As new products and production processes are being developed, the use of lasers is often considered and in many cases when compared to alternative technologies the laser is found to be an indispensible tool. This has undoubtedly proven to be the case in the production of Mono and Polycrystalline solar cells.
The use of solar technology for the production of energy is becoming more important as we strive to become less dependent upon diminishing supplies of fossil fuels. As the production levels of solar cells increase so too does the use of lasers. The attributes of non-contact processing, flexible beam delivery and precise control make the laser the ideal tool for processing these fragile components.
With a number of different micro-machining processes to be performed during the production of Mono and Polycrystalline solar cells, it is essential that the most appropriate laser source is used.
With a complete range of lasers available including CO2, Nd:YAG, Nd:Vanadate, Disc, Fibre and Diode in Multi/Monomode, Q-switched and frequency-multiplied formats, Rofin has the correct laser source for each part of the solar cell production process.
In order to obtain high efficiency and performance from a solar cell, the front and rear sides of the product must be electrically isolated at the edges. The separation of p-type layers is achieved by cutting grooves using Q-switched Nd:YAG or Q-switched Nd:Vanadate lasers. When compared to the alternative plasma etching process, the productive advantages of laser scribing include better inline processing and improved material flow. A further advantage of the laser is that there is no need for expensive etching gases and their subsequent disposal.
High power density is necessary in order to effectively eject the melt out of the kerf and to avoid re-deposition of the molten material. Using Rofin’s StarDisc Laser the material is ablated at impressive speed using multiple passes, with very high peak laser power, achieving rapid material removal rates. The StarDisc Laser does not require assist gas and the multiple pass process provides efficient material removal, which is expelled upwards with minimal melt creation. Typical speeds for edge isolation are in the range of 400 to 800mm/s.
Drilling Wafers to Achieve Reverse Contacting
The efficiency of solar cells can be further improved by eliminating the front side contact grids and bus bars, which would otherwise block quite a substantial amount of light. Using EWT (Emitter Wrap Through) and MWT (Metal Wrap Through) concepts, the electrical contacts on the front side are effectively transferred to the reverse of the wafer. This process requires holes of different diameters and numbers to be drilled within the product.
Holes with diameters between 30-100μm are produced by percussion drilling. Larger holes require a relative movement between laser beam and wafer and in this instance these larger holes are produced by a trepanning process. The extremely high removal rates seen in this process are also achieved using the Rofin Q-switched StarDisc Laser which has very high TEM00 average power and an ideal range of pulse widths. The StarDisc Laser can achieve processing speeds of up to 5,000 holes/second for percussion drilling and up to 25 holes per second for trepanning.
High speed cutting of Mono and Polycrystalline silicon wafers can be performed with extremely high precision and low heat input by using the same ablation process as described for edge isolation and drilling. Historically, flash-lamp pumped Nd:YAG lasers were used to melt cut the silicon in a single pass with a coaxial gas jet. However, due to the rapid cooling of the melt layer at the cut edge, micro cracks were formed. Further evaluation indicated that a multi-pass cutting process without the use of assist gas provided a better surface quality at the edge. Using Rofin’s Q-switched StarDisc Laser, users can expect typical cutting speeds of up to 150mm/s for a wafer thickness of 0.200μm.
With wafer thicknesses above 400μm and in production areas which have a low level of automation, silicon wafers are not cut completely, but scribed to a depth of 30-50% of the cross section. To separate the wafer, a subsequent snapping operation either manual or fully-automated, is required to complete the process. Typical scribing speeds for this application are in the range of 50-300mm/s.
When it comes to encoding and marking solar cells with a laser, demands on the marking quality are high. “Microglyph” codes are innovative 2 dimensional codes. Different to any conventional matrix or barcode, the basic principle of this technology uses tiny, 45 degree diagonal lines (the micro "glyphs") to encode binary data onto the solar cell surface without impairing its electrical conductivity. The resultant encoding is fully readable despite of the reflection properties of polycrystalline silicon.