Most of the PV manufacturers focus on CIGS TF solar cell technology, while improving the solar cell efficiencies and reducing the manufacturing costs. CIGS solar cells consist of a glass substrate as a back contact coated with metallic Molybdenum TF. In general, green/IR DPSS Q-switched lasers are used to process electrically isolating scribes on Molybdenum thin films. Figure 1 shows the schematic diagram of CIGS thin film solar cell structure.
Figure 1. Schematic of CIGS thin film solar cell structure
Although it is a well-known fact that the quality of Molybdenum scribes depends on the techniques of film deposition, researchers from Spectra-Physics have now identified the influence of laser pulse width on scribe quality. They have demonstrated the processing of electrically isolating good quality scribes in Molybdenum TF without causing damage to the underlying glass substrate.
Molybdenum Scribing in CIGS TFPV Solar Cell Using HIPPO™ 532-11 Laser
Figure 2 shows the microscope picture of scribe on a 600nm thick Molybdenum thin film. Spectra-Physics HIPPO™ 532-11 laser was used to generate 50µm wide P1 scribes at 2m/s scan speed with the help of f-theta lens and galvanometer scanner.
Figure 2. Microscopic image and 3D, 2D depth profile trace of P1 Moly scribe generated at 2m/s scan speed
Besides using techniques for optical inspection of scribes, the researchers also characterized the quality of scribes using mechanical stylus depth profilometry. It was evident from the corresponding 2D and 3D depth profiles shown in Figure 2, electrically isolating high quality scribes were achieved without causing damage to the underlying glass substrate and without any cracking and lifting of film along the edges of scribe.
The HIPPO laser’s short pulse width facilitated the processing of cleaner scribes with reduced burr at shorter pulse durations i.e. 8 to 15ns, when compared to longer pulse durations i.e.>15ns.
Spectra-Physics short-pulse Q-switched lasers provide better results with a number of Molybdenum TF thicknesses. Based on the throughput requirements, scribe width and material removal threshold, a wide range of Spectra-Physics 355nm, 532nm and 1064nm Q-switched products including HIPPO and Navigator™ can be used.
The Spectra-Physics Navigator lasers have low pulse widths of 8ns, high repetition rates and high processing speeds with a smaller heat-affected zone. These characteristics deliver higher throughput and higher manufacturing yields.
Spectra-Physics’ HIPPO 1064nm and 532nm lasers however, feature 50kHz optimal pulse repetition frequency making it suitable for PV scribing with high-speed beam scanning. In addition, higher power output of these lasers ensure beam- splitting to achieve higher system throughput. They also offer short pulse width, good pulse-to-pulse energy stability and excellent beam quality. Table 1 shows the characteristics of various Spectra-Physics laser models.
Table 1. Characteristics of various Spectra-Physics laser models
||Repetition Rate (nominal)
||<8 ns at 35 kHz
||<15 ns at 50 kHz
||<13 ns at 50 kHz
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This information has been sourced, reviewed and adapted from materials provided by Spectra-Physics.
For more information on this source, please visit Spectra-Physics.