Contactless and material-selective laser processes are of critical importance for the production of sensitive electronic components such as solar cells. Lasers can achieve a decisive step to greater efficiencies and lower manufacturing costs for crystalline and thin-film photovoltaics. With this goal in mind, the Fraunhofer Institute for Laser Technology ILT is developing industrial-scale processes, e.g. for the high-resolution structuring of thin layers, along with corresponding mechanical components to raise production throughput rates. Selecting the most suitable laser to optimize such processes plays a predominant role in these research activities.
Competitive process engineering for the production of electronic components calls for high speeds, small structure sizes, and large-scale applicability. In organic electronics, structured printing currently allows feature sizes as small as 10 micrometers at high speeds. A significantly higher resolution and productivity can be obtained with structuring by laser. Especially important here is choosing a laser that is ideally suited to the requirements of the particular application. “Most companies in the solar industry don’t know how much time and costs they can save by using the right laser for manufacturing thin-film solar modules or crystalline solar cells,” explains Dr. Malte Schulz-Ruthenberg, project manager at Fraunhofer ILT. “For example, completely different beam guiding and shaping approaches are required for high-speed drilling of back-contact solar cells than those used to create complex structures on electronic circuits at high processing rates .”
Consequently, researchers at Fraunhofer ILT are investigating different approaches to improve process efficiency in a range of different projects. One of these projects is exploring the possibility of multiple beam splitting using diffractive optical elements, which can dramatically increase production throughput rates. A polygon scanner is also being developed, which enables two-dimensional structuring of thin layers at extremely high speeds of several hundred meters a second. Fraunhofer ILT will be presenting the demonstrator of this polygon scanner to a professional audience at the joint Fraunhofer booth in Hall 3/G22 at the European Photovoltaic Solar Energy Conference and Exhibition, EU PVSEC, in Frankfurt from September 24 – 28, 2012. Combined with modern beam sources working at high repetition rates, the polygon scanner can significantly increase production throughput. It can be used for processing both thin-film solar modules and crystalline solar cells.
Series connection for rigid and flexible solar modules
In addition to machine technology, one focus of research at Fraunhofer ILT is on further developing structuring processes for thin-film solar modules. These modules require small strips of cells connected in series in order to reduce current densities, which in turn reduces electrical losses within the module. What many companies are still carrying out by means of mechanical scribing can be done quicker and more cleanly by means of laser radiation. The challenge ILT researchers now face is to do this without impairing the functionality of the layers of conducting, semi-conducting, or insulating materials, which have thicknesses ranging from a few nanometers to a few micrometers. If, for example, residues of ablated material or thermal damage to neighboring areas occur during processing, the extreme thinness of these layers can lead to their degradation and cause the entire solar module not to work. The laser structuring processes therefore have to be adapted to the different characteristics of each individual layer. Ultrashort pulse lasers can be used for physical processes that are not feasible at longer pulse durations. This opens up new process windows, and paves the way towards new industrial-scale processes.
In the FlexLas project, funded by the European Commission and the state government of North Rhine-Westphalia, a laser structuring technique for organic solar cells on flexible film substrates is being developed at Fraunhofer ILT. This type of solar module is considered an economical, forward-looking product in the field of solar energy. It might well be possible one day to make textiles or handbags with flexible solar cells, which could be used to charge a cell phone. The laser structuring processes being developed in Aachen can also be applied to other products with multiple-layer systems, such as smart phone screens and flat lighting elements.
Production technology for crystalline solar cells
Scientists in research and development are currently working on a variety of laser processes for manufacturing crystalline solar cells. For example, a technique developed at Fraunhofer ILT allows you to drill upward of 10,000 holes a second into silicon wafers. Thin passivation layers can be removed with scarcely any effect on electrical functionality. And thanks to innovative beam-shaping optics, laser-based module manufacturing drives soldering times down to less than a second.
Using the right beam source here can significantly improve the production process. ILT researchers are currently testing a variety of different beam sources in order to fulfill the widest possible range of parameters relating to pulse duration, wavelength, process-adapted intensity distribution, etc. while minimizing laser-related damage.
The researchers in Aachen are also busy devising innovative approaches to producing high-efficiency cells. To create a texture that reduces reflection and maximizes use of the sun’s radiation, an ablation-free laser process is combined with a subsequent etching stage. This reduces laser-related material damage to a minimum and maximizes process speed, contributing in turn to a significant reduction in production costs.
For further information
Dr. Malte Schulz-Ruthenberg
Group Micro and Nanostructuring
Phone +49 241 8906-604
Fraunhofer Institute for Laser Technology ILT, Aachen
Dr. Alexander Olowinsky
Head of the Group Micro Joining
Phone +49 241 8906-491
Fraunhofer Institute for Laser Technology ILT, Aachen