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Electrically conductive composites consisting of carbon nanotubes (CNTs) in a poly(2,6-dimethyl-1,4-phenylene ether)/polystyrene (PPE/PS) blend are prepared by latex technology. The latter consists of 4 steps: preparation of an aqueous dispersion of CNTs, mixing with a polymer latex, solvent removal by freeze–drying and processing. In the first route, PPE/PS blend latexes are directly used to prepare nanocomposites by latex technology. The CNT/PPE/PS composites display an increased glass transition temperature compared to unfilled material and a percolation threshold as low as 0.3¿wt% of CNTs. Secondly, CNT/PS masterbatches prepared by latex technology are mixed with PS/PPE blend pellets by extrusion. This approach constitutes a potential first step towards an industrially viable process.

We demonstrate a route toward the preparation of healthier fruit juice infused chocolate candy. Up to 50 wt% of the fat content in chocolate, that is cocoa butter and milk fats, is replaced with fruit juice in the form of emulsion droplets using a quiescent Pickering emulsion fabrication strategy. Fumed silica particles are used in combination with chitosan under acidic conditions (pH 3.2-3.8) to prepare water-in-oil emulsions, the oil phase being sunflower oil, molten cocoa butter, and ultimately white, milk, and dark chocolate. Adsorption of the polycationic chitosan molecules onto the surface of the silica particles influenced the particle wettability making it an effective Pickering stabilizer, as shown by cryogenic scanning electron microscopy analysis. The formation of a colloidal gel in the continuous (molten) oil phase provided the system with a yield stress, hereby giving it a gel-like and thus quiescent behaviour under low shear conditions, as determined by rheological measurements. This warrants a homogeneous distribution of emulsion droplets as settling through gravity upon storage under molten/liquid conditions is arrested. In our low-fat chocolate formulations the cocoa butter has the desired polymorph V structure, and neither sugar nor fat bloom was observed upon storage of the fruit juice containing chocolate confectionaries. This journal is © The Royal Society of Chemistry.

In the present review, the main degradation mechanisms occurring in the different layer stacking (i.e. photoactive layer, electrode, encapsulation film, interconnection) of polymeric organic solar cells and modules are discussed. Bulk and interfacial, as well as chemical and physical degradation mechanisms are reviewed, as well as their implications and external or internal triggers. Decay in I-V curves in function of time is usually due to the combined action of sequential and interrelated mechanisms taking place at different locations of the device, at specific kinetics. This often makes the identification of specific root causes of degradation challenging in non-model systems. Additionally, constant development and refinement in terms of type and combination of materials and processes render the ranking of degradation mechanisms as a function of their probability of occurrence and their detection challenging. However, it clearly appears that for the overall stability of organic photovoltaic devices, the actual photoactive layer, as well as the properties of the barrier and substrate (e.g. cut of moisture and oxygen ingress, mechanical integrity), remain critical. Interfacial stability is also crucial, as a modest degradation at the level of an interface can quickly and significantly influence the overall device properties. © 2011 Elsevier B.V. All rights reserved.

26th European Photovoltaic Solar Energy Conference and Exhibition; 214-217, To reduce the costs of polymer solar cells it is important to develop a fully solution processed route. Although several successful generic routes to solution processed hole collecting electrodes have been shown, this is not the case for the electron collecting electrode of polymers cells, as it is hard to find a solution processable low workfunction, low ohmic, electron collecting electrode. Here an efficient recombination layer is described formed by the combination of ZnO and PEDOT and applied in an electron collecting layer. The ZnO contacts the conduction band of the acceptor material to collect electrons from the absorber layer. Since ZnO is a wide bandgap material, it also acts as an exciton and hole blocking layer. Electrons collected by the ZnO layer easily recombine with holes from the highly doped, high conductive PEDOT layer. The performance of a device with a recombination layer is compared in a standard as well as inverted polymer solar cells. The performance of the device with the PEDOT/ZnO recombination layer deviates less than 10 % from similarly processed standard and inverted device structures.

The presence of a transparent conductive electrode such as indium tin oxide (ITO) limits the reliability and cost price of organic photovoltaic devices as it is brittle and expensive. Moreover, the relative high sheet resistance of an ITO electrode on flexible substrates limits the maximum width of a single cell. Holst Centre and ECN have developed an alternative ITO-free transparent anode, based on a solution processed high conductive and transparent PEDOT:PSS layer in combination with a printed current collecting grid. Screen printed silver grids can yield sheet resistances down to 1 Ohm/Sq with a surface coverage of only ca 5%. The efficiency of a flexible device with an active area of 4 cm2 with such a grid is much higher than a similar device based on ITO. Furthermore, as this composite anode is solution-processed, it is an important step forward towards low-cost large area processing. Moreover, initial experiments indicate that the stability of ITO-free devices is higher than the stability of standard devices based on ITO. This work will ultimately contribute toward fully printed devices, which will provide low-cost manufacturing and improved stability of organic photovoltaic modules.

By using a latex-based technology, it is possible to efficiently disperse both single and multi wall carbon nanotubes (SWNTs and MWNTs) within most of the polymers that are produced by emulsion polymerization, or that can be brought into the form of an emulsion. The first step of the process consists of exfoliating NTs in an aqueous surfactant solution. This step can be monitored by UV-Vis spectroscopy. Four experimental techniques have been developed in order to determine the minimum amount of surfactant, which is necessary to reach the highest degree of exfoliation of the NTs. The key step of this process is the mixing a stable dispersion of NTs covered by surfactant molecules with a polymer latex. In the nanocomposites obtained using this concept, preponderantly individualized NTs are homogeneously dispersed into the polymer matrix. This leads to the obtaining of conductive nanocomposites with a percolation threshold of about 0.3wt% of SWNTs in a high molecular weight polystyrene (PS) produced by free radical emulsion polymerization. Several procedures were explored in order to improve the properties of the materials, like tuning the characteristics (molecular weight and particle size distribution) of the polymer matrix.

Several methods have been developed over the last few years to achieve the incorporation of carbon nanotubes (CNTs) into a polymer matrix in order to obtain electrically conductive nanocomposites. The key factors for producing such composites with low CNT loadings comprise the quality of the wetting between the filler and the polymer matrix, as well as the state of dispersion of the CNTs throughout the matrix. The final target is to manufacture easilyprocessable, low density conductive plastics, that in the future would be able to replace metals in applications for which these are still preferred. Nevertheless, as produced CNTs are either stuck together in thick bundles in the case of singlewall carbon nanotubes (SWCNTs), or are highly entangled in the case of multi-wall carbon nanotubes (MWCNTs). As a consequence, one of the main bottlenecks for the production of high performance CNT/polymer nanocomposites remains the resistance of CNTs against individualization. The work presented in this Ph.D. thesis focuses on the study of: - the various steps of a process based on the application of latex technology, which is utilized to prepare CNT/polymer nanocomposites. The key step of this process is the mixing of two colloidal solutions, being a dispersion of mainly individual CNTs covered by surfactant molecules, and a polymer latex, i.e. an aqueous dispersion of submicron polymer particles. The latter are also covered with surfactant molecules. - the nanocomposites produced in this way, in particular their electrical properties, which are governed by the formation of a percolating network of CNTs, and are therefore strongly dependent on the concentration of conductive fillers dispersed in the matrix. The first step of the process consists of debundling CNTs in an aqueous surfactant solution (typically sodium dodecyl sulfate, SDS) in order to obtain a stable dispersion of CNTs covered by surfactant molecules. The achievement of this first step is crucial: in order to get conductive films with filler loadings as low as possible, it is imperative to achieve a good dispersion of the CNTs in the polymer matrix. Consequently, it is very important to control and to monitor the CNT debundling during the first step of the process. However, CNT debundling does not guarantee a proper dispersion of the CNTs in the final nanocomposite. It was demonstrated that this sonication-driven step can be monitored by UV-Vis spectroscopy, see Chapter 3. This method is based on the fact that individual CNTs absorb light in the wavelength region between 204 200 and 1200 nm. The debundling of SWCNTs or MWCNTs results in an increase of the concentration of individual CNTs, and finally in an increase of the UV-Vis signal. A leveling off of the UV-Vis absorbance, recorded as a function of time and/or the total amount of ultrasonic energy supplied to the system, indicates that the maximum degree of exfoliation has been achieved and that, accordingly, further energy input can be stopped in order to prevent unnecessary damage to the CNTs. These results were confirmed with cryo-Transmission Electron Microscopy (cryo-TEM) and Scanning Electron Microscopy (SEM). In addition, four different experimental techniques, based on UV-Vis spectroscopy, surface tension measurements, thermogravimetry and a modified version of the Maron’s titration, have been developed in order to determine the lowest amount of surfactant necessary to reach the highest degree of exfoliation of the CNTs (Chapter 4). The results obtained enabled us to estimate a lower limit of the specific surface area of exfoliated SWCNTs, as well as an estimation of the specific surface covered by one surfactant molecule when adsorbed on the SWCNT surface. It is worth mentioning that these procedures are in principle applicable to a large range of surfactant-particle systems. In a second step, the stable aqueous SDS-CNT dispersion is mixed with a polymer latex. The mixture obtained is freeze dried and subsequently compression-molded. Before melt processing, the system typically consists of closely-packed latex particles (polystyrene latex particles for our model system) mixed with CNTs, which are confined in the interstitial space between the polymer particles. Since flow of the polymer occurs during the compression molding step, CNTs can move through the polymer melt. As a result, the processing conditions have a large influence on the conductivity, as well as on the percolation threshold of the nanocomposite, see Chapter 5. According to SEM analysis, in the nanocomposite films obtained, the CNTs are homogeneously dispersed in the polymer matrix, and form a network of preponderantly individualized CNTs. In particular, electrically conductive nanocomposites with a percolation threshold of about 0.3 wt% (resp. 0.9 wt%) of SWCNTs (resp. MWCNTs) dispersed in a high molecular weight polystyrene produced by free radical emulsion polymerization can be obtained in this way. This latex-based process is extremely versatile since it enables us to disperse SWCNTs and MWCNTs into most of the polymers produced by emulsion polymerization or polymers which can be artificially brought into a latex form. Thanks to this method, CNTs were successfully dispersed in another amorphous polymer than polystyrene, viz. poly(methyl methacrylate), a semi-crystalline polymer, i.e. polypropylene, or in a polymer blend, namely a poly(2,6-dimethyl- 1,4-phenylene ether)/polystyrene blend (see Chapter 7). Additionally, several procedures were explored in order to improve the properties of the composites, i.e. to lower the percolation threshold and increase the conductivity. We demonstrated that tuning the characteristics of the polymer matrix (in particular its molecular weight distribution, see Chapter 5) is a promising way to optimize these properties. It was also shown that the choice of the polymer matrix itself is a relevant parameter (see Chapter 7). Also the characteristics of the CNTs themselves (among others: their type (SWCNTs or MWCNTs), their intrinsic conductivity, their degree of purity, their diameter, and their aspect ratio) are of importance. Summarizing, we can state that the latex-based production of electrically conductive nanocomposites leads to well-defined materials with preponderantly individual CNTs homogeneously dispersed in a polymer matrix. Interestingly, the electrical properties of these materials are strongly influenced by the characteristics of the polymer matrix and the CNTs chosen, as well as by the process parameters.

A versatile, latex-based concept for efficiently dispersing Single Wall Carbon Nanotubes (SWNTs) in a highly viscous polystyrene matrix is described Crucial for the entire process is the ultrasound-driven exfoliation of the as produced SWNT bundles in water. The energy input must be sufficiently high to achieve the desired individualization, but at this stage sonication should be stopped to avoid damaging the SWNTs, which would reduce both electrical conductivity and strength. With UV/VIS spectroscopy the unraveling of the NT bundles can easily be monitored, a process which was simultaneously visualized with cryo-TEM and SEM images. These images were in agreement with the collected UV/VIS data. Depending on the molar mass distribution of the PS matrix, the well-dispersed SWNTs significantly raise the Tg of the PS. A low molar mass PS fraction, probably acting as a wetting agent for the SWNTs, seems to be required for an improved PS/SWNT interaction.

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25th European Photovoltaic Solar Energy Conference and Exhibition / 5th World Conference on Photovoltaic Energy Conversion, 6-10 September 2010, Valencia, Spain; 312-318, The presence of a transparent conductive electrode such as indium tin oxide (ITO) limits the reliability and cost price of organic photovoltaic devices as it is brittle and expensive. Moreover, the relative high sheet resistance of an ITO electrode on flexible substrates limits the maximum width of a single cell. Holst Centre and ECN have developed an alternative ITO-free transparent anode, based on a solution processed high conductive and transparent PEDOT:PSS layer in combination with a printed current collecting grid. Screen printed silver grids can yield sheet resistances down to 1 Ohm/Sq with a surface coverage of only ca 5%. The efficiency of a flexible device with an active area of 4 cm2 with such a grid is much higher than a similar device based on ITO. Furthermore, as this composite anode is solution-processed, it is an important step forward towards low-cost large area processing. Moreover, initial experiments indicate that the stability of ITO-free devices is higher than the stability of standard devices based on ITO. This work will ultimately contribute toward fully printed devices, which will provide low-cost manufacturing and improved stability of organic photovoltaic modules.