22nd International Symposium on Plasma Chemistry
July 5-10, 2015; Antwerp, Belgium
Evaluation of TiO 2 and N-TiO 2 blocking layer to improve the efficiency in
dye-sensitized solar cells
C. Stegeman1, R.S. Moraes1, E. Saito2, D.A. Duarte3,4, A.L.J. Pereira1, A.S. da Silva Sobrinho1, D.M.G.Leite1 and
M. Massi1,2
1
Technological Institute of Aeronautics, Plasmas and Processes Laboratory, São José dos Campos, SP, Brazil
2
Federal University of São Paulo, Institute of Science and Technology, São José dos Campos, SP, Brazil
3
Federal University of Santa Catarina, Center of Mobility Engineering, Joinville, SC, Brazil
4
Catholic University of Santa Catarina, Joinville, 89203-005, SC, Brazil
Abstract: Blocking layer is the name of a very thin film used to improve the efficiency of
dye-sensitized solar cells due to decreasing of the dark current. In this work, we grow up
these thin films made with TiO 2 e N-TiO 2 by plasma depositions that were carried out by
DCMS and HiPIMS. The structural analysis of the films was made by profilometry,
ellipsometry and goniometry. After assembly of the cells, their characterization were
performed by current-voltage curve, from which we observed an increase of 10% in the
overall conversion efficiency.
Keywords: dye sensitized solar cells, blocking layer, HIPIMS, DCMS
1. Introduction
Over the years you may notice a decrease in the
availability of fossil fuels for energy, which stimulates the
studies on the development and improvement of
renewable techniques such as the use of solar energy. In
light of these ideas in this work we present results of the
study with dye-sensitized solar cells [1], fitted with a
system called blocking layer (BL). This system consists
of inserting a semiconductor nanometric thin film layer
between the nanoporous TiO 2 film and the transparent
conductive oxide (TCO) [2]. The aim is to prevent charge
recombination at the electrolyte and TCO interface.
The materials indicated for efficient BL have to present
a high work function, proper band alignment, low
resistivity and high transparency [3]. The most widely
used materials for BL are usually titanium oxides such as
TiO x , Nb-doped TiO 2 , N–TiO 2 , but also different metal
oxides, such as Nb 2 O 5 , SiO 2 and ZrO 2 , have been
successfully used [3].
A compact TiO 2 BL can be prepared by different
methods such as electron beam evaporation, chemical
vapor deposition from precursors including Ti 3 O 5 under
oxygen partial pressure or from other precursors like tetra
isopropyl orthotitanate or by the aerosol pyrolysis
deposition [4]. ]. Plasma processes can be used to produce
films which shows very particular characteristics, for
example, high compression and intense decrease in the
number of defects.
In this work TiO 2 e N-TiO 2 films have been obtained
by direct current magnetron sputtering (DCMS) and high
power impulse magnetron sputtering system (HiPIMS)
[5,6]. This is an industrial technique which allows highly
reproducible results, over large surfaces, a relatively low
deposition rate, and as a consequence, highly accurate
control of thickness as well as compositional uniformity
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can be achieved. We present the results of structural film
blocking layer through goniometry, profilometry and
ellipsometry. The cell characterization has been studied in
the framework of the Grätzel-type dye sensitized solar
cell, through the current-voltage curve.
2. Materials and methods
2.1. Blocking layer deposition
In this study, the blocking layer of TiO 2 and N–TiO 2
films were deposited on Si (100) and glass covered with a
thin transparent conductive coating layer of tin oxide
doped with fluorine (FTO) with resistivity of 7 ohm / sq
(Solaronix TCO22-7) by sputtering technique using
sources DCMS and HiPIMS without substrate heated.
A high purity (99,95%) titanium target was sputtered by
both DCMS and HiPIMS in order to produce the films.
Table 1 shows the deposition conditions and the
experimental set up is shown in Fig. 1.
Fig. 1. Experimental set up of the BL depositions.
1
Table 1. Deposition Conditions
Parameter
DCMS
HiPIMS
ma power
300 W
Frequency
0
Acting time
-
120 Hz
150 ms
Target-substrate distance
110 mm
TiO 2 deposition time
40 min
120 min
N-TiO 2 deposition time
20 min
40 min
1.5 nm/min
0.5 nm/min
3 nm/min
1.5 nm/min
TiO 2 deposition rate
N-TiO 2 deposition rate
TiO 2 flow rates
10.4 sccm of Ar
3.5 sccm of O 2
N-TiO 2 flow rates
10.4 sccm of Ar
0.6 sccm of O 2
10 sccm of N 2
Working pressure
5 mTorr
2.2. DSCs fabrication
After BL deposition, a nanoporous TiO 2 layer (T-90
Dyesol Transparent Titania Paste), with approximately 18
ηm thickness and 1 cm2 area, was deposited on the BL
and sensitized for 24 hours in a ruthenium based N3 dye
(Solaronix Ruthenizer 535), as illustrated in Fig 2a.
Fig. 2. Working electrode (a) and cell (b).
FTO is a thin layer of platinum (Solaronix Platisol T),
which was sintered with a heating ramp of 15 °C/min to
450 °C and maintained in this temperature during 10
minutes. Electrode and the counter electrode were
attached by clips, which guaranteed no direct contact
between them, as show in Fig 2b.
Finally, we injected the electrolyte (Solaronix MPN-50)
with a micropipette. The cells were not sealed; however,
were analyzed immediately after their assembly and,
subsequently, discarded. The cell was assembled as
schematically shown in Fig. 3.
2.3. Layers characterization
The thickness of BL was measured by profilometry
(KLA Tencor Alpha-Step D-600) on Si (100) substrate.
The gap energy was calculated by the Tauc extrapolation
method [7] by using ellipsometry (Horiba Jobin Ivon
UNISEL 2) on Si (100) substrate. The contact angle
analyzes were performed by goniometry (Ramé-Hard
Model 500 - Advanced Goniometer) with deionized water
with drops of 6 μl.
2.4. DSCs Functional characterization
The functional properties of the cells have been
investigated under simulated sun light irradiation using a
solar simulator at 1 sun with an AM1.5 G filter (Orbital
Engenharia
Simulador Solar
Contínuo SOLSIM).
Before each test, incident light intensity has been
carefully checked by a calibrated silicon cell.
3. Results and Discussion
3.1. Structural and optical results
For analysis of the blocking layer films, profilometry
technique was used to measure thickness of the layers, as
shown in Fig. 4 for TiO 2 DCMS sample.
Fig. 4. Thickness profile obtained by mechanical
profilometry for TiO 2 sample made by DCMS.
Fig. 3. Schema of multilayered structure and of the cell
assembling procedure. From the left, glass substrates
covered with FTO, next layer is the BL and followed by a
typical solar cell composition, with an active layer made
by nanoporous oxide of titanium, liquid electrolyte and a
counter-electrode made by glass with FTO covered by Pt.
The nanoporous film was sintered with a heating ramp
of 10 °C/min to 500 °C and maintained in this
temperature during 30 minutes. The counter electrode
2
All films are approximately 60 nm. As were done with
different compositions and techniques, we observed
variation in deposition rates, and consequently the
deposition time changed. The deposition rate varied as
shown in Table 1. This difference between the depositions
rates of the two techniques is due to the difference
between the plasmas sources in which the HIPIMS
plasma provides more intense, high degree of ionization
and off-normal transport ionized particles [8], that
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promotes a sputtering of produced films, reducing their
thicknesses.
For the analysis of ellipsometry we observed that the
energy gap decreases for samples produced by HIPIMS,
especially the doped sample, which has the lowest value,
as shown in Table 2. The nitrogen doped samples show
this effect due to the orbital N 2p in the structure of the
films [9].
Table 2. Structural and optical results.
Only FTO
Gap
energy
(eV)
Contact
angle
(deg)
Surface
energy
(mJ/m2)
-
66.4±2,4
100.8
TiO 2 DCMS
3.,17±0,04
57.5±1,1
110.7
TiO 2 HiPIMS
3.09±0,08
19.9±7,2
139.7
N-TiO 2 DCMS
3.21±0,18
69.7±0,5
96.9
N-TiO 2 HiPIMS
2.86±0,40
83.8±0,8
79.8
From the contact angle it is possible to evaluate the
surface wettability as well as surface energy, which was
calculated from the classical Young-Dupre equation [10].
The larger contact angle was obtained for sample N-TiO 2
HiPIMS. This is an indication that the surface has few
imperfections and, consequently, few oxygen deficiencies
because of the higher values of contact angle. It indicates
that only a small amount of H 2 O was decomposed in the
film surface and this effect decreases the presence of
hydroxyl radicals in film showing a hydrophobic behavior
and low surface energy. All these results are shown in
Table 2.
3.2. Functional and characterization
Fig. 5 shows the current-voltage curve obtained for all
the cells assembled and studied in this work.
efficiency with BL films, which indicates a decrease in
the recombination of electrons between electrolyte and
BL. As expected, there was no significant change in the
fill factor [11].
Based on the same results, we see that the short-circuit
photocurrent increases after incorporation of nitrogen in
the film lattice and the cell built the film deposited by
HiPIMS has the highest value. This is caused by the better
crystallinity degree for films deposited by HiPIMS in
comparison to other sputtering sources [6]. On the other
hand, the open-circuit photovoltage decreased slightly for
N-TiO 2 deposited by HiPIMS. This was caused by cracks
of the nanoporous TiO 2 during lab analyses, which
increased the BL exposition to the electrolyte solution
and, as consequence, increased the dark current.
However, the photovoltage decreasing was suppressed by
the photocurrent increasing, which evidenced a
reasonable increase of approximately 10% in the overall
cell efficiency in comparison to the cell without BL and
above 2% after nitrogen incorporation.
Table 3. Functional parameters of the cells.
Isc (mA)
Voc (V)
FF
η (%)
Without BL
4.80
0.56
0.65
1.76
TiO 2 DCMS
4.89
0.57
0.66
1.84
TiO 2 HiPIMS
4.99
0.57
0.66
1.88
N-TiO 2 DCMS
4.69
0.60
0.67
1.87
N-TiO 2 HiPIMS
5.01
0.59
0.65
1.93
4. Conclusion
The results show that BL is an effective way to increase
dye-sensitized solar cell efficiency. All samples showed
improvement in the overall conversion efficiency
compared to the cell without BL. The film doped with
nitrogen produced by HiPIMS showed the best results. It
had different optical properties, such as low energy gap,
and high contact angle, showing a flatter surface and
probably without defects. The BL acts effectively to
prevent back electron recombination and it is the key
factor in enhancing the overall cell performance. This
layer is an interesting solution for improving efficiency of
DSCs.
5. Acknowledges
The authors thank the financial support of FAPESP
(Grant 2011/50773-0), CNPq (Grant 555.686/2010-8) and
CAPES.
Fig. 5. Current-voltage curve for all cells studied in this
work.
From Fig. 5 the functional parameters of the cells were
calculated. Table 3 presents the short circuit current (J SC ),
open circuit voltage (V OC ), fill factor (FF) and cell
efficiency (η). We observe an increase in the cell
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