News Details
DPSS Lasers Overcome Glass Process Challenges 4
2012/9/20 13:55:24
Glass processing for PV solar panels
Glass also is used extensively in thin-film photovoltaic (TFPV) manufacturing. With up to meter-size sheets of 1- to 3-mm-thick glass as a substrate, solar panels are produced with a variety of solar absorber materials, such as amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium gallium (di)selenide (CIGS). A well-known success story for Q-switched DPSS lasers in TFPV manufacturing is in scribing thin films of material �C referred to as P1, P2 and P3 scribing �C to create the panels' monolithic serial interconnection architecture. But a typical TFPV panel is >99 percent glass by composition (before module packaging), so it would seem logical that laser glass processing could have potential somewhere along the production line.
Such an opportunity does arise in the final packaging of some TFPV panels if the same glass substrate on which the photovoltaic device is built becomes part of the rear side of the encapsulation package. When the panel is fully sealed �C which includes metal framing around the perimeter �C a through hole in this glass is needed to access the devices electrical contacts.
Laser processing offers advantages for fabricating these conduit through-vias. Unlike mechanical drilling, the laser enables a noncontact process, which means low or no vibration is generated that can potentially weaken the devices thin films. Also, laser cutting of glass can be done as a completely dry process, which is important for thin-film materials such as the moisture-sensitive solar absorber layer.
Processing challenges for glass
Laser processing of glass in general is a fairly delicate operation because care must be taken to avoid excessive melting, chipping and cracking. Achieving the proper quality and throughput compromise requires a significant process development effort. This is particularly true when a higher-power laser is used for processing. The higher laser power enables faster processing, but it also increases the thermal loading on the glass, which can result in melting, cracking and even invisible weakening of the glass in the form of residual stress, any of which could cause product failure in the field.
To achieve proper thermal management in the glass, equipment and methods for focusing and scanning the laser beam must be carefully selected. One approach uses a high-speed two- or three-axis scanning galvanometer system with a flat-field f-Theta objective, which allows fast scanning of a tightly focused laser beam over a stationary glass plate. Todays galvo scanner products allow speeds on the order of meters per second to be generated with relatively small scanning dimensions, down to 1 to 2 mm. Such rapid motion of the focused laser spot goes a long way toward minimizing heat buildup in the glass. Very thin glass can be cut with single or multiple overlapping scans of the focused beam. As glass thicknesses increase to the range of hundreds of microns, however, the laser cutting process becomes self-limiting. In this case, multiple adjacent beam scans are required to widen the kerf, facilitating debris removal and, ultimately, full cut-through of the thick glass. For still thicker glasses, it becomes necessary to translate the focused beam along the Z-axis through the bulk of the glass. At this point, the laser "cutting" process has clearly transformed into a 3-D laser milling process.
When applied to thin-film photovoltaic panel glass via drilling, the equipment and techniques detailed above result in quality through holes drilled in a matter of seconds for the requisite diameters of ~4 to 6 mm. The image in Figure 4 shows sample cross sections of cutouts generated with the Spectra-Physics Pulseo 532-34 laser system on 1-, 2- and 3-mm-thick glass substrates. Rated for 34-W output at 120-kHz pulse repetition frequency (PRF), the laser outputs >350 µJ at 100-kHz PRF; the combination of high pulse energy and fast pulse output enables rapid processing. And the <30-ns pulse duration allows controlled heat input into the material.
Glass also is used extensively in thin-film photovoltaic (TFPV) manufacturing. With up to meter-size sheets of 1- to 3-mm-thick glass as a substrate, solar panels are produced with a variety of solar absorber materials, such as amorphous silicon (a-Si), cadmium telluride (CdTe) and copper indium gallium (di)selenide (CIGS). A well-known success story for Q-switched DPSS lasers in TFPV manufacturing is in scribing thin films of material �C referred to as P1, P2 and P3 scribing �C to create the panels' monolithic serial interconnection architecture. But a typical TFPV panel is >99 percent glass by composition (before module packaging), so it would seem logical that laser glass processing could have potential somewhere along the production line.
Such an opportunity does arise in the final packaging of some TFPV panels if the same glass substrate on which the photovoltaic device is built becomes part of the rear side of the encapsulation package. When the panel is fully sealed �C which includes metal framing around the perimeter �C a through hole in this glass is needed to access the devices electrical contacts.
Laser processing offers advantages for fabricating these conduit through-vias. Unlike mechanical drilling, the laser enables a noncontact process, which means low or no vibration is generated that can potentially weaken the devices thin films. Also, laser cutting of glass can be done as a completely dry process, which is important for thin-film materials such as the moisture-sensitive solar absorber layer.
Processing challenges for glass
Laser processing of glass in general is a fairly delicate operation because care must be taken to avoid excessive melting, chipping and cracking. Achieving the proper quality and throughput compromise requires a significant process development effort. This is particularly true when a higher-power laser is used for processing. The higher laser power enables faster processing, but it also increases the thermal loading on the glass, which can result in melting, cracking and even invisible weakening of the glass in the form of residual stress, any of which could cause product failure in the field.
To achieve proper thermal management in the glass, equipment and methods for focusing and scanning the laser beam must be carefully selected. One approach uses a high-speed two- or three-axis scanning galvanometer system with a flat-field f-Theta objective, which allows fast scanning of a tightly focused laser beam over a stationary glass plate. Todays galvo scanner products allow speeds on the order of meters per second to be generated with relatively small scanning dimensions, down to 1 to 2 mm. Such rapid motion of the focused laser spot goes a long way toward minimizing heat buildup in the glass. Very thin glass can be cut with single or multiple overlapping scans of the focused beam. As glass thicknesses increase to the range of hundreds of microns, however, the laser cutting process becomes self-limiting. In this case, multiple adjacent beam scans are required to widen the kerf, facilitating debris removal and, ultimately, full cut-through of the thick glass. For still thicker glasses, it becomes necessary to translate the focused beam along the Z-axis through the bulk of the glass. At this point, the laser "cutting" process has clearly transformed into a 3-D laser milling process.
When applied to thin-film photovoltaic panel glass via drilling, the equipment and techniques detailed above result in quality through holes drilled in a matter of seconds for the requisite diameters of ~4 to 6 mm. The image in Figure 4 shows sample cross sections of cutouts generated with the Spectra-Physics Pulseo 532-34 laser system on 1-, 2- and 3-mm-thick glass substrates. Rated for 34-W output at 120-kHz pulse repetition frequency (PRF), the laser outputs >350 µJ at 100-kHz PRF; the combination of high pulse energy and fast pulse output enables rapid processing. And the <30-ns pulse duration allows controlled heat input into the material.