Raphaël Charvet

The Electronic Role of the TiO2 Light-Scattering Layer in Dye-Sensitized Solar Cells

Dye-sensitized solar cells have been fabricated with different kinds of TiO2 films including SiO2 coated and bare TiO2 microparticles as light-scattering layer (LSL). Results show that the LSL barely affects the cells dark current, whereas under illumination, it made a significant contribution to the total photocurrent. Photo-voltage decay measurements performed under bias illumination show the density of electronic states (DOS) of the LSL to be two times smaller than that of a transparent nanoparticle layer (TNL). Strikingly, DSCs fabricated from a 4.5μm thick LSL alone showed a conversion efficiency of 5% despite of being pale pink in color. This impressive performance is attributed to light containment in the LSL and a low density of defect electronic states.

Femtosecond laser assisted 3-dimensional freeform fabrication of metal microstructures in fused silica

Femtosecond laser exposure of fused silica combined with chemical etching has opened up new opportunities for three-dimensional freeform processing of micro-structures that can form complex micro-devices of silica, integrating optical, mechanical and/or fluidic functionalities. Here, we demontrate an expansion of this process with an additional fabrication step that enables the integration of three-dimensional embedded metallic structures out of useful engineering metals such as silver, gold, copper as well as some of their alloys. This additional step is an adaptation of the pressure infiltration for the insertion of high conductivity, high melting point metals and alloys into topologically complex, femtosecond laser-machined cavities in fused silica. This produces truly 3-dimensional microstructures, including microcoils and needles, within the bulk of glass substrates. Combining this added capability with the existing possibilities of femtosecond laser micromachining (i.e. direct written waveguides, microchannels, resonators, etc.) opens up a host of potential applications for the contactless fabrication of highly integrated monolithic devices that include conductive element of all kind. We present preliminary results from this new fabrication process, including prototype devices that incorporate 3D electrodes with aspect ratios of 1:100 and a feature size resolution down to 2μm. We demonstrate the generation of high electric field gradients (of the order of 1013 Vm-2) in these devices due to the 3-dimensional topology of fabricated microstructures.