This paper presents a low temperature, solution-based processing method of highly transparent, sparse networks of carbon nanotubes via annealing process that dramatically improves the conductivity of thin films of octadecylamine functionalized highly soluble single-wall carbon nanotubes by up to five orders of magnitude. This increase in conductivity obtained at low temperatures allows for the creation of transparent conducting carbon nanotube (CNT) films via printed deposition of contacts for photovoltaic, light emitting, and display devices. An increase in films conductivity has been shown with process temperatures of 200°C at normal atmospheric pressure. The dependence between the sheet resistance of CNT layers and the annealing parameters is analyzed together with Raman and FTIR data, suggesting a relationship between the loss of octadecylamine functional groups along with the healing of CNT defects during the annealing process and the dramatic conductivity improvement of CNT layers.
Flexible radiation dosimeters have been produced incorporating thick films (>1 μm) of the semiconducting polymer
poly([9,9-dioctylfluorenyl-2,7-diyl]-co-bithiophene). Diode structures produced on aluminium-metallised poly(imide)
substrates, and with gold top contacts, have been examined with respect to their electrical properties. The results suggest
that a Schottky conduction mechanism occurs in the reverse biased diode, with a barrier to charge injection at the
aluminium electrode. Optical absorption/emission spectra reveal a band gap of 2.48 eV for the polymer. The diodes have
been used for direct charge detection of 17 keV X-rays, generated by a molybdenum source. Using operating voltages of
-10 and -50 V respectively, sensitivities of 54 and 158 nC/mGy/cm3 have been achieved. Increasing the operating
voltage shows that the diodes are stable up to approximately -200 V without significant increase in the dark current of
the device (<0.2 nA).
A common strategy to improve the electrical performance of organic field effect transistors is to optimize the charge carrier mobility of the semiconducting thin film. Polymer semiconductor transport properties have shown a dependence on the chain length, due principally to the strong influence of molecular weight on the thin film microstructure. In this work, we report on a study of the influence of increasing molecular weight of poly(2,5-bis(3-docecylthiophen-2-yl)thieno[3,2-b]thiophenes) (pBTTT-C12) on the polymer bulk thermal properties, thin film microstructure and the electrical performance of thin film field effect transistor devices. Clear differences can be observed within a number average molecular weight range of 8,000 - 18,000 Dalton. A Liquid crystalline phase was only observed at the highest molecular weight, different thin film morphology was observed within the molecular weight range, and the field effect mobility was shown to increase with increasing molecular weight.
The development of p-type semiconducting polymers demonstrating good stability under ambient operation is of importance for the development of low cost, printed electronics. We present here the synthesis and full characterisation of two soluble terthiophene polymers, and examine the effect of introducing a fused aromatic heterocycle, thieno[2,3-b]thiophene, into a terthiophene polymer backbone. This heterocycle contains a cross-conjugated central double bond, and its inclusion was shown to have a marked influence on the optical, thermal and electrical properties of the terthiophene polymer. Transistors were fabricated from both polymers, and the operation and storage lifetime under ambient operation was compared.
This work describes the development of solution processable liquid crystalline semiconductors and their applications in field-effect transistors. The relationship between liquid crystal molecular structure, its corresponding phase behaviour and electrical performance is examined. Molecular design methodology is employed to control the liquid crystalline morphology. The thermal, optical and electrical behaviour of these materials is characterised and X-ray diffraction scattering technique is used to reveal details of morphology and molecular orientation.
New polymerisable liquid crystalline organic semiconductors based on small molecule "reactive mesogens" are reported. These molecules, comprising p-conjugated cores with reactive endgroups, were designed, synthesised then solution processed into thin films. The mesogenic morphology was attained thermally and fixed through a post fabrication photopolymerisation reaction. Thermal, optical and electrical properties of these thin films were characterised to reveal details of morphology and molecular orientation. Both bulk (time-of-flight) and surface (field-effect) charge carrier mobilities were measured. The relationship between molecular structure, corresponding macrostructure processability and charge mobility is discussed. Fabrication and characterisation of field-effect transistors based on reactive mesogens are presented and discussed.
Recent studies of lasing and stimulated emission in luminescent (pi) -conjugated polymers performed by our group are presented. Optical properties of cylindrical, high-Q, polymer microcavities are discussed. The emission spectra of plastic microring and microdisk lasers were measured and analyzed. Cylindrical light emitting polymer microdiodes, as possible candidates for electrically-driven plastic lasers have been fabricated. Stimulated emission and lasing were also demonstrated for polymer solutions infiltrated in opal photonic crystals. In addition, two unusual regimes of stimulated emission characterized by narrow laser-like spectral lines were found in thin waveguiding polymer films. These regimes may be associated with random optical feedback introduced by light scattering inside the polymer films and amplified Raman scattering, respectively.
We have studied resonant and non-resonant Raman scattering spectra in thin films of novel disubstituted acetylene polymers such as poly(1-ethyl-2-phenylacetylene) (PEtPA), poly(1-n-hexyl-2-phenylacetylene) (PHxPA) and poly(1-phenyl- 2-p-n-butylphenylacetylene) (PDPA-nBu), which possess high photoluminescence (PL) quantum efficiency. We found that the Raman scattering frequency dispersion is smaller in disubstituted acetylene polymers than in other acetylene polymers, in agreement with many other strongly luminescent polymers. Assuming the model of short polyene conjugation length in these acetylene polymers, we can obtain the conjugation length (N) for each polymer from the respective phonon frequency of the carbon-carbon double bond; we obtained N equals 7 for PDPA-nBu, and N equals 5 or 6 for PHxPA and PEtPA. The related energies of 11Bu and 21Ag can be estimated from these N and are in good agreement with the respective absorption and PL spectra of the various disubstituted polymers.
The Z-scan technique is used to extract the real and imaginary third order nonlinear susceptibilities, Re(chi )(3) and Im(chi )(3), respectively. A series of Z scans were conducted at 590 nm (near one-photon resonance) on a cumulene-containing polymer, poly(p- phenylene-1,4-diphenyl-1,2,3-butatriene) or PPC3, prepared in solution. At very high peak focal point intensities two- photon absorption is seen superimposed on a one-photon saturation signature in open aperture Z scans. A negative real third-order nonlinear susceptibility is also observed in closed aperture Z scans. We describe a procedure to extract the desired third-order nonlinear susceptibilities by conducting Z scans at various peak focal point intensities and then fitting the results by adjusting Re(chi )(3) and saturation intensity. Molecular second hyperpolarizability, is calculated to be (-1.4 + 2.2i) X 10-29 esu which is almost 40 times larger than that measured in a monomer equivalent, and scaling nonlinearly with chain length. Z scans were also conducted at 780 nm, (below one-photon resonance) on PPC3 and also gives a molecular second hyperpolarizability that is greater than that seen in the monomer. The higher value of molecular second hyperpolarizability measured near one-photon resonance is attributed to the existence of a real state at the first transition in a two-photon process.
We have investigated the lasing properties of several luminescent conducting polymers, i.e. DOO-PPV and the bi- substituted polyacetylenes PDPA-nBu, and PHxPA, dissolved in various polar and non-polar solvents. PPV polymers emit with high quantum efficiencies in broad emission bands cantered in the orange/red region of the spectrum, depending on the solvent, and the PDPA polymers emit in the blue/green region. Our tested laser resonators include polymer solutions excited with 100 ps pulses from a regeneratively amplified mode-locked Nd:YAG laser. We obtain pulsed, low-threshold laser operation with repetition rate of up to 1 kHz. Resulting mainly from recent reported originally in the literature. The dependencies of threshold pump energy and output versus input power characteristics on material parameters are investigated for a fixed optical gain length. The results are compared with the standard Rhodamine 590 organic dye system used in the same wavelength regions. We have observed that the well know phenomenon of 'concentration quenching' in dye molecules does not happen in polymers. Spectral narrowing in PDPA-nBu solution, emitting near 500 nm, is also obtained for the first time.
We have investigated the optical emission characteristics of poly(2,5-dioctyloxy-p-phenylenevinylene) (DOO-PPV) thin films under high intensity ps pulsed laser excitation (Ipump > 1 MW/cm2). We observed that the emission spectrum consist of a spectrally narrowed emission (SNE) band (FWHM approximately 10 nm) and superimposed sharp spectral lines. High resolution spectral measurements conducted at room and liquid nitrogen temperatures show that the spectral width of these lines is less than 1 nm. The lines peak positions are independent of film thickness, however, they shift upon changing the excitation wavelength. Also, the excitation intensity dependence of these narrow lines is found to be different from that of the SNE band. We argue that the narrow lines are due to Raman scattering gain of the 1st, 2nd and 3d order Raman active modes of DOO-PPV.
We discuss both cooperative radiation and stimulated emission and consider their role in spectral narrowing of luminescent conducting polymers. We argue that cooperative radiation is favored in films with poor optical confinement. On the other hand, directional stimulated emission can be observed in dilute solution and thin films with superior optical confinement. Spectral narrowing in this case can be achieved by increasing either the excitation length or excitation intensity. The optical gain and loss coefficients are measured. Narrow line (approximately 1.5 cm-1) laser emission is observed in cylindrical microcavities formed by thin polymer films coated around glass fibers in the red and green spectral ranges. The cavity quality factors of these plastic lasers are mainly determined by selfabsorption and estimated to be about 5000.
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