A simple and efficient approach to a printable silver conductor for printed electronics

Article abstract of DOI:10.1021/ja067596w

A simple and efficient approach to a printable silver conductor for organic electronics is described. Conductive silver features were prepared by annealing thin-film features deposited from an alcoholic solution of silver acetate, ethanolamine, and a long-chain carboxylic acid. High electrical conductivity similar to that of a vacuum-deposited silver conductor could be achieved at a relatively low processing temperature. Organic thin-film transistors with silver source/drain electrodes prepared by this approach provided excellent field-effect transistor properties. Copyright

Full text of DOI:10.1021/ja067596w

Published on Web 01/27/2007
A Simple and Efficient Approach to a Printable Silver Conductor for Printed
Electronics
Yiliang Wu, Yuning Li, and Beng S. Ong*
Materials Design & Integration Laboratory, Xerox Research Centre of Canada, Mississauga,
Ontario, Canada L5K 2L1
Received October 24, 2006; E-mail: beng.ong@xrcc.xeroxlabs.com
Scheme 1
Printed thin-film transistor (TFT) circuits have in recent years
received fast growing interest as low-cost alternatives to conven-
tional silicon-based technologies to enable ubiquitous large-area,
flexible, and ultra-low-cost electronics.1 One of the critical materials
for printed TFT circuits is a solution-processable electrical conduc-
tor. However, the latter has so far received disproportionally far
less attention than printable semiconductors,2 despite the fact that
it is equally important as conductive features (e.g., electrodes,
reduction of Ag(I) to Ag(0). The complexation of the amine function
conductive lines, and interconnects, etc.) in enabling low-cost TFT
of hydroxyalkylamine to Ag(I) would ensure its availability for
circuits.3-5 Scattered earlier works on solution-processable conduc-
reduction during annealing, while its low molecular weight would
tive materials focused largely on doped conjugated polymers such
enable its facile evaporation off the silver film after reduction.
as polyanilines,5a polypyroles,5b PEDOT,5c etc., which are not suited
We found that addition of a carboxylic acid to the silver salt
for these applications due to low electrical conductivity and poor
solution improved the continuity of the resulting silver film, leading
electrical/thermal stabilities. On the other hand, noble metals such
to significantly increased conductivity. Specifically, a spin-coated,
as gold and silver, which possess high conductivity (∼104-105 S
clear silver acetate/ethanolamine film on a glass substrate gave a
cm-1) and operational stability, require high-temperature vacuum
silver film with a conductivity of about 1.0-2.0 × 103 S cm-1 on
deposition. Gold nanoparticles have been utilized in printing highly
conductive elements for TFTs,3 but the high cost of gold has
overshadowed the attributes of this otherwise appealing approach.
Solution-processable silver materials have been investigated as
annealing in air at 200 °C for 10 min. On the other hand, a similar
silver salt film provided a significantly higher conductivity of 1.0-
2.0 × 104 S cm-1 under similar annealing conditions when a small
amount of oleic acid was added to the solution. Figure 1 shows
an alternative approach to low-cost printable conductors.4 Silver
the conductivity of the resulting silver film as a function of chain
pastes and inks generally provide low thin-film conductivity,4c while
length of the aliphatic carboxylic acid additive. The conductivity
commercial silver nanoparticles gave high conductivity for thin-
increased dramatically from a chain length of 7 to about 10 carbon
film features (<200 nm) only at high sintering temperature due to
atoms and then leveled offsan increase in conductivity of as much
relatively large particle sizes or presence of strong stabilizers.
as 1 order of magnitude was observed. A high conductivity (>104
Electroless plating of silver has been used with microcontact
S cm-1) could also be achieved at a lower annealing temperature
printing to fabricate TFT electrodes,4d but this method appears to
of 150 °C given sufficient annealing time (∼45 min). The resulting
be too complex to be practical. Recently, we reported the design
silver films showed good adhesion on glass and silicon wafer
of amine- and acid-stabilized sub-10 nm silver nanoparticles which
substrates.
could be converted to highly conductive, functionally capable
The effects of acid additive could be understood by studying
electrical conductors for TFTs.4a,b However, the preparation of such
the surface morphology of resultant silver film using optical
silver nanoparticles for low-temperature coalescence to conductive
microscopy. With no acid or an acid additive of a shorter chain
elements remains relatively challenging synthetically. The low
length (eC7), the resultant silver film contained numerous crystal-
yields or poor stability of silver nanoparticles may lead to increased
line domains (Figure 1, inset a) whose boundaries were believed
cost and potentially preclude their adoption in practical applications.
We describe here a simple solution process to silver conductor
for low-cost printed electronics using a solution of a silver(I) salt
(e.g., silver acetate), an organoamine (e.g., ethanolamine), and a
long-chain carboxylic acid (e.g., oleic acid) in an alcohol solvent
(e.g., n-butanol). The solution was first deposited as a thin film on
a substrate via various solution techniques (e.g., coating, stamping,
printing, etc.), and then thermally converted directly to conductive
elements without going through a silver nanoparticle intermediate.
Although silver salts are known to thermally self-reduce to metallic
silver, such reduction generally requires a high temperature.6 Strong
reducing agents, such as NaBH4, hydrazine, and aldehyde, on the
other hand, would cause rapid reduction of silver salt solution,
Figure 1. Conductivity of resultant silver film as a function of chain length
of the carboxylic acid additive. Insets: optical images of resultant silver
films from silver salt solution with (a) no acid additive showing numerous
leading to uncontrolled precipitation. Since both organoamine and
alcohol are mild reducing agents for silver salts,7 we therefore
utilized a low molecular weight hydroxy-substituted alkylamine for
crystalline domains and (b) oleic acid additive showing homogeneous film.
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10.1021/ja067596w CCC: $37.00 © 2007 American Chemical Society

C O M M U N I C A T I O N S
Figure 3. (a) Drain current IDS versus source/drain voltage VDS as a function
of gate voltage VG for an illustrative TFT with silver source/drain electrodes
from the silver salt solution (channel length and width are, respectively, 25
Figure 2. Thin-film X-ray diffraction of a silver salt solution annealed at
150 °C for 45 min. Inset is a SEM image of the resulting silver thin film.
and 1340 µm). (b) IDS and (-IDS)
1/2 versus VG at a constant VDS ) -60 V
used for calculation of the mobility and current on/off ratio.
to impede electrical conduction in the film, thus lower electrical
conductivity. With a longer carboxylic acid additive (>C8), a
homogeneous thin film with no discernible crystalline domain
boundaries was formed (Figure 1, inset b), thus the observed higher
electrical conductivity. Shorter-chain acids showed no effects since
they were too volatile and would be evaporated off during
annealing. Longer-chain acids slowed down on silver crystallization,
leading to a more homogeneous silver film containing less but larger
crystalline domains.
Highly conductive silver thin films prepared by this approach
were further characterized using X-ray and SEM analyses (Figure
2). X-ray diffraction displayed diffraction peaks at 2θ ) 38.10,
44.20, 64.34, and 77.39°, which were identical to those of a vacuum-
deposited silver film.8 SEM image showed a continuous silver film
of coalesced silver particles having an average particle size of
50-100 nm.
The performance of solution-processed silver conductors of the
present approach was evaluated as the source/drain electrodes in a
TFT device. Unlike single-layer conductive tracks such as antennas
for RFID tags or conductive lines for electronic interconnects, a
multilayered TFT structure would present a more challenging
environment for evaluating the functional performance of conduc-
tive elements. Poor interfacial contacts and/or intermixing of organic
semiconductor with the silver electrodes during deposition and
annealing would adversely affect the device performance. TFT
devices of a bottom-gate, bottom-contact structural configuration
were built on an n-doped silicon wafer using silver source/drain
electrodes generated from the silver acetate solution and a channel
semiconductor comprised of a spin-coated PQT-12 thin film.2a All
the experimental TFT devices exhibited excellent field-effect
transistor (FET) characteristics, which conformed closely to the
conventional gradual channel model in both the linear and saturated
regimes (Figure 3). The devices provided high FET mobility of
0.10-0.15 cm2 V-1 s-1, high current on/off ratio of 107, close to
zero turn-on voltage, and low threshold voltage of -6 V. These
were similar to those of reference TFTs with vacuum-deposited
gold electrodes2a and were significantly superior to those of
reference TFTs with vacuum-deposited silver electrodes (mobility
of ∼0.06 cm2 V-1 s-1; on/off ratio of 106-107). No contact
resistance was observed with smaller channel TFT devices (∼20
µm) even though it was present in similar devices with vacuum-
deposited silver source/drain electrodes. The absence of contact
resistance is believed to be due to the doping of PQT-12
semiconductor at the electrode/semiconductor interface by the
residual carboxylic acid on the surface of the resultant silver
electrodes.4b
relatively low temperature which is compatible with the structural
and dimensional integrity of commercial plastic substrates. The
silver acetate/hydroxylamine/carboxylic acid/n-butanol solution used
for the preparation of the silver conductor was very stable at room
temperature in the dark as no precipitation in the solution was
observed after being stored for over 6 months.
In conclusion, we have developed a simple approach to a
printable silver conductor for microelectronics. Our silver conduc-
tive features exhibited high electrical conductivity similar to that
of a vacuum-deposited silver conductor, yet worked far more
efficiently as electrodes in TFTs.
Acknowledgment. The authors are grateful to Sandra Gardner
for assistance in recording XRD and SEM images.
Supporting Information Available: Instrumentation, silver precur-
sor formulation, device fabrication. This material is available free of
charge via the Internet at http://pubs.acs.org.
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This approach is particularly significant for flexible electronics
in that the formation of metallic silver could be accomplished at a
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