We demonstrate a novel method to visualize individual molecular chains in the surface layer of organic semiconducting polymer films of arbitrary thickness. The method is based on scanning force microscopy and employs a combination of a sensitive phase detection system and metal coated tips with a low (∼1 N/m) spring constant and a relatively high Q factor (∼200). The molecularly resolved morphology is observed in phase images that are taken simultaneously with the topography. Surprisingly, we found that, when the tip apex radius exceeds the intermolecular spacing in the surface layer, surface aggregates –when present- are visualized. In both cases, the phase contrast is shown to result from van der Waals interaction between the conjugated backbone of the polymer chains and the metallic tip, and can quantitatively be described by a simple harmonic oscillator model. We have employed this method to study the morphology of poly(p-phenylene vinylene) (PPV) derivatives with different substitutions.

White light-emitting devices were fabricated using the blends of blue, green, red emissive polymers of poly[9,9-(2'-ethylhexyl)fluorene] (PEHF), poly(9,9-dioctylfluorene-2,7-vinylene) (POFV), poly(2-(3',7'-dimethyloctyloxy)-5-methoxy-1,4-phenylene-1-cyanovinylene) (CNPPV) and poly(2-(2'-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene) (MEH-PPV). Inefficient energy transfers between the blue polymer and the green or the red polymer, which was observed in PL spectra of the blend films, provides a good opportunity to get white light emission due to better blend ratio controllability. The EL devices were fabricated with ITO/PEDOT/blend/Ca/Al configuration and good white light-emission was obtained for a certain blend ratio. The light-emission was observable from 4V and a maximum brightness of 400 cd/m2 was obtained with the device.

Organic semiconductors exhibiting complementary-type carrier mobility are the key components for the development of the field of “gplastic electronics” We present here a novel series of α,ω- and isomerically pure ββ'-diperfluorohexyl-substituted thiophene and study the impact of fluoroalkyl substitution and conjugation length vis-a-vis the corresponding fluorinefree analogues. Trends between the fluorinated and fluorine-free families in molecular packing, HOMO-LUMO gap, and π-π interactions are found to be strikingly similar. TFT measurements indicate that all members of the fluorinated series are n-type semiconductors

In this work, we study the workfunction alignment of organic compounds and electrodes evaporated on top of these organic materials by means of electroabsorption (EA). The organic materials of this study are poly[2-methoxy-5-(3',7'-dimethyloctyloxyl)]-1,4-phenylene vinylene (MDMO-PPV) and methanofullerene [6,6]-phenyl C61-butyric acid methyl ester (PCBM). The electrodes are Al, Au, LiF/Au and LiF/Au, the latter two with a 1 nm thick layer of LiF, as well as with co-evaporation of LiF and the metal. In case of the MDMO-PPV, LiF enhances the built-in potential and thus reduces the electron injection barrier. Equal built-in potentials for a co-evaporated LiF:metal contact suggest doping is happening at the MDMO-PPV/metal interface. Unlike predicted by the Au and Al workfunction value, the built-in potentials for the PCBM/Al and PCBM/Au interface are equal. This suggests that Fermi level pinning occurs at these interfaces. The presence of a thin LiF layer screens the charge transfer from the metal to the PCBM.

Organic Light Emitting Diodes (OLED) based on metal chelate complexes (Alq3) as the emmiting layer has been prepareded and studied in order to characterise their optical and electrical properties. Electrical measurements have been performed in order to establish the nature of carrier transport. The results point to a trap charge-limited conduction (TCL) with characteristic trap energy near 0.13 eV. In electroluminescence a large green band with a maximum at 2.4 eV was found as a result of three overlapping bands that are correlated with the organic layers. The origin of this luminescence spectrum was analysed.

A new solid phase synthetic strategy for the production of organic semiconductors has been developed. The strategy uses a germanium-based linker and Suzuki-type cross-coupling protocols and has been demonstrated for the iterative synthesis of both a regio-regular oligo-3-alkyl-thiophene and an oligoarylamine. The process also incorporates a novel “doublecoupling” after each iteration which minimizes deletion sequences. The key steps exploits the susceptibility of α-silyl- or α-silyloxy- but not germyl-substituted derivatives toward nucleophilic ipso-protodemetalation.

We have investigated the charge carrier lifetimes for electrons and holes in smectic mesophases of a 2-phenylnaphthalene derivative, 6-(4'-octylphenyl)-2-dodecyloxynaphthalene (8-PNP-O12) by time-of-flight (TOF) measurement. For the negative charge carriers, we found two transits in different time range, which are attributed to electronic and ionic conduction. With the aid of liquid-like fluidity of the material, we could prepare very thick samples over 500μm, and it enables us to determine the carrier lifetimes, which are governed by different regimes: the hole lifetime is governed by recombination of charge carriers and estimated to be 10-2 sec; on the other hand, the electron lifetime is dominated by trapping at deep states and estimated to be on the order of 10-5 sec for both SmA and SmB phases.

In a novel modification of transient differential transmission spectroscopy, the first excited state S1 is reexcited via a second laser pulse towards a higher lying state Sn. The dynamics of the relaxation of this state Sn as well as the states created from Sn are revealed by a broad-band probe pulse.

We find that the charge carrier generation efficiency from Sn is higher compared to S1. The push and probe durations below 20 fs enable the temporal resolution of the ultrafast relaxation of the Sn state and enables us to identify the two main contributions to enhanced charge generation from Sn, energy migration towards sites of high dissociation probability, and exciton dissociation during vibrational relaxation.

In this work we present the synthesis and characterization of several novel osmium complexes of the form [Os(N-N)2 L-L]2+ 2Ts- or [Os(L-L)2 N-N]2+ 2Ts- designed for organic light emitting device (OLED) applications. In the complex notation N-N represents a derivative of 2,2'-bipyridine or 1,10-phenanthroline and L-L represents a strong π-acid arsine or phosphine ligand. Several counterions have been used and include tosylate, hexafluorophosphate, triflate, heptafluorobutyrate, chloride, bromide, and iodide. The complexes feature 3MLCT emission that ranges from 611-650 nm, which makes them suitable as an emission source for red OLEDs. Phosphorescent quantum yields as high as 45% and emission lifetimes as short as 400 nanoseconds have been reached. The complexes were incorporated into OLED and give off red, orange, yellow, and green electrophosphorescence. Red devices have been created using polyvinylcarb/2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole, polyvinylnapthalene/2-tert-butylphenyl-5-biphenyl-1,3,4-oxadiazole, and polyfluorene host materials. OLEDs have been created which give brightness up to 3000 cd m-2 and efficiency up to 2.2% at 632 nm emission.

Polymer-dispersed liquid crystal (PDLC) is a composite material that consists of sub micron-size droplets of liquid crystalline (lc) material dispersed in a polymer matrix. A device is usually assembled by sandwiching PDLC between ITO-coated glass plates. The electro optical properties of such a device can be controlled by the application of electric fields; the device acts as a “switch” for the transmission of light. The transmission of light through the device can be controlled between almost zero and 100% by applying ac electric fields. The dielectric properties of the lc and polymer, shape and size distribution of the lc droplets in the PDLC, amplitude and frequency of the applied electric field are among the important parameters for optimizing the performance of such a device. We have investigated the optical properties of PDLC devices fabricated by using several different phase-separation techniques and different lc/polymer materials. We present results for the switching characteristics of these devices as functions of applied electric fields and the size-distribution of the lc droplets. We also present results that reinforce the influence of the interfacial charges on the optical properties of the PDLC devices.

Polymeric, planar waveguides of pyrromethene dye 1,3,5,7, 8-pentamethyl-2, 6-diethylpyrromethene-BF2 (PM567)-doped poly(methylmethacrylate) (PMMA) were prepared by spin coating (SC) and solution diffusion (SD) methods. Both techniques produced films that, under laser illumination, show strong spectral narrowing due to amplified spontaneous emission (ASE). Samples prepared by SD showed higher gain than the SC films. Also, compared to the SC samples, the smooth surface of the SD samples reduced scattering at the surface and therefore guding losses. The high dye concentration present in the SD samples may be responsible for the increased photostability (more than 105 pulses). The SD method appears to offer a simple alternative for the production of planar wave-guides for amplifier devices.

We report EL characteristics of blue organic electroluminescent (EL) devices with a hole transport layer (HTL) as an emitter i.e. ITO/NPD/BCP/Alq3/Cs(0.5 nm)/Al. Here, ITO, NPD, BCP, and Alq3 are abbreviations for indium-tin-oxide, 4,4'-bis[N-(1-naphthyl)-N-phenylamino]- biphenyl, 2,9-dimethyl-4,7-diphenyl-1,10-phenanthroline, and tris(8-hydroxyquinoline) aluminum, respectively. Three different chemically modified ITO electrodes were used. As a hole-injecting buffer layer, copper phthalocyanine (CuPc) or 4,4',4''-tris-(3-methylphenylphenylamino) triphenylamine (MTDATA) was used in this study. We found that the luminance increased less than linearly with an increase in current for all EL devices studied even when hole and electron injection were enhanced. The luminance loss in the device is attributed to quenching singlet excited states (1NPD*) by large excess radical cations (NPD+.) accumulated in the emission zone due to large overlap between a fluorescence spectrum of 1NPD* and an absorption spectrum of NPD+.. In addition, 1NPD* can be also quenched by radical anions (NPD-.) when excess electrons are injected.

Scanning-tunneling spectroscopy experiments have been performed on conjugated polymer films and have been compared to a three-dimensional numerical model for charge injection and transport. It is found that field enhancement near the tip apex leads to significant changes in the injected current, which can amount to more than an order of magnitude, and can even change the polarity of the dominant charge carrier. As a direct consequence, the single-particle band gap and band alignment of the organic material can be directly obtained from tip height-voltage (z-V) curves, provided that the tip has a sufficiently sharp apex.

A series of photon harvesting lanthanide complexes have been synthesized by chelating dendritic â-diketone ligands to Eu+3. The ligands are modified benzoyltrifluoroacetone units (BTFA) with poly(aryl ether) dendron attachments. These ligands impose a very large asymmetric nature to the first coordination sphere of europium compared to previous dendritic compounds. These complexes exhibit europium emission with no residual emission from the ligands as the dendritic subunits increase from G-0 to G-3. This is due to both antenna and site isolation effects. These complexes mimic light harvesting organisms seen in Nature. When these intensities are compared throughout the series, an increase in intensity is seen where [(G2)BTFA]3Eu is the brightest, with [(G3)BTFA]3Eu being less intense than the G-2 complex. However, the lifetime data indicates that the G-3 complex is the longest lived species.

In this work, we report on pressure induced structural changes in crystalline oligo(paraphenylenes) containing two to six phenyl rings. Revisiting the crystal structures at ambient conditions reveals details in the packing principle. A linear relationship between the density at ambient conditions and the number of phenyl rings is found. Energy dispersive X-ray diffraction has been performed in a systematic study on polycrystalline powders of biphenyl, paraterphenyl, p-quaterphenyl, p-quinquephenyl and p-sexiphenyl under hydrostatic pressure up to 60 kbar. Our investigations not only yield pressure dependent lattice parameters and hints towards pressure induced changes in the molecular arrangement, but also allow for an analysis of the equations of state of these substances as a function of oligomer length. We report the previously unknown bulk modulus of p-quaterphenyl, p-quinquephenyl, and p-sexiphenyl (B0 = 83 kbar, 93 kbar, and 100 kbar respectively) and its pressure derivative (B0' = 6.4, 7.5, and 5.6). A linear dependence of the bulk modulus on the inverse number of phenyl rings in the molecules is found.

L We have investigated the electronic, structural, and charge injection properties of interfaces formed between three electroactive conjugated organic materials, i.e., N, N'-bis-(1-naphthyl)-N, N'-diphenyl1-1,1-biphenyl1-4,4'-diamine (á-NPD), pentacene, p-sexiphenyl, and two high work function electrode materials, i.e., gold and the conducting polymer poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). Ultraviolet photoelectron spectroscopy shows that the hole injection barrier between the three organic materials and PEDOT:PSS is lower by 0.6-1.0 eV as compared to Au, despite a similar work function of the pristine electrode material surfaces (ca. 5 eV). This very large difference is due to an effective change of the metal work function due to the deposition of organic molecules, i.e., a decrease of the Au surface dipole due to adsorption. Accordingly, model device structures built from á-NPD and pentacene on the two different electrode materials show much higher current densities for hole injection from PEDOT:PSS than from Au. Hole injection from Au for á-NPD devices is independent of deposition sequence and substrate. Pentacene devices exhibit significant asymmetries in that respect, due to a strong dependence of the morphology and preferred molecular orientation of the crystalline material on the substrate, as shown by atomic force microscopy and X-ray diffraction. Consequently, great care must be taken when modeling current-voltage characteristics of devices comprised of crystalline organic solids, especially when the influence of film thickness or different substrate materials is to be studied.

The aim of this study is to investigate the crystalline structure of anthracene C14H10 under high pressure performing angle dispersive x-ray diffraction experiments using synchrotron radiation in combination with Rietveld refinements and rigid body approximation. High hydrostatic pressure was applied up to 27.8 GPa using a diamond anvil cell. Full structural information (molecular orientations and lattice constants) is given up to a pressure of 20.3 GPa. At the highest pressure of 22.7 GPa the unit cell volume is decreased by 36.8%. Fourier transformation of the diffracted intensities reveals the electron density distribution within the unit cell. A pressure induced increase of the electron densities between adjacent molecules is observed. These findings are shown to be in agreement with theoretical calculations and hint towards the evolution of the anisotropic conductivity with pressure.

We have investigated a series of α,α'-oligothiophenes with chromophore length ranging from four to six units, and with side chains comprised of ten, six, two, and zero alkyl units. We have fabricated top-contact and bottom-contact organic TFTs with thermally evaporated active layers and found that the TFT performance depends critically on the length of the side chains and on the contact configuration (with mobilities ranging from 0.07 to 1.1 cm2 / V-s), but is relatively insensitive to the chromophore length. We have also fabricated ring oscillators with didecylsexithiophene and measured a signal propagation delay of 30 μsec per stage. In addition, we have fabricated oligothiophene-TFTs with ultra-thin self-assembled monolayer (SAM) gate dielectrics, with the intent to evaluate if long alkyl side chains contribute usefully to the effective thickness of a SAM gate dielectric. We have measured carrier mobilities as large as 0.05 cm2 / V-s and subthreshold swing as low as 200 mV / decade.

We report on light extraction experiments with polyfluorene based organic light emitting diodes (OLEDs). The intensity distribution of emission within the substrate and within air is determined experimentally. For the devices investigated in this study, we find that the external quantum efficiency can be increased by a factor of 1.82, in the case of complete suppression of waveguiding within the substrate. The experimental data is in good agreement with modeling results based on dipole emission within a multilayer system. As a consequence of these findings, we modified the device topology by introducing light extraction structures between glass substrate and indium tin oxide anode. This results in redirection of photon trajectories and thus in a higher probability to extract waveguided radiation. At a luminance of 100 cd/m2, the photometric efficiency for emission in forward direction is enhanced by a factor of 1.45 to 8.9 cd/A.

Sensorineural hearing loss (profound deafness) is a result of the inability of the transductory structures in the cochlea (organ of Corti) to convert the mechanical displacement of the basilar membrane to neural signals. A class of devices known as Cochlear Implants can significantly enhance the hearing ability in these patients. Fundamentally different from the existing cochlear implant technology, are the totally implantable piezoelectric based devices that are being developed. The unit is completely self-contained, designed to work without any signal amplifiers or transmission elements, greatly simplifying the stimulation process, and enhancing the cosmetic appearance of the patient. These devices utilize the bending piezoelectric effect. Device design consists of arrays of elements of piezoelectric polymer films with conducting polymer electrodes embedded in a flexible substrate with the whole device coated with an insulating layer. The incoming mechanical energy (pressure waves) into the cochlea generates electrical charge by virtue of the piezoelectric effect of the film. The generated charge is fed to electrical connections evaporated on the substrate and is used to stimulate surviving nerve fibers in the cochlea. In certain environments where acoustic impedance matching is limited by size constraints and conducting liquid medium, the advantage of polymer based devices over ceramics and metal based devices, are their flexibility, low acoustic impedance, and high sensitivity. However, in order to utilize these useful properties, the electrode material is an important issue, since the conventionally used metal electrodes, have high acoustic impedance and also impose mechanical clamping on the soft polymer which can significantly reduce the electromechanical efficiency of the transducer. Due to its flexibility, strong coherent interfaces, and significantly improved acoustic transparency, such an all-polymer electroactive system is compared to a metal-polymer system of similar design and also compared to the current technology.