AVS 62nd International Symposium & Exhibition
    Energy Frontiers Focus Topic Tuesday Sessions
       Session EN+AS+EM+SE+SS-TuM

Paper EN+AS+EM+SE+SS-TuM2
Optical and Surface Properties of Semiconductor Nanowires for Solar Fuels

Tuesday, October 20, 2015, 8:20 am, Room 211B

Session: Photocatalysis
Presenter: Eleonora Frau, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
Authors: E. Frau, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
J. Vukajlovic, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
A. Dalmau-Mallorqui, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
A. Fonctuberta i Morral, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
E. Alarcon Llado, Ecole Polytechnique Fédérale de Lausanne (EPFL), Switzerland
Correspondent: Click to Email
Semiconductor nanowires (NWs) are filamentary crystals with new properties from their bulk counterparts. Their large versatility makes them excellent candidates as building blocks for contributing to solving the energy problem in the near future. In this work, we will assess two main properties of semiconductor NWs that have an impact to solar energy conversion.

First, it is known that light is strongly absorbed by NW arrays since light resonances give rise to effective absorption cross-sections that are much larger than the geometrical ones. Optical resonances depend on NW geometry and dielectric environment, and can result into absorption effective diameters up to 25 times larger than the geometrical for certain wavelengths. We have used finite-difference time-domain (FDTD) electromagnetic simulations to understand and design NW-based sunlight scavengers. For instance, a GaAs NW array that is only covering 3% of the surface can generate more photocurrent than a planar film, considering a 30% reflectivity (see figure1). Also that thanks to optical resonances, an indirect-bandgap material such as Si is capable of absorbing most of the light within a 2um long NW array that only covers 7% of the device surface.

On the other hand, it is also known that surface states and traps detriment device performances. However, in case where solar energy is directly converted into fuel (such as hydrogen) in a photoelectrochemical (PEC) cell, the large surface-to-volume ratio of NW forests is an important asset. Since the electrochemical reactions happen at the semiconductor surface, NWs enable the use of low-cost catalysts (e.g. MoSx) even though they exhibit lower performances than noble metals (e.g. Pt). In order to assess the effects of nanostructuring photo-electrodes for solar fuel generation, we have studied photo-cathodes based on Silicon nanopillar structures. The photo-cathodes were fabricated by using a top-down approach and their diameters range from ~200 to 900nm and lengths ~2um. We observe that reducing the size of the nanostructure, increases the overpotential, and thus the overall efficiency (see figure 2). By coating the surface with thin TiO2 layers, the performance is improved in terms of overpotential and fill factor. We explain these findings by using an electrico-kinetic model of the semiconductor-water junction. We find that the TiO2 layers actually act as a hole blocking layer, preventing recombination.