L. A. DOBRZAÑSKI et al.: CHARACTERISTICS OF DYE-SENSITIZED SOLAR CELLS WITH CARBON NANOMATERIALS
649–654
CHARACTERISTICS OF DYE-SENSITIZED SOLAR CELLS WITH
CARBON NANOMATERIALS
ZNA^ILNOSTI NA FIKSIRANO BARVO OB^UTLJIVIH SOLARNIH
CELIC Z OGLJIKOVIMI NANOMATERIALI
Leszek Adam Dobrzañski, Agnieszka Mucha, Marzena Prokopiuk vel Prokopowicz,
Marek Szindler, Aleksandra Dryga³a, Krzysztof Lukaszkowicz
Silesian University of Technology, Konarskiego St. 18A, 44-100, Gliwice, Poland
krzysztof.lukaszkowicz@polsl.pl
Prejem rokopisa – received: 2014-07-30; sprejem za objavo – accepted for publication: 2015-09-21
doi:10.17222/mit.2014.134
Dye-sensitized photovoltaic cells consisting of a layered structure have been developed for 20 years and they are a basis for the
new development trend of photovoltaics. One of the examined aspects of their application is building-integrated photovoltaics.
Dye-sensitized photovoltaic cells (DSSCs) were developed by Michael Grätzel and Brian O’Regan in 1991 and have been
intensively examined ever since. Because of their low production costs, easy transfer, the relatively high efficiency of the photon
conversion to the current and an easy production technology, dye-sensitized cells might represent an alternative to silicon cells.
Basically, a dye-sensitized photovoltaic cell consists of five elements: a mechanical base covered with a layer of transparent
conductive oxides (TCOs), a semiconductor film, e.g., TiO2, dye absorbed on the semiconductor’s surface, an electrolyte
including a redox carrier, and a counter electrode suitable to regenerate a redox carrier, e.g., platinum. As part of this work we
produced dye-sensitized solar cells. First, the glass with transparent conductive oxides was thoroughly cleaned. Then, the glass
with TCO was coated with a layer of TiO2 using the doctor-blade technique, and fired in a furnace at 450 °C. The plate prepared
in this way was then sensitized in a ruthenium-based dye. The counter electrode was obtained by applying it on the glass with
TCO carbon nanomaterials, including graphite, carbon black and carbon nanotubes. The photo-anode and the counter electrode
were combined and between them was injected the redox electrolyte. This paper provides an analysis of the microstructure and
electrical properties of nanostructural coatings with the carbon nano-element of the integrated dye-sensitized photovoltaic cells.
Keywords: dye-sensitized solar cell, carbon elements, counter electrode
Na fiksirano barvo ob~utljive fotovoltai~ne celice sestojijo iz plastovite strukture in so zadnjih dvajset let tematika razvoja na
tem podro~ju, predstavljajo namre~ nov razvojni trend v fotovoltaiki. Eden od raziskovanih vidikov njihove uporabe je
fotovoltaika, integrirana v zgradbe. Na fiksirano barvo ob~utljive fotovoltai~ne celice (DSSC), sta razvila Michael Grätzel in
Brian O’Regan leta 1991 in so od tedaj pogost predmet raziskav. Zaradi nizkih stro{kov njihove proizvodnje, enostavne
prenosljivosti, relativno visoko u~inkovite konverzije fotonov v tok in enostavne proizvodnje, so na fiksirano barvo ob~utljive
celice lahko nadomestek silicijevim celicam. V osnovi na fiksirano barvo ob~utljive fotovoltai~ne celice sestojijo iz 5
elementov: mehanska podlaga je prekrita s plastjo prosojnih, prevodnih oksidov TCO, polprevodnega sloja, npr. TiO2, fiksne
barve absorbirane na povr{ini polprevodnika, elektrolita z vklju~no redoks nosilcem nasprotna elektroda, ki je sposobna
regeneracije redoks nosilca, kot je npr. platina. Kot del tega dela, so bile izdelane na fiksirano barvo ob~utljive solarne celice.
Najprej je bilo steklo s presevnimi prevodnimi oksidi dobro o~i{~eno. Nato je bilo steklo s TCO, z uporabo tehnike kirur{kih
no`ev, prekrito s plastjo TiO2 in `gano v pe~i pri 450 °C. Tako pripravljena plo{~a je bila ob~utljiva za barve na osnovi rutenija.
Nasprotna elektroda je bila dobljena z nanosom TCO ogljikovih nanomaterialov, vklju~no z grafitom, ~rnim ogljikom in
ogljikovimi nanocevkami na steklu. Fotoanoda in nasprotna elektroda sta bili kombinirani in med njiju je bil vbrizgan redoks
elektrolit. ^lanek predstavlja analizo mikrostrukture in elektri~nih lastnosti prevlek z nano strukturo, z ogljikovimi
nanoelementi integrirane fotovoltai~ne celice, ob~utljive na fiksirano barvo.
Klju~ne besede: solarna celica, ob~utljiva na fiksirano barvo, ogljikovi elementi, nasprotna elektroda
1 INTRODUCTION
The increasing rate of energy consumption in the
world is directly related to the increase in the human
population. The progress of civilization is associated
with an increase in the demand for energy, and particu-
larly the most useful of its forms – electricity. Nowadays,
mankind consumes 13,500 GW of power. The solar
power reaching the Earth is 170,000,000 GW. If even a
part of this energy could be used it could reduce envi-
ronmental problems involving atmospheric pollution,
arising as a result of the excessive use of conventional
energy sources. Photovoltaics is an alternative and envi-
ronmentally friendly technology for electricity produc-
tion. Photovoltaic cells (also known as solar cells or PV
cells) are used to convert solar energy into electricity,
and this phenomenon is called the photovoltaic effect.
The main advantage that enhances the development of
organic photovoltaics is the potentially several times
lower price of energy production per unit cell area than
conventional solar cells based on silicon. Other
advantages include 1–4 much better aesthetics compared
to silicon solar cells, low toxicity, high transparency,
possibility to choose the colour, flexibility, low dead-
weight, low power loss due to the unfavourable angle of
incidence of the sunlight, which is used in BIPV
(Building Integrated Photovoltaics), working under
reduced radiation (cloudy, darkening), where these cells
Materiali in tehnologije / Materials and technology 50 (2016) 5, 649–654 649
UDK 67.017:620.3:621.383.51 ISSN 1580-2949
Original scientific article/Izvirni znanstveni ~lanek MTAEC9, 50(5)649(2016)
have a much better performance than silicon solar cells,
a low production price due to the use of small amounts
of material and the simplicity of production technolo-
gies, and the performance is independent of temperature
changes in the range of 25–65 °C.
The construction of a dye-sensitized solar cell is
based on a layered structure, which consists of two trans-
parent glass plates with a Transparent Conductive Oxide
(TCO) on it, placed parallel to each other and spaced
about 40 μm apart (Figure 1). On one of the plates is
applied a nanocrystalline titanium oxide layer coated
organometallic photosensitive dye (photosensitizer) –
this system retrieve in the cell function photo-anode
(illuminated anode). On the surface of the second plate
glass with TCO is usually nanoplatinum, which is a cata-
lytic layer – this system is in the cell cathode. The space
between the plates is filled with an electrolyte containing
a redox system I–/I3–. Each component shows the depen-
dence between many other materials. If at least one
element in dye-sensitized solar cells is changed, e.g., the
dye, the particle size of the TiO2, the film thickness, or
the composition of the electrolyte, the DSSC cell re-
quires adjustment to ensure optimal system perfor-
mance.5,6
The operating principle of a dye-sensitized solar cell
is shown in Figure 2. The dye and the electrolyte are
essential components of the cell.
The task of the counter electrode is to gather elec-
trons flowing from the outer current and to catalyse the
reduction of the triiodide ions. Platinum is the most
common material used as a counter electrode. Despite
the fact that platinum shows a high catalytic activity, its
shortage in resources, high costs and corrosion possibi-
lity through a triiodide solution, inhibit its application on
a large scale in the future.7 For this reason, there is a
need for research on alternative materials that are charac-
terized by electrochemical activity and chemical stabi-
lity. So far platinum8, carbon,9,10 conductive plastics,11
CoS,12 WO2,13, Mo2C and WC,14 TiN,15 have been used
as the counter electrodes. There are many publications
about the methods of shaping the surface and structure of
materials to improve their properties.16–18
Carbon nanotubes conduct electricity. They are
almost transparent, flexible and strong, which makes
them the ideal material for transparent electrodes for
DSSC. The only drawback is that photo-generated
charge carriers in the nanotube may recombine with ions
in the dye, which reduces the power-conversion efficien-
cy of the solar cell.
In the present work inexpensive and available carbon
materials such as carbon black, graphite, and carbon
nanotubes have been used as alternative materials to
platinum because of their high corrosive resistance, high
reactivity for triiodide reduction and low costs.7,9,10,19–25
The disadvantages in catalytic activity in comparison to
platinum may be compensated for by increasing the
active surface of a catalytic layer using the porous struc-
ture.7,25 Forming high-quality carbon bands on a sub-
strate gives us prospects for using carbon as a counter
electrode.
L. A. DOBRZAÑSKI et al.: CHARACTERISTICS OF DYE-SENSITIZED SOLAR CELLS WITH CARBON NANOMATERIALS
650 Materiali in tehnologije / Materials and technology 50 (2016) 5, 649–654
Figure 1: Construction of dye-sensitized solar cell6
Slika 1: Zgradba solarne celice ob~utljive na fiksirano barvo6
Figure 2: The operating principle of dye-sensitized solar cells6
Slika 2: Princip delovanja solarnih celic, ob~utljivih na fiksirano barvo6
Figure 3: The production steps of dye-sensitized solar cells
Slika 3: Proizvodni koraki pri solarnih celicah, ob~utljivih na fiksi-
rano barvo
2 EXPERIMENTAL PROCEDURE
The production steps for dye-sensitized solar cells
were shown in Figure 3. As the counter electrodes we
used three carbon materials: carbon black, graphite and
carbon nanotube. The dye-sensitized solar cells have the
following arrangement of layers (Figure 4):
• FTO glass/TiO2/dye/electrolyte/carbon black/FTO
glass,
• FTO glass/TiO2/dye/electrolyte/graphite/FTO glass,
• FTO glass/TiO2/dye/electrolyte/nanotube/FTO glass,
where:
FTO glass is glass with a layer of fluorine-doped tin
oxide (FTO).
2.1 Fabrication of the photoanode
Glass plates with dimensions 30 mm × 30 mm
(10
/sq.) were used. In order to remove any surface con-
tamination and to degrease, the FTO glasses were dipped
and held in an ultrasonic, deionized water, acetone,
ethanol and isopropanol. In order to reduce the active
surface of the dye-sensitized solar cells the FTO glass
was covered with a layer of tape (Figure 5). In order to
obtain a TiO2 paste nitric acid with pH 3–4 was mixed
with ethanol.26–28 To the solution was added titanium
oxide nanopowder. This solution was stirred until a
uniform paste was obtained. A drop of TiO2 paste was
applied to the FTO glass and then it was uniformly
spread over the glass surface using the doctor-blade
technique (Figure 6). After removing the tape, the glass
plate with the titanium paste was annealed in a furnace at
450 °C in air and then air-cooled. In order to sensitize
the electrode it was immersed in the dye solution with
absolute ethanol for 24 h at room temperature, without
access to light. Once removed, the electrode was washed
with ethanol to remove any excess dye and allowed to
dry. A glass plate with a layer of titanium oxide, with
and without dye, is shown in Figure 7.
2.2 Fabrication of the counter electrode
The preparation of the carbon nanotubes’ surface
layers on the silicon substrates was performed using
EasyTube®2000. During the process the default settings
were used. EasyTube®2000 is an advanced, turnkey,
thermal catalytic chemical vapor deposition CVD pro-
cess tool for the synthesis of a wide variety of nano-
structured materials. The catalyst needed for the CNT
growth was a transition metal plus iron, and the catalyst
was introduced to the process together with the CNT
precursor.
It this case the counter electrodes were prepared by
spring carbon nanotubes on the FTO glass. The CNT
solution was prepared by direct mixing of the acid highly
conductive PEDOT:PSS and applied on the glass surface
with FTO. The second electrode that was used for this
experiment was the electrode with carbon black. The
L. A. DOBRZAÑSKI et al.: CHARACTERISTICS OF DYE-SENSITIZED SOLAR CELLS WITH CARBON NANOMATERIALS
Materiali in tehnologije / Materials and technology 50 (2016) 5, 649–654 651
Figure 5: FTO glass covered with a layer of tape
Slika 5: FTO-steklo, prekrito s plastjo traku
Figure 6: Photo-anode preparation
Slika 6: Priprava fotoanode
Figure 4: Schema of DSSC with: a) carbon black, b) graphite, c) car-
bon nanotube
Slika 4: Shema DSSC z: a) ~rnim ogljikom, b) grafitom, c) ogljiko-
vimi nanocevkami
carbon black is cheap in industrial mass production. Ti is
also used in printing toners, so we can easily spray it on
to FTO glass.
A thin layer of carbon materials, i.e., carbon black
(Figure 8a), graphite (Figure 8b), carbon nanotube
(Figure 8c), were deposited on the FTO glass that was
previously cleared of impurities.
2.3 Fabrication of the DSSC
An anode and a cathode were combined with the
sealing strip, which simultaneously serves as a separator.
Careful electrode bonding is a very important step in the
preparation of DSSC cells. It prevents the leakage and
evaporation of the electrolyte.
The last step was the placement the electrolyte,
which is a solution of iodine and iodide in an organic
solvent containing a redox couple I–/I3–, between the
photo-anode and a counter electrode.
2.4 Measurements
Because the used carbon elements and titanium oxide
consist of nano-metric structural units or single carbon
layers, modern research equipment was used. First of all,
an Atomic Force Microscope, a High Resolution Trans-
mission Electron Microscope and a Scanning Electron
Microscope.
Atomic force microscopy (AFM, XE-100, Park
Systems) was used to observe the surface morphology of
the TiO2 layer with and without the dye.
The counter electrodes’ morphology was observed
using the scanning electron microscope (SEM; SUPRA
35, ZEISS).
The High Resolution Transmission Electron Micro-
scope S/TEM (TITAN 80-300, FEI) was used to observe
the surface morphology of the carbon nanotubes.
The voltages of the DSSCs were recorded using a
multimeter (Meter Link Extech EX845) as a source
measure unit, which was connected between the FTO
glass and the counter electrode.
3 RESULTS AND DISCUSSION
Figure 9 shows the AFM images of the TiO2 layer
and the dye-adsorbed TiO2 layer. The size and the dis-
L. A. DOBRZAÑSKI et al.: CHARACTERISTICS OF DYE-SENSITIZED SOLAR CELLS WITH CARBON NANOMATERIALS
652 Materiali in tehnologije / Materials and technology 50 (2016) 5, 649–654
Figure 8: Counter electrode with a layer of: a) carbon black, b) graphite, c) PEDOT: PSS with carbon nanotubes
Slika 8: Nasprotna elektroda s plastjo: a) ~rnega ogljika, b) grafita, c) PEDOT: PSS z ogljikovimi nanocevkami
Figure 9: AFM images of the: a) TiO2 layer and b) dye-adsorbed TiO2
layer
Slika 9: AFM-posnetek: a) plast TiO2 in b) s fiksirano barvo adsorbi-
rana plast TiO2
Figure 7: FTO glass with TiO2 layer: a) before dying, b) after dying
Slika 7: FTO steklo s plastjo TiO2: a) pred su{enjem, b) po su{enju
tribution of the particles characterize the dye-absorbed
surface of the photo-anode. The SEM images show that
TiO2 surface with the absorbed dye is smoother than the
surface without the dye.
Figures 10 to 12 show the SEM surface images of
various counter electrodes on the FTO glass. Figure 10
shows the microstructure of the carbon black, where a
rectangular atomic arrangement can be observed. Be-
cause the fluorine-doped tin oxide film deposited on the
glass has a rough surface, in Figure 11 where the gra-
phite is shown, we can also observe the rough pyramid
microstructure. Figure 12 shows the microstructure of
the carbon nanotubes’ electrode, where severely agglo-
merated CNTs are observable. We can see that the sur-
face with the CNT and the highly conductive
PEDOT:PSS has a uniform structure and fewer pores
were generated in the structure compared with the micro-
structure of the carbon black or the graphite.
By adding carbon nanotubes to the electrode it is
possible to obtain a larger surface area and therefore a
larger contact surface than the graphite and carbon black
counter electrode.
Table 1 shows the electrical parameters by means of
voltage for the three dye-sensitized solar cells with three
different counter electrodes. As was expected after the
SEM images, the highest voltage comes from the DSSC
with carbon nanotubes as a counter electrode.
Table 1: Electrical properties of the investigated DSSCs
Tabela 1: Elektri~ne lastnosti preiskovane DSSC
Type of counter electrode Voltage (mV)
carbon black 40
graphite 35
carbon nanotubes 52
4 CONCLUSIONS
In this study, for the purpose of decreasing the cost of
dye-sensitized solar cells (DSSCs), we investigated the
effect of carbon materials, i.e., carbon nanotubes, gra-
phite and carbon black, added as a counter electrode.
Three different low-cost DSSCs (CNT-counter electrode
DSSC, carbon black-counter electrode DSSC, graphite-
electrolyte DSSC) were fabricated.
The costly platinum electrode is able to be replaced
by the modified counter electrode (using CNTs), with
only little change in the efficiency of the cell. The effect
of the carbon nanotubes on the performance of the DSSC
showed that the DSSC fabricated with the CNT and
PEDOT:PSS had the highest photovoltaic performances.
These results are attributed to increasing the surface area
of the counter electrode.
The conductivity of the carbon black is lower than
carbon materials such as the graphite and carbon nano-
tubes, because those carbon nanotubes have a high con-
ductivity and a large surface area. The literature proves
that the carbon nanotubes with PEDOT:PSS layers have
a good adhesion, so it can be used on the DSSC
L. A. DOBRZAÑSKI et al.: CHARACTERISTICS OF DYE-SENSITIZED SOLAR CELLS WITH CARBON NANOMATERIALS
Materiali in tehnologije / Materials and technology 50 (2016) 5, 649–654 653
Figure 12: SEM images of carbon nanotubes
Slika 12: SEM-posnetek ogljikovih nanocevk
Figure 10: SEM image of carbon black
Slika 10: SEM-posnetek ~rnega ogljika
Figure 11: SEM image of graphite
Slika 11: SEM-posnetek grafita
electrodes. In the future it is planned to perform tests of
the adhesion for the products’ layers of low-cost DSSCs.
The investigated effect of the CNT counter electrodes
on the efficiency of DSSC showed that these kinds of
solar cells had the best efficiency for the counter electro-
des.
Acknowledgment
The project was funded by the National Science
Centre on the basis of the contract No. DEC-2013/09/B/
ST8/02943.
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