We have calculated J-V characteristics of an organic conducting sample (containing traps) including the Poole-Frenkel Effect (PFE). Both shallow and exponentially distributed traps are considered. We show that our approach is equivalent to combining the effect of trapping and using the field dependent mobility in one unified model. For shallow single level or shallow Gaussian traps, inclusion of PFE or using the (well-known) field dependent mobility gives the same dependence of current on voltage at a given temperature. However the value of zero field mobility µ0 comes out to be different. We have fabricated and measured the J-V curves of the ITO/MEH-OPV5/Al diodes. An extremely fast rise with voltage V is observed at small voltages, which can be interpreted either by the Schottky contact limited Shockley like current or by bulk space charge limited current with PFE. The correct mechanism can be determined by making J-V measurements at different temperatures.

We report on the fabrication of step index polymer optical fibers that use a novel conjugated polymer and fluorescent stilbenoid compounds for lasing and amplification applications in the blue-green regions. The green-emitting polymer, Thianthrene-MEH-PPV, and the blue-emitting compounds, 1,2-bis(4-diphenylaminophenyl)-ethylene and 1,4-bis(4-diphenylamino-styryl)-benzene, are designed for the blue-green region of the spectrum and have very high quantum yields and large Stokes shifts. Significant spectral narrowing and superlinear increase of output intensity are observed under photoexcitation at 355 nm, which are indicative of the occurrence of amplified spontaneous emission. By gain spectroscopy, a high gain coefficient of 36 cm−1 at 494 nm has been obtained for the 1,4-bis(4-diphenylamino-styryl)-benzene-doped fiber when it is transversely photoexcited at 12 mJ/cm2.

Polyethylene terephthalate (PET) has been modified by Cr ion implantation with a dose range from 1×1016to 2×1017ions /cm2 using a metal vapor vacuum arc MEVVA source. The surface morphology was observed by atomic force microscopy (AFM). The Cr atom precipitation was found. The changes of the structure and composition have been observed with transmission electron microscope (TEM). The TEM photos revealed the presence of Cr nano-meter particles on the implanted PET. It is believed that the change would cause the improvement of the conductive properties and wear resistance. The electrical properties of PET have been improved after metal ion implantation. The resistivity of Cr ion implanted PET decreased obviously with an increase of ion dose. When the metal ion dose with 2×1017cm−2 was implanted into PET, the resistivity of PET could be less than 0.1 Ωm. But when Si or C ions with same dose are implanted PET, the resistivity of PET would be up to several Ωm. The result show that the resistivity of Cr ion implanted sample is obviously lower than that of Si- and C-implanted one. After Cr implantation, the surface hardness and modulus could be increased. The property of the implanted PET has modified greatly. The hardness and modulus of Cr implanted PET with dose of 2×1017/cm2 is 9.5 and 3.1 times greater than that of pristine PET. So we can see that wear resistance improved greatly. The Cr ion beam modification mechanism of PET will be discussed.

Electro-active organic materials based on conjugated molecules, showing promising optical features in the UV/Vis region combined with high electron mobility and fluorescence quantum yield, are of great interest for both, scientific and commercial use. Since the electro-optical properties of these materials are critically influenced by their the crystal and molecular structures, the goal of our work is to study the nature of the intermolecular interactions by applying hydrostatic pressure. X-Ray powder diffraction experimets were carried out to determine the lattice parameters. These experimental data then served as an input for an optimization of the atomic positions and molecular orientation based on total -energy and atomic-force computations within density functional theory.

In this paper we present a combined protocol for ultrathin film formation and assembly of Φ- conjugated polymer materials using the layer-by-layer (LbL) technique and a precursor polymer approach to electrodeposition. While much work has been done on investigating the chemistry (synthesis) and properties (physics) of these materials, little focus has been given on the multilayer and ordering issues associated with ultrathin film formation especially using novel “wet-processing” approaches. This work details our molecular assembly approaches on conjugated polymer systems for polyfluorene and polythiophene derivatives. This was demonstrated in the following systems: a) ionene precursor polyfluorene polymers deposited as ultrathin films using the LbL technique from an aqueous solution and b) electrodeposition of polysiloxane modified precursor polymers. Important strategies are outlined for combining the two techniques as a novel method for preparing ultrathin films and patterning.

Investigations on microscopic and photovoltaic properties of polyfluorene blends are presented here. The length scale of lateral phase separation is manipulated by control of solvent evaporation conditions. Photoluminescence efficiency measurements show that charge transfer is more effective in blends phase separated on the nanometer scale. Vertically segregated structures are obtained by a combination of solution viscosity and spin coating conditions. The external quantum efficiency of photovoltaic devices fabricated with vertically segregated blend is found to be 4 times higher than that of devices made with laterally segregated blends.

This paper overviews recent work that examined the optical properties of mono-disperse, PPE-based π-conjugated oligomers that contain a 2,2-bipyridine-5,5'-diyl metal coordinating unit. The photophysical properties of the free oligomers and metallated oligomers that contain the -Ru(bpy)22+ and -Re(CO)3Cl transition metal complexes coordinated to the bpy-diyl unit have been characterized by absorption, photoluminescence and transient absorption spectroscopy. In addition, we report the photophysics of a novel organometallic “square” that features “sides” consisting of a PPE-type π-conjugated oligomer and “corners” that consist of (dbubpy)PtII(acetylide)2 units (where dbubpy = 4,4'-di-tert-butyl-2,2'-bipyridine).

We have studied the structure and growth regularities of highly ordered para-sexiphenyl (C36H26) thin films deposited by Hot Wall Epitaxy on mica. In particular, atomic force microscopy (AFM) was used to investigate the early growth stage of these films, in order to find the process controlling parameters. It was shown that the substrate temperature and the growth time are important parameters for control of the film morphology, in terms of the degree of anisotropy and long range order. X-ray diffraction pole figure technique and transmission electron microscopy were also used to characterize the crystallographic structure of the thicker films. We have shown that the highly ordered crystallites of para-sexiphenyl (showing needle-like morphology by AFM) are oriented with their (11 1 ) or (11 2 ) crystallographic planes parallel to the substrate surface. For each of these two orientations there are two opposite directions for growth of crystallites reflecting the two-fold symmetry of the mica surface.

The synthesis of a new class of π-conjugated based on N-alkyl-2,7-carbazole is reported. Two different synthetic pathways were used to obtain 2,7-functionalized carbazole units. The first pathway leads to N-alkyl-2,7-dichlorocarbazole which undergoes a Yamamoto coupling to obtain the homopolymer. The second synthetic method provides N-alkyl-2,7-diiodocarbazole. This monomer allows the utilization of more versatile synthetic tools such as Suzuki and Stille couplings to obtain highly regioregular copolymers. Poly(N-(2-ethylhexyl)-2,7-carbazole) (PEHC) prepared using Yamamoto coupling shows strong pure blue photoluminescence (437 nm) and electroluminescence (432 nm, 225 cd/m2) with no excimer formation.

The non-linear optical properties of poly[1,4-phenylene-1-phenyl vinylene], poly[1,4-phenylene-1,2-di(4′-phenoxyphenyl) vinylene], poly[2,5-dioctyloxy-1,4-phenylene vinylene] and poly[2- methoxy -5- (2'-ethylhexyloxy)-1,4-phenylene vinylene] MEH-PPV with different molecular weights were studied by the Third Harmonic Generation (THG) technique. The dispersion of the χ3 coefficient was measured in the wide infra red range from 1.2 up to 2.1 μm. It was shown that the highest χ3 value, observed for the high molecular weight MEH-PPV (Mw 230.000), reaches the maximum of 8x10-11 esu at 1.77 μm. At the same time, the medium molecular weight MEH-PPV (Mw 95.000) demonstrated not significantly different χ3 value of 6x10-11 esu. The dispersion of the χ3 coefficient was compared to the absorption of the third harmonic beam and strong χ3 enhancement relative to the excitonic absorption was observed. However, a surprising shift of about 70 nm towards the longer wavelengths between the two peaks was always observed for all studied PPV derivatives. The results of THG measurements were compared to those obtained by conventional Zscan technique in the same range. It was shown that the THG χ3 values in infrared range were up to one order of magnitude lower than those of direct Z-scan measurements.

From admittance spectroscopy measurements on poly(p-phenylene vinylene) based light-emitting diodes various relaxation processes can be observed. At low bias inductive contributions due to the transit of charge carriers dominate, at high bias capacitive contributions as a result of charge redistribution appears. These processes reveal a transition from space-charge limited to recombination limited behavior, as is also indicated by the current-voltage characteristics.

We have prepared and characterized a light emitting diode (LED) with the active film formed by two polymers emitting in two different regions: in the yellow-green and in the red range of the visible spectrum. The two polymers are polyparaphenylenevinylene (PPV) and poly[3-(6-methoxyhexyl)thiophene] (P6OMe) respectively. P6OMe was spin coated on a PPV layer previously oriented and was also oriented by means of the rubbing technique in the direction perpendicular to the PPV orientation. The polarized electroluminescence obtained from this device shows yellow-green emission from PPV, or red from P6OMe depending on the orientation of the analyzer.

We present our recent findings on the development of organic light emitting diodes (OLEDs). One of the keys to highly efficient phosphorescent emission in organic light-emitting diodes is to confine triplet excitons generated within the emitting layer. To confine triplet excitons, we employ perfluorinated phenylene dendrimers (C60F42) as a both hole- and exciton-block layer, and a hole-transport material (4,4',4”-tris(N-carbazolyl) triphenylamine [TCTA]) as a host for the phosphorescent dopant, Ir(ppy)3, in the emitting layer. The maximum external quantum efficiency reaches to 19.2%, and is over 15% even at high injection current densities of 10 to 20 mA/cm2, where the brightness of the device reaches to approximately 10,000 cd/m2.

Self-assembled monolayers (SAMs) with a variety of structures and terminal groups were evaluated as underlayers for pentacene deposition. It was found that the most critical factor in the formation of highly oriented thin film pentacene with large grain size was the geometric structure of the monolayer. Monolayers with terminal bonds parallel to the surface produce large pentacene grains with an angular rather than the dendridic structure normally observed on octadecyltrichlorosilane (OTS) coated substrates. The grain size, X-ray scattering, carrier mobility, and current on/off ratios are all improved with monolayers of the appropriate geometry.

Integrated plastic circuits (IPCs) will become an integral component of future low cost electronics. For low cost processes IPCs have to be made of all-polymer Transistors. We present our recent results on fabrication of Organic Field-Effect Transistors (OFETs) and integrated inverters. Top-gate transistors were fabricated using polymer semiconductors and insulators. The source-drain structures were defined by standard lithography of Au on a flexible plastic film, and on top of these electrodes, poly(3-alkylthiophene) (P3AT) as semiconductor, and poly(4-hydroxystyrene) (PHS) as insulator were homogeneously deposited by spin-coating. The gate electrodes consist of metal contacts. With this simple set-up, the transistors exhibit excellent electric performance with a high source-drain current at source - drain and gate voltages below 30V. The characteristics show very good saturation behaviour for low biases and are comparable to results published for precursor pentacene. With this setup we obtain a mobility of 0.2cm2/Vs for P3AT. Furthermore, we discuss organic integrated inverters exhibiting logic capability. All devices show shelf-lives of several months without encapsulation.

Ferroelectric polymer thin films are attractive for a wide range of applications such as MEMS, IR sensors, and memory devices. We present the results of a recent investigation on the thickness dependence of the ferroelectric properties of poly(vinylidene fluoridetrifluoroethylene) copolymer spin cast films on electroded Si substrate. We show that as the film thickness is reduced, there exist two thickness regions. For films at thickness above 100 nm, the thickness dependence of the ferroelectric properties can be attributed to the interface effect. However, for thinner films, there is a large change in the ferroelectric properties such as the polarization level, the coercive field, and polarization switching speed, which is related to the large drop of the crystallinity in the ultrathin film region (below 100 nm). The results from Xray, dielectric measurement, and AFM all indicate that there is a threshold thickness at about 100 nm below which the crystallinity in the film reduces abruptly.

The intramolecular photoinduced energy and electron transfer within a fullereneoligothiophene-fullerene triad with nine thiophene units (C60-9T-C60) and an oligo(p-phenylene vinylene)-fullerene dyad with four phenyl groups (OPV4-C60) is investigated with femtosecond pump-probe spectroscopy with sub-10 fs and 200 fs time resolution in solvents of different polarity. Photoexcitation of the π-conjugated oligomer moiety in the triad and dyad results in an ultrafast singlet-energy transfer reaction to create the fullerene singlet-excited state with a time constant of 150-190 fs, irrespective of the polarity of the medium. In a polar solvent, intramolecular electron transfer occurs from the oligomer moiety to the C60 moiety with a time constant of 10-13 ps as a secondary reaction, subsequent to the ultrafast singlet-energy transfer. The charge-separated state has a lifetime of 50-80 ps and recombines to the ground state.

Thermal deposition of small amounts of various salts at the interface between the active layer and the aluminum cathode was shown to alter the performance of bulk heterojunction photovoltaic devices. LiF and LiBr were found to enhance the power conversion efficiency as compared to devices with no interfacial salt, but Cs and K compounds were found to severely diminish the device performance. It is suggested that the Li compounds preferentially align to produce a bulk dipole moment at the interface, whereas the Cs and K compounds do not.

Polarons and triplet excitons in conjugated polymers exhibit sub-gap absorptions which allow them to be identified spectroscopically. We have used quasi-steady-state induced absorption techniques on working polymer light-emitting diodes to study charge carriers and triplet excitons in devices. We identify absorptions due to charges, and at low temperatures can also resolve features due to triplet excitons. From the magnitude of the absorption features, we study the charge and triplet densities as a function of applied voltage. We obtain a value for the triplet generation rate at low temperatures and an estimate of the singlet exciton formation probability. Analysis of the triplet lifetime as a function of charge density reveals the presence of triplet-polaron interactions and we obtain a rate constant for this triplet annihilation process.

A novel mercury ion selective electrode has been constructed to have strongly coordinating macrocyclic ligand binding site covalently attached to polythiophene film using 1-1,4,10-trioxa-7,13-diazacyclopentadecane-1-thiophenylmethane (TDCD-thiophenylmethane) as a specific chelator. In amperometric mode, the current is proportional to mercury (II) concentrations down to 2 ppb. Reversible cyclic voltammetric waves show that both mercury (I) and mercury (II) are stabilized by the ligand. Monomethyl mercury and vapor phase elemental mercury are also electrochemically determined. Dissolved oxygen and heavy metal cations show no significant interference even at ten times the mercury concentration. The polymer surface shows robust performance for two years or longer. Fabrication strategies are being developed to optimize the probe performance. The variables under investigation include substrate surface roughness, the ratio of derivitized thiophene (TDCD-thiophenylmethane) to underivatized thiophene in the surface polymer, ratio of thiophene with two chelating rings to thiophene with only one, number and thickness of electrodeposited polymer layers, composition of polymer layers, and number of -CH2- units separating the chelating ring from the thiophene monomer units. The probe exhibits linear response to mercury concentration, and detects mercury in several forms.