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Substrates for Flexible Electronics: A Practical Investigation
on the Electrical, Film Flexibility, Optical, Temperature, and
Solvent Resistance Properties
Valerio Zardetto, Thomas M. Brown, Andrea Reale, Aldo Di Carlo
C.H.O.S.E. (Centre for Hybrid and Organic Solar Energy), Department of Electronic Engineering,
University of Rome-Tor Vergata, via del Politecnico 1, 00133 Rome, Italy
Correspondence to: T. M. Brown (E-mail: Thomas.Brown@uniroma2.it)
Received 21 January 2011; accepted 2 February 2011; published online 1 March 2011
DOI: 10.1002/polb.22227
ABSTRACT: Designing
and developing flexible electronics
requires a thorough investigation of the substrates available for
the fabrication of devices. Here, we present a practical study on
a variety of significant substrates: polyethylene terephthalate
(PET), its heat-stabilized (HS) derivative, HS-PET, and polyethylene naphthalate (PEN) plastic insulating films; indium tin oxide
(ITO)-coated ITO/PEN and ITO/PET transparent conducting films;
rigid ITO/glass and FTO/glass substrates; stainless steel and titanium foils. We put the substrates through a range of tests these
actually undergo during device fabrication to determine their
optical, mechanical flexibility (under different types of tensile
and compressive stress bending with and without a PEDOT:PSS
conducting polymer layer), solvent resistance, stability to temperature treatment (conductivity and deformation), and to UV
irradiation. We highlight issues and propose solutions to
improve substrate response. The results and thresholds
extracted reveal limitations and windows of opportunity useful
for the designer of flexible optoelectronics in determining manufacturing processes and the final applications under everyday
C 2011 Wiley Periodicals, Inc. J Polym Sci Part B:
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Polym Phys 49: 638–648, 2011
INTRODUCTION In the past decade, the interest from both
the industry and consumers toward flexible electronic and
optoelectronic devices has risen enormously due to the
unique properties of these being lightweight, bendable, conformable, rugged, and not easily breakable. The enthusiasm
is also related to the fact that the films used as substrates
enable web roll-to-roll processing leading to very substantial
gains in manufacturing throughput and economy.1 These features promise the future spread of flexible electronics in
many applications such as photovoltaics,2–7 organic lightemitting diodes,8–10 displays,11–13 lighting,14,15 e-paper,16,17
radiofrequency identification systems (RFID),18,19 thin-film
transistor electronics (TFT),19–23 pressure, image and chemical sensors,24–26 logic memories,27 and a raft of yet-to-bedevised new applications. The technologies are being developed with a great variety of materials, ranging from
solution-processed polymers, evaporated organic molecules,
thin-film inorganic semiconductors, and oxides to hybrids
materials and multilayers.
Thin polymeric foils are extremely attractive, because, first,
they can be made lightweight and inexpensive making it possible to target high-volume and low-cost commercial products. Processing of these materials being usually carried out
at temperatures below 180–120 C makes them particularly
attractive for organic technology based on low-temperature
printing or evaporation techniques. Secondly, they can be
made transparent: an essential prerequisite for applications
where light has to either enter (e.g., photovoltaics and photodetection) or exit (displays, lighting) the device. Finally, these
polymer foils are inherently plastic enabling a high degree of
mechanical flexibility and conformability.28
A variety of foils is being developed as substrates for flexible
electronics espousing the mechanical/optical properties of
these with the requirements arising from the various manufacturing processes and operation conditions of the finished
applications. In general, two classes of substrates are considered: plastic foils and thin metal foils.
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KEYWORDS: flexible films; indium tin oxide (ITO); polyethylene
naphthalate (PEN); polyethylene teraphthalate (PET); substrates
A number of plastic films are available including thermoplastic amorphous polymers such as polycarbonate, polyethersulfone, and high-transition glass temperature materials (Tg)
such as polyimide.29 The latter shows excellent thermal
properties but due to its low transparency, it is usually used
as an opaque substrate (e.g., on the back electrode side for
inorganic CIGS and a-Si solar cells, TFTs, and light-emitting
and reflective displays).30–32 The other aforementioned
transparent amorphous polymers do not always exhibit the
desirable mechanical and solvent resistance properties exhibited instead by semicrystalline polyethylene terephthalate
(PET) and polyethylene naphthalate (PEN) polymers. It is
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these materials (PET and PEN) that constitute the two main
transparent flexible substrates used today in the development of flexible electronics. Not only they provide good resistance to solvents and a discrete tolerance to temperature
but also their intrinsic clarity makes them apt as transparent
substrate electrodes when coated with indium tin oxide
(ITO) for all those optoelectronic applications that require
transmission of light and electrical conduction.
Thin metal foils represent an alternative to plastic films
where higher temperatures are required during manufacturing (for instance, in the fabrication of inorganic thin film
a-Si, CIS, CIGS solar cells, or silicon transistors). They also
provide excellent conductivity, good chemical resistance, and
a better barrier to moisture or oxygen ingress compared to
plastic sheets, which often require additional special encapsulation where long-lasting operation is a must.33 Stainless
steel (SS) is a common choice,34–38 or even titanium foil for
chemical inertness.39 For many high-volume applications
such as the aforementioned displays and photovoltaics, however, at least one substrate has to be transparent. For all
these ‘‘front interfaces,’’ PET, and/or PEN films have often
become the substrate of choice.
Building a quantitative and comprehensive knowledge of the
characteristics of these substrates becomes crucial for
ongoing research and development into the manufacturing
processes one can use and for designing applications that
can withstand every day use. Some studies have been carried
out focusing on particular aspects such as those relating to
the bare nonconductive polymer substrates,28 or on the
mechanical stability of ITO,40–42 or on the thermal stability
and optical properties of conductive plastic foils.43,44 The
aim of this work is to carry out a broad, comprehensive systematic investigation encompassing mechanical flexibility,
thermal and electrical stability, solvent-resistance, and optical properties of PET and PEN substrates, both in their bare
form but importantly also when coated with ITO.
The investigation consists in putting the substrates through
a range of reproducible tests, which the substrates actually
undergo during device fabrication in the laboratory. Furthermore, we have also endeavored to extend the practical investigation to other situations that simulate possible manufacturing processes and operation conditions of the final device.
First, we investigated the effect of a temperature treatment
(from room temperature to 250 C) on the sheet resistance
of flexible foils (PET/ITO, PEN/ITO, Ti, and SS). We compared these with the behavior of ITO and fluorine tin oxide
(FTO)-coated glass, which were able to withstand higher
temperatures. ITO and FTO glass represent the transparent
substrates usually used for ‘‘rigid’’ optoelectronics. Subsequently, we quantified the deformation arising from temperature treatments in bare PET, its heat stabilized (HS) derivative (HS-PET), and HS-PEN (here listed in inverted order of
stability according to our experiments). PET/ITO and PEN/
ITO films (sheet resistance of 15 X/h) showed poorer temperature stability compared to their bare counterparts. As an
example, to highlight the importance of temperature treat-
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ments on an optoelectronic device that is also being developed on flexible substrates,45 experimental data of the conversion efficiencies of dye solar cells, as a function of the
sintering temperature of the TiO2 layer will be presented.
For binder-free pastes, we found that the power conversion
efficiency g increases monotonically with T.
We evaluated the mechanical stability of substrates monitoring how the sheet resistance of both PET/ITO and PEN/ITO
conductive substrates changed under compressive and tensile stress for different radii of curvature r (down to 3 mm).
Because ITO is rigid and brittle, it cracked when it was bent
and stretched, leading to a dramatic decrease in its conductivity. Cracking occurred under tensile stress for bending
radii <14 mm, depending on flexing conditions. Tensile
stress was more damaging than compressive stress.
We spin-coated poly(3,4-ethylenedioxythiopene) poly(styrenesulfonate) (PEDOT:PSS) layer over ITO and observed
improvement in the electrical stability of the system under
mechanical stress with the conductive polymer layer also
furnishing an upper limit to the overall sheet resistance of
ITO/PEDOT:PSS even after the most demanding bending
tests.
Optical transmission data for all the substrates are also
reported, highlighting the differences between PET and PEN
and their conductive ITO counterparts (including ITO and
FTO glass). Optical degradation was monitored after UV
treatment leading to a loss of transparency and some yellowing in particular in PEN. Finally, we show that the PET/ITO
and PEN/ITO films show remarkable stability to treatments
under the most common polar and a-polar solvents used in
the fabrication of plastic electronic devices.
EXPERIMENTAL
Substrates
PET (Melinex506), heat-stabilized HS-PET (Melinex ST506),
and HS-PEN (Teonex Q65FA) films (125-lm thick) were supplied by Dupont-Tejiin Films; PET/ITO (LR15 175 lm thick)
and PEN/ITO (125 lm thick) by Solutia Performance Films
(sheet resistances of 15 X/h); glass/ITO (10 X/h thick 1.1
mm) by Kintec and glass/FTO (8 X/h 3.1 mm and 15 X/h
3 mm) by Pilkington. Stainless steel (FE220270) and titanium (TI000380) foils by GoodFellows; additional PET/ITO
by Sigma Aldrich (639303). Before all experiments, the substrates were rinsed in beakers containing acetone and ethanol. For resistance measurements, the substrates were cut to
a dimension of 2 2.5 cm2 for plastic and metal samples
and 2 2 cm2 for glass. Silver paste (RS 186-3600) was
applied precisely to two sides. The resistance, RCAL, was
measured between the two silver strips. The sheet resistance
(RSHEET) was estimated using RCAL(W/L), and the values
obtained were equivalent to those provided on the specification sheets by the suppliers.
Thermal Stability
Plastic films, metal foils and conductive glass were processed
in a precision hotplate with closed lid to increase uniformity
(Titan Typ PZ 28-3TD) for 30 min at a particular temperature.
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The oven used was Lenton VHT6/120. Sheet resistance value
was always measured at room temperature (after the 30-min
treatment), when the temperature returned back to RT. Temperature-induced plastic deformation was gauged estimating
the radius of curvature r. r was calculated by using the trigonometric Chord Theorem and Law of sines based on two
experimentally measured dimensions: projection of the substrate length on the ground plane (measured with a caliper)
and the substrate height from the plane (measured by monitoring the focus distance difference between the curved
surface and a reference flat one with an Olympus BX51M Optical microscope with a 50 magnification lens). For the most
severe deformation (i.e., 2pr < substrate length), the diameter
of the rolled up substrate was measured directly with the
caliper.
Mechanical Flexibility
To gauge the bending effects on the electrical characteristics
of the conductive substrates, the samples were applied conformally to cylinders of different radii ranging from 30 to
3 mm. For tensile (compressive) stress, the ITO side was facing externally (internally) from the cylindrical surface.
Resistance was monitored in two situations: in the first,
measurements were carried out flat after bending cycles (1,
50, 100, and 150); in the second, when the sample was kept
fixed on the cylinders. For practicality, the substrate lengths
and width were p*r and 0.75p*r, respectively. Micrographs
were taken with an Olympus BX51M optical microscope with
a 50 magnification lens. PEDOT:PSS CLEVIOS P HC-V4 was
deposited from a 2.7-lm filtered solution by spin-coating
yielding (110 6 10) nm thick films. The samples were
annealed at 120 C for 10 min in a Nitrogen glove box.
Optical Investigation
Transmittance spectra were measured with a spectrophotometer (Shimadzu UV–vis 2550, with integrating sphere,
MPC220). The effect of UV irradiation on the plastic samples
was carried out by subjecting these to a 150-min UV treatment under a (5000-EC by Dymax) lamp (with a power of
225 mW/cm2). Total luminous AM1.5G light transmittance
(sTOT) was calculated with the following equation:
sTOT ¼
Z760
380
, Z760
TðkÞ DðkÞ VðkÞ dðkÞ
DðkÞ VðkÞ dk
380
where T(k) is transmittance data measured, D(k) is solar
irradiance spectra,46 and V(k) is human eye sensitivity.47
Solvent Resistance
PET/ITO and PEN/ITO samples were immersed in an
ultrasonic bath of acetone (10 min) and ethanol (10 min).
Samples were dipped also for 3 h in ethanol (414608 by
Carlo Erba), acetone (406974 by Carlo Erba), and 2 h in
xylene, toluene (488555 by Carlo Erba), and chlorobenzene
(270644 Sigma Aldrich).
Preparation of Dye Solar Cells
Preparations of the devices were those described in ref. 48,
except for dye (N719 from Dyesol), electrolyte (EL-HSE from
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Dyesol), and for the TiO2 layer. TiO2 layer was prepared by a
binder-free paste containing TiO2 nanoparticles of 25-nm
diameter (P25 Degussa) and 100-nm diameter (Sigma
Aldrich 634662V) mixed in a ratio of (7:3) in water and
ethanol (at 20% wt) solution (1:1.5). After deposition via
blade coating (5 5 mm2 cm2) on FTO-glass, the TiO2 was
sintered in the hotplate at different temperatures (RT to
600 C) before completing the device fabrication. All the
devices were tested under a sun simulator (Solar constant
1200 KHS) at AM 1.5 1000 W/m2 calibrated with a Skye
SKS 1110 sensor.
R
RESULTS AND DISCUSSION
The substrates investigated were of four types: (1) bare
polymer films, PET, HS-PET, HS-PEN, provisioned from the
same manufacturer (Dupont Teijin), to highlight the difference between the substrates rather than variations that can
arise from different suppliers; (2) PET/ITO and PEN/ITO
films (N.B. where not specifically mentioned all PET/ITO and
PEN/ITO data consisted in Solutia Performance Films foils.
Any additional data from different suppliers will be specified
in the text); (3) Stainless steel and titanium foils and (4)
glass/ITO and glass/FTO. The results of each data point
represent the average over three different samples, and the
error bars correspond to the standard deviation.
Thermal Properties of Substrates
Two important characteristics to gauge when choosing a
substrate for temperature stability arising from thermal
processes during device fabrication and/or operation are the
deformation (for flexible substrates) and electrical degradation (for conductive substrates).
Electrical Stability After Thermal Treatment
Thermal stability of the conductive plastic (PET/ITO, PEN/
ITO), metal foil (Ti, SS), and glass (glass/ITO and glass/FTO)
substrates was evaluated by monitoring the sheet resistance
as a function of treatment temperature (30 min) as shown
in Figure 1.
Starting with glass substrates, the sheet resistance (RSHEET)
of ITO stays roughly constant (10 X/h) till 250 C, after
which it starts to increase, doubling at 325 C and reaching
four times its original value at 600 C. Interestingly instead,
the sheet resistance of glass/FTO does not increase but stays
constant till about 450 C and then actually decreases to
7 X/h at 600 C making it a more viable option for those
applications, which require stability for temperatures above
250–300 C. The behavior for these conductive glass substrates is consistent with work by Kawashima et al.49
PEN/ITO shows good electrical stability up to 235 C up to
which the sheet resistance value remains stable (15 X/h).
At 250 C, however, the resistance increases almost 20-fold
due to partial melting of the substrate. For PET/ITO, the
sheet resistance remains stable (15 X/h) only up to 150 C,
increasing by 20% at 200 C (18.1 X/h). Above 200 C,
RSHEET increases dramatically until the substrate has melted
substantially at 250 C. The higher thermal stability of PEN/
ITO compared to PET/ITO can be mainly ascribed to the
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measurements were carried out: in the first case, the probes
were carefully placed on the Ti surface; for the second curve,
the probes were applied with force, thereby, (partially)
boring through the thermal oxide. This effect was not present in stainless steel.
Thermal Deformation
We found that the films curve when heated. The deformation
of plastic substrates is reported in Figure 2(a) in terms of
radius of curvature (and its reciprocal) of the deformed
substrate as a function of treatment temperature (30 min).
Obviously, the smaller the radius, the larger the temperatureinduced deformation undergone by the substrate. Results are
presented for PET, HS-PET, HS-PEN, PET/ITO, and PEN/ITO.
For the glass/ITO, glass/FTO, and the Ti and SS metal foils,
we did not observe any appreciable deformation.
At 115 C, the PET films bend to a radius of curvature of
r ¼ 1 m. The more lenient threshold of r ¼ 0.5 m is attained
at 152 C. Improvement was observed for heat stablized HSPET for which the thresholds are 180 C (r ¼ 1 m) and
183 C (r ¼ 0.5 m). HS-PEN, whose thresholds are 180 C
(r ¼ 1 m) and 193 C (r ¼ 0.5 m), shows a less pronounced
deformation above 180 C. At 250 C, both types of PET
melt loosing all mechanical consistency, whereas PEN still
retains some of its consistency albeit signifcantly deformed.
FIGURE 1 (a) Room temperature sheet resistance of conductive
PET/ITO, PEN/ITO foils and glass/ITO, glass/FTO substrates
after undergoing a thermal treatment at different temperatures
for 30 min; (b) resistance as a function of treatment temperature (30 min) for stainless steel (SS) and titanium (Ti). For Ti
(1), the electrical probes were gently placed on the Ti surface;
for Ti (2), these were applied with significant pressure (see
text).
The higher temperature stability of heat stablized HS-PET, in
comparison to PET, is due to the heat-stabilization process
that increases the dimensional stability of polymer films,
because the internal strain in the film itself is relaxed by
exposure to high temperature while under minimum line
thermal stability of the underlying polymer films, as we will
show below.
Stainless steel shows a constant resistance (R ¼ 0.4 X) for
the entire range of temperatures (RT to 600 C). With
respect to the resistance measured on the PET/ITO films
(R ¼ 16.5 X, equivalent to RSHEET ¼ 15 X/h), the resistance
R measured for SS of the same dimension is, thus, a factor of
41 lower (useful for applications where high current densiies
are present). It is interesting to note that the resistance of
the Ti of the same dimension (0.3 X at RT) only remains stable up to 325 C and then increases considerably. Two
curves are reported in Figure 1(b). The first increases
steeply with T (R > 2 M X at 600 C). The other has a less
pronounced slope (R ¼ 5 X at 600 C). The increase of
resistance is likely due to the growth of a thin layer of insulating oxide whose thickness depends on annealing temperature (and time) when the process is carried out in air.50 The
difference between the two curves lies in the way the two
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FIGURE 2 (a) Deformation of plastic substrates (PET, HS-PET,
HS-PEN, PET/ITO, PEN/ITO) in terms of radius of curvature
(and its reciprocal) as a function of treatment temperature (30
min). The ‘‘PET/ITO constr.’’ samples consisted in PET/ITO
mechanically constrained on two sides of the film onto rigid
substrates; (b) image of PET/ITO substrates after the thermal
treatment.
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tension.51,52 In addition, HS-PEN is more stable than HS-PET
due to the presence of naphthalene rings instead of the
benzenes, which increases the Tg of the material.53
The behavior in temperature changes when one considers
the same substrates coated with an ITO layer. First, the substrates show some (albeit not significant) curvature even at
room temperature. This may be due to residual stresses contained in the foils arising from the deposition process of the
ITO on the polymer.40 The films start to bend more rapidly
(i.e., at lower temperatures) than their neat PET and PEN
counterparts. The presence of ITO leads to more pronounced
deformation as the radii of curvature were about an order of
magnitude greater than the neat polymer film counterparts.
Above 150 C, PET/ITO shows a more pronounced deformation compared to PEN/ITO, which shows a dramatic increase
in curvature only above 180 C. PET/ITO films actually
tended to completely roll up at 220 C (and to melt at
250 C) as shown in Figure 2(b).
Although in most laboratories, the substrates are processed in
hotplates or ovens as sheets (RSHEET show the same values in
oven and hotplate, whereas thermal deformation is slightly
higher in the oven), it is likely that for high-volume production,
the substrates will be processed in roll-to-roll continuous web
form. Here, tensile forces along the substrate axis will constrain
the film (along the ends of the two rolls). We simulated the
application of these tensile forces by constraining two sides of
the flexible films to rigid substrates using tape. As can be
noted, the constraint improves the ruggedness of the flexible
substrates by helping to reduce significantly the deformation
caused by the thermal treatments: for PET/ITO, the onset of
significant deformation is delayed to 150 C. We observed that
the ITO films always bend convexly both in the oven and hotplates [in Fig. 2(b), the ITO is on the top side of the substrates], as a result of internal stresses and differences in the
coefficient of linear thermal expansion of polymer and ITO.40
Finally, to highlight the importance of this temperature study,
we report the conversion efficiencies (g) of dye solar cells as
a function of sintering temperature for the binder-free TiO2
layer on FTO-glass substrates in Figure 3. For these binderfree pastes, differently from pastes that include binders,54 g
increases monotonically with T because higher temperatures
lead to improved electromechanical bonding between the
TiO2 nanoparticles. Thus, the availability of substrates that
withstand higher temperatures can be important for improved
performance of devices (and ultimately of cost/KWp). At
the moment, until R&D provides even more dimensionally
stable alternatives, opaque metal foils have to be used for
temperatures above 180 C,2,55 or alternative sintering
methods identified, such as mechanical compression,6 microwave,56 UV,57,58 or raster scanning laser processing.48,59
Flexibility and Resistivity
One of the most valuable properties of polymer films is their
inherent flexibility. This is particularly appealing for electronic applications, which can be made flexible or conformal
to an underlying curved surface but also during manufacturing (the films are wound-up in rolls). Flexibility becomes
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FIGURE 3 Power conversion efficiencies of dye solar cells as a
function of sintering temperature of the binder-free nanocrystalline TiO2 layer on glass/FTO substrates. Also reported
vertically are our estimates for the upper treatment temperature (for 30 min) applicable for some of the substrates investigated in our study without leading to significant degradation
of the substrates. For PET/ITO this depends on heating procedure as explained in the text; for stainless steel and FTO, this
may lay even higher than the 600 C range of our studies.
appealing only when there is little loss in the conductance of
the conductive films on bending or flexing the substrates.
We monitored the sheet resistance of PET/ITO and PEN/ITO
films under compressive and tensile stress for varying radii
of curvature (from 30 to 3 mm) in two different situations:
in the first, the samples underwent bending cycles and were
then measured when flat; in the second, the substrates were
measured when still fixed on the curved cylindrical surface.
Naturally, the results depend greatly on the properties of the
conductive film used. ITO is a brittle inorganic electrode.
PEDOT:PSS, first used in OLEDs as anode60 leading to
improved device performance in combination with ITO61
mainly due to a lowering of the injection barrier (increase of
the built in potential)62 has become a widely used conductive polymer,8,60–63 for a great variety of other optoelectronic
devices including solar cells and TFTs.23,64 We will show that
PEDOT:PSS films on curved flexible foils, both alone and in
combination with ITO, dramatically improves the electrical
stability of the substrates on flexing.
ITO
In Figure 4(a), the sheet resistance of PET/ITO is reported
as a function of the radius of curvature, r, during the bending
process. Under all our bending conditions, the conductivity
of the TCO remained unaltered for r > 14 mm. For smaller
radii of curvature, we observed a big difference in behavior
between the substrates that underwent tensile stress (i.e.,
ITO on the outer surface) to those that underwent compressive stress (i.e., ITO on the inner surface). Immediately below
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a surface). For one cycle, degradation of the conductance
only occurs at r ¼ 5 mm. We also measured RSHEET for 50,
100 bending cycles. RSHEET increased monotonically under
repeated flexing (from 1 to 150 cycles).
The degradation of ITO conductivity when increasing curvature is related to the fact that the ITO film is brittle. The
optical microscopy images of Figure 5 (relating to 150 cycle
tests) show that the ITO coating has damaged considerably
after undergoing those tensile and compressive stresses that
degrade its conductance. The number of visible cracks
increases for smaller radii of curvature (thus, leading to an
increase in resistance). For tensile stresses (Fig. 5), the
cracks are less visible although more numerous per cm2
compared to those arising from compressive stress. Furthermore, degradation not visible at the optical microscopy level
may also occur. The fact that ITO substrates are more sensitive to tensile stresses is also evident in Figure 5 and confirmed in other studies.65,66 It has been reported that for
tensile stress, the main failure is usually due to cracking/
channeling, whereas for compressive stress, debonding may
become important.67,68 Debonding will depend also on the
quality of adhesion between ITO and the polymer films.69
FIGURE 4 Sheet resistance of PET/ITO (a) and PEN/ITO (b) as a
function of radius of curvature under tensile (in black) and
compressive (in red) stress for three different situations: measured on a curved cylindrical surface (squares-line); after 150
(triangles-dashed) and 1 (diamonds-dotted) bending cycles
measured flat.
r ¼ 14 mm, both the PET/ITO substrates that underwent
fixed tensile stress (i.e., the film was measured when still
bent—useful for conformal applications) and bending for
150 cycles (measured flat), show a marked degradation in
conductance. At r ¼ 11 mm, the resistance has already
increased by about an order of magnitude. The situation
aggravates at even smaller r. Compressive stress is much
more lenient on the substrates: degradation of the conductivity occurs around r ¼ 8 mm for fixed compressive and
around r ¼ 11 mm for 150 cycles stress. Resistance values
measured in fixed tensile stress are higher than after
150 cycles flat, because in the former case, the failures or
cracks are stretched under measurement. The stretching is
not as present under fixed compressive stress as can be
noted from the different behavior.
Figure 4(a) also shows the data measured on substrates that
underwent only one bending cycle as it consists in a useful
data point for applications or manufacturing processes
where the substrate finds itself bent only once (e.g., in a production or storage roll before being applied flat or curved to
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PEN/ITO substrates have a similar behavior to that of PET/
ITO films in that tensile stress degradation occurs for r < 14
mm, whereas for compressive stress, degradation occurs for
r < 8 mm (at r ¼ 5 mm RSHEET increased by 50% after 150
cycles); for one cycle degradation occurs around r ¼ 5 mm.
However, it is evident that the electrical degradation on flexing is less pronounced. Although different adhesion properties cannot be excluded, the different behavior (particularly
evident under tensile stress) can be ascribed to the fact that
the PEN films (125 lm) were thinner than the PET films
(175 lm) because strain is related to film thickness.65,69
We also found that the same flexing tests carried out on
Sigma Aldrich PET/ITO, which (presumably) has a thinner
ITO film (due to its higher sheet resistance of 60 X/h),
improves the flexibility versus resistivity behavior of the
films in all conditions (no variations of RSHEET observed till
r ¼ 7.5 mm). Thus, for electronic devices for which high
current densities are not developed, thinner ITO coatings
lead to better mechanical properties (at the expense of conductivity) as also reported by Chen et al.65
Our investigations included different types of measurements,
which may be useful in designing the structure and the position of a conductive layer in a flexible device and the maximum bending that can be tolerated. Bending cycles simulate
all applications in which the device will be rolled up or
flexed during operation. A sample fixed on a curve surface
simulates a (conformal) device, which remains curved on
operation. We note that, in some applications, where two
opposite conductive substrates are used (e.g., plastic dye
solar cells) one electrode will undergo tensile stress at the
same instant when the opposite electrode will undergo a
compressive one. In this case, to avoid any type of failure
mechanisms in the conductive layer, our data show that the
radius of curvature must be limited to a value 14 mm.
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FIGURE 5 Optical microscopy images (50) of the ITO surface (on PET) after 150 bending cycles under compressive (C) and tensile
(T) stresses for different radii of curvature: (a) r ¼ 14 mm; (b) r ¼ 7.5 mm; (c) r ¼ 3 mm.
PEDOT:PSS and ITO/PEDOT:PSS
In Figure 6, the sheet resistance of PEDOT:PSS on PET,
PEDOT:PSS on PET/ITO are compared to that of PET/ITO as
a function of the radius of curvature, r, during the fixed
tensile stress experiment (i.e., the film was measured when
still curved). No appreciable variation in the sheet resistance
(5.6 103 X/h) was observed for PET/PEDOT:PSS substrates for the whole range of curvatures investigated down
to r ¼ 3 mm). This is in contrast with the PET/ITO sample,
which although starting from a greatly lower value, shows a
dramatic increase in resistance for all r < 14 mm. The polymer film shows its obvious ‘‘plastic’’ properties as opposed
to the degree of britleness inherent in the ITO coatings.70,71
the latter case, this is because the overall resistance of the
ITO/PEDOT:PSS system is a parallel of the two contributions
(the one due to ITO becoming very large). Note that smaller
cracks may appear for the larger r (e.g., r > 10 mm) and
that PEDOT:PSS may also fill these acting as an electric
bridge. Furthermore, if the crack size is comparable to the
length of the polymer chains, the conductance enhancement
In many applications, PEDOT:PSS is deposited onto ITO,
using the latter for its better conductive properties and the
former for its electronic properties (that lead to better hole
injection or collection).61 Figure 6 shows that degradation of
the sheet resistance in PET/ITO/PEDOT:PSS films is delayed
compared to the bare PET/ITO films and starts only for r <
11 mm (instead of r < 14 mm) and also is less marked.
Although the resistance for PET/ITO continues to monotonically increase when reducing r (reaching values of 106 X/h
for r ¼ 3 mm), the resistance of the PET/ITO/PEDOT:PSS
does not exceed the intrinsic value of the PEDOT:PSS resistance even for r ¼ 3 mm.
The introduction of PEDOT:PSS layer over the ITO coating
has three positive effects. Firstly, it delays the onset of degradation to lower radii of curvature; secondly, the rate of
resistance increase is reduced; thirdly, it limits the highest
resistance value to that of the intrinsic PEDOT:PSS film. In
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FIGURE 6 Sheet resistance of PEDOT:PSS on PET (squares)
and PEDOT:PSS on ITO/PET (empty triangles) and ITO/PEN
(empty circles) compared with the bare PET/ITO (full triangles)
and PEN/ITO (full circles) as a function of the radius of curvature during the fixed tensile stress test (i.e., the film was measured when curved).
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added by the PEDOT:PSS overlayer will be more effective
(high r) that in the case, where the cracks become much
wider (lower r). If this particular PEDOT:PSS layer is
replaced with a different and more conductive variety, it is
likely that the asymptotic value would decrease to the intrinsic sheet resistance achievable.72–75 The closer this value to
that of the more brittle ITO, the more appealing it would
become for flexible electronics.
Optical Investigation
Transparency of the substrate is one of the crucial requirements for flexible optoelectronics where light has to either
penetrate or exit the device (e.g., displays, photodiodes).
Here, we study the transmittance spectra of the materials
and estimate light transmission in the visible range (380–
760 nm).76 Also, we investigated the optical stability under
UV exposure, important for outdoor applications or for
particular manufacturing processes (e.g., photolithography or
UV-ozone treatments).
Transmittance Spectra
Transmittance spectra for HS-PET and HS-PEN and their
ITO-covered counterparts are reported in Figure 7(a) in the
330–800 nm range.
Neat HS-PET films show the higher transmittance over all
the visible range is flat and around 90% (sTOT¼90.5%), only
decreasing appreaciably in the UV (k < 380 nm). HS-PEN
films show a lower transmittance (sTOT¼88.7%) especially in
the blue region of the visible spectrum (transmittance drops
decisevely below 400 nm). The difference between the two
materials is ascribed to the naphthalene rings in the PEN
main chain, which start absorbing at longer wavelengths
compared to the benzene rings in the PET.77,78 Note that
nonheat stablized films are more transparent than the HSPET.
The addition of ITO to the PET and PEN foils brings about
an overall reduction of sTOT (85.2 and 82.3%, respectively)
particularly evident in the blue and especially in the UV part
(k < 420 nm) of the visible specrtum. The spectra show a
maximum at 560 nm (87.3%) and 576 nm (84.6%), respectively (N.B., the peak sensitivity of the photopic human eye
is at 555 nm).79
FIGURE 7 (a) UV–vis trasmittance spectra for PET/ITO and PEN/
ITO and also HS-PET and HS-PEN (before and after UV exposure). (b) UV–vis trasmittance spectra for PET/ITO (15 X/h),
PEN/ITO (15 X/h), glass/ITO (10 X/h), glass/FTO (8 X/h and 15
X/h) conductive susbtrates.
Figure 7(b) shows a comparison of the transmittance spectra
of conductive plastic (PET/ITO PEN/ITO), with those of conductive glass (glass/ITO 10 X/h and glass/FTO 8 X/h and
15 X/h). The glass substrate covered with ITO shows a total
light transmittance of 86%, while glass covered with FTO
78.6% and 83% in the visible range, for the more and less
conductive versions respectively (note: however, that the
glass substrates covered with different conductive layers are
also different in thickness, see Experimental section). The
spectra also show that glass substrates are more transparent
in the UV range compared to the plastic films and also that
FTO shows a flatter transmittance in the visible range.
exposure (150 min). When HS-PEN is exposed to a significant flux of UV light, transparency decreases (sTOT from 88.7
to 87.7%) especially in the blue range of wevelengths,
380 nm < k < 500 nm.76 PET is more resistant to UV exposure (sTOT from 90.6 to 90%). Indeed, we noticed a discernible yellowing of PEN after UV irradiation, not appreacible
with PET (even if the transmission spectra in the violet
range is slightly lower post-treatment even for PET). Yellowing is due to photo-oxidation, triggered by energetic UV photons leading to scission of bonds in the backbone of polymer,
formation of new complexes that can lead to yellowing, loss
of elasticity, and even cracks and material loss.80–82 The difference is mainly due to the presence in PEN of the naphthalene structures (instead of the benzenes in PET), which are
more easily affected by UV radiation.80 Processes for UV
stabilization of polymers and UV-stabilized films can be considered where increased UV stability is sought.83
Stability Under UV Exposure
Figure 7(a) shows a comparison between the transmission
spectra of HS-PET, and HS-PEN before and after a UV-light
Solvent Resistance
Before fabricating the various electronic devices over the
flexible substrates, these typically undergo a cleaning step to
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remove any contaminants.84 A common wet-cleaning procedure involves subjecting the substrates to ultrasonic baths in
acetone and ethanol (or isopropanol). We monitored the
PET/ITO and PEN/ITO samples resistivity before and after
an ultrasonic bath (10 min each) and a longer immersion
(3 h each) in these two solvents, and no visible damage and
variation in the conductivity of ITO was observed.
The substrates need to also withstand the action of more
a-polar solvents that are often used to deposit the active
layers over these substrates (e.g., via printing techniques).
After 2 h of complete immersion in xylene, toluene, and
chlorobenzene, the samples showed no visible damage or
any variation in the sheet resistance for either substrate
(PET/ITO, PEN/ITO).
CONCLUSIONS
A practical investigation was carried out on the electrical,
thermal and mechanical flexibility, thermal deformation,
optical transmittance, and solvent resistance properties of
plastic film substrates (PET, PEN and ITO/PET, ITO/PEN).
Where applicable, these properties were compared with rigid
transparent ITO/glass and FTO/glass substrates, flexible
titanium and stainless steel (SS) metal foils. The results
aimed to provide useful information and insights for the
design and fabrication of flexible electronic devices.
After a thermal treatment of 30 min, the PET films spontaneously bent to a radius of curvature of 1 m at 115 C. The
more lenient threshold of r ¼ 0.5 m was attained at 152 C.
Improvement was observed for high stability HS-PET and
HS-PEN for which the thresholds were 180 and 180 C (r ¼
1 m) and 183 and 193 C (r ¼ 0.5 m), respectively. For the
ITO-coated films, the deformation was much more pronounced compared to neat substrates and at considerably
lower temperatures. We found that by constraining two sides
of these films (i.e., simulating the tensile forces of web processing) deformation can be contained yielding good mechanical and electrical stability up to higher temperatures
(150 C for PET/ITO). We observed that the limiting factor
was not the electrical stability (PET/ITO showed no variations of sheet resistance up to 180 C while PEN/ITO until
235 C) but the physical deformation of the plastic samples.
Stainless steel guaranteed a constant resistance in the whole
range of T we studied (RT to 600 C), whereas thermal oxidation of Ti needs to be taken into account for temperatures
above 325 C. No appreciable thermal deformation of these
opaque metal foils occurred in T. For ITO/glass, the sheet resistance stayed roughly constant (10 X/h) till 250 C, after
which it started to increase, doubling at 325 C. For FTO/
glass no increase in RSHEET was observed up to 600 C.
Flexibility tests for PET/ITO and PEN/ITO showed that
tensile stress degrades the conductivity more significantly
than compressive stress both when the substrates were fixed
on a curved surface and after 150 bending cycles measured
flat (electrical degradation increased monotonically with
number of bending cycles). In all cases, no electrical degradation was seen when flexing the substrates down to r ¼
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14 mm, which, thus, constitutes a ‘‘safe’’ bending radius. For
compressive tests, no changes in RSHEET occurred down to
r ¼ 11 mm for PET/ITO, and r ¼ 8 mm for PEN/ITO. Optical
microscopy highlighted the influence of failures such as
cracks appearing in the ITO coating upon different degrees
of flexing. When applying a PEDOT:PSS film over the ITO/
PET and ITO/PEN substrates, the electrical flexibility of the
device improved also giving an upper limit to RSHEET in the
cases where dramatic degradation (r < 5 mm) of the ITO
occurred.
Optical transmittance spectra showed that PET is more
transparent than PEN (with and without ITO coating layer).
Application of a 15 X/h ITO overlayer reduces absolute
transmission in the visible by 5%. UV-induced degradation
was evident in the yellowing of PEN, but not noticeable in
PET. Common solvents of different polarity used for either
cleaning procedures (acetone and ethanol) or processing
(xylene, toluene, and chlorobenzene) did not affect the electrical characteristics of the ITO-coated plastic foils.
ACKNOWLEDGMENTS
The authors thank Girolamo Mincuzzi, Luigi Salamandra, Gianpaolo Susanna, and Gabriele De Angelis for assistance in laboratory equipments and materials, Solutia Performance Films
(Seube Thierry) and Dupont Tejiin Films (Angela Gardner), for
providing plastic substrates, This work was financially supported by a grant from Regione Lazio ‘‘Polo Solare Organico
Regione Lazio.’’ T. M. Brown thanks MIUR for a ‘‘Incentivazione
alla mobilità di studiosi residenti all’estero’’ fellowship.
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