Frank Nüesch

Fabrication of organic light-emitting diode pixels by laser-assisted forward transfer

Fabrication of a polymer light-emitting device was achieved by a laser forward transfer technique using the decomposition of a thin triazene polymer film by a XeCl excimer laser. The dry deposition process allows transfer of a bilayer consisting of the electroluminescent polymer poly[2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] covered with an aluminum electrode onto a receiver substrate. The soft transfer results in laterally well resolved pixels (≈500μm), whose fluorescence as well as electroluminescence spectra remain unaltered. The rectifying and smooth current-voltage characteristics add to the merits of this laser-based transfer method that opens up the possibility of direct-writing heat- and UV-sensitive materials.

Effect of coadsorbent on the photovoltaic performance of squaraine sensitized nanocrystalline solar cells

The effect of chenodeoxycholic acid as the coadsorbent with a squaraine sensitizer on TiO(2) nanocrystalline solar cells was investigated, and it was found that the coadsorbent prevents the squaraine sensitizer from aggregating on the TiO(2) nanoparticles but reduces dye loading leading to an interdependent photovoltaic performance. Analysis of the absorption spectra, and incident monochromatic photon-to-current conversion efficiency data showed that the load of squaraine sensitizer as well as the appearance of H-aggregates is strongly dependent on the molar concentration of chenodeoxycholic acid coadsorbent. The open circuit voltage of the solar cells with chenodeoxycholic acid increases due to the enhanced electron lifetime in the TiO(2) nanoparticles coupled with the band edge shift of TiO(2) to negative potentials.

Release of carbon nanotubes from an epoxy-based nanocomposite during an abrasion process.

The abrasion behavior of an epoxy/carbon nanotube (CNT) nanocomposite was investigated. An experimental setup has been established to perform abrasion, particle measurement, and collection all in one. The abraded particles were characterized by particle size distribution and by electron microscopy. The abrasion process was carried out with a Taber Abraser, and the released particles were collected by a tube for further investigation. The particle size distributions were measured with a scanning mobility particle sizer (SMPS) and an aerodynamic particle sizer (APS) and revealed four size modes for all measured samples. The mode corresponding to the smallest particle sizes of 300-400 nm was measured with the SMPS and showed a trend of increasing size with increasing nanofiller content. The three measured modes with particle sizes from 0.6 to 2.5 μm, measured with the APS, were similar for all samples. The measured particle concentrations were between 8000 and 20,000 particles/cm(3) for measurements with the SMPS and between 1000 and 3000 particles/cm(3) for measurements with the APS. Imaging by transmission electron microscopy (TEM) revealed that free-standing individual CNTs and agglomerates were emitted during abrasion.

Panchromatic Response in Solid-State Dye-Sensitized Solar Cells Containing Phosphorescent Energy Relay Dyes

Running relay: Incorporating an energy-relay dye (ERD) into the hole transporter of a dye-sensitized solar cell increased power-conversion efficiency by 29 % by extending light harvesting into the blue region. In the operating mechanism (see picture), absorption of red photons by the sensitizer transfers an electron into TiO2 and a hole into the electrolyte. Blue photons absorbed by the ERD are transferred by FRET to the sensitizer.

Frequency dependent dielectric and mechanical behavior of elastomers for actuator applications

The low frequency mechanical and dielectric behavior of three different elastomers has been investigated by dynamic mechanical analysis and dielectric spectroscopy, with the aim of accounting for the frequency dependence of the characteristics of the corresponding dielectric elastomer actuators. Satisfactory agreement was obtained between the dynamic response of the actuators and a simple model based on the experimental data for the elastomers, assuming that the relatively large prestrains employed in the actuators to have little influence on the frequency dependence of their effective moduli. It was thus demonstrated that the frequency dependence of the actuator strain is dominated by that of the mechanical response of the elastomer, and that the frequency dependence of the dielectric properties has a relatively minor influence on the actuator performance.

Woven Electrodes for Flexible Organic Photovoltaic Cells

A stable, conductive, transparent, and flexible electrode based on a precision fabric with metal wires and polymer fibers woven into a mesh is presented. Organic solar cells on woven mesh electrodes and on conventional glass/ITO substrates with very similar performance characteristics are demonstrated.

Improvement of toughness and electrical properties of epoxy composites with carbon nanotubes prepared by industrially relevant processes

The addition of carbon nanotubes (CNTs) to polymeric matrices or master batches has the potential to provide composites with novel properties. However, composites with a uniform dispersion of CNTs have proved to be difficult to manufacture, especially at an industrial scale. This paper reports on processing methods that overcome problems related to the control and reproducibility of dispersions. By using a high pressure homogenizer and a three-roll calendaring mill in combination, CNT reinforced epoxies were fabricated by mould casting with a well dispersed nanofiller content from 0.1 to 2 wt%. The influence of the nano-carbon reinforcements on toughness and electrical properties of the CNT/epoxies was studied. A substantial increase of all mechanical properties already appeared at the lowest CNT content of 0.1 wt%, but further raising the nanofiller concentration only led to moderate further changes. The most significant enhancement was obtained for fracture toughness, reaching up to 82%. The low percolation thresholds were confirmed by electrical conductivity measurements on the same composites yielding a threshold value of only about 0.01 wt%. As corroborated by a thorough microscopic analysis of the composites, mechanical and electrical enhancement points to the formation of an interconnected network of agglomerated CNTs.

Red-green-blue polymer light-emitting diode pixels printed by optimized laser-induced forward transfer

An optimized laser-induced forward transfer (LIFT) technique has been used to fabricate tri-color organic light-emitting diode (OLED) pixels. At reduced pressures, and with a defined donor-receiver gap, patterned depositions of polyfluorene-based OLED pixels have been achieved. OLED pixel functionality has been demonstrated and compared with devices made using conventional deposition techniques. In addition, improved functionality has been obtained by coating the cathode with an electron-injecting layer, a process not possible using conventional OLED fabrication techniques. The OLED pixels fabricated by LIFT reach efficiencies on the range of conventionally fabricated devices and even surpass them in the case of blue pixels.

Phosphorescent energy relay dye for improved light harvesting response in liquid dye-sensitized solar cells

Energy transfer from phosphorescent ruthenium complex N877 dissolved in the liquid electrolyte to the squaraine sensitizer SQ1 anchored on the titanium oxide surface resulted in a four fold increase in external quantum efficiency in the blue part of the visible spectrum despite complete quenching of the luminescence of N877 by the iodine in the electrolyte.

Improved performance of cyanine solar cells with polyaniline anodes

Significant progress is being made in the photovoltaic energy conversion using soluble small organic molecules. We report the fabrication of layered heterojunction solar cells with 3% power conversion efficiency consisting of a solution-processed cyanine dye, C60 and doped polyaniline anode layers that match the cyanine energy level and facilitate hole extraction.