The competitive nature of the manufacturing of solar cells is largely influenced by the zero defect approach of high speed automation. To this end, YAG lasers have been called upon to provide the precise laser scribing of the panels. Once the various layers of photovoltaic materials have been laminated to the glass, the laser is needed to scribe a series of channels that eventually become each of the individual voltage producing cells.
The quality, shape, size and intensity of the laser must be closely monitored and controlled for this process to produce a solar cell that meets specification and cost requirements. There are two variables when using a laser for scribing that must be monitored and controlled; the intensity or energy of the beam and the size and shape of the laser beam.
If the beam does not possess the correct amount of energy, then the laser will not ablate the photovoltaic material. If not done, then any residual material can allow shorts to occur between these subpanels, causing the panel to be rejected and scrapped. The size and shape of the laser beam must also be controlled. If the beam is not maintained to a critical size and shape, then the channel scribed by the laser beam will not be of the uniform width necessary, and, again, any variances in this width can allow electrical shorts to occur as the manufacturing process is completed resulting in scrap material. The other need for control of the laser beam size and shape is the requirement to maximize the total area of the photovoltaic material thereby producing the greatest amount of wattage possible. If the laser beam is not controlled and allow to be larger than necessary, then there is less area to produce electricity and the panel is less efficient.
The following application pictures show an assembly where multiple laser beams are employed simultaneously to scribe a coated glass panel. In this example, there are 2 lasers, one on each side, and each laser is split 4-ways, thereby allowing a total of 8 laser beams to scribe the panel as it passes beneath this laser optics assembly.
In this set up, there was a requirement to establish 8 uniform laser beams, meaning the energy per pulse for each of the 8 were within tolerance, and the size, shape, and intensity of the 8 beams were within specification as well.
The first effort was to profile each of the 8 beams using the Spiricon SP620U, USB CCD beam profiler camera (4.4um x 4.4um pixels ) 1600 x 1200. This camera is quite versatile for all types of applications, including solar cell manufacturing. For this measurement, and to establish a baseline value, the Spiricon camera was positioned below each of the 8 focusing lens thereby allowing the beam to be imaged, measured and graphically displayed.
Through this approach a measurement of beam size using a 1/e2 ( 13.5% of Peak) was established and prepared as a baseline for future measurements. The use of this measurement helps in the diagnosis of failure in the transmission optics, the quality of the beam splitters as well as the longer term deterioration of the supply laser.
The 2nd measurement for each of the 8 laser beams was to understand the energy per pulse value for each beam. As with the USB SP620U profile camera, the Ophir PE10 pyroelectric energy sensor ( 2uJ to 10mJ) was position under each beam with the quantitative measurement taken. One measurement was taken to insure the expected energy per pulse is within spec, and then two to insure that all 8 beams were within tolerance to each other.
One characteristic about the use of energy sensors, is that all laser failures will produce only a drop in measurements. Therefore, should the focusing lens become coated with dust, or the beam splitting wedges crack or become coated with ambient residue, the energy measurement will drop. And of course, should the power supply inside either of the lasers fail, even marginally, this change will be detected by these measurement.
This two set process is being conducted at the beginning of each process, and then once manufacturing personnel have confidence in the stability of these settings, they will likely reduce the frequency of the measurements to twice a week or eventually once a week.
"Measurement is the first step that leads to control and eventually to improvement. If you can't measure something, you can't understand it. If you can't understand it, you can't control it. If you can't control it, you can't improve it." —H. James Harrington