Highest efficiency Si solar cells realised with n-Si — Prof. Weber, Fraunhofer ISE

Prof. Eicke R. Weber, director, Fraunhofer ISE, Germany, will be delivering the opening keynote at the inauguration of the SOLARCON India 2009 and India Semiconductor Association (ISA) PV Conclave next Monday in Hyderabad.

Thanks to ISA’s help, I was able to get into a conversation with him, where he elaborated on the capabilities of n-type solar cells, how the solar PV industry is dealing with recession, and well, lessons to learn for India.

Why n-type is better than p-type solar cells?
Researchers at Fraunhofer Institute for Solar Energy Systems ISE have developed new methods and cell concepts for the manufacture of n-type silicon solar cells. What are the capabilities of n-type silicon solar cells that makes them better than p-type?

Prof. Weber said: “The main challenge in the operation of a solar cell is to collect the minority carriers created by sun illumination. Silicon solar cells are commonly made from p-type silicon, mainly because the carrier mobility of the minority carriers, electrons for p-Si, is higher than that of holes, the minority carriers in n-type Si.

“However, if contamination plays a role, which is especially the case for highest-efficiency cells, n-type silicon has distinct advantages because metal atoms like to form electrically active pairs with acceptor dopants that degrade performance.

In n-type silicon are donor dopants instead of the acceptor dopants in p-Si, so that carrier lifetime is better than in p-type Si. This can more than compensate the disadvantage of lower minority carrier mobility. Therefore, highest efficiency Si solar cells are usually realised with n-Si.”

Fraunhofer CSE has also opened a new PV module lab for research, development, testing and evaluation of new materials and production processes for photovoltaic solar modules with the aim to increase module energy yield, reduce cost, and extend module durability.

There is a Fraunhofer CSE in Boston and a Fraunhofer ISE in Freiburg. At the CSE in Boston, it is just starting with the process to establish such a new testing facility. However, this facility is already in place at the Fraunhofer ISE in Freiburg.

How can solar makers turn around?
Solar manufacturers are said to be already losing money this year and the capacity utilization is 27.9 percent. Also, the days of inventory are currently 122, up from 71 days in 2008. If they continue to add new capacity, things will only worsen, exasperating the recession, it can get difficult for them to turn this around.

Prof. Weber said that right now, there is already again a shortage in the module market.

“In Germany, we will see in 2010 at least a 10 percent decrease of the feed-in rate for new systems, so that after the substantial price drops in the first half of 2009 the second half of 2009 offered an unusually lucrative opportunity for the installation of PV systems.

“It remains to be seen whether this market pull will be carried into 2010. On the two-three year time scale the key question will be when the US market really starts, at best by offering in selected staters a lucrative feed-in rate.

“I am convinced, if this demonstrates quick market penetration as we experienced in Germany it will quickly spread throughout the USA, generating a PV market much larger than the German market as the US enjoys many high-sunshine regions in the South.

Lessons for India
Obviously, there are lessons to learn for the Indian solar/PV industry.

As per Prof. Eicke Weber, the key issue for increasing the PV market is to create attractive options for investors.

“In grid-connected systems this can be best done by attractive feed-in rates that have to be tailored for the respective region, and should offer 8-12 percent annual return from the produced PV electricity.”

In off-grid systems, the state might offer investment or tax incentives to allow the creation of lucrative investment opportunities, he added.

Bringing down solar production cost per watt!
It was also interesting to find out what the Fraunhofer ISE, as well as the industry, is doing to bring solar production cost per watt down.

Prof. Weber pointed out that two issues are involved — lower costs and higher efficiencies, or, at best both.

“We work on increasing the efficiency of Silicon and triple junction solar cells for CPV concentrator applications, and on the use of lower-cost feedstock material and cell processes for Si technology.

“It is actually impressive to see that today we are able to produce 15-17 percent efficient Si solar cells in multicristalline technology, and 20-23 percent cells in cost-effective high efficiency processes with single crystal silicon.

“In CPV, we achieved in January of this year a 41.1 percent world record cell efficiency.”

According to him, the price drop in 2009, caused by a lack of investment money with the global financial crises, has helped a lot to accelerate the way towards grid parity of PV costs with household electricity rates in Germany.

In Bavaria, there are already whole counties that generate 10 percent of their electricity from PV.

“The price decrease is spearheaded by thin film modules, that have a considerably lower efficiency. Si modules are forced to compete with the price of thin film technologies although their price per Watt installed can stay somewhat above thin film modules because they offer larger energy harvest from a given area," he concluded.