Time-domain tailored-pulse laser enables scribing of CIGS solar modules

Matt Rekow, Richard Murison, PyroPhotonics Lasers, a subsidiary of ESI, Inc, Montreal, QC, Canada

Laser scribing is an enabling process for the formation of monolithic series interconnects for large panel thin film solar cells. Interconnect formation typically requires three scribing operations known in the industry as P1, P2 and P3. A fourth operation, often referred to as P4, removes shunts at the edges and prepares the module for electrical interconnect and encapsulation. Laser scribe processes based on nanosecond (ns) pulsed lasers are preferred for both a-silicon and CdTe thin film PV manufacturing. CIGS (copper indium gallium selenide) is a different story. In this case, the shunt and series resistance of laser processed modules suffers unacceptably due to thermal degradation when traditional ns pulse lasers are used.

Success has been claimed utilizing picosecond (ps) lasers for CIGS P2 and P3, however, negative thermal effects still cannot be avoided as is evident in Fig. 2a. Furthermore, such lasers are much more expensive, much larger and considered far less industrially robust than ns lasers. Consequently, they have yet to gain broad acceptance across the solar industry. Because of these concerns, mechanical stylus scribing continues to be the industry norm for CIGS P2 and P3 scribing.

The tailored-pulse laser

The invention of a new type of fiber laser has enabled discovery and exploitation of a previously unknown process window for the CIGS P2 and P3 process. The laser is a 1064nm pulse programmable fiber laser [1]. Unlike more traditional lasers, this laser technology allows the pulse duration to be varied from approximately two to several 100s of nanoseconds, independent of the laser repetition rate, which can be varied up to 500kHz. In addition, each pulse can be arbitrarily programmed to generate a specific desired temporal profile of instantaneous laser power. Pulse trains comprised of these shaped pulses can be applied to the scribing process at high repetition rates. It is precisely these attributes that are not available in Q-switched or ps lasers that enable this novel and effective process.