Comment

2 June 2009

Lasers can boost solar cell manufacturing …

Almost everyone agrees electricity generated from solar power has an incredible future.

Just look at the benefits: It is renewable and available in unlimited quantities, and it does not produce any gases detrimental to the climate.

Its only drawback right now is the price. Electric power currently produced by solar cells in northern Europe remains subsidized so it can compete against the household electricity generated by traditional power plants.

But by adding a laser into the picture, there may be a way to optimize the manufacturing costs and efficiency of solar cells.

Cell phones, computers, MP3 players, kitchen stoves, and irons all have one thing in common: They need electricity. And in the future, more and more drivers will use electricity to fuel their cars. If the latest forecast from the World Energy Council is correct, global electricity requirements will double in the next 40 years. At the same time, prices for the dwindling resources of petroleum and natural gas are climbing.

“Rising energy prices are making alternative energy sources increasingly cost-effective. Sometime in the coming years, renewable energy sources, such as solar energy, will be competitive, even without subsidization,” said Dr. Arnold Gillner, head of the microtechnology department at the Fraunhofer Institute for Laser Technology in Aachen, Germany.

“Experts predict that grid parity will be achieved in a few years,” Gillner said. “This means that the costs and opportunities in the grid will be equal for solar electricity and conventionally generated household electricity.”

Gillner is developing technologies now that will allow faster, better, and cheaper production of solar cells in the future. “Lasers work quickly, precisely, and without contact. In other words, they are an ideal tool for manufacturing fragile solar cells. In fact, lasers are already being used in production today, but there is still considerable room for process optimization.”

In addition to gradually improving the manufacturing technology, physicists and engineers in Aachen are working with solar cell developers on new engineering and design alternatives.

Researchers can now drill holes into silicon cells at a rate of more than 3,000 holes within one second. Because it is not possible to move the laser source at this speed, the experts have developed optimized manufacturing systems that guide and focus the light beam at the required points.

“We are currently experimenting with various laser sources and optical systems,” Gillner said. “Our goal is to increase the performance to 10,000 holes a second. This is the speed that must be reached in order to drill 10,000 to 20,000 holes into a wafer within the cycle time of the production machines.”

The tiny holes in the wafer, with their diameter only 50 micrometers, open up possibilities for the solar cell developers.

“Previously, the electrical contacts were arranged on the top of the cells. The holes make it possible to move the contacts to the back, with the advantage that the electrodes, which currently act as a dark grid to absorb light, disappear. And so the energy yield increases. The goal is a degree of efficiency of 20% in industrially-produced emitter wrap-through (EWT) cells, with a yield of one-third more than classic silicon cells,” Gillner said. The design principle itself remains unchanged: In the semi-conductor layer, light particles, or photons, produce negative electrons and positive holes, each of which then wanders to the oppositely poled electrodes. The contacts for anodes and cathodes in the EWT cells are all on the back, there is no shading caused by the electrodes, and the degree of efficiency increases. With this technique, it may one day be possible to use unpurified “dirty” silicon to manufacture solar cells that have poorer electrical properties, but that are cheaper.

For related information, go to www.isa.org/manufacturing_automation.