J. Escarré

Micro- and nanostructuring of poly(ethylene-2,6-naphthalate) surfaces, for biomedical applications, using polymer replication techniques

Here we investigate the formation of superficial micro- and nanostructures in poly(ethylene-2,6-naphthalate) (PEN), with a view to their use in biomedical device applications, and compare its performance with a polymer commonly used for the fabrication of these devices, poly(methyl methacrylate) (PMMA). The PEN is found to replicate both micro- and nanostructures in its surface, albeit requiring more forceful replication conditions than PMMA, producing a slight increase in surface hydrophilicity. This ability to form micro/nanostructures, allied to biocompatibility and good optical transparency, suggests that PEN could be a useful material for production of, or for incorporation into, transparent devices for biomedical applications. Such devices will be able to be autoclaved, due to the polymers high temperature stability, and will be useful for applications where forceful experimental conditions are required, due to a superior chemical resistance over PMMA.

Hot Embossing of Polymer Substrates for Thin Silicon Cell Applications

The use of polymer substrates for thin film solar cells is becoming an issue of great interest, as they facilitate monolithic interconnection of the cells to produce modules and can be used in continuous roll-to-roll processes. However, to reach high efficiencies of thin film silicon cells on polymer substrates, the development of efficient light confinement strategies has to be improved. In this work, hot embossing is used to produce a suitable surface morphology on PEN substrates. Three morphologies have been studied by using three selected masters. To evaluate the quality of the embossing, the morphology of the masters and that of the stamped PEN samples have compared. Finally, the stamped polymers have been covered with thin Ag and transparent conductive oxide (TCO) layers and whole reflectance experiments have been performed to asses the efficiency of the fabricated back reflectors

Cyclically Varying Hydrogen Dilution for the Growth of Very Thin and Doped Nanocrystalline Silicon Films by Hot-Wire CVD

An important issue for the massive implementation of thin silicon technology in photovoltaic is the use of plastic substrates, which allow the use of roll-to-roll deposition systems and planar monolithic interconnection between the cells. However, the use of plastic substrates require a fully low temperature process; especially critical in the deposition of the thin amorphous (a-Si:H) or nanocrystalline (nc-Si:H) layers. Hot-Wire Chemical Vapour Deposition (HW-CVD) technique has demonstrated to be a good alternative to deposit quality thin films at low temperature. In this paper we focus our study on very thin (50 nm) n- and p-doped nc-Si:H films deposited at low substrate temperature around 100°C. We have observed that, in this low temperature deposition conditions, the promotion of an a Si:H incubation layer leads to a poor doping efficiency and poor electrical properties of the films. Hence, in addition to the optimization of the deposition conditions, we deposited doped layers by cyclically varying the hydrogen dilution (CVH) during deposition process. This CVH method promotes a layer-by-layer growth and inhibits the formation of the incubation layer. Several doped nc-Si:H layers have been deposited with and without this CVH method. The structural, electrical and optical properties of these films and advantage of CVH in improving the device quality of the thin doped layers are reported.

Characterization of UV laser ablation for microprocessing of a-Si:H thin films

Hydrogenated amorphous silicon has been widely studied last years, both from the basic research and industrial points of view, due to the important set of potential applications that this material offers, ranging from Thin Films Transistors (TFTs) to solar cells technologies. In different fabrication steps of a-Si:H based devices, laser sources have been used as appropriate tools for cutting, crystallising, contacting, patterning, etc., and more recent research lines are undertaking the problem of a-Si:H selective laser ablation for different applications. The controlled ablation of photovoltaic materials with minimum debris and small heat affected zone with low processing costs, is one of the main difficulties for the successful implementation of laser micromachining as competitive technology in this field. This work presents a detailed study of a-Si:H laser ablation in the ns regime. Ablation curves are measured and fluence thresholds are determined. Additionally, and due to the improved performance in optolectronic properties associated to the nanocrystalline silicon (nc-Si:H), some samples of this material have been also studied.