is attributable to electron transfers from ITO to MoO₃ and from α-NPD to MoO₃. Moreover, we demonstrated that the Ohmic hole injection is realized at the interfaces of ITO/rubrene and ITO/N,N'-di(m-tolyl)-N,N'-diphenylbenzidine (TPD) using an ultrathin MoO₃ layer as well.

Interface and its engineering are critical to achieve efficient organic light-emitting diodes (OLEDs). In this presentation, we would like to present a new type of electron transport layer (ETL) based on metal oxide between emissive layer and metal electrode. The metal oxide synthesized by sol-gel process and can be solution processed on top of the emissive layer. The efficiency of OLEDs based on green polyfluorene polymer has found increased significantly, from 15 lumen/watt to 19 lumen/watt compared with our previous cathode structure of Cs₂CO₃/Al. The electrochemical properties and electronic structure of the ETL, and energy alignment at the interface are examined to understand the function of this ETL and the contribution to the improvements for device performances. We will present the details of the analysis and the composition of the materials in the presentation.

Organic light emitting diodes (OLED) are expected to be used in next generation solid state lighting sources serving as an alternative to conventional incandescent bulbs and fluorescent lamps. OLEDs will provide the environmental benefits of possible considerable energy savings and elimination of mercury, as well as some other advantages such as thin flat shape, planar emission, and no UV emission. Recently, important properties of OLEDs such as efficiency and lifetime have been greatly improved. Additionally, for lighting applications, a high color rendering index (CRI) at the desired CIE chromaticity coordinates, high luminance and large area uniform emission, and high stability over long time operation are also required. In this paper, we describe the development and performance of our high CRI OLEDs: the conventional OLED with multiple emissive layers and the multi-unit OLED with only two emissive units (a fluorescent blue emissive unit and a phosphorescent green / red emissive unit). Related technologies for OLED lighting to obtain uniform emission at high luminance in large areas are also described.

Organic light emitting diodes (OLEDs) provide potential for power-efficient large area light sources that combine revolutionary properties. They are thin and flat and in addition they can be transparent, colour-tuneable, or flexible. We review the state of the art in white OLEDs and present performance data for three-colour hybrid white OLEDs on indexmatched substrates. With improved optical outcoupling 45 lm/W are achieved. Using a half-sphere to collect all the light that is in the substrate results in 80 lm/W. Optical modelling supports the experimental work. For decorative applications features like transparency and colour tuning are very appealing. We show results on transparent white OLEDs and two ways to come to a colour-variable OLED. These are lateral separation of different colours in a striped design and direct vertical stacking of the different emitting layers. For a striped colour tuneable OLED 36 lm/W are achieved in white with improved optical outcoupling.

This contribution examines in details the effects of dopants on the hole transporting properties of N,N'- diphenyl-N,N'-bis (1-naphthyl) (1,1'-biphenyl)- 4,4'diamine (NPB). Dopants for NPB are copper phthalocyanine (CuPc), 4-(dicyanomethylene)-2-methyl-6-(pdimethylaminostyryle)-4H-pyran (DCM1), 4-dicyanomethylene-2-methyl- 6-[2-(2,3,6,7-tetra-hydro-1H,5H-benzo[ij] quinolizin-8-yl)vinyl]-4H-pyran (DCM2), 2-(4-biphenyl0-5-(4- tertbutylphenyl)-1,3,4-oxadiazole (tBu-PBD) and 2,9-dimethyl-4, 7-diphenyl-1,10-phenanthroline (BCP). The effects of these dopants on the hole transport of NPB will be presented. Generally, the dopant molecules behave like hole traps or scatterers. Their detailed behaviors are determined by their highest occupied molecular orbitals relative to that of NPB. Traps are found to induce significant reduction in hole mobility. However, hole scatterers only alter the mobility slightly. Two different underlying charge transport mechanisms are proposed and then it is further examined by temperature dependent measurements.

The effectiveness of carrier injection in electron transport layers has been investigated for high efficiency organic light emitting devices. Via ultraviolet and x-ray photoemission spectroscopy (UPS and XPS), the carrier band structures, interfacial interactions and electron-injection mechanisms are discussed. Acting as a good hole blocking layer with higher mobility for electrons, 4,7-diphenyl-1, 10-phenanthroline (Bphen) was chosen to be the electron transport layer. The performance of device used Rb₂CO₃ doped into Bphen is obviously better than the device even used LiF with aluminum as cathode. According to the UPS spectra, the Fermi level of Bphen after doped with the ratio of 2% and 8% rubidium carbonate (Rb₂CO₃) shifts toward the lowest unoccupied molecular orbital as a result of charge transfer from rubidium atom to Bphen, showing that electron-injection ability would be improved based on strong n-type doping effect. Moreover, when aluminum is deposited as a thin layer on the surface of Bphen doped with Rb₂CO₃, the peak around 5 eV, which is attributed to the delocalized Pi-electrons decreases as gap states appear around 2.8 eV at the top of the highest occupied molecular orbital. There are changes in the binding energy of core levels of rubidium, nitrogen and aluminum, which indicates a negative charge transfer to Bphen at the interface that could have the reduction of electroninjection barrier height. Thus, the interfacial chemical reaction leads to the excellent electron injection ability could be demonstrated.

Admittance spectroscopy is a simple yet powerful tool to determine the carrier mobility of organic compounds. One requirement is to have an Ohmic contact for charge injection. By employing a thin interfacial layer of tungsten oxide or molybdenum oxide we have found a possibility to efficiently inject holes into organic materials with a deep highest occupied molecular orbital level down to 6.3 eV. These results considerably enhance the application range of the admittance spectroscopy method. The measured mobility data are in excellent agreement with data obtained by the time-of-flight technique. To efficiently inject electrons into materials with an ionization potential of up to 2.7 eV we thermally evaporated an intermediate layer of cesium carbonate and discuss the extracted electron mobilities.

Semiconducting polymers are very promising optoelectronic materials enabling the simple fabrication of devices such as light-emitting diodes, lasers and solar cells. However, the development of polymer lasers has been hampered by the low charge mobility of these materials preventing electrically driven lasers. We find that this problem can be overcome by taking advantage of the complementary properties of inorganic semiconductors. We show that by separating the charge transporting and lasing regions in a structure combining an indium gallium nitride light-emitting diode with a semiconducting polymer distributed feedback laser, an electrically pumped hybrid polymer laser can be made. This provides a new route to simple, convenient, compact and low-cost visible lasers with the potential for applications in security, sensing, spectroscopy, and medical diagnostics.

In this work modulation of laser emission from polymer nano-structured lasers was explored through three different optical techniques. We show all optical control of polymer distributed feedback lasers based on polyfluorenes (PFO and F8BT) by applying a gating pulse, which completely switch-off emission in the sub-ps time scale. The switching mechanism is assigned to photo-injection of charge carriers induced by the gate transition. This is a resonant non-linear process, that might work at high bit-rate, paving the way toward plastic, large-scale integrated, ultrafast optical logic. New opportunities may also be offered by two other techniques: Two-photon two-color pumping allows lasing action only in presence of two different pulses, one in the visible and one in the IR, resonant with the second telecommunication window. This may allow to convert telecommunication signal from a fiber to visible range and thus to Plastic Optical Fibers for organic photonics. Another technique we explored uses a blend of F8BT and a photochromic material, (1,2-bis-(5-phenyl-2-methyl-3-tienyl) perfluocyclopentene))(C4). With a UV pulse we are able to change C4 structure, thus overlapping its absorption spectrum with F8BT emission and modulating yellow ASE emission.

We demonstrate that organic thin film transistors (OTFTs) can act as an alternative tool for carrier mobility evaluation in amorphous organic electronic materials. OTFT is a three terminal device which can be operated with an active layer of film thickness thinner than 100 nm. The materials under investigation are phenylamine-based (PA) compounds, which are amorphous hole transporting materials widely used in organic light emitting diodes (OLEDs). The field effect (FE) mobilities of PA compounds (hole) were determined in a TFT configuration. For the case of N,N'-diphenyl-N,N'-bis(1-naphthyl) (1,1'biphenyl)-4,4'diamine (NPB), the FE mobility was found to be 2 × 10^-5 cm²/Vs. It is about one order of magnitude smaller than that obtained from independent time-of-flight (TOF) technique (2 x 10^-4 cm²/Vs) using a thick film of ~ 5 μm. Temperature dependent measurement was performed under temperature ranging from 235 to 360 K. The extracted energetic disorder by means of the Gaussian Disorder Model from OTFT was 85meV, which was larger than that of TOF (~74 meV). Similar observations were found in other PA compounds. The increase in the extracted disorder parameter in TFT configuration was one of the origins of the discrepancy between the FE and TOF mobility. OTFTs can be regarded as a useful tool for carrier mobility evaluation with little material consumption.

We developed a novel host material, CheilGH-01 reinforced with electron transporting property to perform high luminance efficiency and low driving voltage with long lifetime in green phosphorescent OLED without HBL (hole blocking layer). The device with a structure of ITO/NPB/TCTA/Host:Ir(ppy)₃/Balq/Alq₃/Liq/Al using CheilGH-01 as a host showed better performance than the device with CBP as a host; At luminance of 1000 cd/m², driving voltages, current efficiencies and power efficiencies are 5.4 V, 60 cd/A and 35 lm/W for CheilGH-01 and 7.2 V, 42 cd/A and 18.8 lm/W for CBP. It takes 240 hours for CheilGH-01 and 177 hours for CBP from the initial luminance of 3000 cd/m² to the luminance of 1500 cd/m² (55% of the initial). In the case of device with a structure of ITO/NPB/TCTA /Host:Ir(ppy)₃/Alq₃/Liq/Al, CheilGH-01 showed 300% enhancement in luminance efficiency and more than 800% longer lifetime than CBP; At luminance of 1000 cd/m², driving voltages, current efficiencies and power efficiencies are 6.3 V, 51 cd/A and 26 lm/W for CheilGH-01 and 8.6 V, 21 cd/A and 7.8 lm/W for CBP. It takes 411 hours for CheilGH-01 and 82 hours for CBP from the initial luminance of 3000 cd/m² to the luminance of 1500 cd/m². By using our novel host material, we are able to eliminate HBL from the conventional green phosphorescent OLED and still provide OLED with longer lifetime and excellent luminance efficiency.

We examined the driving mechanism of indium-tin oxide (ITO)/4,4-bis[N-(1-naphthyl)-N-phenyl-amino]biphenyl (α-NPD)/Tris-(8-hydroxiquinolate) aluminum (Alq₃)/cathode type organic light-emitting diode (OLED) by using a displacement current measurement (DCM). The DCM enables us to directly calculate the amount of accumulated charge. There exists a maximum value in the amounts of the blocked holes at α-NPD/Alq₃ interface. The maximum value was about 120 nC/cm², this value is consistent with the density of the fixed interfacial charge proposed by Brütting et al. By using hole-only device with Au cathode, we also investigated the hole blocking and the subsequent overflow of hole current beyond the interface. The observed feature can be explained by the hole blocking due to the interfacial charge rather than by that due to the HOMO mismatch at the interface.

We have studied the effects of hole transporting layers and electron transporting layers on efficiencies of Iridium(III)bis [(4,6-di-fluorophenyl)-pyridinato-N,C2'] picolinate (FIrpic) doped 3,5'-N,N'-dicarbazole-benzene (mCP) host blue PHOLEDs. We found that the device efficiency is very sensitive to the hole transporting materials used and both the triplet energy and carrier transport properties affect the device efficiency. On the other hand, there is no apparent correlation between the device efficiency and the triplet energy of the electron transporting material used. Instead, the device efficiency appears to be determined by the electron mobility of the electron transporting layer only.

Despite its wide application in devices, the mechanism of improvement induced by the LiF insertion layer remains controversial and to be fully resolved. We report our study of the interface formation when gold or Al is deposited onto 5 Å LiF covered Alq using X-ray and ultraviolet photoemission spectroscopy (XPS, UPS). We found that initial Au deposition produced a small shift of energy levels toward higher binding energy, which was reversed by subsequent Au coverages. The energy level positions finally reach those of the pristine Alq, resulting in a flat-band situation in the interface region. This is in sharp contrast to the Al/LiF/Alq interface, where ~1 eV downward shift of the energy levels substantially reduces the electron injection barrier. The observation of the overall flat-band condition in the interface region explains well why for thin LiF interlayer, the metal overlayer material is critical for the improvement of charge injection. As we observed here for Au, the low reactivity of the deposited metal atoms do not result in substantial n-doping of the Alq in the interface region, in contrast to more reactive metals like Al and Mg that can cause substantial n-doping of Alq, signified by the ~1 eV energy level shifts toward higher BE and emergence of the gap state, and reduce the electron injection barrier as a result.

New electroluminescent copolymers with fluoro groups in vinylene unit, poly(9,9-di-hexylfluorene-2, 7-vinylene-co-pphenylenedifluorovinylene) (PFVPDFV), have been synthesized by the GILCH polymerization. The fluoro groups were introduced on vinylene units to increase the electron affinities of the copolymers. The PFVPDFVs exhibit absorption spectra with maximum peaks at 371 ~ 413 nm. In the PL spectra of PFVPDFVs, as the PDFV content increases up to 50% in the copolymer system, fwhm was decreased by 4 - 38 nm as compared to PFV. The HOMO energy levels of the copolymers were about 5.25 - 5.50 eV, and the LUMO energy levels were about 2.67-2.97 eV. The polymer LEDs (ITO/PEDOT/polymer/Al) of PFVPDFVs showed emission with maximum peaks at around 472 - 538 nm. By adjusting the feed ratios of PDFV in the copolymers, it was possible to tune the emission colors from greenish yellow to orange depending on the obtained CIE coordinates. The luminescence efficiencies of the copolymers at room temperature are about 0.1-1.47 cd/A. The introduction of up to 50 % of PDFV in PFVPDFVS can enhance the device performance to result in high current density, brightness and efficiency due to the increased electron injection ability caused by the presence of fluoro groups in the vinylene units.

Organic light-emitting diodes (OLEDs) have great potential for displays and lighting applications. For large area displays the ideal materials would be both phosphorescent and solution processible. These requirements mean that the materials need to be able to be patterned and the most advanced method for forming pixelated displays is inkjet printing. Light-emitting phosphorescent dendrimers have given high efficiency monochrome displays with the emitting layer deposited by spin-coating. However, the viscosity of the dendrimer solutions is insufficient for inkjet printing. We report the development of a new class of light-emitting materials, namely poly(dendrimers) in which a green emissive phosphorescent dendrimer is attached to a poly(styrene) backbone. Free radical polymerization of a dendrimer-styrene monomer gave a poly(dendrimer) with a weight average molecular weight of 24000 and a polydispersity of 3.6. A dilute solution of the dendrimer had a viscosity 15% higher than the neat solvent. Comparison of the photophysical studies of the poly(dendrimer) versus a model monomer dendrimer showed that the PL spectrum was broader and red-shifted, and the PL quantum yield around 50% lower. This was attributed to intermolecular interactions of the emissive dendrimers, which are held closely together on the polymer backbone.