Laser Fired Contact (LFC) technology for the production of highly efficient silicon solar cells.
Dr. Ralf Preu
Director of the division PV Production Technology and Quality Assurance Fraunhofer Institute for Solar Energy Systems ISE Freiburg, Germany
Members of the Project Team
• Dr. Ralf Preu, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, D (Team Representative)
• Dr. Jan Nekarda, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, D
• Dipl.-Phys. Martin Graf, Fraunhofer Institute for Solar Energy Systems ISE, Freiburg, D
Areas of Application
Major sectors of industry which are profiting from the innovation:
• Solar cell manufacturers
• Laser tool manufacturers
• Laser manufacturers
• Reduced manufacturing costs for existing products
• Improved quality in existing products
• Reduced processing costs
Crystalline silicon solar cells benefit strongly from the implementation of a dielectric passivation layer on the rear side. The so called PERC-cell (passivated emitter and rear contact cell) was already suggested more than 25 years ago by Andrew Blakers at the Australian University of New South Wales. Such layers, which usually consist of a 100 nm thin material stack made from aluminum oxide, silicon nitride or silicon oxide reduce electrical and optical losses of the device and hence can increase the output power by up to 5% relative. For that to happen a variety of local structures is required interconnecting the electrode through the passivation layer with the silicon wafer. Ideally the contact pattern holds a density of approximately 400 contacts / cm² which yields 100.000 contacts per standard wafer size (156x156 mm²). For more than 10 years a procedure containing photolithographic structuring, wet chemical etching and metal evaporation was used to obtain these contacts.
In 2000, Ralf Preu et al. invented the laser fired contact (LFC) technology. The electrode, realized by usually 2 μm thin layers made of evaporated aluminum, is locally irradiated and heated by means of single laser pulses. The recoil pressure subsequently drives the aluminum through the passivation layer creating an aluminum-silicon alloy underneath with very good contact properties. After recrystallization of the alloy a highly doped p-type region is formed around the contact, which is of fundamental relevance for high contact and cell quality. This process was the first simple and fast alternative to the former used elaborate and expensive multi process structuring procedure.
In order to adapt this technology for industrial mass production, in a first step the process was transferred to a galvo-scanning-system. By alloying several thousand contacts per second with this system the former process time was reduced to about 2 seconds per wafer. By 2008, the industrial circumstances for the implementation of PERC-cells changed seriously, since the implementation of a 2 μm thin evaporated electrode into mass production became more and more unlikely.
Instead the already established screen printed and fired electrode, featuring a 20-30 μm thick porous aluminum layer, turned out to be feasible also for the production of PERC-cells. Consequently the laser process had to be adjusted fundamentally. This development was successfully carried out within the project “Laser Fired Contact – Cluster” (LFCC) funded by the German Federal Ministry for the Environment, Nature Conservation and Nuclear Safety BMU. The consortium involved the Fraunhofer Institute for Solar Energy Systems ISE plus the five largest German solar cell manufacturers by this time, which all acquired licenses for the LFC patent. A key factor for success was the use of longer laser-pulses up to the μs-range to be able to penetrate through the porous screen printed layer. On the other hand pulse energies of several millijoules were necessary in order to create a sufficient amount of silicon-aluminum-alloy at the interface. The combination of both - long pulses with high energy – had become available in the last years through the commercialization of disc lasers.
The LFC process was set up at the R&D facility of the solar cell manufacturer Q-Cells in a follow up project in 2009-2010 in order to test the process under pilot line condition. As a consequence the manufacturer successfully integrated and tested this process as well as the produced solar modules extensively for more than one year. By the use of the LFC process Q-cells set several world records regarding cell and module efficiencies such as 19,5% for a large area multi-crystalline silicon solar cell and 18,5% for a standard sized module were achieved. Finally the LFC technology was implemented in a mass production line with a capacity of 200 MW in the year 2012 and approximately 1 million photovoltaic modules with Laser Fired Contacts are produced and installed meanwhile. The outstanding performance of these modules compared to standard technology has been confirmed in several field tests and the highest performance ratio of 93,3% for PV modules from multi-crystalline silicon solar cells was just recently affirmed in a test of 151 modules.