By: Henrikki Pantsar1, Tim Lauterborn1, Annerose Knorz2, Hans Herfurth1, Stefan Heinemann1
1 Fraunhofer USA, Center for Laser Technlogy,
46025 Port Street, Plymouth, MI 48170
2Fraunhofer Institute for Solar Energy Systems
Heidenhofstrasse 2, 79110 Freiburg, Germany
The greatest challenge for photovoltaic solar cells is to reduce the price per watt for terrestrial applications. In silicon panel production this can be accomplished by economies of scale, developing automation, improving cell efficiency and reducing material costs either by using thinner wafers and/or lower-quality materials. Larger, thinner substrates enable processing of more active area per step and reduce the consumption of material per cell. Back contact solar cell concepts such as the Emitter Wrap Through (EWT) adapt well to these requirements. The respective challenge in manufacturing these cells is the large number of through holes that are needed per cell. One cell can comprise up to 25,000 holes.
Due to the required cycle times drilling techniques such as percussion drilling are not fast enough for production. In order to reach highest possible drilling rates high rate drilling techniques, such as using a scanner with pulse synchronization are needed. This processing strategy allows high beam duty cycles and thus faster processing times. The drilling process has to be optimized to reach best material removal efficiency. In this aspect MOPA fiber lasers have shown to be efficient tools due to their property of allowing independent adjustment of pulse parameter such as pulse width and frequency.
The Center for Laser Technology of Fraunhofer USA, Inc. in Plymouth, MI has developed laser-based techniques for texturing and drilling Si-wafers at extremely high rates using MOPA fiber lasers. Contrary to q-switched lasers, the pulse parameters; pulse energy, pulse width and pulse frequency can be adjusted independently opening a parameter space in which the process can be optimized for specific process or material. Drilling efficiencies of 150 holes per Joule have been demonstrated in 200 μm silicon wafers using process-optimized pulse parameters. Using a 5 W average power a number of approximately 800 holes per second were drilled. The results have been published in the Proceedings of the ICALEO 2009 conference held in Orlando, Fl. Since then, the process has been demonstrated using 20 W laser power reaching more than 3,100 holes per second. The high drilling rate is based on pulse parameter optimization and a unique FPGA based pulse synchronization controller. Further improvement in drilling rates is yet expected with manipulating the pulse shape.
The above brief overview was extracted from its original abstract and paper presented at The International Congress on Applications of Lasers & Electro-Optics (ICALEO) in Orlando, FL. To order a copy of the complete proceedings from this conference click here