Cutting-edge solar cells

There is certainly no shortage of energy on earth. In just half an hour, the sun sends more energy to earth than all the people living on the planet consume in an entire year. Despite this fact, researchers aim to use every square millimeter of solar cell area as they strive to transform valuable solar radiation into climate neutral electricity.
 
And they do so with good reason. The production costs of solar cells must be reduced further if solar electricity is to compete with traditional electricity generation techniques such as burning coal or splitting atoms. “Until now, industry has mainly cut production costs by automating manufacturing processes,” says Ingo Köhler, who heads a unit at Merck that researches innovative structuring concepts for the production of solar cells. But, according to Köhler, this approach is now reaching its limits, which is why researchers are now intensifying their efforts to increase the solar cells’ electricity yield.

At the end of 2010, the market’s leading crystalline silicon solar cells, which are made of polycrystalline silicon, had an efficiency of 16 percent. The equivalent figure for those made of monocrystalline silicon was 18 percent. That leaves a lot of unused potential. The aim must therefore be to significantly increase the amount of incident sunlight that becomes usable electric energy within the cell. In this context, scientists are delighted when they achieve even minor progress.

Researchers at Merck therefore began to look for ways in which they could increase the electricity-generating area of a solar cell without increasing the cell's size. For example, they considered moving structures that cast shadows on the front side of the cell—which faces the sun—to the cell’s rear. And they soon found one such structure: the channel that electrically isolates the front and rear sides of the cell from one another.

Some background information: A solar cell fundamentally consists of a light-absorbing semiconductor wafer that converts sunlight into electricity. To enable the generated electricity to flow out of the wafer, the two sides of the device are covered with differently charged layers. One side is positively biased, the other is negatively biased. When a wafer leaves a coating facility, however, the two sides are still short circuited. To enable the cell to work at all, the sides first have to be electrically isolated from each other.

This isolation is achieved using a channel. Until now, such channels have been burnt into the wafer’s surface using a powerful laser. The result is a non-conducting barrier between the layers on the wafer’s front and rear sides. However, around 1.2 percent of the usable area is eliminated in the process.

Although this loss may appear minor at first glance, when you calculate potential savings on the basis of 100 megawatts of solar output, the financial gain is more than $500,000.