Silicone based materials have attracted considerable attention from Light Emitting Diode (LED) manufacturers. In LEDs, silicones can function in several roles that include optical lenses, stress relieving encapsulants, mechanical protection and light path materials. The key attributes of silicones that make them attractive materials for high brightness (HB) LEDs include their excellent transparency in the UV-visible region, their non-discoloring behavior and their stable thermo-mechanical properties. The first part of this paper/presentation will describe recent silicone materials development efforts directed towards providing LED manufacturers with silicone materials solutions for LED device fabrication. Injection molding of novel silicone resin based materials will be discussed as a viable route for high throughput LED device manufacturing. For other portions of the light spectrum, specifically at telecom wavelengths, the performances of silicone based materials are also verified and this makes them attractive materials for numerous photonics device applications. The second part of this paper/presentation will describe recent demonstrations of siloxane for use as waveguides for datacom and telecom applications. A Variable Optical Attenuator (VOA) utilizing silicone based waveguides (exploiting dn/dT property) and an Optical Backplane built from silicone waveguides and out-of-plane mirrors built on glass and FR-4 substrates are discussed.

OLED devices with electrically doped transport layers show low operating voltage, high efficiency and long lifetime [1-3, 5, 7]. This paper demonstrates that the concept of p- and n-type electrical doping can be applied under manufacturing conditions. It shows that handling of dopants, adjustment of doping concentrations, and preparation of p-i-n type OLED stacks is possible with the worldwide first vertical in-line set-up. An in-line-manufactured highly efficient RGB-OLED-system is presented.

The gravure printing technique is currently under investigation as a possible method for the roll-to-roll production of OLEDs in the 6th framework EU funded project entitled ROLLED - "Roll-to-roll manufacturing technology for flexible OLED devices and arbitrary size and shape displays". The objective in the project is to fabricate an entire OLED structure by using roll-to-roll manufacturing methods and to examine, how the commercial production could be set up and integrated into an existing printing process. In order to attain a roll-to-roll compatibility, all the materials, inks and device structures need to be suitable for printing. Since, such OLED device structures are very sensitive to moisture and oxygen, high barrier materials to be applied as wet chemical coatings on transparent polymer films such as PET by common roll-to-roll coating techniques have been investigated. The barrier films on their respective substrates act as front and back side encapsulation materials, where the front side encapsulation material is to be used as a transparent and flexible substrate for OLED fabrication. The transmission rates to be achieved for both front and back side encapsulation for oxygen and water vapour are 5 mg m^-2day^-1 (corresponding to 7 cm³m^-2day^-1 for O₂). In this paper, we show how light-emitting devices manufactured by gravure printing operate compared to the ones manufactured by traditional methods. Furthermore, we present recent results on the development of ITO nanoparticle coatings, cathode inks and flexible barrier materials.

Bulk-heterojunction solar cells made from blend films of regioregular poly(3-hexylthiophene) (P3TH) and methanofullerene (PCBM) exhibit a ten-fold increase of the power conversion efficiency upon post production annealing. We have applied a numerical model to explain this dramatic enhancement of the efficiency. The mobilities of electrons and holes were selectively determined, and it was found that the hole mobility of P3HT in the blend increases by more than three orders of magnitude. Moreover, upon annealing the absorption spectrum of P3HT:PCBM blends undergo a strong red-shift, improving the spectral overlap with the solar emission, which result in an increase of more than 60% in the generation rate of charge carriers. Our numerical model demonstrates that the unannealed devices suffer from the buildup of space-charge, as a consequence of the low hole mobility. Furthermore, at short-circuit the dissociation efficiency of bound electron-hole pairs at the donor/acceptor interface is close to 90%, which explains the large quantum efficiencies measured in P3HT:PCBM blends.

In this work, we report on efficient heterojunction organic solar cells containing a new oligothiophene derivative α,α'-bis-(2,2-dicyanovinyl)-quinquethiophene (DCV5T) as donor (D) and fullerene C₆₀ as acceptor (A). The oligothiophene carries electron withdrawing substituents which increase the ionization energy and even more strongly the electron affinity. In thin films, the absorption is significantly broadened compared to solution and the optical gap is reduced to 1.77 eV. Nevertheless, the material shows strong fluorescence with low Stokes shift (peak at 1.71 eV), i.e. low energy loss upon reorganisation in the excited state. At the heterointerface between the low band-gap oligothiophene and fullerene C₆₀, photogenerated excitons from both materials are efficiently separated into electrons on the LUMO of C₆₀ and holes on the low-lying HOMO of the oligothiophene. This step involves only low energetic losses since both the HOMO and the LUMO offset of the two materials are below 0.6 eV, close to the expected exciton binding energy. We can thus reach high open circuit voltages of up to 1.0 V. The most efficient solar cells with power efficiencies around 4 % are obtained when the photoactive heterojunction is embedded between a p-doped hole transport layer on the anode side and a combination of a thin exciton blocking layer and aluminium on the cathode side. However, due to the high ionization energy of the oligothiophene (approx. (5.6 ± 0.1) eV), hole injection from any anode or hole transport layer is difficult and the IV curves thus show a characteristic S-shape which reduces the fill factor FF. It is found that the actual FF sensitively depends on the work function of the p-doped hole transport layer, that can be influenced by doping.

One of the limitations of present organic solar cells is the relatively poor spectral overlap of their absorption bands with the solar spectrum. Semiconducting polymers as poly(3-hexyl thiophene) have a bandgap higher than 2.0 eV (600 nm), thereby limiting the maximum possible absorption of the solar spectrum to about 30%. A way to overcome this limitation is a tandem solar cell where two bulk heterojunction single cells are stacked in series, each with a different bandgap. The combined absorption then covers a broader region of the solar spectrum. So far, solution-processed tandem solar cells have not been realized due to incompatibility of the solvents. We demonstrate a solution-processed polymer tandem cells by stacking two single cells in series. The tandem cell consist of two bulk heterojunction subcells separated by a thin semitransparent electrode of gold. This middle electrode serves in three different ways; as a charge recombination centre, as a protecting layer for first cell during spin coating of the second cell, and as a semitransparent layer that creates optical cavities, which allows tuning of the optical transmission through the first (bottom) cell to optimize the optical absorption of the second (top) cell. To cover a broader region of the solar spectrum we combined a small bandgap polymer (λ_max ~ 850 nm) with a large bandgap polymer (λ_max ~ 550 nm). These sub cells are electronically coupled in series, which leads to an open-circuit voltage that equals the sum of each sub cell. A high open-circuit voltage of 1.4 Volt is achieved. The current density of the tandem cell follows the current of the top cell, which has a lower, limiting current. The tandem architecture and proper materials give us the possibility to cover a very broad spectral range of the solar spectrum to make highly efficient organic solar cells in the near future.

We report on the experimental observation of polarization splitting and terahertz oscillations in transmission and laser emission from optically anisotropic microcavities. A guest-host composite of tris-(8-hydroxyquinoline) aluminium (Alq3) and 4-(dicyanomethylene)-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran (DCM) serves as active laser material. The anisotropy is attributed to oblique columnar structures in the distributed Bragg reflector mirrors of our microcavity, resulting from sample fabrication. A splitting of 0.2 nm occurs in the laser emission from an organic vertical cavity surface emitting laser at a wavelength of 612.6 nm, and a splitting of 2.5nm is obtained from a sample for Ti-Sapphire laser transmission at 781 nm. Split modes are perpendicularly polarized. An upconversion setup allows temporally resolved studies of transmission and emission behavior, showing an oscillation at a frequency of 1.25THz in transmission, and 0.18THz in emission, respectively. The temporal behavior of laser emission is modelled by a set of rate-equations and extended to account for the resulting oscillations. Our observations suggest that a phase-coupling mechanism between both occuring modes is present in the laser emission from our microcavity.

Ambipolar organic field-effect transistors (FET) are interesting as building blocks for low power complementary circuits in organic electronics. Another intriguing feature of ambipolar FETs is the recombination of holes and electrons within the channel, which leads to the formation of excitons that can relax radiatively and thus emit light. We have recently demonstrated that ambipolar charge transport is a generic feature in a wide range of polymer semiconductors when appropriate injection electrodes and trapfree dielectrics are used. Among these materials are those that are generally used in light-emitting diodes and thus show high photoluminescence efficiencies. Here we demonstrate ambipolar light-emitting field-effect transistors based on the conjugated polymer OC₁C_10-PPV (poly(2-methoxy-5-(3,7-dimethyloctoxy)-p-phenylenevinylene)) as the semiconducting and emissive layer. OC₁C_10- PPV shows efficient electron and hole transport with field-effect mobilities of 3⋅10^-3 cm²/Vs and 6⋅10^-4 cm²/Vs, respectively. Electrons and holes are injected from calcium and gold source and drain electrodes, respectively, and recombine radiatively within the transistor channel leading to visible light emission. We can actively control the position of the recombination zone through the applied gate and source-drain bias in both constant and variable current mode and thus move the emission zone from the source through the channel to the drain electrode and vice versa. The intensity of light emitted from the channel is proportional to the drain current with efficiencies comparable to those of LEDs based on OC₁C_10-PPV.

We have realized a light-emitting organic field-effect transistor (LEOFET). Excitons are generated at the interface of an n-type and a p-type organic semiconductor heterostructure inside the transistor channel. The dimensions and the position of the p-n heterostructure are defined by photolithography. The recombination region is several microns from the metal electrodes. Therefore, the exciton quenching probability in this device is reduced. Numerical simulations show that the recombination region can move within the transistor channel by changing the biasing conditions.

We propose a new type organic light emitting transistor (OLET) combining static induction transistor (SIT) with double hetero junction type organic light emitting diodes (OLED) using n-type zinc oxide (ZnO) films which works as a transparent and electron injection layer. The device characteristics of newly developed OLED and ZnO-SIT showed relatively high luminance of about 500 cd/m² at 7.6 mA/cm² and is able to control by gate voltage as low as a few volts, respectively. The crystal structures of the ZnO films as a function of Ar/O₂ flow ratio and the basic characteristics of the thin film transistor (TFT) and SIT depending on the ZnO sputtering conditions are investigated. The results obtained here show that the OLET using ZnO film is a suitable element for flexible sheet displays.

An experimental and modelling study has been carried out of the current-voltage-luminance (J-V-L) characteristics of blue polyfluorene-based organic light emitting devices, with a PEDOT:PSS anode and a Ba/Al cathode. The polymer contains copolymerized hole transporting units that facilitate hole injection. The luminous efficacy for perpendicular emission as a function of the voltage shows a pronounced peak; for an 80 nm thick device, it is equal to 3.3 cd/A at 8 V. At the peak voltage, the external quantum efficiency is 2.2 %. We are working on a comprehensive device model that should provide a framework within which these results can be understood, and present in this paper our intermediate results. Hole and electron transport were studied using devices with a Au and Al cathode and anode, respectively. For hole-only devices a fair description of the temperature and layer thickness dependent J-V curves could be obtained by using a 'conventional' model for the mobility, involving a Poole-Frenkel factor for the field-dependence. For electron-only devices, the analysis is complicated by the presence of an approximately 0.5 eV injection barrier. We have found a parametrization scheme that provides a good description of the experimental J-V curves. A double carrier model that is based on the results of these studies of single-carrier devices provides a good description of the J-V curves of double carrier devices. We have developed a numerical model for the light outcoupling from the optical cavity. For the model parameters assumed, the calculated peak position and shape of the lumunous efficacy as a function of V are in good agreement with the experimental results at room temperature. An analysis is given of the factors that determine the peak height. We argue that a solid physical basis for the model used to describe the electron injection and mobility is still lacking, so that continued electron transport studies will be required.

In this contribution, we discuss several research results which have contributed to the vision of a broad application of organic light emitting diodes in displays and lighting. We discuss the factors which determine the efficiency of OLEDs. First, we briefly discuss work on the controlled molecular doping of organic semiconductors, having the same advantageous influence on the device properties than in inorganic semiconductor devices, in particular in reducing the operating voltage. By comparison with a simple theory, we show that the voltages achieved with doped OLEDs are already very close to the thermodynamic limit. We then show, with the example of red phosphorescent OLEDs, how many properties of the OLED are improved by doping.

We report on white organic light emitting diodes with three stacked emitter layers comprising the fluorescent blue emitter Spiro-DPVBi, the phosphorescent green emitter system TCTA:Ir(ppy)₃ and the phosphorescent red emitter system NPB:Ir(MDQ)₂(acac). A thin additional layer of mixed TCTA and TPBi separates the fluorescent and phosphorescent emitting regions, simultaneously confining excitons efficiently and letting electrons and holes easily pass. Furthermore, phosphorescence quenching by Dexter transfer to the non-radiative triplet state of Spiro-DPVBi is suppressed. Devices were optimized to get color coordinates very close to the warm white standard illuminant A. Best devices have a current efficiency of 13.8 cd/A, CIE color coordinates of (0.45, 0.42), and a color rendering index of 91 at a brightness of 1000 cd/m². Due to the use of electrically doped charge transport layers, the voltage needed for 1000 cd/m² was only 3.0 V, which leads to a power efficiency of 14.4 lm/W assuming Lambertian emission.

Organic electrophosphorescent materials and devices are the prime focus of organic light-emitting diodes research due to their high external quantum efficiency and power efficiency. The host materials with both high triplet energy level and high thermal stability are especially formidable for blue phosphorescent emitters. Herewith we report a novel triplet host material based on fluorene, 9,9-bis(4'-carbazol-phenyl)fluorene (CPF), in which two phenyl-carbazole moieties are connected to C9 carbon of the fluorene. This compound possesses not only desirably high triplet (2.9 eV) energies, but also extremely high glass transition temperature (Tg = 165 °C) and thermal stability. By using CPF as the host material, blue-emitting phosphorescent devices exhibited much higher efficiency and longer lifetime than those with CBP host.

Today, organic light emitting diodes are used in small to medium displays in portable electronic equipment like MP3 players and mobile phones. Their thin form factor, together with good readability due to low angular dependence of the emission makes them attractive for these applications. The rapid progress in the last years has lifted the performance of OLEDs to a level where one can seriously start to consider applications in lighting markets. Whereas it is obvious that first applications will be in less demanding niche markets, clearly the most interesting target is the general illumination market. In this report, first applications requirements will be described, followed by a brief review of state of the art monochrome OLEDs. The main part deals with the various ways in which monochrome devices can be combined into white ones, giving examples of existing solutions. The conclusion is that for the white OLED design, there no clear winner yet. Given the rapid progress in material and device development, one can expect that within a few years white OLEDs will be available which can start to penetrate the general lighting market.

An OLED device suitable for automobile ceiling lights has been designed, fabricated and evaluated. The OLED structure is fabricated on a thin (120 µm), flexible polymer foil with integrated micro-optics to achieve customized beam shaping in the far field. A pixelated OLED structure matched to a patch-pad microlens matrix was used to convert the OLED Lambertian emission into a Gaussian-shaped illumination beam. Both refractive and diffractive microlenses were investigated. The integrated micro-optics OLED architecture reduces light losses due to waveguiding effects and effectively increases the light extraction by up to 70%.

OLEDs gain more and more interest in the field of lighting applications. The OLED technology provides striking advantages and covers completely new application fields offering a new freedom in design for next generation lighting. Large area OLEDs might act as a 2-dimensional light source which is thin, flat and lightweight generating diffuse, nonglaring illumination. In the first part of our report we investigate small scale inhomogeneities of polymer based OLEDs. Devices were monitored during operation by taking pictures of the active area at constant periods of time. These pictures were analyzed by a software tool with respect to the occurrence and evolution of defects. Initially induced inhomogeneities are growing and dominate the performance with increasing operation time. Within the error margin of the setup no additional spots are generated during operation. The voltage drop inside the ITO anode due to a high resistivity plays an important role for the brightness homogeneity of large area devices. The voltage drop causes a brightness fall-off towards the center of the device. It is maintaining with increasing average current density and luminance, respectively. At a brightness of 1000cd/m² the deviation at the center exceeds 30%. The homogeneity of luminance is improved by incorporation of additional metal lines on the anode layer. The best results were achieved with 200nm thick aluminum structures with a pitch of 1mm and a width of 60μm of each line. At an average current density of 45mA/cm² the decay towards the center of the device is only half of the decrease without any additional metallization.

Charge injection in organic light emitting diodes (OLEDs) is studied by impedance spectroscopy on a solution processable polymer based OLED (PLED) using different metallic cathodes. A negative capacitance is observed in organic light-emitting diodes (OLEDs) at low frequencies which can be explained using a detailed kinetic model based on sequential injection through surface states at the metal/organic interface. In this paper the methodology used to derive this model and its application to experimental data is presented.

We report on the experimental observation of large magnetoresistance in polyfluorene organic light-emitting diodes (OLEDs). Very similar magnetic field effects (MFEs) of comparable magnitude were also observed in electroluminescence and photocurrent measurements. We provide a comprehensive overview of these three types of MFE. To the best of our knowledge, the mechanism causing these MFE is currently not known. Moreover, we show that these experiments do not allow determination whether the MFE acts on the carrier density or carrier mobility making any attempt of explaining it ambiguous. As a remedy, we performed magnetoresistance measurements in holeonly OLEDs and show that the MFE acts on the carrier mobility rather than carrier density.

We propose a new electrooptic antenna design using organic polymer to receive microwave signals. In this paper we present the characterization of an electrooptic organic polymer. We measured the dielectric constant at microwave frequencies, and the electrooptic coefficient. We measured values of r₃₃ of 3.35 pm/V at 1310 nm and 1.98 pm/V at 1550 nm. The goal of the electrooptic antenna design is to obtain maximum microwave and optical interaction. We propose a novel approach based on resonance effect in both optical and microwave domain. For the optical resonance effect we use a Fabry-Perot cavity, and a patch structure as microwave resonator.

New fluorinated poly(arylene ether sulfone)s (FPAES) and poly (arylene ether ketone)s (FPAEK) with two types of NLO chromophores as pendant groups were obtained by a polycondensation reaction carried out using very mild reaction conditions. The glass transition temperature (Tg) of these copolymers is between 168 and 190°C. The SHG intensity was measured during thermal corona poling : the dipole orientation starts at 100°C, and reaches a maximum at 160°C, which is lower than Tg. During cooling of the film under the applied electric field, we observed an unusual drop of the SHG intensity starting at 120°C and reaching 70 % of the maximum value. The physical origin of this drop has been investigated and is possibly attributable to a secondary phase transition of the copolymers. Despite this decrease, d33 coefficients measured after poling vary between 2 and 15 pm/V at 1907 nm fundamental wavelength, depending on the first hyperpolarisability of the NLO chromophore. The dipole orientation of all these copolymers is very stable provided the temperature of the films is kept below the temperature threshold of 120 °C, which is below the Tg of the polymers.

Dye-loaded zeolite L host-guest materials were already successfully used in the realisation of efficient light-harvesting antenna systems. A new hierarchy of structural order is introduced by arranging the zeolite L crystals into densely packed, oriented monolayers on a substrate. In device engineering, a high degree of supramolecular organisation is a prerequisite for achieving desired macroscopic properties. The methods we developed to synthesise such monolayers, to fill them with dyes and to terminate them with a luminescent stopcock will be discussed as well as their influence on the design of novel materials. By subsequent insertion of two different types of dye molecules in a zeolite L monolayer, the first unidirectional antenna system was realised. UV-VIS absorption as well as NIR luminescence spectroscopy was carried out on dye-loaded zeolite L monolayers. We also report a novel concept for the preparation of thin layer, silicon based solar cells.

We report waveguide amplification of spontaneous emission and coherent random laser action in individual self-assembled organic nanofibers grown by high-vacuum deposition. The interpretation of the experimental results is given on the basis of simple models, including transfer matrix calculations in one-dimensionally disordered structures. We present also the numerical results for light scattering from a nanofiber which can be used as a basis for further experiments.

Single-walled carbon nanotubes dispersed in a polymer matrix are studied via cw photomodulation spectroscopy. The spectrum is dominated by a modulation of the absorption lineshape, which we assigned to electro-absorption caused by local electric fields arising from trapped photoinduced charges. The lack of selectivity in the excitation and the dominance of the contribution by low energies point to an efficient migration of the photoexcited states, either the singlet excitons or the charges resulting from their dissociation.

Photo-induced phenomena were investigated in photoresposive organic field-effect transistors (photOFETs) based on conjugated polymer/fullerene solid-state mixtures as active semiconductor layer and divinyltetramethyldisiloxane-bis(benzocyclobutene) (BCB) as gate dielectrics. The devices were characterized both in under dark showing n-type transistor behaviour with linear and saturated mobility of 1.7 x 10^-3 cm²/Vs and 2.7 x 10^-2 cm²/Vs respectively, and under white light illumination condition, where large shifts in the threshold voltage in the transfer characteristics were obtained. A typical phototransistor behaviour in a wide range of illumination intensities are observed in these devices.

A poly(phenyleneethynylene) polymer bearing amino acid pendant groups is used as enantioselective active layer in solid-state sensing devices. The chiral analyte in the present study is menthol in both the natural (-) and synthetic (+) enantiomers. The polymer bearing amino acid chiral sites is demonstrated to interact more favorably with the natural menthol than the synthetic one in a quartz crystal microbalance revealing system. Promising perspectives are seen for the use of such polymers in chiral discriminating, chemically sensitive resistors or even transistors.

Now organic thin film transistor technology gains growing maturity, high performance organic photodetectors are the missing link towards full organic photosensitive sensor arrays, needed for the realization of applications like organic scanners and organic cameras. In the borderline of the research in organic solar cells, research in organic photodetectors has mainly been limited to organic photodiodes. However, phototransistors offer the possibility to reach higher sensitivities, thanks to the internal current gain of a transistor structure. This document focuses on organic field-effect phototransistors (organic photoFETs) where illumination can be used as an optical gate signal in addition to the electrical gate signal. An experimental description of pentacene photoFETs during broadband illumination is given. A distinction between the direct photocurrent and the additional current enhancement by a thresholdshift can be made, and a relation for this thresholdshift in function of time is revealed. Consequently, the diffculty to compare different organic photoFETs without any information about the illumination time is highlighted and a comparison based on this power law is proposed. Subsequently it is pointed out that by the stability of this thresholdshift after illumination, the pentacene photoFET does not only act as a light detector but also as a memory element, capable to store information about illumination power and duration for an extended amount of time.

An increase in stochastic molecular vibrations of a solid due to heating above the melting point leads to a decrease in its long-range order and a loss of structural symmetry. Therefore conventional liquids are isotropic media. We report on a light-induced isothermal phase transition of polymer materials from an isotropic solid to an anisotropic liquid state in which the degree of mechanical anisotropy can be controlled by light. Whereas during irradiation by circular polarized light the materials behave as an isotropic viscoelastic fluid, they display considerable fluidity only in the direction parallel to the light field vector under linear polarized light. In this case the polymers behave visually as a liquid along one direction and as a solid in others (one-direction liquid-state), demonstrating thus extraordinarily anisotropic mechanical properties. The fluidisation phenomenon is related to photoinduced motion of azobenzene-functionalised molecular units, which can be effectively activated only when their transition dipole moments are oriented close to the direction of the light polarization. We also show that the selective excitation of chromophores orientated parallel to the light polarization not only has great influence on the anisotropy of the mechanical properties of the materials, but also induces an anisotropic expanding force, which can be used for polarization-selective opto-mechanical actuators and sensors.

Polyaromatic dyes were used to synthesize molecular nanocrystals in sol-gel thin films for sensor applications. Fluorescence Confocal Microscopy (FCM) and Transmission Electron Microscopy (TEM) experiments showed the advantages of our nanocrystallization process compared with microcrystallization in free solutions. Indeed, we obtained well-dispersed and spherical nanocrystals with a narrow size distribution, exhibiting a good crystallinity. Time-resolved fluorescence spectroscopy allowed us to measure fluorescence lifetimes of nanocrystals in presence of molecular probes. Then, chemical sensoring properties of these molecular nanocrystals were demonstrated.

Two dimeric trimeric phenylenvinylene derivative 2, 5, 2', 5'-tetrakis (4'-fluorostyryl) biphenyl (P-F-TSB) and 2, 5, 2', 5'-tetra (p-trifluoromethylstyryl)-biphenyl (TFM-TSB) have been synthesized as new electroluminescent (EL) materials. P-F-TSB exhibits good color purity, high luminance of blue light-emission in organic light-emitting devices. Maximum brightness and luminous efficiency are of 1828 cd/m² and of 1.92 cd/A, respectively. CIE coordinates are x=0.20 and y=0.22. Interestingly, we can fabricate single layer white light-emitting device using TFM-TSB as emitting layer. The broad electroluminescence emission band may attribute to long-wavelength excimer and electromer emission in addition to the blue component from singlet excited state of individual TFM-TSB molecule. Furthermore, white-light emission can also be obtained with a typical three-layer structure of ITO/ NPB (50 nm)/ TFM-TSB (50 nm)/ Alq3 (30 nm)/LiF/Al device. The maximum brightness of this device is 809 cd/m² at 217 mA/cm² and 13V, and the maximum luminous efficiency is 1.49 cd/A at 11 mA/ cm² and 8V.

In this study, we presented high-performance flexible organic light-emitting diodes (FOLEDs); to do so, we prepared indium-tin oxide (ITO) thin layers by ion beam sputtering (IBS) on polyethylene terephtalate (PET) substrates in soft low temperature conditions. The IBS technology seems well adapted to us to adjust the conduction level of the interface films to the one of the various organic materials making up the fabrication processes of the organic optoelectronic components; moreover this technique does not require a high substrate temperature or an annealing after ITO deposition to crystallize the obtained layers. Because of the great number of deposition parameters (oxygen flow, substrate temperature, deposition rate...) playing interdependent roles and strongly influencing the electrical, optical and structural properties of the layers, we optimized the effects of these different parameters separately by using electrical and optical characterizations as well as X-ray diffraction analyses. The performances of FOLEDs on PET substrate with different ITO thicknesses were investigated and compared to the ones of a conventional organic light-emitting diode realized on glass substrate and according to the same device configuration.

Tris(8-hydroxyquinoline) aluminum (Alq) has been widely used as the electron transporting layer as well as the green emitter layer in organic light emitting diodes (OLEDs) since it is thermally and morphologically stable to evaporate into thin films. Alq exhibits strong green emission but its spectrum is broad (~85 nm FWHM) which results in inferior colour purity. On the other hand, rare-earth metal ions exhibit sharp spectral band which corresponds to 5Dx-7Fx transitions. In the case of terbium, this results in a sharp emission in green. In this work, organic light emitting diodes based on a rare-earth metal complex [Tris(3-methyl-1-phenyl-4- trimethyl-acetyl-5-pyrazoline) terbium III] were fabricated by thermal evaporation. The basic device structure is indium tin oxide(ITO) / 4,4'-N,N'-dicarbazole-biphenyl (CBP) / 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) / tris(8- hydroxyquinoline) aluminum (Alq) / Lithium Floride(LiF) / Aluminium (Al). The terbium (Tb) complex was doped into the hole transporting material (CBP) and the electron transporting material (Alq) to investigate the effect of dopant in different layers on the device performance. The effects of different dopant concentration on the photoluminescence (PL) and electroluminescence (EL) emission spectra were also investigated. Sharp emission in green (545 nm) was observed for optimum device structure and doping concentration.

The optical and electronic characteristics of thick OLEDs were examined by varying the thickness of the carrier transport layers (NPB and Alq₃) for the first time. For having better mode guiding, more than 285nm thickness device was studied in this paper. The experimental results show the threshold voltage is proportional to the total thickness but the breakdown electric fields decrease. I-V curves can be fitted with a second-order polynomial form, and these fit-ting curves can be used for modeling the carrier transport in thick OLEDs. Eventually, a comprehensive model is pro-posed to describe the carrier transport due to the trap effect in bulk organic materials. It shows this model is in a good agreement with our measured I-V curves.

The synthesis and properties of 3,6-bis(triphenylsilyl)carbazole as an effective host material for blue electrophosphorescence are reported. The electrochemically active C3 and C6 sites of carbazole are non-conjugated blocked with the steric, bulky and large-gap triphenylsilyls, the resulting new compound retains the large triplet energy of carbazole yet exhibits much enhanced morphological stability and electrochemical stability in comparison with previous carbazole-based host materials. Using this new host material, blue phosphorescent OLEDs having high efficiencies up to 16%, 30.6 cd/A and 26.7 lm/W are demonstrated.

Photophysical studies and photovoltaic devices on a low bandgap, high charge-carrier-mobility Poly(Thienylene Vinylene) (PTV), prepared from a soluble precursor polymer synthesised via the 'dithiocarbamate route', are reported. In composites with an electron acceptor ([6,6]-phenyl C₆₁- butyric acid methyl ester (PCBM), a soluble fullerene derivative) photoinduced absorption (PIA) characteristic for charged excitations together with photoluminescence (PL) quenching are observed indicating photoinduced electron transfer. The "bulk heterojunction" photovoltaic devices using PTV and PCBM composites show short circuit currents up to 4 mA/cm² under AM 1.5 white-light illumination. The photocurrent spectrum of the photovoltaic device shows an onset at about 1.65 eV (750 nm) which corresponds to the absorption spectrum of the polymer.

In the present work the infrared-transmission of PEDOT:PSS (poly(3,4-ethylenedioxythiophene):poly(styrensulfonat)) is being investigated using three different kind of electrochromic devises. PEDOT:PSS is an electrochromic conducting polymer able to change its optical properties when it is doped and undoped. The doping is achieved by a voltage applied across the cell. The optical properties are reversible if the polarity of the voltage is changed. We report here, to our knowledge, the first cell with adaptive transmission in the infrared wavelength range. The issue for this type of cell is that all the layers in the device must be transmissive in the whole relevant wavelength range. The results were further verified by construction of similar cells with different configuration giving adaptable reflectivity.

We investigated organic bulk-heterojunction solar cells based on an absorber blend of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl C61-butyric acid methyl ester (PCBM) by electrical impedance spectroscopy (EIS). A strong neck in the modulus plot of the EIS-spectra indicates that the absorber is divided into two regions of different conductivities. A similar behaviour was observed for pure P3HT-diodes. Hence, it can be concluded that the PCBM:P3HT absorber is pdoped by impurities of P3HT, so that a Schottky-like contact with aluminium is formed. It is known from literature, that annealing of PCBM:P3HT solar cells leads to drastic improvement of the photovoltaic performance. We compared the current-voltage characteristics and impedance spectra before and after consecutive annealing steps. After the annealing an expansion of the depletion region was observed, indicating that volatile dopants were evaporated out of the absorber. This contributes to an improved photovoltaic performance as the separation of the generated charges in the depletion region is more efficient than in the non-depleted region. Also an improved rectification behaviour might be caused by a lower doping level.

In this work we study the internal electric field (V_int) present in devices based on an intrinsically semiconducting polymer. Intermediate layers between the indium-tin-oxide and Al electrodes and the photoactive layer are able to influence and alter this electric field. The two commonly used intermediate layers, namely poly(3,4- ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) and LiF, are subject of this study. Their influence is studied with Electroabsorption (EA) spectroscopy as well as transient photocurrent measurements under applied bias. While PEDOT:PSS has no significant influence on V_int, introducing LiF increases V_int close to the bandgap of the studied semiconducting polymer. However, using PEDOT:PSS directly influences the spectral EA response. The interface between PEDOT:PSS and the conjugated polymer is studied by impedance spectroscopy. We interpret the results in terms of the presence of charges at the interface.

Spin-coated thin films of poly[(9,9-dioctylfluorenyl-2,7-diyl)-co-5,5-(4',7'-di-2-thienyl-2',1',3'-benzothiadiazole)] (APFO-3) blended with [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) are used as the active material in polymer photovoltaic cells. Such blends are known for their tendency to phase separate during film formation. Tuning the morphology of the blend in a controlled way is one possible road towards higher efficiency. We studied the effect of adding chlorobenzene to chloroform-based blend solutions before spin-coating on the conversion efficiency of APFO-3:PCBM photodiodes, and related that to the lateral and vertical morphology of thin films of the blend. The lateral morphology is imaged by atomic force microscopy (AFM) and the vertical compositional profile is obtained by dynamic secondary ion mass spectrometry (SIMS). The profiles reveal compositional variations consisting of multilayers of alternating polymer-rich and PCBM-rich domains in the blend film spin-coated from chloroform. The vertical compositional variations are caused by surface-directed spinodal waves and are frozen in during the rapid evaporation of a highly volatile solvent. With addition of the low-vapour pressure solvent chlorobenzene, a more homogeneous vertical composition is found. The conversion efficiency for solar cells of this blend was found to be optimal for chloroform:chlorobenzene mixtures with a volume-ratio of 80:1. We have also investigated the role of the substrate on the morphology. We found that blend films spin-coated from chloroform solutions on PEDOT:PSS-coated ITO show a similar compositional structure as the films on silicon, and that changing the substrate from silicon to gold only affects the vertical phase separation in a region close to the substrate interface.

We compare different structures of organic solar cells based on zinc phthalocyanine (ZnPc) and fullerene derivatives as electron donor and acceptor materials, respectively. Bilayer devices are fabricated and characterized by current-voltage and spectrally resolved photocurrent measurements. In a novel approach, the ZnPc was combined with soluble fullerene derivatives. With a pyrrolidinofullerene bearing chelating pyrridyl-groups we observed a complexation between donor and acceptor molecules. Due to a favorable structuring of the donor-acceptor interface this leads to a significant enhancement of the solar cell performance compared to similar devices where no complexation takes place. Coevaporated bulk heterojunction mixed-layers are introduced between the pristine layers. In these optimized structures short circuit currents up to 13 mA/cm² are observed. We investigate the voltage dependence of the spectrally resolved photocurrent of ZnPc / Buckminsterfullerene bilayer solar cells and interpret the results in terms of the Gartner model.

Standard organic light emitting diodes (OLEDs) are usually bottom-emitting, i.e. they emit light through a transparent and electrically conductive substrate. Usually, indium tin oxide (ITO) is used for this purpose. However, as indium is a very expensive metal, replacing it is of vital interest for cheap OLED mass production, especially when it comes to lighting applications. We suggest the use of a polymer instead of ITO, carrying out both hole transport and injection. In contrast to conventional approaches, which use a conductive polymer on top of ITO as smoothening and hole injection layer, we employ solely a highly conductive polymer in combination with an OLED comprising doped charge transport layers. This allows us to renounce the ITO layer underneath. We use a new, highly conductive formulation of PEDOT:PSS, called Baytron® PH 500, with a conductivity of typically 500 S/cm, providing a smooth and electrically well-conductive substrate for the OLED stack. The use of such a polymeric injection layer and of a doped small-molecule OLED stack results in a low operating voltage of the devices. The charge transport layers of the OLED consist of MeO-TPD (N,N,N',N'-tetrakis(4-methoxyphenyl)-benzidine) doped with a low percentage of F4-TCNQ (2,3,5,6-tetrafluoro-7,7,8,8-tetracyano-quinodimethane) for the hole transport layer and of Bphen (4,7-diphenyl-1,10-phenanthroline) co-evaporated with Caesium for the electron transport layer. We demonstrate both fluorescent and phosphorescent monochromic OLEDs based on Baytron® PH 500 which achieve good efficiencies. The OLEDs made on Baytron® PH 500 are compared with devices made on an ITO anode. Although the polymer possesses a somewhat lower conductivity than ITO, efficient devices can be fabricated. For example, using the blue emitter Spiro-DPVBi (2,2',7,7'-tetrakis(2,2-diphenylvinyl)spiro-9,9'-bifluorene), we achieve an efficiency of up to 5.1 cd/A. As another example, we discuss green OLEDs based on the triplet emitter Ir(ppy)3 (fac tris(2-phenylpyridine) iridium) doped in a wide gap material. In this case, even a higher efficiency than on ITO is reached: 62 cd/A at a luminance of 100 cd/m², corresponding to an increase in external quantum efficiency by 15% as compared to ITO.

In this work we discuss improvements of organic solar cells based on poly(3-hexylthiophene-2,5-diyl) : C_61-butyric acid methyl ester (P3HT:PCBM) blends. The polymer layer is combined with various electron transport materials and different cathodes. We were able to utilize the good charge carrier separation and transport properties of the P3HT:PCBM blend together with the flexibility of evaporated heterostructures. The systematic use of different cathodes such as calcium, aluminium/lithiumfluoride and organic intermediate layers resulted in higher fill factors and open circuit voltages compared to simple aluminium cathodes. In particular we studied the influence of additional layers of electron transport layer consisting of C₆₀, lithium doped bathophenanthroline (BPhen:Li) 2-(4-tert-butylphenyl)-5-(4-biphenylyl)-1,3,4-oxadiazole (PBD) and 2,9- dimethyl-4,7-diphenyl-1,10-phenantrolene (BCP) on the cell properties. Solar cells with power conversion efficiencies well above 3% have been fabricated.

In order to obtain a red emission with organic electroluminescent device, we realized multilayer structures using various Europium complexes as emitting layer. The multilayer structures were obtained with the emitting layer deposited on a ITO substrate; a 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP) layer was used as hole blocking layer and a tris-(8-hydroxyquinoline) aluminium (Alq3) layer as electron transport layer. Cathode of the devices was realized with a calcium layer covered with an protective aluminium layer. Two different europium complexes have been tested: Eu (TTA)₃ phen (TTA = thenoyltrifluoroacetone), Eu (DTP)₃ (dipphen) (DTP = 1,3-di(2-thienyl)propane-1,3-dione, dipphen = 4,7-Diphenyl-1,10-phenanthroline) The Europium complexes were inserted in a poly-9-vinylcarbazole (PVK) matrix and the "spin-coating" process was used to obtain the emitting layers. The maximum emission of the OLEDs are centered around 610 nm according to the ⁵D₀ → ⁷F₂ transition of the rare earth. Europium complexes absorption spectra in chloroform and electroluminescence spectra are presented. Current-voltage and luminance-voltage characteristics are also presented and we discuss about the results obtained with various molar concentration of Europium complexes in the PVK matrix.

The active layer morphology of vacuum deposited organic solar cells based on phthalocyanine-perylene donor-acceptor couple have been studied, comparing coevaporated bulk heterojunction with multilayer structures consisting in alternated donor and acceptor very thin layers. These latter devices show current-voltage behaviour indicating that their active layer acts as an interpenetrated network. They show better photovoltaic parameters, with power conversion efficiencies 2.5 time higher than the bulk heterojunctions, and better external quantum efficiencies. A compromise with vacuum evaporation growth rate has to be taken in order to realize the multilayer solar cells: high growth rate produces a better bulk interpenetrated structure giving rise to a better external quantum efficiency, but low growth rate favours the planar molecular arrangement that gives a better absorption of the perylene molecules. From the comparison between measurements held in open air and under dry nitrogen atmosphere, we think that multilayer solar cells seem to be far less sensible to open air condition than the coevaporated solar cells.

Three fluorene-based copolymers carrying reactive side groups (oxetanes) were synthesised and characterised. Their optical properties are essentially controlled by the introduction of either fluorene or lower energy gap (benzothiadiazole or terthiophene) comonomers, randomly distributed along the polymer chain. We show that, in the presence of a small amount of a photoacid dispersed in the polymer films, these are converted into insoluble polymeric networks upon UV irradiation and heating. Absorption and emission spectra of the polymeric networks are similar to those of the starting polymers, showing that the cross-linking, due to the polymerisation of the oxetane groups, does not interfere. This ability to induce their insolubility upon UV irradiation is used to fabricate multi-layer light-emitting diodes. Using a transmission electron microscope grid as a shadow mask we show that micrometer-size patterns can be created.

This paper investigates the thermal stability of organic bulk heterojunction solar cells, with a special focus on the thermal ageing of both photovoltaic parameters and morphology of the active layer. The photovoltaic parameters of a set of bulk heterojunction solar cells were determined by IV-characterization and their bulk morphology was investigated with transmission electron microscopy (TEM). A link could be made between the degradation of the short circuit current under a thermal treatment and the corresponding change in bulk morphology. A possible improvement of the thermal stability of bulk heterojunction solar cells is presented through the use of a polymer with higher glass transition temperature.

The Erythemal Response Spectrum is a scientific expression that describes the sensitivity of the skin to the ultraviolet radiation. The skin sensitivity strongly depends on the UV wavelength: a long exposition to UV radiation causes erythema once a threshold dose has been exceeded. In the past years several devices have been developed in order to monitor the UV exposure, most of them are based on inorganic materials that are able to mimic the human skin behaviour under UV radiation. We present a new device based on liquid crystals technology. The sensor is based on a liquid crystalline mixture that absorbs photons at UV wavelength and emits them at a longer one. This system presents several innovative features: the absorption range of the mixture can be varied to be sensitive to different wavelengths, the luminescence intensity can be tuned, the system can be implemented on flexible devices.

We report the characterization of the interface formation of fullerene (C₆₀) on hafnium (Hf) using x-ray and ultraviolet photoelectron spectroscopy (XPS and UPS). The valence band, C 1s and Hf 4f spectra were measured during the deposition of C₆₀ on a clean Hf surface in a stepwise manner. After enough deposition of C₆₀ layers on Hf, XPS measurements indicate that there is no chemical reaction between C₆₀ on Hf, and band bending exists at the C₆₀/Hf interface. From UPS measurements, the energy level difference between Fermi level of Hf and the onset of the highest occupied molecular orbital of C₆₀ is 2.01 eV. The vacuum level of Hf was shifted toward low binding energy (0.95eV) as the C₆₀ deposition. These valence spectra results indicate that there exist an interface dipole and small electron injection barrier at the C₆₀/Hf interface. We provide the complete energy band diagram of the C₆₀/Hf interface and confirm that a small electron injection barrier can be achieved by inserting a low work function metal in a C₆₀ field-effect transistor.

We present the systematic study of charge carrier mobility in 4,4'-N,N'-dicarbazole-biphenyl (CBP) films doped with a triplet-emitter material tris(2-phenylpyridine) iridium (Ir(ppy)₃) - 1 wt% and 10 wt%. The hole mobility was investigated with the time-of-flight (TOF) technique, as a function of electric field in the range 10⁵ - 2 × 10⁶ V/cm. For a CBP film doped with 1% of Ir(ppy)3 the hole mobility was also measured as a function of temperature from 20° C to 70° C. The obtained hole mobilities in CBP film doped with 1% of Ir(ppy)₃ are in the range 5 × 10-10 - 3 × 10-7 cm²/Vs. The measured dependence of the hole mobility on temperature and electric field is explained in the framework of the Gaussian disorder model of Bassler. Calculated values for the effective energetic and the positional disorder are 162 meV and 4.2, respectively. The activation energy at zero field is 0.58 eV. The mobility extrapolated to zero field and infinite temperature amounts to 6 × 10-4 cm²/Vs. In CBP film doped with 10% of Ir(ppy)₃ the hole mobilities are in the range 2 × 10-8 - 3 × 10-7 cm²/Vs. The observed increase of the hole mobility in Ir(ppy)₃-doped CBP films with increasing doping concentration can be attributed to hopping transport of charge carries via dopant molecules.

Among the growth techniques employed for preparing the active layer of organic semiconductors, ultra-high vacuum ones have permitted to achieve the best results in terms of control of thickness of the layer, purity, and for the possibility to modulate its morphological and structural properties by employing different growth conditions. Organic molecular beam epitaxy (OMBE) represents a natural choice for the fabrication of high quality field effect transistors especially when small conjugated oligomers are used as the active material. In spite of the advantages of this technique, the intrinsic properties of molecular materials (high diffusivity, weak interactions with most substrates, strong tendency to desorb, etc.) hinder the possibility to achieve a stable layer-by-layer growth, which is on the basis of the technological development of inorganic devices. Here, we show how the problem can be addressed and solved: by using properly prepared organic single crystals as substrate, a stable layer-by-layer epitaxial growth of organic semiconductors with the OMBE technique is demonstrated, exploiting solely the weak van der Waals interactions acting between substrate and overlayer. With this method, organic heterostructures of oligothiophenes and oligocenes are shown to be easily grown with a control at the monolayer level, as demonstrated through the discussion of a detailed morphological and optical characterisation. These results represent the starting point for the development of a technology based on all-organic nanostructures.

Depositing a monolayer of molecules on top of the most important semiconductor material, i.e. silicon, in order to obtain hybrid heterostructures is the main purpose of this work. Aromatic and heteroaromatic molecules with a vinylic group reacting with hydrogenated silicon atoms on a Si(100) surface by a radical reaction promoted by heat are studied. The molecular grafting occurs through the vinylic function while preserving the original molecular structure and molecular properties. After depositing the organic materials samples are analysed with atomic force microscopy (AFM) in order to verify the coverage level, film thickness and topographic features.

Ultra high vacuum operated prototypical organic light emitting diodes have been investigated by using chemically sensitive x-ray photoelectron microscopy. The mechanism of dark spot formation and degradation of organic light emitting devices have been imaged and spectroscopically measured. The morphology and the chemical composition of the Al cathode reveal the formation of volcano like defects as a result of local micro-explosions. The chemical maps and micro-spot spectra identify a release of volatile In-, Sn- and C-containing species, including metallic In, supporting the evidence that the degradation process is driven by local decomposition of the ITO/organic interface.

One of the most studied systems in the field of organic photovoltaic devices are bulk heterojunction solar cells based on poly[2-methoxy-5-(3',7'-dimethyloctyloxy)-1,4-phenylenevinylene] (MDMO-PPV) as electron donor and methano[60] fullerene [6,6]-phenyl C61 butyric acid methyl ester (PCBM) as electron acceptor. A striking feature of these types of solar cells is that at the optimal device thickness of typically 100 nm only 60% of the incident light is absorbed. It is evident that the absorption can be enhanced by increasing the thickness of the active layer. However, in spite of an increased absorption the overall power conversion efficiency shows a negative trend when increasing the device thickness beyond 100nm. From device point of view the reduced performance with increasing thickness mainly originates from a decrease of the fill factor. We demonstrate that this fill factor decrease is a result of the formation of space-charges in thick devices and charge recombination.

The open-circuit voltage (Voc) of polymer/fullerene bulk heterojunction solar cells is investigated as a function of light intensity for different temperatures. The observed photogenerated current and V oc are at variance with classical p-n junctionbased models. The influence of light intensity and recombination strength on V oc is consistently explained by a model based on the notion that the quasi-Fermi levels are constant throughout the device, including both drift and diffusion of charge carriers. The light intensity dependence of the short-circuit current density (J sc) is also addressed. A typical feature of polymer/fullerene based solar cells is that Jsc does not scale exactly linearly with light intensity (I). Instead, a power law relationship is found given by Jsc∝ Iα, where α ranges from 0.9 to 1. In a number of reports this deviation from unity is attributed to the occurrence of bimolecular recombination. We demonstrate that the dependence of the photocurrent in bulk heterojunction solar cells is governed by the build-up of space charge in the device. The occurrence of space-charge stems from the difference in charge carrier mobility of electrons and holes. In blends of poly(3-hexylthiophene) and 6,6- phenyl C61-butyric acid methyl ester this mobility difference can be tuned in between one and three orders of magnitude, depending on the annealing conditions. This allows us to experimentally verify the relation between space charge build-up and intensity dependence of Jsc. Model calculations confirm that bimolecular recombination leads only to a typical loss of 1% of all free charge carriers at Jsc for these devices. Therefore, bimolecular recombination plays only a minor role as compared to the effect of space charge in the intensity dependence of J sc.

The charge transport in white light-emitting polymers was investigated using hole-only devices of a series of copolymers. The series consists of three spirobifluorene copolymers with a common backbone. The polymers contain an increasing number of dyes that are included into the backbone to achieve white light emission. Because the dyes have different band gaps than the backbone, it is expected that they will hamper the charge transport in the polymer since they will result in electron traps or hole traps, or both. In this experiment we demonstrate that the dyes in the polymers under investigation have no influence on the hole transport. We therefore conclude that the dyes have their HOMO levels aligned and are thus likely to function as electron traps.

Several polymer molecules have structures which are suitable for the non-linear optic applications. We report on the design and fabrication of a high performance electro-optic modulator made of polymer thin films. The polymer we study contains a chromophore based on Disperse Red One covalently grafted to a host-matrix. The polymer materials are deposited in thin layers on a glass substrate by chemical solution deposition, either by spin-coating or by dip-coating. The thickness of the films is ranging from a hundred nanometers to several micrometers. Initially, the polymer molecules are randomly oriented and the films are isotropic, hence no electro-optic effect can be observed. In order to break the symmetry and align the chromophores, the films are submitted to the so-called corona poling process. As a result, their structure become non-centrosymmetric and the second-order susceptibility is no longer zero. The corona poling method consists of applying a high electric field to the polymer by means of a needle electrode, placed above the polymer film which is posed on a grounded sample support electrode. Thermal regulation of the support electrode allows to control the temperature during the poling of the films. Once the poling process has been established, a chemical cross-linking function is thermally activated in order to fix the orientation of the chromophores in the polymer matrix. The orientation and its stability in time is evaluated with a Second Harmonic Generation measurement set-up using the Makers Fringes configuration. We studied the influence of the poling temperature, the distance between the corona needle electrode and the sample, the high voltage applied, and the duration of the poling process on the efficiency of chromophore orientation in order to optimize the poling procedure. Finally, aging of poled polymer samples has been investigated at elevated temperatures, confirming the stability of the cross-linking process.

A derivative of 2-methylindole, namely 3-[2-(4-nitrophenyl)ethenyl]-1-(2-ethylhexyl)-2-metylindole (NPEMI-E) has been synthesized. Materials obtained from this molecule have been studied as thin films between two ITO layers. The study revealed that NPEMI-E collects in itself both photoconductivity and NLO characteristics. Differential Scanning Calorimetry (DSC) measurements showed the formation of stable glass films characterized by a T_g temperature lower than room temperature. Blends with the photoconductive poly-N-vinyl-2,3-dimethylindole (PVDMI) were also studied, giving again stable glass films independently of the wt.% contents of NPEMI-E. Photorefractivity measurements on both pure and blended NPEMI-E allowed to measure a value of the optical gain Γ = 627 cm^-1 at an applied electric field E = 60 V/µm. This high value of Γ corresponds to a sharp maximum of the experimental trend of Γ as a function of the wt.% content of NPEMI-E. The corresponding content was wt.% = 91.5. The presence of this maximum induced us to make the hypothesis that, besides the well known reorientational contribution to the photorefractivity, a further mechanism (recently theoretically studied) is active in our blends. This mechanism arises in the interactions among the NLO polarized and polarizable moieties (cooperative effect). It can produce a rapid variation of some of the electrooptical parameters conditioning the extent of the photorefractivity. This can happen at a well defined mean intermolecular distance and hence at a well defined concentration of the NLO molecules.

The photophysical properties of DAB-dendrimers from 1^st to 4^th generation as well as Diaminohexane - all substituted with the in maximum achievable quantity of pheophorbide a (Pheo) molecules were studied in comparison with a novel hexapyropheophorbide a - fullerene hexaadduct (FHP6) and a fullerene [6:0]-hexaadduct which carries twelve pyropheophorbide a units (FHP12) using both steady-state and time-resolved spectroscopic methods. It was found that neighboring dye molecules covalently linked to one DAB- or fullerene moiety due to the length and high flexibility of carbon chains could stack with each other. This structural property is the reason for the possibility of formation different types of energy traps, which were resolved experimentally. The dipole-dipole resonance F&diaero;rster energy transfer between the dye molecules coupled to one complex caused a very fast and efficient delivery of the excitation to a trap. As result the fluorescence as well as the singlet oxygen quantum yields of the different complexes were reduced with increasing number of dye molecules per complex. Nevertheless in every case the singlet oxygen generation was less influenced then the fluorescence quantum yield, exposing the complex to a non-negligible amount of excited oxygen in the singlet state. While the fullerene complexes turned out to be stable under these conditions, the DAB-dendrimer-backbones were completely fragmented to small rudiments carrying just one or a small number of dye molecules.

Guest-host polymer system with potential use in electro-optic devices is discussed. The polymer host is a polyimide and the guest chromophore is Disperse Red 19 (DR 19). Decay times of poled order after corona poling procedure have been derived from the behaviour at elevated temperatures below glass transition temperature T_g using Arrhenius plots. An extrapolated lifetime greater than 30 years at room temperature has been obtained.

Second order non linear optical (NLO) properties of sPMMA based host-guest systems containing eight dimethylaminobenzylidene -1, 3 - indandione (DMABI) related chromophores have been investigated by means of quantum chemical calculations and SHG experimental characterization. Ab initio calculations with basis set 6-31G were used for molecular geometry determination as well as in the calculations of molecular hyperpolarizability by FF approach. Influence of the chromophore concentration on the host-guest film NLO performance was obtained by SHG Maker fringe experiments. The highest value of d⁵³²₃₃ =80 pm/V (frequency corrected value d⁰₃₃ =12 pm/V) at chromophore concentration 15 %wt have been recorded for tret-butyl substituted DMABI chromophore.

We report the characterization of novel oligothienoacenes with five and seven fused thiophene rings; materials with potential applications in organic electronics. The compounds were investigated, both in their neutral and doped forms, by UV-Vis-NIR and FT-Raman spectroscopies. Density functional theory (DFT) calculations have been also performed to assess information regarding the minimum-energy molecular structure, atomic charges distribution, topologies and energies of the frontier molecular orbitals (MOs) around the gap, vibrational normal modes related to the main Raman features, and vertical one-electron excitations giving rise to the main optical absorptions.

We demonstrate that the aggregation tendency of dye molecules in a host polymer can be significantly reduced by exploiting non-covalent interactions between the host polymer and guest dye molecules. Such interactions occur spontaneously with no need for chemical synthesis, and could thus be utilized to combine the ease of processing of traditional guest-host systems with the high dye concentrations achievable in covalently linked systems. We studied the aggregation properties of the common azo-dye Disperse Red 1 in polymer matrices with different functional groups. The dye aggregation tendency is substantially reduced in a polar polymer matrix and, most importantly, by forming a polyelectrolyte-dye complex, a dye monomer can be attached to approximately each polymer unit. This indicates that polymer-dye complexes provide a facile route for new type of optical materials, which could lead to applications in various fields of optics and photonics.

We report the synthesis and characterization of perfluorinated tetracene; a material with potential applications in organic electronics. The electrochemical behaviour of the compound is analyzed by differential pulse voltammetry, and compared with that of tetracene. The structure of perfluorotetracene is planar as observed for pentacene. We also report a comparative Raman spectroscopic study of tetracene and perfluorotetracene in relation to their π-conjugational properties. Density functional theory (DFT) calculations have been also performed, at the B3LYP/6-31G** level, to assess information regarding the topologies and energies of the frontier molecular orbitals (MOs) around the gap, and about the vibrational normal modes associated with the Raman features selectively enhanced by the π-conjugation.

The optically induced switching of organic materials with large dipole moments consisting of acceptor and donor groups bridged by a delocalized π-electron system is important for investigations of optoelectronic effects. To the contrary of azobenzene molecule, photoinduced change of dipole moment of indandione molecule does not need free volume in thin films. To modify the dipole moment and also to optimise properties of film formation, a number of novel indandione derivatives is investigated. Reversible switching of the surface potential of polymer films with incorporated or chemically attached indandione molecules is observed at irradiation in photoinduced intramolecular electron transfer wavelength.

In this work we report on hybrid organic/inorganic sol-gel based optical waveguides in slab configuration grown both on glass and silicon substrates by spin-coating process. The samples were obtained using Zr(IV)-propoxide as inorganic precursor and 3-glycidoxypropyltrimethoxisilane (GLYMO) as organic component in molar ratio of 3/1. Optical characterization of each layer has been accomplished performing refractive index measurements at different wavelengths in the visible and near infrared regions using an home-made experimental setup developed in our laboratory. Measurements have evidenced a difference between the in-plane and out-of-plane refractive index. We found that the amount of the optical anisotropy depends on the speed of spinning deposition and on the curing temperature. In particular the anisotropy increases with the increasing of speed deposition and with the increasing of curing temperature. Results were also confirmed by spectroscopic ellipsometric measurements. We attributed the optical anisotropy to a preferential orientation of the organic groups of the alkoxysilane precursor (GLYMO) during spin coating deposition.

In this work we report on the preliminary results of compatibility, fabrication and characterization of two planar polymeric multilayer structures for electro-optic applications. The structures are composed by three polymeric layers grown on glass substrate. Buffer and cladding layers were synthesized using polymethylmethacrylate (PMMA). The guiding layers were composed by Polysulfone (PSU) and by a mixture of PMMA and polycarbonate (PC) both doped with Disperse Red 1 (DR1) molecules as nonlinear active units. Linear and nonlinear optical characterization of the structures by refractive index, absorption, optical losses, Second Harmonic Generation (SHG) and time stability measurements have been carried out.

The concentration dependence of amplified spontaneous emission (ASE) in optically pumped polystyrene (PS) films containing a variable concentration (between 2.5 and 100 % by weight (wt)) of the luminescent and hole-transporting organic molecule N,N'-Bis(3-methylphenyl)-N,N'-diphenylbenzidine (TPD) is studied. It is observed that the photoluminiscence (PL) efficiency, the ASE threshold and the linewidth above threshold, decrease with concentration up to 20 wt % doped films and then keeps a constant value up to concentrations of 100 wt % (neat films). The position of ASE can be tuned between 413nm and 421 nm by changing the concentration of TPD. It was also possible to tune the ASE position (between 404 and 417 nm) trough control of the film thickness (between 100 and 200 nm). The observed shifts of the ASE position due to changes in concentration are determined by the PL efficiency (not by the waveguiding characteristics of the films). On the other hand, the observed shifts in the ASE position due to changes in film thickness depend on the shape of the PL spectrum and on cut-off thickness limitations. In this case, the ASE thresholds depend on the different confinement of the propagation modes due to thickness variations.

A hybrid organic-inorganic sol-gel material based on 3-(glycidoxypropyl)-trimethoxysilane (GPTMS), an organically modified alkoxide containing an epoxy ring in the organic functional group, and titanium isopropoxide Ti(OPrⁱ)₄ has been synthesised. The presence of Ti alkoxide is important in order to increase the refractive index and to improve the mechanical and physical properties of the film. A basic catalysis has been used to perform the hydrolysis/condensation reactions of the inorganic network and to preserve the presence of unreacted epoxy rings. The epoxy groups are thus available in the final solid state films for the UV-photopolymerization process. The photopolymerization allows one to increase the refractive index and to pattern the film directly. The modification of the hybrid sol-gel TiO₂ matrix microstructure during the thermal treatment and the interaction with the UV light have been investigated by FT-IR and UV-Vis spectroscopies, DTA and TGA thermal analysis. The sol-gel film thickness and roughness have been analyzed by a profilometer: the films possess high optical quality and well controlled thickness, ranging from 500 nm to 2 μm. The refractive index of sol-gel thin films was measured by m-line spectroscopy and ellipsometry. An increase of the refractive index in the range 0,02÷0,1 was achieved, depending on the UV irradiation dose.

Organic materials possessing quadratic non linear optical (NLO) properties are now part of the optical material panel available for applications. Up to now, the patterning of their optical properties for device fabrication is commonly achieved by the lithographic techniques derived from the microelectronic domain. We propose here an alternative method that allows a direct patterning of the NLO properties. The objective is to manufacture quasi phase matched devices for optical frequency conversion in guided mode and in the regime of free propagation.

Polymers doped with non-linear optical (NLO) molecules are key materials in the elaboration of organic NLO devices. In this field, there is an ongoing need for chromophores with large dipole moments and optical non linearities. Here, we consider pyridinium phenoxides, a type of zwitterionic biphenyl-like molecule. A combination of mathematical modelling and some preliminary experimental measurements indicate that the NLO properties of these molecules depend on the twist angle existing between the two aromatic rings. In order to corroborate this structure/activity relationship, different sterically hindered pyridinium phenoxides were synthesized using the Suzuki coupling reaction involving a boronic ester and an aryl halide. We analyze the solvatochromism of the substituted zwitterions in details, determine the chemical equilibrium of protonation and perfom nonlinear optical measurements which are interpreted with the help of semi-empirical calculations.

The complex dielectric tensor of quaterthiophene crystals is reported as deduced from ellipsometry measurements on single crystals and compared with that calculated on the basis of the microscopic theory. The internal reflectivity at the interface between the crystal and air has been derived, enabling discussion of the effect of self-waveguiding of the emission on the basis of the components of the material dielectric tensor.

Light scattering processes in unconfined nematic liquid crystals have been investigated by means of time-resolved transmittance and static light scattering techniques. In time-resolved experiments, sample temperature has been modulated and different modalities of propagation have been probed. The results here obtained show that a nematic polydomain liquid crystal exhibits scattering properties strongly different from a nematic monodomain and, in a sense, it behaves as an optically thin sample. Such a behaviour may be ascribed to the strong backscattering exhibited by the polydomain sample, as indicated by static light scattering experiments, where the angular dependence of the scattered intensity was measured, revealing a preferential backward direction. The two experimental techniques here adopted have shown their complementary role to understand the actual scattering regime. The diffusion constant D has been determined by means of time-resolved measurements and a comparison with the results obtained in monodomain nematic liquid crystals has been attempted.

Five new elecdtroluminescent compounds based ont tris(8-quinolinolate)Al(III) and lithium quinolate with thiophene substituents at C-2 and C-5 position were synthesized and characterized. Lithium complexes showed blue emission, whereas aluminum complexes gave green fluorescence in solution. Double-layer devices were fabricated and found to emit light with brightness in excess of 3000 cd/m² with an efficiency of 1.36 cd/A. These preliminarily electroluminescent results demonstrate that the novel oligothiophene substituted 8-hydroxyquinolines are promising emissive and electron transport materials for organic electroluminescent devices.

The goal of the present study is to underline the relationships that may exist between the molecular arrangement in thin films and the OLED efficiency. For this purpose, we have synthesized three new silole derivatives with incrementally flexible structure to tune their packing ability. Moreover, these siloles were especially designed to possess both electron- and hole-transporting properties in order to be used in single-layer OLEDs. This architecture was chosen since the absence of organic layers interfaces allows a better evaluation of the role of the molecular arrangement in the active layer. The examination of the EL properties gives evidences of the prominent role of the molecular organization on the OLED efficiency. A crystalline-like organization of the molecules allows high current density but low luminance efficiency since an excessive electron current flow is involved compared to the hole one, and the recombination rate is poor. On the contrary, disordered assemblies of molecules allows better performances by avoiding unfavorable π-stacking, while keeping good intermolecular orbital overlaps to support charge carrier transport. Thus single-layer devices based on silole 9 exhibit luminance efficiency as high as 0.75 Cd/A associated with both good chemical and film stabilities that are very promising applications for long life OLED application.

The nonlinear optical performance of several Fluorene-based molecules was studied using different measurement methods and pulse durations. We used picosecond pulses at 532 nm and, femtosecond pulses tunable from 532-810 nm for performing open and closed aperture Z-scans, and we used femtosecond 570-930 nm pulses for two-photon induced fluorescence (2PF) spectroscopy. The observed nonlinear losses were compared using the three methods. The results exhibit much stronger nonlinear absorption with picosecond pulses due to the additional excited-state absorption processes involved. Also the nonlinear refractive index was found to be higher for the picosecond measurements. In addition using a femtosecond white-light continuum (WLC) pump-probe method we showed that by proper tuning of pump and probe wavelengths an intermediate state resonance enhancement (ISRE) of the 2PA can be observed yielding the same overall nonlinear absorption observed using picosecond pulses.