Non-steady state operation of polymer/TiO2 photovoltaic devices

We present data on the initial period of operation of Gilch-route NMH-PPV/TiO2 composite solar cells (CSCs) which show that during this period the CSCs operate in a non-steady state regime. The behavior is complex and may include a gradual rise of the open circuit voltage (V-oc) and of the short-circuit current density (J(sc)) with time, a passage through a maximum of either or both parameters, and even a sign reversal. The mechanisms most probably contributing to the transient processes are: i) diffusion driven redistribution of charges resulting in the build up of a quasi steady state charge density profile across the device; ii) photo-doping resulting in a relatively slow increase of the average charge carrier concentration and consequently of the conductivity of the device. The latter is responsible for a strong decrease in V-oc, and is evidenced by the significant increase in dark current after device illumination

Nanocomposite titanium dioxide/polymer photovoltaic cells: effects of TiO2 microstructure, time and illumination power.

Nanocomposite titanium dioxide/polymer photovoltaic cells have been fabricated using poly[2-(2-ethylhexyloxy)-5-methoxy-1,4-phenylenevinylene] (MEHPPV). Two different types of titanium dioxide were used, one synthesized using a sol-gel method, the other was a commercial paste. The crystal structure, porosity and absorption spectra of the titanium dioxide layers were measured, and the titanium dioxide synthesized using the sol-gel method had a much lower level of anatase. The photovoltaic properties of the ITO/TiO2/MEHPPV/Au cells, which were similar for both types of TiO2, were measured as a function of illumination power and compared with equivalent circuit models. A simple equivalent circuit model incorporating a diode, two resistances and a light induced current was inconsistent with the illumination - dependent data and was improved by adding an illumination dependent shunt resistance. A very long lived, photo-induced increase in dark current was observed, which could not be explained by a polymer degradation mechanism or an increase in temperature under illumination, but was more likely to be due to trapped charge

Aluminium oxide barrier films on polymeric web and their conversion for packaging applications

In recent years, inorganic transparent barrier layers such as aluminium oxide or silicon oxide deposited onto polymer films have emerged as an attractive alternative to polymer based transparent barrier layers for flexible food packaging materials. For this application, barrier properties against water vapour and oxygen are critical. Aluminium oxide coatings can provide good barrier levels at thicknesses in the nanometre range, compared to several micrometres for polymer-based barrier layers. These ceramic barrier coatings are now being produced on a large scale using industrial high speed vacuum deposition techniques, here, reactive evaporation on a 'boat-type' roll-to-roll metalliser. For the thin barrier layer to be useful in its final packaging application, it needs to be protected. This can be either via lamination or via an additional topcoat. This study reports on acrylate topcoats, but also undercoats, on aluminium oxide coated biaxially oriented polypropylene films. The effect of the acrylate layer on barrier levels and surface topography and roughness was investigated. The acrylate was found to smooth the substrate surface and improve barrier properties. Furthermore, the activation energy for water vapour and oxygen permeation was determined in order to investigate barrier mechanisms. The oxide coated film was, additionally, converted via adhesive lamination, which also provided improvement in barrier levels. © 2013 Elsevier B.V. All rights reserved

High Performance PbS Quantum Dot/Graphene Hybrid Solar Cell with Efficient Charge Extraction.

Hybrid colloidal quantum dot (CQD) solar cells are fabricated from multilayer stacks of lead sulfide (PbS) CQD and single layer graphene (SG). The inclusion of graphene interlayers is shown to increase power conversion efficiency by 9.18%. It is shown that the inclusion of conductive graphene enhances charge extraction in devices. Photoluminescence shows that graphene quenches emission from the quantum dot suggesting spontaneous charge transfer to graphene. CQD photodetectors exhibit increased photoresponse and improved transport properties. We propose that the CQD/SG hybrid structure is a route to make CQD thin films with improved charge extraction, therefore resulting in improved solar cell efficiency

Exceptional Performance of Room Temperature Sputtered Flexible Thermoelectric Thin Film Using High Target Utilisation Sputtering Technique

The High Target Utilisation Sputtering technique (HiTUS) is of interest for industrial processes, including in roll-to-roll manufacturing. This study marks the first application of HiTUS to thermoelectric materials, exemplified by bismuth telluride. The HiTUS technique separates the sputtering power into the plasma power and the target power, with additional kinetic energy in the sputtering particles from the applied electrical field, thus enabling a much wider sputter parameter space to modify the film performance. This study investigates how plasma power, target power, and substrate bias in HiTUS intricately influence crystal orientation/size, elemental composition, surface morphology, and other film properties. These factors subsequently affect carrier density/mobility, and consequently the thermoelectric performance of the bismuth telluride film. These deposited films reach a power factor of 6.5 × 10−4 W m−1 K−2 with a figure of merit ≈0.14 at room temperature, the highest value for room-temperature sputtered un-doped bismuth telluride. Subsequent post-deposition annealing significantly enhances the crystallinity of the film (highly polycrystalline), further improving the power factor to 23.5 × 10−4 W m−1 K-2, with a figure of merit ≈0.45 at room temperature. The excellent performance of the HiTUS fabricated thermoelectric film opens opportunities for the large-area manufacture of thin-film thermoelectric materials and devices

In-situ AFM study of near-surface crystallization in PET and PEN

The surface crystallization behaviour of PET (poly(ethylene terephthalate)) and PEN (poly(ethylene 2,6-naphthalate)) spin-coated thin films was compared by means of AFM (atomic force microscopy) with an in-situ heating stage. As the films were heated up stepwise, characteristic surface crystals appeared at a crystallization temperature (Tc) in the near-surface region which is about 15 ºC under the bulk Tc, and were replaced by bulk crystals when the temperature was increased to the bulk Tc. In the case of films whose thickness is less than 70 nm (PET) and 60 nm (PEN), significant increases in the bulk Tc were observed. SFM (scanning force microscopy) force-distance curve measurements showed that the glass transition temperature (Tg) of the near-surface region of PET and PEN were 22.0 and 26.6 ºC below their bulk Tg (obtained by DSC). After the onset of surface crystallization, edge-on and flat-on crystals appeared at the free surface of PET and PEN thin films, whose morphologies are very different to those of the bulk crystals. Although the same general behaviour was observed for both polyesters, there are significant differences both the influence of the surface and substrate on the transition temperatures, and in morphology of the surface crystals. These phenomena are discussed in terms of the differences in the mobility of polymer chains near the surface

In-situ AFM study of near-surface crystallization in PET and PEN

The surface crystallization behaviour of PET (poly(ethylene terephthalate)) and PEN (poly(ethylene 2,6-naphthalate)) spin-coated thin films was compared by means of AFM (atomic force microscopy) with an in-situ heating stage. As the films were heated up stepwise, characteristic surface crystals appeared at a crystallization temperature (Tc) in the near-surface region which is about 15 ºC under the bulk Tc, and were replaced by bulk crystals when the temperature was increased to the bulk Tc. In the case of films whose thickness is less than 70 nm (PET) and 60 nm (PEN), significant increases in the bulk Tc were observed. SFM (scanning force microscopy) force-distance curve measurements showed that the glass transition temperature (Tg) of the near-surface region of PET and PEN were 22.0 and 26.6 ºC below their bulk Tg (obtained by DSC). After the onset of surface crystallization, edge-on and flat-on crystals appeared at the free surface of PET and PEN thin films, whose morphologies are very different to those of the bulk crystals. Although the same general behaviour was observed for both polyesters, there are significant differences both the influence of the surface and substrate on the transition temperatures, and in morphology of the surface crystals. These phenomena are discussed in terms of the differences in the mobility of polymer chains near the surface

Mesoscale modelling of processing rubber-toughened acrylic polymers

The use of modelling in reproducing the microstructure of rubber-toughened acrylic polymers has been examined. A modelling procedure has been carried out in three stages. In the first stage, a rheological constitutive equation for rubber-toughened poly (methyl methacrylate) (RTPMMA) has been developed. In the second stage, the flow of RTPMMA melt has been modelled simulating common industrial polymer processing techniques such as extrusion and injection moulding. For the final stage, a mesoscale model has been built in order to reproduce the RTPMMA microstructure. Comparison with experimental observations has shown that the code has successfully predicted that rubber particle elongation is much more pronounced during injection moulding than during extrusion. It has been shown that particle elongation is greater near the walls of the mould, whereas the particles are more or less spherical in the central region of the mould. The 'shear' region (where the particles deform at an angle of approximately ±45°) has been reproduced. Particle distribution across the width and length of the mould has been found to be fairly uniform. Such a mesoscale model not only provides a better explanation of experimental observations for toughened polymers, but it can also provide the polymer industry with the ability to make useful suggestions for possible routes to improved materials for a wide range of multi-phase systems

The solvation of poly(vinyl alcohol)

The possible incorporation of water molecules within the crystal structure of poly(vinyl alcohol) is discussed. Modelling of the crystal structure suggested that water could be incorporated without severe disruption, and the effect on the X-ray powder diffraction trace was simulated. The effect of variation in tacticity is discussed in terms of the nature of the hydrogen bonding. Simulated traces are compared with experimental data from atactic samples in which a change in the diffraction peak intensities is observed for samples crystallised with water present. This is compared with samples produced from nonaqueous solutions

Quantitative ATR-IR analysis of anisotropic polymer films: Surface structure of commercial PET

A commercial PET film has been characterized using variable-angle ATR-IR spectroscopy to create a depth profile of the surface. The anisotropic optical constants (n, k) of the film were extracted from the reflectivity spectra assuming that the depth region probed at each angle of incidence is homogeneous. The refractive index n, absorption index k, and absorption coefficient K spectra of the film were then employed to calculate the trans / gauche content of PET and the extent of trans conformer orientation as a function of depth of penetration. The trans-PET content of the commercial film we analyzed changes from 78% at a depth of penetration of 0.4 μm to 61% at a depth of penetration of 1.1 μm. Results of the orientational analysis of the film are presented, and problems specific to ATR-IR spectroscopy in such analysis are also discussed. © 2005 American Chemical Society