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MANUFACTURINGLASERS


ablating, cutting, and even melting materials used in the production of silicon based and thin film solar cells and panels. They have been used in advanced manufacturing processes, such as semiconductors, for some time and the lessons learnt from these industries are starting to have an impact in the solar and PV manufacturing industries.


L


The issues are not as clear cut as replacing existing methods with the latest laser capabilities. Due to initial costs the industry requires continual market growth as well as more developments within the laser industry to provide the type of manufacturing tools that will assist the industry in developing the required devices that increase output at a lower cost. In 2011 the appropriate conditions may finally coalesce making this the year of the laser for PV and solar manufacturing.


Photovoltaic fabrication requires between 11-15 processing steps including deposition of metallic and semiconductor layers, passivation, annealing, scribing and placing the interconnections. For c-Si cells it is required to create zones of light and heavy phosphor doping, the later is used for connections (ohmic contacts). To produce such structures a grid of dielectric masks pattern is created, via laser ablation openings allowing the doping to take place are developed, the drive-in and diffusion steps form the desired doping gradients.


Where can lasers perform? Lasers have the capacity to enter the manufacturing process in a surprising number of ways. The solar market can be divided into mono- or polycrystalline and thin film solar cells. Laser based tools are particularly strong in finer precision manufacturing (such as in via hole drilling or thin- film scribing) and in flexible manufacturing (such as laser marking or mask repair). Lasers can also be used in more generic processes but they are often more complex or expensive than the current manufacturing technology.


In solar manufacturing lasers are essential in areas such as scribing thin film cells. In other processes, laser-based tools are seen as an attractive alternative to the current methods but not necessarily required so the issue is about the cost of the laser process. In those processes, the laser- based tool has to displace a more established and sometimes more cost-effective technology (such as wet or dry etching, or even sandblasting).


asers have the potential of offering a non contact method of scribing,


Integration of interconnects within solar cells like CIGS remains a challenge for manufacturers and in most cases a metallic grid is printed on top to complete the process. Research into the use of picosecond lasers with a correct wavelength of radiation is offering new opportunities for lasers in areas such as the selective removal (scribing) of thin films in complex structure of materials with different thermal properties.


Areas of consideration Laser edge isolation: For the achievement of high opto-electronic efficiency, differently doped front and rear sides of crystalline solar cells need to become isolated at the edges. The latter layers are electrically isolated by scribing with lasers. Laser edge isolation is alternatively used compared to traditional processes such as edge grinding, which on the one side provides a good isolation, but on the other side has higher wafer breakage rates.


Grooving (Laser grooved buried contact): In the LGBC concept, the front-side contact of the solar cell is positioned in prescribed laser grooves. Shadowing effects are reduced.


Laser Drilling for Back-Contact Cell Design: Employing the EWT (emitter wrap-through) and the MWT (metal wrap-through) concept, electrical contacts are being transferred from the front side towards the rear side to reduce shadowing effects. This is achieved by drilling holes of different sizes using lasers.


Laser Structuring (Contact Structuring, Surface Structuring): The efficiency of solar cells is considerably influenced by processes applied for structuring nitride layers deposited onto the wafer,


Trumpf’s True Micro 5000 aimed towards PV maufacturing


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www.solar-pv-management.com Issue I 2011


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