Simple setup to measure electrical properties of polymeric films
R. K. Hiremath, M. K. Rabinal, and B. G. Mulimani
Citation: Rev. Sci. Instrum. 77, 126106 (2006); doi: 10.1063/1.2403937
View online: http://dx.doi.org/10.1063/1.2403937
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REVIEW OF SCIENTIFIC INSTRUMENTS 77, 126106 共2006兲
Simple setup to measure electrical properties of polymeric films
R. K. Hiremath, M. K. Rabinal,a兲 and B. G. Mulimani
Department of Physics, Karnatak University, Dharwad, Karnataka 580 003, India
共Received 14 April 2006; accepted 12 November 2006; published online 20 December 2006兲
A simple method to measure electrical conductivity of conducting organic films has been described.
A setup, based on four-probe technique, is specifically designed and fabricated for nondestructive
electrical conductivity measurements of freestanding thin films. The current-voltage and
temperature dependent characteristics of thin films of polyethylenedioxythiophene and polypyrrole
and thick wafers of germanium have been used to test the setup. The results obtained are highly
reproducible and are in good agreement with the reported values in the literature, employing
different techniques. © 2006 American Institute of Physics. 关DOI: 10.1063/1.2403937兴
Organic metals and semiconductors are conjugated carbon solids having a broad range of electrical conductivity
from 10−9 to 103 共⍀ cm兲−1.1–5 The partially delocalized pielectrons present in these materials are important in controlling optoelectronic properties. Conducting polymers constitute an important subclass of the above solids and their
conductivities can be varied from an insulating state to an
almost metallic state by chemical modifications.1,2 The incorporation of certain functional groups in the main chain of
polymers can convert them from being completely intangible
to processible materials.6 As a result, these materials find
wider applications in modern electronics such as light emitting diodes, thin film transistors, chemical and biological
sensors, etc.7 In contrast to metallic films, conducting polymers are lightweight, more flexible, noncorrosive, and lowcost materials. Intensive research has been carried out to explore different methods of measuring electrical conductivity
of solids in general8 and that of organic materials in particular. These techniques can be broadly classified into two
kinds: one is the conventional method of attaching electrical
contacts to the sample and another is without electrical contacts to the sample, also called contactless method. The latter
method is limited only to metallic materials.
In the first case, the contacts are established either by
evaporation of metals or by pressing sharp metallic needles
共point contacts兲 against the sample surface. Evaporated contacts are expensive, time consuming, and are fragile for making further electrical contacts to the sample. The point contact method has the advantage that contacts can be removed
and repeatedly reattached to the sample. But in the case of
soft and thin films, the technique severely damages the continuity of films and leads to erroneous results. Although it is
a widely accepted technique for electrical conductivity measurements of bulk materials, it is not suitable for soft and
thin films of polymers. Many times, these films are characterized by making contacts with silver or carbon pastes. The
significant limitations of these pastes are that they contain
organic solvents that may react with polymer films and that
silver migrates under the influence of electric field, leading
a兲
Author to whom correspondence should be addressed; electronic mail:
[email protected]
0034-6748/2006/77共12兲/126106/3/$23.00
to changes in the composition of the original material. The
migration of certain ions into bulk semiconductors under the
influence of electric field has been well reported.9–12 Therefore, it is desirable to have a suitable technique to measure
the conductivity of polymeric films in which the contacts can
easily be attached and detached without damaging the film
quality and which also avoids the use of silver and carbon
pastes. Considering the above facts, in this article we suggest
a simple method to measure the electrical conductivity of
polymer films as well as of other materials using gold-plated
flat contacts on insulating material loaded with symmetrical
spring pressure.
The conductivity setup is shown in Fig. 1. It has a
printed circuit board 共PCB兲 with four gold-coated flat electrodes, retrieved from the hard disk of an old computer. The
patterned electrodes created by conventional methods can
also be used. Each electrode has a width of 0.55 mm, length
of 3.5 mm, and height of 0.05 mm, and the gap between the
electrodes is 0.5 mm. This PCB rests on an aluminum platform 共3.5⫻ 4.5 cm2兲 amidst the spring stand 共insulated from
platform using Teflon washers兲, and comprises two brass
plates with springs of appropriate sizes between them. The
conductivity of the film is measured by loading it on the
electrode pattern, and a thin sheet of mica is covered over the
film for electrical insulation. The brass plates from the spring
stand are lowered on the mica spacer, and the springs are
symmetrically compressed to the desired pressure to establish electrical contact between the polymer film and metal
electrodes. The compression is done by turning the nuts on
the top brass plate. The patterned electrodes are connected to
the outer contacts with a wider separation for easy accessibility, as shown in Fig. 1. The present setup has the advantages that the four electrodes are flat and have smaller interelectrode separations. This ensures that films are better
supported, with more than 50% contact area between film
and electrodes. As a result, the thin films do not buckle in
between the electrodes under the loading pressure. In the
present case the estimated pressure on each electrode was
around 2 atm 共estimated for the contact area and the weight
required to compress the springs to the desired level兲.
All the measurements were carried out in open air using
regulated dc power supply, Keithley-197A DMM for current
measurements and Keithley-6512 electrometer for voltage
77, 126106-1
© 2006 American Institute of Physics
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126106-2
Hiremath, Rabinal, and Mulimani
Rev. Sci. Instrum. 77, 126106 共2006兲
FIG. 1. Experimental setup to measure electrical conductivity of thin films:
共1兲 spring stand, 共2兲 brass plate, 共3兲 aluminum plate, 共4兲 mica sheet, 共5兲 thin
film sample, 共6兲 printed circuit board, 共7兲 contact electrodes, and 共8兲 aluminum base.
measurements. Current is passed through the two outer electrodes and voltage is measured at the inner electrodes. Observations are recorded for the current ranging from
−200 to + 200 ␮A. For high temperature conductivity measurements, the conductivity cell is placed in a small furnace
共home-built兲 and the readings are scanned between room
temperature and 373 K. The electrical resistivity is calculated using the formula ␳ = ␳0 / G7共w / s兲, where ␳0
= 共V / I兲2␲s for nonconducting bottom, w is the film thickness, s is the separation between the electrodes, and G7共w / s兲
is the correction factor.13
Conductivity measurements were carried out on the
films of polyethylenedioxythiophene 共PEDOT兲 and polypyrrole 共PPY兲 and also on thick samples of germanium. Readings were scanned for positive and negative cycles of current
between −200 and +200 ␮A, and corresponding voltages
were noted.
Thin freestanding films of PEDOT-PSS were prepared
by slow evaporation of commercial solution 共supplied by
Baytron P, Bayers, Germany兲 in small flat-bottom crucibles.
Uniform films of size 6 ⫻ 2 mm2 and 180 ␮m thickness were
cut and used for conductivity measurements. The thickness
of the films was measured by the peacock dial gauge 共Ozaki
Mgf. Co. Ltd., Japan兲. Figure 2共a兲 shows repeated currentvoltage measurements at room temperature on as-prepared
film 共filled squares兲 and on annealed 共at 373 K兲 film 共open
hexagons兲. The curves are Ohmic in nature and are quite
reproducible. In each of these figures, five sets of data have
been overlapped to show the reproducibility. This shows that
the thermal annealing of PEDOT-PSS improves both conductivity and reproducibility of I-V curves. There are reports that
the annealing of PEDOT-PSS films at optimum temperatures
results in improvement of electrical conductivity and also of
the optical properties. It is shown by atomic force micros-
FIG. 2. 共a兲 Room temperature I-V curves 共five sets兲 of PEDOT:PSS, asprepared film 共filled squares兲, and annealed film at 373 K 共open hexagon兲.
共b兲 Room temperature I-V curves 共five sets兲 of PPY 共filled squares兲 and
germanium 共open circles兲. 共c兲 Temperature dependent resistivity of PEDOT
共filled squares兲, PPY 共open circles兲, and germanium 共inset兲.
copy 共AFM兲 and x-ray diffraction 共XRD兲 measurements that
annealing helps improve the microcrystallinity of the
polymer.14,15 The average electrical conductivity values calculated from these curves are 1.55 共⍀ cm兲−1 for as-prepared
film and 1.70 共⍀ cm兲−1 for annealed film. The reported values of in-planar electrical conductivity of PEDOT films lie in
the range of 1 – 10 共⍀ cm兲−1.16 There is a report on microscopic conductivity of PEDOT that has been measured by
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126106-3
Rev. Sci. Instrum. 77, 126106 共2006兲
Notes
AFM, i.e., the PEDOT composite films show morphological
inhomogeneities that lead to large anisotropy in electrical
conductivity measured at microscopic level. The variations
in conductivity may be attributed to the structural
inhomogeneities.17 Figure 2共c兲 shows a plot of resistivity as a
function of temperature for PEDOT-PSS film. Resistivity decreases with temperature between 300 and 340 K, which is a
signature of semiconducting behavior. Above 340 K the resistance increases with temperature.
The next sample chosen for the measurement was an
electrochemically synthesized film of PPY, 30 ␮m thick.
Five sets of room temperature current-voltage curves recorded for PPY are shown in Fig. 2共b兲 共filled squares兲. Reproducibility is evident from the overlapping nature of these
curves. The estimated electrical conductivity at room temperature resulting from these graphs corresponds to
1.53 共⍀ cm兲−1. Resistivity as a function of temperature is
measured and it shows a semiconducting behavior as can be
seen in Fig. 2共c兲 共open circles兲. The electrical conductivity of
PPY films highly depends on their methods of preparation
and the resulting morphology of composite films. Therefore,
it is rather difficult to compare our results with the published
data. In-plane conductivity values for PPY films reported under ambient conditions lie between 10 and 1000 共⍀ cm兲−1.18
To check the reliability of the setup, we also measured
the conductivity of commercially available germanium
samples 共n type兲; the current-voltage curves are highly symmetric and overlapping that is shown in Fig. 2共b兲 共open
squares兲. The calculated value of conductivity from these
curves is 0.182 共⍀ cm兲−1, which is quite close to the values
given by the supplier 0.16 共⍀ cm兲−1. The inset in Fig. 2共c兲 is
a plot of the temperature dependent resistivity of the germanium sample that shows extrinsic and intrinsic regions of
conductivity.
We also created similar patterns 共four probes兲 on indium
tin oxide 共ITO兲 coated glass 共supplied by Merck, Germany兲
by etching in 1M HCl solution containing zinc dust to develop photoconductivity measurements of polymer films.
PEDOT films mounted on such setup were stable in the beginning, but gradually became unstable. On the removal of
the polymer film, it was found that the ITO contacts were
partially etched and peeled off from the glass substrate. Subsequently, these measurements were repeated for few more
times with PEDOT-PSS and the same behavior was observed. In case of organic light emitting diodes, PEDOT-PSS
coated ITO are used as hole-injecting electrodes; under the
continuous operation of these devices, it is observed that this
type of interfaces degrade with time.19,20
In conclusion we have designed and fabricated a simple
setup for electrical conductivity measurements using printed
circuit board. The setup is tested successfully for conjugated
conducting polymer films and germanium samples. The
highly reproducible I-V curves provide the test of reliability
of the setup. The conductivity values of the tested samples
are within the observed limits and their semiconducting behavior is evident from the resulting graphs. Another feature
of this setup is that it can also be miniaturized to the required
dimensions to conduct the experiments under vacuum.
The authors acknowledge the Department of Science and
Technology, Govt. of India, for the financial assistance. One
of the authors 共R.K.H.兲 is grateful for the fellowship given
under the Physics Department’s UGC-DSA program 共Phase
III兲. The authors are grateful to Dr. Madan Mitra, Dept. of
Physics, Indian Institute of Science, Bangalore, for supplying
PPY films.
D. de Leeuw, Phys. World 31, 31 共1999兲.
C. K. Chiang, C. R. Fincher, Jr., Y. W. Park, A. J. Heeger, H. Shirakawa,
E. J. Louis, S. C. Gau, and A. G. Mac Diarmid, Phys. Rev. Lett. 39, 1098
共1977兲.
3
R. B. Kaner and A. G. Mac Diarmid, Sci. Am. 258, 60 共1988兲.
4
C. D. Dimitrakopoulos and D. J. Mascaro, IBM J. Res. Dev. 45, 11
共2001兲.
5
H. E. Katz, A. J. Lovinger, J. Johnson, C. Kloc, T. Soegrist, W. Li, Y.-Y.
Lin, and A. Dodbalapur, Nature 共London兲 404, 478 共2000兲.
6
W. Yin and E. Ruckenstein, Synth. Met. 108, 39 共2000兲.
7
G. Horowitz, Adv. Mater. 共Weinheim, Ger.兲 10, 365 共1998兲.
8
A. V. Ermakov and B. J. Hinch, Rev. Sci. Instrum. 68, 1571 共1997兲.
9
I. Lyubomirsky, M. K. Rabinal, and D. Cahen, J. Appl. Phys. 81, 6684
共1997兲.
10
T. Heiser and A. Mesli, Appl. Phys. A: Solids Surf. 57, 325 共1993兲.
11
L. Chernyak, K. Gartsman, D. Cahen, and O. M. Stafudd, J. Phys. Chem.
Solids 56, 1165 共1995兲.
12
I. Lyubomirsky, V. Lyahovitskaya, R. Triboulet, and D. Cahen, J. Cryst.
Growth 159, 1148 共1996兲.
13
D. K. Roy, Physics of Semiconducting Devices, 2nd ed. 共University Press,
Hyderabad, India, 2004兲, p. 78.
14
S. Cho and K. Lee, J. Korean Phys. Soc. 46, 973 共2005兲.
15
W. H. Kim, G. P. Kushto, H. Kim, and Z. H. Kafafi, J. Polym. Sci., Part B:
Polym. Phys. 41, 2522 共2003兲.
16
L. B. Groenendaal, F. Jones, D. Pielartzik, and J. R. Reynolds, Adv. Mater.
共Weinheim, Ger.兲 12, 481 共2000兲.
17
C. I. Zannetti, A. Mechler, S. A. Carter, and R. Lal, Adv. Mater. 共Weinheim, Ger.兲 16, 385 共2004兲.
18
A. Paul, D. Sarkar, and T. N. Misra, J. Appl. Phys. 28, 899 共1995兲.
19
D. Vaufrey, M. Ben Khalifa, J. Tardy, C. Ghica, M. G. Blanchin, C.
Sandu, and J. A. Roger, Semicond. Sci. Technol. 18, 253 共2003兲.
20
C. Nunes de Carvalho, G. Lavareda, E. Fortunato, and A. Amaral, Thin
Solid Films 427, 215 共2003兲.
1
2
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