How can we harvest the energy of the sun at a better quality and at a cheaper cost? To find out, Anna Fontcuberta and her team in the STI Laboratory of Semiconductor Materials (LMSC) at EPFL are working on novel solutions to produce the solar cells of tomorrow.

The research of Fontcuberta, a professor in the STI LSMCL, focuses on new ways to engineer semiconducting structures, mainly with the use of nanotechnologies.

Semiconductors, thanks to their physical properties, have increased the functionality of many objects in our daily lives (microwave ovens, cars, DVD player or computers e.g.) and at the same time our quality of life.

The LSMC works on new geometries using nanowires. These are needle-like crystals of a diameter between 20 and 100 nm and several microns long.

The objective is to increase their functionality by understanding their properties and finding new ways to fabricate them. Among the many applications using nanowires is one of a higher interest to Fontcuberta and her team: solar cells.

Because of the world's urgent need to harvest greener energies, nanowire solar cells have a huge societal and industrial potential for the future.

"We are working on nanowire solar cells using GaAs in their core, a high conducting material which absorbs light at the ideal range with respect to the solar spectrum", explains Fontcuberta.

For example, in the 1990's, GaAs solar cells took over from silicon devices in photovoltaic arrays for satellite applications, or power the robots that are exploring the surface of Mars.

In the LMSC, gallium and arsenide atoms are engineered (or "tricked") in a way that they organise themselves to form wires rather than horizontal layer on layer structures (which they tend to do naturally).

This 3 dimensional geometry is a novelty as it enables the trapping of more light than planar structures, such as silicon solar devices, and with less material.

Each vertical nanowire becomes a device that produces current. The combination of the nanowires' small scale (one micron) and revolutionary 3D geometry (a little bit like hair standing up), enables a significant decrease of the solar cell's cost per watt - compared to commonly used solar cells.

Apart from enhancing the light absorbtion, Fontcuberta and her team are working on ways to optimise it. For example, they combine the nanowire's GaAs core with other nanoscale materials in both axial and radial directions. As an example, InAs quantum dots (or "islands") on the nanowire play the role of stimulants for a better absorbtion of the light.

Nanowire solar cells represent the 3rd generation of solar cells because they are made on a very small scale and can be combined in many different ways, enabling the extraction of more energy at a lower cost.

Despite encouraging applications, "It might take ten more years before nanowires can be found on the market", explains Fontcuberta.

This is an objective, which the LMSC will pursue at EPFL.