In a polymer light-emitting diode, the fraction of excitons formed as singlets is of crucial importance in determining the quantum efficiency. We have shown that it is possible to measure excited state absorptions due to triplet excitons and polarons in working polymer LEDs, and we are able to quantify the triplet generation rate by measuring the strength of the triplet absorption. Here, we show that by careful study of singlet emission and triplet absorption in an LED based on a poly(p-phenylenevinylene) derivative we can obtain an accurate value of 83±7% for the singlet formation probability, significantly higher than the value of 25% predicted by simple spin statistics. We extend these measurements to devices based on poly(dioctyl-fluorene), where we find similarly high values for the singlet formation probability. In devices using the polyfluorene copolymer F8BT, we find that the triplet absorption is extremely small, consistent with even higher singlet formation probabilities.

In this work the origin of the open-circuit voltage is studied in bi-layer devices based on polyfluorenes. The devices were fabricated using a lamination technique, which avoids the problems that usually arise when making well-defined bi-layers from soluble polymers. Often the open-circuit voltage in devices is attributed to the difference in the workfunctions of the electrodes. Whilst a cathode dependence of the open-circuit voltage was observed here, an additional constant voltage of ∼1V was also present in the devices investigated. We conclude that this additional open-circuit voltage is largely a result of a photo-induced dipole at the donoracceptor interface. This dipole is formed by charges that are separated as a result of exciton dissociation when the device is illuminated. Devices where the order of the polymer layers was reversed showed very different open-circuit voltages which confirms that the open-circuit voltage is heavily dependent on the structure of the active layer in the device. The effects of device thickness and excitation intensity on open-circuit voltage are also discussed.

We have investigated the field-effect mobility of three kinds of low molecular weight spirolinked compounds, namely 2,2',7,7'-tetrakis (diphenylamino)-9,9'-spirobifluorene (spiro-TAD), 2,2',7,7'-tetrakis(biphenyl-4-yl)-9,9'-spirobifluorene (spiro-6φ) and 2,7-bis-(N,N-diphenylamino)- 2',7'-bis-(biphenyl-4-yl)-9,9'-spirobifluorene (spiro-X2). The field-effect mobilities of these materials in the saturation region are 8 x 10-4 cm2V-1s-1, 5 x 10-5 cm2V-1s-1 and 4 x 10-4 cm2V-1s-1 respectively. The atomic force microscopy images show that films prepared from these materials are amorphous with a very smooth surface and the limited field-effect mobility is due to the intrinsic behaviour of amorphous materials.

We report on highly efficient molecular organic light-emitting diodes (MOLEDs) using two novel silole derivatives as emissive and electron transport materials. A silole derivative, namely 2,5-di-(3-biphenyl)-1,1-dimethyl-3,4-diphenylsilacyclopentadiene (PPSPP), which shows blue fluorescence with a high photoluminescence quantum yield of 85% in the solid state, was used as the emissive material. Another silole derivative, namely 2,5-bis-(2‘2“-bipyridin-6-yl)-1,1- dimethyl-3,4-diphenylsilacyclopentadiene (PyPySPyPy), that exhibits a non-dispersive high electron mobility of 2x10-4 cm2/Vsec was used as the electron transport material. MOLEDs using these two siloles and a common hole transport material show blue-green emission centered at 495 nm. This red-shifted electroluminescence (EL) band relative to the blue fluorescence of PPSPP is assigned to a PPSPP:NPB exciplex. A low operating voltage of 4.5 V was measured at a luminance of 100 cd/m2 and an EL quantum efficiency of 3.4% was achieved at 100 A/m2. To our knowledge, this is the highest EL quantum efficiency ever reported based on exciplex emission.

Hydroxypropylcellulose (HPC) systems have been actively investigated as flexible transparent substrates for use in LCD technology. HPC solid films (20-60 μm) obtained from aqueous liquid crystalline solutions present tunable topography. The need for transparent contacts in working electronic devices leads to the study of the properties of Indium Tin Oxide (ITO) deposited on these flexible substrates. Substrates obtained from a 30% w/w solution of HPC in water (HPC30) were found to have an isotropic topography with a roughness (Ra ) of 18.4 nm as measured by atomic force microscopy (AFM). Substrates with an anisotropic topography were obtained from a 65% w/w HPC solution (HPC65). AFM measurements show a Ra = 29.1nm. Two types of peridiocities can be observed in HPC65 substrates. A primary set of large bands, perpendicular to the shear direction is characterized by a periodicity of 1.8-2.1 μm and an average peak-to-valley height of 74-87 nm. This first set of bands lies under a secondary set of smoother band texture with a periodicity of 0.35 – 0.40 μm and an average peak-to-valley height of 2.9-4.4 nm. Transport properties of ITO deposited on HPC30 substrates (ITO+HPC30) show a resistivity around 3x10-2 Ω.cm independent of orientation. ITO films deposited onto HPC65 substrates (ITO+HPC65) are conductive and show a resistivity which depends strongly on current direction about two orders of magnitude. The resistivity measured along the shearing direction is 3.8x10-2 Ω.cm and along a perpendicular direction is 2.7 Ω.cm. The anisotropic resistivity of ITO+HPC65 cannot be a direct consequence of the current path difference between both directions due to surface topography. The optical microscope observation of the ITO+HPC65 surface reveals the presence of cracks in the ITO film along the shear direction, which could be induced by the anisotropic mechanical properties of HPC65 substrates. The resistivity difference in ITO+HPC65 could be attributed to these cracks.

A new series of symmetric two-photon absorbing chromophores with various л-bridges were synthesized. It has been shown that the spectra of two-photon absorption and two-photon induced fluorescence can be fined tuned by structural variations in the electronic nature and twist angle of л-bridges of D-л-D type chromophores. These chromophores exhibited large twophoton absorption cross-sections (up to 1800 GM), which were determined by two-photon induced fluorescence technique using femtosecond pulse excitation. The colors of two-photon pumped emission of these chromophores varied from blue to red (433 ∼ 604 nm) as the structure of л-bridges changed.

In this paper, a clear view on the bulk microstructure of MDMO-PPV:PCBM blends as used in bulk hetero-junction organic solar cells is obtained by means of TEM (Transmission Electron Microscopy). Using TEM, 3-dimensional information is acquired on phase separated regions, formed during casting. Particle statistics illustrate quantitatively that a.o. drying conditions and choice of solvent dramatically influence the blend structure. More information about the lateral blend structure and distribution is obtained in cross-sectional view. Since blend morphology is strongly related to photovoltaic performance, TEM can be a powerful tool for understanding today's photovoltaic performances and screening new sets of materials.

One promising method to pattern full color polymer Organic Light-Emitting Diode (OLED) displays is to print dye from a pre-patterned organic film onto a spin-cast polymer and then diffuse the dye into the film at room temperature in a solvent vapor environment. This method utilizes the well-known tendency for a polymer film to absorb solvent vapor, which depresses the glass transition temperature of the polymer and dramatically increases diffusion the dye. In this work, we have studied the temperature dependence of this process. The dye coumarin 6 (C6) was transferred onto films consisting of 2-(4-biphenylyl)-5-(4-tert-butylphenyl)- 1,3,4-oxadiazole (PBD) mixed with the polymer poly(9-vinylcarbazole) (PVK). Samples were then placed on a heated stage in a chamber and exposed to acetone vapor to diffuse the C6 into the polymer film. The profile of the diffused dye was determined by depthdependent photoluminescence measurements and Secondary Ion Mass Spectroscopy. We observed that the amount of diffused dye decreased at higher temperatures, in contrast to conventional thermally-driven diffusion. The results are understood by noting that the decrease in the polymer glass-transition temperature and the corresponding rapid increase in dye diffusivity depend on the quantity of solvent absorbed by the polymer, which decreases as the temperature of the polymer is raised.

We report on directed self-assembly in supramolecules containing rod-like polymers. The comb-shaped supramolecules consist of poly(2,5-pyridinediyl), camphorsulfonic acid, and hydrogen bonded amphiphilic side chains. Locally they form highly ordered lamellar structures in which the polymers are stacked. The liquid crystalline state allows facile overall alignment yielding absorption dichroism and polarized photoluminescence. After alignment, cleavage of the amphiphiles results in solid films which retain this optical anisotropy with the high photoluminescence quantum yield.

We present a study of aqueous and plasma anodised aluminium oxide (Al2O3) and its performance in thin film transistors (TFTs). The current through the oxide was measured with aluminium electrodes and with one of the electrode replaced by poly(3-hexylthiophene)(P3HT). The current increased by up to 2 orders of magnitude with P3HT. The current increased further when the polymer was doped with different percentages of 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ). It was also found to be dependent on the thickness of the polymer film. Surprisingly, the oxide current fell to its initial value when the polymer film was removed. Two mechanisms may explain the behaviour in these devices: charge injection and/or displacement. C-V plots were obtained from the MOS capacitors and were frequency dependent. They also showed substantial hysteresis, with a lateral shift along the voltage axis. This indicates the presence of a mobile species that increases with the concentration of dopant. We deduce that much of the increased gate current is associated with displacement currents induced by ion motion.

We investigate the relationships among all currently known X-ray structures of crystalline pentacene by calculating their “inherent” structures of minimum potential energy. We are thus able to show that two distinct bulk crystalline phases of pentacene exist, with very subtle but clear di.erences. We then assess the effects of temperature on the crystal structures, by including both inter- molecular and low-frequency intra-molecular phonons in the framework of quasi harmonic lattice dynamics methods. In this way we properly reproduce the experimental thermal expansion, and obtain a reliable description of the phonon dynamics and of its temperature dependence. The calculated phonon frequencies compare well with the experimental Raman spectrum.

The performance of thermally evaporated pentacene TFTs is strongly affected by defects. The influence of the preparation conditions on the level of defects were studied by Photoconductivity (CPM) and Photothermal Deflection Spectroscopy (PDS). CPM and PDS are powerful tools to provide insights in the band structure, the sub band gap absorption, distribution of defects and the ordering of the material. The influence of the preparation conditions on the morphology and the structural properties will be discussed and correlated with experimental data of CPM and PDS. In particularly, the influence of the deposition temperature on the absorption spectra of the polycrystalline films will be addressed. Furthermore, the material analyse will be correlated with the device performance of pentacene TFTs.

In this work, thin films (thickness ∼ 0.5 μm) were obtained by plasma polymerization of pyrrole (Ppy) and thiophene (Pth) at 25-30 W and 0.1-0.2 mbar of pressure. Further doping with iodine was carried out to some of the Ppy and Pth films (Ppy/I2, Pth/I2) in order to enhance their electrical conductivity properties.

Structural and morphological characterization of both Ppy and Pth as well as of Ppy/I2 and Pth/I2 was performed using Infrared Spectroscopy (IR), X-ray Photoelectron Spectroscopy (XPS) and Atomic Force Microscopy (AFM).

In the light of the information given by IR, XPS and AFM techniques, exhaustive and accurate description of both undoped and I2/doped Ppy and Pth films obtained by Plasma Polymerization is attained.

The electro-optical characteristics of a polymer light emitting diode (PLED) with a strongly reduced hole injection have been investigated. The device consists of a poly-p-phenylene vinylene semiconductor with a Ag hole injecting contact, which has an injection barrier of about 1 eV. It is observed that the light and current density of such an injection-limited PLED strongly exceed the expected device characteristics. Numerical calculations of the injection-limited PLED show that the enhanced performance can be explained by a very high electric field at the hole injecting contact, due to trapped electrons.

The influence of different dielectrics on the structural and electronic properties of pentacene films and TFTs is discussed. The pentacene films were thermally evaporated on inorganic dielectrics compatible with flexible substrates. A strong correlation between morphology and structural properties of the pentacene films and the mobility of the TFTs was observed for all the dielectrics studied. In the case of plasma enhanced chemical vapor deposited (PECVD) silicon nitride and silicon oxide dielectrics the growth of pentacene is mainly determined by the roughness of the dielectric. The roughness inhibits the ordering of pentacene molecules on the surface. However, by optimizing the fabrication process of the dielectrics, we have achieved similar pentacene mobilities on PECVD dielectrics and thermal oxide (0.4 cm2/Vs), without employing self-assembled monolayers like octadecyltrichlorosilane (OTS). An OTS treatment of oxide based dielectrics leads to an increase of the mobility by a factor of 2-3 up to >1cm2/Vs for thermal oxide. Pentacene films on inorganic dielectrics exhibit mobilities from of 0.2-1.2 cm2/Vs and high on/off ratios between 107 and 108.

Organic field-effect transistors (OFETs) are “soft material” versions of accumulationmode silicon-based FETs, where a gate field across a dielectric induces a conductive charge channel at the interface of the dielectric with a semiconductor, between source and drain electrodes. Charge carrier mobilities >0.01 and on/off ratios >10,000 are routinely obtained, adequate for a few specialized applications such as electrophoretic pixel switches but well below standards established for silicon microprocessor technology. Still, progress that has been made in solution-phase semiconductor deposition and the printing of contacts and dielectrics stimulates the development of OFET circuits for situations where extreme low cost, large area, and mechanical flexibility are important. Circuits with hundreds of OFETs have been demonstrated and a prototype OFETcontrolled black-on-white “electronic ink” sign has been fabricated.

We have prepared and characterized thin film field effect transistors (FETs) of bis-(1, 2, 5-thiadiazolo)-p-quinobis(1, 3-dithiole) (BTQBT) and its derivatives. Preparation and characterization of the films were carried out under ultrahigh vacuum condition. Most materials examined showed p-type semiconducting behaviors. Among p-type molecules, BTQBT films deposited at room temperature showed the highest mobility and on/off ratio of 0.2 cm2/Vs and 108, respectively, at optimal film growth conditions. These performances are almost comparable to those of pentacene and polythiophene thin films, indicating that BTQBT molecule is a prominent semiconducting material for high-speed organic transistors. It was also found that a tetracyanoquinodimethane (TCNQ) derivative showed an n-type semiconducting behavior with an electron mobility of 8.9 x 10-4 cm2/Vs.

Novel fluorene-based conjugated light-emitting polymers have been designed and synthesized. By varying the compositions of the polymer backbone, the charge-injecting and -transporting properties of the polymers were significantly improved. The light-emitting diodes (LEDs) using these polymers as the emissive layers exhibited low turn-on voltage, a high external quantum efficiency, and high brightness due to balanced electron and hole conductivity.

The transport properties of poly(2,5-thienylene vinylene) (PTV) field-effect transistors (FET) have been investigated as a function of temperature under controlled atmosphere. In a disordered semiconductor as PTV the charge carrier mobility, dominated by hopping between localized states, is dependent on the charge carrier density. The transfer characteristics of PTV FET have been modeled considering the distribution of charge carriers and mobility over the accumulation channel. Good agreement with the experimental data is obtained.

This work describes the properties of an organic-inorganic hybrid gel with an incorporated I-/I3-redox couple that serves as a solid electrolyte for a dye-sensitized TiO2 photoelectrochemical (DSPEC) cell with an efficiency of 4% [1]. The influence of the AcOH on the hydrolysis and condensation reactions of the gels was studied with the help of infrared spectroscopy, while the presence and the formation of the I3 - species were established from the UV-visible spectra of the gels and independently evaluated from the confocal micro-resonance Raman spectra of the electrolyte encapsulated in the DSPEC during open- and short-circuit conditions. Finally, the same gel electrolyte was used for the construction of an electrochromic (EC) window without a counter electrode with intercalation properties (transmission and reflective configuration).