MANUFACTURINGLASERS
Laser edge deletion is alternatively used compared to traditional processes such as sandblasting, which main drawbacks are reduced product quality and higher operating costs.
Metal-wrap-through (MWT): is a new development that allows the placement of connections in the back side of the cell with the following benefits. It reduces the resistance of the cells and therefore more current is able to flow.
All processing associated with the interconnection buses are done in the backside and shading is reduced and more active area is available from this process. Up to 10% and more. To make connections with the front side of the device, lasers drill the holes and molted metal is used to connect.
UV lasers are also gaining ground for manufacturing as they offer overlap with
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like laser patterning, superficial laser structuring, and laser contact structuring. The technology of laser fired contacts (LFC) aims at the enhancement of the solar-cell efficiency by the realization of a highly efficient, dielectrically passivated back contact. Using the latter approach, shadowing losses of a standard cell can be reduced from between 10 and 15 % to between 2 and 3 %.
Laser micromachining processes being used with thin film solar cells include, e.g. laser edge deletion, laser surface structuring in process steps P1, P2 und P3, and laser marking.
Laser Structuring (Selective Ablation): Upon fabrication of thin-film solar cells, the substrate is coated with conducting and non-conducting layers. The latter layers, consisting of TCO, silicon, or other active media, and metal, reach maximum thicknesses of a few microns and can be machined with lasers of different wavelengths (IR, VIS, UV) in processing steps P1, P2, and P3. Laser edge deletion: In order to electrically isolate and seal the modules, it is required to completely ablate all layers from the edges of a thin film solar cell on glass. Therefore, lasers featuring a flat-top beam profile are utilized. The maximum machining speed amounts to 4,000 mm/s, with a typical width of isolation trenches between 10 and 100 µm.
absorption bands in c-Si and SiNxSiO2 dielectric layers. Picosecond lasers can reduce the thermal impact on the silicon base. Eliminating laser damage to solar cells is a key factor in enabling their use in manufacturing of any type of cell and one where laser manufacturers are working to improve the ability to control laser sources. Lasers can provide efficiency gains in the order of 0.5% and 1.5% as an immediate impact on device performance.
Challenges for lasers For the more obvious applications of laser manufacturing the challenges are fairly simple. Improve output and reduce costs. For other areas of manufacturing where lasers may have an impact the challenges are a mixture of financial and technological concerns. Many areas of manufacturing that lasers can impact on require current entrenched technologies to be pushed aside. In some cases this is unlikely as the laser option is currently more expensive than the current technology.
From the laser company perspective the issues are all about cost, including convincing low cost manufacturers that lasers can be more cost effective over the longer term. For laser manufacturers the challenge can be simply to make a product or process low cost enough for market entry. When considering the technical challenges, each process is, of course, unique with its own specific issues. In general, however, there are no special breakthroughs required for the better understood manufacturing processes whether it is thin-film scribing, via hole drilling or laser soldering. There are however challenges for
www.solar-pv-management.com Issue I 2011
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