The emerging mass production of solar cells,

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1 photovoltaics It was the use of lasers which made the EFG process possible: The coherent beam cuts the wafers from the octagonal silicon ingots. Photos (2): Schott Solar The emerging mass production of solar cells, paired with the efforts undertaken by the branch to reduce costs, has enabled laser technologies to gain a foothold in the solar industry. This applies equally for the manufacturing of both crystalline cells and thin film modules. Broad scope of applications Laser systems precision and speed The implementation of laser technologies permits a further optimisation of manufacturing processes and promotes the development of new cell concepts. In the thin film sector, it is in the meantime accepted practice. It is still the case that photovoltaic cells are for the most part based on thin wafers of silicon. This domination of the market is also unlikely to crumble in the near future. But if the current pace of development is to be maintained, it is imperative to bring down the cost of the silicon wafers. In the final analysis, the target is to reduce the costs per watt of output. This can be achieved above all by raising cell efficiency, but at the same time also by using thinner wafers. The present benchmark is an efficiency of more than 20 % from wafers significantly thinner than 200 μm. As crystalline silicon is a very brittle material, however, the risk of breakage increases with each reduction in wafer thickness. A variety of laser-based manufacturing methods are established in industrial production today, and research institutes are also working on innovative laser-based approaches, for example the German Fraunhofer Institute for Solar Energy Systems (ISE) in Freiburg and the German Institute for Solar Energy Research (ISFH) in Hameln/Emmerthal, which cooperates with the Hannover Laser Centre (LZH). In the past, laser technologies were used above all in the processing of solar cells. In most cases, this involved the removal of dielectric layers to enable contacts to be formed at the points uncovered. Further applications were processes for structuring of the wafer surface and for diffusion. These possibilities also remain relevant today, but have now been joined by laser-assisted edge isolation, laser soldering methods and techniques for selective doping to modify local emitter structures. Innovative contacting methods are another field. The industry is currently introducing laser drilling of the holes necessary for the manufacturing of back-contacted solar cells. When it comes to laser texturing of the cells, however,»the implementation in industry is progressing rather sluggishly,«says Aart Schoonderbeek, project manager at the Hannover Laser Centre,»because the available lasers are still too slow for cost-effective production.«off-the-shelf laser technology The laser manufacturers, in the meantime, are pressing forward with their developments. The German company Trumpf in Ditzingen is considered a world market and technology leader. It sells a whole range of micro- 114 Sun & Wind Energy 2/2008

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3 photovoltaics Quality control during manufacturing: Close-up inspection of an ASI OEM module after laser structuring. The module is orange in colour until the contacts are made on the rear. The final product is black. With an average laser power of 40 W, the TruMicro 3140 is designed for high productivity in precision machining. Its nanosecond pulses are delivered with a high pulseto-pulse stability. Photo: Trumpf processing lasers, as does its main competitor, the U.S.- German Rofin Group. With its TruMicro series, which covers average laser powers between 40 and 500 W, Trumpf has launched a new generation of products interesting for use in solar manufacturing. The TruMicro 3140, for example, is suited for both edge isolation tasks and for drilling in connection with back-contacted cell designs. In the former case, the laser is operated with a pulse duration between 15 and 80 nanoseconds (1 ns = 10-9 s, see box text) to produce a groove along the cell edge and thus to prevent electrical short-circuits between the front and rear sides. This isolation groove, typically up to 80 μm wide and between 10 and 20 μm deep, results from vaporising of the cell material. The company also has a suitable solution on offer for the thin film technology. The TruMicro 7050 system can be used to deposit the active layers onto a glass substrate. On the basis of a Q-switched disc laser, a pulse duration of the order of 1 μs is realised with an average laser power of over 500 W. The systems can also ablate the edge regions of the layer structures as protection against environmental influences and overvoltages. In edge isolation, the TruMicro achieves an ablation rate of 20 cm² per second. The lasers of the TruMark 6000 series are similarly destined for use in thin film solar technologies. With their two wavelengths 532 nm (average laser power > 6.5 W) and 1064 nm (average power > 20 W) they offer suitable specifications for the structuring of modules. In addition, Trumpf offers diode-pumped, picosecond disc lasers (TruMicro 5000) for applications where it is imperative to avoid burrs or deposits. If pulses in the nanosecond range are used, then fused material, fissuring, and delamination are typically found on the molybdenum layer along the ablated track. These phenomena can be avoided by using picosecond pulses. The TruMicro delivers peak pulse powers of up to 50 MW, at a pulse duration less than 10 ps and with a pulse repetition rate of 200 khz. The Rofin Group has also developed a new generation of Q-switch lasers through to market maturity. In this technology, an active element, the Q-switch, stands as an optical block in the resonator beam path. Lasers of this type permit a second resonator cavity to be integrated into the same laser housing. Devices of the Q- switched series offer maximum powers of up to 850 W. In edge isolation, a laser of this type achieves an ablation rate of 50 cm 2 per second. The StarDisc Q-switch disc laser, similarly a Rofin development, is reported by the manufacturer to increase productivity in the through-contacting of solar cells to three to five times the previous level. With such laser systems, the output range for disc lasers is expanded to cover powers of 60 to 3000 W. The StarDisc laser is suitable for the manufacturing of back-contacted cells by either the MWT (Metal Wrap Through) or EWT (Emitter Wrap Through) processes. 116 Sun & Wind Energy 2/2008

4 With the MWT method, the conductors necessary for interconnections within the module are shifted to the rear side of the solar cell. To this end, 25 to 50 holes with diameters between 300 and 500 µm are drilled in a grid pattern on each solar cell, and later filled with conducting material. Rofin s StarDisc lasers here achieve drilling rates of 25 holes per second. The EWT process, on the other hand, transfers all the electrical contacts of the negative-doped layer to the cell reverse, for which it is necessary to drill around 15,000 holes with individual diameters of 60 to 70 μm. In this case, a StarDisc laser manages around 3,000 holes per second. A further field of application for the StarDisc laser is the cutting and scribing of silicon. With the typical solar wafer thicknesses of between 200 and 250 μm, the cutting speed is 100 mm per second, whereas scribing is accomplished at 200 mm per second, for a scribing depth of 400 μm. The benefit of a laser-assisted process for the cutting and scribing of silicon is the fact that micro cracks are practically eliminated. Consequently, the breakage losses in further processing are considerably reduced. Manufacturers focus on exploiting the benefits A number of leading German cell manufacturers have already been able to exploit these and other benefits. Thalheim-based Q-Cells, for example, is using lasers for the manufacturing of its solar cells. And Schott Solar in Alzenau has chosen laser technologies for both crystalline-cell and thin-film applications. At Ersol in Erfurt, lasers play an important role in edge isolation, and also handle A RISE cell held up to the sun: The laser perforation is clearly visible. Photos (3): ISFH three of the eight process steps for Ersol Thin Film.»Laser techniques are efficient methods which function excellently in automated processes,«says Lars Waldmann, press spokesman at Schott Solar. The company has been using lasers to cut silicon wafers for several years now. The starting point for a wafer is an octagonal ingot drawn from a crucible of molten silicon. The laser subsequently»slices«this silicon block into individual wafers. Using the EFG process, Schott Solar achieves an annual capacity of around 100 MW in Alzenau. In the thin film field, at least according to Waldmann, there is no real alternative to laser technologies. Schott Solar uses lasers for the structuring of amorphous solar cells. The new thin film line in Jena offers an annual production capacity of 33 MW. Competitors Ersol, who produce crystalline solar cells, use lasers to achieve edge isolation. For technical director Rüdiger Schulz, the advantages of this method are evident:»it is an extremely reliable technology. At the same time, a laser can be controlled with much greater precision than a wet-chemical process.«schulz is also satisfied with the speed of the laser-based system, with cycle times of just three to four seconds. Another field of applications which Ersol envisages for lasers in the future is laser marking. Laser soldering, on the other hand, plays no role for the company at the moment. Ersol Thin Film has been using solid-state lasers (power: 20 W) in the manufacturing of thin film modules since August last year. Three production steps are now handled by UV lasers. Front and rear TCOs (Transparent Conductive Oxides) are cut with a laser with a wavelength of 355 nm, the amorphous silicon Build larger Fabs but how? We can solve the special technical challenges. Visit us in Munich at Photovoltaic Technology Show 2008 Europe Hall C1 Booth H17 M+W Zander FE GmbH Business Division Photovoltaics Lotterbergstr Stuttgart, Germany Phone pvinfo@mw-zander.com Sun & Wind Energy 2/

5 photovoltaics A silicon wafer being mounted in the laser-structuring tool at the German Institute for Solar Energy Research. Positioning aid for laser structuring: The red areas assist automatic detection of the wafer position. layer with a green laser (532 nm). These cuts are made in such a way that the resultant cells are series connected in groups of 150 cells. Lasers are also the technology of choice for Christian Koitzsch, managing director of Ersol Thin Film:»We are working with large-format substrates. An adequate processing speed is thus an important consideration.«the throughput rate of a laser process is higher than with other technologies. The use of lasers also serves to reduce costs. And it must not be forgotten that, unlike wet-chemical etching or mechanical scribing, laser is a dry process. This is especially significant in view of the cleanroom atmosphere required for the production equipment. Research institutes testing new laser methods In the meantime, several research facilities in Germany are contributing innovative concepts aimed at further expanding the use of lasers in solar cell manufacture. The Fraunhofer ISE is working on further development of a relatively new contacting method. LFC (laserfired contact) is the basis for a novel rear-surface structure for conventional solar cells.»as the material is only contacted at individual points, we can achieve a higher level of cell efficiency, currently 19.3 %,«says Andreas Grohe, head of the laser and PVD group. The Fraunhofer institute is cooperating with solar cell manufacturers on this project. The ISE is using above all diode-pumped, pulsed solid-state lasers with typical wavelengths in the UV range. According to Grohe, one strong trend is moving towards picosecond lasers, because»with ultrashort pulses, it is possible to work very close to the surface and with a minimum of unwanted damage.«in his opinion, laser technologies offer four key benefits in respect of solar cell manufacture. In contrast to full-surface structuring and wet-chemical processes, a laser can be applied locally, and can furthermore be used to produce fine structures within this local range. In addition, laser-based processes cause no mechanical stresses, as there is no physical contact with the materials. This means that it is possible to process very thin specimens. And in view of the wide range of available wavelengths and pulse durations, a high degree of process selectivity is also obtained. The Hannover Laser Centre (LZH), too, is develop - ing new laser processes suitable for the solar industry and maintains a strategic partnership with the Institute for Solar Energy Research (ISFH). Picosecond lasers are also a focus of current interest in Hannover.»This laser has the advantage that there is no heating of the material from its short pulses, and also no damaging of the crystalline structure. This, in turn, minimises the efficiency losses in the final cell,«says Aart Schoonderbeek. The production speeds of such ultrashort-pulse lasers, however, are at present still to slow for industrial-scale use. Research at the ISFH is currently looking into the cost-saving potential of laser technologies in the manufacturing of high-efficiency silicon-based solar cells. One project which the institute has been promoting for some time is the so-called RISE process. A RISE cell is a back-contacted cell whose illuminated front side is nonmetallic. The cell front is textured and coated with a silicon nitride layer for surface passivation and to minimise reflection. Isolation is provided between the phosphor-doped emitter regions and the boron-doped base regions on the rear with a thermally produced SiO 2 barrier. Small openings in this barrier permit local contacting of the doped regions by way of metal fingers. Lasers are used at several stages in the manufacturing of such cells. Through laser ablation of the silicon, a pattern of troughs is created on the rear surface of the cell. This permits exact definition of the base and emitter regions, as well as reliably self-aligning contact separation after all-over evaporation coating. The metal damage caused during laser ablation is removed by way of wet-chemical surface treatment. The depth of actual damage can be controlled by choosing corresponding laser types. Whereas an infrared laser with a wavelength of 1064 nm results in damage down to a depth of more than 20 μm, crystal damage can be limited to below 5 µm with a frequency-converted solidstate laser (532 nm or 355 nm). 118 Sun & Wind Energy 2/2008

6 The laser as a universal tool A laser device comprises a beam source, an electrical power supply and a cooling unit to dissipate the excess pump energy. The beam source, in turn, comprises a laser-active medium, the pump source and a resonator. The decisive element is the optical unit, in other words the arrangement for guidance or formation of the laser beam. This optical unit is amalgamated with fixtures to position and handle the workpiece, and with measuring and control facilities to make up a complete laser machining system. The workpiece alignment for machining can be realised in one of two different ways. Either the workpiece is adjusted by way of linear and rotary axes to match a fixed laser beam, or else the laser beam is adjusted relative to the stationary workpiece. The laser beam is here shifted by way of a mirror scanner. Such scanner systems permit beam advance rates of several metres per second. The favoured laser type for solar cell manufacture is the solid-state laser. The typical pulse durations are measured in microseconds, nanoseconds and picoseconds: 1 μs = 10-6 s, 1 ns = 10-9 s, 1 ps = s. 10 years experience in PV factory design and still loving it With the aid of laser techniques, the institute has succeeded in raising the efficiency of the RISE cell to 22 %. First, the SiO 2 layer on the rear was opened locally. A laser was then used to ablate the organic coating applied to the rear surface at the points where the metal fingers were later to contact the silicon. The RISE process is currently undergoing testing in a pilot series. Another topic addressed by the ISFH concerns the use of laser soldering for PV modules. As silicon wafers become thinner and thinner, and consequently all the more brittle, lasers represent an interesting alternative also for the electrical interconnection of the solar cells. After all, a reduction in the thermal and mechanical stresses to which the cells are subjected enables the breakage losses to be minimised. Soldering with lasers provides for a locally contained energy input, fast process times and a high degree of flexibility. For its soldering tests on PV modules, the institute uses a highpower laser (200 W) emitting a continuous beam at a wavelength of 980 nm. The results obtained to date indicate that a soldering duration of just 300 ms already produces connections between the tinned copper strips and solar cells with a bonding strength of more than 10 N/cm. Laser soldering is thus a suitable method to produce repeatable, low-resistance connections. There is still plenty of scope for the use of laser technologies in the manufacturing of solar cells. One future target for the branch is to raise the processing speed in cell manufacture with the aid of lasers.»the trends are pointing to larger and larger silicon wafers. In a few years, these wafers will need to be running off the lines by the second. This, in turn, will mean connecting up two or three parallel laser systems,«says research assistant Rainer Grischke of the ISFH. It can be expected that laser systems will soon be working twice as fast as they are today. Grischke s prophesy:»i am sure that, over the next few years, we will see laser technology advancing into many more new fields with potential for industrial applications.«y Further information: Anette Weingärtner Wherever our customers are, we are present. Now in San Francisco, USA. ib vogt consulting Inc. 160 Spear Street San Francisco, CA info@ib-vogt.com