MANUFACTURINGLASERS
chemicals. Currently one of the largest laser applications in c-Si solar cell production is edge isolation. The p-doped wafer is coated with an outer layer of n-doped silicon to form a large area p-n junction. However, this outer layer (n-doped) covers the entire wafer to create an unacceptable recombination pathway between front and back
Figure 1: Schematic representation of laser edge isolation for crystalline silicon solar cell
surfaces. This pathway can be terminated by edge isolation, i.e. a trench is created for electrical isolation as shown in Figure 1. In order to maximize the active surface area and efficiency, this trench has to be as narrow and as close to the edge as possible. Typical dimensions for laser edge isolation are 30-50µm wide and 10-20µm deep.
JPSA systems provide a variable width and depth to meet the customer’s requirements. The high throughput isolation cuts are performed at rates up to 500 mm/sec with minimum recast as shown in Figure 2. The system offers fully integrated debris control via JPSA’s vortex debris control tool. This high throughput is achieved with only 20 W of power on target using an optimized beam delivery setup at 355 nm. The low power and short wavelength combine to ensure minimal thermal affectation while still leading to high throughput when comparing to solutions that use longer wavelengths [5].
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Figure 2: Top view of edge isolation trench by JPSA laser system
Figure 3: Cross section of thick silicon through cut by JPSA laser system
Laser dicing/wafer downsizing Standard solar cells can be cut for use in concentrator applications. The wafer is downsized and sectioned to fill the area of the concentrator optics field of view. Similarly to the laser edge deletion technique, at JPSA we optimize laser wavelength, power and beam delivery system to achieve extremely narrow cut widths with high throughput and minimal thermal affectation. At JPSA typical performance with 20 W of available laser power only versus often hundreds of watts used by other laser companies is a 7 micron wide cut, 150 microns deep at 150 mm/sec or 100 microns deep at 300 mm/sec [6]. By using high performance motion stages, accuracies of +/-3 microns for the cuts are achieved. Additionally, laser dicing is a non-contact method and may be used to cut through the metal traces which can be used for wafer alignment and complete automation of the process.
Figure 4: Laser drilled vias in silicon
Typical solar cells have screen-printed metal for the front-side contacts which block a significant area from receiving sunlight. To minimize losses from shading of the silicon by the front metal traces, techniques such as Metal Wrap Through (MWT) or Emitter Wrap Through (EWT) are employed. For MWT, the emitter busbar is relocated to the rear of the cell while the contact fingers remain on top surface, and laser-drilled vias are used to connect the two. Typically up to 200 holes are required, with diameter around 100-200 microns. For EWT, all electrical contacts are relocated on the rear of the cell. Typically around 10,000 vias are required with <100 microns diameter. The challenge from a
www.solar-pv-management.com Issue II 2011
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