Flexible silver paste enables thin-film photovoltaic flex solar cells
Flexible silver paste enables
thin-film photovoltaic flex
solar cells
Dr. Hong-Sik Hwang, Lee Kresge, James Slattery, and Dr. Ning-Cheng Lee, Indium Corporation, Clinton, NY
Introduction
paste (Table 1). Paste A was a commercial Ag
A flexible high performance
With fossil fuel price soaring, solar energy
paste and was used as a control. Other than
Ag metallization paste, LT-
becomes a vital energy alternative. Although
the control, all pastes contained 92 w/w %
TF-6363, has been developed
silicon photovoltaic solar cells account for
Ag for the binder evaluation. Once a binder
for thin film photovoltaic
over 95% of the solar cells produced today,
was selected, the paste was further optimized
copper-indium-gallium diselenide (CIGS),
flex solar cells. The binder of
in Ag content.
amorphous silicon and cadmium telluride
the paste is soft epoxy-based
The potential as a flexible silver paste
(CdTe) thin film cells hold promise for rapid
was assessed by measuring the contact resis-
resin system. Compared with
growth. These thin film photovoltics can be
tance of cured paste on ITO-coated CIGS
a thermoplastic paste system,
deposited not only on glass, plastic or metal
substrate after humidity and bending treat-
LTTF-6363 exhibits supe- substrates but also on flexible substrates,
ment. Apparently, besides flexibility, other
rior adhesion and is flexible. which offer the advantage of roll-to-roll
properties also have to meet the require-
These features enable the
processing, thus significantly reducing
ments, such as contact resistance stability
deployment of flex solar pan-
manufacturing costs. Furthermore, solar cell
against thermal cycling and UV exposure,
els where tolerance against
on flexible substrates allows for easy deploy-
bulk resistance, volume resistivity, solderabil-
rolling or bending is critical.
ment of solar panels, leading to much wider
ity, printability and slump-resistance. The ex-
LTTF-6363 also displays
applications. However, the marginal flexibil-
perimental methods for the aforementioned
ity performance of silver metallization pastes
excellent print characteristics
tests are described in the following section.
in use today severely restricts the practical
and non-slump performance.
use of flexible solar cells. In addition, the
Experimental
This is extremely important
slump of the pastes after print reduces the
1. Volume resistivity
for maximizing the effective
effective exposure area to sun considerably.
A line (width: 2 mm, length: 4.5 cm) of the
open areas on solar cells. This paper describes a new highly flexible,
metallization paste was printed through
LTTF-6363 exhibits very non-slump silver metallization paste which
a stencil (thickness: 0.1 mm) on a glass
good solderability, thus allows
can be cured at temperatures below 200˚C.
substrate and cured at 165˚C for 10 min.
easy soldering connection
Also discussed are physical properties of the
The bulk resistance was measured for 2 cm
with other electronic devices.
uncured and cured paste, including contact
length of the line with a four-wire micro-
resistance, adhesion stability after humidity
ohm meter (Biddle Instruments, Blue Bell,
exposure and substrate bending and UV
PA). Volume resistivity was calculated by the
resistance.
following equation:
Keywords: Photovoltaic, Solar Volume resistivity (mΩ∙cm) = [measured
Cell, Metallization Paste, Flex,
Quest for flexible silver paste
bulk resistance (mΩ) x line width (0.2 cm) x
Thin Film, Volume Resistivity,
Several polymeric binder systems were inves-
line height (cm)] / line length (2 cm).
Contact Resistance
tigated for their potential as a flexible silver
Paste Binder system Ag load (w/w %)
A Thermoplastic – based 93
B Medium epoxy-based 92
C Soft epoxy-based 92
D Hard epoxy based 92
E Epoxy resin/acrylate resin-based 92
F Soft epoxy resin with smaller Ag flakes 92
Table 1. Ag metallization pastes studied.
6 – Global Solar Technology – November/December 2008 www.globalsolartechnology.com
issue_2.1.indd 6 2/22/09 9:39:44 PM
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