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other areas of manufacturing as well as integrating lasers into the process.

A key challenge for wafer based solar cells is the expected continual thinning of wafers from the current 180-200 µm. Combine this with the fragility of thinner wafers and manufacturers will have to be able to handle and transfer them to the laser processing area with the required accuracy and also with no breakage or micro cracks. Today there is a breakage loss rate limit expectation for uptime fulfillment of < 0.5 % and this will be expected to improve. Technical challenges arise from two areas for laser manufacturers. Beam quality and the intensity profile of the lasers. Special beam shaping optics are used to transform laser energy distribution to a profile more suited for the specific manufacturing need.

Another challenge during laser processing is prevention of thermal damage on the material caused by the duration of laser pulses. Depending on the application the correct wavelength and pulse width needs to be applied in order to avoid damage. Once the right parameters are found there remains the challenge of removing ablated particles during and after laser processing needs to be solved to prevent contamination.

Processes such as laser drilling, the large-scale removal of silicon oxide, and the removal of metal materials are critical, as many particles are created that need be removed with the help of sophisticated exhaust systems. In thin film photovoltaics, toxic absorber materials such as Cadmium Telluride, which are ablated by laser technology, also need be removed thoroughly. Companies often work with specialist exhaust companies to resolve these issues. The patterning of CIG-CIGS thin-film cells can develop issues with melting. Cracking and exfoliation of the films can be a problem so to avoid such issues ultra-short (picosecond pulses) are used. This in particular true for processing molybdenum which has a propensity to crack and melt under continued or extended laser pulses.

Unlike some etch processes, most of these laser applications are necessarily serial; each scribe line or drilled hole is created in sequence. Laser engineers have to match PV plant throughputs in order not to introduce a bottleneck in production. Achieving the right speed has become a major challenge. The solution has been to optimise the laser source or develop rapid ‘galvo’ beam delivery systems to meet the specific needs of an application. This is becoming commercially worthwhile as PV manufacturing matures into a

business which has the scale to justify such investment in R&D.

Another technical challenge requires achieving the correct resolution demanded by a particular process. This can involve resorting to more expensive frequency doubled (green) or tripled (UV) solid-state lasers.

Laser companies also develop specific ‘PV’ versions of solid state lasers. These provide sufficient power at the right wavelength for thin film patterning and deliver extraordinary pulse-to-pulse stability at the necessary high Q-switch frequencies of operation. So there is a focus on continuous improvement of sources to better meet the demands of the PV engineers. A great deal of effort is going into optimising current lasers to specific manufacturing needs.

Presently, lasers still have a way to go in order to consolidate their position in the thin-film solar panel business. Throughput at a much lower cost is probably the best way to resume the main challenge in order to incorporate lasers in production lines in a longer extent.

For isolation and connection scribes high quality beam lasers with precise beam conformation and different wavelengths and increasing pulse frequencies must be made available in compact, faster, more flexible and costly effective solutions. For edge deletion, semitransparent layer manufacturing, glass processing the focus must be on process capacity and probably approaches based on higher energy per shot can provide the desired increases in productivity. The flexibility of lasers performing these tasks (semitransparent layers or glass processing) can be highly suited to BIPV demands where flexibility in module sizes, transparencies and geometries are very high. Although the technical solution is known, as the


some etch processes, most of

these laser applications are

necessarily serial; each scribe line or drilled hole is

created in sequence

21 Issue I 2011

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