A. DRYGA£A et al.: A CARBON-NANOTUBES COUNTER ELECTRODE FOR FLEXIBLE DYE-SENSITIZED SOLAR CELLS
623–629
A CARBON-NANOTUBES COUNTER ELECTRODE FOR
FLEXIBLE DYE-SENSITIZED SOLAR CELLS
ELEKTRODA IZ OGLJIKOVIH NANOCEVK ZA TANKOPLASTNE
BARVNO OB^UTLJIVE SON^NE CELICE
Aleksandra Dryga³a1, Leszek Adam Dobrzañski1, Marzena Prokopiuk vel
Prokopowicz1, Marek Szindler1, Krzysztof Lukaszkowicz1, Marian Domañski2
1Institute of Engineering Materials and Biomaterials, Silesian University of Technology, Konarskiego St. 18a, 44-100, Gliwice, Poland
2Centre of Polymer and Carbon Materials, Polish Academy of Sciences, M.Curie-Sk³odowskiej St. 34, 41-819, Zabrze, Poland
aleksandra.drygala@polsl.pl
Prejem rokopisa – received: 2016-07-15; sprejem za objavo – accepted for publication: 2017-01-20
doi:10.17222/mit.2016.206
Dye-sensitized solar cells (DSSCs) are an attractive alternative to conventional crystalline silicon solar cells because of their
low-cost, relatively high photon-to-current conversion efficiency for low energy consumption and simple fabrication process.
The dye-sensitized solar cell consists of the following components: a photoanode, a dye, an electrolyte, and a counter electrode.
The counter electrode is a crucial element, in which the triiodide is reduced to iodide by electrons flowing through the external
circuit. Platinum is the most used material for a counter electrode in DSSCs, due to its electrocatalytic activity towards I3–
reduction. However, the use of platinum may not be a suitable option because of its high cost. Additionally, to achieve wide-
spread application of next-generation photovoltaics, it is important to develop flexible devices. Given this, the paper presents the
influence of mechanical stress arising from the bending of a flexible substrate on the morphology, the resistance of counter
electrode based on carbon nanotubes as well as the electrical properties of dye-sensitized solar cells.
Keywords: photovoltaics, dye-sensitized solar cells, flexible substrates, counter electrode, carbon nanotubes
Fleksibilne tankoslojne barvno ob~utljive son~ne celice (angl. DSSCs) so privla~na alternativa konvencionalnim kristalnim
silicijevim son~nim celicam zaradi nizke cene in relativno visoke u~inkovitosti pretvorbe foton-tok, z nizko porabo energije in
enostavnim postopkom procesom proizvodnje izdelave. Tankoplastna son~na celica je sestavljena iz naslednjih komponent:
fotoanode, barvila, elektrolita in tokovne elektrode. Tokovna elektroda je klju~ni element, na kateri se trijodid reducira na jodid
z elektroni, ki te~ejo skozi zunanji tokokrog. V solarnih celicah je platina najpogosteje uporabljan material za tokovne elektrode
zaradi svoje elektrokataliti~ne aktivnosti (redukcija I3– ionov). Zaradi visoke cene pa uporaba platine vendarle ni optimalna
izbira. Torej, da bi dosegli {iroko uporabo naslednjih generacij fotovoltaike je pomembno razvijati fleksibilne naprave. ^lanek
predstavlja vpliv mehanskih napetosti zaradi upogibanja fleksibilnega substrata na morfologijo in odpornost tokovne elektrode
iz ogljikovih nanocevk, kot tudi elektri~ne lastnosti tankoslojne son~ne celice.
Klju~ne besede: fotovoltaika, tankoslojne barvno ob~utljive son~ne celice, fleksibilni substrati, tokovna elektroda iz ogljikovih
nanocevk
1 INTRODUCTION
The utilisation of renewable energies is of significant
importance because of the increase in fossil energy costs
in combination with carbon dioxide reduction preventing
global warming. The importance of solar energy can be
considered as a sustainable energy that may successfully
satisfy a part of the energy demand of future gene-
rations.1,2 Photovoltaics seems to be the most promising
new electric energy source. What is needed to transform
solar power from a marginalised technology to a main-
stream source of energy is cheaper materials. At present,
the main scientific effort is made to lower the production
costs of PV systems and improve their parameters. It is
believed that in the not so distant future, thanks to new
materials, solar cells could be ubiquitous and one of the
cleanest energy sources all over the world. 2,3 There are
many publications about the methods of shaping the sur-
face and structure of materials to improve their proper-
ties.4–12 Some materials exhibit a property known as the
photovoltaic effect that causes them to absorb photons of
light and excite an electron or other charge carrier to a
higher-energy state. When these free electrons are
captured, an electric current results that can be used as
electricity. In recent years, most of the solar cells are
based on a silicon substrate.11–15 Although the cost per
peak watt of crystalline silicon solar cells has signi-
ficantly dropped, it is still expensive compared to the
conventional grid electricity resources.15
Dye-sensitized solar cells (DSSCs) are an inexpen-
sive alternative to the conventional p-n junction solar
cells. The use of DSSCs is one of the most promising
approaches towards the realisation of both high perfor-
mance and low cost, thanks to their low material cost and
ease of manufacturing.16–18 One of the challenges of this
technology is, however, expensive, the heavy and non-
elastic glass substrate typically used in DSSCs. There-
fore, scientists transfer the DSSC technology from glass
substrates to lightweight, cost-efficient, and flexible
plastic foils and metal sheets. Additionally, flexible
dye-sensitized solar cells built on elastic substrates have
Materiali in tehnologije / Materials and technology 51 (2017) 4, 623–629 623
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
UDK 621.311.243:621.365.3:621.38 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 51(4)623(2017)
attracted great industrial interest because they can be
roll-to-roll printed, which is well suited for scale mass
production (accelerate production and reduce cost).19–21
In this paper we report on flexible DSSCs based on a
carbon-nanotubes counter electrode. The influence of
mechanical stress arising from the bending of the flex-
ible substrate on the quality and resistance of the depo-
sited layers was investigated.
A fundamental property of insulators is resistivity.
The resistivity can be used to determine the dielectric
breakdown, dissipation factor, moisture content, mecha-
nical continuity and other important properties of the
material. The volume resistivity of some materials, such
as sapphire and Teflon®, can be as high as 1016 to 1018
·cm. Because of such large magnitudes, measuring the
resistivity of insulators can be difficult unless proper test
methods and instrumentation are used. One test method
often used for measuring the resistivity of materials is
ASTM D-257, "DC Resistance or Conductance of
Insulating Materials." Instruments called electrometers
are used to make this measurement because of their
ability to measure small currents. Two methods are most-
ly used to measure high resistance, i.e., the constant-volt-
age method and the constant-current method.22–23
In the constant-voltage method, a known voltage is
sourced and a picoammeter or electrometer ammeter is
used to measure the resulting current. The basic configu-
ration of the constant-voltage method is shown in Fig-
ure 1. In this method, a constant-voltage source, V, is
placed in series with the unknown resistor, R, and an
electrometer ammeter, A. Since the voltage drop across
an electrometer ammeter is negligible, essentially all the
voltage appears across R. The resulting current is
measured by the ammeter and the resistance is calculated
using Ohm’s law, R=V/I.23–24
In the constant-current method, a constant current is
forced through the unknown resistance and the voltage
drop across the resistance is measured. The basic
configuration for the constant-current method is shown
in Figure 2. Current from the constant-current source, I,
flows through the unknown resistance, R, and the voltage
drop is measured by the electrometer voltmeter, V. Using
this method, resistances up to about 1014  can be
measured. Even though the basic procedure seems
simple enough, some precautionary measures must be
taken.22–24
One of the components in dye-sensitized solar cells is
the counter electrode. The role of the counter electrode is
to act as a catalyst for reducing the redox species, which
are the mediators for regenerating the sensitizer after the
electron injection, or for collecting the hole from the
hole-transporting materials.25–28 This paper presents the
influence of mechanical stress arising from the bending
of a flexible substrate on morphology, the resistance of
the counter electrode based on carbon nanotubes with the
addition of poly(3,4-ethylenedioxythiophene)-poly(sty-
renesulfonate) PEDOT:PSS and polyvinylpyrrolidone
PVP as well as electrical properties of dye-sensitized
solar cells.
2 EXPERIMENTAL PART
Multi-walled carbon nanotubes (MWCNTs) were
dispersed in anhydrous ethyl alcohol using ultrasonic
dispersion methods. Then the poly(3,4-ethylenedio-
xythiophene)-poly(styrenesulfonate) PEDOT:PSS was
added. Carbon nanotubes are materials that strongly
agglomerate. Therefore, they were exposed to dispersion
for 45 min. To prevent the aggregation of carbon nano-
tubes, the mixture of 95 % of mass fractions of active
material and 5 % of mass fractions of polyvinylpyrro-
lidone PVP was used. Using a spin coating methods, the
mixture was deposited on the polymer polyethylene
terephthalate (PET) with a thin layer of indium tin oxide
(ITO) and dried at 60 °C for 25 min. The platinum thin
film was deposited on a PET foil in a device based on the
sputtering method (Physical Vapour Deposition – PVD)
in an Ar atmosphere. The sputtering time was 90 s, the
current was 50 mA and the voltage was 50 V.
The photoelectrode was prepared on a flexible
ITO-PEN substrate by a doctor-blade technique using a
commercially available TiO2 nanocrystalline powder
P 25 Degussa mixed with ethyl alcohol and distilled
water. After that the films were sintered at 120 °C for 4
h. The photoelectrodes were immersed in the anhydrous
ethanol solution of 0.5-mM N3 (Cis-diisothiocyanato-
bis(2,2’-bipyridyl-4,4’-dicarboxylic acid) ruthenium(II),
Solaronix) for 24 h at room temperature. The internal
space between the photoanode and the counter electrode
was controlled at 25 μm by Surlyn foil (Solaronix). The
photoanode and counter electrode were sealed with a
25-μm-thick Surlyn frame at 100 °C for 15 s. After
sealing the cells were filled with the electrolyte solution
A. DRYGA£A et al.: A CARBON-NANOTUBES COUNTER ELECTRODE FOR FLEXIBLE DYE-SENSITIZED SOLAR CELLS
624 Materiali in tehnologije / Materials and technology 51 (2017) 4, 623–629
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Figure 2: The constant-current method for measuring resistance 22–24
Figure 1: The constant-voltage method for measuring resistance 22–24
with redox couple I–/I3– through the hole in a counter
electrode.
The influence of the mechanical stress arising from
the bending of the flexible substrate on the quality and
resistance of counter electrode based on carbon
nanotubes with the addition of PEDOT:PSS and PVP.
The samples were bent with the tester for measuring the
bending strength of polymeric materials (Figure 3).
The morphology of the counter electrodes based on
platinum and carbon nanotubes deposited on a PET foil
was performed using a scanning electron microscope
(Zeiss Supra 35). In order to obtain images of the surface
topography, the detection of secondary electrons by the
detector In Lens was used.
The resistance of the prepared samples was measured
using a Keithley meter. The meter was connected to a
specialised adapter for testing thin films. This was in
order to better contact and more accurately measure on
the edge of the layers applied silver contacts connected
to external electrodes. The measurement block diagram
used for our measurements is presented in Figure 4.
Resistance measurements were made using a con-
stant-voltage measurement in real time with the current
running. The chosen method allows us to obtain reliable
results only for layers deposited on a polymer foil. It can
be assumed that the resistance of the layers is the same
or similar, regardless of the substrate.
Electrical parameters of manufactured flexible
dye-sensitized solar cells with ca ounter electrode based
on platinum and carbon nanotubes with the addition of
PEDOT:PSS and PEDOT:PSS/PVP were characterised
by measurements of I-V illuminated characteristics on
PV Test Solutions Tadeusz ¯danowicz Solar Cell I-V
Tracer System under standard test condition (AM 1.5,
1000W/m2). The level of irradiance was determined
using the reference cells with a KG5 filter.
The construction of flexible dye-sensitized solar cells
is illustrated in Figure 5.
The structure of produced DSSCs consists of compo-
nents:
• working electrode – dye molecule (N3) coated
nanocrystalline porous TiO2 deposited on PET/ITO
substrate,
• counter electrode – carbon nanotubes with the
addition of PEDOT:PSS and PVP deposited on
PET/ITO substrate,
• electrolyte (Iodolyte Z-150, Solaronix) containing an
I–/I3– redox couple.
3 RESULTS AND DISCUSSION
Figure 6 shows the morphology of the PET/ITO foil
coated with the Pt film. It illustrates that the substrate is
uniformly covered with the Pt film.
A detailed inspection of scanning electron micro-
scope micrographs of PET/ITO foils coated with the Pt
film after bending revealed microcracks and crevices
(Figure 7). This indirectly indicates that platinum film
A. DRYGA£A et al.: A CARBON-NANOTUBES COUNTER ELECTRODE FOR FLEXIBLE DYE-SENSITIZED SOLAR CELLS
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Figure 3: Tester for measuring the bending strength of polymeric
materials
Figure 5: Construction of dye-sensitized solar cell
Figure 4: General measurement system set-up
A. DRYGA£A et al.: A CARBON-NANOTUBES COUNTER ELECTRODE FOR FLEXIBLE DYE-SENSITIZED SOLAR CELLS
626 Materiali in tehnologije / Materials and technology 51 (2017) 4, 623–629
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Figure 9: SEM images of counter electrode based on carbon nano-
tubes with addition PEDOT:PSS/PVP after 100 bending cycles
Figure 7: SEM images of platinum on PET/ITO foil after 100 bending
cycles
Figure 8: SEM images of counter electrode based on carbon nano-
tubes with addition: a) PEDOT:PSS/PVP, b) PEDOT:PSS after 100
bending cycles
Figure 6: SEM images of platinum on PET/ITO foil
can have poor properties for catalytic activity at the
counter electrode and electrical conductivity for the
charge transfer. The cracks are formed mainly along the
bending line. Moreover, the PET foil coated with ITO
and Pt after 100 bending cycles in some places delami-
nates and breaks away from the elastic substrate (Fig-
ure 7).
Figure 8 shows the SEM images of a counter elec-
trode based on carbon nanotubes with the addition of
PEDOT:PSS and PEDOT:PSS/PVP. The addition of 5 %
of mass fractions of polyvinylpyrrolidone PVP prevents
the agglomeration of carbon nanotubes. It was observed
that, after the same number of bending cycles, the sur-
face of the counter electrode based on carbon nanotube
with the addition of PEDOT:PSS/PVP is much more
damaged than the surface of the counter electrode with-
out PVP (Figures 9 and 10). However, the amount of
damage and the size of the defects in the carbon-nano-
tubes counter electrode is much smaller than in the
platinum foil. It can be seen that carbon nano-elements
reduce the spread of cracks.
The resistance of the carbon-nanotubes thin films
with the addition of the PEDOT:PSS polymer was about
4·103  (Figure 11). The appearance of small cracks
registered using a scanning electron microscope did not
affect significantly the results of electrical measure-
ments. Mechanical stress caused by the bending of the
substrate slightly affected the resistance of the tested
layer. After 50 cycles of the bending, the resistance value
increased to about 5.08·103. Further bending does not
result in significant changes in resistance and maintained
at the similar level.
The addition of PVP polymer, which prevents
agglomeration, caused a significant deterioration of the
resistance to about 7·1010  (Figure 12). The results of
the electrical properties measurement are correlated with
the surface studies conducted on a scanning electron
microscope. The impact of mechanical stress increased.
The appearance of micro-cracks after 100 cycles of
bending of the substrate increased the resistance to about
1.6·1011 . Therefore, registered increases the resistance
by more than one order of magnitude.
Table 1: Conversion efficiency of dye-sensitized solar cells with
counter electrodes based on platinum and carbon nanotubes with the
addition of PEDOT:PSS and PEDOT:PSS/PVP
Number
of
bending
cycles
Type of counter electrode
Platinum
Carbon nanotubes
with the addition
of PEDOT:PSS/PVP
Carbon nanotubes
with the addition
of PEDOT:PSS
0 4.02 % 3.61 % 3.42 %
100 2.42 % 2.65 % 2.96 %
A. DRYGA£A et al.: A CARBON-NANOTUBES COUNTER ELECTRODE FOR FLEXIBLE DYE-SENSITIZED SOLAR CELLS
Materiali in tehnologije / Materials and technology 51 (2017) 4, 623–629 627
MATERIALI IN TEHNOLOGIJE/MATERIALS AND TECHNOLOGY (1967–2017) – 50 LET/50 YEARS
Figure 12: Influence of mechanical stress on the resistance of
carbon-nanotubes thin films with the addition of PEDOT:PSS and
PVP deposited on a PET foil
Figure 10: SEM images of counter electrode based on carbon nano-
tubes with addition PEDOT:PSS after 100 bending cycles
Figure 11: Influence of mechanical stress on the resistance of
carbon-nanotubes thin films with the addition of PEDOT:PSS
deposited on a PET foil
The efficiency of dye-sensitized solar cells with
different types of counter electrodes is presented in Table
1. These results indicate that the photovoltaic properties
of dye-sensitized solar cells after bending is decreased.
Platinum is a preferred material for the counter electrode
because of its high conductivity and catalytic activity.
However, dye-sensitized solar cells based on a platinum
counter electrode subjected to the impact of mechanical
stress demonstrate the lowest efficiency. This is the result
of damage introduced into the platinum thin film depo-
sited by a sputtering method and its delamination from
the flexible substrate (Figure 7). These damages have a
detrimental influence on the proper working of the solar
cell and reduce its properties, which could be a conse-
quence of poorer charge transfer. It can be observed that
the carbon-nanotubes counter electrode is more resistant
to bending compared to the platinum counter electrode.
4 CONCLUSION
Flexible dye-sensitized solar cells built on elastic
substrates have attracted great interest as they are light-
weight and can be roll-to-roll printed to accelerate pro-
duction and reduce costs. The study showed the
possibility of replaceming the standard platinum counter
electrode by carbon nanotubes deposited on the elastic
substrate. In the frame of this work, we investigated the
influence of mechanical stress arising from the bending
of a flexible substrate on the morphology, the resistance
of the counter electrode based on carbon nanotubes as
well as the electrical properties of dye-sensitized solar
cells.
It was found that the bending has a significant
influence on the morphology of platinum layer deposited
on a PET foil. It was shown that on the platinum surface
after bending cracks were formed and in some places the
layer was delaminated and broken away from the sub-
strate. The addition of PVP to the counter electrode
based on carbon nanomaterials with PEDOT:PSS
prevents agglomeration of carbon nanotubes, but after a
bending test the resistivity of the prepared layer and
electrical properties of produced dye-sensitized solar
cells are reduced. The amount of damage and the size of
defects in the carbon nanotubes counter electrode is
much smaller than in the platinum foil. Moreover, it can
be seen that carbon nano-elements reduce the spread of
cracks. Mechanical stress slightly effects the resistance
of studied carbon nanotubes layers with the addition of
PEDOT:PSS. Only the addition of the PVP polymer,
which prevents agglomeration, caused a significant
deterioration of the resistance as well as the increased
influence of mechanical stress. These damages have a
detrimental influence on the operation of solar cells and
reduce their efficiency.
Acknowledgements
The project was funded by the National Science
Centre on the basis of the contract No. DEC-2013/
09/B/ST8/02943. This publication was co-financed by
the Ministry of Science and Higher Education of Poland
as the statutory financial grant of the Faculty of Mecha-
nical Engineering SUT.
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