PTB >>Disk Lasers in the Photovoltaic Industry

Diverse types of lasers, such as nanosecond, pulsed, and excimer, have been considered for various applications in the photovoltaic industry, including edge isolation, edge deletion, drilling for back contact, cutting of Siwafer, and patterning of crystalline solar cells. High power lasers, with high stability and high efficiency in addition to high beam quality, are needed now more than ever. A possible solution to this demand for technology comes in the form of the unique design of the disk laser, which offers power scalability and high beam quality.

Understanding the Disk Laser
heat flow. Therefore, the thermal gradient is parallel to the laser propagation and the internal thermal lensing is effectively eliminated. This allows true scalability of the laser output power. Diode laser light is used to pump the thin disk. Typically a beam parameter product of about 500mm mrad is desired to pump a high average power thin disk laser. Therefore, fiber coupled laser diodes are possible, as well as homogenized laser diode arrays. In a real world application, the latter approach is preferred because of the much-reduced cost per watt of pump power. With a simple light path arrangement, the pump light is guided up to 20 times through the disk, which ensures the high efficiency of the disk laser.

Figure 1 illustrates the design of a disk cavity, where the disk is mounted in the center and pumped by the diode laser light. Today industrial continuous disk lasers are available with an output of up to 5.5 kW from one disk and with an optical efficiency of above 60%. If additional power is necessary, up to four cavities can be used in one resonator. The output power of the laser device increases to a maximum of 16 kW without changing the beam quality. This thin disk technology,

Figure 2. Microscopic picture of an ablated area done with square fiber.

due to its high beam quality and high scalable power, not only allows for cw, but also for high power nanosecond and picosecond lasers. For pulsed operation, various tech nologies can be applied, ranging from q-switched or cavity dumping oscillators to mode-locked Master-Oscillator- Power-Amplifier (MOPA) lasers.

Due to the high upper state lifetime of Yb:YAG and the relatively small gain in the thin disk laser, the typical pulse duration of a simple q-switched disk laser is in the order of one microsecond. Through employing a cavity dumped configuration, a wider range of pulse durations is accessible. Hence, the cavity dumped TruMicro 7050 provides tunable pulse width between 25 and 700 nanoseconds and a repetition rate of up to 100 kHz. Using a pockels cell inside the cavity and ejecting the pulses through a thin film polarizer, the laser is able to produce an average output power of 750 watts with a beam quality that allows a core diameter of 100 um or larger in the delivery fiber.

Edge Deletion by High-Power Nanosecond Thin Disk Laser
To avoid short circuits at the edge of thin film, a solar module laser can be used to ablate an area of about 5 to 15 mm in width at the edges of the solar modules. Compared to the current standard process of sand blasting, the laser process has several advantages. First, the laser process is a non-contact method. The substrate, in most cases a glass substrate, will not be damaged by a practical beam or tooling. Second, due to the process being performed primarily through the glass, the ablated material can be exhausted easily. Third, the focus spot of the laser defines the geometry of the ablation and a laser beam cannot blunt. The area of ablation per pulse is constant during the production process.

The main criterion for acceptance of the edge deletion process is given by the

Figure 3. Ablation of molybdenum layer with nanosecond and picosecond laser pulses.

resistance of the ablated surface, which should be at 100M􀀱 or above. In addition, appearance and cleanness of the ablated area can influence the process setting. Laser ablation of thin layers requires short pulse duration in the nanosecond range with high average power in order to realize an ablation rate in the range of 10 to 50 cm2/s. Due to the facts that the layer structure of the thin film solar cells, the glass substrate, and the cycle time are different, you have to adjust the laser ablation process. Typically, spot diameters between 500 and 1000 um with a pulse to pulse overlap between 5 and 50% and process speed of several meters per second are necessary. With the availability of different core shapes for delivery fibers, an additional degree of freedom appears. Figure 2 shows an ablated area processed with a TruMicro 7050, 620 x 620 um square fiber, and the TRUMPF scanner system Programmable Focus - ing Optic (PFO). The advantage of this square fiber core shape for the application is that we were able to increase the ablation rate by minimizing the pulse overlap and increasing the homogeneity of the ablated area at the same time.

Patterning With High-Power Picosecond Thin Disk Laser.
While a nanosecond laser can be used for scribing processes of amorphous silicon or cadmium-telluride solar cells, picosecond pulses open an additional application field for scribing copperindium- selenium (CIS) solar cells. Due to the short pulse length of six picoseconds, you can use the laser to remove the molybdenum in CIS modules at a higher speed and without cracks (Figure 3). This "cold" laser ablation processing of the photo-active layer with ultra-short laser pulses also offers high quality and productivity advantages compared to mechanical processes or laser processes with nanosecond and microsecond pulses.

New concepts of crystalline silicon solar cells use passivation layers to minimize the recombination rate of electron hole pairs, not only at the front side, but also at the back side of the solar cell. Manufacturers use the dielectric SiOx and SixNy layers between the silicon and the metal contacts. The laser technology allows you to

Figure 4. Ablation of a passivation layer with single pulses.

achieve contact through the isolating passivation layer. There are two ways to do this. Either you use the laser to shoot the metal contacts through the passivation layer onto the wafer, known as laser fired contacts, or you use the laser to remove the insulating layers before the metal coating is applied. With selective removal of the insulating layer, ultra-short pulses operate without added heat creating disruptions in the silicon. Figure 4 shows single shot ablation of a passivation layer on crystalline silicon.

Conclusion
The disk laser has the ability to scale the average power by simply enlarging the modal cross-section of the laser beam, thereby keeping internal intensities constant. Especially important for pulsed laser systems are the inherently low nonlinearities of the disk laser. This enables high peak power pulses at high average power. MOPA systems based on disk technology are powerful picosecond lasers. These lasers are used in the manufacturing of solar cells for sophisticated surface treatment, such as highly productive and precise removal. Cavity dumping, in combination with thin disk laser technology, enables industrial fiber-coupled nanosecond lasers of up to 750 watts, used to ablate or remove largearea layers quickly. All in all, disk laser technology improves the quality and speed of standard laser processes in the production of solar cells, and offers solutions to the challenge of economically producing future cell designs.

This article was written by Juergen Stollhof, Technical Sales Manager, and Hoonhee Lee, Application and Project Manager, Micro Processing, TRUMPF Laser Technology Center (Farmington, CT). For more information, contact Mr. Stollhof at juergen.stollhof@ us.trumpf.com, or visit http://info.hotims.com/22926-201.