Fiber laser with adjustable pulse duration enables new ablation processes P2 and P3 in the production of CIGS photovoltaic modules
In the production of thin-film photovoltaic modules, coating processes and ablation processes, in this case the writing of lines, are carried out alternately on a glass plate. This is how the individual solar cells are created with the two electrodes at the top and bottom and the photovoltaic layer in the middle as shown below. CIGS cells are typically built on a Mo electrode layer. The Mo layer on the glass (P1), the CIGS layer on the Mo (P2) and the transparent electrode layer with the CIGS on the Mo (P3) are removed in three ablation steps P1, P2 and P2. So far, P1 has typically been carried out with ns laser pulses, while P2 and P3 have been realized by mechanically scribing with needles. Since the needles wear out quickly, various laser processes are currently being examined for their suitability for P2 and P3.
A new fiber laser from ESI-Pyrophotonics can be programmed in terms of pulse duration and pulse shape in the range of 2-250 ns with a resolution of 1 ns. Investigations on CIGS modules at Pyrophotonics, NRC Canada and the National Center for Photovoltaics* have shown that 1064 nm pulses with a well-defined pulse duration of 5-10 ns produce reliable P2 and P3 cuts without thermal melting, which leads to annoying short circuits, or bulging in melt zones.
The underlying process here is less an ablation than a laser-induced blasting off of the CIGS layer. The laser light at 1064 nm penetrates the CIGS layer, is absorbed at the Mo interface and the resulting gas pressure caused by Se gas causes the CIGS layer to flake off. This creates a break edge without melting, as can be seen in the electron microscopic images above.
EDX images showed that only minimal remnants of Cu, In and Ga remain at the bottom of the track, while Se is ubiquitous, supporting the proposed removal mechanism. If the pulse duration is correctly selected, the process window in terms of energy density and depth of focus is large enough to ensure a stable industrial process.