Pinning down the defects of silicon solar panels

17-12-2009

Many of the rooftops of new apartment blocks in Europe these days are covered by solar panels that aim at picking up the rays of the sun and converting them into energy. New guidelines point this as a cheap way of producing clean energy. Despite the success of solar panels, there is still room for a better energy output, and a team from Germany has used the ESRF with the aim to achieve a longer life for solar panels. In their studies, they found out that metal nano-colonies perturb the correct functioning of the silicon solar cells and they also identified their location.

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About 90% of the world solar panels are made of silicon, a cheap semiconductor material which converts around 15-22% of the energy received. Solar panels that are currently in use produce energy for at least 10-20 years. Sometimes, however, they are destroyed prematurely due to so-called “hot spots”, for which diode breakdown is a possible cause. A better understanding of the breakdown behaviour of solar cells is an important step towards keeping solar panels alive for a longer time, increasing the total energy output of solar panels. Scientists from the Fraunhofer Institute for Solar Energy Systems (Germany) and the ESRF are working on preventing the early destruction and thus making solar panels more resistant, giving them a longer life.

The team spotted where flaws occur in silicon solar cells and what kind of defects they are. They noticed that grain boundaries house iron precipitate colonies. These groups are transition metal clusters that can cause local diode breakdown, which, in the worst case, destroys the solar panel.

 
photovoltaic

The image most to the left is a part of the solar cell which shows recombination active features in dark. Except for the straight black lines running from left to right, they belong to the solar cell and are situated at the surface of the silicon wafer. The large image to the right (gray) is an SEM image of the part of the solar cell which is marked by the rectangle. The metal precipitates are the two 3D images. Credits: W. Kwapil.

In a previous study, the team observed that the breakdown behaviour of solar cells does not depend on the element of the impurity but on its concentration. Their latest results show that the breakdown seems to be induced by individual large clusters of more than 30 nanometres rather than by smaller impurity particles finely distributed along the grain boundaries.

The team used X-ray microfluorescence at the nanoimaging station of ID22 to take measurements of two breakdown sites. Thanks to a beam of 100x100 nanometres and the high flux of the beamline, researchers could detect transition metal precipitates with a diameter of the order of some ten nanometers.

The next step for the team is to identify the physical mechanism lying behind the observed breakdown behaviour. Inspired by their latest results, the researchers assume that the presence of an electrical charge near metal precipitates can increase the local electric field and induce the local pre-breakdown.

 

Reference: Kwapil, W. et al, Appl. Phys. Lett. 95, 232113 (2009).

Top image: Many houses in Europe already use photovoltaic cells as a source of energy. Credits: G. Admans.