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The optical properties of 1,2;8,9-dibenzopentacene molecules in different environments, namely solutions, solid solutions, and thin films, focusing on the shift of the electronic levels as a function of the dielectric properties of the surrounding media are investigated. In all cases, we find that the optical gap of 1,2;8,9-dibenzopentacene is somewhat larger than that of pentacene (by ~0.21 eV in solution) in spite of the larger extent of the π-electron system in the former molecule. This a priori unexpected finding is rationalized on the basis of quantum-mechanical simulations.

Blue emitting GaN-based light emitting diodes (LEDs) show a distinct spectral behavior with respect to temperature and injection current density. Operating LEDs with short current pulses of 500 ns provides a steady state situation, which allows investigating the emission behavior of LEDs at a certain device temperature thereby maintaining thermal equilibrium. The LEDs were examined in a temperature range between 4.2 and 400 K and in a current density range between 2 and 50 A cm−2. Low temperature investigations showed a blue shift of the electroluminescence spectra (EL) with respect to junction temperature, which is assigned to the radiative recombination of localized excitons. In the elevated temperature region a distinct red shift due to energy gap shrinkage was observed. Further, we expect an exciton lifetime reduction at 4.2 K. Additionally, the influence of the driving parameters (pulse injection current or direct current (DC)) in the presence of piezoelectric fields is discussed and separated into band filling effect and occurrence of fields, which screen the quantum confined Stark effect. Low temperature investigations indicate that band filling is mainly responsible for the blue shift of the EL spectra with respect to the injection current.

The evaporation of quinacridone from a stainless steel Knudsen cell leads to the partial decomposition of this molecule in the cell, due to its comparably high sublimation temperature. At least one additional type of molecules, namely indigo, could be detected in the effusion flux. Thermal desorption spectroscopy and atomic force microscopy have been used to study the co-deposition of these molecules on sputter-cleaned and carbon-covered silicon dioxide surfaces. Desorption of indigo appears at temperatures of about 400 K, while quinacridone desorbs at around 510 K. For quinacridone, a desorption energy of 2.1 eV and a frequency factor for desorption of 1 × 10(19) s(-1) were calculated, which in this magnitude is typical for large organic molecules. A fraction of the adsorbed quinacridone molecules (∼5%) decomposes during heating, nearly independent of the adsorbed amount, resulting in a surface composed of small carbon islands. The sticking coefficients of indigo and quinacridone were found to be close to unity on a carbon covered SiO2 surface but significantly smaller on a sputter-cleaned substrate. The reason for the latter can be attributed to insufficient energy dissipation for unfavorably oriented impinging molecules. However, due to adsorption via a hot-precursor state, the sticking probability is increased on the surface covered with carbon islands, which act as accommodation centers.

In this study, we report on an innovative residue-free nanoimprint lithography process for the patterning of submicron-spaced contacts, used as source and drain electrodes in downscaled organic thin-film transistors. The method is based on thermally initiated radical polymerization of a novel imprint resist whose outstanding chemical and physical properties are responsible for the excellent results in processability and structure transfer. In combination with a pretreated stamp the thermally curable resist enables residue-free imprinting, thus making etching obsolete. In addition, this process implies only moderate temperature budgets and it is eco-friendly due to a water-based lift-off. To validate the process, source and drain patterned submicron organic thin-film transistors with pentacene as the semiconductor were fabricated, which show excellent transistor behavior parameterized by a low switch-on voltage of Vso = 3 V, an on–off ratio of more than 10 5

In this study we report on an innovative nanoimprint process for the fabrication of entirely patterned submicron OTFTs in a bottom-gate configuration. The method is based on UV-Nanoimprint Lithography (UV-NIL) combined with a novel imprint resist whose outstanding chemical and physical properties are responsible for the excellent results in structure transfer. In combination with a pretreated stamp the UV-curable resist enables residue-free imprinting thus making etching obsolete. A subsequent lift-off can be done with water. The UV-NIL process implies no extra temperature budget, is time saving due to short curing times, eco-friendly due to a water-based lift-off, simple because it is etch-free and completely r2r compatible. It works perfectly even if ultra-thin organic and hybrid films are used as gate dielectrics. On this basis entirely patterned functional submicron OTFTs with pentacene as the semiconductor are fabricated showing clear saturation, low switch-on voltage (∼3 V) and a sufficiently high on–off ratio (10 3 ).

We present a simulation procedure based on optical ray-tracing in order to optimize the angular homogeneity of the light emitted from the color conversion element (CCE) in a phosphor conversion-based white LED. The blue LED and the yellow CCE light have rather different emission characteristics; so the geometry of the CCE as well as its phosphor concentration have to be carefully adjusted in order to achieve equal irradiance and/or radiant intensity distributions on a photo-detector surrounding the LED. The simulations identify the optimal CCE geometries and material compositions in order to obtain angular homogeneity for a broad range of color temperatures.

We report on the micro-fabrication of diffractive optical elements (DOEs) such as 1D, 2D and concentric grating structures inside the volume of thin silicone films by femtosecond laser direct writing. In addition, we show that such structures can also be integrated into silicone films that act as encapsulation layers of high power light-emitting diodes. The latter strategy opens new possibilities to homogenize and to control the light emitted from such devices.

We report on an experimental set-up based on a confocal principle in order to acquire the light-intensity distribution (XZ and XY optical sections) of high-power LEDs. To be able to record the emission characteristics of millimeter-sized LEDs and to carry out the measurements with high precision the set-up consists of a moving stage and stationary rather than scanning optics, along with a lock-in amplifier in combination with a photodiode as a detection unit. The optical sections recorded provide valuable information on the light-intensity distribution and the light propagation both within transparent substrates (in case of flip-chip LEDs) as well as in the ambient of the LEDs. In order to evaluate the accuracy of the measurement technique, the impact of the numerical aperture of the objective lens on the shape of the optical sections recorded was tested for a set of different objective lenses. The method reported provides new opportunities for a direct determination not only of the amount but also the directionality of the light extraction from LEDs that are processed in order to improve the light-extraction efficiency.

Here we report on the fabrication and characterization of ultra-thin nanocomposite layers used as gate dielectric in low-voltage and high-performance flexible organic thin film transistors (oTFTs). Reactive sputtered zirconia layers were deposited with low thermal exposure of the substrate and the resulting porous oxide films with high leakage currents were spin-coated with an additional layer of poly-alpha-methylstyrene (P alphaMS). After this treatment a strong improvement of the oTFT performance could be observed; leakage currents could be eliminated almost completely. In ellipsometric studies a higher refractive index of the ZrO(2)/P alphaMS layers compared to the "as sputtered" zirconia films could be detected without a significant enhancement of the film thickness. Atomic force microscopy (AFM) measurements of the surface topography clearly showed a surface smoothing after the P alphaMS coating. Further studies with X-ray photoelectron spectroscopy (XPS) and transmission electron microscopy (TEM) also indicated that the polymer definitely did not form an extra layer. The polymer chains rather (self-)assemble in the nano-scaled interspaces of the porous oxide film giving an oxide-polymer "nanocomposite" with a high oxide filling grade resulting in high dielectric constants larger than 15. The dielectric strength of more than 1 MV cm(-1) is in good accordance with the polymer-filled interspaces.

In this study we report on the optimization of the contact resistance by surface treatment in short-channel bottom-contact OTFTs based on pentacene as semiconductor and SiO 2 as gate dielectric. The devices have been fabricated by means of nanoimprint lithography with channel lengths in the range of 0.3 μm < L < 3.0 μm. In order to reduce the contact resistance the Au source- and drain-contacts were subjected to a special UV/ozone treatment, which induced the formation of a thin AuO x layer. It turned out, that the treatment is very effective (i) in decreasing the hole-injection barrier between Au and pentacene and (ii) in improving the morphology of pentacene on top of the Au contacts and thus reducing the access resistance of carriers to the channel. Contact resistance values as low as 80 Ω cm were achieved for gate voltages well above the threshold. In devices with untreated contacts, the charge carrier mobility shows a power-law dependence on the channel length, which is closely related to the contact resistance and to the grain-size of the pentacene crystallites. Devices with UV/ozone treated contacts of very low resistance, however, exhibit a charge carrier mobility in the range of 0.3 cm 2 V -1 s -1 < μ

The performance of organic thin-film transistors (OTFT) for flexible, low cost and disposable “plastic” electronic products advances rapidly: various organic semiconductors display hole or electron carrier mobilities that compare favorably with those of hydrogenated amorphous silicon, the inorganic counterpart for such applications as flexible displays, smart cards and radio frequency identification tags, nonvolatile memories and sensors. The possibility for tailoring functional organic materials, bears potential towards novel electronic products such as smart skins, smart textiles and “invisible electronics”, where multiple functionalities, portability and ubiquitous integration is requested. In this context diverse properties of organic thin-film devices are inevitable such as lightweight, low power consumption, low operationvoltage and compatibility with diverse substrates. Reducing the threshold voltage and the subthreshold swing is essential for operating OTFTs at low-voltage levels. When combined with very low gate leakage currents, OTFTs may also become a key element in high-end sensor applications, such as flexible touch pads and screens or thermal imaging tools for night vision, surveillance or for the detection of undesired heat loss paths in buildings. The aforementioned transistor parameters not only critically depend on the thickness and the dielectric properties of the gate insulator, but also on the trapped charge densities at the interface between these materials. The selection of semiconductors and gate insulators with excellent interface properties is currently the challenge in the quest for improving the performance of OTFTs. Here we show that bottom-gate OTFTs based on the organic semiconductor pentacene and high-k nanocomposite gate dielectrics, exhibit transistor performances with very low gate leakage currents, subthreshold swings close to the theoretical limit, and low-voltage battery operation. The subthreshold swings of OTFTs with different organic and hybrid gate dielectrics follow an inverse dependence on the gate capacitance as is expected by standard MOS theory. The trapped charge carrier density at the interface between the semiconductor and the dielectric surpasses that of the SiO2-pentacene interface, being close to the average trap densities in the SiO2–Si interface in metal oxide semiconductor transistors. [15] We also report the first application of these OTFTs in an optothermal light sensor. We describe the transistor, the temperature sensitive fluorinated polymer, their combination in an integrated circuit, and the application of this circuit as a thermal infrared sensor and as a switch that can be operated by a laser pointer. Figure 1 shows the structure of low-voltage organic transistors with high dielectric constant (high-k) oxide–polymer nanocomposites. Al2O3 or ZrO2 were chosen as high-k dielectric materials, combined with poly(a-methyl styrene) (PaMS) or poly(vinyl cinnamate) (PVCi) to form a smooth and dense nanocomposite gate dielectric. Pentacene is used as the organic semiconductor material, the gate electrode is based on Al, while Au source and drain electrodes are employed. Figure 1b, c, and d show atomic force microscopy (AFM) images of a ZrO2/PaMS nanocomposite gate dielectric based transistor. The bare ZrO2 metal oxide surface is displayed in Figure 1b, the nanocomposite in (c) and the pentacene layer grown on top of the nanocomposite dielectric in (d). The AFM images clearly reveal that the rough (surface rms-roughness = 1.5 nm) and less dense ZrO2 layer, which is composed of regularly clubbed grains (Fig. 1a), smoothens by forming the nanocomposite (rms-roughness = 0.4 nm). The substrate roughness critically influences the growth dynamics of pentacene molecules on top of dielectric surfaces, grain sizes typically increase with decreasing surface roughness. For rmsroughness values below 0.5 nm the pentacene morphology is characterized by dentritic crystallites of several microns C O M M U N IC A IO N

Based on comprehensive studies by scanning electron microscopy (SEM), atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS) we report on the morphology and the ion–ion as well as the ion–polymer interaction properties of thin spin-cast films comprising an oligo(ethylene oxide) (OEO) grafted poly(p-phenylene) blended with either lithium- or tetrabutylammonium trifluoromethanesulfonate. Assuming that the ionic species intercalate within the OEO side-chain matrix, the covalent attachment of OEO appears to be an appropriate strategy to overcome the generally observed phenomenon of large-scale phase-separations between conjugated polymers and solid-state electrolytes. XPS turns out to be a suitable approach to study the basics of the ion–ion and the ion–polymer interactions of these model systems directly for the thin, spin-cast films.

A sol-gel-based fabrication route for obtaining ferroelectric P(VDF-TrFE) thin films via a spin-coating technique at low temperatures was developed. The ferroelectric and pyroelectric properties of IR-sensors based on these layers were determined by hysteresis loop measurements, by dielectric characterization and by the laser intensity modulation method. On-chip stepwise poling at room temperature turned out to be very effective in increasing the pyroelectric current response. Finally, the sensors were directly integrated with organic thin film transistors to guarantee a low impedance read-out of the response.

Thin films of quinacridone deposited by physical vapor deposition on silicon dioxide were investigated by thermal desorption spectroscopy (TDS), mass spectrometry (MS), atomic force microscopy (AFM), specular and grazing incidence X-ray diffraction (XRD, GIXD), and Raman spectroscopy. Using a stainless steel Knudsen cell did not allow the preparation of a pure quinacridone film. TDS and MS unambiguously showed that in addition to quinacridone, desorbing at about 500 K (γ-peak), significant amounts of indigo desorbed at about 420 K (β-peak). The existence of these two species on the surface was verified by XRD, GIXD, and Raman spectroscopy. The latter spectroscopies revealed that additional species are contained in the films, not detected by TDS. In the film mainly composed of indigo a species was identified which we tentatively attribute to carbazole. The film consisting of mainly quinacridone contained in addition p-sexiphenyl. The reason for the various decomposition species effusing from the metal Knudsen cell is the comparably high sublimation temperature of the hydrogen bonded quinacridone. With special experimental methods and by using glass Knudsen-type cells we were able to prepare films which exclusively consist of molecules either corresponding to the β-peak or the γ-peak. These findings are of relevance for choosing the proper deposition techniques in the preparation of quinacridone films in the context of organic electronic devices.

Polymer electrets from the polyvinylidene family are highly suitable materials for plastic electronics. The high dielectric constant of such polymers in both the ferro- and non-ferroelectric phase is most suitably employed in gate dielectrics in organic field-effect transistors. The pyro- and piezoelectric response makes ferroelectric polymer electrets also an ideal choice for organic temperature, infrared radiation and pressure sensors. Here we report steps towards transparent organic pyroelectric infrared sensors: The infrared sensor element is based on poly (vinylidene fluoride), the top contact pentacene transistor on poly(vinylidene-fluoride tetrafluoroethylene hexafluoropropylene) and poly(vinyl cinnamate) as gate dielectric. Steps for combining the ferroelectric polymer sensors and transistors to a fully organic temperature and infrared sensing element are outlined. The sensor concept may be easily scaled up to the production of sensor arrays for thermal imaging

Well-ordered thin films of p-oligophenylenes, p-quaterphenyl (4P), p-quinquephenyl (5P) and p-sexiphenyl (6P), were prepared on a Cu(1 0 0) single crystal substrate by vacuum vapor deposition. Their structures were investigated by infrared reflection absorption spectroscopy (IRRAS) and low energy electron diffraction (LEED). In the monolayers, p-oligophenylenes are physisorbed with their axes parallel to the substrate and form a regular two-dimensional arrangement. This structure is unaltered even in multilayers up to a thickness of a few nanometers. In the potassium (K)-doped 4P films, on the other hand, the arrangement is disrupted due to the perturbation by the dopant. The IRRAS investigation including observed and calculated spectra suggests that the radical monoanions are formed upon low-level doping, followed by saturation of the radical monoanion concentration and appearance of the dianions on increasing the amount of K.

We report on a strategy to reduce the size scale of the phase-separation in blends of conjugated polymers and solid state electrolytes. Blending a blue-green emitting conjugated polymer (methyl substituted ladder-type poly(p-phenylene), mLPPP) with a crown ether-based solid state electrolyte (Dicyclohexano18crown6 (DCH18C6), as an ion co-ordinating and transporting component and a Li salt, Lithium trifluoromethanesulfonate (Li triflate, LiTf for short) that provides the ionic species) evokes pronounced phase-separation. Endowing the backbone of mLPPP with convenient structural units largely reduces the size scale of this phase-separation. In particular we show that carbazole groups, which are generally applied in organic electronics to facilitate charge carrier transport, accessorily abate the tendency for phase-separation from a micro- to a sub-micro-scale. This attitude is investigated by UV/VIS/NIR absorption measurements, infrared spectroscopy, atomic force microscopy (AFM) and X-ray analysis by means of energy dispersive X-ray spectrometry (EDXS) in the scanning electron microscope (SEM).

We report on the opto-electronic characteristics (in particular with respect to the efficiency behaviour) of organic light-emitting diodes (oLEDS) and organic light-emitting electrochemical cells (oLECS), both prepared from a blue-green emitting conjugated polymer, (methyl substituted ladder type poly (p-phenylene), mLPPP), as electronically conductive component. Different from the oLEDs, the oLECs are characterized by a distinct decrease of efficiency upon bias voltage increase. From scan rate dependent studies we conclude that this efficiency drop is not only due to device decomposition upon operation above the window of electrochemical stability but also reflects (and thereby sheds some more light on) the intrinsic mode of oLEC operation. Due to the declining width of the intrinsic region upon ongoing pin-junction formation field induced and electrode induced luminance quenching processes also play a decisive role.

Silicon nitride (SiNx) thin films of various stoichiometries (x) were prepared on Si (100) substrates applying the Nd:YAG (λ=1064 nm) pulsed laser deposition (PLD) process from pure Si targets in the “shaded off-axis” technique at room temperature. The specific arrangement of this technique with perpendicular target and substrate surfaces and a metallic screen in between guarantees very low particulate (droplet) deposition and, thus, excellent surface qualities. The about 80- to 100-nm-thick silicon nitride films have very smooth surfaces (∼0.5–1.5 nm roughness) and dense structures. The N2 partial pressure strongly influences the nitrogen content and the silicon bonding structure of the films analyzed by means of secondary ion mass spectroscopy (SIMS) and X-ray photoelectron spectroscopy (XPS), resp. As a consequence, the optical properties examined by spectroscopic ellipsometry are tailorable in a wide spectral range between 250 and 1200 nm.

We report on the efficiency behavior of light-emitting electrochemical cells (LECs) fabricated from a methyl-substituted ladder-type poly(p-phenylene) (mLPPP) that was blended with a crown ether based solid state electrolyte. Unlike organic light-emitting diodes (oLEDs) utilizing mLPPP as an active layer, the LECs suffer from a loss of efficiency at elevated current densities. From scan rate dependent studies we deduce that this efficiency drop is not only due to device decomposition upon high voltage operation and we also reveal the intrinsic mode of LEC operation. The decreasing width of the intrinsic region between the p- and n-type doped zones upon ongoing pin-junction formation causes distinct (either field or electrode induced) luminance quenching effects.

Alkoxy substituted derivatives of poly- and oligo-[(m-phenylenevinylene)-alt-(p-phenylenevinylene)] were synthesized via the Wittig (P1, OPV1, OPV2) and the Wittig-Horner (P2, OPV3, OPV4) condensation routes. The polymers were characterized by 13 C NMR, 1 H NMR, FT-IR spectroscopy and GPC. 1 H NMR was a convenient tool to distinguish between the cis and trans double bonds in the compounds. Poly [(4-decycloxy-1,3-phenylenevinylene)-alt-(1,4-phenylene)] (P1) contained cis and trans double bonds in significant amounts, the vinylene configuration of poly [(4-decycloxy-1,3-phenylenevinylene)-alt-(2,5-dipentyloxy-1,4-phenylenevinylene)] (P2) was nearly exclusively trans. Model compounds (OPV1-4) were also synthesized to support the structural and optical characterization. UV-vis absorption, photoluminescence (PL) and cyclic voltammetry measurements have been performed to investigate the influence of the positions and the number of substituents on electronic levels. The polymers exhibited an intensive solid-state emission in the blue-green (P1) and the green (P2) region of the spectrum. Light emitting diodes have been fabricated consisting of ITO, PEDOT:PSS, P2 and Ca/A1. They exhibited high luminance of 100 cd.m -2 at 5.9 V and low onset voltages (4.3 V) for the electroluminescence (EL).

We report on surface composition studies of thin spin-cast films of a polymeric mixed ionic electronic conductor (PMIEC). The polymeric constituent consists of a hydrophobic conjugated poly(p-phenylene) (PPP) backbone functionalized with hydrophilic oligo(ethylene oxide) (OEO) side-chains to supply the electronically conducting conjugated polymer with an ion transporting functionality. In particular a lithium salt (lithium trifluoromethanesulfonate), the prototype salt applied in solid-state electrolytes, was chosen for the investigations. Identification of the elements and their quantification was done by XPS for different salt concentrations in order to address the still open question of surface-induced effects on the ion distribution in thin films of solid-state electrolytes. No enrichment or depletion of the ionic species at the free surface could be observed for the concentrations investigated within the error of the measurement given.

We report on the morphological aspects of thin films prepared from a blue–green light-emitting conjugated polymer, (methyl-substituted ladder-type poly(p-phenylene, mLPPP)), blended with a solid-state electrolyte composed either by a crown ether, dicyclohexano-18-crown-6 (DCH18C6), or a high-molecular-weight poly(ethylene oxide) (HMWPEO), and a Li salt, lithium trifluoromethanesulfonate (LiCF3SO3, Li triflate (LiTf)), as they have been successfully applied in light-emitting electrochemical cells (LECs). The surface morphologies of the blend layers were investigated using atomic force microscopy (AFM) in tapping mode, and the ion distribution was probed using X-ray analysis by means of energy-dispersive X-ray spectrometry (EDXS) in the scanning electron microscope (SEM). We show that the two different phase-separation processes, the complexation tendencies of the ionic species as well as the ionic transport numbers, have tremendous influence on the performances of the corresponding LECs, revealing either rectifying or symmetric optoelectronic characteristics in forward and reverse bias directions. This opens up new possibilities for tuning the optoelectronic properties of ion-supported organic electronic devices.

We report on investigations of the morphological aspects of blend layers prepared from a blue-green light-emitting conjugated polymer (methyl-substituted ladder-type poly(p-phenylene), mLPPP), blended with either a crown ether, Dicyclohexano18crown6 (DCH18C6) or a high-molecular-weight poly(ethylene oxide) (HMWPEO), and a Li salt, lithium trifluoromethanesulfonate (LiCF3SO3, Litriflate, LiTf for short) as a solid state electrolyte as they were successfully applied as active layers in light-emitting electrochemical cells (LECs). The surface morphologies of the blends were investigated by atomic force microscopy (AFM) in Tapping mode© and the ion distribution with X-ray analysis by energy-dispersive X-ray spectrometry (EDXS) in the scanning electron microscope (SEM). The specific phase morphologies of the blends, the complexation of the ions and the ionic transport numbers were found to have tremendous influence on the performances of the corresponding LECs revealing either rectifying or symmetric optoelectronic characteristics in forward and reverse bias directions.

An important characteristic of thin films of state-of-the-art conjugated polymers and small molecule materials is energetic and morphological disorder. For the photophysics in these electroactive thin films, the disorder has an important consequence: a single and multi-step (migration) excitation energy transfer processes of the charge-neutral states, such as singlet and triplet excitons (TEs) towards lower energetic sites can be observed prior to radiative recombination. Yet as found migration assisted processes, which are rather pronounced in the solid state of organic molecules and conjugated polymers can be held responsible for a large number of nonradiative deactivation channels as they enable quenching at chemical defects or other excited states such as polarons. Furthermore, the ability to migrate enlarges the action radius of singlet excitons (SEs) up to 20 nm in the solid state which makes it necessary to include the migration process for describing the overall energy transfer behaviour of conjugated polymers.

Amorphous, hydrogenated carbon thin films with varying silicon content (a-Si(1−x)Cx:H) for optical and tribological applications were deposited by means of pulsed laser deposition (PLD) from silicon targets with a high power pulsed Nd:YAG laser of 1064-nm wavelength in argon and C2H2 containing atmospheres. To minimize the deposition of particulates and droplets originating in the laser ablation process of the pure silicon targets, a perpendicular arrangement of the target and substrate surface as well as a screen between them was applied (‘shaded off-axis’ geometry). The chemical composition was investigated employing X-ray photoelectron spectroscopy, revealing carbon contents between 80 and 97 at.% and a mainly CC4 bonded structure. A distinct effect of the carbon content on the hardness and elastic modulus of the films determined by nanoindentation was observed. In contrast, the optical behavior of the films investigated employing spectroscopic ellipsometry was found to be nearly independent on the carbon content. A comparison of the optical properties for all films with literature data revealed a hydrogenated diamond-like carbon bonding structure.

In this study, we use photoinduced absorption (PIA) spectroscopy to show that the triplet exciton properties of an oligo(ethylene oxide) (OEO) grafted conjugated polymer (PPP-R10) are very sensitive to the presence of ionic species. Investigations were performed on thin spin-cast films of this polymeric mixed ionic–electronic conductor. Since the ion-coordinating side-chains are directly attached to the conjugated backbone, an ion induced order–disorder transition affects the characteristics of the conjugated polymer, which is observable as a blue shift in the π–π * absorption, the triplet exciton absorption spectra, and also manifests in other triplet exciton properties, like their lifetimes and the bimolecular annihilation parameter. Based on the sensitivity of the triplet exciton properties such polymers can be envisioned in the application in chemical sensor systems.

We report on investigations of the morphology of spin-coated thin films of an organic mixed ionic-electronic conductor consisting of the conjugated polymer methyl substituted ladder type poly(para- phenylene) (mLPPP) blended with a solid state electrolyte (Dicyclohexano18crown6 (DCH18C6), complexed with lithium trifluoromethanesulfonate (LiCF 3 SO 3 )). This blend system was successfully applied as active layer in light-emitting electrochemical cells (LECs). While thin films blends of the conjugated polymer and the pristine crown ether show a very smooth surface, the addition of LiCF 3 SO 3 causes a pronounced surface roughening. Since such a distinct surface roughness can be the reason for a device failure mechanism that limits the device lifetime, this attitude is investigated by tapping mode atomic force microscopy (AFM). These studies are complemented with X-ray analysis by means of energy dispersive X-ray spectrometry (EDXS), in the scanning electron microscope (SEM), in order to get a better insight into the ion distribution within the blend layer and its influence on the surface roughness formed.

We report on the Raman behavior of methyl substituted ladder type poly(p-phenylene) (mLPPP), a conjugated polymer with a high application potential for optoelectronic devices. A funded explanation of the experimentally observed Raman modes based on ab initio calculations is given, which takes also the influence of the side chains into account. Experimental investigations were performed with different laser excitation wavelengths on mLPPP in solution and films drop-casted on silicon substrates. From the studies of the polymer in solution the differential Raman scattering cross-section of mLPPP were determined on the basis of the known values of different solvents.

We report on investigations of thin films of homopolymers and copolymers consisting of a conjugated poly(p-phenylene) (PPP) backbone and oligo(ethylene oxide) (OEO) side chains of varying chain length in order to supply a conjugated backbone with an ion-transporting functionality. The optical properties of these polymeric mixed ionic–electronic conductors (PMIECs) reveal a response upon complexation of the OEO side chains with an alkali metal salt (lithium triflate). In addition, the rather polar nature of the OEO side chains and the non-polar nature of the PPP backbone can be applied to control the polymer/substrate interfacial properties. These topics were investigated by means of UV/VIS/NIR absorption measurements, along with photoinduced absorption spectroscopy. Infrared and Raman spectroscopy complemented this work to determine the amount of ion dissociation.

Pulsed laser deposition (PLD) is a physical vapour deposition coating technique for the production of thin films with complex chemical compositions. One of the main advantages of PLD is that excellent coating properties can be achieved even at low deposition temperatures. However, particulate defects in the growing films resulting from the evaporation process are often mentioned as the most important disadvantages of the PLD process. Unfavourable optical, thermo-physical and mechanical properties of the target material evaporated by laser radiation promote the formation of particulate defects. This paper presents some results on silicon-based PLD-films with reduced density of particulates. Silicon, SiO x and SiO 2 thin films were deposited by laser ablation from silicon targets with a high power pulsed Nd:YAG laser of 1064 nm wavelength in argon and oxygen containing atmospheres. The substrates were arranged in shaded off-axis geometry. The chemical composition and structure of the films were investigated employing transmission electron microscopy (TEM), secondary ion mass spectroscopy, X-ray photoelectron spectroscopy and ellipsometry. The results demonstrate the capability of PLD for the deposition of SiO x films with varying composition (0⩽ x ⩽2) by shaded off-axis PLD. The results of TEM and spectroscopic ellipsometry are indicating amorphous film structures in all cases.

The ability to grow inorganic thin films with highly controllable structural and optical properties at low substrate temperature enables the manufacturing of functional devices on thermo-sensitive substrates without the need of material postprocessing. In this study, the authors report on the growth of zinc oxide films by direct plasma-enhanced atomic layer deposition at near room temperature. Diethyl zinc and oxygen plasma were used as the precursor and coreactant, respectively. The process was optimized with respect to the precursor and coreactant dosing as well as to the purging times, which ultimately resulted in saturated atomic layer deposition growth. The so-obtained films exhibit a polycrystalline pattern with a (100) texture and low amount of incorporated carbon. Furthermore, the possibility to tune crystallite size, refractive index, and bandgap of the films by adapting the plasma radio-frequency power is demonstrated.

We report on combined investigations on indium–tin oxide (ITO) coated glass substrates in order to reveal the influence of chemical and physical treatments on the morphology and the elemental distribution at the ITO surface. The latter was studied by X-ray photoelectron spectroscopy while a combination of atomic force microscopy (AFM) and transmission electron microscopy was used to study the morphologic aspects. We find that etching ITO with chromosulphuric acid significantly reduces the amount of carbon contamination present at the surface of as received ITO substrates whereas the morphology of ITO remains almost the same. Additionally, we present ellipsometric studies of an organic semiconducting oligomer, para-hexaphenyl (PHP), grown on different substrates by physical vapour deposition. This method enables the determination of the optical parameters and the film thickness. In order to understand the growth mechanism of PHP in more details, its growth on silicon (Si) was also studied. We present AFM images of the first two as well as of several monolayers of PHP on Si giving evidence for the formation of PHP islands on the surface. 2002 Elsevier Science B.V. All rights reserved.

We report the synthesis of novel phenylenevinylene derivatives that allow for the introduction of meta versus para connections on the phenylene rings, as well as the incorporation of nitrogen atoms within the conjugated backbone and the attachment of electroactive substituents. We assess the impact of the various derivatization schemes on the electronic and optical properties by means of gas-phase ultraviolet photoelectron spectroscopy (UPS) and optical absorption and emission measurements; the evolution of the experimental data is further supported by the results of quantum-chemical calculations. We demonstrate that the electronic and optical properties of phenylenevinylene chains, and by extension those of many conjugated materials, can be tuned over a large energy range by a tailored design of the molecular structures.

Ab initio calculations of the Raman spectra of ladder type oligo (p-phenylenes) (LOPP) were done in order to study the influence of the side chains attached on the methine bridge and the aromatic rings. We found that the aromatic CC stretching modes and the interring CC stretching modes are significantly influenced. Due to the attachment of an aromatic side chain on the methin bridge, an additional strong mode appears in the interring CC stretching region. Side chains on the aromatic rings cause a splitting of the aromatic CC stretching mode.

We report on Raman spectra of ladder type oligo(p-phenylenes) of different chain lengths (5LOPP and 7LOPP) and make a comparison to the Raman spectrum of the corresponding polymer, ladder type poly(p-phenylene) (LPPP). For a better understanding of the vibrational behavior of LPPP, several LPPPs with different substituents (methyl, allyl, n-butyl, 1-adamantyl) were investigated. The recorded Raman spectra of these LPPPs show significant changes in the interring CC stretching regions. An experimental analysis of the frequencies, intensities and line-shapes of significant modes is given in dependence on the substituents and chain lengths.

The electroluminescence spectra both in forward and reverse directions of a light-emitting electrochemical cell prepared from a methyl-substituted ladder-type poly(paraphenylene) (mLPPP), blended with a crown ether, Dicyclohexano18crown6 (DCH18C6), and a Li salt, lithium trifluoromethanesulfonate (litriflate, LiTf for short) are compared. Different from the forward direction, in reverse direction the spectrum is strongly influenced by self-absorption effects. We discuss this behavior by different degrees of doping of the p- and n-type doped zones. This effect can be applied for a new method to realize bias-dependent color emission from single-layer devices at low voltages that can be prepared by only one simple and easy spin-coating process.

Noise spectroscopy is presented to be a powerful tool to investigate the current flowing in organic light emitting diodes (oLEDs) with high sensitivity. Measurements can be performed over the whole bias range of interest, from reverse bias up to high values of forward bias voltage. From these measurements one can gain insight into the microscopic conduction processes dominating the device current and obtain valuable information for improved device modeling. In particular it is shown that the low frequency power spectrum of the tested oLEDs has a power law dependence around 1/f 1.3 almost irrespective of device characteristics and of measurement conditions. Additionally, noise spectra are also proposed as a means to sense the initial state and the growth of degradation phenomena in these devices. The onset of degradation is shown to be signaled by current spikes that reflect on a net increase of the white noise component of about three orders of magnitude in the power spectral density, when degradation is just hardly beginning to be visible as dark spots on the emitting surface.

The properties of polyfluorenes for use in luminescent devices have been improved by a variety of approaches. Their conjugation length may be extended by using more planar indenofluorene units. Formation of long wavelength emitting aggregates is suppressed by attachment of bulky dendron groups to give stable blue emission. The emission colour can be tuned across the visible spectrum by incorporation of perylene dye units on the main chain or as substituents. Rod-coil copolymers have been made in order to achieve control of the supramolecular order. Polyfluorenone prepared by a precursor route shows improved electron injection and transport properties.

The photoluminescence (PL)-detected magnetic resonance (PLDMR) of various π-conjugated materials, such as methyl-substituted ladder-type poly( p -phenylene), para -hexaphenyl (PHP) films and ladder-type oligophenylenes are described. The optical measurements are compared to a thermally stimulated current (TSC) study of defects in m-LPPP and PHP. As TSC probes the density of mobile charge carriers after detrapping and PLDMR reveals the influence of trapped charges on the PL, their combination yields the concentration of traps, their energetic position, and their contribution to PL quenching. The TSC measurements reveal trap densities≥l.6×10 16 and l.4×10 14 cm −3 in m-LPPP and PHP, respectively. From a comparison of the PLDMR and TSC results one finds that the interaction and hence the nonradiative quenching of singlet excitons (SE's) at polarons is stronger in PHP than in m-LPPP due to a higher diffusivity of SE's in PHP. All of the results are in excellent quantitative agreement with a rate-equation model in which the positive (PL-enhancing) spin 1/2 PLDMR is due to the role of polarons in nonradiative quenching of SE's. The results also suggest that this quenching process is very significant in luminescent π-conjugated materials and organic light-emitting devices, and should be taken into account, especially at high excitation densities such as in lasing action.

The effect of vacuum ultraviolet radiation on the valence electronic structure of the electroluminescent organic materials p-sexiphenyl (6P) and tris-(8-hydroxy quinoline) aluminum (Alq 3 ) was investigated by ultraviolet photoemission spectroscopy (UPS). The intense radiation of an undulator at the storage ring BESSY II (Berlin) caused a loss of conjugation in 6P, evidenced by a decrease in intensity of delocalized π-orbitals in the UPS spectra. Depending on the degree of film degradation, surface charging was observed for both materials. It is shown that by illuminating the sample with laser light that can be absorbed by the organic/metal substrate system the surface charging could be compensated. Thus, the generation of free charge carriers by optical means appears to be a useful substitute for the use of an electron flood-gun in photoemission experiments, whenever sensitive samples could suffer from irradiation with electrons.

We present photocurrent excitation cross-correlation experiments which provide time-resolved information on the dissociation of photoexcited states in a prototypical π-conjugated polymer. A first pump pulse generates singlet exciton states (S 1 ) which are re-excited to a higher lying state by a second time delayed pulse resonant with a singlet exciton transition (S 1 →S n ). Dissociation is monitored via the change of photocurrent response in single layer devices. Our experiment probes diffusion driven, ultrafast dissociation of inter-chain-hot excitons during inter-chain-migration to sites lower in energy. Once this process has concluded intrinsic dissociation occurs from a higher lying state reached after the second excitation.

Hydrostatic pressure is used to modulate the intermolecular interactions in the conjugated oligophenyl, para-hexaphenyl. These interactions affect the structural properties and also cause changes i...

Recent studies of frequency resolved DF due to T-T annihilation to singlet excitons (1S*) in π-conjugated polymers are reviewed, and the significance of this process is compared to that of other long-lived processses affecting the PL, in particular the nonradiative quenching of 1S* by polarons or triplet excitons. While the DF studies suggest that T-T annihilation typically accounts for up to ∼3% of the total PL, PLDMR and other studies suggest that singlet quenching by polarons and triplets may be a very significant process in the films and organic light emitting devices (OLEDs), especially at high excitation densities.

We have used ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) to study the changes in the valence electronic structure of oligophenylenes upon the deposition of alkali metals. We have fabricated thin films of p -quarterphenyl (4P) and p -sexiphenyl (6P) by thermal evaporation under ultrahigh vacuum conditions, resulting in partly oriented organic films. Subsequently, increasing amounts of alkali metals (Na, K, Rb, Cs in individual experiments) were deposited onto those in a stepwise manner. After each step, UPS and MAES spectra were recorded. We find new emissions within the formerly empty energy-gap of the organic materials induced by the charge transfer from the alkali metals. These are attributed to the formation of bipolarons. We find that the deexcitation probability of He ∗ (2 1 S) atoms on the doped organic surfaces exceeds that of He ∗ (2 3 S) by up to four times.

The photoluminescence (PL)-detected magnetic resonance (PLDMR) of various π-conjugated materials, including poly(p-phenylene vinylenes) (PPVs) isolated PPV chains in a nano-structured liquid crystal host, methyl-substituted ladder-type poly(p-phenylene), and oligophenylenes are described. All of the results are in excellent quantitative agreement with a rate-equation model in which the positive (PL-enhancing) spin 1/2 PLDMR is due to the role of polarons in nonradiative quenching of singlet excitons. The results also suggest that this quenching process is very significant in luminescent π-conjugated materials and organic light-emitting devices, and should be taken into account, especially at high excitation densities such as in lasing action.

We have performed momentum dependent electron energy-loss studies of the low lying electronic excitations in various π conjugated oligomers. Our experiment directly probes the dynamic structure factor S(q,ω) and the momentum dependent intensity variation of the excitations observed can be used to extract the size of the electron-hole pair created in the excitation process. We show that the primary gap excitations in a number of such model systems are essentially delocalized over the entire molecules. The extension of the corresponding exciton scales with the molecule length only and is independent of the details of the corresponding molecular structure.

Highly oriented thin films of hexaphenyl - which are used in organic opto-electronic applications - are characterised in terms of their crystal structures. Two different crystal structures of hexaphenyl (C 36 H 26 ) are observed when the films are prepared by physical vapour deposition at various substrate temperatures. If the substrate is kept at room temperature, hexaphenyl crystallises within a structure which is already known from single crystal investigations. However, when the thin films are grown at a substrate temperature of 160°C a new crystalline phase appears. This structure was characterised by X-ray and transmission electron diffraction. Due to the strong preferred orientation of the crystallites within the thin films, the lattice constants as well as main features of the new crystal structure could be determined. The lattice is indexed as monoclinic with: a = 7.98 A, b = 5.54 A, c = 27.64 A and β = 99.8°. The new crystal structure has high similarity to the already known crystal structure. Both structures are built by layers of hexaphenyl molecules, within one layer the aromatic planes of the hexaphenyl molecules are packed in a herringbone pattern The characteristic feature of the new structure is that the long axes of the hexaphenyl molecules are arranged absolutely perpendicular to the layers, whereas within the already known structure the long axes show an tilt angle of 17° to the layer normal direction.