Flexible silver paste enables thin-film photovoltaic flex solar cells
ing results in many aspects, the contact
Substrate LTTF-6363 Paste A
resistance of this paste was measured by
TLM (transmission line model) and com-
Contact resistance on AZO/glass, Ohm•cm2 0.18 0.21
pared with the control paste A (Table 2).
Contact resistance on ITO/glass, Ohm cm2 0.08 0.06
Discussion
Table 2. Contact resistances of LTTF-6363 and paste A on AZO/glass and ITO/glass measured by TLM
It is interesting to note that, while the
method.
contact resistance of the two pastes is
about comparable, LTTF-6363 displays a
lower bulk resistance but a higher volume Paste LTTF-6363 Paste A
resistivity than that of Paste A, as shown
Ag % (w/w of paste) 94 93
in Table 3. First question, how does this
happen? Second question, for solar cell
Volatile (volume % of paste) 7.90 24
application, is the bulk resistance or the
Contact resistance, ohm 3.42 3.23
volume resistivity more meaningful for
higher energy efficiency?
Bulk R measured, ohm 0.63 0.80
Volume resistivity was calculated based Volume resistivity, micro-ohm•cm 26.6 18.0
on theoretical shrinkage by volatile evapo-
ration (shrinkage factor: 100% - volatile
Volume resistivity was calculated based on theoretical shrinkage by volatile evaporation (shrinkage
factor: 100% - volatile volume %)
volume %)
To answer the first question, the pro-
Table 3. Paste composition and electrical properties. Tests done with ITO/glass substrate.
cess of the two metallization pastes can be
reviewed, as shown in Figure 21.
Upon printing, the wet paste volume
Although Paste A exhibits a higher
of the two pastes, LTTF-6363 and A, was
bulk resistance, its cross-sectional area is
identical, as dictated by the stencil pat-
substantially smaller. With line length
Dr. Hong-Sik Hwang is a research
tern. However, after curing, the dry paste
being identical for both pastes, this eventu-
chemist with Indium Corporation. He
volume is no longer identical, with Paste A
ally results in a lower volume resistivity for
recently earned his Lean Six Sigma
being considerably smaller than LTTF-
Paste A than LTTF-6363.
Black Belt from the Thayer School
6363. This is a result of a greater volatile
To answer the second question, since
of Engineering at Dartmouth, New
content in Paste A than that of LTTF-6363.
paste line is used for collecting current,
Hampshire, USA.
By definition,
and the current allowed is inversely propor-
tional to the bulk resistance (according to
Lee Kresge is a research chemist with
Volume Resistivity = Bulk Resistance
relation V = I x R), a lower bulk resistance
Indium Corporation.
X Line Cross-section area /Line
means a greater current allowed. Volume
length
resistivity becomes meaningful only when
James Slattery is Indium Corporation’s
the dry paste volume is the same for pastes
vice president, technical support.
being compared.
Dr. Ning-Cheng Lee is vice president of
Conclusion
technology with Indium Corporation.
The flexible high performance Ag metal-
lization paste LTTF-6363 was developed
for thin film photovoltaic flex solar cells.
The binder of the paste is soft epoxy-based
resin system. Compared with thermoplastic
paste system, LTTF-6363 exhibits superior
adhesion and is flexible. These features
enable the deployment of flex solar panels
where tolerance against rolling or bending
is critical. LTTF-6363 also displays excel-
lent print characteristics and non-slump
performance. This is extremely important
for maximizing effective open area on solar
cell. LTTF-6363 exhibits very good solder-
ability, thus allowing easy soldering connec-
tion with other electronic devices.
Figure 21. Schematic of metallization paste process.
www.globalsolartechnology.com Global Solar Technology – November/December 2008 – 11
issue_2.1.indd 11 2/22/09 9:39:52 PM
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48