Photovoltaic cells have been known for a long time. In semiconductors such as silicon crystals sunlight creates an electric current, which can directly be fed into the power grid. The sun has enough power to supply the whole earth with electricity. But in Europe solar cells make only a vanishing small share of renewable energy sources. Solar cell production is still an expensive process. And the photovoltaic cells can only exploit about 16 percent of the energy of the sunlight.

Physicist Dr. Sylvain Nicolay at the École Polytechnique Fédérale de Lausanne (EPFL) in Neuchatel, Switzerland, is working with a different type of solar cells, the so called thin film solar cells.

Which are the advantages of thin film solar cells?
Producing them consumes less raw materials, energy and time than producing common solar cells. That makes the thin film solar cells more ecologic and economic than the thicker predecessor.

Where could they be eventually applied?
One of the typical applications for the thin film silicon solar cells is their use in the “building integrated photovoltaic”, for which a solar cell is integrated into a classic window. We will then perforate this solar cell in order to get holes that will let the light pass through the window. This will give then the transparency necessary in such a building. In parallel, the area without hole becomes a solar cell that will absorb the light in order to create electricity.

How efficient are thin solar cells today?
Unfortunately their efficiency is currently about 40 percent lower than in conventional solar cells. Only seven percent of the sunlight can be exploited. To improve the efficiency we want to change the structure of the solar cell’s glass on the top. A layer of nano sized crystals from a so called transparent conductive oxide (TCO) is deposited onto the glass. Until recently these TCO layers had to be imported from Japan which made the production of such solar cells very expensive. Now we are testing nano sized TCO layers we have developed on our own. The aim is to make them better and cheaper and thus reduce costs significantly.

How is nanotechnology used in these TCO layers?
In order to scatter the light, we use some rough features on the surface of our films which are in the order of a few hundred of nanometers. This allows us to trap the light along wavelengths between 500 nanometers and few microns. Because of the high scattering effect the absorption of light is increased in the cell. As the light beam has a longer distance to travel through the cell, it generates more electrons on its way. The pyramidal structure scatters the light beam and diffuses it crossways through the cell. The scattering depends on the size of the pyramids: optimal is a combination of both nano and micro sized structures which scatter light in a wide wavelength range. This results in highest solar energy harvesting. Normal glass does not scatter light. The nano-tuned surface can refract the light in an angle from 15 to 30 degrees.

When will thin solar cells reach the market?
Although the new layer with nano crystals is already used in industrial production, the manufacturing of solar cells is still comparatively expensive. Further research needs to be done.