Lasers Enable PV

Solar electricity has a future: 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: the electric power currently being
produced by solar cells in northern Europe must
be subsidized if it is to compete against the household electricity generated by
traditional power plants. Fraunhofer researchers believe laser technology can
contribute to optimising 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 cars will also be fuelled by electric power. If
the latest forecast from the World Energy Council WEC can be believed, 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,” explains 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. This means that the costs and
opportunities in the grid will be equal for solar electricity and conventionally
generated household electricity.”

Together with his team at the
Fraunhofer Institute for Laser Technology ILT in Aachen, this researcher 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, the physicists and engineers
in Aachen are working with solar cell developers
- for example, at the Fraunhofer Institute for Solar Energy Systems ISE in
Freiburg - on new engineering and design
alternatives.

Researchers have recently
demonstrated how lasers can drill holes into silicon cells at breathtaking
speed: The ILT laser system drills 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 which guide and focuses the light
beam at the required points.

“We are currently experimenting with
various laser sources and optical systems,” Gillner
explains. “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 - their
diameter is only 50 micrometers - open up undreamt-of 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 percent% in industrially-produced emitter
wrap-through (EWT) cells, with a yield of one-third more than classic silicon
cells,” Gillner explains. 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. 

Drilling holes into silicon cells is
only one of many laser applications in solar cell manufacturing. In the EU
project Solasys - Next Generation Solar Cell and
Module Laser Processing Systems - an international research team is currently
developing new technologies that will allow production to be optimized in the
future. ILT in Aachen is coordinating the six million euro
project. “We are working on new methods that make the doping of semiconductors,
the drilling and the surface structuring of silicon, the edge isolation of the
cells, and the soldering of the modules more economical,” project coordinator
Gillner explains.

For example, “selective laser
soldering” makes it possible to improve the rejection rates and quality of the
contacting, and so reduce manufacturing costs. Until now, the electrodes were
mechanically pressed onto the cells, and then heated in an oven.

“But silicon cells often break
during this process,” Gillner knows. “Breakage is a
primary cost factor in production.”

On the other hand, however, with
“selective laser soldering” the contacts are pressed on to the cells with
compressed air and then soldered with the laser. The mechanical stress
approaches zero and the temperature can be precisely regulated.  Laser technology is also helping to optimize
the manufacture of thin film solar cells. The extremely thin film packages made
of semiconducting oxide, amorphous silicon, and metal that are deposited onto
the glass panels still have a market share of only ten percent.

But as Gillner knows, “This could be higher, because thin film
solar cells can be used anywhere that non-transparent glass panels can be
mounted, for example, on house facades or sound-insulating walls. But the
degrees of efficiency are comparable low at five to eight percent, and the
production costs are comparatively high.”

The laser researchers are working to
improve these costs. Until now, the manufacturers have used mechanical methods
or solid-state lasers in the nanosecond range in order to structure the active
layers on the glass panels. In order to produce electric connections between the
semiconductor and the metal, grooves only a few micrometers wide must be
created.

“The ultrashort pulse laser is an ideal tool for ablating thin
layers: It works very precisely, does not heat the material and, working with a
pulse frequency of 80 MHz, can process a 2-by-3 meter glass panel in under two
minutes,” Gillner reports. “The technology is still
very new, and high-performance scanning systems and optical systems adapted to
the process must be developed first. In the medium term, however, this
technology will be able to reduce production
costs.”

The rise of laser technology in
solar technology is just taking off, and it still has a long way to go. “Lasers
simplify and optimize the manufacture of classic silicon and thin-film cells,
and they allow the development of new design alternatives,” Gillner continues. “And so laser technology is making an
important contribution towards allowing renewable energy sources to penetrate
further into the energy market.”