Flexible silver paste enables thin-fi lm photovoltaic fl ex solar cells
ohm-cm around 96%, then increases again be discerned, as demonstrated in Figure 18
with increasing Ag content. The minimal for 94% Ag paste. Accordingly, the optimal
value is higher than Paste A. Ag content is determined as 94% w/w
when considering electrical property, stabil-
2. Flexibility ity against humidity, and flexibility. This
The cured pastes with Ag content equal to optimal composition for low temperature
or greater than 96% were fairly brittle and thin film Ag metallization paste is desig-
hence were ruled out for further evalua- nated as LTTF-6363.
tion. For Ag content ranging from 92% to
95%, the contact resistance upon bending Slump
treatment without humidity pre-condi- The soft epoxy-based binder system
tioning is shown in Figure 16. No breakup LTTF-6363 was formulated to resist cold
!
could be discerned for all samples, and all slump after the print. Furthermore, it gels
Figure 18. Contact resistance of paste with soft
epoxy resin binder and 94% Ag when tested against
pastes showed decreasing contact resistance quickly upon heating, thus preventing hot
bending treatment. Samples were pre-conditioned at
with increasing bending treatment, with slump as well. Figure 19 shows the photos
85/85 from 0 to 14 days. 92% Ag sample exhibiting most significant of pastes printed for LTTF-6363 and the
decrease in contact resistance. control thermoplastic-based Paste A. LTTF-
However, for samples pre-conditioned 6363 held the print shape very well, while
to 98% (w/w). Also shown are the bulk
at 85oC/85%RH, damage caused by that of Paste A experienced significant
resistance (0.80 ohm), contact resistance
humidity was noticeable for 95% Ag, as slump. The gap in the slump behavior
(3.23 ohm) and volume resistivity (18.0 µ
shown in Figure 17. After 85oC/85%RH between the two materials further enlarged
ohm-cm) of the control Paste A, with each
pre-conditioning for seven days, breakup after curing.
value marked on the Y and Y’ axis.
in the paste line was recognizable after 8X Figure 20 shows the cross-sectional pic-
The contact resistance is virtually a
bending treatment, although contact resis- tures of cured thin lines of LTTF-6363 and
constant regardless of the Ag content and
tance did not deteriorate significantly. Af- Paste A printed through 50 microns thick,
is slightly higher than Paste A. The bulk
ter pre-conditioning for 14 days, the paste 250 microns wide stencil. After curing,
resistance shows a very moderate concave
line was broken and electrical discontinuity the width of Paste A was about 60% wider
curve, with minimum value 0.63 ohm
was registered after 8X bending. than that of LTTF-6363. In other words,
around 94-95% Ag which is lower than
When the Ag content was decreased Paste A blocked 60% more sun light when
Paste A. The volume resistivity decreases
down to 94% or below, no electrical dis- compared with LTTF-6363.
initially, reaching the minimal value 24 µ
continuity or physical deterioration could As the LTTF-6363 showed very promis-
Figure 19. Photos of LTTF-6363 (left) and Paste A (thermoplastic, right) after print with the use of slump stencil IPC-A-20 specified in J-STD-005. The aperture openings are
0.20 x 2.03 mm and 0.33 x 2.03 mm, and the stencil thickness is 0.10 mm.
Figure 20. Cross-section pictures after printing a paste through 50 microns thick, 250 microns wide stencil and then curing at 165 oC for 10 min. LTTF-6363 (left), Paste A
(thermoplastic, right).
10 – Global Solar Technology – November/December 2008 www.globalsolartechnology.com
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