1. Rationale:
a: Describe the need or problem addressed by this activity.
(Indicate the customer, what benefits they will receive, and if possible, quantify the impact on the return on investment
[ROI] if the Document is implemented.)
Metal-Wrap-Through (MWT) technology is one of the most effective solutions for reaching
higher cell efficiency and higher module power output (Figure 1). Not only the cell efficiency is
improved by reducing front side metallization area (without busbars any more), but also the ohmic
losses of the module interconnection can be potentially decreased. Laser drilling of vias is one of
the unique processes for MWT solar cells. Via metallization can then be done for electrical
conduction.
Fingers on front side
P-type Si
Via
Aluminium Base
Positive Point Contacts
Negative Point Contacts
Figure 1: MWT cell structure
MWT cells may be designed differently at both front (Figure 2) and back (Figure 3) sides.
Front design 1
Front design 2
Front design 3
Front design 4
Figure 2: different front design of MWT cells
Back design by traditional
soldering
Back design by conductive paste and
conductive backsheet
Figure 3: different back design of MWT cells
Both front and back sides of MWT cells can be designed differently as shown above.
However, the metalized vias are common to all MWT cells, which are shown in Figure 1.
The metallization within vias plays an important role for the performance of MWT solar cells
because the generated current collected by the front fingers will be transferred through the vias to
the rear side of the cells. If vias metallization is not fully optimized, there will be poor contact
between the front finger and rear pad. This kind of contacts increases series resistance which
results in the decreasing of the fill factor and finally bad cell performance. Methods to reduce
series resistance of via metallization have been widely discussed in practice, including via paste
modification, screen printing process, additional vacuum step during printing. Figure 4 shows the
status of via hole at optimized and non-optimized conditions. When the cell was fabricated at nonoptimized condition, fill factor dramatically dropped due to the high series resistance.
For both MWT cell manufacturers and MWT module makers, they need standardized test
method to test electrical resistance of metalized vias.
Figure 4: Cross section image of vias after metallization process
(Left: optimized condition, right: non-optimized condition)